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Robert Tulip
2010-Jun-25, 01:51 AM
At http://www.bautforum.com/showthread.php/104931-What-is-the-meaning-of-the-claim-quot-time-is-an-illusion-quot?p=1750467#post1750467 Ken G said
“It is clearly not an "objective fact" that gravity is a force. If you think that is an objective fact, there are serious lapses in your knowledge of modern physics that would represent a significant sidebar to correct.”
Is gravity a force?

forrest noble
2010-Jun-25, 02:47 AM
Robert Tulip,

The prevailing theory of gravity, General Relativity, does not consider gravity as a force. It accordingly is caused by a warp of space caused by matter, the result of a change in space geometry. Without naming them, most other "competing theories" also do not consider gravity as a force today. Gravity as a force has its foundation in Newtonian Physics.

jlhredshift
2010-Jun-25, 02:58 AM
I too was taught that gravity is an effect of the warpage of spacetime and consequently, gravity waves seemed to make sense, but we have never detected them. I have not been able to understand, then, why gravitons are postulated?

Jens
2010-Jun-25, 03:03 AM
I think probably the reason that people tend to use "fundamental interactions" in the place of "fundamental forces" is that there is ambiguity about whether gravity is a force or not. I think that perhaps on a deeper level it's not so clear what a "force" is in the first place, so the question may perhaps be answered by the question, what is a force?

Geo Kaplan
2010-Jun-25, 03:06 AM
I have not been able to understand, then, why gravitons are postulated?

The short answer: In a quantum theory of gravity, there would be a force carrier, the graviton. Thus far, though, gravity has stubbornly resisted efforts at unification with the other "forces."

forrest noble
2010-Jun-25, 03:42 AM
Jens,

I agree with your question.


...........on a deeper level it's not so clear what a "force" is in the first place, so the question may perhaps be answered by the question, what is a force?

Following that question might come another, is there really any such a thing as a fundamental a priori (apriori) force of any kind in the first place or are such theorized forces such as the Strong Force, Weak Force, gravity, magnetism -- each of these so-called forces really caused by matter and its interactions by one means or another? -- even by means maybe far less complicated than the warping of spacetime.

EDG
2010-Jun-25, 04:07 AM
The short answer: In a quantum theory of gravity, there would be a force carrier, the graviton. Thus far, though, gravity has stubbornly resisted efforts at unification with the other "forces."

Couldn't that just mean that we're looking for something that doesn't exist? It may be that gravity simply doesn't work in a way that requires a "force carrier".

Geo Kaplan
2010-Jun-25, 04:39 AM
Couldn't that just mean that we're looking for something that doesn't exist? It may be that gravity simply doesn't work in a way that requires a "force carrier".

Yes, it is certainly possible that a grand unification is a pipedream -- we cannot rule that out. But many soldier on, nonetheless.

Robert Tulip
2010-Jun-25, 06:19 AM
Thanks for responses. Given the power of gravity to hold planets in their orbits with the inverse square law, I still wonder what it means for modern physics to say gravity is not a force. Pretty strong "interaction" that keeps Jupiter and all the planets zooming around at pretty well constant distances from the sun. My GUT feeling is there is one universe, so all forces are ultimately consistent with each other.

Jens
2010-Jun-25, 06:35 AM
Thanks for responses. Given the power of gravity to hold planets in their orbits with the inverse square law, I still wonder what it means for modern physics to say gravity is not a force.

To use an imperfect analogy, suppose that the universe was a frictionless hilly terrain with marbles rolling over it. Whenever there was a depression, the marbles would seem to be attracted to the object at the middle of the hole. But you wouldn't call it a force. The only force would be the momentum of the marbles.

Geo Kaplan
2010-Jun-25, 06:36 AM
Thanks for responses. Given the power of gravity to hold planets in their orbits with the inverse square law, I still wonder what it means for modern physics to say gravity is not a force. Pretty strong "interaction" that keeps Jupiter and all the planets zooming around at pretty well constant distances from the sun. My GUT feeling is there is one universe, so all forces are ultimately consistent with each other.

I'm not sure what you mean by "consistency", and I don't see how that notion would necessarily follow from the proposition that there is one universe.

EDG
2010-Jun-25, 06:36 AM
Thanks for responses. Given the power of gravity to hold planets in their orbits with the inverse square law, I still wonder what it means for modern physics to say gravity is not a force. Pretty strong "interaction" that keeps Jupiter and all the planets zooming around at pretty well constant distances from the sun. My GUT feeling is there is one universe, so all forces are ultimately consistent with each other.

Well, (a) nobody (on this thread) has actually defined what a "force" really is (Jens asked earlier) and (b) I'm not really sure if it's accurate to call gravity an "interaction" at all - at least not between two similar things. Electromagnetism involves particles interacting with eachother, the Strong and Weak forces (AFAIK) involve subatomic particles interacting with eachother... but what about Gravity? At one level one could see it as masses interacting with other masses, but I don't think that's an accurate representation of what's really going on - it seems to me that gravity is a result of masses existing in spacetime, but as far as we know right now there aren't "particles of spacetime" for matter to interact with.

agingjb
2010-Jun-25, 10:23 AM
I'm sure that there is some precise definition of "force" that may, or may not, exclude gravity. But "force", as a word, has an astonishing variety of meanings - many of which long preceded its use as a precise term in physics. To say that gravity is not a force seems to me to be making a distinction that must require further elaboration.

There is a book to written on the use of "force", "energy", and other words in various contexts, and their misuse by attempting to attribute and confuse their meanings in physics with their meanings in more general discourse.

Ken G
2010-Jun-25, 01:18 PM
To say that gravity is not a force seems to me to be making a distinction that must require further elaboration.

Or equally to the point, to say that it is a force requires further elaboration as well, because our current best theory of gravity neither treats it as a force, nor unifies it with the other forces. Many expect that will someday not be the case, but the only objective fact here is that the day has not yet come. I'd say the deeper issue here, in regard to what is objective fact, is that concepts like "what is a force" do not exist as statements of objective fact independently of some theory that can give meaning to those words. In Newton's theory, gravity is a force, in Einstein's, it isn't. The objective facts are the degrees to which these theories agree with various experiments-- not whether or not gravity is a force. There is an important difference, with respect to objective facts, between physical phenomena, and the language we use to understand physical phenomena. We do not capture the essence of a phenomenon by hanging a label on it. It may be an objective fact that we have chosen to hang a label, and it may be an objective fact that the label will someday prove wanting, but the label itself is never an objective fact.

AriAstronomer
2010-Jun-25, 01:24 PM
As EDG asked two posts above (for some reason my posting with quote isn't working), I think we already know that there cannot be any particles responsible for interactions between space-time and the material world. Space-time is currently accelerating, and (unless we have made a terrible rounding error) plans to accelerate to and beyond the speed of light. If particles were responsible, then they would also have to travel FTL to relay the information back to the spot that was 'doing the interaction'. They're are whacky ideas about tachyons and other particles able to travel FTL, but these seem to be still reserved for Sci-Fi at the moment.
The curvature of space-time is also what I have been taught as the responsible mechanism for gravity, although gravitons are very hopeful at the same time. Maybe another wave-particle duality (aka we don't know what it is)? Particles and curvature responsible for mediating gravity? I don't know.

agingjb
2010-Jun-25, 01:53 PM
Well it's up to you folks. If you want to use words in that way, then don't expect too many people to be able to see the distinctions that you are making. I'll admit that, although I take it on trust that there is a valid distinction, I'm not capable of making it myself.

jlhredshift
2010-Jun-25, 01:55 PM
In the movie "War Games" there came a point in the movie where the General was seeing multiple incoming warheads on his screens and to him it was an objective reality. His data was good as far as he was concerned. His instruments were telling him that he was under attack. Dr. Faulkin pleaded with him that it was a computer generated fantasy and asked "does it make any sense?"

The work of Perlmutter and Filippenko is above reproach, and awesome, but does an accelerating universe make any sense?

Ken G
2010-Jun-25, 03:39 PM
Well it's up to you folks. If you want to use words in that way, then don't expect too many people to be able to see the distinctions that you are making. I'll admit that, although I take it on trust that there is a valid distinction, I'm not capable of making it myself.
The distinction is actually rather easy to make. It would become very clear, for example, if and when a new theory of time comes along and makes the old concept seem naive. Then you will be able to tell the difference, yes? Note this has already happened three times even in just the last 100 years (once when we found that time is not just measured by a clock, it is owned by a clock, again when naive ideas that time could have no beginning were dispelled by the Big Bang model, and yet again when macroscopic time emerged from quantum mechanics as a kind of conceptual behavior of an aggregate of effects that are not so easily time-ordered-- as in delayed quantum erasure). To say that the distinction between our concept of time, and what time "really is", is tantamount to an admission of being unaware of even the last 100 years of physics.

agingjb
2010-Jun-25, 04:43 PM
Sorry folks, but "gravity is not a force" is simply incomprehensible to the vast majority of people, and it seems to me that I would do well to admit that general relativity and quantum dynamics are now actually beyond my own comprehension and pay no further attention to them.

Nereid
2010-Jun-25, 04:58 PM
Well it's up to you folks. If you want to use words in that way, then don't expect too many people to be able to see the distinctions that you are making. I'll admit that, although I take it on trust that there is a valid distinction, I'm not capable of making it myself.
This has been said before, but deserves repeating: in physics, as a science, technical terms have precise, specific, meanings. Sometimes these meanings are similar to, or overlap, the everyday English meanings, but sometimes they do not.

As this Q&A section is set up (its aims, etc), the answers to questions here need to be in line with the best, widely-accepted physics (or astronomy, etc) today.

And the most direct answer to the question in the OP is that, in GR, gravity is not a force.

A slightly longer answer might include the fact that, in Newtonian mechanics (etc), gravity is, indeed, a force. However, we have known for nearly a century now that the Newtonian theory is inconsistent with good observations (and, for nearly half a century, experimental results too). At the level of everyday, personal, experience, gravity seems to behave just like a force, on par with magnetic and electrical forces.

cosmocrazy
2010-Jun-25, 05:00 PM
Sorry folks, but "gravity is not a force" is simply incomprehensible to the vast majority of people, and it seems to me that I would do well to admit that general relativity and quantum dynamics are now actually beyond my own comprehension and pay no further attention to them.

This being so, as an amateur scientist and enthusiast I would prefer to modify or improve on what my old school teacher taught me and accept what the best minds consider as correct at present. Yes I have my own ideas on what gravity might or might not be, but the truth is know body really knows. What we do know is that observation suggests that on the macroscopic level gravity appears to be the warping of space-time or what ever that is, geometrical. At the microscopic level QM theory suggests that gravity could be caused by the interaction of matter in the form of force carrying particles named gravitons (yet to be observed).

I'll sit on the fence and see which route science takes as observation and experiments are improved.

agingjb
2010-Jun-25, 05:14 PM
Yes Cosmocrazy, that used to be my view, but it is now becoming rather obvious that I'm not even clever enough to make even that judgement on trust. The best minds in many fields are beyond me; in this case the distance seems to be too great for me to sustain an an interest,

cosmocrazy
2010-Jun-25, 05:20 PM
Yes Cosmocrazy, that used to be my view, but it is now becoming rather obvious that I'm not even clever enough to make even that judgement on trust. The best minds in many fields are beyond me; in this case the distance seems to be too great for me to sustain an an interest,

Well I believe thats the beauty of BAUT! We have some extremely knowledgeable members here who are happy to explain current science and theory into layman's terms which we should be able to get the gist of (well most of it anyhow). Even some of the best scientists struggle with fully understanding current QM advancements. But thats no different than how relativity was perceived approx 100 years ago.

Ken G
2010-Jun-26, 05:07 AM
Yes Cosmocrazy, that used to be my view, but it is now becoming rather obvious that I'm not even clever enough to make even that judgement on trust. The best minds in many fields are beyond me; in this case the distance seems to be too great for me to sustain an an interest,You are giving up to easily, your intellect is perfectly capable of understanding how gravity might not be a force. Let me take a different tack and see where it gets us, and ask you, is the force that makes you slide forward in your seat when you jam on the car brakes, is that indeed a force?

EDG
2010-Jun-26, 05:14 AM
You are giving up to easily, your intellect is perfectly capable of understanding how gravity might not be a force. Let me take a different tack and see where it gets us, and ask you, is the force that makes you slide forward in your seat when you jam on the car brakes, is that indeed a force?

Define "force". Until you do that, nobody can answer the question.

mugaliens
2010-Jun-26, 05:28 AM
Define "force". Until you do that, nobody can answer the question.

Any influence that causes a free body to undergo an acceleration." - Wikipedia (http://en.wikipedia.org/wiki/Force). I would add "...measurable by onboard accelerometers," as this is the key difference between gravity and a force. Just as gravity is not considered a force, it's effect is not classical acceleration. Thus, while it's convenient to calculate it's effect using F=m*a, since it's not a force, it's neither an acceleration, and the converse is true as well.

Ken G
2010-Jun-26, 05:36 AM
Define "force". Until you do that, nobody can answer the question.Like everything in physics, forces are defined by the theory that uses them (and although mugaliens' definition is indeed a good one for many classical theories of dynamics, we don't actually need a definition of force to know that gravity isn't one in general relativity). In general relativity, gravity is not used as a force, simply because the theory does not even need to define force to treat gravity. Your objection is mistaken-- in general relativity, a universe that contains nothing that the theory would call a force could still have gravity. Physics in that universe would not even have a concept of force, yet would have a concept of gravity. Indeed, this is more or less the situation (in general relativity) with most of the matter in our own universe-- the so-called "dark matter." As far as we now know, dark matter supports no concept of "force" at all, and is treatable entirely by general relativity. That may not always be true, but remember, the claim here is that it is not an "objective fact" that gravity is a force-- the claim is not that it is an "objective fact" that gravity isn't a force. What's more, the claim is that the only objective fact when a theory defines and uses a term is that we have created a theory that defines and uses that term. The term is never itself an objective fact, under any circumstances, and for any definitions. Supplying those definitions is therefore quite irrelevant.

agingjb
2010-Jun-26, 05:41 AM
I'll try just once more. Yes scientists need to use specialised language, but when they use common words in specialised ways so that precise formulations become nonsense in ordinary language, then they risk losing their audience.

Most people neither know nor care that what they perceive when they heft a weight has, or may have, such a different type of cause from the tug of a magnet that they ought to place it in a completely different category.

It's not just physics, and not just the borrowing of common words to use in precise senses that are not generally understood. Science, it seems to me, is moving away from lay understanding in many ways. There's another book to be written about the way exact taxonomy is making scientific nomenclature less stable than informal language.

Ken G
2010-Jun-26, 05:55 AM
I'll try just once more. Yes scientists need to use specialised language, but when they use common words in specialised ways so that precise formulations become nonsense in ordinary language, then they risk losing their audience.
But you didn't answer my question. When you slide forward in your seat in a braking car, is that because there is a force on you, or isn't it? You can use common words here, I'm not looking for nitpicky technical terms-- if you are explaining why someone slides forward, to an "ordinary person", is it a force, or not? It turns out that gravity, in general relativity, is just like that-- so if your answer is "it depends, I could explain it either way", then I'll say "yup, same for gravity." So does that mean it is an objective fact that gravity is a force? Does this require any "specialized language" to see?


Most people neither know nor care that what they perceive when they heft a weight has, or may have, such a different type of cause from the tug of a magnet that they ought to place it in a completely different category.And what about sliding in car seat? Do many people know or care if that's the same as magnetism, or not? So when you explain why that happens, how do you do it? If the person doesn't even care to have an explanation, so be it, but this is a science forum, so we assume they do want an explanation-- and we want to understand what we are explaining.



