View Full Version : Our Understanding of Gravity

spratleyj

2008-Jul-15, 01:50 AM

I was recently reading through Kip Throne’s book Black Holes & Time Warps, in which he states that we no longer think of gravity in the Newtonian sense, that is we don't view it "mass" "pulling" on other "mass" rather it’s the bending of space-time… is this a mainstream concept? and am I missing something here?

BTW I don't want to discuss ATM theories here, just what is generally accepted :lol:

Chris Hillman

2008-Jul-15, 01:57 AM

How does Thorne's view conflict with my recent post in a thread in which you participated about the status and nature of gtr?

If it helps, Kip Thorne is a highly regarded leader in gravitation physics, not a crank ;)

spratleyj

2008-Jul-15, 02:03 AM

hmmm... I'm not talking about the "nature" or "status" of GTR, I'm just saying that I was always "taught" ...

"between any two objects in the Universe there acts a gravitational force that is proportional to the product of their masses and inversely proportional to the square of their separation"

While GTR defines it as the "curvature" of space-time... so are both of these concepts correct? or is Newton's "out of date"?

Chris Hillman

2008-Jul-15, 02:08 AM

hmmm... I'm not talking about the "nature" or "status" of GTR, I'm just saying that I was always "taught" ...

"between any two objects in the Universe there acts a gravitational force that is proportional to the product of their masses and inversely proportional to the square of their separation"

While GTR defines it as the "curvature" of space-time... so are both of these concepts correct? or is Newton's "out of date"?

I wouldn't say that Newtonian gravity is "incorrect" but gtr is "correct". I'd say that both are theoretically and experimentally well founded theories, but that gtr is known to have a wider domain of applicability. That is, there are many scenarios in which Newtonian gravity is perfectly adequate, and in those situations it is appropriate to use it. In other situations, gtr is known to be adequate but Newtonian gravity is known to be inadequate, so we use gtr in those situations. In this sense, gtr is said to have "replaced" Newtonian gravity, a characterization which is (unfortunately) easily misunderstood.

In still other situations, it is expected that gtr will also break down, but as yet no viable replacement for gtr is known which can be used in those situations.

Not to misunderstand: so far, there is no experiments or observerations which are generally agreed to demonstrate a failure of gtr.

Any quantum theory of gravity will probably be more difficult to use than gtr, just as gtr is more difficult to use than Newtonian gravitation. For this reason, it is likely that Newtonian gravity (and gtr) will continue to be taught as core topics to physics students even after a quantum theory of gravity appears. Obviously, human intellect is becoming inadequate; for these and other reasons, humans will no doubt be phased out ASAP.

spratleyj

2008-Jul-15, 02:12 AM

So, specifically does the "inverse square law" hold?

Jens

2008-Jul-15, 02:18 AM

I was recently reading through Kip Throne’s book Black Holes & Time Warps, in which he states that we no longer think of gravity in the Newtonian sense, that is we don't view it "mass" "pulling" on other "mass" rather it’s the bending of space-time… is this a mainstream concept? and am I missing something here?

My understanding, which may be wrong, is that that is the way that Einstein understood gravity. Whereas quantum physics tends to see it differently. And that is one of the reasons why there is the attempt to make a "grand unifying theory" that unites gravity with the other forces. But to be honest, I'm not sure where this leaves gravitons, because I think that Einstein believed they existed, right?

Chris Hillman

2008-Jul-15, 02:20 AM

So, specifically does the "inverse square law" hold?

I think this is an example of what Nereid was complaining about.

As you continue to study, one thing I think you will come to appreciate is that simple questions often do not have simple answers--- at least, not honest ones.

I was trying to explain above that physics is not about "absolute truth". I think you still expect that physics (or science generally) results in eternal incontrovertible answers which stand for all time. Black and white, as Nereid put it. But this expectation is naive. In fact, science is constantly evolving, but because new science is always firmly grounded in old science (this is a key part of what makes science so strong!), old scientific theories (if they are any good) are usually not simply tossed out entirely; they remain valuable within well prescribed boundaries, for specific uses.

So, to take your example:

mathematically speaking, the inverse square force law holds in Newtonian gravitation (including the classical field theory reformulation, which was created by French mathematicians in the early nineteenth century),

Newtonian gravitation is well established as an excellent theory which is sufficiently accurate to use in many situations,

in other situations, it is known to be inadequate (e.g. it fails to accurately predict the observed precession of Mercury, and it fails to account for light bending, gravitational red shift, Shapiro light delay, orbital decay in binary pulsars, it fails to predict Hubble expansion, and so on).

