Einstein's Theory of Relativity

[AJ Allen]

I've just recently researched a bit of his work with this and just wanted some clarifications:

1. Is Einstein redefining gravity or is he saying it doesn't exist
2. Is it right to say that he is saying gravity is the distortion of the fabric of time and space
3. If gravity is just the distortion of time and space does this apply to simple things such as a apple falling? Or does this theory only address astronomical entities such as the sun or other planets?

 

[phinds]

Yes, Einstein's General Theory of Relativity does say that gravity is an effect of the curvature of space-time, with that curvature being caused by mass. It applies everywhere except with the exception of the center of a black hole where the GR math breaks down and gives a physically impossible answer.

BUT ... in terms of practical matters, nothing on a scale as small as a planetary object requires GR except for the MOST precise correct answer. Bridge builders never use GR and NASA does not use it when calculating trajectories to the moon but on the other hand, if GR is not accounted for in the GPS System, it would have you driving in corn fields and into the side of buildings instead of on the roads.

 

[mathman]

Einstein's general relativity is a more precise description of the phenomenon of gravity and it applies in all situations. However for most practical problems, such as the Apollo prgram or a falling apple, Newton's gravity is accurate enough, and is much simpler to use.

 

[Drakkith]

I've just recently researched a bit of his work with this and just wanted some clarifications:

1. Is Einstein redefining gravity or is he saying it doesn't exist
2. Is it right to say that he is saying gravity is the distortion of the fabric of time and space
3. If gravity is just the distortion of time and space does this apply to simple things such as a apple falling? Or does this theory only address astronomical entities such as the sun or other planets?

1. It's redefining gravity as a geometrical effect instead of a "force".
2. That's good enough of an explanation for a layperson. The "fabric" of GR is a geometrical explanation of 4-dimensional spacetime, unlike a 2-dimensional fabric you usually think of.
3. It applies to everything. Apples, people, and black holes.

 

[bapowell]

Regarding 2., it might be of interest to point out that some practitioners prefer not to encourage the literal geometric interpretation of the theory. For instance, Weinberg in his seminal text "Gravitation and Cosmology" chooses not elevate geometry to a staring role, but rather emphasizes that the "peculiar empirical properties", like Einstein's equivalence principle, just happen to be expressed in the mathematical language of differentiable manifolds. This has implications for whether we view gravity as a force or merely as a "geometrical effect".

Consider electromagnetism, which is based on U(1) gauge symmetry in the Standard Model. It is perfectly reasonable to cast this theory as one of geometry: we find that the vector potential plays the role of the connection, describing how "vectors" twist and contort as they are parallel transported through the admittedly abstract U(1) internal space. Further, the electromagnetic field strength tensor literally measures the curvature of this internal space. Do these ideas stop us from also considering the vector potential as a gauge field whose quanta are the literal mediators of the electromagnetic force? No: the geometric interpretation of the U(1) theory is not emphasized, favoring instead its particle physics incarnation in terms of gauge bosons.

General relativity is no more "geometric" than the gauge theories of particle physics, the primary difference being that the geometry of general relativity is actual space and time, rather than abstract "internal" group spaces, and so it is emphasized. Emphasis aside, if there really is no fundamental distinction, why do we single out gravity as being a "geometric effect" while the other interactions are particle mediated?

 

[phinds]

... why do we single out gravity as being a "geometric effect" while the other interactions are particle mediated?

Thanks for posting this discussion; it was very interesting.

 

[ShayanJ]

Regarding 2., it might be of interest to point out that some practitioners prefer not to encourage the literal geometric interpretation of the theory. For instance, Weinberg in his seminal text "Gravitation and Cosmology" chooses not elevate geometry to a staring role, but rather emphasizes that the "peculiar empirical properties", like Einstein's equivalence principle, just happen to be expressed in the mathematical language of differentiable manifolds. This has implications for whether we view gravity as a force or merely as a "geometrical effect".

Consider electromagnetism, which is based on U(1) gauge symmetry in the Standard Model. It is perfectly reasonable to cast this theory as one of geometry: we find that the vector potential plays the role of the connection, describing how "vectors" twist and contort as they are parallel transported through the admittedly abstract U(1) internal space. Further, the electromagnetic field strength tensor literally measures the curvature of this internal space. Do these ideas stop us from also considering the vector potential as a gauge field whose quanta are the literal mediators of the electromagnetic force? No: the geometric interpretation of the U(1) theory is not emphasized, favoring instead its particle physics incarnation in terms of gauge bosons.

General relativity is no more "geometric" than the gauge theories of particle physics, the primary difference being that the geometry of general relativity is actual space and time, rather than abstract "internal" group spaces, and so it is emphasized. Emphasis aside, if there really is no fundamental distinction, why do we single out gravity as being a "geometric effect" while the other interactions are particle mediated?