It's not just physics, and not just the borrowing of common words to use in precise senses that are not generally understood. Science, it seems to me, is moving away from lay understanding in many ways. There's another book to be written about the way exact taxonomy is making scientific nomenclature less stable than informal language.
If scientific language is moving farther from common people, then we have an educational challenge to close the gap by helping people understand the distinctions being made in that language-- in a simple and widely approachable way. It is certainly no solution to just let the language get sloppy on the grounds that people couldn't understand more precise usage, because then we run the risk of promoting misconceptions rather than promoting education. Many people may not wish to understand general relativity at all, true enough, but if they do want to understand it, the first place to start is the understanding of why gravity is not a force in that theory.

EDG
2010-Jun-26, 06:28 AM
"Scientific Language" isn't necessarily moving farther from common people, it's the people who are using that scientific language who are moving away from common people. As soon as anyone says "well, it's obvious that...'' or "clearly, it's...." or even "it's a simple concept" then that sets off warning bells for me, because it means they're assuming knowledge that isn't necessarily there in the audience.

You aren't explaining what a force is, and yet you're attempting to convince people that gravity isn't one. You haven't even really clearly explained WHY gravity isn't a force in the first place - you're just assuming that the audience knows all about relativity and why gravity isn't a force there. Start from the beginning before you try to break concepts that haven't even been explained to the audience yet.

It's all about building the conceptual scaffolding first.

EDG
2010-Jun-26, 06:31 AM
Any influence that causes a free body to undergo an acceleration." - Wikipedia (http://en.wikipedia.org/wiki/Force). I would add "...measurable by onboard accelerometers," as this is the key difference between gravity and a force. Just as gravity is not considered a force, it's effect is not classical acceleration. Thus, while it's convenient to calculate it's effect using F=m*a, since it's not a force, it's neither an acceleration, and the converse is true as well.

Do the Strong and Weak "forces" cause free bodies to undergo accelerations?

forrest noble
2010-Jun-26, 06:45 AM
Robert Tulip,


Given the power of gravity to hold planets in their orbits with the inverse square law, I still wonder what it means for modern physics to say gravity is not a force. Pretty strong "interaction" that keeps Jupiter and all the planets zooming around at pretty well constant distances from the sun. My GUT feeling is there is one universe, so all forces are ultimately consistent with each other.

It would seem that you may not be happy with the GR explanation concerning the warping of space as an explanation of gravity. What seems intuitive to one person may seem absurd to another. I don't like the explanation of warped space but dislike the idea of a priori (apriori) forces even more. To me all ideas of apriori pulling forces in general seem like 5,000 year old magical ideas having their origins in mysticism and magic (even though Newton was one of my idols). Whether warped space is correct or any idea concerning apriori forces captures ones intellect should be based upon logical interpretations and insight concerning cosmic observations and Earthly experiments, not just believing a particular theory for generally unknown reasons.

Hopefully someday proof of this or that explanation of gravity will be generally shown to be valid, and that the "forces of nature" will also be simply and logically explained without the need for them being apriori forces. My feeling is that these ideas concerning so-called apriori forces (gravity, magnetism, strong interaction, weak interaction) in general will not stand the test of time. That in the end everything that exists will have simple explanations, unlike warped space or quantum theory, without any kind of apriori forces at all -- but maybe that's just wishful thinking.

Robert Tulip
2010-Jun-26, 06:55 AM
Can this debate about the meaning of force possibly be seen as an example of the old distinction from Aristotle between the sublunary microcosm and the superlunary macrocosm? In the macrocosm, general relativity applies and gravity is not a force. In the microcosm of earth, which of course also obeys the law of the macrocosm, things fall, and force equals mass times acceleration, with gravity a strong force affecting entropy and cyclic patterns. Gravity is a practical force, for example it drives the energy of the tides, in perhaps a too simple mechanical Newtonian view of physics.

Ken G
2010-Jun-26, 09:26 AM
Do the Strong and Weak "forces" cause free bodies to undergo accelerations?Certainly, radioactivity involves dramatic acceleration of free bodies (tiny particles) via those forces, albeit not in exactly classical ways. So if your point is that terms like "force" don't exist independently of specific theories, that is certainly true. Quantum mechanics as a whole doesn't really use a concept of force, it uses a concept of energy (and even then a rather bizarre one). That's why the answer to the OP must be made more precise: gravity is not a force in general relativity.

agingjb
2010-Jun-26, 09:34 AM
Something slams into me; I slam into something; either way I get bruised.

I wait with interest to see what replaces the term "G-forces" in writing about aviation and driving.

Ken G
2010-Jun-26, 09:36 AM
In the macrocosm, general relativity applies and gravity is not a force. In the microcosm of earth, which of course also obeys the law of the macrocosm, things fall, and force equals mass times acceleration, with gravity a strong force affecting entropy and cyclic patterns.Is not the action of GPS satellites part of the "microcosm of Earth"? Yet they require GR corrections to function. The issue is not the "cosm", it is simply the level of accuracy you need. Some theories are more accurate than others, so we choose the theory based on the desired accuracy.

Gravity is a practical force, for example it drives the energy of the tides, in perhaps a too simple mechanical Newtonian view of physics.There is no doubt that imagining that gravity is a force serves us in many contexts and many desired levels of accuracy. That's why Newton's gravity is still used, even more often than Einstein's, in practical applications. But the point is that these are all representations of gravity, chosen for their usefulness, and statements like "gravity is a force" or "gravity is not a force" are not objective claims about reality, they are context-dependent value judgements, and therefore highly subjective and need-based. So it is with physics, in all cases, no exceptions.

ScientificBoysClub
2010-Jun-26, 09:37 AM
Gravity is the force which is consequence of geometry of Space and time.
Gravity is the distortion of space and time caused by mass !

dgavin
2010-Jun-26, 10:08 AM
I'm going to side with QM on this one and say that Gravity is a quantifiable force. There have been experiments demostrating gravitomagetic effects, which would indicate that like EM effects, GM effects can be induced in certain materials. It almost has to be a real QM force for this to happen, otherwise you have a case of materials sponaeously altering thier warping of spacetime without an increase of mass.

kevin1981
2010-Jun-26, 10:22 AM
My feeling is that these ideas concerning so-called apriori forces in general will not stand the test of time.

What are "apriori forces"? I do not understand this sentence.

antoniseb
2010-Jun-26, 11:45 AM
I wait with interest to see what replaces the term "G-forces" in writing about aviation and driving.

Excellent. The fact is that most of the above is discussing what's a force, and what is gravity, how is it expressed. If you are making calculations using the Newtonian model, then for your purposes, YES, gravity is a Force. If you are doing something else, maybe you call it a force whether out of tradition, or lack of a better monosyllabic term.

loglo
2010-Jun-26, 11:49 AM
I'm going to side with QM on this one and say that Gravity is a quantifiable force. There have been experiments demostrating gravitomagetic effects, which would indicate that like EM effects, GM effects can be induced in certain materials. It almost has to be a real QM force for this to happen, otherwise you have a case of materials sponaeously altering thier warping of spacetime without an increase of mass.

The point is that you don't have to choose sides at all. And if you do that says more about you than the nature of gravity!

Nereid
2010-Jun-26, 12:26 PM
I'm going to side with QM on this one and say that Gravity is a quantifiable force. There have been experiments demostrating gravitomagetic effects, which would indicate that like EM effects, GM effects can be induced in certain materials. It almost has to be a real QM force for this to happen, otherwise you have a case of materials sponaeously altering thier warping of spacetime without an increase of mass.
This is certainly an attractive viewpoint, and may well help someone (or someones) develop a better theory of gravity.

However, in the context of the (new) Q&A policy, it is misleading (at best).

The various (weird) GR effects which have now been demonstrated - in observations and experiments - are completely consistent with GR, and AFAIK there is no 'QM-like' theory which can account for these. Gravity is still not a force within GR.

Nereid
2010-Jun-26, 12:31 PM
Something slams into me; I slam into something; either way I get bruised.

I wait with interest to see what replaces the term "G-forces" in writing about aviation and driving.
When you go round a tight bend, at sufficiently high speed that your tyres squeal, you get slammed into the door (or would, if not restrained by seatbelts). What is the force that's slamming you?

agingjb
2010-Jun-26, 01:20 PM
When you go round a tight bend, at sufficiently high speed that your tyres squeal, you get slammed into the door (or would, if not restrained by seatbelts). What is the force that's slamming you?

Who knows? The consensus here seems to be against my instinct that what's slamming me is some sort of force. I bow to that consensus.

Nereid
2010-Jun-26, 02:29 PM
"Scientific Language" isn't necessarily moving farther from common people, it's the people who are using that scientific language who are moving away from common people. As soon as anyone says "well, it's obvious that...'' or "clearly, it's...." or even "it's a simple concept" then that sets off warning bells for me, because it means they're assuming knowledge that isn't necessarily there in the audience.

You aren't explaining what a force is, and yet you're attempting to convince people that gravity isn't one. You haven't even really clearly explained WHY gravity isn't a force in the first place - you're just assuming that the audience knows all about relativity and why gravity isn't a force there. Start from the beginning before you try to break concepts that haven't even been explained to the audience yet.

It's all about building the conceptual scaffolding first.
First, this would definitely take us away from the narrow scope of the OP's question ... but then, as happens so often in Q&A threads, we're already well away anyway (despite what the new Q&A policy says).

Do you mean something like this?

In everyday English, force means something like {insert words}.

This is not too far from what force means in much of classical physics, in particular Newtonian mechanics, and classical electromagnetism. {maybe add a few words about some differences?}

However, a little over a century ago there were two revolutions in physics, with the advent of relativity (first special, then general) and quantum mechanics (today, perhaps, quantum field theory).

Neither of these revolutions changed the amazing effectiveness of classical physics in explaining almost everything a person can see, or feel, in their everyday lives ... except for applications involving these revolutionary new theories, such as the GPS and computers.

Within these revolutionary new physics theories, force has a quite different meaning than it has in classical physics (and everyday life).

In QM, force means something like {insert words here}

And in General Relativity (GR), there are no forces at all. Instead, what we call gravity is explained in terms of the curvature of space-time, produced by the presence of mass-energy.

Now despite the gobsmacking success of both QM and GR {insert words describing that here}, they are mutually incompatible, at a deep, fundamental level {insert words describing that here}. So we already know that one, or possibly both, fail as accurate descriptions of reality (whatever you choose to take that word to mean).

But, within the framework of the new Q&A policy, we cannot speculate on what a theory, or theories, of physics might be, that replaces, or extends, either QM or GR; so we are not permitted (as some have already done in this thread) to speculate on whether, at some time in the future, gravity will again be a force.

Nereid
2010-Jun-26, 02:36 PM
When you go round a tight bend, at sufficiently high speed that your tyres squeal, you get slammed into the door (or would, if not restrained by seatbelts). What is the force that's slamming you?Who knows? The consensus here seems to be against my instinct that what's slamming me is some sort of force. I bow to that consensus.
The point of my question is to highlight a difference between the everyday use of the word force and its technical meaning in physics.

In this case, what most people would, instinctively?, describe as a force ('slamming' is a good, everyday, criterion for deciding if a force is present) is called a 'pseudo-force' in classical mechanics, and an explanation would involve something like frames of reference, inertia, and so on.

At a deeper level, the explanations provided by physics only make sense within the framework of those explanations; ergo, to understand those explanations, one needs to be prepared to accept the technical definitions that are an essential part of the relevant theories.

Finally, and again, questions here can be answered only within the bounds of the (new) Q&A policy ... and the bedrock of that is contemporary science.

FarmMarsNow
2010-Jun-26, 02:56 PM
p29
If scientific language is moving farther from common people, then we have an educational challenge to close the gap by helping people understand the distinctions being made in that language-- in a simple and widely approachable way. It is certainly no solution to just let the language get sloppy on the grounds that people couldn't understand more precise usage, because then we run the risk of promoting misconceptions rather than promoting education. Many people may not wish to understand general relativity at all, true enough, but if they do want to understand it, the first place to start is the understanding of why gravity is not a force in that theory.

p36
There is no doubt that imagining that gravity is a force serves us in many contexts and many desired levels of accuracy. That's why Newton's gravity is still used, even more often than Einstein's, in practical applications. But the point is that these are all representations of gravity, chosen for their usefulness, and statements like "gravity is a force" or "gravity is not a force" are not objective claims about reality, they are context-dependent value judgements, and therefore highly subjective and need-based. So it is with physics, in all cases, no exceptions. Nicely said.

Ken G
2010-Jun-26, 03:58 PM
Something slams into me; I slam into something; either way I get bruised.
Yes, but this is a reflection of what Newton called "action/reaction pairs." You will note, however, that fictitious forces, of the type that make you slide in your seat, do not come in action/reaction pairs. Hence, they are forces that do not even obey the same Newton's laws used to define the meaning of forces. They are called "coordinate forces", which appear from the very language you are using to describe what is happening, not from the invariant reality of the situation. (And by the way, when you actually slam into the door of the car, that is considered a force, and does come in an action/reaction pair. We are talking about the fictitious "force" that leads you toward the door in your own internal coordinate system inside the car, but does not register on an accelerometer in your pocket. We are not talking about the force from the door when you finally get there, which will indeed register on an accelerometer in your pocket.)


I wait with interest to see what replaces the term "G-forces" in writing about aviation and driving.When our labels think for us, we should not expect deep insights.

EDG
2010-Jun-26, 04:11 PM
In everyday English, force means something like {insert words}.

This is not too far from what force means in much of classical physics, in particular Newtonian mechanics, and classical electromagnetism. {maybe add a few words about some differences?}

However, a little over a century ago there were two revolutions in physics, with the advent of relativity (first special, then general) and quantum mechanics (today, perhaps, quantum field theory).

Neither of these revolutions changed the amazing effectiveness of classical physics in explaining almost everything a person can see, or feel, in their everyday lives ... except for applications involving these revolutionary new theories, such as the GPS and computers.

Within these revolutionary new physics theories, force has a quite different meaning than it has in classical physics (and everyday life).

In QM, force means something like {insert words here}

And in General Relativity (GR), there are no forces at all. Instead, what we call gravity is explained in terms of the curvature of space-time, produced by the presence of mass-energy.

Now despite the gobsmacking success of both QM and GR {insert words describing that here}, they are mutually incompatible, at a deep, fundamental level {insert words describing that here}. So we already know that one, or possibly both, fail as accurate descriptions of reality (whatever you choose to take that word to mean).

But, within the framework of the new Q&A policy, we cannot speculate on what a theory, or theories, of physics might be, that replaces, or extends, either QM or GR; so we are not permitted (as some have already done in this thread) to speculate on whether, at some time in the future, gravity will again be a force.

OK, good. Now, if you could fill in the blanks where you said "{insert words here}", you'd be making this whole thing a lot clearer to everyone.

Ken G
2010-Jun-26, 04:20 PM
I believe the reason Nereid didn't fill in those blanks is that what goes there gets pretty technical, and her point holds independently of those details. But I'll give it a go to keep it simple-- in everyday English, force means pretty much the same thing as influence, and indeed I can force you to do things by threatening you if you don't, which is certainly not the Newtonian meaning. In QM, force is an almost nonexistent concept, because QM uses energy instead, and is based on the Hamiltonian formulation of classical mechanics. The Hamiltonian formulation is also centered around energy, not force, and though you can define a "generalized force", it often stems from the dynamics rather than leading to the dynamics. So in QM, forces are a way of talking about what happened, they are not a driver of what happens, and we would be hard pressed to afford them with the same fundamental status they enjoy in the Newtonian framework. Indeed, the fact that QM is inconsistent with GR means that one or both theories must fundamentally be just ways of talking about what happens, rather than what drives what happens. Nor should this bother us-- it is sheer myth that physics is about determining what drives the phenomena we observe, in actual fact physics has always been about devising a quantitative language for talking about and predicting what happens. Hopefully that fills in the blanks well enough to see the point that she was making-- we can choose to talk about gravity using the language of forces, but our best current theory of gravity does not take that tack.

