PDA

View Full Version : Understanding Gravity.



astromark
2010-Aug-19, 10:34 PM
I want a better understanding of this subject. WHAT IS GRAVITY ?

I do not want or invite any argument as to the general relativity or Newtonian schools of thinking.:wall:

I am most interested in understanding the warping of space by what power ? force ? Presence ?

That a body of any mass has a effect upon bodies of mass needs some explanation.

I am fully exceptive of the science that gravity is NOT a force but is a effect.

I want to better understand the mechanics of it please.

I am going to be vigilant and will report any uncivil response ...

Please have regard for opposing views and make a effort to a tolerance of understanding.:clap: mark.

kevin1981
2010-Aug-19, 11:20 PM
I hope you don't mind me asking this Mark. Space/Time is warped by Mass/Energy and we have a very good mathematical model on how this works. The same goes for the 'expanding' universe. But is Gravity a very good theory to describe what we observe mathematically? What i mean is, physically, do we really know what 'Gravity' is ?

dgavin
2010-Aug-20, 12:15 AM
What Gravity is is still an unknown, other then being able to be determined by a set of formulas.

If gravity is mediated by a force, then it should be quantifiable under quantum mechanics. So far all attempts to quantify gravity and then unify it with the other QM forces have failed.

So Gravity could be one of the following:
1. A effect of warped space/time and not actually a force. (General Relitivity)
2. A field/force that is mediated by a spin 2 quantum particle called a Graviton, and possibly emitted by the mass carying higgs bosum. (Quantum Mechanics)
3. A 5d through 11d extradimensional force that manifests itself on our space time continum as a 4d warpage of space time. (String Theory)
4. Something that when it's discoveredwhat it really is will not fit in with any known physics systems.

5. Magic.... (this a joke, but it about describes what it is with out current knowlege right now)

caveman1917
2010-Aug-20, 12:59 AM
A field/force that is mediated by a spin 2 quantum particle called a Graviton, and possibly emitted by the mass carying higgs bosum. (Quantum Mechanics)

Why would it be linked to the Higgs boson?
Gravity doesn't depend on the mass in se, but on the energy. We commonly think of it as related to mass because rest mass is the major contributor to energy, but any energy will do.
For example a photon has a gravitational effect too, but it doesn't interact through the Higgs mechanism.


3. A 5d through 11d extradimensional force that manifests itself on our space time continum as a 4d warpage of space time. (String Theory)

I thought that string theory also takes the graviton-approach instead of the curvature one.

astromark
2010-Aug-20, 01:15 AM
Yes 'Kevin1981' thats my understanding also.. and 'dgavin' as I do not except magic :lol:its one of the others....?

and I am still asking... How does the presence of mass distort space near to it., and some distance.

Just as I do not know... is not a expectable conclusion is it ?

jfribrg
2010-Aug-20, 01:18 AM
I'm with you astromark. I have no idea what gravity is. Didn't Einstein call it "spooky action at a distance"? Perhaps if the existence of gravitons and/or gravitational waves is some day proven, then maybe the mathematical formulae might give some insight into exactly what it is. Until then, you have different theories (string theory, MOND) without any idea what is correct and what isn't. Perhaps we need a better understanding of space-time in order to better understand what space-time warping is.

Ken G
2010-Aug-20, 01:27 AM
I don't think anyone can answer this question at the moment, and what's more, I don't even think it is the kind of question science answers. I could say "what is an atom, really", or "what is spin, really", and no one knows the answers to those either. That's because all of these concepts, including gravity, only take on specific meaning in the context of how a particular theory uses the concept. It is usually easy to see when two theories are basically talking about the same thing (like Newton's and Einstein's gravity), but they involve different definitions in the two theories, so they are fundamentally different things. Or you could try a phenomenological definition, like "gravity is whatever makes things fall", but that's no better-- because if someone presses you to define "fall" in some general kind of way, you will always end up defining "falling" as "what gravity makes things do." That's obviously circular! So the phenomenological definitions only work in common parlance, not scientific parlance. For that, you need a theory, and there's really no such thing as gravity outside of the particular theory that gives the word meaning.

astromark
2010-Aug-20, 01:35 AM
" Spooky action at a distance." I like that...

So space time is bent. Yes., and that it is the presence of mass that is doing it. Yes.,

but that seems to ask more questions than it answers...

I have not found a graviton or even the reason why anyone thinks one exists.

I do not think a sub atomic particle has been tied to the distortion of space by mass.

By what mechanism does space get bent ?

caveman1917
2010-Aug-20, 01:37 AM
and I am still asking... How does the presence of mass distort space near to it., and some distance.

Just as I do not know... is not a expectable conclusion is it ?

Your guess is as good as anyone's. Nobody knows how the presence of energy curves spacetime near to it, it just does.
It's not even clear what is cause and what is effect here. Consider a black hole, the energy is localized inside the EH. Yet its 'effect' reaches outside of the EH.
Perhaps one way to look at it is to say that gravity is energy - just different perspectives on the same thing.


Didn't Einstein call it "spooky action at a distance"?

That was about quantum entanglement.


Perhaps if the existence of gravitons and/or gravitational waves is some day proven, then maybe the mathematical formulae might give some insight into exactly what it is. Until then, you have different theories (string theory, MOND) without any idea what is correct and what isn't.

Gravitational waves stand on a firm footing since the discovery of that pulsar binary. Theoretically they were a part of GR from its inception.
What would be interesting is that if they were found to not exist.

Swift
2010-Aug-20, 01:58 AM
I have moved this thread from Q&A. Given the instructions about Q&A given in this thread (http://www.bautforum.com/showthread.php/105054-About-this-section-of-the-forum-%28for-the-natives%29), some of which is quoted below, it seems apparent that this will be an extended discussion and is more appropriate here

This section of the forum is for astronomy and space exploration questions with straightforward, generally accepted answers.

Questions that are likely to lead to extended discussion about the correct answer, or that have no clearcut correct answer, should be posted in the forum most appropriate to the topic of the question.

Ken G
2010-Aug-20, 02:10 AM
Yes.,

but that seems to ask more questions than it answers...Scientific theories always ask more questions than they answer, that's part of their purpose. There's some strange belief that theories demystify the universe, yet every single theory we've ever had actually replaced more superficial mysteries with more profound ones. That's what they do.


By what mechanism does space get bent ?That's what I meant by a question science does not answer. By what mechanism did Newton's gravity, the "law" of the cosmos used for 200 years, generate action at a distance? And if gravitons are found to work, and gravity is unified with the other forces, then what will be the mechanism by which mass emits gravitons? Or take quantum electrodynamics, a very successful theory with spectacular accuracy, and ask an expert, by what mechanism do charges emit virtual photons? These questions do not have answers, science just makes up these theories, without any underlying mechanisms. If a theory has an underlying mechanism, then it is not a fundamental theory-- the underlying mechanism will be the fundamental theory that does not have an underlying mechanism.

WaxRubiks
2010-Aug-20, 02:43 AM
But if someone asks how chess pieces get around a board, the mechanism, at one level, will be the mechanics of a human hand, and arm etc....where at other levels the mechanism wouldn't be know, like why the hand even exists.

What about the Anthropic principle?
If there were no gravity then we wouldn't exist.

Delvo
2010-Aug-20, 02:54 AM
Einstein did not call gravity spooky action at a distance. He called quantum entanglement that.

There are at least two theories on gravity. One, relativity, says that it's curved space. The other, quantum mechanics, doesn't have curved space, and needs intermediary force particles such as photons for any one particle of matter to affect another, so gravity has to be the action of particles called "gravitons". They're incompatible; both can't be right, so at least one has to be wrong. The problem is that, according to every test we can do so far, they're both right. Some parts of our daily lives even depend on their being right. So, until somebody is able to run a test in which the two theories give contradictory answers about what the results should be, and finds out which one gave the wrong prediction, and finds another way to explain the predictions that that "wrong" theory has always made accurately before, both theories will remain viable. So it will remain equally valid to describe gravity in either of the two mutually contradictory ways.

astromark
2010-Aug-20, 03:11 AM
I am pleased to have asked and even more pleased to have such a level of response...

Thanks to Swift, KenG, Caveman1917, and others... With a slight reservation re; Anthropomorphic...

can you expand on that, just a little more:eh: It sounds like something the tablets are for... ?:lol:

For if its something we do not know yet seems to be at the very root of existence ..

It is as if that just the presence of mater. Distorts space.,

and because I can not define space absolutely I am lost to understand by how this action works... Yes thats expectable.

WaxRubiks
2010-Aug-20, 04:12 AM
Thanks to Swift, KenG, Caveman1917, and others... With a slight reservation re; Anthropomorphic...

can you expand on that, just a little more:eh: It sounds like something the tablets are for... ?:lol:


.

Yes, you can get the pills at Boots..

I have my doubts about it myself, thinking about it....I think that there is more meaning to the universe than that.

Ken G
2010-Aug-20, 05:58 AM
But if someone asks how chess pieces get around a board, the mechanism, at one level, will be the mechanics of a human hand, and arm etc....where at other levels the mechanism wouldn't be know, like why the hand even exists.Sure, if you didn't see the hand moving the pieces, you'd wonder about why the pieces move. But if you did see the hand, then you'd wonder how that works. And if you knew about muscles and nerves, then you'd wonder about how those work. Every "mechanism" you discover will involve some other mechanism that is mysterious. There's no end to this process of wondering, and there isn't supposed to be.


What about the Anthropic principle?
If there were no gravity then we wouldn't exist.It is questionable whether or not such a principle "explains" anything, instead of simply having to be true.

Ken G
2010-Aug-20, 06:00 AM
They're incompatible; both can't be right, so at least one has to be wrong.Right, "at least"-- and why anyone would suspect even for an instant that either is not wrong, I cannot imagine.

EDG
2010-Aug-20, 06:16 AM
Not that this says anything about the "rightness" or "wrongness" of either theory, but personally I just can't get my head around the idea that a mass can emit particles that pull something toward it. Curved space seems much more intuitive and easy to visualise and understand (again, I am aware that "intuitive" doesn't necessarily mean "correct").

Then again, I'm not really sure how magnetic fields are supposed to work on a quantum level either (I get electrical attraction and repulsion, but are there such things as "magnetons"?). Could gravity and magnetism work in a similar way at the quantum level perhaps (OK, I know one seems to be attractive only while the other is both attractive and repulsive, but could there be some fundamental similarity in underlying mechanism somehow)?

publius
2010-Aug-20, 06:47 AM
EDG,

Actually the "particle exchange" model of forces is not that simple. At first blush, one thinks that does away with the ephemeral notion of a "field". It doesn't really, as the "particles" being exchanged are stranger than the field. Indeed, note the name of the modern quantum theory, Quantum *FIELD* Theory. :)

Even that is about fields, just fields that have some mathematical doohickies in them that can be called particles. Trying to apply the QFT prescriptions to GR just doesn't work (basically due to the non-linearity of the EFE). Indeed, and this is far beyond my understanding and based on what I read from those who (I hope) know what they're talking about, curved space-time backgrounds throw a monkey wrench in that process of declaring certain doohickies to be particles. I've read it said that the notion of "particle" just doesn't mean much against a curved background and one has to go with "pure field". Whatever that really means.

So, a quantum theory of gravity may not even have the notion of "gravitons" being exchanged at all, just the will-o-the-wisp of the quantum field doing whatever it does.

IOW, right along with what Ken says, I'll bet you dollars to doughnuts (or better, gold to Treasury bonds) that a quantum theory of gravity will be even stranger, more fundamentally mysterious, than anything to date, and make asking questions like "why does mass-energy curve spacetime) seem so boring by comparison. Our theories will become better at predicting what nature will do, but they become worse at telling us *why* nature does what it does.

astromark
2010-Aug-20, 06:52 AM
Edg has presented a aspect that seems just plainly wrong. I have not considered gravity or the bending of space time to be

The action of electromagnetic principals. BUT, do I know it to be wrong ? NO, I can only ask for more proof.

I can not see where gravity might fit on the spectrum.. smaller than a planked length. I need more than this.

But its started me thinking... great.

Jeff Root
2010-Aug-20, 07:46 AM
Why are gravity and the electric force so similar?

That question has fascinated me since the 1970's.

-- Jeff, in Minneapolis

cosmocrazy
2010-Aug-20, 08:47 AM
Why would it be linked to the Higgs boson?
Gravity doesn't depend on the mass in se, but on the energy. We commonly think of it as related to mass because rest mass is the major contributor to energy, but any energy will do.For example a photon has a gravitational effect too, but it doesn't interact through the Higgs mechanism.

My thinking also.

cosmocrazy
2010-Aug-20, 08:51 AM
Your guess is as good as anyone's. Nobody knows how the presence of energy curves spacetime near to it, it just does.
It's not even clear what is cause and what is effect here. Consider a black hole, the energy is localized inside the EH. Yet its 'effect' reaches outside of the EH.
Perhaps one way to look at it is to say that gravity is energy - just different perspectives on the same thing.

If gravity affects everything including light then I agree it has to be energy related in some way.

cosmocrazy
2010-Aug-20, 08:54 AM
EDG,

Actually the "particle exchange" model of forces is not that simple. At first blush, one thinks that does away with the ephemeral notion of a "field". It doesn't really, as the "particles" being exchanged are stranger than the field. Indeed, note the name of the modern quantum theory, Quantum *FIELD* Theory. :)

Even that is about fields, just fields that have some mathematical doohickies in them that can be called particles. Trying to apply the QFT prescriptions to GR just doesn't work (basically due to the non-linearity of the EFE). Indeed, and this is far beyond my understanding and based on what I read from those who (I hope) know what they're talking about, curved space-time backgrounds throw a monkey wrench in that process of declaring certain doohickies to be particles. I've read it said that the notion of "particle" just doesn't mean much against a curved background and one has to go with "pure field". Whatever that really means.

