Is General Relativity More General than Gravity?

[Demystifier]

TL;DR Summary

Is there a difference between general relativity and gravity?

One obvious difference is that gravity is more general, because there are alternative theories of gravity that differ from general relativity. In this sense general relativity can be thought of as a subset of gravity.

But I am interested in a different type of difference. I am interested in a perspective, if any, in which general relativity can be thought of as more general than gravity. So are there aspects of general relativity that are not aspects of gravity? For instance, could we say that a study of general transformations of spacetime coordinates is a study of general relativity but not necessarily a study of gravity?

 

[PeroK]

Summary: Is there a difference between general relativity and gravity?

One obvious difference is that gravity is more general, because there are alternative theories of gravity that differ from general relativity. In this sense general relativity can be thought of as a subset of gravity.

But I am interested in a different type of difference. I am interested in a perspective, if any, in which general relativity can be thought of as more general than gravity. So are there aspects of general relativity that are not aspects of gravity? For instance, could we say that a study of general transformations of spacetime coordinates is a study of general relativity but not necessarily a study of gravity?

I'd say that General Relativity is the theory and study of curved spacetime. And, gravity is a phenomenon (or several phenomena) associated with curved spacetime - or explained by it.

But, GR is more general in the sense that there are phenomena that are not directly seen as the result of "gravity": universal expansion, redshift, closed time-like curves etc.

Gravity itself on the other hand may straddle several theories: Newtonian, GR, Quantum; and, therefore, is more general from that perspective.

 

[Ibix]

Is this off the back of the "Escape Velocity, Gravitational Velocity & Time Dilation" thread that's running at the moment?

My instinct is that gravity is "that which makes stuff drop to the floor when I let go of it". That got formalised by Newton, whose theory was then broken by special relativity. The fix, general relativity, is a much more widely applicable theory than Newton's, and includes large scale descriptions of the cosmos and phenomena such as event horizons that you can't even conceive of under Newtonian gravity. Whether this, and future theories replacing GR, are "theories of gravity" is surely just a matter of taste - or, more precisely, conventional usage by physicists.

I personally regard GR as a theory of gravity, generalising the use of the word "gravity" somewhat. MTW is properly titled "Gravitation", so there's some support there. But it's an English-as-she-is-spoke thing, as far as I'm aware.

 

[Dale]

Summary: Is there a difference between general relativity and gravity?

I am interested in a perspective, if any, in which general relativity can be thought of as more general than gravity.

General relativity is a theory and gravity is a phenomenon or set of phenomena. I don't think that comparing their generality makes sense. They are just different categories of things.

I would use the comparison "A is more general than B" to mean that all of B is a part of A, and even then only for classes where the attribute "generality" makes sense. Gravity and GR don't seem to be related in that way.

 

[Demystifier]

General relativity is a theory and gravity is a phenomenon or set of phenomena.

So it's something like Abelian gauge theory and electromagnetism.

 

[martinbn]

Summary: Is there a difference between general relativity and gravity?

Why?

General relativity is a theory and gravity is a phenomenon or set of phenomena. I don't think that comparing their generality makes sense. They are just different categories of things.

But he probably means general relativity and the theoretical description of gravity. So is everything the GR describe gravity or does it describe other things? On the other hand is gravity completely described by GR or are there aspects of gravity that need more than GR, say quantum stuff?

 

[Mister T]

Summary: Is there a difference between general relativity and gravity?

Gravity is the name we give to a naturally-occurring phenomenon. General relativity is an invention of the human mind. The latter is an attempt at an explanation of the former.

 

[Demystifier]

Analog gravity https://en.wikipedia.org/wiki/Analog_models_of_gravity , that is physical phenomena (such as acoustic waves in a moving fluid) that can be described in terms of an effective curved spacetime, can be thought as examples of applied general relativity without gravity.

 

[haushofer]

Analog gravity https://en.wikipedia.org/wiki/Analog_models_of_gravity , that is physical phenomena (such as acoustic waves in a moving fluid) that can be described in terms of an effective curved spacetime, can be thought as examples of applied general relativity without gravity.

The same can be said of holography and supergravity.

 

[Dale]

Analog gravity https://en.wikipedia.org/wiki/Analog_models_of_gravity , that is physical phenomena (such as acoustic waves in a moving fluid) that can be described in terms of an effective curved spacetime, can be thought as examples of applied general relativity without gravity.

Aren’t you a proponent of the idea that different interpretations are different theories? Don’t you recognize that this is at most the same mathematical framework and a different minimal interpretation? Even by my anti-interpretation stance this would be a different theory since it changes the mapping between the mathematical framework and experiment, so surely you would not consider it the same theory either.

 

[Demystifier]

Aren’t you a proponent of the idea that different interpretations are different theories? Don’t you recognize that this is at most the same mathematical framework and a different minimal interpretation? Even by my anti-interpretation stance this would be a different theory since it changes the mapping between the mathematical framework and experiment, so surely you would not consider it the same theory either.

I'm not sure I understood your point, but note that in those analog gravity examples the curved spacetime geometry does not satisfy the Einstein equation. So it is general relativity in the sense of being physics described in terms of curved spacetime geometry, but it is not gravity in the sense that the curvature is not determined by the matter energy-momentum tensor.

 

[Dale]

So it is general relativity in the sense of being physics described in terms of curved spacetime geometry

GR is not just pseudo-Riemannian geometry. You, of all people, should recognize that.

 

[martinbn]

So it is general relativity in the sense of being physics described in terms of curved spacetime geometry, but it is not gravity in the sense that the curvature is not determined by the matter energy-momentum tensor.

But then it is only an analog of GR, not GR. So analog gravity is described by analog GR.

 

[Demystifier]

GR is not just pseudo-Riemannian geometry. You, of all people, should recognize that.

Well, in a sense that's what this thread is about, to define what exactly GR is. If GR includes the Einstein equation, which indeed what most textbooks implicitly say, then you are absolutely right. But in that case "general relativity" is a misnomer, because the name "general relativity" suggests that it is supposed to be just a principle of relativity that generalizes Einstein special relativity from 1905 and Galilean relativity, which are two theories of relativity that really deserve their names. So if the Einstein equation is included, then "general relativity" is more than a generalized principle of relativity.

This is somewhat similar to the name "quantum mechanics" which suggests a theory of discrete energies, while today the name "quantum mechanics" is used for the theory in which discrete energies are not essential at all. In the case of quantum mechanics, we know that the name stuck due to historical reasons. Perhaps something similar happened with general relativity? Perhaps Einstein coined the name "general relativity" when he realized that the equivalence principle can be explained by pseudo-Riemannian geometry, before he said anything specific about the relation between curvature and matter?

 

[Demystifier]

But then it is only an analog of GR, not GR. So analog gravity is described by analog GR.

See my post above.

 

[Dale]

If GR includes the Einstein equation, which indeed what most textbooks implicitly say, then you are absolutely right. But in that case "general relativity" is a misnomer, ...

With this we will close this thread. The textbooks define what “General Relativity” means, not your personal view on what you think those two words should mean. A semantic debate about whether or not the words “General Relativity” capture the entirety of the theory is not the purpose of this forum.

PS my point wasn’t about the EFE at all, but about the minimal interpretation of GR