Does this mean that gravity is or is not a force? It means that questions like that aren't asking what they seem to be asking on a superficial level. A question whose answer can be "yes" or "no" is clearly not well enough posed to achieve clarity. The work to supply the question with meaning is just as important as the work to answer it.

agingjb
2010-Jun-26, 04:22 PM
Well folks, I give up. I would have advised a usage where the word "force" is explicitly qualified in a statement like "gravity is not a force". My advice is apparently unwelcome in BAUT.

Ken G
2010-Jun-26, 04:29 PM
Well folks, I give up. I would have advised a usage where the word "force" is explicitly qualified in a statement like "gravity is not a force". My advice is apparently unwelcome in BAUT.You are simply ignoring the fact that the usage has been perfectly well qualified. To repeat: in our best theory of gravity, no concept of force is ever introduced in order to treat gravity. What more clarification is needed-- that suffices to establish that gravity is not a force in that theory. Gravity is not a lot of things in GR, the theory need not go through all the things gravity is not-- it only has to provide what it is (at the simplest level, curvature of spacetime). Your fundamental disconnect is that you want "force" to have a precise scientific meaning that is universal to all theories. Physics language simply doesn't work that way-- all its terms are meaningful only in the context of particular theories. Even a term like "electron" has a very different meaning in different physical theories, and if you ask ten theoretical physicists "how do you like to think about what electrons are", expect 10 different answers, most of which would sound like gobbledy-gook. Yes they are all in some sense referring to the same phenomena, just as all language about gravity is referring to the same phenomena, but what a force is, and whether gravity is one, is all dependent on the larger context of what theory is being used. I'm sorry that scientific language requires this, but does it really require that much sophistication to accept this?

EDG
2010-Jun-26, 04:36 PM
Does this mean that gravity is or is not a force? It means that questions like that aren't asking what they seem to be asking on a superficial level. A question whose answer can be "yes" or "no" is clearly not well enough posed to achieve clarity. The work to supply the question with meaning is just as important as the work to answer it.

So, putting that into more understandable plain english, you're saying that gravity is a consequence of something else going on in Quantum Mechanics? OK, so what are these "dynamics", that you refer to?

agingjb
2010-Jun-26, 04:42 PM
No. I do not want "force" to have a a precise scientific meaning; I want the common word "force" not to have its meaning dictated by its usage in a science. But I said that. Time I quit here.

Geo Kaplan
2010-Jun-26, 05:24 PM
So, putting that into more understandable plain english, you're saying that gravity is a consequence of something else going on in Quantum Mechanics? OK, so what are these "dynamics", that you refer to?

No, gravity is not a consequence of anything going on in QM; gravity is apart from QM.

The effects of gravity are handled by GR.

Nereid
2010-Jun-26, 05:44 PM
In everyday English, force means something like {insert words}.

This is not too far from what force means in much of classical physics, in particular Newtonian mechanics, and classical electromagnetism. {maybe add a few words about some differences?}

However, a little over a century ago there were two revolutions in physics, with the advent of relativity (first special, then general) and quantum mechanics (today, perhaps, quantum field theory).

Neither of these revolutions changed the amazing effectiveness of classical physics in explaining almost everything a person can see, or feel, in their everyday lives ... except for applications involving these revolutionary new theories, such as the GPS and computers.

Within these revolutionary new physics theories, force has a quite different meaning than it has in classical physics (and everyday life).

In QM, force means something like {insert words here}

And in General Relativity (GR), there are no forces at all. Instead, what we call gravity is explained in terms of the curvature of space-time, produced by the presence of mass-energy.

Now despite the gobsmacking success of both QM and GR {insert words describing that here}, they are mutually incompatible, at a deep, fundamental level {insert words describing that here}. So we already know that one, or possibly both, fail as accurate descriptions of reality (whatever you choose to take that word to mean).

But, within the framework of the new Q&A policy, we cannot speculate on what a theory, or theories, of physics might be, that replaces, or extends, either QM or GR; so we are not permitted (as some have already done in this thread) to speculate on whether, at some time in the future, gravity will again be a force.OK, good. Now, if you could fill in the blanks where you said "{insert words here}", you'd be making this whole thing a lot clearer to everyone.
It was intended as nothing more than a template, with the objective being to see if the general approach was what you had in mind.

I'm glad that it is just the sort of thing you had in mind.

I'll comment further when I've digested the subsequent posts in this thread.

Ken G
2010-Jun-26, 05:46 PM
So, putting that into more understandable plain english, you're saying that gravity is a consequence of something else going on in Quantum Mechanics? OK, so what are these "dynamics", that you refer to?The dynamics are the Schroedinger equation, and like Nereid said, that does not treat gravity at the moment.

Nereid
2010-Jun-26, 05:47 PM
No. I do not want "force" to have a a precise scientific meaning; I want the common word "force" not to have its meaning dictated by its usage in a science. But I said that.
Then this part of BAUT is not the place to have such a discussion.

May I suggest that you start a thread in the Feedback section, on what your expectations are, and inviting discussion on how they might be met?


Time I quit here.
At the risk of taking this thread way OT, may I ask what you expected, both in this thread and this Q&A section in general?

agingjb
2010-Jun-26, 06:28 PM
Yes, it is very clear that I am in the wrong place.

dgavin
2010-Jun-26, 07:03 PM
This is certainly an attractive viewpoint, and may well help someone (or someones) develop a better theory of gravity.

However, in the context of the (new) Q&A policy, it is misleading (at best).

The various (weird) GR effects which have now been demonstrated - in observations and experiments - are completely consistent with GR, and AFAIK there is no 'QM-like' theory which can account for these. Gravity is still not a force within GR.

Not trying to be missleading. In the end we still don't know what gravity is. There isn't a real answer yet if gravity is a quantifiable force (with requisit QM force carrying particles) or if it's a simple warp of space/time due to mass. But those gravitomagnetic experiments that have been happening in the last decade, do seem to support that there is a QM solution to gravity. It just hasn't been found yet....

Nereid
2010-Jun-26, 07:27 PM
Not trying to be missleading. In the end we still don't know what gravity is. There isn't a real answer yet if gravity is a quantifiable force (with requisit QM force carrying particles) or if it's a simple warp of space/time due to mass. But those gravitomagnetic experiments that have been happening in the last decade, do seem to support that there is a QM solution to gravity. It just hasn't been found yet....(bold added)

How do you figure that?

After all, the effects are all predicted within GR, and AFAIK there is no QM-type theory of gravity which predicts them at all. FWIW, seeing an analogy, or link, here is a bit like saying that the blue-green colour of Neptune is similar to the bioluminescent blue-green, therefore maybe there's life on Neptune?

Nereid
2010-Jun-26, 07:41 PM
I believe the reason Nereid didn't fill in those blanks is that what goes there gets pretty technical, and her point holds independently of those details.
Well, it may have had more to do with laziness ... :p

But the main point was to see if the template worked, and that should work (or not) independently of the details (as you said).


But I'll give it a go to keep it simple-- in everyday English, force means pretty much the same thing as influence, and indeed I can force you to do things by threatening you if you don't, which is certainly not the Newtonian meaning.
I was thinking of a more narrow everyday meaning for force, a bit like agingjb's existence of slamming as a criterion for determining if there's a force present (or not).

There are certainly still significant differences - for example, if the wind isn't blowing, are there forces acting on trees, for example; or if you float in a body of water, are there any forces? - but there's a rough correspondence.


In QM, force is an almost nonexistent concept, because QM uses energy instead, and is based on the Hamiltonian formulation of classical mechanics. The Hamiltonian formulation is also centered around energy, not force, and though you can define a "generalized force", it often stems from the dynamics rather than leading to the dynamics. So in QM, forces are a way of talking about what happened, they are not a driver of what happens, and we would be hard pressed to afford them with the same fundamental status they enjoy in the Newtonian framework.
I'm glad you took a shot at describing force in QM! :)

I wonder if anyone can provide an equally accurate, yet simpler, description (I'm not sure I could)?


Indeed, the fact that QM is inconsistent with GR means that one or both theories must fundamentally be just ways of talking about what happens, rather than what drives what happens. Nor should this bother us-- it is sheer myth that physics is about determining what drives the phenomena we observe, in actual fact physics has always been about devising a quantitative language for talking about and predicting what happens. Hopefully that fills in the blanks well enough to see the point that she was making-- we can choose to talk about gravity using the language of forces, but our best current theory of gravity does not take that tack.
Yep.

And this gets to a key part of the OP's question: is it about what we, today, understand gravity to be (as in, what is the best theory we have of it, and how well does it account for all relevant observational and experimental results)? Or is it more about the extent to which it is useful to regard gravity as a force (in some, vaguely defined, sense)?

Perhaps the OP can comment, but I took it as very clear that the former was being asked (the latter consideration came from others' posts, and, strictly speaking, should not have been allowed, under the new Q&A policy).


Does this mean that gravity is or is not a force? It means that questions like that aren't asking what they seem to be asking on a superficial level. A question whose answer can be "yes" or "no" is clearly not well enough posed to achieve clarity. The work to supply the question with meaning is just as important as the work to answer it.
Well said.

forrest noble
2010-Jun-26, 07:53 PM
kevin1981,


What are "apriori forces"? I do not understand this sentence.

Kevin, apriori is Latin for prior or existing beforehand. The meaning in this context are the supposed forces of nature. The theory is that these are unexplainable natural forces rather than explainable forces caused by field or matter interactions of some kind. The four apriori pulling forces theorized by today's physics are the strong interaction, weak interaction, magnetism, and possibly gravity. Gravity was the first theorized apriori force.

Geo Kaplan
2010-Jun-26, 08:07 PM
Kevin, apriori is Latin for prior or existing beforehand.

Perhaps the misspelling is what is confusing him. It should be two words: "A priori."

publius
2010-Jun-26, 09:20 PM
Gravitomagnetic effects are as much "fictitious" forces as the familiar "gravitoelectric" part of gravity. The coriolis, -2w x v, acceleration "fields" of rotating frames is a perfect example. It is a magnetic like, velocity dependent coordinate "force". In the strong field, even the more familiar gravitoelectric part has velocity dependent terms.

Considering the new Q&A rules, I'll add the following as sidebar: The coriolis force is a pure coordinate effect from non-inertial frames in flat space-time. As we know, what really counts in GR is "tidal gravity". And that goes for gravitomagnetic (frame dragging) effects as well. We can transform away the coriolis force by simply stopping rotating around in our coordinate system. In GR, "real" frame dragging is the kind that that can't be transformed away -- that part is represented by a type of invariant space-time curvature.

-Richard

EDG
2010-Jun-26, 10:57 PM
The dynamics are the Schroedinger equation, and like Nereid said, that does not treat gravity at the moment.

OK, so what is gravity then? A warping of spacetime, right? But what does that warping do to objects that are affected by it?

Ken G
2010-Jun-26, 11:23 PM
OK, so what is gravity then? A warping of spacetime, right?Gravity is a word. Like all words in physics, it has a meaning that depends on the context. For example, the term "gravity" means many different things-- to an observation, it is a phenomenon. To Newton's theory, it is an action-at-a-distance force. To Einstein's theory, it is a curvature of spacetime. Which meaning is being invoked must be specified if it is not clear from the context-- we must avoid imagining that the term "gravity" means something directly about reality that is independent of these kinds of considerations.
But what does that warping do to objects that are affected by it?It determines their path. What does the curvature of an apple "do to" an ant that is trying to crawl a straight line across it?

Robert Tulip
2010-Jun-26, 11:27 PM
this gets to a key part of the OP's question: is it about what we, today, understand gravity to be (as in, what is the best theory we have of it, and how well does it account for all relevant observational and experimental results)? Or is it more about the extent to which it is useful to regard gravity as a force (in some, vaguely defined, sense)?

I think the thread has answered my question, many thanks to all, to establish that GR makes no use of the concept of force, GR provides the framework for our macroscopic reality, and therefore gravity is not a force as far as current understanding of the nature of space-time is concerned.

If I can pose an observation as a question: Do the matters raised about reconciling GR and QM illustrate that, with the traditional scientific framework of 'four forces' seeking to explain a grand unified theory of reality, gravity is anomalous, as a warp rather than a force?

Nereid's question about explaining useful observation is apt, as the disconnect between warping of space-time and an apple falling on your head is hard to bridge. The acceleration of a falling object under the Newtonian force of gravity contains a real natural power and energy. Finding a mathematical way to explain the practical observation of the power of gravity to hold the planets in their orbits and attract objects within a field seems to benefit, prima facie, from description of gravity as force, in the sense that gravity is the 'force of attraction' between objects. Indeed, this force is quantified by Newton's inverse square law, and accurately describes observation with the refinement of GR. So, I am attracted to the idea that there is a fundamental natural force of the universe that causes gravity.

EDG
2010-Jun-27, 12:14 AM
Gravity is a word. Like all words in physics, it has a meaning that depends on the context. For example, the term "gravity" means many different things-- to an observation, it is a phenomenon. To Newton's theory, it is an action-at-a-distance force. To Einstein's theory, it is a curvature of spacetime. Which meaning is being invoked must be specified if it is not clear from the context-- we must avoid imagining that the term "gravity" means something directly about reality that is independent of these kinds of considerations.

Why? If Einstein's theory is more accurate, then gravity is curvature of spacetime. It's still the same thing in Newton's theory, it's just that all Newton could say was that it's a property of mass that attracted other masses towards it - he didn't know it was a result of the curvature of spacetime then. He could determine the equations to show how something moved in a gravitational field in most scenarios, and they're not really any different from how it works at speeds well below c in Einstein's theory are they?

Ken G
2010-Jun-27, 12:32 AM
Do the matters raised about reconciling GR and QM illustrate that, with the traditional scientific framework of 'four forces' seeking to explain a grand unified theory of reality, gravity is anomalous, as a warp rather than a force?Yes, that's right, gravity does seem anomalous, and that's why it has so far not been unified with the others. Feynman wrote a whole book trying to construct gravity as a quantized field theory, and even he couldn't do it. Einstein couldn't do it either, but he was working from the side of trying to "fix" quantum mechanics to fit better with GR.

Robert Tulip
2010-Jun-27, 12:33 AM
Does the inverse square law (http://en.wikipedia.org/wiki/Inverse-square_law) measure the force of gravity?


The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance. The force is always attractive and acts along the line joining them.

Ken G
2010-Jun-27, 12:41 AM
Why? If Einstein's theory is more accurate, then gravity is curvature of spacetime. That doesn't logically follow. All we can say is that Einstein's theory is more accurate than Newton's, and makes the same predictions as Newton in the limit of low speeds.
He could determine the equations to show how something moved in a gravitational field in most scenarios, and they're not really any different from how it works at speeds well below c in Einstein's theory are they?The quantitative predictions are the same in that limit, but the predictions are arrived at with a different model. That's also true about Newtonian mechanics and special relativity in the absence of gravity-- but neither is an exact treatment of nature because there is always gravity. No theory ever gets to make a claim to being the actual reality, there's always something going on that isn't in the theory.

Tobin Dax
2010-Jun-27, 12:55 AM
Does the inverse square law (http://en.wikipedia.org/wiki/Inverse-square_law) measure the force of gravity?