So, a quantum theory of gravity may not even have the notion of "gravitons" being exchanged at all, just the will-o-the-wisp of the quantum field doing whatever it does.

IOW, right along with what Ken says, I'll bet you dollars to doughnuts (or better, gold to Treasury bonds) that a quantum theory of gravity will be even stranger, more fundamentally mysterious, than anything to date, and make asking questions like "why does mass-energy curve spacetime) seem so boring by comparison. Our theories will become better at predicting what nature will do, but they become worse at telling us *why* nature does what it does.

Gets me thinking that gravity could be considered to have wave/particle duality like light?

cosmocrazy
2010-Aug-20, 08:57 AM
Why are gravity and the electric force so similar?

That question has fascinated me since the 1970's.

-- Jeff, in Minneapolis

Me to Jeff, and I think that gravity and EM forces have some kind of common connection that we seem to be missing somewhere, if gravity affects light then surely there is some common ground i.e the presence of any form of energy including light warps space. I just wish I understood them both better.

Strange
2010-Aug-20, 09:49 AM
What i mean is, physically, do we really know what 'Gravity' is ?

Do we, and can we, know what anything "really" is? (cf the other thread on "what is reality").

You know you are pushing the boundaries when someone like publius says it is beyond his understanding. :)

undidly
2010-Aug-20, 10:44 AM
Gravity is not a force.
It is the name for what happens when things slide DOWN the GRADIENT of space near to mass.

This idea explains ALL the characteristics of gravity.

Strange
2010-Aug-20, 10:46 AM
Gravity is not a force.
It is the name for what happens when things slide DOWN the GRADIENT of space/time.

This idea explains ALL the characteristics of gravity.

I'm not sure it explains everything. For example:

Don't you need gravity to define "down"?

What causes the gradient?

undidly
2010-Aug-20, 11:20 AM
I'm not sure it explains everything. For example:

Don't you need gravity to define "down"?

What causes the gradient?

Down is the direction opposite to the direction in which the observer is accelerating.
That means that an observer ON Earth is accelerating UPWARDS at 1 G.(Equivalence of acceleration and gravity,GR.)

One answer per post.
Gradient explanation later.

Ken G
2010-Aug-20, 02:17 PM
Gravity is not a force.
It is the name for what happens when things slide DOWN the GRADIENT of space near to mass.

This idea explains ALL the characteristics of gravity.Yeah, except it uses nonsense phrases like the "gradient of space."

dgavin
2010-Aug-20, 06:44 PM
Why would it be linked to the Higgs boson?
Gravity doesn't depend on the mass in se, but on the energy. We commonly think of it as related to mass because rest mass is the major contributor to energy, but any energy will do.
For example a photon has a gravitational effect too, but it doesn't interact through the Higgs mechanism.



I thought that string theory also takes the graviton-approach instead of the curvature one.

I mean that like photons being linked to the energetic state changes of atoms, gravitons if correct, might be linked to higgs bosun interactions. It's a logical possibly to consider from a QM point of view. It's still an unknown though.

On the other one i might of been mixing string theory with brane theory. I seem to remeber them both tryign to solve gravity as an >4 dimensional contruct and havign similar issues when renormalizing in both techniques.

dgavin
2010-Aug-20, 06:48 PM
Gets me thinking that gravity could be considered to have wave/particle duality like light?

Not only that, but if the QM/QFT pans out, it leads to the consept of anti-graviton's, which could be behind the accelerating expansion of the universe. There is still so much that is unkown about gravity!

EDG
2010-Aug-20, 06:57 PM
Gets me thinking that gravity could be considered to have wave/particle duality like light?

That's an interesting idea.

My (vague, armwavy and probably not-based-on-much) opinion is that gravity is the most basic, fundamental "force" that there is. It's hard for me to explain really, but I think gravity is what everything else ultimately boils down to - as if the other forces are just "higher order" versions of gravity, and if you could strip them down (at really high energies and pressures) then they just 'break down' into gravity. I'm not sure if that maybe is based on any ideas about what happens when all the forces are unified or something. But like I said, that's all just vague armwaving on my part really.

caveman1917
2010-Aug-20, 10:38 PM
I mean that like photons being linked to the energetic state changes of atoms, gravitons if correct, might be linked to higgs bosun interactions. It's a logical possibly to consider from a QM point of view. It's still an unknown though.

It is indeed an unkown, but i fail to see how it is logical. Only some particles interact through the Higgs mechanism, but all interact gravitationally.


On the other one i might of been mixing string theory with brane theory. I seem to remeber them both tryign to solve gravity as an >4 dimensional contruct and havign similar issues when renormalizing in both techniques.

I am by no means an expert on string theory, but my understanding is that M theory treats gravity as spin-2 particles (closed strings to be exact), in other words as standard gravitons.
What you may be remembering is that this 'closed-string' nature of gravitons means they are not bound to the 4dimensional brane everything else is bound to, and thus are free to 'leak' in multidimensional space. In the same go solving the hierarchy problem.


Not only that, but if the QM/QFT pans out, it leads to the consept of anti-graviton's, which could be behind the accelerating expansion of the universe.

This is simply not true. Gravitons are their own antiparticles. There is no such thing as an antigraviton.


There is still so much that is unkown about gravity!

That one is true :)

Tensor
2010-Aug-21, 12:05 AM
Just to throw a wrench into everyones thinking, why consider gravitons as interacting between masses? Why not consider gravitons as what is interacting with spacetime, to cause the warping of spacetime? Or, possibly interacting with the Higgs field, to cause a change in the way masses interact with the Higgs field, making the appearance of a warping of spacetime?

I want to point out that these arew extremely speculative and in no way should be considered as a serious idea

Cosmo, if gravity is ever shown to be part of Quantum Theory, of course there would be a wave/particle aspect. Gravity waves could even be considered to have a particle aspect. Of course, since we haven't even directly detected gravity waves, detecting the particle aspect of those waves is even more difficult.

caveman1917
2010-Aug-21, 01:17 AM
Just to throw a wrench into everyones thinking, why consider gravitons as interacting between masses?

Their effect is highly correlated with the mass distribution, it would seem natural to link them somehow and the easiest way of doing that is to consider it as interacting between masses.


Why not consider gravitons as what is interacting with spacetime, to cause the warping of spacetime?

But if spacetime can be curved in the graviton description, won't that bring in the regular problems of background (in)depence?


Or, possibly interacting with the Higgs field, to cause a change in the way masses interact with the Higgs field, making the appearance of a warping of spacetime?

If photons do not interact with the Higgs field, neither would they be affected by gravitons, and thus we would see no gravitational lensing.

undidly
2010-Aug-21, 02:15 AM
Yeah, except it uses nonsense phrases like the "gradient of space."

The gradient is the angle of that part of the gravitational well.

Equivalent to slope (near the mass)in the rubber sheet gravity analogy.

A gravitational well has physical dimensions that are measurable.

The increasing gradient near to mass is why "gravity " is stronger near the mass.

Jeff Root
2010-Aug-21, 03:49 AM
Gravitons are their own antiparticles. There is no such thing as
an antigraviton.
Why do you say this? Since it isn't known whether gravitons
exist, how can you be so certain about antigravitons?

-- Jeff, in Minneapolis

Ken G
2010-Aug-21, 04:54 AM
The gradient is the angle of that part of the gravitational well.

Equivalent to slope (near the mass)in the rubber sheet gravity analogy.The rubber sheet isn't an analogy, it's more like a toy picture, not to be taken too seriously.



A gravitational well has physical dimensions that are measurable.This is the kind of statement that comes from taking the toy picture too seriously.


The increasing gradient near to mass is why "gravity " is stronger near the mass.You are not even clear if you are trying to explain gravity with some kind of gradient, or the gradient of gravity with some kind of gradient. It's a muddle. That's not too surprising-- it took Einstein about 10 years to figure it out, once he came up with the equivalence principle. So no, it's quite a bit more than a "gradient in space."

dgavin
2010-Aug-21, 07:13 AM
Why do you say this? Since it isn't known whether gravitons
exist, how can you be so certain about antigravitons?

Speaking from a QM point of view only, there would either need to be an antigraviton, or a fifth fundemental force carrying particly called something like a repulseton to explain the accelerating expansion. An antigraviton would be simpler explaintion I think. But again, we simply don't know yet. So this is really just conjection of what might be found.

There is also the posibility that gravity will remain a complete unknow, as both it and gravitational waves turn out to be nondetectable by much experimentation. Course that gives me a headache just thinking about it, so i hadn't suggested it before.

undidly
2010-Aug-21, 07:17 AM
To Ken G

undidly "A gravitational well has physical dimensions that are measurable".

Ken G " This is the kind of statement that comes from taking the toy picture too seriously".

Do you mean a gravity well does not have physical dimensions or that they are not measurable?

caveman1917
2010-Aug-21, 01:13 PM
Why do you say this? Since it isn't known whether gravitons
exist, how can you be so certain about antigravitons?

-- Jeff, in Minneapolis

It is known that gravitons, if they exist, must exhibit some theoretical properties such as zero rest mass, spin 2, etc.
They also wouldn't be charged (in the general sense, electric charge, lepton charge, ...)
Thus under time reversal (which makes particles into antiparticles) the graviton remains the graviton.
It is its own antiparticle, just like the photon. That can be deduced from the properties it must have - wether it is found to exist or not.

Ken G
2010-Aug-21, 02:37 PM
To Ken G

undidly "A gravitational well has physical dimensions that are measurable".

Ken G " This is the kind of statement that comes from taking the toy picture too seriously".

Do you mean a gravity well does not have physical dimensions or that they are not measurable?It means that gravity is quite a lot more complicated than "measuring the dimension of a gravitational well." Your language does not even distinguish Einstein's gravity from Newton's. What are you saying about Einstein's gravity that does not hold in Newton's?

astromark
2010-Aug-21, 09:16 PM
Just looking across the last few posts you can see some confusion creeping into this normally sterile subject :lol:

Please refrain from discussion of unknowns... Particles called gravitons or there negatives...

its bad enough that we know so little of gravity that we invent what it might be...

A understanding of FACTS known is all I want. I have a wonderfully active imagination that does not require feeding...

As Ken has so aptly put 'its complicated' I am not alone in wanting to understand just what gravity is.

That it can have effect from deep inside a black hole exerting its distortion of space time upon its surrounding while light can not escape...

from just that I want to know... how and why. and that the answer is not yet up in the things we know list.... :lol:

publius
2010-Aug-21, 09:37 PM
I'll take issue with "we know so little of gravity". If knowing means predicting what it will do, General Relativity passes that test with flying colors. If by "knowing" means the "why" that we've discussed before, then we know little of everything. QED tells us no more of why there's this stuff called charge that does what it does than gravity does what it does.

The reason many say gravity is so ill known is because it hasn't been quantized yet. But, as far as predictive power, gravitational effects are nil at the sale of the very small, and classical GR dominates in the macro realm.

And finally, there is nothing all that mysterious about "gravity escaping" from a black hole, even though light does not within GR. There is no "action at a distance". The mass-energy leaves it's imprint on the spacetime behind. Same thing with a charged black hole. The charged particles that fall in leave their electric field behind so to speak.

A test charge interacts with the local field, and a test mass interacts with the local spacetime, both of which are shaped by what happened before (but in GR, by invoking "before", I'm imposing a time slicing.....)

-Richard

Ken G
2010-Aug-21, 10:54 PM
And finally, there is nothing all that mysterious about "gravity escaping" from a black hole, even though light does not within GR. There is no "action at a distance". The mass-energy leaves it's imprint on the spacetime behind. Same thing with a charged black hole. The charged particles that fall in leave their electric field behind so to speak. Yes, and in the quantum field theory view, as far as I know, the idea is that the forces from charges (virtual photons) and the forces from the mass-energy (virtual gravitons) are virtual particles, so are not constrained by the rules that say they cannot escape the black hole. So I'm told, I never use virtual particles for anything and they still seem a bit magical to me.


A test charge interacts with the local field, and a test mass interacts with the local spacetime, both of which are shaped by what happened before (but in GR, by invoking "before", I'm imposing a time slicing.....I think the real issue is that some people cling to the idea that some physical models are what is really happening, while others are mysterious and we still don't know what they are. But they are mistaking the basic familiarity that comes from repetition or experience with an ephemeral (or even nonexistent) concept of genuine understanding-- even things we are all as familiar with as mother's milk are actually completely mysterious if we stop and think about what it is that we actually know about them.

Tensor
2010-Aug-21, 11:21 PM
A test charge interacts with the local field, and a test mass interacts with the local spacetime, both of which are shaped by what happened before (but in GR, by invoking "before", I'm imposing a time slicing.....)

-Richard

It took me a long time and a lot of puzzling over the idea of gravity regenerating itself to account for the warping of spacetime for the area around a black hole. It was completely mysterious for me, until I grasped the idea of non-linearity. THAT, IMHO, is one of the things that makes understanding gravity so, bang your head against the wall or desk difficult. That it doesn't show up in any kind of large amount (I'm not counting Mercury's precession as a large amount, although it's measurable) could be one of those reasons it doesn't make sense.

undidly
2010-Aug-21, 11:59 PM
It means that gravity is quite a lot more complicated than "measuring the dimension of a gravitational well." Your language does not even distinguish Einstein's gravity from Newton's. What are you saying about Einstein's gravity that does not hold in Newton's?

So gravity is more complicated than measuring the dimensions of a gravity well.

Just tell me the dimensions of Earths gravity well and I will be satisfied.

Newton never claimed to understand or explain gravity.
Newtons math gave the right answers and that was all.

Einstein did explain gravity as a curvature of space/time.
I am asking for the angle,gradient,direction or anything about the curvature.