There is a classic paper by the distinguished mathematician Mark Kac on mathematical models in biology which was published many years ago in Science, which I think you would do well to find and read. Sorry I can't recall the exact reference right now.

My understanding, which may be wrong, is that that is the way that Einstein understood gravity. Whereas quantum physics tends to see it differently. And that is one of the reasons why there is the attempt to make a "grand unifying theory" that unites gravity with the other forces. But to be honest, I'm not sure where this leaves gravitons, because I think that Einstein believed they existed, right?

spratleyj posted in numerous threads and also started this one today. Jens mentioned several issues which have already come up and been sorted out, plus one more: gravitons. They have also recently been discussed, but although I am getting a bit worn out, FWIW:

gravitons are hypothetical particles which arise, roughly speaking, from a naive quantization of gtr (by applying machinery developed by the middle of the twentieth century for "quantizing" a classical field theory, producing a quantum field theory with the required classical limit),

this naive quantization is known not to yield a true quantum theory of gravity (although the machinery just mentioned works fine for turning Maxwell's theory of EM into a viable quantum field theory, QED),

it is probably best to regard the notion of gravitons as an idealization valid for some purposes, and probably suggestive of some features of a deeper as yet unknown quantum theory of gravity, but they certainly are not well-established compared to electrons or photons.

HTH

spratleyj

2008-Jul-15, 02:28 AM

I'm not suggesting that a law is always correct or incorrect, I was asking if the "inverse square" law was one of Newton's laws that is only sometimes correct, so there is room for grey

Chris Hillman

2008-Jul-15, 02:34 AM

Do you understand the distinction between "correct" and "accurate"?

Newtonian gravitation is sufficiently accurate for many purposes,

Newtonian gravitation is inadequate for some situations in which gtr is accurate, so in those situations we use gtr instead,

Sometimes we use a mixture of Newtonian gravitation and gtr (this requires great knowledge and skill); sometimes we use a mixture of QFT and gtr (this requires even more knowledge and skill),

Newton's law holds exactly in Newtonian gravity (including the field theoretic version),

Newton's law doesn't even make sense in gtr,

gtr in some sense includes Newtonian gravity as a limiting case,

none of these claims contradict each other.

But now I think I'm beginning to repeat myself...sigh...

spratleyj

2008-Jul-15, 02:36 AM

I think so :) what would you say the distinction is?

Chris Hillman

2008-Jul-15, 02:40 AM

I'll give it one more shot. Let's consider some examples.

It would be incorrect to say according to Newtonian gravitation, a point mass test particle orbiting an isolated massive object will traverse an orbit which is a cubic curve. The correct statement replaces "cubic" with "quadratic". These are mathematical statements about a particular mathematically expressed theory, so the incorrect statement is provably incorrect, while the correct one is provably correct.

Newtonian gravitation is inaccurate for preparing an ephemeris for the motion of the planets in our Solar System, because it fails to account for the so-called "extra-Newtonian precession" of Mercury, Venus, and other planets (in our solar system, this term is only important for the inner planets and some asteriods). This statement involves observation and experiment, not just mathematics.

spratleyj, some things in science--- many things actually--- are just plain subtle. Let's call it quits for now, and agree that you will continue to learn more physics/math from good books. Ultimately, you need to know the math to appreciate the points I am trying to convey. Fair enough?

spratleyj

2008-Jul-15, 02:47 AM

ok... hopefully I have not frustated you guys too badly :)

Sam5

2008-Jul-15, 03:33 AM

ok... hopefully I have not frustated you guys too badly :)

Did you ever get a straight answer to your questions? Or were you criticized for asking them? :)

spratleyj

2008-Jul-15, 03:42 AM

I really never got a "complete" answer... my question was does the "inverse square law" apply to "us"... all I got was that relativity and newton's laws were accurate for certain things... so not to sound rude, but I didn't really learn what I was trying to...

Jens

2008-Jul-15, 06:03 AM

Neither of the theories are correct. They both fail to understand what gravity really is.

Getting back to the accurate vs. correct thing, which applies here as well, it would be "accurate" to say that a computer screen is 14 inches, but certainly not "correct". Because that implies that it's true, which it isn't. It might be 14.01, for example. But 14 inches is certainly accurate. It works well as a measure.