Newton's Law of Universal Gravitation is an example of an inverse-square law, in that the magnitude of the gravitational force depends on the reciprocal of the square of the distance between two masses. The inverse-square law does not measure [Newtonian] gravitational force by any sense of the word that I can think of.

As it says at your link, the inverse-square law is an effect of geometry, describing the change of flux with distance for something emanating radially from a point source. It applies to many phenomena other than Newtonian gravity.

Robert Tulip
2010-Jun-27, 01:18 AM
The inverse-square law does not measure [Newtonian] gravitational force by any sense of the word that I can think of. When large objects are close their gravitational attraction is measured by the inverse square law as strong, while when small object are far apart the law says their attraction is weak, measured by the gravitational constant (http://en.wikipedia.org/wiki/Newtonian_gravitational_constant).

the inverse-square law is an effect of geometry, describing the change of flux with distance for something emanating radially from a point source.Is the radial flux of gravity an energy emanating from every point source?

Andrew D
2010-Jun-27, 02:43 AM
Is the radial flux of gravity an energy emanating from every point source?

It's not really an "energy." It's technically the integral of the vector field over a surface, or more simply, it describes the field density at a location within the field. For gravity (and as mentioned above, many other natural phenomena) it tends to 0 like 1/r^2 as the radius tends to infinity, so the density at any point in the field is proportional to 1 divided by the square of the distance between that point and the 'point source'.

EDG
2010-Jun-27, 02:57 AM
That doesn't logically follow. All we can say is that Einstein's theory is more accurate than Newton's, and makes the same predictions as Newton in the limit of low speeds. The quantitative predictions are the same in that limit, but the predictions are arrived at with a different model. That's also true about Newtonian mechanics and special relativity in the absence of gravity-- but neither is an exact treatment of nature because there is always gravity. No theory ever gets to make a claim to being the actual reality, there's always something going on that isn't in the theory.

Doesn't matter. Newton's a somewhat distant approximation to reality, Einstein's a closer one, and whatever we come up with next will be even closer, and eventually (maybe in a thousand years, maybe in a million, who knows) we'll get to a theory that perfectly describes actual reality. Sooner or later, we'll reach the bottom of the proverbial barrel - I'm quite sure that whatever gravity actually is will be understood eventually, assuming we (or someone else who is capable of such thought) are around long enough.

Tobin Dax
2010-Jun-27, 02:59 AM
When large objects are close their gravitational attraction is measured by the inverse square law as strong, while when small object are far apart the law says their attraction is weak, measured by the gravitational constant (http://en.wikipedia.org/wiki/Newtonian_gravitational_constant).
I don't define "the inverse square law" as equivalent to Newton's Law of Universal Gravitation. Neither does Wikipedia. Your assertion also reads like two situations where more than just distance is different. The inverse square law affect each situation, as do other factors.
Additionally, it seems like we don't agree on the use of "measure."


Is the radial flux of gravity an energy emanating from every point source?
I'd say no, but after all this discussion what force is, I really don't want to start the same thing about energy.

swampyankee
2010-Jun-27, 03:00 AM
I'm going to (more or less) quote one of my physics professors: it acts like a force, so you can call it a force.

publius
2010-Jun-27, 03:23 AM
Yes, the inverse square law can be seen as a result of geometry. In generalized "source charge" and "field" terms, a point source "gives off" an amount of "flux" equal to it's source charge, which spreads out evenly. At some distance r away from the source, the surface area of the sphere there is proportional to r^2, and the flux density is inversely proportional to this surface area. A constant relates this flux density to the strength of the force at such a point.

However, there's no reason why flux should spread out like that. The color force, responsible for the strong nuclear interaction doesn't spread out like that, but that is firmly in quantum territory and no classical field limit exists.

Robert Tulip asked was this flux density "energy". Well, in classical EM it is indeed proportional. THe field contains energy and there is a point energy density associated with each point in space of the field, which is given by 1/2(E dot D) and 1/2(H dot B) for a linear medium. So when a field is established around a point source, it fills all of space with an energy density going as 1/r^4. For a point source, the integral of that blows up, which is the infamous "infinite self-energy problem" of classical EM. Quantum theory doesn't fix that either, although the integral doesn't diverge as bad. It is hoped a unified field theory will fix it. :).

But we've got a problem with trying to fit gravity in this field energy density picture. Gravity is attractive, with a minus sign on the force law, meaning like charges attract of course.

Consider assembling a group of + charges onto a sphere say. As you put more and more charge on the sphere, the field increases and the energy density of the field does as well. And since like charges repel, you have to do work pushing the additional charges together, and that energy goes into the field. Let the charges fly apart and the field and energy is reduced, that energy going back into the mechanical energy of the particles carrying the charge. (Same thing with the magnetic forces on current loops). If let a negative charge fall into the original + sphere, the field energy is also reduced. This is all makes sense.

But note how the attractive nature of gravity throws a monkey wrench in this picture. Two masses tend to clump together, *giving energy up*, but the field is *increased*. As we assembly masses, we get energy out, but yet the field increases. The only way this can make sense is if the gravitational field energy is negative (and mathematically, that indeed follows from the minus sign on the force law, and incidently a gravitomagnetic field, in the GEM approximation would have to be negative energy as well).

That just doesn't make sense classically. Energy has to be positive. So, you can imagine putting some offset on the energy expression. Free space has some constant baseline energy density and the gravitational field subtracts from that. But since the field could be arbitrarily large, that constant would have no upper limit.

So right there, we've got a big monkey wrench in treating gravity as an EM-like field. Both Maxwell and Heaviside dabbled with what is now known as the GEM approximation (but they didn't know about that factor of 2 that comes from GR). Maxwell couldn't abide with the negative energy business and didn't try to develop it. Heaviside did, thinking the gravitomagnetic field might explain Mercury's procession, but it was way too small and he lost interest in it.

Gravity and EM seem similiar on the surface, but when you dig into it, you find they are very different. And exploring that led to Einstein and GR.

-Richard

astromark
2010-Jun-27, 05:55 AM
I'm going to (more or less) quote one of my physics professors: it acts like a force, so you can call it a force.

And all the other arguments are out of touch with that reality... That a effect can be measured so a force is apparent.
A field of gravitational influence. That space can be detected as bending by the presence of mass is by that definition a measure of gravity force.
I can not find a gravity particle or graviton. I can not show or even explain what process of action is being detected. Other than to measure a 'Force'...
I do not see any movement here from that view. It might be correct to state gravity is not a force.
As a direct action of that 'Force' I will fall on my face if tripped. Tell me that is not so and I will listen...
I am struggling to comprehend a other view... some of us have faulty wiring.

Nereid
2010-Jun-27, 07:33 AM
I'm going to (more or less) quote one of my physics professors: it acts like a force, so you can call it a force.And all the other arguments are out of touch with that reality... That a effect can be measured so a force is apparent.
A field of gravitational influence. That space can be detected as bending by the presence of mass is by that definition a measure of gravity force.
I can not find a gravity particle or graviton. I can not show or even explain what process of action is being detected. Other than to measure a 'Force'...
I do not see any movement here from that view. It might be correct to state gravity is not a force.
As a direct action of that 'Force' I will fall on my face if tripped. Tell me that is not so and I will listen...
I am struggling to comprehend a other view... some of us have faulty wiring.
I think these two posts nicely encapsulate a deep challenge those who answer Q&A questions face*.

Of course, as has been said many times in this thread, for many situations, circumstances, phenomena, etc you can treat gravity as a force, and that is perfectly good (for those situations etc).

However, that was not the question asked.

It's also been said, several times, that, at an intuitive level, it is difficult (or impossible) for some to grasp the concepts in GR, which lead to the conclusion that gravity is not a force (in GR).

The root of this difficulty, is, I suspect, quite deep; it is not easily addressed in the format we have here (FWIW, I think the core issue has to do with the nature of physics, what its limits are, and so on). I mean, how do you get across the idea that one's intuitive feelings concerning the nature of gravity, and forces, are irrelevant in terms of understanding GR? And that GR is the best we have, today, concerning gravity? And that "best" is mind-blowingly astonishing^?


Anyone is welcome to answer questions, but any answer should accurately reflect the generally accepted scientific mainstream. Some speculation is permissible if it is clearly indicated as such and if it can be traced to peer-reviewed science, i.e., if there's currently no clearcut mainstream answer, but mainstream scientists have proposed a possible answer that has yet to be verified or widely accepted.(bold added)

The answer to the OP's question is, I submit, one of most clear-cut you could wish for, in terms of "generally accepted scientific mainstream"; and that answer is "gravity is not a force in GR, and GR is the only currently accepted mainstream theory of gravity". Now there is a lot that can be clarified in this - what is a force? what is gravity in GR? how does 'curvature of spacetime' appear like a force? and so on - and we have had several post exchanges of this kind, in this thread.

However, we have also had quite a few posts which contain answers that clearly, and obviously, violate the "Some speculation is permissible if it is clearly indicated as such and if it can be traced to peer-reviewed science" policy. Yet no mod actions in sight.

(Perhaps a mod could split this post out, to form a new thread in the Feedback section?)

* the challenge is also embedded in several other, recent, posts too
^ Sky&Telescope has a recent article on the double pulsar; it gives a brief vignette of just how accurately GR accounts for natural phenomena

Nereid
2010-Jun-27, 07:53 AM
this gets to a key part of the OP's question: is it about what we, today, understand gravity to be (as in, what is the best theory we have of it, and how well does it account for all relevant observational and experimental results)? Or is it more about the extent to which it is useful to regard gravity as a force (in some, vaguely defined, sense)?I think the thread has answered my question, many thanks to all, to establish that GR makes no use of the concept of force, GR provides the framework for our macroscopic reality, and therefore gravity is not a force as far as current understanding of the nature of space-time is concerned.

If I can pose an observation as a question: Do the matters raised about reconciling GR and QM illustrate that, with the traditional scientific framework of 'four forces' seeking to explain a grand unified theory of reality, gravity is anomalous, as a warp rather than a force?

Nereid's question about explaining useful observation is apt, as the disconnect between warping of space-time and an apple falling on your head is hard to bridge. The acceleration of a falling object under the Newtonian force of gravity contains a real natural power and energy. Finding a mathematical way to explain the practical observation of the power of gravity to hold the planets in their orbits and attract objects within a field seems to benefit, prima facie, from description of gravity as force, in the sense that gravity is the 'force of attraction' between objects. Indeed, this force is quantified by Newton's inverse square law, and accurately describes observation with the refinement of GR. So, I am attracted to the idea that there is a fundamental natural force of the universe that causes gravity.(bold added)

Well, we have the OP elaborating on the question in the OP, so I guess it's OK to pursue this, in this Q&A thread.

First, thanks Robert Tulip for closing the loop, and letting us know that the question in the OP (yours) was answered satisfactorily.

I think the logic in the last two sentences is flawed; in any case I think you have not grasped a fundamental aspect of contemporary physics.

Suppose we re-cast these last two sentences in the context of QM; something like this perhaps:


Indeed, the electromagnetic force is quantified by Maxwell's equations, and accurately describes observation with the refinement of QED. So, I am attracted to the idea that there is a fundamental natural wave in the universe that causes electromagnetism.

Of course we, human beings, are most sensitive to phenomena that have a characteristic scale not too many orders of magnitude different than our own - we readily comprehend trees blowing in the wind, for example.

But why should the universe run according to human-scale perceptions? Surely one of the most humbling lessons from the history of physics is that we hold no special place in the universe? That things which happen at scales we can readily comprehend are not special in any way*?

Viewed in this light, the success of Newton's theory of gravity is a mere historical accident; as a scifi story, we could imagine intelligent beings who grew up in an environment where strong gravitational fields ruled derived GR first.

Put this another way: past successes in physics theories are not reliable guides to future performance.

* I'm not talking about how best to learn some aspect of physics here

astromark
2010-Jun-27, 08:18 AM
It has NOT been established that the question; " Is gravity a force ?" That being the OP. Can not be answered, yes...

If this warping of space by mass is not gravity, why do we call it so and what else is it ?

Do you want a effect of unknown cause.. shall I call it Dark Attraction... that does not seem to break this foolishness.

No, no no. I shall call it the weak force 'Gravity' As a alternate has not been provided or the justification for one established...

It is not I that steps out of the mainstream scientific view.... surly. Mark wondering what I have missed ?

Strange
2010-Jun-27, 01:46 PM
No. I do not want "force" to have a a precise scientific meaning; I want the common word "force" not to have its meaning dictated by its usage in a science. But I said that. Time I quit here.

The common word force will [probably] not have its meaning changed by science. That isn't how language works.

Many words have specific meanings in science or engineering which differ slightly from non-technical use. The meaning of those words can also vary from one discipline to another. I sometimes wonder what "normal" people mean when they use a word such as "virtual" (if the word even exists in their lexicon); it is almost certainly different from what it means to me in a work environment.

If you want to say gravity is a force out in the "real world", then that is OK. In the same way that it is convenient, if thechnically incorrect, to talk about centrifugal force. Just don't be surprised if some smart alec tells you it isn't!

Ken G
2010-Jun-27, 01:47 PM
Sooner or later, we'll reach the bottom of the proverbial barrel - I'm quite sure that whatever gravity actually is will be understood eventually, assuming we (or someone else who is capable of such thought) are around long enough.Why people persist in believing in this kind of mythology is beyond me. I'd say about the only thing that science allows us to be certain of is the falseness of this hope. But everyone is entitled to their hopes.

Ken G
2010-Jun-27, 01:53 PM
I'm going to (more or less) quote one of my physics professors: it acts like a force, so you can call it a force.You can call it John if you want. The issue is, what do you gain by doing so, and what do you lose? In this case, you lose the huge insight of general relativity. That insight works like this. If you drop your pen right now, in Newton's view, the pen will be the only thing in the room that has a net force on it, so the pen will be the only thing that is accelerating. Trouble is, if you build an accelerometer in deep space, and then attach it to the pen, it turns out the pen will be the only thing in the room that registers zero on that accelerometer-- everything else will have a nonzero accelerometer reading.

General relativity explains why that is-- it turns out the the pen is the only thing in the room that does not have a net force on it and is not accelerating. If your professor were right, and it's fine in all situations to call gravity a force because it "acts that way", then you would miss this insight. Probably, what your professor was saying translates better to something like "you can call gravity a force if you like, and it will have certain advantages, and certain disadvantages. Part of your education should be to learn what those advantages and disadvantages are-- rather than a blind adherence to a dogmatic assertion that gravity is a force."

Nereid
2010-Jun-27, 02:14 PM
It has NOT been established that the question; " Is gravity a force ?" That being the OP. Can not be answered, yes...

If this warping of space by mass is not gravity, why do we call it so and what else is it ?

Do you want a effect of unknown cause.. shall I call it Dark Attraction... that does not seem to break this foolishness.

No, no no. I shall call it the weak force 'Gravity' As a alternate has not been provided or the justification for one established...

It is not I that steps out of the mainstream scientific view.... surly. Mark wondering what I have missed ?
Let's take this slowly, step by step, OK?

The question in the OP is starkly simple and direct: "Is gravity a force?"

Now, per the new Q&A policy, we are required to answer this question (those of us who choose to attempt an answer*) in a pretty clearly defined way; namely, "Anyone is welcome to answer questions, but any answer should accurately reflect the generally accepted scientific mainstream."

And, as I have said, this is about as clear-cut a case you could wish for, in terms of what an accurate reflection of the generally accepted scientific mainstream is; namely, we have an astonishingly good theory of gravity (and only one; GR), and in that theory, gravity is not a force.