Jeff Root
2010-Aug-22, 12:24 AM
It is known that gravitons, if they exist, must exhibit some
theoretical properties such as zero rest mass, spin 2, etc.
They also wouldn't be charged (in the general sense, electric
charge, lepton charge, ...)
Thus under time reversal (which makes particles into antiparticles)
the graviton remains the graviton.
It is its own antiparticle, just like the photon. That can be deduced
from the properties it must have - wether it is found to exist or not.
Ah! So you are saying that there are no *known* properties that
gravitons could have that could be reversed to give antigravitons.

Could you go into more detail about this in another thread
(which I'll start in Q&A) regarding photons rather than gravitons?

-- Jeff, in Minneapolis

astromark
2010-Aug-22, 01:51 AM
The explanations I am reading do not answer my confusion.. After reading and re reading... it does not altar the facts for me...

Publius I do understand much is 'known' of the gravity. Calculations and predictions are not made of guess work.

I can not answer my question. Thats why I asked it.

I want to better understand how over such great distances a shift in the space time seems to work.

Ken G
2010-Aug-22, 02:39 AM
Einstein did explain gravity as a curvature of space/time.
I am asking for the angle,gradient,direction or anything about the curvature.To be honest, I have no simple explanations of the use of spacetime curvature in general relativity. The mathematics is quite difficult, and pretty much only people who work with it regularly can be counted on to not hold misconceptions (and I'll bet even those people slip up from time to time). One picture that seems to work pretty well is to endow space with real characteristics, even though no such theory of space actually exists, and then state that a source of gravity makes space fall. To see how fast it makes space fall, release an object at infinity and watch what it does, and note that Newton's law of the acceleration of gravity will actually be correct for determining the rate of falling space (in the case of a point mass, anyway). What is significant is that an observer who is hovering a fixed distance from the source of the gravity, so not falling with the space, is the one that has a force on it, and is the one that is accelerating upward through that falling space-- the released object is inertial, has no force on it, and is not accelerating, it is just "doing what the space it occupies is also doing." If the object is not released from infinity, but from some finite distance from the source, it starts out with a large velocity "relative to the falling space", and will maintain that fixed relative velocity as it falls with the space.

That picture gets you pretty far, and might even be correct for a point source, but I'm not sure how literally you can get away with it. But it is just a picture, mind you-- the mathematical statement involves curvature of spacetime, but that gets into tensors and some very tricky math.

astromark
2010-Aug-22, 04:46 AM
I do like and generally find informative the posts of the above... Ken G.. and this subject is a good case study of that. Excellent.

I note that asking to define the gravity well of Earth or anything would be dependent on your ability to detect any such curvature from 0.

We seem to have grasped well the fact that objects of mass distort the space they are in and near to.

That significant objects have a space bending power is very interesting... No wave, No particle.. but what I wonder ?

undidly
2010-Aug-22, 05:06 AM
I do like and generally find informative the posts of the above... Ken G.. and this subject is a good case study of that. Excellent.

I note that asking to define the gravity well of Earth or anything would be dependent on your ability to detect any such curvature from 0.

We seem to have grasped well the fact that objects of mass distort the space they are in and near to.

That significant objects have a space bending power is very interesting... No wave, No particle.. but what I wonder ?

""objects of mass distort the space they are in""

Yes.

But how much and in which direction?

publius
2010-Aug-22, 05:31 AM
""objects of mass distort the space they are in""

Yes.

But how much and in which direction?


The problem is that question "how much and what direction" is meaningless and comes from that fact you're trying to paint a picture that doesn't work like you're thinking.

Spacetime curvature is just that. It is the curvature, an abstract mathematical concept (which derives from simpler concepts of curvature that can be visualized geometrically), of a 4 dimensional geometry. And the base "flat" 4 dimensional geometry, Minkowski, doesn't even work like you might think. Minkowski is a non-positive definite manifold, where the metric, the notion of distance, can be negative as well as positive. First, we can't visualize any more than 3 dimensions, and second, it's a strange geometry that doesn't have the same rules as positive definite Euclidean geometries do anyway. Then you add curvature to that mix. What you get is an abstract mathematical framework.

Point in the direction of tomorrow. Point in the direction of yesterday. You can't do it. That's what you're asking when you're asking what direction is spacetime curved toward.


-Richard

Jeff Root
2010-Aug-22, 06:47 AM
I don't see any problem with answering that the curvature is toward
yesterday or toward tomorrow. If spacetime curvature posited four
spatial dimensions, then I would't be able to visualize it. But three
spatial dimensions and one time dimension-- I can do that.

-- Jeff, in Minneapolis

astromark
2010-Aug-22, 07:20 AM
In the case of this galaxy we know that answer... its towards the central mass. And in the case of the universe near here...

That would be the Virgo cluster... Why is there some confusion here ?

No I can not point at yesterday or tomorrow but, I can detect the slope of the gravity well.

By detection of that slope do we measure the mass of a object... Yes the image is getting clearer.

I can not and would not dare speak for others but, I am finding that as we do not know of the mechanism of that slope., but

but that I can visualize and have a working concept of it. The gravity well is not such a unknown...

and with many questions as yet to be answered.

My explanation is of what is gravity is not a startling new concept. I hope it is the best that science can offer.

I am simply looking for a better explanation than I have yet found... 'Understanding Gravity.'

I want to pull into this the obvious next question.. 'If space is empty what is bending ?' Then its not empty is it ?

Jeff Root
2010-Aug-22, 08:00 AM
As far as I've been able to tell so far, space is probably empty.
What is "bending" or "curved" is not "space itself" (since space
is not a material "thing"), but the directions that things in that
space accelerate, and the distances between those things.
The distances and directions change when masses are nearby,
so we say the space is curved by those masses.

The mass alone doesn't tell you the curvature. The distribution
of the mass is equally important.

-- Jeff, in Minneapolis

cosmocrazy
2010-Aug-22, 08:52 AM
As far as I've been able to tell so far, space is probably empty.
What is "bending" or "curved" is not "space itself" (since space
is not a material "thing"), but the directions that things in that
space accelerate, and the distances between those things.
The distances and directions change when masses are nearby,
so we say the space is curved by those masses.

The mass alone doesn't tell you the curvature. The distribution
of the mass is equally important.

-- Jeff, in Minneapolis

Could this mean that the distortion of space which results in the acceleration you mention is due to increased energy of the system from the presence of mass? By which I mean the increased energy is observed by us as acceleration of the masses involved, an effect we call gravity.

astromark
2010-Aug-22, 10:05 AM
Well.... it sort of but only just... and sort of and maybe is that those objects of detectable mass are seen to be accelerated

by the presence of mass. By the stretching and destortions of space. I understand this.

Now if thats not confusing my name is not Masuthla... and its not. I do not except that space is empty. Just nearly empty...

BUT JUST Hang on a moment... with no insult to any and all Masuthla's intended. Space Is for all intents empty. No argument there.

Its the space itself that is doing the movement we measure as gravity... How you might say that and the way I just did are not the same.

No mater to that... please. This energy you talk of that you think might be just the energy of what ? Mass. ?

So we detect motion or force and attribute it to the presence of mass. I think that is clear. Prey tell how ?

How does it work ? Are you going to tell me that sub atomic imbalances are driving space distortion... gravity. really ?

I am pleased some of us understand me...

Ken G
2010-Aug-22, 02:53 PM
Point in the direction of tomorrow. Point in the direction of yesterday. You can't do it.That's why I've always felt that the spacetime concept cannot be the complete story. The way displacements in time contribute to the metric is different from displacements in distance, yet the spacetime mathematics unifies these in such a way that it is tempting to say "time is just another dimension like space." But that can't be the whole story, or we could "point toward yesterday." There remains something different about time, something more fundamental to the experience of observers, who in turn design the physics. That's why it was such a shock that time was unifiable with space at all-- but we must not go overboard and begin to imagine it is somehow the same thing as space. I feel that the spacetime manifold concept has huge unifying and mathematical power, but it must be a replacement of reality with something else, so it must be just a model that breaks down at some point. Perhaps that's why it does not unify with quantum mechanics.

Jerry
2010-Aug-22, 07:34 PM
As others have stated, Newton describe gravity mathematically in a way that still explains gravimetric attraction most of the time.

Maxwellian electromagnetic field equations are so similar to gravity; it was immediately assume gravity is an electromagnetic effect and experiments were conducted that were expected to proof this. The predictions were wrong.

Relativity - a replacement theory - also makes predictions about the behavior of gravity that are slightly revised from the Newtonian predictions and better aligned with experimental observations. It is wrong to say relative has passed every test; because there are many cases where it has not. At this time it is generally assumed the experiments that did not match expectations were flawed experiments. (For example, the Cold Dark Matter that is currently hypothesized to exist is based upon the assumption that all attempts to observe dark matter have failed due to lack of sensitivity of our detectors; and that there is no basic flaw in our conception of gravity.)

Current quantum theory makes predictions about the force carriers for gravity. It is interesting that the roots of the BB hypothesis rely upon a force carrier (or very similar) explanation for gravity; because one essential element of the theory is that there was a time in the very early universe that the gravitational force did not exist.

"Renormalization" is a black box approach to physics: you put one set of functions on one side of the box, and you pull out a new set of functions on the other side of the box. It is not so much an explanation as a concession that we cannot explain certain observations in physical terms or simple mathematical abstractions.

There are currently on-going tests that are shrinking the parameter space of both relativistic and particle physics explanations for gravity; meaning that the predictions about the behavior of gravity in both astronomical space and particle space are running out of room.

It is a very interesting time, because the electromagnetic explanation was dropped when early 20th century tests eliminated the predicted parameter space of an electromagnetic eather. This does not rule out an electromagnetic causal relationship with gravity; it only means that if there is such a relationship; the nature of the relationship lies outside of the parameter space predicted in the 19th century. We are running out of parameter space using the replacement theories as well.

Ken G
2010-Aug-22, 10:00 PM
Maxwellian electromagnetic field equations are so similar to gravity; it was immediately assume gravity is an electromagnetic effect and experiments were conducted that were expected to proof this. The predictions were wrong.I am not aware of any mainstream physicist of the day who expected gravity to be an electromagnetic effect. Indeed, the experiments that gave birth to relativity (especially general relativity) resulted in gravity being much closer to the same as electromagnetism than the Newton's theory said it would be, and that's why general relativity allows for gravitational radiation.

It is wrong to say relative has passed every test; because there are many cases where it has not.I believe the claim that is generally made is that it has passed every test that has been put to it. We can't hold it responsible for not passing tests it has not been subjected to. Still, it is true that it may someday fail a test, that seems inevitable in any theory.


At this time it is generally assumed the experiments that did not match expectations were flawed experiments. (For example, the Cold Dark Matter that is currently hypothesized to exist is based upon the assumption that all attempts to observe dark matter have failed due to lack of sensitivity of our detectors; and that there is no basic flaw in our conception of gravity.) I wouldn't say it is assumed the experiments are flawed, I'd say it is recognized that the experiments are not suitably controlled. For controlled experiments, you want to control the sources of gravity, and when that has been done successfully, there has been no problem. But in galactic astronomy, we cannot control the sources, we are stuck with what nature gives us. That's not exactly a "flaw", but it is endemic to astronomy. You are right that we would have liked general relativity to work even with the gravity sources we already know about, and it is unsettling to have to introduce new ones, but it is not failing an experimental test. GR cannot be held responsible if the experiments are not suitably well controlled.


It is interesting that the roots of the BB hypothesis rely upon a force carrier (or very similar) explanation for gravity; because one essential of the theory is there was a time in the very early universe that the gravitational force did not exist.That is not an accurate characterization of the Big Bang model-- no such assumption is "essential" in the Big Bang, that theory is based in general relativity. But certain added elements get talked about, like the inflationary epoch, which are particle-physics applications fed into general relativity in highly speculative ways. If they were discovered to be wrong, the Big Bang model would likely survive anyway.

Jerry
2010-Aug-23, 05:10 AM
Many many gravitational wave antenna have failed to detect a single gravitational wave, starting with Weber in the 1970's. These results are unique, in that the experiments have not failed to achieve the designed sensitivity limits; gravitational waves simply have not been observed within the parameter space in which they were thought to be detectable. These tests have been 'put to' General Relativity on an exhaustive scale; consuming hundreds of millions of research dollars.

Disappointing, because as soon as gravitational waves were detected, I was going to apply for a patent on a gravitational wave camera/interferometer;)

Ken G
2010-Aug-23, 05:33 AM
Many many gravitational wave antenna have failed to detect a single gravitational wave, starting with Weber in the 1970's. But none of them should have received a detectable signal, before LIGO. On the other hand, binary pulsars should spin down because of gravitational radiation-- in just the way that they are seen to do. Thus, so far, gravitational waves are not a failed test of GR, they are a passed test of GR. If LIGO goes a lot longer without finding anything, that situation might change.

These results are unique, in that the experiments have not failed to achieve the designed sensitivity limits; gravitational waves simply have not been observed within the parameter space in which they were thought to be detectable.No, past attempts were hoping there'd be much larger signals than anyone had any reason to expect. Expected detections are quite the modern state of the art, and even then it's a crapshoot when you get an event to happen.


These tests have been 'put to' General Relativity on an exhaustive scale; consuming hundreds of millions of research dollars. The only experiment with that price tag is LIGO, which is very much an ongoing experiment. It's too soon to take stock of the return on that investment. The Dodgers spent 20 million dollars on Manny Ramirez this season, and he's been injured for most of it. How many hundreds of millions of dollars have been wasted by sports fans for that reason alone?


Disappointing, because as soon as gravitational waves were detected, I was going to apply for a patent on a gravitational wave camera/interferometer;)Then you're the one at fault! They are trying to confound you.

astromark
2010-Aug-23, 08:16 AM
There I am standing at the top walkway of the Telescope structure, pondering waving at the gravity of distant objects

that do not seem to wave back... and being careful not to become a example of gravity at work... and, No. Cancel that patent application.

dgavin
2010-Aug-23, 12:43 PM
Actually the lack of detection of gravitiational waves by LIGO and other experiments simply means that gravity waves are not detectable by current methods of experimentation.