So yes, I would agree that the theories are not "correct" in the sense that they do not describe what gravity "really is." In fact, there may not be any way of doing that. We cannot describe what anything "really is". That enters the realm of philosophy rather than science.

Chuck

2008-Jul-15, 06:48 AM

Calvin & Hobbes (http://picayune.uclick.com/comics/ch/2008/ch080714.gif)

pzkpfw

2008-Jul-15, 07:49 AM

In fact, there may not be any way of doing that. We cannot describe what anything "really is". That enters the realm of philosophy rather than science.

In this case, I believe Cruiser Dude is referring to the ATM views he has.

(Similar to what Motor Daddy used to post).

Stuart van Onselen

2008-Jul-15, 02:58 PM

I really never got a "complete" answer... my question was does the "inverse square law" apply to "us"... all I got was that relativity and newton's laws were accurate for certain things... so not to sound rude, but I didn't really learn what I was trying to...The inverse square rule doesn't really "apply", in the sense that Kentucky State law applies to Kentucky, but not to California.

However, for "us", if we're trying to measure most things, then the inverse square law is accurate enough to be useful. It is accurate enough for us to predict the orbits of the planets, except for Mercury.

But when we try to measure some of the stranger phenomena mentioned here, the inverse-square law becomes more and more inaccurate when measuring these phenomena, while still being sufficiently accurate for most of the phenomena around us.

You may think of it as "horses for courses".

Of course, we first try to apply Newtonian gravity (with its attendant inverse-square rule) because it is much, much easier to do the math. Only when scientists can't get a match between Newtonian predictions and the phenomenon they're studying, do they reluctantly turn to the hairy math involved in GTR.

Same as classical and quantum mechanics. If classical is accurate enough, in a given realm, people use it. Only when classical mechanics starts slowly diverging from observation, as the things being observed get smaller and smaller (sub-atomic small) do they have to reach for the quantum formulae or the wave formulae to get accurate answers. At the expense of working with the very difficult math involved.

spratleyj

2008-Jul-15, 04:04 PM

Thanks for the reply Stuart van Onselen... that's exactly the kind of response I was waiting for...

Stuart van Onselen

2008-Jul-15, 04:23 PM

Please call me Stuart, or even "Van", which is a kinda universal nickname for any of the gadzillions of South Africans with "Van" in their surname.

However, only my boss or my lover can call me Stu or (especially) Stewie. :)

But seriously: My one physics lecturer told of one of his post-grad students who wanted to do a project based on a speech recently given by an eminent quantum physicist. It was, obviously enough, a project based on quantum mechanics.

But my lecturer took a quick look at the phenomenon being investigated, did a back-of-an-envelope calculation, and declared "But this is a classical phenomenon!" i.e. the energies involved were large enough that classical mechanics could adequately describe it.

So this undergrad completed her project using classical mechanics. She managed to find two consequences that the eminent physicist had missed, and even corrected one mistake he had made!

So even though both classical and quantum theories could give accurate predictions in this case, the complexity of quantum math was such that the error and the extra findings got lost.

Why did the other professor not see this? Because, as his speciality was quantum mechanics, he automatically reached for his quantum "tool-case", without even thinking to do the two-minute calculation needed to see that it was actually a classical phenomenon.

John Mendenhall

2008-Jul-15, 05:05 PM

Look at it this way: ordinary mechanics (including Newtonian Mechanics) is a subset of Special Relativity, which is a subset of General Relativity, which is probably a subset of some theory of everything (which will subsume Quantum Mechanics, also).

For our ordinary day-to-day purposes, inverse suare works fine (it applies to us). For some calculations (nodes of Mercury's orbit), you need SR. For others (orbiting a BH), you need general.

Easy to see, easy to remember. 1, 2, 3.

spratleyj

2008-Jul-15, 05:33 PM

thanks for the responses! I think you guys did a excellent job answering my question....

cosmocrazy

2008-Jul-15, 05:39 PM

Look at it this way: ordinary mechanics (including Newtonian Mechanics) is a subset of Special Relativity, which is a subset of General Relativity, which is probably a subset of some theory of everything (which will subsume Quantum Mechanics, also).

For our ordinary day-to-day purposes, inverse square works fine (it applies to us). For some calculations (nodes of Mercury's orbit), you need SR. For others (orbiting a BH), you need general.

Easy to see, easy to remember. 1, 2, 3.

This is a nice simple way to understand the different but valid models used today. I agree John, and i think this was the point Chris, and others were trying to get across. :)

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