What you seem to be doing, in your post (that I'm quoting) is defining 'force' in such a way as to make gravity a force. There's nothing wrong with that ... except that it involves your own, personal, views and does not accurately reflect the generally accepted scientific mainstream (and so, per the Q&A policy, is not permitted).

The question of what forces are, in GR, has been briefly addressed earlier in this thread; perhaps I (or someone else) could write a rather lengthy post on whether GR includes the concept of force (and if so, how is it treated/understood), on how the fundamental forces in contemporary physics (i.e. the electroweak and the strong) are treated in GR, on what the equivalents of pseudo-forces (or fictitious forces or ...) are in GR, etc? That might help you - and other readers - understand better why gravity is not a force in GR.

It is certainly true that the idea that gravity is not a force, but rather the curvature of space-time, is a quite dramatic change from the Newtonian (and, to some extent, intuitive) one, and takes quite an effort to accept. However, that does not change the fact that the answer to the OP's question is very simple and straight-forward.

* FWIW, my take on the new policy is that all BAUTians should refrain from sidebar discussions, and stick to answering the OP's question

EDG
2010-Jun-27, 03:23 PM
Why people persist in believing in this kind of mythology is beyond me. I'd say about the only thing that science allows us to be certain of is the falseness of this hope. But everyone is entitled to their hopes.

I think you'd be wrong about that.

EDG
2010-Jun-27, 03:27 PM
You can call it John if you want. The issue is, what do you gain by doing so, and what do you lose? In this case, you lose the huge insight of general relativity. That insight works like this. If you drop your pen right now, in Newton's view, the pen will be the only thing in the room that has a net force on it, so the pen will be the only thing that is accelerating. Trouble is, if you build an accelerometer in deep space, and then attach it to the pen, it turns out the pen will be the only thing in the room that registers zero on that accelerometer-- everything else will have a nonzero accelerometer reading.

Relative to what's around it though, the pen is accelerating downwards towards the floor. Therefore (from the perspective of anyone around it), there is a force on it.

Spaceman Spiff
2010-Jun-27, 06:25 PM
...Trouble is, if you build an accelerometer in deep space, and then attach it to the pen, it turns out the pen will be the only thing in the room that registers zero on that accelerometer-- everything else will have a nonzero accelerometer reading.

General relativity explains why that is-- it turns out the the pen is the only thing in the room that does not have a net force on it and is not accelerating....

Swampyankee and EDG:
They don't call it "free-fall" for nothing! Think about that for a minute, and maybe a minute or two more. (And, yes, I am expanding the usual Newtonian definition.)

Our best understanding of the macroscopic world is General Relativity -- it describes how mass-energy 'moves' through 4-D space-time due to the presence of mass-energy -- along a geodesic (http://en.wikipedia.org/wiki/Geodesic) in the absence of the 3 fundamental forces. Because the Earth has significant mass with a particular effect on the geometry of space-time, then all other "point masses" (to keep things simple), such as yourself or your pencil, should be moving through space-time in a particular manner (along a geodesic). Trouble is, the electromagnetic "force" (and various quantum mechanical effects to do with Fermions), as manifested in the atoms of a planet, is "in the way" and is thus the accelerating agent. That is the only force on you and the only acceleration you are undergoing, if sitting in your chair, for example.

The freely falling pencil is moving through space-time along a path of maximal aging (http://www.eftaylor.com/pub/GRtoPLA.pdf) (a principle of "least" action (http://www.eftaylor.com/leastaction.html); see here (http://www.eftaylor.com/pub/QMtoNewtonsLaws.pdf), here (http://www.eftaylor.com/pub/CallToAction.pdf) and here (http://www.eftaylor.com/pub/ForceEnergyPredictMotion.pdf), in particular) -- ignoring the fact that that the individual atoms (or, rather, quarks and electrons) in the pencil are held together and thus not completely free to move through space-time as determined by its geometry due to the presence of non-uniform mass-energy (I'm also neglecting air-resistance).

Another example. A pencil falling freely in the vicinity of Earth is for all intents and purposes the same situation as the pencil off by itself, say 100 million of light years away from any important source of mass-energy (with the above noted caveats). A pencil in free-fall feels no force. If it were you suddenly finding yourself in intergalactic space, you would have the same feeling of "butterflies in the stomach" that you feel when you fall here on the Earth or as the astronauts feel falling (virtually) freely around the Earth. Gravity could be considered as another fictitious (coordinate) force, if you insist on associating gravity with force.

GR as we presently know it is unlikely to be the last word, but I hope the above helps in illustrating our present understanding.

mugaliens
2010-Jun-27, 07:24 PM
Something slams into me; I slam into something; either way I get bruised.

What's slamming into you that's keeping your feet planted firmly on the ground? (or your backside planted in the chair)


I wait with interest to see what replaces the term "G-forces" in writing about aviation and driving.

We just call it "G's." As for the phrase "times the acceleration due to gravity," that's where the misunderstanding arises, as it's not gravity that's doing the acceleration. Rather, it's the force on your feet that's accelerating you at 1 G, upwards, through the inertial frame which would have you freefall towards the center of the Earth. Thus, in freefall there's no acceleration with respect to the inertial frame. Rather, the acceleration arises when you're countering the effects of the intertial frame.

EDG
2010-Jun-27, 07:34 PM
Swampyankee and EDG:
They don't call it "free-fall" for nothing! Think about that for a minute, and maybe a minute or two more. (And, yes, I am expanding the usual Newtonian definition.)

Free-fall is when the only force acting on an object is gravity. I'm not sure what anything else you've said in this post has to do with that.


Another example. A pencil falling freely in the vicinity of Earth is for all intents and purposes the same situation as the pencil off by itself, say 100 million of light years away from any important source of mass-energy (with the above noted caveats). A pencil in free-fall feels no force. If it were you suddenly finding yourself in intergalactic space, you would have the same feeling of "butterflies in the stomach" that you feel when you fall here on the Earth or as the astronauts feel falling (virtually) freely around the Earth. Gravity could be considered as another fictitious (coordinate) force, if you insist on associating gravity with force.

Isn't a pencil in free-fall experiencing gravity though? If Earth and the pencil were the only matter in the universe, then a pencil sitting in space (even 100 million ly) from Earth would feel a tiny pull towards the Earth because Earth is sitting there and warping space time, and gravity has infinite range. The pencil would fall towards Earth as a result, under the sole influence of gravity. Wouldn't it?


GR as we presently know it is unlikely to be the last word, but I hope the above helps in illustrating our present understanding.

I'm afraid it didn't help me much. Got anything clearer?

astromark
2010-Jun-27, 07:57 PM
Thanks to Nereid. For showing me the explanation... being humbled by error is a skill I have mastered...

So this effect of a distortion of space is called gravity but is in fact only a effect of the distortion of space by action upon the cosmic web...

Now please consider the point; I have a observatory filled with young inquiring children. We are talking of escape velocities and re-entry speeds...

and now its not gravity as a force... That might be the case. Its not good enough is it ? The distortion of space by mass is not what I want understood.

I will use gravity force as a better tool of explanation than some regardlessly more accurate dribble.

My point is this. If a distortion of space by the presence of a warping of space by the action of a force... can you see some confusion ?

To the physicist no such gravity force is real. Its a distortion of space by the presence of mass...

Sorry, but 'MY' gravity force will survive. It is a fair argument that being correct is not always right.

As a space vehicle burns fuel so as to be slowed significantly as to allow the gravity force of Earth to accelerate the capsule back through the atmosphere...

Every time I see the words gravity force I WILL see the error... but still use it.

Tensor
2010-Jun-27, 08:53 PM
Every time I see the words gravity force I WILL see the error... but still use it.

And there is nothing wrong with that. Even NASA uses the idea of "gravity as a force" to move probes around solar system, even though they know that General Relativity is more correct. As Nereid noted in her post:


Anyone is welcome to answer questions, but any answer should accurately reflect the generally accepted scientific mainstream. Some speculation is permissible if it is clearly indicated as such and if it can be traced to peer-reviewed science, i.e., if there's currently no clearcut mainstream answer, but mainstream scientists have proposed a possible answer that has yet to be verified or widely accepted.

I don't think that any of the mods or admins would be upset with an answer that went:

To give you a straight answer, no, gravity is not a force. However, for over three hundred years gravity was treated as a force, using the mathematics found by Isaac Newton. In 1915 Albert Einstein developed the General Theory of Relativity(GR). This is our current best theory of gravity. Under this theory, gravity is not thought of as a force, gravity is a result of the curvature of spacetime, and objects follow geodesics, when in free fall, though spacetime. GR has been shown to be more correct than Newtonian gravity, which is why we say that gravity is not a force. However, in most cases, calculations using Newton's force equations, are close enough for the correct answer that they are used, even by NASA, to move space probes around the solar system.

The reason I don't think that there would be a problem here is that, while the mainstream is well aware that GR is the standard Gravitational Theory, the mainstream(as in NASA and most other groups that calculate the positions of the planets) also mainly use the Newtonian gravitational eqautions. Now, if any other questions arise out of that answer, then the person asking the question could ask another question. Also, it would depend on the level of the person asking the question whether or not I would go into some of the deeper answers that have been posted here.

Spaceman Spiff
2010-Jun-27, 10:31 PM
Free-fall is when the only force acting on an object is gravity. I'm not sure what anything else you've said in this post has to do with that.

That's Newtonian mechanics and gravity you're referring to. GR tells us that you should think of free-fall as a point mass like body moving along its geodesic in space-time. This motion is dictated by the geometry of space-time which is determined by the mass-energy.

If you insist on defining the relevant terms (force, acceleration, gravity) in a Newtonian sense, you should not be surprised that you keep seeing a Newtonian picture.



Isn't a pencil in free-fall experiencing gravity though? If Earth and the pencil were the only matter in the universe, then a pencil sitting in space (even 100 million ly) from Earth would feel a tiny pull towards the Earth because Earth is sitting there and warping space time, and gravity has infinite range. The pencil would fall towards Earth as a result, under the sole influence of gravity. Wouldn't it?

I'm afraid it didn't help me much. Got anything clearer?

If what you mean by "experiencing gravity" is motion along a geodesic (in the free-fall limit) through curved 4D space-time due to the presence of mass-energy, then I suppose you could say this. And of course, the two bodies you refer to would move through space-time according to its geometry as determined by the mass-energy (and any intrinsic curvature on cosmological scales, which we will ignore for these purposes). They would be in free fall -- as Einstein would describe this concept.

However, what I was trying to do in that example was to provide an illustration of the big picture and ignore the tiny, tiny details, the latter of which you preferred to look at ("gravity has infinite range"). So try tossing aside these infinitesimally small details and read and think about this again:


If it were you suddenly finding yourself in intergalactic space, you would have the same feeling of "butterflies in the stomach" that you feel when you fall here on the Earth or as the astronauts feel falling (virtually) freely around the Earth.

You believe that one's natural state is to sit on one's behind, and ascribe something special to a falling apple (or falling moon) and call that something a force. GR turns this on its head and does away with the force concept altogether(**). Instead GR speaks in the language of 'motion' through 4D space-time due to the presence of mass-energy.



(**) One could consider it to be a fictitious force.

EDG
2010-Jun-27, 10:40 PM
That's Newtonian mechanics and gravity you're referring to. GR tells us that you should think of free-fall as a point mass like body moving along its geodesic in space-time. This motion is dictated by the geometry of space-time which is determined by the mass-energy.

If you insist on defining the relevant terms (force, acceleration, gravity) in a Newtonian sense, you should not be surprised that you keep seeing a Newtonian picture.

OK, so define that motion then. How can you describe it without mentioning acceleration if it's well... accelerating? Something is moving towards something else, and in a way that it's gaining more speed every second. That's acceleration.



If what you mean by "experiencing gravity" is motion along a geodesic (in the free-fall limit) through curved 4D space-time due to the presence of mass-energy, then I suppose you could say this. And of course, the two bodies you refer to would move through space-time according to its geometry as determined by the mass-energy (and any intrinsic curvature on cosmological scales, which we will ignore for these purposes). They would be in free fall -- as Einstein would describe this concept.

OK, but in practical terms... the pencil's still accelerating toward the Earth. Yes, OK, the Earth's slightly affected by the mass-energy of the pencil, sure, but the "geometry of spacetime as determined by the mass-energy of the Earth" is what's really dominating the relative motions here, right? While your method of describing it may be more accurate, I'm not really convinced it adds anything to understanding what's going on - you're just referring to "acceleration due to gravity" in a more complicate way aren't you?

Ken G
2010-Jun-27, 11:50 PM
OK, so define that motion then. How can you describe it without mentioning acceleration if it's well... accelerating?You need to understand the difference between proper acceleration and coordinate acceleration, that will answer all your questions.


OK, but in practical terms... the pencil's still accelerating toward the Earth.Actually, in GR, the Earth is accelerating toward the pencil (not the whole Earth, mind you, but the ground under the pencil)-- at least that is the proper acceleration. You are referring to coordinate acceleration, which has no physical meaning independent from the coordinate choice. Proper acceleration is what an accelerometer measures, for example.


Yes, OK, the Earth's slightly affected by the mass-energy of the pencil, sure, but the "geometry of spacetime as determined by the mass-energy of the Earth" is what's really dominating the relative motions here, right? Yes, and that is what is responsible for the fact that the pencil is not accelerating and the Earth is (along with internal pressure forces inside the Earth, of course).


I'm not really convinced it adds anything to understanding what's going on - you're just referring to "acceleration due to gravity" in a more complicate way aren't you?No, in a more fundamentally true way. It isn't just accuracy we are gaining, it is just a better theory of gravity, applying better concepts.

Robert Tulip
2010-Jun-28, 12:09 AM
Is not the action of GPS satellites part of the "microcosm of Earth"? Yet they require GR corrections to function. The issue is not the "cosm", it is simply the level of accuracy you need. Some theories are more accurate than others, so we choose the theory based on the desired accuracy.What else requires GR correction to function? Where is Newtonian mechanics insufficient for terrestrial purposes?

There is no doubt that imagining that gravity is a force serves us in many contexts and many desired levels of accuracy. That's why Newton's gravity is still used, even more often than Einstein's, in practical applications. But the point is that these are all representations of gravity, chosen for their usefulness, and statements like "gravity is a force" or "gravity is not a force" are not objective claims about reality, they are context-dependent value judgements, and therefore highly subjective and need-based. So it is with physics, in all cases, no exceptions.How key is this to the view that the theory of relativity produced a change of paradigm in scientific understanding?
Yes, that's right, gravity does seem anomalous, and that's why it has so far not been unified with the others. Feynman wrote a whole book trying to construct gravity as a quantized field theory, and even he couldn't do it. Einstein couldn't do it either, but he was working from the side of trying to "fix" quantum mechanics to fit better with GR.
Does this illustrate how the shift from classical mechanics to general relativity involved a shift in frame of reference from a terrestrial bound observation of 'what works', to a universal reference based on the mathematical properties of the universe?

Nereid
2010-Jun-28, 12:12 AM
Thanks to Nereid. For showing me the explanation... being humbled by error is a skill I have mastered...

So this effect of a distortion of space is called gravity but is in fact only a effect of the distortion of space by action upon the cosmic web...

Now please consider the point; I have a observatory filled with young inquiring children. We are talking of escape velocities and re-entry speeds...

and now its not gravity as a force... That might be the case. Its not good enough is it ? The distortion of space by mass is not what I want understood.

I will use gravity force as a better tool of explanation than some regardlessly more accurate dribble.

My point is this. If a distortion of space by the presence of a warping of space by the action of a force... can you see some confusion ?