There is also the posibility gravity waves might not be detectable by normal expeirments, it could be that gravity waves are moving outisde of normal space/time and thus would remain undetectable by most methods experiments would use.

Jerry
2010-Aug-23, 05:28 PM
Actually the lack of detection of gravitiational waves by LIGO and other experiments simply means that gravity waves are not detectable by current methods of experimentation.

There is also the posibility gravity waves might not be detectable by normal expeirments, it could be that gravity waves are moving outisde of normal space/time and thus would remain undetectable by most methods experiments would use.
These explanations are akin to saying "we don't know what a gravity wave looks like"; and while it may be true, it is inconsistent with the anticipated results of experiments specifically designed to test one aspect of general relativity.

Here is another curiousity:

http://arxiv.org/abs/1008.3526v1

Testing Newtonian gravity with distant globular clusters: NGC1851 and NGC1904


Globular clusters are useful to test the validity of Newtonian dynamics in the low acceleration regime typical of galaxies, without the complications of non-baryonic dark matter. Specifically, in absence of disturbing effects, e.g. tidal heating, their velocity dispersion is expected to vanish at large radii. If such behaviour is not bserved, and in particular if, as observed in elliptical galaxies, the dispersion is found constant at large radii below a certain threshold acceleration, this might indicate a break down of Newtonian dynamics.
To minimise the effects of tidal heating in this paper we study the velocity dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904. ...

These data allow to trace the velocity dispersion profile up to ~2r0, where r0 is the radius at which the cluster internal acceleration of gravity is a0 = 10e-8 cm/s/s. It is found that in both clusters the velocity dispersion becomes constant beyond ~r0. These new results are fully in agreement with those found for other five globular clusters previously investigated as part of this project. Taken all together, these 7 clusters support the claim that the velocity dispersion is constant beyond r0, irrespectively of the specific physical properties of the clusters: mass, size, dynamical history, and distance from the Milky Way. The strong similarly with the constant velocity dispersion observed in elliptical galaxies beyond r0 is suggestive of a common origin for this phenomenon in the two class of objects, and might indicate a breakdown of Newtonian dynamics below a0.

I haven't had time to read this yet, but it sounds like the wheels are falling off of the wagon.

Ken G
2010-Aug-23, 07:59 PM
I haven't had time to read this yet, but it sounds like the wheels are falling off of the wagon.If their interpretation is correct, then I would have to agree with you. Of course, it's always possible, maybe even likely, that what is happening is the wheels are falling off their interpretation of what is actually some form of experimental error. I haven't read it either, so I'll keep open both possibilities-- the very fact that both possibilities are open makes it something interesting to keep an eye on. Red flag to keep an eye out for: if repeating the same study with higher observational precision gives the same result but with a smaller a0.

Tensor
2010-Aug-23, 09:50 PM
If their interpretation is correct, then I would have to agree with you. Of course, it's always possible, maybe even likely, that what is happening is the wheels are falling off their interpretation of what is actually some form of experimental error. I haven't read it either, so I'll keep open both possibilities-- the very fact that both possibilities are open makes it something interesting to keep an eye on. Red flag to keep an eye out for: if repeating the same study with higher observational precision gives the same result but with a smaller a0.

I am reading it (very difficult as I am traveling and don't have my library with me). However, I have noticed these two interesting quotes:


What is different is the interpretation of the data. The common wisdom goes that in galaxies it is the dark matter that alters the dynamics, while in globular clusters something else does it. The most viable explanation being tidal heating, the increase of the stellar velocity dispersion due to the differential acceleration produced by the Milky Way in different position of the globular cluster. It would be, however, more logic(sic) to invoke a common origin of the phenomenon, that is a failure of Newtonian dynamics below a0 as claimed within the frame work of MOND (Milgrom 1983).

In the conclusion of the paper:

This picture would be in good agreement with the prediction of the MOND hypothesis, in the case of no strong gravitational external field (Milgrom 1983). However, according to the MOND original formulation in none of these clusters we should observe deviations from Newtonian dynamics, be- cause the external field of the Milky Way is close or above a0. Therefore our results appear formally in disagreement with MOND predictions.

So while they may have an interesting observation (I stress the may here, see next sentence). The conclusion notes that the velocity dispersion of each cluster is not enough to invalidate Newtonian mechanics. However, the authors then go about combining all seven of the studied clusters. Not explaining whether or not the clusters are alike enough to do this. Then, they limit the number of points and present their results and suddenly, the velocity dispersion observation pops out. Does that mean that their methods and interpretations are wrong. Not necessarily.

Ken G
2010-Aug-24, 12:02 AM
The conclusion notes that the velocity dispersion of each cluster is not enough to invalidate Newtonian mechanics. However, the authors then go about combining all seven of the studied clusters. Not explaining whether or not the clusters are alike enough to do this. Then, they limit the number of points and present their results and suddenly, the velocity dispersion observation pops out. Does that mean that their methods and interpretations are wrong. Not necessarily.Yes, either they have seen the tip of the iceberg of a phenomenon that will just get clearer and clearer with future data, or the future data will make their interpretation disappear into the aether. It's good science either way, but only if they don't overinterpret what could only be described as a highly tentative finding. There's a common "red flag" for bogus results, which is that new data constantly requires different and more nuanced massaging to avoid having the original effect disappear.

caveman1917
2010-Aug-24, 03:01 AM
I find it rather odd they found an effect predicted by a theory, in a location where that same theory says it shouldn't be in the first place.

Jens
2010-Aug-24, 04:45 AM
Just as a slightly different take on this, though very much to do with the nature of gravity, has anybody read the paper by Verlinde about gravity as an entropic force, or as an emergent force? He is arguing that it isn't fundamental, but emerges as a result of something else (to do with information). I don't want to say too much about it simply because I don't understand it all that well so I'm sure I would mis-characterize.

Ken G
2010-Aug-24, 01:47 PM
I find it rather odd they found an effect predicted by a theory, in a location where that same theory says it shouldn't be in the first place.
It certainly makes you worry about a similar contaminant existing in data used to bolster the other theory.

Ken G
2010-Aug-24, 01:53 PM
Just as a slightly different take on this, though very much to do with the nature of gravity, has anybody read the paper by Verlinde about gravity as an entropic force, or as an emergent force? He is arguing that it isn't fundamental, but emerges as a result of something else (to do with information). I don't want to say too much about it simply because I don't understand it all that well so I'm sure I would mis-characterize.It's been discussed on other threads, but none of us has the expertise to add much to the buzz about it-- reactions by some are guardedly optimistic, others are highly critical. The idea seems to hinge on the possibility that spacetime itself is organized entropically-- it is in some sense "composed" of some kind of highly complex random process that plays out in the most probable configuration, related to thermodynamic issues of how black holes seem to "store entropy" in their event-horizon membranes, which can then be radiated as Hawking radiation.

At some level, this has to be true, because all fundamental processes must be entropic in nature-- nothing that happens spontaneously can happen for any other reason than it is the most probable outcome, that tells us less about the universe than it tells us about the use of the concept of probability. But the real question is, what do we gain by taking this perspective for gravity? That wasn't so clear, it seemed highly pedagogical rather than predictive.

astromark
2010-Aug-27, 02:39 AM
Over complication of the simple facts... That the very presence of mass distorts the space it is near and in.

How does this impart any information other than clues of mass and proximity to it I have no idea...

could a little explanation be forthcoming ? please. I obviously have a problem seeing the link between these ideas.

Tensor
2010-Aug-27, 04:45 AM
Over complication of the simple facts... That the very presence of mass distorts the space it is near and in.

How does this impart any information other than clues of mass and proximity to it I have no idea...

could a little explanation be forthcoming ? please. I obviously have a problem seeing the link between these ideas.

First off, energy and pressure produce the warping, not just mass (the equivalent energy of the mass). Second, there is nothing in General Relativity that explains exactly what, in English words, about energy and pressure, causes the warping. The only thing linking the two are the Einstein Field Equations. Where the Stress Energy Tensor, plus a constant equals the warping of a manifold as described by the Ricci curvature tensor, the scalar curvature and the metric tensor. I'm not sure there is any easier way to explain it except by saying energy and pressure warps spacetime. Mathematically, I don't know of anything easier than here (http://people.hofstra.edu/Stefan_waner/diff_geom/Sec14.html). Unfortunately, you also need to know quite a bit of math, just to understand the basics of GR.

astromark
2010-Aug-27, 05:09 AM
@ Tensor; I asked the question. I started this thread. This force you talk of that is energy and pressure driven .. your words not mine...

Is what we are talking of. Gravity is not in itself a force. Excepted. I use the term effect.

If I place a object of mass completely motionless some place.. any place.

It will, given time... begin to fall down towards a object that is distorting space time...

AND let it be clear that... that link opened by the word 'here'... ( thank you. ) does cover the maths very well. Completely maybe.

BUT... Is a perfect example of why I am now hiding behind the couch whimpering like a frightened child.,:eh: :lol: and is not in any way helpful.

Jeff Root
2010-Aug-27, 05:59 AM
Tensor, Mark, and all,

I started the recent references to gravity arising from mass rather than
energy and pressure in post 57. That was a deliberate simplification,
knowing that in all cases we have actually observed so far, mass is
essentially the only factor. The difference between mass and all forms
of energy plus pressure in those cases being practically insignificant,
and the complications (to me, at least) being monumental.

Which doesn't mean that I don't approve of your nitpicking. I do! :-)

-- Jeff, in Minneapolis

kevin1981
2010-Aug-28, 06:11 PM
Mathematically, I don't know of anything easier than here (http://people.hofstra.edu/Stefan_waner/diff_geom/Sec14.html).
Personally, I think that is where natural born ability comes into play. You can't learn mathematics like that unless you are gifted in the first place.

The way i look at gravity now is this. It is a mathematical concept to understand why galaxy's, stars, planets and other bodies behave like they do. Physically how gravity and GR work, we don't know. Gravity/GR are mathematical models to describe observations, they are not physical models. Am i in the right ball park here ?

Thanks

Tensor
2010-Aug-28, 06:39 PM
Personally, I think that is where natural born ability comes into play. You can't learn mathematics like that unless you are gifted in the first place.

While being gifted may help, if you're willing to put in a lot of work, you don't have to be particularly gifted. The big question is how badly do you want to understand.


The way i look at gravity now is this. It is a mathematical concept to understand why galaxy's, stars, planets and other bodies behave like they do. Physically how gravity and GR work, we don't know. Gravity/GR are mathematical models to describe observations, they are not physical models. Am i in the right ball park here ?

Thanks

I would say that's a pretty good assessment.

Strange
2010-Aug-28, 11:59 PM
The way i look at gravity now is this. It is a mathematical concept to understand why galaxy's, stars, planets and other bodies behave like they do. Physically how gravity and GR work, we don't know. Gravity/GR are mathematical models to describe observations, they are not physical models. Am i in the right ball park here ?

I'm not sure that is significantly different from any other theory in physics though.

astromark
2010-Aug-29, 05:51 AM
The very presence of mass. Distorts space. Any mass entering a distorted space will appear to fall towards that mass.

We have wrongly thought ( me :eh: )... its a force. Now I know its not and all seems expectable and good science.

But I still want to know by what means that happens. Am I missing something... I might not be able to do or understand that

mathematics commutation I saw back there under the 'here'... and have just crawled out from hiding. So be gentle.

Jeff Root
2010-Aug-29, 08:50 AM
Mark,

I'm going to sort of go back to a very basic level of your question and
say that whether gravity is a force or not depends as much on what you
mean by "force" as it does on the nature of gravity.

Now, having said that, I will explain that I said it not for you, but mainly
for people who don't know as much about gravity as the participants in
this thread, who read the assertion that gravity is not a force and react
by saying, that's nonsense-- of course gravity is a force. And they are
right that gravity is a force, because of how they think of force, which, in
connection with gravity, depends in turn on how they think of acceleration,
which depends in turn on how they think of relative motion, which depends
in turn on whether they are just interested in the local effects of gravity, or
are trying to get a Big Picture of how gravity works in a universe that can
be described by a universal theory such as general relativity. For almost
all purposes the local effects are enough, and gravity can be considered a
force. But if you are trying to understand what gravity actually *is*, you
have to look beyond local effects and consider that Big Picture, in which
case you can usefully consider gravity to be something that is not a force.

-- Jeff, in Minneapolis

astromark
2010-Aug-29, 09:04 AM
Yes Jeff thats good... and you know its the * is * that bothers me... :eh:

Strange
2010-Aug-29, 11:23 AM
Yes Jeff thats good... and you know its the * is * that bothers me... :eh:

But why does the "is" of gravity bother you more than, say, electricity? Or light, or time, or space, or love, or beauty, or ...

cosmocrazy
2010-Aug-29, 11:58 AM
But why does the "is" of gravity bother you more than, say, electricity? Or light, or time, or space, or love, or beauty, or ...

Thats a fair point and is something I raised sort of in my thread on reality in this section of the forum. I guess if Mark wants to understand what gravity "is" then it will also be required to understand what space "is" since in GR its the space thats warped which effects the path which matter and energy travel along, thus an effect we call gravity.

Tensor
2010-Aug-29, 01:35 PM
Yes Jeff thats good... and you know its the * is * that bothers me... :eh:

But, and this is the key to it, we don't have an explanation for the *is*. Any more than we have an explanation, as Strange points out, fundamentally for the *is* of charge, or the *is* of color charge, or the *is* of the weak force. All of those *is*es are mathematical models. With the word explanations a imprecise explanation of what the math is saying. For instance, what causes an what we call an electron to emit what we call a photon and how does it do it? We don't know. We can describe the process mathematically, explain it somewhat with words, but the actual process of the when and how of the emission, is one of those *is* things.