There are many things that make a geometrical interpretation of GR the most natural one, sometimes even the only one!
One thing is that for other interaction, a geometrical interpretation requires the introduction of a new spooky space which is not desirable. But about GR, we have space-time which I directly observable.
Also, when I think about the effects that gravitation can have on space-time, it seems very natural to me to have a geometrical interpretation for GR. So natural that I think its being suggested by nature itself. If we say that gravitation is, like other interactions, a field on Minkowski space-time, then there should be some extra explanation that how it has the observed effects on space and time.
Also all attempts at deriving Einstein's equations using QFT methods, concluded that the field(s) behaved so much like the geometrical properties of space-time that no one bothers to name them differently. And now we have at our disposal a space(space-time) and a theory describing its properties. Not accepting that gravity is the result of the geometry of space-time and looking for another point of view seems a child's nag to me!(No disrespect intended but that's really how it seems to me.)
For one good thing, you can look at section 7.3 of Gravitation by Misner, Thorne and Wheeler.
This is my idea about GR but I should confess that I think its more wrong to prevent a physical idea to grow. So, although I like the idea that gravitation is caused by space-time's geometry and think this is GR's correct interpretation and will oppose other interpretations, But I'll be happy to see such theories to grow and be discussed. Its strange that I have this two sided feeling that I oppose it but don't want it to be absent but that's how I feel.
I should also say that this is my idea about GR in case I believe GR is the correct theory for gravity. I know its the mainstream theory for gravity, so I think its probably the correct one but I like to investigate its alternatives too. So if one day, one of GR's alternatives turns out to be better, I will change to that and so, that day, I won't attach gravity to space-time's geometry.
(oops...did I talk too much?!...sorry!:D)

 

[PAllen]

Regarding 2., it might be of interest to point out that some practitioners prefer not to encourage the literal geometric interpretation of the theory. For instance, Weinberg in his seminal text "Gravitation and Cosmology" chooses not elevate geometry to a staring role, but rather emphasizes that the "peculiar empirical properties", like Einstein's equivalence principle, just happen to be expressed in the mathematical language of differentiable manifolds. This has implications for whether we view gravity as a force or merely as a "geometrical effect".

Consider electromagnetism, which is based on U(1) gauge symmetry in the Standard Model. It is perfectly reasonable to cast this theory as one of geometry: we find that the vector potential plays the role of the connection, describing how "vectors" twist and contort as they are parallel transported through the admittedly abstract U(1) internal space. Further, the electromagnetic field strength tensor literally measures the curvature of this internal space. Do these ideas stop us from also considering the vector potential as a gauge field whose quanta are the literal mediators of the electromagnetic force? No: the geometric interpretation of the U(1) theory is not emphasized, favoring instead its particle physics incarnation in terms of gauge bosons.

General relativity is no more "geometric" than the gauge theories of particle physics, the primary difference being that the geometry of general relativity is actual space and time, rather than abstract "internal" group spaces, and so it is emphasized. Emphasis aside, if there really is no fundamental distinction, why do we single out gravity as being a "geometric effect" while the other interactions are particle mediated?

Weinberg said, in later essay, that he had lifelong regrets about the de-emphasis of geometry in "Gravitation and Cosmology". However I do think (as I guess you do) that whatever the successful successor theory to GR and SM is, it will not involve spacetime curvature except as an emergent approximation.

 

[ShayanJ]

However I do think (as I guess you do) that whatever the successful successor theory to GR and SM is, it will not involve spacetime curvature except as an emergent approximation.

Yeah, that's expected but this one I didn't expect: Maybe everything is emergent!
(So where does the entropy come from in the first place man?)

 

[nitsuj]

General relativity is no more "geometric" than the gauge theories of particle physics, the primary difference being that the geometry of general relativity is actual space and time, rather than abstract "internal" group spaces, and so it is emphasized. Emphasis aside, if there really is no fundamental distinction, why do we single out gravity as being a "geometric effect" while the other interactions are particle mediated?

Ahh so we should interpret all "forces" as "geometric", I thought "force carriers" seemed silly.

 

[aleazk]

"It is possible that this problem—the non renormalizability of general relativity*—has arisen because the usual flat-space formalism of quantum field theory simply cannot be applied to gravitation. After all, gravitation is a very special phenomenon,
involving as it does the very topology of space and time"

Weinberg, 1980s.

*i.e., in the perturbative approach in which the metric is written as the sum of a background metric and a dynamical part.

 

[CKH]

"It is possible that this problem—the non renormalizability of general relativity*—has arisen because the usual flat-space formalism of quantum field theory simply cannot be applied to gravitation. After all, gravitation is a very special phenomenon,
involving as it does the very topology of space and time"

Is this still considered a real problem in quantum theory? I suppose it is.

I recently read about a new theory that revives the ether thus providing a preferred frame with absolute time and space. The paper was exceptionally abstract so I don't pretend to understand it. But, the author claimed that the preferred frame in ether theory can solve this problem in QM. For example, in the deterministic, realistic but non-local De Broglie–Bohm theory of QM, the theory might provide a absolute definition of simultaneous that makes sense in the context of non-local actions.

The theory provides a different equation for gravity. According to the author, the theory approaches GR in some limit.

It sounded promising. He claims to eliminate the BH singularity and the BBT singularity (substituting a cyclic universe). It's not a theory of everything yet, but a different possible approach to one and provides an alternate to GR. EEP is derived in his theory.

In the theory, everything is an effect within the ether. The reality that we experience is comparable to the waves on the ocean. All that we experience are waves. All that actually exists is ocean (ether). There should be experiments that can falsify the theory. It is not exactly the same as GR. Perhaps observations of black holes.

Apparently something has got to give to get GR and QM into a single theory. Besides, the singularity implied by the BBT is a serious limitation for existing physics.

Here's a link to the paper.

 

[R Naveen]

I have a question related to relativity. Time dilation (both velocity and gravitational) is explained with the example of clocks. How does time dilation actually affect humans physiologically?

 

[russ_watters]

I have a question related to relativity. Time dilation (both velocity and gravitational) is explained with the example of clocks. How does time dilation actually affect humans physiologically?

Strictly speaking, time dilation does not "affect" things, it is only an observation of the fact that time is reference frame dependent. So if you travel at high speed away from Earth and come back, you will come back younger than a twin on earth, because you traveled along a different path in space-time. But locally, for you, everything is normal.