To the physicist no such gravity force is real. Its a distortion of space by the presence of mass...

Sorry, but 'MY' gravity force will survive. It is a fair argument that being correct is not always right.

As a space vehicle burns fuel so as to be slowed significantly as to allow the gravity force of Earth to accelerate the capsule back through the atmosphere...

Every time I see the words gravity force I WILL see the error... but still use it.
I think your confusion (if that's what it is) has to do with the very considerable differences between the sorts of questions you get asked - directly or indirectly - by your inquiring young children, and the question in the OP.

One of the key points in the lengthy discussion we had about how the Q&A policy should change is that the answers given should be appropriate to the intended audience. For your intended audience, treating gravity as a force is very likely to be entirely appropriate.

However, the audience which is the OP is very, very different from yours; Robert Tulip is a long standing BAUTian, and if you read through some of his posts, you'll see that he is very familiar with the distinction between Newtonian gravity and GR (as theories). So, for him, an answer that is satisfactory to your inquiring young children would almost certainly be entirely unsatisfactory.

Nereid
2010-Jun-28, 12:24 AM
I hope Ken G won't mind if I take a shot at answering some of your questions ...


Is not the action of GPS satellites part of the "microcosm of Earth"? Yet they require GR corrections to function. The issue is not the "cosm", it is simply the level of accuracy you need. Some theories are more accurate than others, so we choose the theory based on the desired accuracy.What else requires GR correction to function? Where is Newtonian mechanics insufficient for terrestrial purposes?
HIPPARCOS, GAIA, GPB, APOLLO, LISA, accurate measurements of time (e.g. the clock syncronisation of telecoms networks), accurate measurements of mass, ...



There is no doubt that imagining that gravity is a force serves us in many contexts and many desired levels of accuracy. That's why Newton's gravity is still used, even more often than Einstein's, in practical applications. But the point is that these are all representations of gravity, chosen for their usefulness, and statements like "gravity is a force" or "gravity is not a force" are not objective claims about reality, they are context-dependent value judgements, and therefore highly subjective and need-based. So it is with physics, in all cases, no exceptions.How key is this to the view that the theory of relativity produced a change of paradigm in scientific understanding?
I'm not sure what you're asking, but GR is about as radical a break with previous theories (with similar domains of applicability) as any, in the history of physics.



Yes, that's right, gravity does seem anomalous, and that's why it has so far not been unified with the others. Feynman wrote a whole book trying to construct gravity as a quantized field theory, and even he couldn't do it. Einstein couldn't do it either, but he was working from the side of trying to "fix" quantum mechanics to fit better with GR.
Does this illustrate how the shift from classical mechanics to general relativity involved a shift in frame of reference from a terrestrial bound observation of 'what works', to a universal reference based on the mathematical properties of the universe?
YMMV, but I think not.

The most critical step was taken by Newton, when he united the heavens and Earth, with his theory (as we'd call it today) of universal gravitation.

EDG
2010-Jun-28, 01:08 AM
You need to understand the difference between proper acceleration and coordinate acceleration, that will answer all your questions.

Then I'll need someone (preferably not you, because you're terrible at explaining anything) to explain clearly, concisely, and in plain english what the difference is. Telling me to understand it won't help, without that difference being explained.

As it is, Spiff seems to be a lot better at explaining these things than you are, so I'd rather let him continue (Thanks, Spiff).

swampyankee
2010-Jun-28, 01:12 AM
You can call it John if you want. The issue is, what do you gain by doing so, and what do you lose? In this case, you lose the huge insight of general relativity. That insight works like this. If you drop your pen right now, in Newton's view, the pen will be the only thing in the room that has a net force on it, so the pen will be the only thing that is accelerating. Trouble is, if you build an accelerometer in deep space, and then attach it to the pen, it turns out the pen will be the only thing in the room that registers zero on that accelerometer-- everything else will have a nonzero accelerometer reading.

General relativity explains why that is-- it turns out the the pen is the only thing in the room that does not have a net force on it and is not accelerating. If your professor were right, and it's fine in all situations to call gravity a force because it "acts that way", then you would miss this insight. Probably, what your professor was saying translates better to something like "you can call gravity a force if you like, and it will have certain advantages, and certain disadvantages. Part of your education should be to learn what those advantages and disadvantages are-- rather than a blind adherence to a dogmatic assertion that gravity is a force."

Yeah, but if I called it "John," nobody would know what I'm talking about. Language is a medium for communication, and words require commonly accepted meanings for them to be useful. Since -- by relativity -- I cannot tell with any local test whether an object is sitting on a planet, with gravity providing the "force" that holds me to the floor or if the object is being accelerated by a rocket, the thrust of which is providing the force which is holding me to the floor, I'm not sure that saying gravity is or is not a force is anything but sophistry.

publius
2010-Jun-28, 01:37 AM
Proper acceleration is a invariant measure of the deviation from the geodesic. It is what a local accelerometer measures. When you're sitting in you chair, such an accelerometer will measure 1g. Now, go into free fall. That accelerometer now registers 0. An accelerometer on an object in orbit will measure 0 as well.

Consider a rocket powered platform out in free space. Fire the rockets so an accelerometer on the platform registers 1g. The situation there is exactly the same as that on earth. Drop something, and it accelerates away from the platform at 1g, yet feels no force.

In the language of GR, both the platform and the surface of the earth are deviating from the local geodesic by 1g. Objects in free fall are following their geodesic. 1g is the "proper acceleration". Indeed in GR, we can still write Newton's Second Law as F = ma, but 'a' is now something a bit more involved, but yet much more elegant. It is the deviation from the local geodesic. That geodesic is determined by the distribution of mass-energy-momentum via the Einstein Field Equation (EFE).

-Richard

EDG
2010-Jun-28, 02:11 AM
OK... I'm kinda following you. But how does the accelerometer know what the geodesic should be though? Wouldn't it have to be calibrated to the geodesic beforehand?

publius
2010-Jun-28, 02:12 AM
Well, well. I see Wiki now has a pretty darn good article on proper acceleration:

http://en.wikipedia.org/wiki/Proper_acceleration

The author there did a pretty good job.

-Richard

publius
2010-Jun-28, 02:22 AM
OK... I'm kinda following you. But how does the accelerometer know what the geodesic should be though? Wouldn't it have to be calibrated to the geodesic beforehand?

It doesn't have to know. Objects naturally follow their geodesics. The simplest accelerometer is simply a ball weight ball attached to springs. If the springs stretch or compress, you know your are deviating from your geodesic. Imagine such a contraction on our rocket platform.

We can modify the famous statement of Newton thus:

An object will follow its geodesic unless acted on by a external force.


-Richard

Swift
2010-Jun-28, 02:24 AM
Then I'll need someone (preferably not you, because you're terrible at explaining anything) to explain clearly, concisely, and in plain english what the difference is.
EDG,

That is completely inappropriate. If you want to select the members who answer your questions, PM them. You will not express your opinions about which members are good at what in a public thread.

Ken G
2010-Jun-28, 02:41 AM
Since -- by relativity -- I cannot tell with any local test whether an object is sitting on a planet, with gravity providing the "force" that holds me to the floor or if the object is being accelerated by a rocket, the thrust of which is providing the force which is holding me to the floor, I'm not sure that saying gravity is or is not a force is anything but sophistry.Reread the thread. There is ample information in it to help you see that it is not sophistry to understand why gravity is not a force in general relativity. I would just be repeating my explanations, but I'll summarize that forces generate acceleration as measured on an accelerometer. (If you do review the detais, you may find them more useful than EDG does.)

astromark
2010-Jun-28, 04:14 AM
This is getting very silly... and the word pedantic springs to mind. I have read and understand much of what I have been told here.

I am in no way challenging what I have been told here and, recognise the accuracy of those facts...

That Gravity is NOT a force as such..., but the deeper I dig into the thinking behind accelerating mass and proper motion and velocity and vectors.

The space my ship is speeding through is warped by the near by object of mass. That Saggaterious A., black hole...

That a measurable action is detected a force has been applied. I seem to have arrived at a impasse.. self imposed logic.

and Do I understand the argument that has unfolded here. Yes. Across the reaches of interstellar space a detectable movement of force is recognise as gravity.

That gravity is not a force.

I shall continue to assert gravity as a force.

The argument is of by what mechanics does it work. By the mechanism of warped space.
What bends the space ? Not gravity but mass.

Spaceman Spiff
2010-Jun-28, 01:10 PM
Proper acceleration is a invariant measure of the deviation from the geodesic. It is what a local accelerometer measures. When you're sitting in you chair, such an accelerometer will measure 1g. Now, go into free fall. That accelerometer now registers 0. An accelerometer on an object in orbit will measure 0 as well.

Consider a rocket powered platform out in free space. Fire the rockets so an accelerometer on the platform registers 1g. The situation there is exactly the same as that on earth. Drop something, and it accelerates away from the platform at 1g, yet feels no force.

In the language of GR, both the platform and the surface of the earth are deviating from the local geodesic by 1g. Objects in free fall are following their geodesic. 1g is the "proper acceleration". Indeed in GR, we can still write Newton's Second Law as F = ma, but 'a' is now something a bit more involved, but yet much more elegant. It is the deviation from the local geodesic. That geodesic is determined by the distribution of mass-energy-momentum via the Einstein Field Equation (EFE).

-Richard

If only we were taught this in 1st year physics -- and then none of this confusion would cloud our thinking. :clap:

Nereid
2010-Jun-28, 02:42 PM
This is getting very silly... and the word pedantic springs to mind. I have read and understand much of what I have been told here.

I am in no way challenging what I have been told here and, recognise the accuracy of those facts...

That Gravity is NOT a force as such..., but the deeper I dig into the thinking behind accelerating mass and proper motion and velocity and vectors.

The space my ship is speeding through is warped by the near by object of mass. That Saggaterious A., black hole...

That a measurable action is detected a force has been applied. I seem to have arrived at a impasse.. self imposed logic.

and Do I understand the argument that has unfolded here. Yes. Across the reaches of interstellar space a detectable movement of force is recognise as gravity.

That gravity is not a force.

I shall continue to assert gravity as a force.

The argument is of by what mechanics does it work. By the mechanism of warped space.
What bends the space ? Not gravity but mass.(bold added)

Help me out here, please, astromark.

In GR, the best theory we have, today, of gravity, gravity is not a force.

In Newtonian, or classical, mechanics (or some subset thereof), gravity is a force.

While Newton's law of universal gravitation is a good approximation, throughout a very large region of parameter space, it is, very clearly, inconsistent with all relevant observational and experimental results. On the other hand, GR is fully consistent with that body of results (within the formal uncertainties).

What is pedantic about the above?

If you want to understand better why gravity is not a force, in GR, beyond what's already been posted in this thread, perhaps you could start a new thread, focussing clearly and specifically on just what it is you do not understand? Or, you could go to the Physics Forums Library (http://www.physicsforums.com/library.php) (per the link in the sticky at the top of this section).


I shall continue to assert gravity as a force.
Yet here's what the Q&A policy states:

Anyone is welcome to answer questions, but any answer should accurately reflect the generally accepted scientific mainstream. Some speculation is permissible if it is clearly indicated as such and if it can be traced to peer-reviewed science, i.e., if there's currently no clearcut mainstream answer, but mainstream scientists have proposed a possible answer that has yet to be verified or widely accepted.
YMMV, but it seems to me that your assertion is impermissible, under this policy.

Spaceman Spiff
2010-Jun-28, 03:25 PM
Let me add to publius' explanation with a thought experiment.

You and a scale that measures weight (a force) are upon a platform extending over an edge of a 30 meter deep chasm near the top of a very large vacuum chamber (you are obviously assisted by a breathing apparatus). You stand on the scale. The springs press against your feet, decreasing the length of the coil extension. The scale determines F = k * delta(x) and calls this your weight W, where k is the (ideal) spring constant and delta(x) is the amount the coil extension decreased after you stepped on the scale.

Suddenly, the platform supporting you and the scale vanishes. Being scientifically-minded, despite knowing what fate awaits, you look down at the scale and what do you see? Your weight has dropped to zero. i.e., you are weightless because there are no forces acting -- you and the scale are in "free" fall in the truest sense. You experience what "weightless" astronauts do in orbit about the Earth and the hypothetical astronaut does in intergalactic space.

EDG:
If you are really interested in learning and understanding, at least at a conceptual level, what our best models tell us about the world around us from this forum, I suggest some patience in both the (re)reading of and thoughtful reflection upon the posts of Ken G and publius. IMHO they are the dynamic duo :dance: when it comes to looking deeply into our scientific models to elucidate and illuminate what they actually say (and don't say) about our universe, using a razor to shave off the cruddy veneer. It's easy to toss off simple caricatures to describe our understanding of some process. However, upon further reflection these explanations often lead to confusion. Translating the precise mathematical language of our models into English is not easy, and I give these guys (and many other contributors to this forum) a lot of credit. :clap:

sabianq
2010-Jun-28, 03:38 PM
well, according to Einstein's' theory (GR), gravity is not a force. rather gravity is caused by the architecture or shape of space time.

mass causes space time to warp and like a bowling ball on a suspended sheet, it causes a well where other things want to go toward it.

Swift
2010-Jun-28, 05:10 PM
I think the thread has answered my question,
Since the OP says his question has been answered (back in post # 68) and this has become a general discussion, I'm moving this thread to Astronomy.

Swift
2010-Jun-28, 05:13 PM
This is getting very silly... and the word pedantic springs to mind.
astromark,

This comment seems inappropriate. If you have questions, ask them. If you have concerns about a post, Report them.

forrest noble
2010-Jun-28, 07:30 PM
Thought this might be an interesting comment. Cannot find the quotes but will paraphrase the ideas.

When Newton was asked: --- we understand Mr. Newton that in your great book gravity pulls things downward but might you comment on why you think that it does so? Newton simply said, gentleman I propose theories concerning how nature works, not hypotheses on details concerning how it might be. It would seem that he was saying that although gravity may have a cause beyond his force idea, he would not openly speculate concerning such matters.

Einstein was asked a similar question in an interview. He was asked: We understand Mr. Einstein that matter warps the space that surrounds it but could you comment on why you think it does so. Einstein's answer was similar to Newton's. He said that at the present time this is the full extent of my gravitational theory and I see no reason at this time to speculate beyond the theory concerning further detail, which would be seemingly unrelated to the mathematics involved. Again in his explanation, like Newton, he stated that he did not wish to publicly speculate on such matters.

astromark
2010-Jun-28, 08:48 PM
I used the word pedantic because that is my perception of what I am seeing... a little tiff between Edge and Ken G. and I have a valid point...

That it may well be correct and well understood that 'Gravity' is not a force. What I am attempting to say is this;

That the measured force detected as gravity might actually be a distortion of space by a object of mass...

That is excepted. I have no argument with that FACT. Am I still allowed to use this word gravity ?

For the general description of attracting masses. A perceived action but wrong.

For insertion into orbit about a object there are figures I need for that calculation... Mass, velocity, and can I use the word 'gravity.' ?

Has it suddenly become redundant.

I step onto a scale and measure my weight... Hmmm... lets call it the distortion of space by the mass differential... Now it is I whom is being pedantic.

Space is distorted by the presence of mater. Can I correctly call that gravity. Should it be a 'force' or a 'effect' ?

Yes I can be said to be caught in a gravity well...

Nereid
2010-Jun-28, 09:44 PM
Thought this might be an interesting comment. Cannot find the quotes but will paraphrase the ideas.