Tensor
2010-Aug-29, 01:50 PM
Thats a fair point and is something I raised sort of in my thread on reality in this section of the forum. I guess if Mark wants to understand what gravity "is" then it will also be required to understand what space "is" since in GR its the space thats warped which effects the path which matter and energy travel along, thus an effect we call gravity.

Which is why that "here" page isn't even the whole story. When working with the math of the model called GR, space is represented by an abstract four dimensional object called a manifold. The "here" page assumes that you know about manifolds. There are various requirements for that manifold to be used in GR, the biggest is that it is smooth, or continuous. Or, in math terms, it has to be differentiable over volume you want to investigate. Use a different model, you get a different representation of space. Quantum Loop Gravity (QLG) uses a non-smooth manifold or a lattice work of points to model the manifold. So, which is real space? Probably neither. Both are simply mathematical models that help us make quantifiable predictions, and thus useful as a tool to explain what we see around us.

kevin1981
2010-Aug-29, 03:40 PM
I'm not sure that is significantly different from any other theory in physics though.

I have'nt read the posts below 81 as i wanted to answer this before reading anymore. But if you push a marble and it moves across a table, then you can describe why the marble is moving by saying you 'physically' pushed it. Where with the warping of space, space warping, bending are analogies. Space does not warp or bend 'physically' as it is not a physical entity. Space is exactly that, space.

So we describe general relativity using mathematical models, but how it actually works 'physically', we do not know. At least, this is my understanding.

Strange
2010-Aug-29, 04:42 PM
I have'nt read the posts below 81 as i wanted to answer this before reading anymore. But if you push a marble and it moves across a table, then you can describe why the marble is moving by saying you 'physically' pushed it.

But as soon as you want to explain how we can push a marble when both the finger and marble consist almost entirely of empty space, you are in the realm of quantum theory and no better off...

kevin1981
2010-Aug-29, 05:11 PM
But as soon as you want to explain how we can push a marble when both the finger and marble consist almost entirely of empty space, you are in the realm of quantum theory and no better off...



To my understanding, even though atoms are mostly empty space, we still have a 'physical' theory of how this happens, which is backed up by the mathematics.

The atoms have electrical charges that repel each other, so even though there is a lot of space, the electric fields occupy it. As you know i am sure, this is why my hand will not go through a wall which is fundamentally empty space.

All i am saying is, we know how GR works mathematically but not physically.

I am talking classically here. This has nothing to do with quantum gravity, yet naturally it leads to it.

astromark
2010-Aug-29, 07:34 PM
So... my original point, 'Understanding Gravity.' Has been well answered and, thank you all.

By careful mathematical modeling of the observed facts we have arrived at the same place I started from...

We have moved forward. I can now say I have a understanding of what gravity is...

I do not know by what mechanism it transpires... and it would seem we are all on the same page.

caveman1917
2010-Aug-29, 10:05 PM
To my understanding, even though atoms are mostly empty space, we still have a 'physical' theory of how this happens, which is backed up by the mathematics.

The atoms have electrical charges that repel each other, so even though there is a lot of space, the electric fields occupy it. As you know i am sure, this is why my hand will not go through a wall which is fundamentally empty space.

But what *is* an electric field? Or an electrical charge for that matter?

Tensor
2010-Aug-29, 10:21 PM
To my understanding, even though atoms are mostly empty space, we still have a 'physical' theory of how this happens, which is backed up by the mathematics.

Which is exactly the same situation as GR. We have a kinda sorta physical description of all forces or things that can be considered forces, which is backed up by the mathematics.


The atoms have electrical charges that repel each other,

How do they repel each other? Theory says that they exchange virtual photons. How does the exchange of virtual photons cause charges to repel in some situations and attract in others? Physcially, what is the process? For that matter, how do they know how and when to exchange those virtual photons? What is the physical process?


so even though there is a lot of space, the electric fields occupy it.

Physically, what is an electric field?


As you know i am sure, this is why my hand will not go through a wall which is fundamentally empty space.

Well, we are sure, but we don't know, physically, what the electric field is, do we (unless, of course, you have an explanation for the last question)? So how do we "know" physically, in the theory that our hand will not go through? Physically, describe the process.


All i am saying is, we know how GR works mathematically but not physically.

Which is exactly the same as the EM, weak (or electro-weak if you prefer) and color.


I am talking classically here. This has nothing to do with quantum gravity, yet naturally it leads to it.

Even classically, we don't have good physical explanation for the EM force. (color and weak fields were never classical as they weren't really "discovered" until quantum theory had taken over. And, as far as quantum field theory goes, no one, has any idea, as to exactly what the wave amplitudes represent, physically. There are lots of ideas philosophically, however.

kevin1981
2010-Aug-29, 11:23 PM
How do they repel each other? Theory says that they exchange virtual photons. How does the exchange of virtual photons cause charges to repel in some situations and attract in others? Physcially, what is the process? For that matter, how do they know how and when to exchange those virtual photons? What is the physical process?

I do not know the answers to those questions. I did'nt realize virtual photons had anything to do with atoms repelling each other.


So how do we "know" physically, in the theory that our hand will not go through? Physically, describe the process.

I have no worries in admitting i am out of my depth here. If you don't ask questions then you don't get answers. I am sure you understand. Though i really thought it was a simple case of certain atoms or molecules repelling each other. But are you saying that mathematically, this can be answered. But what actually happens 'physically', we do not know?


Even classically, we don't have good physical explanation for the EM force. (color and weak fields were never classical as they weren't really "discovered" until quantum theory had taken over. And, as far as quantum field theory goes, no one, has any idea, as to exactly what the wave amplitudes represent, physically. There are lots of ideas philosophically, however.

This has surprised me a little. And it has also caught me out. I thought we had a good understanding of EM and QFT. But as i understand it, are you saying that mathematically we have very good models of how these concepts work, and they make excellent predictions in which the results match. But 'physically', like GR, we do not know 'how' Electromagnetism does what it does?

Also, i have a quote from Ken, which makes a lot of sense to me. It may even be the answer to these questions we are talking about.

I would like to borrow Ken's brains for a while so i could understand these deeply fundamental questions :D


These questions do not have answers, science just makes up these theories, without any underlying mechanisms. If a theory has an underlying mechanism, then it is not a fundamental theory-- the underlying mechanism will be the fundamental theory that does not have an underlying mechanism.

Jeff Root
2010-Aug-29, 11:39 PM
I am out of my depth, too. It is good for people who are out of their
depth to ask questions, and to speculate.

Mr. A and Mr. B both agree on exactly what information is in your head.
But Mr. A says that you understand the physics, while Mr. B says that you
do not understand the physics. Whether you understand the physics or
not depends on what it means to understand something, and there is
quite a bit of disagreement about that.

I tend to agree with Mr. A, but I am completely unable to refute any of
Mr. B's arguments, so I have to go along with him, too.

I think that the beginning of an answer to the question of *when*
electric charges exchange virtual photons is that they exchange them
constantly. But why do they exchange them at the particular rate
they do, and not some other rate? Good question...

-- Jeff, in Minneapolis

caveman1917
2010-Aug-29, 11:45 PM
But if a mechanism is explained by adding a deeper mechanism, and science only does mechanisms, how could one get out of the problem of always having an 'unexplained most basic' mechanism?

Ken G
2010-Aug-29, 11:48 PM
But as soon as you want to explain how we can push a marble when both the finger and marble consist almost entirely of empty space, you are in the realm of quantum theory and no better off...Exactly. We must avoid the temptation to confuse familiarity with understanding. We are all familiar with "pushing marbles", and we are much less familiar with "bending spacetime." Does that mean we understand the former and not the latter? I would say it just means the first is closer to our direct experience, so we can communicate around it using less sophisticated language than the second. Using less sophisticated language means that more people can connect their own familiarities with what we are saying, but is that closer to understanding? It seems rather the opposite-- less sophisticated language should be farther from understanding, not closer, because deeper understanding is more sophisticated understanding.

For example, we say we "push a marble." Our hand comprises of atoms and fields (in our models of a hand), and so does the marble. What made the hand move? Brain impulses. Do we understand how those got generated, how we decided to push the marble? Hardly. Then when our hand "made contact" with the marble, did anything really "touch"? No, the fields became interconnected, and what came to pass was a complex combination of electromagnetic interactions, spin-orbit and spin-spin interactions, and exchange energies involving the identical electrons in our hand and in the marble. And that's just how we think about what happened there, what actually happened is anyone's guess. How could people 500 years ago "push a marble", or play tiddlywinks, just as well as we can, all before Newton's concepts of forces, or van der Waal's investigation of molecular forces, or Heisenberg's uncertainty principle, or Pauli's exclusion principle? They had no idea what "pushing a marble" was, in comparison to how we think about it today, and we may have no idea what pushing a marble is compared to how humans 500 years from now will think about it.

So to what sense do we "understand" what pushing a marble is? Only insofar as we can do it, and the models we use to do it, in our daily lives, don't involve any of that-- essentially no physics at all. We know this because children do it-- their models are all they need, thank you, yet are very far from "understanding." Why would we imagine science is somehow different, rather than just more sophisticated?

Ken G
2010-Aug-29, 11:57 PM
But if a mechanism is explained by adding a deeper mechanism, and science only does mechanisms, how could one get out of the problem of always having an 'unexplained most basic' mechanism?By not asking to get out of that problem-- it isn't a problem. I think the real problem is that there is a kind of lie told about science-- it is sometimes claimed that the goal of science is to demystify the universe. But science has never done that, has never tried to do that, and is not about to start now. Instead, the goal of science has always been to replace a wide array of seemingly unconnected and rather superficial mysteries with a set of more unified and much more profound ones. Our success is in getting deeper before we have to say "I have no idea", not in avoiding the phrase.

kevin1981
2010-Aug-30, 12:32 AM
Exactly. We must avoid the temptation to confuse familiarity with understanding. We are all familiar with "pushing marbles", and we are much less familiar with "bending spacetime." Does that mean we understand the former and not the latter? I would say it just means the first is closer to our direct experience, so we can communicate around it using less sophisticated language than the second. Using less sophisticated language means that more people can connect their own familiarities with what we are saying, but is that closer to understanding? It seems rather the opposite-- less sophisticated language should be farther from understanding, not closer, because deeper understanding is more sophisticated understanding.

Thanks Ken, i understand what you are saying. If we talked about GR all day and got brought up on it then the language involved would'nt sound so sophisticated as we would be used to it.



For example, we say we "push a marble." Our hand comprises of atoms and fields (in our models of a hand), and so does the marble. What made the hand move? Brain impulses. Do we understand how those got generated, how we decided to push the marble? Hardly. Then when our hand "made contact" with the marble, did anything really "touch"? No, the fields became interconnected, and what came to pass was a complex combination of electromagnetic interactions, spin-orbit and spin-spin interactions, and exchange energies involving the identical electrons in our hand and in the marble. And that's just how we think about what happened there, what actually happened is anyone's guess.

Everything comes down to quantum physics it seems! It is everywhere and a lot of people do not even realize it, i find all this stuff mind blowing. The bold is because again, we come to observer dependence. What we perceive are human made constructs.

I have a question Ken, but would like to pm you if that is ok, as it is completely off topic from the OP.

Ken G
2010-Aug-30, 02:13 AM
Thanks Ken, i understand what you are saying. If we talked about GR all day and got brought up on it then the language involved would'nt sound so sophisticated as we would be used to it. Right, Kip Thorne and Steven Hawking might have GR conversations that sound profound and crazy and unintelligible to us, but to them are like deciding what restaurant to go to.

Everything comes down to quantum physics it seems! Perhaps even the process we use to think about everything, and the circularity of that is I'm sure not lost on you.

I have a question Ken, but would like to pm you if that is ok, as it is completely off topic from the OP.I don't think that matters, you should ask here and let everyone have a reaction.

Tensor
2010-Aug-30, 03:28 AM
I do not know the answers to those questions. I did'nt realize virtual photons had anything to do with atoms repelling each other.

I have no worries in admitting i am out of my depth here. If you don't ask questions then you don't get answers. I am sure you understand.

Kevin, my apologies, I had no intention of putting you on the spot. Yes, you do need to ask questions if you don't know. I was just trying to get across to you that you seemed to think that there were differences about what was known about how GR and the Quantum Field Theories (QFT) worked at the fundamental level. There aren't. Within both GR and the QFT, there are things that we simply don't know, physically, but we can use math to make predictions.


Though i really thought it was a simple case of certain atoms or molecules repelling each other. But are you saying that mathematically, this can be answered. But what actually happens 'physically', we do not know?

That is exactly what I am saying. We really don't know what exactly photons are, or for that matter, electrons or protons. Or, even if the idea of a proton makes sense in light of the idea of quarks. All of those things (along with all of the other particles) are abstract mathematical ideas that allow us to make rather accurate predictions. As Ken points out, the fact that we have more familiarity with the concepts of EM (which can be carried over to the weak and color forces) than the concepts of gravity makes us believe we have a better understanding of what the underlying processes are.


This has surprised me a little. And it has also caught me out. I thought we had a good understanding of EM and QFT. But as i understand it, are you saying that mathematically we have very good models of how these concepts work, and they make excellent predictions in which the results match. But 'physically', like GR, we do not know 'how' Electromagnetism does what it does?

Again, bingo. Once you fully grasp this, you really don't look at science the same way ever again. Will we ever be able to understand or have a model that explains the actual fundamental physical process? Who knows, but the current fundamental EM process includes those silly wave amplitudes, that has no physical meaning.


Also, i have a quote from Ken, which makes a lot of sense to me. It may even be the answer to these questions we are talking about.

