Gravity Mystery: Ask "GR Crowd" for Deeper Understanding

[oldman]

In the cosmology forum I posted this statement:

All we know is that (at a Newtonian level) masses attract but don't repel, and that (at a GR level) mass/energy distorts spacetime so that geodesics converge. But we don't know exactly how mass/energy accomplishes either feat. Hence gravity remains a given mystery.

,

to which a PF mentor kindly replied:

Well, you should ask the GR crowd for a deeper understanding of the concept of a "gravitational field" in modern physics, but gravitation is generally framed in terms of a metric. This gives the geometry of spacetime, which is shaped by the matter and energy contained within. This matter and energy then moves in response to the local geometry.

Loosely, I suppose you can think of the geometry of spacetime as the "gravitational field" ...

I fully accept what he says, but would be interested if any of "the GR crowd" would like to comment further.

What I am asking is: by what means does mass/energy shape the geometry of spacetime? Does anybody know? Or is it one of those questions which one shouldn't ask?

 

[jtbell]

Everything in physics becomes a mystery, when we reach the most fundamental level that we know about. We always come to something that we simply have to assume as a given, without further explanation, at present at least.

In classical general relativity, the Einstein field equation is that level. It tells us how to calculate the curvature of spacetime, given the momentum-energy stress tensor. But we don't (yet) know where it comes from, or the underlying mechanism by which it works. Some people are working on various approaches (string theory, etc.) to answering these questions. As far as I know, none of these have been developed far enough so that we can even test them experimentally.

 

[pervect]

Consider electrical charge. Two like charges repel, and two unlike charges attract. Isn't that also a mystery? We can write down the laws of charge conservation, but why should charge be a conserved quantity? And why should there be forces between charges?

For what it's worth, if you _don't_ think that charges and electromagnetic forces are "mysterious", you can come up with theories of gravity as a spin-2 theory formally similar to electromagnetism. This is not the "standard" approach, but see for instance

http://www.citebase.org/fulltext?format=application/pdf&identifier=oai:arXiv.org:astro-ph/0006423

 

[pmb_phy]

Everything in physics becomes a mystery, when we reach the most fundamental level that we know about. We always come to something that we simply have to assume as a given, without further explanation, at present at least.

In classical general relativity, the Einstein field equation is that level. It tells us how to calculate the curvature of spacetime, given the momentum-energy stress tensor. But we don't (yet) know where it comes from, or the underlying mechanism by which it works. Some people are working on various approaches (string theory, etc.) to answering these questions. As far as I know, none of these have been developed far enough so that we can even test them experimentally.

I couldn't have said it better myself!

Or is it one of those questions which one shouldn't ask?

I urge you to always keep on asking these questions. Otherwise you'll never get answers.

Pete

 

[oldman]

Everything in physics becomes a mystery, when we reach the most fundamental level that we know about. We always come to something that we simply have to assume as a given, without further explanation, at present at least.

In classical general relativity, the Einstein field equation is that level. It tells us how to calculate the curvature of spacetime, given the momentum-energy stress tensor. But we don't (yet) know where it comes from, or the underlying mechanism by which it works. Some people are working on various approaches (string theory, etc.) to answering these questions. As far as I know, none of these have been developed far enough so that we can even test them experimentally.

I agree with pmb-phy about your reply. Thanks very much indeed for this post. I suspected that this was the situation, but needed confirmation in case I had been confused into ignorance by the plethora of mathematical ingenuities in which GR is embedded.

 

[oldman]

Consider electrical charge. Two like charges repel, and two unlike charges attract. Isn't that also a mystery?

Yes it is. Perhaps a more directly perceived one. As jtbell said:

Everything in physics becomes a mystery, when we reach the most fundamental level that we know about. We always come to something that we simply have to assume as a given, without further explanation, at present at least.

.

Thanks for the reference. It looks quite fierce, but interesting.

 

[oldman]

I urge you to always keep on asking these questions. Otherwise you'll never get answers.

Pete

Thanks for the encouragement. I'll keep trundling along!

 

[pmb_phy]

Thanks for the encouragement. I'll keep trundling along!

You're most welcome sir.

General Relativity is often mis-stated as being the relativistic explanation of gravity. It is not. It is a relativistic description of gravity. As Eddington put it (Nature, March 14, 1918, p. 36)

The purpose of Einstein's new theory has often been misunderstood, and it has been criticised as an attempt to explain gravitation. The theory does not offer an explanation of gravitation; that lies outside its scope, and does not even hint at a mechanism.

Pete

 

[oldman]

You're most welcome sir.

General Relativity is often mis-stated as being the relativistic explanation of gravity. It is not. It is a relativistic description of gravity. As Eddington put it (Nature, March 14, 1918, p. 36)

You touch on a distinction close to my heart. I have long maintained that physics is only(!) an amazingly effective and intriguing description of nature; one that uses a rather esoteric language. Thanks for the Eddington quote. I have his "The Nature of the Physical World" on my desk as I write.

 

[Garth]

You touch on a distinction close to my heart. I have long maintained that physics is only(!) an amazingly effective and intriguing description of nature; one that uses a rather esoteric language. Thanks for the Eddington quote. I have his "The Nature of the Physical World" on my desk as I write.

Indeed there will always remain mystery, the scientific process is to push that mystery further and further back, but I'll contend it will always answer one set of questions only by raising another set.

The key distinction of a scientific description, however, is not only to provide natural explanations to natural phenomena but also to be able to make testable and falsifiable predictions.

Garth

 

[oldman]

The key distinction of a scientific description, however, is not only to provide natural explanations to natural phenomena but also to be able to make testable and falsifiable predictions.

Garth

Agreed. But distinction from what?

 

[jtbell]

From a non-scientific description, of course!

 

[pmb_phy]

Indeed there will always remain mystery, the scientific process is to push that mystery further and further back, but I'll contend it will always answer one set of questions only by raising another set.

The key distinction of a scientific description, however, is not only to provide natural explanations to natural phenomena but also to be able to make testable and falsifiable predictions.

Garth

Too many people use the term "explanation" in physics if you ask me. Science starts with observation of nature. When consistencies are found for which one can make wide sweeping assertions then one has a law of nature. But you cannot say that the law 'explains' nature since that would be very circular reasoning. In Fritz Rorhlich's book "Classical Charged Particles" he explained this better than anywhere else in the literature that I've seen.

Pete