When Newton was asked: --- we understand Mr. Newton that in your great book gravity pulls things downward but might you comment on why you think that it does so? Newton simply said, gentleman I propose theories concerning how nature works, not hypotheses on details concerning how it might be. It would seem that he was saying that although gravity may have a cause beyond his force idea, he would not openly speculate concerning such matters.

Einstein was asked a similar question in an interview. He was asked: We understand Mr. Einstein that matter warps the space that surrounds it but could you comment on why you think it does so. Einstein's answer was similar to Newton's. He said that at the present time this is the full extent of my gravitational theory and I see no reason at this time to speculate beyond the theory concerning further detail, which would be seemingly unrelated to the mathematics involved. Again in his explanation, like Newton, he stated that he did not wish to publicly speculate on such matters.
The quotes themselves might be very interesting!

They both seem to be saying what Ken G (and others) have been saying; namely, making clear what the scope of science (physics, in this case) is.

Ut
2010-Jun-28, 09:50 PM
No. I do not want "force" to have a a precise scientific meaning; I want the common word "force" not to have its meaning dictated by its usage in a science. But I said that. Time I quit here.

Since when has the "common word" had its meaning dictated by its usage in a science? You have the right to use the word "force" however you like. You just shouldn't expect the science buffs on a science buff discussion board to be satisfied with ambiguous vernacular word usage in discussions where that word has one or several precise definitions, depending on the context of the discussion.

You may as well walk up to an outspoken evolutionary biologist and should "only a theory" at them.

You can come up with your own definition of "dog" and "cat", too, if you'd like. But you might get some funny looks when you start telling people that your dog ran out of ink, and you're considering buying a mechanical cat to cut down on the number of cat shavings in your sharpener.

publius
2010-Jun-28, 09:54 PM
I step onto a scale and measure my weight... Hmmm... lets call it the distortion of space by the mass differential... Now it is I whom is being pedantic.

Space is distorted by the presence of mater. Can I correctly call that gravity. Should it be a 'force' or a 'effect' ?

Yes I can be said to be caught in a gravity well...


The weight you measure is due to the force, the real force, of the ground pushing you up against your geodesic. The same effect will be seen on the accelerating platfrom in free space. This is the only real force involved, and has nothing to do with gravity itself, but with other, real, external forces, that effect deviation from the geodesic. Your weight has nothing to do with gravity. It has to do with your *inertia*, a local property of resisting proper acceleration, deviation from geodesics.

This is not pedantry, but a key, even earth-shattering insight that led Einstein to GR. This insight has a name, the Equivalence Principle.

Gravity can be defined, in the simplest and most general terms very easily, the tendency of mass to clump together. You can even say attract. But this is not a force, as force is defined and understood as a scientific term. Forces only enter into it when you want to *resist* this attractive tendency.

If you want to get in orbit from the ground, you're going to have to exert some real forces on yourself to do so. Once in orbit, you follow your force free path through the space-time.

And if I wanted to be pendatic about all this, well you ain't seen nothing yet. :)

-Richard

Nereid
2010-Jun-28, 10:19 PM
I used the word pedantic because that is my perception of what I am seeing... a little tiff between Edge and Ken G. and I have a valid point...

That it may well be correct and well understood that 'Gravity' is not a force. What I am attempting to say is this;

That the measured force detected as gravity might actually be a distortion of space by a object of mass...

That is excepted. I have no argument with that FACT. Am I still allowed to use this word gravity ?

For the general description of attracting masses. A perceived action but wrong.

For insertion into orbit about a object there are figures I need for that calculation... Mass, velocity, and can I use the word 'gravity.' ?

Has it suddenly become redundant.

I step onto a scale and measure my weight... Hmmm... lets call it the distortion of space by the mass differential... Now it is I whom is being pedantic.

Space is distorted by the presence of mater. Can I correctly call that gravity. Should it be a 'force' or a 'effect' ?

Yes I can be said to be caught in a gravity well...
astromark, please read the posts, in this thread, by publius, Ken G, Spaceman Spiff, etc. Please read them very carefully.

If you don't understand what they write, by all means start a new thread, in the Q&A section, with any, and all, the questions you have about what they wrote.

FWIW, I think you have still not grasped at least one key aspect of GR; I think you are using your own, intuitive, sense of what a force is, and trying to squeeze that into your understanding of GR.

Now GR is certainly not easy to understand, even at a high level. It certainly involves some subtleties and nuances that may, unless you truly appreciate their profundity, seem like mere pedantry. But, please be assured that they are anything but; as publius just said, the insight which Einstein had is one of the most profound in the history of physics.

publius
2010-Jun-28, 10:33 PM
Yes, it would be splendiferous floccinaucinihilipilification to dismiss this as pedantry. And that's your word of the day, in tribute to the now late senator from the state of West Virginia, who once rose to the floor to condem the Senate for engaging in such.

forrest noble
2010-Jun-29, 12:03 AM
publius,

I have seen by your last posting and otherwise that your pedantic proclivities have grandiose potential if you wished to pursue them, but one must wallow and pander a bit (in my opinion) concerning explanations that can transcend and qualify academic misconceptions which I believe totally dominate theories of physics and cosmology today. Theory in this case having the meaning: to bring out into a public space a play of ideas that might help us better understand our world.

In my opinion Einstein's Equivalence principle , that you mentioned above which would include his verbal explanations thereof, was by far his greatest insight ever that, in my opinion, will outlive every other "insight" that he ever had including all his formulations.

Nereid,

Concerning my paraphrasing of Newton and Einstein -- quotes:


The quotes themselves might be very interesting!

I agree with you. The problem was I couldn't find these quotes. Maybe you or somebody else could find one or the other or both. I'll keep looking. Was hoping that my paraphrasing might be interesting but there's nothing better than the real thing.


They both seem to be saying what Ken G (and others) have been saying; namely, making clear what the scope of science (physics, in this case) is.

I also agree concerning the scope of science: that the scope of science in general has always been (in my opinion) trying to explain reality the best way that is/was presently available by consensus and making those perceptions practical for the advantage of mankind.

Hornblower
2010-Jun-29, 12:53 AM
Yes, it would be splendiferous floccinaucinihilipilification to dismiss this as pedantry. And that's your word of the day, in tribute to the now late senator from the state of West Virginia, who once rose to the floor to condem the Senate for engaging in such.

That was a supercalifragilisticexpialidociously good response.

In my opinion, the GR opinion that gravity is not a force is not pedantic, but rather is counterintuitive to most people who have not mastered physics at a rather advanced level. I see nothing fundamentally wrong with that state of affairs.
___________________

Supercalifragilisticexpialidocious,
Even though the sound of it is something quite atrocious,
If you say it loud enough you'll always sound precocious,
Supercalifragilisticexpialidocious.

Credit to Walt Disney, Mary Poppins, 1964

astromark
2010-Jun-29, 02:53 AM
And there is nothing wrong with that. Even NASA uses the idea of "gravity as a force" to move probes around solar system, even though they know that General Relativity is more correct. As Nereid noted in her post:



I don't think that any of the mods or admins would be upset with an answer that went:

To give you a straight answer, no, gravity is not a force. However, for over three hundred years gravity was treated as a force, using the mathematics found by Isaac Newton. In 1915 Albert Einstein developed the General Theory of Relativity(GR). This is our current best theory of gravity. Under this theory, gravity is not thought of as a force, gravity is a result of the curvature of spacetime, and objects follow geodesics, when in free fall, though spacetime. GR has been shown to be more correct than Newtonian gravity, which is why we say that gravity is not a force. However, in most cases, calculations using Newton's force equations, are close enough for the correct answer that they are used, even by NASA, to move space probes around the solar system.

The reason I don't think that there would be a problem here is that, while the mainstream is well aware that GR is the standard Gravitational Theory, the mainstream(as in NASA and most other groups that calculate the positions of the planets) also mainly use the Newtonian gravitational eqautions. Now, if any other questions arise out of that answer, then the person asking the question could ask another question. Also, it would depend on the level of the person asking the question whether or not I would go into some of the deeper answers that have been posted here.

and that is the position I maintain... thank you Tensor.

Note; No refractive of the Disney quote... ( just more childish rubbish ). Its as if being right is more important than being understood.
So I can still call this bending of the space a distortion of gravity. Knowing what gravity is... No. you do not.
I am well aware of the effect I call gravity. I am no closer to understanding by what process that distortion is propagated.
There seems to be no graviton. Particle or wave. The bending of the fabric of space by the presence of mass.

Tobin Dax
2010-Jun-29, 04:50 AM
Let me add to publius' explanation with a thought experiment.

You and a scale that measures weight (a force) are upon a platform extending over an edge of a 30 meter deep chasm near the top of a very large vacuum chamber . . . .

The weight you measure is due to the force, the real force, of the ground pushing you up against your geodesic. . . . .

My annual "field trip" to the elevator with a bathroom scale might get a little more interesting this year. You guys have said many of these things better than I could. Thanks.

astromark
2010-Jun-29, 06:35 AM
Yes... but, you be careful. Try this... walk amongst a group of well educated people and tell them..

" Gravity is not a force." and duck.

Because as true as we know that is... It will not be gladly received., or excepted.

We. The few people that actually care about these things are a minority group.

That knowing these things does not carry a lot of weight.

Back in page four I upset some with my old school attitude. You can have too much information.

mugaliens
2010-Jun-29, 06:55 AM
Since -- by relativity -- I cannot tell with any local test whether an object is sitting on a planet, with gravity providing the "force" that holds me to the floor or if the object is being accelerated by a rocket, the thrust of which is providing the force which is holding me to the floor, I'm not sure that saying gravity is or is not a force is anything but sophistry.

Actually, there is a local test you can perform. In each case, build a modest tower with a calibrated source of EM emitter with a known wavelength measured first at the base of the tower, then rechecked with the emitter at the top of the tower while the receiver remains at the base.

In the case of a rocket, there's no change in frequency. In the case of a planet, there is a change in frequency.

Of course the other test is to wait until the fuel runs out...

publius
2010-Jun-29, 07:34 AM
Actually, Mugs, there will be a change in frequency on the rocket as well as the planet as measured by observers as you describe. The Equivalence Principle means what it says. Indeed, Einstein first deduced gravitational time dilation and redshift applying SR to what co-accelerated observers would measure via the Equivalence Principle. The accelerated observers on the rocket will even see curvature of a transerve beam of light, etc.

-Richard

Tobin Dax
2010-Jun-29, 11:07 AM
Actually, there is a local test you can perform. In each case, build a modest tower with a calibrated source of EM emitter with a known wavelength measured first at the base of the tower, then rechecked with the emitter at the top of the tower while the receiver remains at the base.

In the case of a rocket, there's no change in frequency. In the case of a planet, there is a change in frequency.

Of course the other test is to wait until the fuel runs out...

There must be a change of frequency with the accelerating rocket. Because light travels at a finite speed, there would be a small relative difference in the speed of the emitter and the speed of the reciever. The Doppler effect would create the same blueshift that you would see on a planet.

Spaceman Spiff
2010-Jun-29, 03:13 PM
Yes... but, you be careful. Try this... walk amongst a group of well educated people and tell them..

" Gravity is not a force." and duck.

Because as true as we know that is... It will not be gladly received., or excepted.

We. The few people that actually care about these things are a minority group.

That knowing these things does not carry a lot of weight.

Back in page four I upset some with my old school attitude. You can have too much information.

astromark:

For someone who is apparently interested in learning how the world works according to our best models of nature, you will not find this to be a useful attitude. We might as well be discussing the fact that space and time intervals are not absolutes -- not invariants -- while a construct known as a space-time interval is an invariant (a "conserved quantity"). In fact it is exactly the same discussion we are having now. Think about that.

astromark
2010-Jun-29, 08:44 PM
Gravity is not a force of nature. Can I say it is a effect. Oh, I just did.

If challenging and questioning the science is understood as not being receptive of it... That would not be my view.

I have the tolerance of understanding to ask the obvious. By doing so I see explanations and discussion that further unravel this mystery.

To just put it out, that gravity is not a force does require some explanation... I am happy with that.

Reading back through the contributions here I am in good company.

Ut
2010-Jul-04, 06:03 AM
Gravity is not a force of nature. Can I say it is a effect. Oh, I just did.

If challenging and questioning the science is understood as not being receptive of it... That would not be my view.

I have the tolerance of understanding to ask the obvious. By doing so I see explanations and discussion that further unravel this mystery.

To just put it out, that gravity is not a force does require some explanation... I am happy with that.

Reading back through the contributions here I am in good company.


Gravity is as much a force as black is a race. While denying either in the company of laypeople will get you some curious looks, the fact remains that both the concepts of gravity as a force and black as a race are accepted by the general public without any explanation at all. So why is one required to refute them?

astromark
2010-Jul-04, 08:53 AM
@ Ut... I said, "Gravity is NOT a force." but I will add that black is a complete absence of light., and never has been a race.

I am in no way responsible for the errors of history... This 134 post long thread has been up this path before... sorry if I confused you.

Andrew D
2010-Jul-05, 12:36 AM
Gravity is as much a force as black is a race. While denying either in the company of laypeople will get you some curious looks, the fact remains that both the concepts of gravity as a force and black as a race are accepted by the general public without any explanation at all. So why is one required to refute them?

Firstly, I'm sure you could have picked a much more manageable parallel than race. :naughty:

Secondly, the general public, unfortunately, often has the wrong idea about a great many things.

Your tactless comparison is useful, however, because the perpetuation of racism and the failure to accept general relativity after nearly a century of observational verification point out both the incredible ignorance of the general public and their need for gentle correction from the scientific community. To withhold such correction for difficulty of explanation is unethical by any standard ... save yours, apparently.

Ut
2010-Jul-05, 10:41 PM
Firstly, I'm sure you could have picked a much more manageable parallel than race. :naughty:

No, I think it's an apt comparison. Both are notions which today are held almost exclusively by the scientific laity. They're the rotten leftover scraps of largely passe scientific ideas. That one of these ideas is tied to a great stain on the social fabric of humanity is no reason to ignore it. Indeed, it does no one any good to pretend such notions don't exist, or to believe they're inappropriate to mention.

That is, after all, how those ideas continue to live. In the shadows of taboo.


Secondly, the general public, unfortunately, often has the wrong idea about a great many things.

Your tactless comparison is useful, however, because the perpetuation of racism and the failure to accept general relativity after nearly a century of observational verification point out both the incredible ignorance of the general public and their need for gentle correction from the scientific community. To withhold such correction for difficulty of explanation is unethical by any standard ... save yours, apparently.

To simply tell someone that they're wrong is a great way to seem pompous, and to elicit an emotional response. You can believe you're being as gentle as an ultrasuade chamois on diamond. People simply hate being told they're wrong.

Better to entice them to justify their own position, and to ask them how they arrived at it. In cases like "race exists" or "gravity is a force", you'll find that the vast majority of people will simply tell you that that's what they were told or taught growing up. \

Thank you, by the way, for declaring me both tactless and unethical. You did a fine job of illustrating exactly what I'm saying.

Robert Tulip
2010-Jul-05, 11:35 PM
rotten leftover scraps of largely passe scientific ideas.... continue to live. In the shadows of taboo.
Ut, your comparison may be relevant to social science but speaking of taboo in physics is not right. Gravity as a force is a useful concept for practical purposes, and there is no taboo about General Relativity. It is just the ideas of warping spacetime are counterintuitive from the normal human perspective that sees gravity in terms of weight and force. For example hydroelectric stations are powered by the force of gravity. It involves difficult physics to understand why the concept of force in this local situation is inconsistent with observation of the universe.

Swift
2010-Jul-06, 01:36 AM
Ut and Roobydo,

You both need to chill a little, this is getting entirely too heated.