That's a good quote. Although I would ask for a definition of not having a underlying mechanism (or what is fundamental. For my part, the not having an underlying mechanism (or being fundamental) would be where we can argue the why or possibly the philosophical reason for the mechanism. Like the wave amplitudes in QFT. We have no explanation as to what the amplitude represents physically. So we can argue the why of the amplitude, or what it represents philosophical (see the various "meanings" of QFT for instance). But the fact that we can explain it, physically, makes it, to me, fundamental.


I would like to borrow Ken's brains for a while so i could understand these deeply fundamental questions :D

That would be a good choice if you want understanding.

Tensor
2010-Aug-30, 03:40 AM
Mr. A and Mr. B both agree on exactly what information is in your head.
But Mr. A says that you understand the physics, while Mr. B says that you
do not understand the physics. Whether you understand the physics or
not depends on what it means to understand something, and there is
quite a bit of disagreement about that.

I tend to find that what most people here (even me, depending on the subject) think they mean by understanding the physics is that they understand the analogies, metaphors, and word pictures used to describe the mathematics. Not the actual mathematical models themselves. To me, understanding the physics means understanding the mathematical models. Of course, others, as you point out, may have other opinions.


I tend to agree with Mr. A, but I am completely unable to refute any of
Mr. B's arguments, so I have to go along with him, too.

I have the same trouble, again, on a lot of subjects. It's a problem we all have, unless we happen to be one of those lucky people who understand everything. Otherwise, our lack of knowledge will cause us to be unable to refute those that know more than us. And that includes not being able to refute either one of those that have opposing opinions on a subject.


I think that the beginning of an answer to the question of *when*
electric charges exchange virtual photons is that they exchange them
constantly. But why do they exchange them at the particular rate
they do, and not some other rate? Good question...

This whole section is a much better paragraph than I had and that last sentence is actually a much better question that the one I asked, thanks Jeff.

Ken G
2010-Aug-30, 03:57 AM
That's a good quote. Although I would ask for a definition of not having a underlying mechanism (or what is fundamental.I just mean that a "fundamental" theory is one that has no deeper theory that explains it-- so that can be a moving target when new theories come along. Newton's laws may be viewed as fundamental because they rely on a fundamental principle, the principle of "least action" (the right relativity has to be used of course, but that doesn't make it any less fundamental). Even so, they also rely on a description of space and time (even if relativistic spacetime is used) that has no explanation, no deeper theory says why they are used that way (some work has looked for thermodynamic explanations, so perhaps someday those will be viewed as more fundamental, but they seem pretty ad hoc). What's more, quantum mechanics uses space and time differently, so we see that "fundamental" does not imply "true".


For my part, the not having an underlying mechanism (or being fundamental) would be where we can argue the why or possibly the philosophical reason for the mechanism. Like the wave amplitudes in QFT. We have no explanation as to what the amplitude represents physically. So we can argue the why of the amplitude, or what it represents philosophical (see the various "meanings" of QFT for instance). But the fact that we can explain it, physically, makes it, to me, fundamental.I think you are saying that what is fundamental to a theory is the testable predictions it makes, not the path it takes to arrive at those predictions when it's not clear if other paths could get to the same place. If so, I'd agree-- for example, I would count quantum field theory as a fundamental theory because its predictions cannot be arrived at via some different approach in a deeper theory. It relates to the correspondence principle-- a "fundamental theory" must be able to predict everything that a less fundamental theory can, while admitting no other theory that can make all of its predictions. I would not count the wave amplitudes themselves as fundamental, because they are just stepping stones used to guide the calculation of the matrix elements-- other guides can be used, to get the same results, like Heisenberg operator formalism where you let the time dependence be carried in the operators rather than in the amplitudes. The matrix elements are the "fundamental" objects, because they are the predictions that can be tested, and there's currently no other theory (AFAIK) that can make those predictions except by calculating those matrix elements.

All I was saying is that using such a definition of "fundamental" already guarantees that we will never understand the whys of our most fundamental theories, and that we shouldn't bemoan that-- this is how science is supposed to work. I think we agree there.

Ken G
2010-Aug-30, 04:14 AM
I tend to find that what most people here (even me, depending on the subject) think they mean by understanding the physics is that they understand the analogies, metaphors, and word pictures used to describe the mathematics. Not the actual mathematical models themselves. To me, understanding the physics means understanding the mathematical models. Which presumably connects with being able to obtain quantitative testable results, i.e., able to "solve the problems on the exam." I agree that this is surely what "understand the physics" means within physics itself, but of course this forum goes well beyond practicing physicists, so perhaps there is value in recognizing two "levels" of understanding the physics-- understanding like a physicist, and understanding like a non-physicist who can nevertheless correctly use the language if not solve the problems ("talk the talk" but not "walk the walk"). The same really holds within the various sub-fields of physics-- we don't have a practitioner in each subfield, so often we have to satisfy ourselves with the second type of "understanding."


This whole section is a much better paragraph than I had and that last sentence is actually a much better question that the one I asked, thanks Jeff.
Indeed, I think the metaphysical problem with virtual particles goes deeper than just not knowing why they get emitted and at what rate-- it is that their very nature is self-contradictory when placed in any model of what is "actually happening". The very term "virtual" appearing in an ontology exposes the problem: we are forced to adopt an ontology that refutes its own ontological status! That's a horrendous state of affairs, that we have simply "learned to live with."

Another approach might be to just "fess up" and stop using language like "the charges exchange virtual particles that obey a different set of laws than real particles", and say instead something more like "we can tell that this cannot be what reality is actually doing, but to get the right answer, a convenient way we can think of is to imagine particles that have an ill-defined ontological status." The reason we know reality is not actually doing that is because if it was actually doing that, the particles would have to be actual rather than "virtual," but the only alternative is to reject ontological language entirely, which is a bit of a shame but might arguably be called for.

In other words, when people draw Feynman diagrams, they can be thought of non-ontologically as a kind of mnemonic for getting the terms right in a calculation, or they can be thought of ontologically as various processes that reality is actually in some sense going through to get the final amplitude. Or do it with two slits-- either say that the photon "goes through both slits", which is an ontological statement, or just say that the calculation of the amplitude accepts contributions from both slits and we make no claims on what the photon is doing that are not experimentally justifiable, which is a non-ontological approach to the same language. In the latter view, "virtual photons" are an overly ontological description of terms in a mathematical calculation.

Strange
2010-Aug-30, 08:29 AM
Exactly. We must avoid the temptation to confuse familiarity with understanding. We are all familiar with "pushing marbles", and we are much less familiar with "bending spacetime." Does that mean we understand the former and not the latter? I would say it just means the first is closer to our direct experience, so we can communicate around it using less sophisticated language than the second. Using less sophisticated language means that more people can connect their own familiarities with what we are saying, but is that closer to understanding? It seems rather the opposite-- less sophisticated language should be farther from understanding, not closer, because deeper understanding is more sophisticated understanding.

Exactly. Also, when you dive down a level from "pushing marbles" to invoke electromagnetic forces, we are still to some extent in the land of the familiar: we all grow up with electrostatic charges, magnetism, electricity, etc and these things seem less mysterious, more tangible. But when you you think about what they *really* are, they are concepts just as evanescent as spacetime.

kevin1981
2010-Aug-30, 01:52 PM
Kevin, my apologies, I had no intention of putting you on the spot.

It's no problem at all. I just thought it would be better to put my hands up and admit i do not know. I am here to learn because i enjoy it and want an understanding of why nature is the way it is. If i do not know something, then theres no shame in that.


Yes, you do need to ask questions if you don't know. I was just trying to get across to you that you seemed to think that there were differences about what was known about how GR and the Quantum Field Theories (QFT) worked at the fundamental level. There aren't. Within both GR and the QFT, there are things that we simply don't know, physically, but we can use math to make predictions.

I did not know this, i just presumed that we knew how atoms repel and why EM does what it does because we have such good theory's. I am a little surprised to find out that fundamentally, we don't really know. I am only surprised because i thought these things were known.

Because i have read that QED is such a great theory with very accurate results, i presumed we knew everything about electricity, atoms repelling and other phenomena.

But now my understanding is these theories are great at predicting results. But how the results 'become' the results is still a mystery. The quest continues..


That is exactly what I am saying. We really don't know what exactly photons are, or for that matter, electrons or protons. Or, even if the idea of a proton makes sense in light of the idea of quarks. All of those things (along with all of the other particles) are abstract mathematical ideas that allow us to make rather accurate predictions. As Ken points out, the fact that we have more familiarity with the concepts of EM (which can be carried over to the weak and color forces) than the concepts of gravity makes us believe we have a better understanding of what the underlying processes are.

So at the deepest fundamental level, we do not know how EM and other quantum phenomena work, but we do have theories that predict great results, mathematically. And the same can be said for General Relativity.




Again, bingo. Once you fully grasp this, you really don't look at science the same way ever again. Will we ever be able to understand or have a model that explains the actual fundamental physical process? Who knows, but the current fundamental EM process includes those silly wave amplitudes, that has no physical meaning.

Great answers, thanks. I have been reading about physics for a few years now. But just bits and bobs on the internet and youtube. But joining this forum has been a real eye opener for me. Especially the threads about the nature of reality, and of course, quantum physics.

It seems the more we dive down and ask these deep questions, the more we realize that the deepest questions can not be answered. They turn into philosophical debates. Also the nature of what reality is and what is going on shocks me.

The nice solid world around us, is one big illusion. I am not saying we imagine it, but it is based on particles of 'something' we call atoms that when isolated in a closed system are everywhere at once!!

That is just one example, but what i am getting at is, things are not really as they appear to be.

This stuff is just completely baffling, yet it is what it is..

astromark
2010-Aug-30, 06:54 PM
:eh:... Yes, quantum whatever, and even general relativity. Electromagnetic repulsion or attraction.

Atomic structure and the strong and weak forces. All have been subjects of great discussion and of gravity.

Gravity. That it has been labeled a weak force is a bit of a myth a ?

cosmocrazy
2010-Aug-30, 07:04 PM
:eh:... Yes, quantum whatever, and even general relativity. Electromagnetic repulsion or attraction.

Atomic structure and the strong and weak forces. All have been subjects of great discussion and of gravity.

Gravity. That it has been labeled a weak force is a bit of a myth a ?

Yes a weak force at quantum level, but as you well know in certain circumstance gravity becomes the dominating force, i.e Black holes

caveman1917
2010-Aug-30, 09:18 PM
By not asking to get out of that problem-- it isn't a problem.

I agree completely, and that was my point. My question was rhetorical :)

Ken G
2010-Aug-30, 10:25 PM
Ah, I see, a leading question!

Strange
2010-Aug-30, 11:11 PM
That is just one example, but what i am getting at is, things are not really as they appear to be.

You've got it!


This stuff is just completely baffling, yet it is what it is..

Never a truer word...

Jens
2010-Aug-31, 04:41 AM
So at the deepest fundamental level, we do not know how EM and other quantum phenomena work, but we do have theories that predict great results, mathematically. And the same can be said for General Relativity.


I just happen to be helping with the translation of a book that is about the history of magnetism. And this precise issue comes up. If I am correct, Descartes believed that it was important to understand the mechanism of why things happen. But then along came Newton, and he made the important contribution that we only need to calculate how they act, not the mechanism. So his theory of gravity basically ignored the question of "why" things attract each other but merely to find out the equation that governs the movements. So in effect, Newton, perhaps as a result of being an occultist, accepted gravity as an occult force. And the world has essentially moved to the Newtonian position, whereas some of us (myself included) still have the Cartesian streak and want to understand the mechanics of it all.

Ken G
2010-Aug-31, 06:06 AM
And the world has essentially moved to the Newtonian position, whereas some of us (myself included) still have the Cartesian streak and want to understand the mechanics of it all.I would say that Descartes was being a bit naive if he really thought that a mechanism can be explained. If I asked you, what is the mechanism of a mousetrap, you would describe springs and catches. But then I'd say, you haven't explained the mechanism, you've just replaced one mechanism I don't understand at all (a mousetrap) with another I understand only dimly (a spring). All you could ever do is connect mechanisms I am not familiar with with those I am-- but how can I really understand any mechanism? Am I not merely mistaking familiarity with the function of a spring for understanding the mechanism of a spring? It would always have to be "mechanisms all the way down". The only alternative is to allow that any familiarity counts as understanding-- so Newton can simply say he is familiar with the force of gravity, and claim that suffices as understanding of its mechanism.

Or perhaps, I am not giving Descartes enough credit, and all he was really objecting to was the action at a distance concept, moreso than requiring that we should "understand the mechanism." That's another kettle of fish-- Newton himself said that he found it completely preposterous that action at a distance would really be possible. He always expected that some deeper theory could explain gravity without invoking action at a distance, he just didn't know how to create such a theory. So we must be clear on just what was "the Newtonian position" that the world has adopted-- we have accepted that the "fundamental mechanism" of any theory will be unexplained, but we have not accepted action at a distance as a fundamental mechanism.

cosmocrazy
2010-Sep-01, 03:55 PM
I would say that Descartes was being a bit naive if he really thought that a mechanism can be explained. If I asked you, what is the mechanism of a mousetrap, you would describe springs and catches. But then I'd say, you haven't explained the mechanism, you've just replaced one mechanism I don't understand at all (a mousetrap) with another I understand only dimly (a spring). All you could ever do is connect mechanisms I am not familiar with with those I am-- but how can I really understand any mechanism? Am I not merely mistaking familiarity with the function of a spring for understanding the mechanism of a spring? It would always have to be "mechanisms all the way down". The only alternative is to allow that any familiarity counts as understanding-- so Newton can simply say he is familiar with the force of gravity, and claim that suffices as understanding of its mechanism.

Or perhaps, I am not giving Descartes enough credit, and all he was really objecting to was the action at a distance concept, more so than requiring that we should "understand the mechanism." That's another kettle of fish-- Newton himself said that he found it completely preposterous that action at a distance would really be possible. He always expected that some deeper theory could explain gravity without invoking action at a distance, he just didn't know how to create such a theory. So we must be clear on just what was "the Newtonian position" that the world has adopted-- we have accepted that the "fundamental mechanism" of any theory will be unexplained, but we have not accepted action at a distance as a fundamental mechanism.

Do we have to accept that the "fundamental mechanism" can not be explained? Or is is just a lack of ability to understand this mechanism, put it into a language that is logic? Can it be possible that the universe operates in such a way that there is no need for a fundamental mechanism for gravity and the like, certainly in the classical way we understand?

Ken G
2010-Sep-01, 04:05 PM
Do we have to accept that the "fundamental mechanism" can not be explained?Yes, but only by definition-- if we explain it, it isn't fundamental any more, because whatever we are using to explain it will supplant it as the fundamental mechanism. In other words, "fundamental" is a fundamentally moving target.

Can it be possible that the universe operates in such a way that there is no need for a fundamental mechanism for gravity and the like, certainly in the classical way we understand?I would say that is certainly possible. What is less clear is if there is any other possibility!

Jeff Root
2010-Sep-01, 06:56 PM
I absolutely will not claim to understand the above conversation ... but it
appears that we have three possibilities: Either that understanding anything
is physically impossible, or that we define it in such a way as to be logically
impossible, or that we don't know what we are talking about when we talk
about "understanding". In any case, it doesn't say much about physics,
only about what it means to understand.

-- Jeff, in Minneapolis

Ken G
2010-Sep-01, 07:38 PM
In any case, it doesn't say much about physics,
only about what it means to understand.
What about what it says about understanding physics? (Re: the thread title.)

Jeff Root
2010-Sep-01, 08:14 PM
Understanding physics being just a specific instance of understanding,
not distinguished from any other area of understanding.

-- Jeff, in Minneapolis

Ken G
2010-Sep-01, 09:19 PM
Understanding physics being just a specific instance of understanding,
not distinguished from any other area of understanding.
Interesting opinion. Others may think that understanding of physics is a particular type of understanding-- that "understanding" is not a generic animal, but one that is tightly connected to the subject matter actually being understood. If it means something different to understand gravity than to understand art, for example, then exploring that difference has been the purpose of the thread. As usual for this forum, such a process always seem to require justification.

astromark
2010-Sep-02, 08:24 AM
I absolutely will not claim to understand the above conversation ... but it
appears that we have three possibilities: Either that understanding anything
is physically impossible, or that we define it in such a way as to be logically
impossible, or that we don't know what we are talking about when we talk
about "understanding". In any case, it doesn't say much about physics,
only about what it means to understand.

-- Jeff, in Minneapolis

Thank you Jeff... Once again you have identified that we quickly descend into explanations of grammar and word usage and yes UNDERSTANDING.

I am not interested in justifying my meaning... You all understand what 'understanding' in this context means...

That is evident by the fact that to the best of your abilities... my question has been answered... or it can not be...

Which ever is the case of your view and mine. Thankyou.

cosmocrazy
2010-Sep-02, 11:16 AM
Thank you Jeff... Once again you have identified that we quickly descend into explanations of grammar and word usage and yes UNDERSTANDING.

I am not interested in justifying my meaning... You all understand what 'understanding' in this context means...

That is evident by the fact that to the best of your abilities... my question has been answered... or it can not be...

Which ever is the case of your view and mine. Thank you.

I think you asked the question knowing that it probably could not be answered with any useful understanding other than we just don't know what gravity is fundamentally. We can measure its effect and predict the results with great accuracy but how, why and what really causes it is yet unknown. The discussion has naturally developed into trying to understand what the foundations & fundamentals of gravity are and how we can interpret the logic and language to explain such, there is now nowhere else to go with the thread.

Ken G
2010-Sep-02, 04:47 PM
The way I would summarize it is, in realizing what we can and cannot "understand" about gravity, we have discovered what it actually means to understand any topic in physics. In other words, rather than concluding "we cannot understand", it makes more sense to reappraise what understanding of physics is, in such a way that it becomes something we can actually do.

swampyankee
2010-Sep-05, 01:05 PM
Einstein did not call gravity spooky action at a distance. He called quantum entanglement that.

There are at least two theories on gravity. One, relativity, says that it's curved space. The other, quantum mechanics, doesn't have curved space, and needs intermediary force particles such as photons for any one particle of matter to affect another, so gravity has to be the action of particles called "gravitons". They're incompatible; both can't be right, so at least one has to be wrong. The problem is that, according to every test we can do so far, they're both right. Some parts of our daily lives even depend on their being right. So, until somebody is able to run a test in which the two theories give contradictory answers about what the results should be, and finds out which one gave the wrong prediction, and finds another way to explain the predictions that that "wrong" theory has always made accurately before, both theories will remain viable. So it will remain equally valid to describe gravity in either of the two mutually contradictory ways.

With that explanation -- GR vs QM -- I now understand why gravity is included as one of the four forces. The two main branches of physics, working top-down (GR) and bottom-up (QM) still don't meet somewhere in the middle and it's not a "force" in one of them :-(.

Tensor
2010-Sep-05, 01:26 PM
With that explanation -- GR vs QM -- I now understand why gravity is included as one of the four forces. The two main branches of physics, working top-down (GR) and bottom-up (QM) still don't meet somewhere in the middle and it's not a "force" in one of them :-(.

Actually, that explanation is not quite right. There are currently two main ways that are being worked on to quantify gravity. One, as noted by Delvo, is to try to define gravity as a force and to quantify the force carrier of gravity. The current favorite among this attempt is Super String Theory.

The other way, is to continue to try to use GR, but to modify the equations to account for spacetime being quantified. This approach is called Loop Quantum Gravity. Interestingly, the only place where the two of them can make a solid prediction, they both agree with GR and each other, so there is currently no way to actually distinguish between each of them or GR. The place where they all make the same prediction has to do with black hole thermodynamics.

kevin1981
2010-Sep-05, 01:46 PM
Why do we call gravity a fundamental 'force' if it is not a force, fundamentally. Where there is energy and pressure, this energy and pressure 'curves' space/time. There is no force there.

Also i thought there were about 20 odd fundamental constants of nature- Light speed being one for example.

So what is the difference between the fundamental forces and the fundamental constants ?

Tensor
2010-Sep-05, 02:58 PM
Why do we call gravity a fundamental 'force' if it is not a force, fundamentally.

Because in Newtonian Physics, gravity is treated as a force (F = ma). GR treats gravity as geometry, although movement of particles or planets will match Newtonian predictions at the low speed, low mass limit. (they have to, that is what's observed)


Where there is energy and pressure, this energy and pressure 'curves' space/time. There is no force there.

Of course not. But most people don't experience geometry, they experience forces (or in the case of GR, what are known as pseudo-forces because they depend on your coordinate choice).



Also i thought there were about 20 odd fundamental constants of nature- Light speed being one for example.

So what is the difference between the fundamental forces and the fundamental constants ?

A force can be thought of a the interaction between two bodies, whether those bodies are large or not doesn't matter. Mathematically, the best way I can think of the difference is that a force is a vector and almost all fundamental constants are either dimensionless or scalars.

kevin1981
2010-Sep-05, 03:22 PM
Because in Newtonian Physics, gravity is treated as a force (F = ma). GR treats gravity as geometry, although movement of particles or planets will match Newtonian predictions at the low speed, low mass limit. (they have to, that is what's observed)

I see, GR is an extension of newtonian gravity and on super large scales we use the Einstein field equations. But in normal everyday experience, newtonian gravity and the mathematics of it can be used using the term 'force' and they are fundamental- Thus using the word 'force' in this context, is absolutely fine.

Is this correct ?

Ken G
2010-Sep-05, 05:45 PM
Yes, that's it.

peteshimmon
2010-Sep-05, 06:26 PM
I expect its a delay phenomenom whereby fundamental
energy particles are held up in a mass concentration
such that when they leave, the density around the
mass is verrry slightly different from the normal
space density due to expansion going on universally.
That makes a negative energy field perhaps.

What! proof? numbers? definitions? :)

Sound of running...

Ken G
2010-Sep-05, 06:48 PM
Do you write for Star Trek?

peteshimmon
2010-Sep-05, 06:58 PM
Noooo....this is reality Ken!

Well..puny, insipid grasps of reality.
We canna help it, we all think of these
things on a technical level.

xylophobe
2010-Sep-13, 04:22 PM
... Spacetime curvature is just that. ...

Wow, this is confusing since some describe gravity as a curvature of space while others describe it as a flowing inward towards mass.

If space is flowing inward like towards a blackhole then I only "see" radial motion and no curvature.

I sort of like the "flow" description because I can not see how a particle of mass can reach out with tentacles and pull us towards it whereas if space is flowing towards the particle then I am being pushed towards the mass.

Is gravity a push or a pull force?

btw: I'm only to the 2nd page so this is probably dropped right into the midst of some other heated conversation.

Ken G
2010-Sep-13, 04:37 PM
Wow, this is confusing since some describe gravity as a curvature of space while others describe it as a flowing inward towards mass.

If space is flowing inward like towards a blackhole then I only "see" radial motion and no curvature.The "flow of space" is just a picture that carries much of the meaning of spacetime curvature, though may not be enough to really "do the problems." There aren't two different ideas here, just different coordinates-- curved spacetime is a coordinate-free way to talk about it, it describes the geometry of the spacetime "manifold" onto which various different coordinates can be used. If you use a "falling space" coordinate, then it's a bit like using a Mercator projection to coordinatize the globe of the Earth onto a flat piece of paper-- it's a way to still use flat-type thinking locally, while letting the squeezing of the coordinates handle the global curvature for you automatically.


I sort of like the "flow" description because I can not see how a particle of mass can reach out with tentacles and pull us towards it whereas if space is flowing towards the particle then I am being pushed towards the mass.
Right, it's kind of nice to give "space" local physical properties in that way. But we can't take them too seriously-- we have no way to measure these local physical properties, it's just a picture. In this picture, mass density acts on the space locally only, causing it to contract whereever there is mass. In between the regions where there is mass, the space simply acts in a smooth and continuous way, like how a rubber sheet would act globally in response to being squeezed in local regions. But if there's no mass locally present, there's no net squeezing of the space-- squeezing laterally is matched with stretching radially (as happens above the surface of the Earth), all while the space "falls in" to "fill in behind" the contracting space within the Earth.

Is gravity a push or a pull force?
In this picture, it is a local contraction on space, and space then has some continuous properties that call for nearby space to fill in where it has been contracted. Objects "embedded" in the space then merely "follow it", so it's neither a push nor a pull on those objects, in literal terms-- because the embedded objects (like a falling rock) aren't "actually moving" at all. Other objects, like the Earth (and like you) that are not "following" the contracting space, require some outward force on them to accelerate through the contracting space (within the Earth) or falling space (outside the Earth).

peteshimmon
2010-Sep-13, 05:01 PM
I may as well complete my input as it may
help some. I was fascinated by a small item
of analysis from Maxwell whereby he applied
his mathematics to describing the action of
a gravitational field. He showed the field
implied an almost infinite energy field that
was modified by matter.

Flash forward 70 years to Quantum theory
and the notion of Zero Point Energy emerged.
The vacuum is an energy field that is
constantly interacting with the electrons
of atoms. In the lowest ground energy state,
atoms can only exist with this constant
agitation or they will radiate away and
fall into the nucleus. Think I have that
right. So it seemed Maxwell has foreseen
the vacuum described by Quantum Mechanics.

Now one cannot help trying to guess the
mechanism of gravity and I wonder about
a modified energy field wherby matter
speeds up near a "lessening" of the energy
density but light slows down as this "ether"
becomes less dense to a waveform.

But "ether" is a naughty word though I
think of a "dynamic" entity.

Now the learned people here say you have
to understand General Relativity completely
to have an opinion that will be politely
considered. You have to understand the
beauty of the Ricci tensor, you must know
your tensor analysis. Well they have a
bloody good point, only being conversant
with the full up to date insight science
has about gravity make one eligable to
pontificate.

But we cannot help it:)

Le Sage will never rest.

caveman1917
2010-Sep-13, 05:19 PM
Wow, this is confusing since some describe gravity as a curvature of space while others describe it as a flowing inward towards mass.

Note that gravity is described as the curvature of spacetime, not just space. The analogy works by noting that the curvature could be seen as describing what space does in relation to time - in a sense giving you the 'behaviour' of space as it unfolds in time, ie giving space a velocity at every single point. Though it's still an analogy of course.


If space is flowing inward like towards a blackhole then I only "see" radial motion and no curvature.

That's the idea ;)
Putting a field of velocity vectors over the extent of (what you can then still consider flat) space is another way of expressing the 4d curvature.
It won't do you much good if you really want to get into the math, but it seems to do the job quite nicely about conceptualizing things.

nokton
2010-Sep-13, 06:14 PM
Wow, this is confusing since some describe gravity as a curvature of space while others describe it as a flowing inward towards mass.

If space is flowing inward like towards a blackhole then I only "see" radial motion and no curvature.

I sort of like the "flow" description because I can not see how a particle of mass can reach out with tentacles and pull us towards it whereas if space is flowing towards the particle then I am being pushed towards the mass.

Is gravity a push or a pull force?

btw: I'm only to the 2nd page so this is probably dropped right into the midst of some other heated conversation.
Xylophobe, you must think in 3 dimensions, not two. Then your question will be answered.
Nokton

xylophobe
2010-Sep-13, 07:11 PM
... In this picture, it is a local contraction on space, and space then has some continuous properties that call for nearby space to fill in where it has been contracted. Objects "embedded" in the space then merely "follow it", so it's neither a push nor a pull on those objects, in literal terms-- because the embedded objects (like a falling rock) aren't "actually moving" at all. Other objects, like the Earth (and like you) that are not "following" the contracting space, require some outward force on them to accelerate through the contracting space (within the Earth) or falling space (outside the Earth).