No, I think it's an apt comparison. Both are notions which today are held almost exclusively by the scientific laity.
No, it is not. It has nothing to do with physics and nothing to do with gravity. And it is entirely too inflammatory for discussion here. Do not bring it up again, unless it is a thread in Science & Technology about the sociology of race.

Robert Tulip
2010-Jul-14, 07:17 AM
Grateful any comment on this article.

A Scientist Takes On Gravity - The New York Times, July 12, 2010
http://www.nytimes.com/2010/07/13/science/13gravity.html?_r=2&pagewanted=1&no_interstitial


Erik Verlinde, 48, a respected string theorist and professor of physics at the University of Amsterdam, whose contention that gravity is indeed an illusion has caused a continuing ruckus among physicists...

Over the last 30 years gravity has been “undressed,” in Dr. Verlinde’s words, as a fundamental force.

This disrobing began in the 1970s with the discovery by Jacob Bekenstein of the Hebrew University of Jerusalem and Stephen Hawking of Cambridge University, among others, of a mysterious connection between black holes and thermodynamics, culminating in Dr. Hawking’s discovery in 1974 that when quantum effects are taken into account black holes would glow and eventually explode.

In a provocative calculation in 1995, Ted Jacobson, a theorist from the University of Maryland, showed that given a few of these holographic ideas, Einstein’s equations of general relativity are just a another way of stating the laws of thermodynamics.

Lee Smolin, a quantum gravity theorist at the Perimeter Institute for Theoretical Physics, called Dr. Jacobson’s paper “one of the most important papers of the last 20 years.”

But it received little attention at first, said Thanu Padmanabhan of the Inter-University Center for Astronomy and Astrophysics in Pune, India, who has taken up the subject of “emergent gravity” in several papers over the last few years. Dr. Padmanabhan said that the connection to thermodynamics went deeper that just Einstein’s equations to other theories of gravity. “Gravity,” he said recently in a talk at the Perimeter Institute, “is the thermodynamic limit of the statistical mechanics of “atoms of space-time.”

Dr. Verlinde said he had read Dr. Jacobson’s paper many times over the years but that nobody seemed to have gotten the message. People were still talking about gravity as a fundamental force. “Clearly we have to take these analogies seriously, but somehow no one does,” he complained.

His paper, posted to the physics archive in January, resembles Dr. Jacobson’s in many ways, but Dr. Verlinde bristles when people say he has added nothing new to Dr. Jacobson’s analysis. What is new, he said, is the idea that differences in entropy can be the driving mechanism behind gravity, that gravity is, as he puts it an “entropic force.”

tnjrp
2010-Jul-14, 07:36 AM
I've heard that "entropic gravity" made some waves some time back but has fallen somewhat on the wayside since. Obviously Professor Verlinde doesn't think it should be allowed to go...

Anyway, not qualified to analyze his claim myself by a long shot, but here's some contrary commentary from folks who just might be.

Firstly, Lubos Motl, known for his robust critiques of various theoretical physics issues:
http://motls.blogspot.com/2010/01/gravity-as-holographic-entropic-force.html

Secondly, something more technical:
http://xxx.lanl.gov/abs/1004.0877

Ken G
2010-Jul-14, 08:39 PM
My basic problem with seeing gravity as an "entropic force" is that we normally think of spacetime as a kind of go-between for talking about how particles affect each other, creating along the way an abstract notion of a spacetime manifold that is not itself the subject of what physics is testably meant to explain. We don't have any way to observe spacetime itself, so we must say that spacetime is a milieu for establishing how the laws of physics affect particles. So if we think of gravity as an "entropic force" that tells spacetime to arrange itself in some kind of maximally entropic way, we should still be able to say that the mass of the Earth causes balls to fall when we drop them.

But if if that is an "entropic force" on the ball, it must mean that the ball has many more ways to be on the ground than in your hand. But if that is so, how does the elastic rubber ball bounce back up to your hand again? Everything has returned to its initial state, so gravity participated in a reversible motion, so it can't be an "entropic force" on the ball, it could only be an "entropic rearrangement" of the spacetime. Is that the proper domain for concepts like entropy to apply, to the constructs we use as middlemen for the influence of the Earth on a ball? It sounds like a kind of metaphysics, where we are applying the laws of physics as if they worked on the laws of physics, rather than on the things we can observe.

Trulialia
2010-Aug-15, 09:46 AM
As far as I know gravity is one of the four basic forces, along with electromagnetic, strong and weak

kevin1981
2010-Aug-15, 08:59 PM
As far as I know gravity is one of the four basic forces, along with electromagnetic, strong and weak

It is, but there is a massive misconception when using the word force. When talking about gravity, most people think that two objects are attracted to each other by some unseen force, like magnetism. But the common view of Einstein's theory of general relativity states that gravity is caused by objects like the earth distorting space/time rather than there being an actual 'force'.

astromark
2010-Aug-18, 08:37 AM
@ Trulialia... I know its a bit long winded and drifts off a few times... :eh: huh ! but if you could read through this whole thread...

Its been well explained. " Is Gravity a force ?" No.

But as a generalization... Yes. and sort of...

Knowing how annoying being both wrong and right can be...:oh:

Not to long ago I would have argued the fact that it is one. I could prattle on about all the reasons I would think that way...

BUT, ... good science has shown me that it is a effect. The direct distortion of space by the mass of objects in it.

Once explained the window opened and I can understand that logic.

Until a better theory comes at me... thats it.

Ken G
2010-Aug-18, 09:45 AM
Another way to say this is that it depends on the theory of gravity you invoke. Newton's gravity was a force, but is known to break down for strong gravity. Einstein's gravity is not a force in the standard sense, it is what is known as a fictitious force, a force in form but not substance, that appears when you fail to recognize that the spacetime is curved. If you do notice the curving of spacetime, then the motion is force-free, it is the inertial path that only appears accelerated when you attempt to pigeon-hole it into locally flat coordinates (and the justification for viewing the acceleration as illusory is that an accelerometer would always read zero in any gravity, that's the "equivalence principle").

However, many modern theorists do not believe Einstein's gravity is the last word on gravity, because it is not consistent with quantum field theory. If we take the perspective, as many do, that quantum field theory is the "ultimate truth" for talking about interactions between particles, then gravity has to once again be viewed as a force, mediated by virtual particles called "gravitons". No such theory has ever been successfully designed in a completely consistent way, but quantum field theorists think it is inevitable that it will. I have no good understanding of why they believe that, so as far as I'm concerned, our best current theory of gravity, Einstein's, is one in which gravity is not a force in the traditional sense.

Jerry
2010-Aug-19, 05:13 AM
Gravity is redefined in GR as a metric tensor, but when you reduce the equation to a simple two body problem at a fixed distance from each other, all of the other factors fall out and (at least unit wise), gravity reduces to a simple force.

GR is not the last word of gravity because it is only a theory of gravity that may or may not be wrong. In engineering, gravity is always considered a force, and you better build your bridge, airplane, zipline accordingly.

Ken G
2010-Aug-19, 07:16 AM
Gravity is redefined in GR as a metric tensor, but when you reduce the equation to a simple two body problem at a fixed distance from each other, all of the other factors fall out and (at least unit wise), gravity reduces to a simple force.Even a simple two-body problem at a fixed distance in GR does not treat gravity as a force. There would only be one force in that problem, whatever is keeping the bodies at a fixed distance, and an accelerometer would testify to the presence of acceleration in that situation, as would a scale. GR centers on the "equivalence principle", which in a nutshell offers a new interpretation of what a bathroom scale does-- in Newtonian gravity, a scale provides the force needed to "balance" the force of gravity, but in GR, a scale provides the necessary acceleration to allow the person to imagine they are "standing still" in their own chosen coordinate system, a coordinate system that fails to account for gravity.


In engineering, gravity is always considered a force, and you better build your bridge, airplane, zipline accordingly.Heaven help us when we rely on engineers to provide our ontologies!

agingjb
2010-Aug-19, 09:21 AM
Ontology? I'd rather define "force" as a class of observed effects with a variety of causes - a definition that has some basis in ordinary language.

But science is so determined to lose its audience that my advice will be futile.

Nereid
2010-Aug-19, 09:24 AM
Gravity is redefined in GR as a metric tensor, but when you reduce the equation to a simple two body problem at a fixed distance from each other, all of the other factors fall out and (at least unit wise), gravity reduces to a simple force.

I'm not sure it does (how did you come to this conclusion?)

For example, if the two simple bodies are in motion - orbit - wrt each other, the orbit is stable under Newton, but decays (due to gravitational wave radiation, GWR) under GR. Can you get a decaying orbit due to GWR from 'a simple force'? I don't know, can you?



GR is not the last word of gravity because it is only a theory of gravity that may or may not be wrong. In engineering, gravity is always considered a force, and you better build your bridge, airplane, zipline accordingly.
What Ken G said. Also, is the construction of a working GPS, as a pure engineering project, possible if you treat gravity solely as a simple force? Again, I don't know, is it?

Ken G
2010-Aug-19, 09:33 AM
Ontology? I'd rather define "force" as a class of observed effects with a variety of causes - a definition that has some basis in ordinary language.Science does tend to look to "ordinary language" for inspiration in its terms, but that practice has its down side-- it leads a lot of people to think they understand something when they do not. For example, your definition of "force" is way too vague to be useful in science, and any attempt to give it a usefully well constrained meaning is going to either invoke ontological constructs, or will end up sounding like a cooking recipe.

Strange
2010-Aug-19, 09:54 AM
Also, is the construction of a working GPS, as a pure engineering project, possible if you treat gravity solely as a simple force? Again, I don't know, is it?

Yes. But you would have to include three unexplained "fudge factors" with a complex dependency on height and velocity...

agingjb
2010-Aug-19, 10:00 AM
Oh well folks, have it your own way. Don't be surprised if non-scientists lose interest. I'd have said either abandon "force" as a term in science, or always specifically qualify it.

So gravity would be a force that, well there's a classification of animals which could serve as a model; gravity could be "force if seen from a distance looks like flies" or "force drawn with with a fine camel-hair brush", according to the theory of the moment.

Strange
2010-Aug-19, 10:00 AM
or will end up sounding like a cooking recipe.

Every area of expertise has its own specialised terminology or uses ordinary words in special ways. This is inevitable. For example, "fold" means something very different in a cooking recipe than it does in origami instructions or in geology. Complaining that one or other is misusing the word or being deliberately obscure is pointless.

Inevitably, some people engaged in these specialist areas will forget that they use words in a special way. One of the skills of a science communicator (or cookery writer) is to know when the general reader is likely to be confused by jargon and either avoid it or explain it.

agingjb
2010-Aug-19, 10:08 AM
And, BTW, those GPS systems. Do their engineers go back to the tensors each time, or do they indeed use fudge factors? And if so, were those fudge factors derived from tensor calculations or estimated from experiment?

Ken G
2010-Aug-19, 10:10 AM
Every area of expertise has its own specialised terminology or uses ordinary words in special ways. This is inevitable. For example, "fold" means something very different in a cooking recipe than it does in origami instructions or in geology. Complaining that one or other is misusing the word or being deliberately obscure is pointless. I agree, but that does not remove us from the need to answer the OP question. Had the OP been, "what does it mean to fold something", then we would need to answer for both origami and cooking. As it was "is gravity a force", we need to answer that from the point of view of the various theories that invoke gravity, and in so doing, recognize that the best and most accurate of those theories does not have gravity as a force. Note that is not the same thing as stipulating categorically that "gravity is not a force", a stipulation that I'm pretty sure no one in this thread made.

agingjb
2010-Aug-19, 10:14 AM
The general reader is always inclined to end up saying "ho hum" if things get too sticky. I would wonder what that underused resource in any specialty, the intelligent and interested lay critic, might say.

Strange
2010-Aug-19, 10:20 AM
And, BTW, those GPS systems. Do their engineers go back to the tensors each time, or do they indeed use fudge factors? And if so, were those fudge factors derived from tensor calculations or estimated from experiment?

They were caclulated from theory. But clearly engineers don't need to study GR. Suitable approximations were developed for use in GPS receivers, which is what system developers use in the software. But it is very clear why they are there.

Ken G
2010-Aug-19, 10:22 AM
The general reader is always inclined to end up saying "ho hum" if things get too sticky. I would wonder what that underused resource in any specialty, the intelligent and interested lay critic, might say.I think we have seen many examples of just what such a person would say on this very thread. Many seem quite interested in the distinctions of how to look at gravity, and are quite capable of seeing how they relate to explanations of why you slide forward in your seat when you slam on the brakes in a car. I still wonder how you would explain that to someone, because if the various models for gravity is a "ho hum" for you, then the sliding on a car seat is still something most of us do have everyday interest in. All the same basic issues appear in that context, that's why it's called the "equivalence principle." Perhaps the equivalence principle is also a "ho hum" for you, but most educated people with interest in science, not to mention an astronomy forum, view that as one of the most astonishing insights in the last 100 years of human inquiry. Maybe you are giving up on understanding the equivalence principle a little too easily.

agingjb
2010-Aug-19, 10:38 AM
It is not my usual reaction to say "ho hum", but I am gradually being persuaded that it ought to be my proper response.

Nereid
2010-Aug-19, 11:46 AM
It is not my usual reaction to say "ho hum", but I am gradually being persuaded that it ought to be my proper response.


I must say that when I first heard that not all bacteria are created equal - i.e. that there are (well then were) three Kingdoms, Bacteria, Archaea, and Eukaryota - my reaction was very much "ho hum"!

However, over the ensuring years I've become increasingly fascinated by this amazing discovery!

Also, when I first heard of Basque (the language), I thought "ho hum, just another minor language in some obscure part of Europe; no doubt its just a dying relic of Latin, somewhat similar to some Spanish or French dialect". Of course, Basque is just a window into some very interesting aspects of linguistics, history, population movements, genetics, ... and I think it was in this connection that I first came across the word "refugia".

Maybe if you get curious about what the (*(&%*&* is a force anyway? you may still have a sense-of-great-wonder moment :)

agingjb
2010-Aug-19, 12:34 PM
Usually I'm interested in everything; in this case I'm being told that I'm stupid if I don't grasp the implications of "gravity is not a force". Believe me there's plenty else to grab my attention, taxonomy and comparative linguistics included.

In fact, up to a point I can follow the argument, but I still think there is something misguided here in its presentation that will drive people away from science.

I suppose I'm just not a BAUT person.

Ken G
2010-Aug-19, 01:07 PM
Usually I'm interested in everything; in this case I'm being told that I'm stupid if I don't grasp the implications of "gravity is not a force".Oh you are being told no such thing. I've already given you the path to understanding this simple point, just describe to me what is happening when you slam on the brakes in your car and you slide forward in your seat. After you've done that, I will tell you in one paragraph what it means to say gravity is not a force, and you will understand quite easily. But you refuse to take even the first step, and blame everyone else.

Sir Knots A Lot
2010-Aug-19, 04:32 PM
Oh you are being told no such thing. I've already given you the path to understanding this simple point, just describe to me what is happening when you slam on the brakes in your car and you slide forward in your seat. After you've done that, I will tell you in one paragraph what it means to say gravity is not a force, and you will understand quite easily. But you refuse to take even the first step, and blame everyone else.

Inertia?

antoniseb
2010-Aug-19, 06:42 PM
I have closed this thread. First because the initial question was a bit ambiguous, and second because several people were piling on one member for not wanting to learn General Relativity.

I think that if the thread had asked "In General Relativity, is Gravity a force" we wouldn't have had this trouble... but it wasn't and so there was a built in argument between the GR people and the Newtonians.

No warnings are being issued, but I am asking all involved to be more sensitive to the others in the thread.