This is why when I am freefalling, like in the space shuttle, then my body feels no acceleration force as opposed to when I'm strapped to a rocket and blasted off. If the local space around me is "stationary" to my body then I feel no force but since my little region of space is caught up in the flow towards the earth then I appear to an earth-bound viewer as an accelerating object.

Ken G
2010-Sep-13, 07:38 PM
Yup, that's the picture that gets you pretty far! It is related to the core concept of Einsteinian gravity: "the equivalence principle."

Jerry
2010-Oct-07, 08:01 PM
-- Venus Express Finds Planet's Atmosphere a Drag
http://www.spaceref.com/news/viewpr.html?pid=31795

"The polar atmosphere of Venus is thinner than expected. How do we know? Because ESA's Venus Express has actually been there. Instead of looking from orbit, Venus Express has flown through the upper reaches of the planet's poisonous atmosphere. Venus Express went diving into the alien atmosphere during a series of low passes in July-August 2008, October 2009, and February and April 2010. The aim was to measure the density of the upper polar atmosphere, an experiment that had never been attempted before at Venus."

Similar foray's into the Poles of Titan by Cassini have consistently produced the opposite result: A thicker atmosphere than expected.

Why this thread? The atmospheric profiles are based upon many variables; including several involving the core density and the distribution of mass within the planet/moon body. Determination of the mass distribution within Titan seem to indicate that the mass distribution is much less centralized than expected.

So why are the results so different from what is expected, based upon our current gravitational laws and other constucts?

Hornblower
2010-Oct-07, 11:53 PM
-- Venus Express Finds Planet's Atmosphere a Drag
http://www.spaceref.com/news/viewpr.html?pid=31795

"The polar atmosphere of Venus is thinner than expected. How do we know? Because ESA's Venus Express has actually been there. Instead of looking from orbit, Venus Express has flown through the upper reaches of the planet's poisonous atmosphere. Venus Express went diving into the alien atmosphere during a series of low passes in July-August 2008, October 2009, and February and April 2010. The aim was to measure the density of the upper polar atmosphere, an experiment that had never been attempted before at Venus."

Similar foray's into the Poles of Titan by Cassini have consistently produced the opposite result: A thicker atmosphere than expected.

Why this thread? The atmospheric profiles are based upon many variables; including several involving the core density and the distribution of mass within the planet/moon body. Determination of the mass distribution within Titan seem to indicate that the mass distribution is much less centralized than expected.

So why are the results so different from what is expected, based upon our current gravitational laws and other constucts?

I would say that we still have a lot to learn about the complex atmospheric dynamics on these alien worlds. I don't see any indication of a gravitational issue here. If the planet had strong enough gravitational asphericities from extreme mascons to create local atmospheric anomalies, I would expect the spacecraft's orbit to be strongly perturbed even without skimming the upper fringe of the atmosphere.

Ken G
2010-Oct-08, 12:42 AM
Yes, there are so many things we might have wrong that could explain the surprises, to focus on probably the one thing we have basically right (gravity) is barking up the wrong tree.

Jens
2010-Oct-08, 03:17 AM
"The polar atmosphere of Venus is thinner than expected.



Similar foray's into the Poles of Titan by Cassini have consistently produced the opposite result: A thicker atmosphere than expected.


Also, you would think that if it were a gravitational effect, it would be consistent. But the fact that it goes one way on Venus and the other way on Titan would seem to argue that it's something about atmospheric dynamics, so maybe the rotation speeds or something like that.

peteshimmon
2010-Oct-08, 06:56 PM
Interesting extension. My own query lately
is the outer ring of Uranus with sub micron
sized particles. I wonder if very accurate
spectral measurements indicate radial
velocities that satisfy Newtonian
gravitation.

naiveharry
2010-Oct-13, 05:11 PM
So if you drop a mass onto the space time fabric ( imagining a taut piece of cloth), it leads to a depression/dimple in the fabric. Now any objects that comes near this mass will have to follow this depression on the space time fabric leading to its acceleration towards the particle, as it is following a space time curve at an angle.
Now imagine the particle has infinite mass, so depression almost tends to -> infinity depression and any object that wanders close to it starts to follow an almost vertical fall into the space time curve and continues to fall forever forever. It could also be treated as -> hole in space time fabric.

This is my understanding of the topic from some readings. Please correct me if I am wrong.

Jerry
2010-Oct-14, 05:30 AM
The stretched sheet analogy describes one aspect of gravity, but not all. The tension is dynamic, gravity is a field effect that includes direction, spin and vibrational energy; as well as the effects on time and space.

Jeff Root
2010-Oct-14, 12:18 PM
naiveharry,

Did you really mean to say "infinite mass"? That seems pretty
extreme. Did you actually mean "infinite density"? The matter
at the center of a black hole approaches infinite density without
limit. However, it shouldn't create a "hole" in the sense that the
imagery of a punctured sheet conjures. The sheet-- representing
spacetime-- should be infinitely elastic, so even if the matter at
the black hole's center *is* infinitely dense, it just means the
sheet in the analogy is infinitely stretched, not punctured.

I'm fairly sure that the "hole" in the term "black hole" refers
to the fact that everything that goes inside the event horizon
is lost from the rest of the Universe and never seen again.

-- Jeff, in Minneapolis

cosmocrazy
2010-Oct-14, 03:59 PM
naiveharry,

Did you really mean to say "infinite mass"? That seems pretty
extreme. Did you actually mean "infinite density"? The matter
at the center of a black hole approaches infinite density without
limit. However, it shouldn't create a "hole" in the sense that the
imagery of a punctured sheet conjures. The sheet-- representing
spacetime-- should be infinitely elastic, so even if the matter at
the black hole's center *is* infinitely dense, it just means the
sheet in the analogy is infinitely stretched, not punctured.

I'm fairly sure that the "hole" in the term "black hole" refers
to the fact that everything that goes inside the event horizon
is lost from the rest of the Universe and never seen again.

-- Jeff, in Minneapolis

Sounds about right to me Jeff. I had heard though that it might be possible that the stretch you mention rather than going to infinity might "tear" (for want of a better word) a hole in space through to another area like in the case of a wormhole effect.

caveman1917
2010-Oct-14, 05:59 PM
Sounds about right to me Jeff. I had heard though that it might be possible that the stretch you mention rather than going to infinity might "tear" (for want of a better word) a hole in space through to another area like in the case of a wormhole effect.

The wormhole effect you mention is possible with a Kerr black hole, but that is due to the singularity being a ring instead of a point, it's not because of the approach to infinite density - that shouldn't by itself create any 'tears', as Jeff mentions.

astromark
2010-Nov-02, 02:01 AM
"But captain we are doing the impossible now...
All the dilithium crystals are out of alignment and its only Tuesday...
We need to realign the fabric converters." and put the wagons in a circle...

We were doing fine until 'peteshimmon' entangled the structure of the universe.

I enjoyed naiveharry's contribution with 'Jeff's' input...added.

Please do not muddy the waters of explanation with wormholes...

The OP with regard to Gravity and explaining the understanding of what it might be.

astromark
2010-Nov-02, 04:04 AM
So... not that I wish to pock fun at or even imply ignorance. We have learned much.

This subject 'Gravity' has us scratching to explain. We know that objects of any mass influence the space near to them.

That with regard to the rules of relativaty any object of mass is influenced by any other object of mass.

That the greater mass objects seem to exhort greater effect...

and with regard to distance and time calculations can be made with some accuracy.

Like I said. We know much... Then why do I feel so void of understanding.

No particle. No wave. No frequency. Leeds me to ask what do we actually know.

With a special note to KenG... Yes, I have been listening., and thankyou for helping my understanding.

My understanding has grown. Yet the explanation for what gravity is still seems a little distant.

This is unacceptable.

Am I looking to hard for a answer that we already have ?

What I mean is. 'Is the simple distortion of space by mass just a property of mass.'

It distorts space. No further action is required... Hmmm...

I will need to consider this with some refreshing beverage on hand...

peteshimmon
2010-Nov-02, 12:52 PM
Its the voices! They say raise a few odd
points, try and tempt the more learned out
of their comfort zones, its jusssst possible
you might find an unexplored rich seam of
pertinent questions.

General relativity remains, however, a
vast feather mattress that does not admit
any discomfort anywhere:)

Thinking about possible experiments
seems a good diversion. Lately it is
what can be found in a very tall pipe
with some hydrogen in it. My gas
physics is sparce yet does gravity
show in laser spectrometry?

Its all going on somewhere!

kevin1981
2010-Nov-02, 02:02 PM
I have learnt a lot from this and other threads too. I think the right term, if i want to get all technical, is, pressure and energy distort space-time to create the effect we call gravity. How space-time distorts at a fundamental level, i don't think anybody knows.

My personal view is, to really understand theory's like GR, QM, EM and so fourth, you need to be able to do the mathematics. Mathematics is the "language" of physics. People like myself, who have a keen interest in science, and want to know why nature/physics work's the way it does can only learn so much without going into the mathematics of these theory's.

I am happy to learn and understand the basic concepts but am also aware that i will never fully understand the workings of gravity as my math skills are really bad.

I do however keep reminding myself that GR is a mathematical theory, that lets us explain observations of an expanding space-time. Space-time is not "physical", so the only way to describe what is happening is through mathematics.

kevin1981
2010-Nov-02, 02:13 PM
What I mean is. 'Is the simple distortion of space by mass just a property of mass.'

I thought it was the other way round. The distortion of space-time by mass is a property of space-time. That's what space-time does in the presence of mass, it distorts.

CosmicUnderstanding
2010-Nov-02, 06:10 PM
My personal view is, to really understand theory's like GR, QM, EM and so fourth, you need to be able to do the mathematics. Mathematics is the "language" of physics. People like myself, who have a keen interest in science, and want to know why nature/physics work's the way it does can only learn so much without going into the mathematics of these theory's.

I am happy to learn and understand the basic concepts but am also aware that i will never fully understand the workings of gravity as my math skills are really bad.

I really feel for you Kevin, because I am in the same boat as you regarding mathematics. I carry with me an extreme interest in understanding and learning how the reality we live in works, but unfortunately I don't feel as though my body is "equipped" with the right gear to comprehend the more advanced mathematics that would need to be learned in order to fully understand these concepts from a physics perspective.

Strange
2010-Nov-02, 08:15 PM
My personal view is, to really understand theory's like GR, QM, EM and so fourth, you need to be able to do the mathematics.

Ah, well put. If only some of the people who post ATM theories had this level of awareness instead of thinking pop science articles are the be all and end all!

astromark
2010-Nov-02, 11:46 PM
The cat is chasing its tail... space time is distorted by mass. A body of mass has a effect on space time in that it distorts it.

Just as space would seem to undergo some distortion when the presence of bodies of mass is apparent.

The key word in this understanding might be 'Effect.'

Regardless of the word soup used... We seem to have reached a point where just excepting the fact is enough.

No amount of mathematical work is going to change this riddle. It will serve to cement the known.

cosmocrazy
2010-Nov-05, 01:54 PM
"But captain we are doing the impossible now...
All the dilithium crystals are out of alignment and its only Tuesday...
We need to realign the fabric converters." and put the wagons in a circle...

We were doing fine until 'peteshimmon' entangled the structure of the universe.

I enjoyed naiveharry's contribution with 'Jeff's' input...added.

Please do not muddy the waters of explanation with wormholes...

The OP with regard to Gravity and explaining the understanding of what it might be.

why not? worm holes are an accepted "idea"! No one knows for sure what happens at the centre of a BH and since a BH is the most warped area of space thus the most extreme case of gravity effects we are aware of then it makes sense to mention any possibilities that are discussed in modern physics.

astromark
2010-Nov-06, 01:53 AM
No. Its not a accepted idea. Not by me anyway. Its a work of fiction with no working model. No basis for understanding other than a proposal.

A proposal without a modal is not the mainstream view... worm holes No.

cosmocrazy
2010-Nov-06, 10:27 AM
Here is a snip it from an article in wiki (found in the link), I know there is no observational evidence for wormholes Mark, but they are an accepted "idea" resulting from some of the equations from GR.

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

"There is no observational evidence for wormholes, but on a theoretical level there are valid solutions to the equations of the theory of general relativity which contain wormholes. The first type of wormhole solution discovered was the Schwarzschild wormhole which would be present in the Schwarzschild metric describing an eternal black hole, but it was found that this type of wormhole would collapse too quickly for anything to cross from one end to the other. Wormholes which could actually be crossed, known as traversable wormholes, would only be possible if exotic matter with negative energy density could be used to stabilize them (many physicists such as Stephen Hawking,[1] Kip Thorne,[2] and others[3][4][5] believe that the Casimir effect is evidence that negative energy densities are possible in nature). Physicists have also not found any natural process which would be predicted to form a wormhole naturally in the context of general relativity, although the quantum foam hypothesis is sometimes used to suggest that tiny wormholes might appear and disappear spontaneously at the Planck scale.[6][7] It has also been proposed that if a tiny wormhole held open by a negative-mass cosmic string had appeared around the time of the Big Bang, it could have been inflated to macroscopic size by cosmic inflation.[8]"

Jerry
2010-Nov-08, 10:59 PM
A search for gravitational waves associated with the August 2006 timing glitch of the Vela pulsar

http://arxiv.org/abs/1011.1357


We present the first direct search for the gravitational wave emission asso
ciated with oscillations of the fundamental quadrupole mode excited by a pulsar timing glitch. No
gravitational wave detection candidate was found.

astromark
2010-Nov-09, 02:31 AM
Look at me... reading that post of 'cosmo's' over and over... looking for what it is I might be missing... Nothing. Its not there.

Oh wait. Its an idea... Oh thats just dandy... As I have no idea. It obviously disqualifies me from ever knowing this.... I surrender. :wall: