Theories for Mechanisms Underlying SR/GR?

[Islam Hassan]

If I understand correctly, SR & GR describe with accuracy (ultra-relativistic) motion, acceleration and gravity but do not explain the underlying mechanisms that may be causing it.

What is the present state of theorising regarding the mechanisms which may underly SR/GR?

To take the speed of light for one, has anyone proposed a mechanism which would explain "why" its speed is invariant in all frames of reference?

IH

 

[Dale]

No. At this time GR is a fundamental theory meaning that there are no theories to explain GR.

 

[exponent137]

No. At this time GR is a fundamental theory meaning that there are no theories to explain GR.

I disagree. Quantum gravity is a theory which could explain GR. But I think that it should not give much more new elements than GR.

would explain "why" its speed is invariant in all frames of reference?

Because all inertial systems are equivalent.

 

[Dale]

Quantum gravity is a theory which could explain GR.

Possibly, sometime in the future. Right now that is merely a hope.

 

[Fredrik]

If I understand correctly, SR & GR describe with accuracy (ultra-relativistic) motion, acceleration and gravity but do not explain the underlying mechanisms that may be causing it.

No theory explains its own "underlying mechanisms". If we had a better theory of gravity than GR, then it might explain GR, but then you'd be asking about its underlying mechanisms instead.

 

[tom.stoer]

Quantum gravity is a theory which could explain GR.

There are several attempts to formulate such a theory but as of today there is not one complete candidate (leave aside experimental testability). Some candidates try to explain gravity in terms of a deeper structure (LQG), some do nothing else but quantize GR (asymptotic safety), string theory tries to harmonize gravity with other forces, but I think we cannot say that it explains gravity.

 

[exponent137]

There are several attempts to formulate such a theory but as of today there is not one complete candidate (leave aside experimental testability). Some candidates try to explain gravity in terms of a deeper structure (LQG), some do nothing else but quantize GR (asymptotic safety), string theory tries to harmonize gravity with other forces, but I think we cannot say that it explains gravity.

Grammar mistake: I think that it will explain GR, when it will be discovered (if it will be discovered.) I hope that you agree with this.

I hope on simpler theories than string theory and even LQG, but this is not important here.

 

[tom.stoer]

Grammar mistake: I think that it will explain GR, when it will be discovered (if it will be discovered.) I hope that you agree with this.

No, no grammar mistake, I don't think so; not in it's current formulation where it fails to (fully) incorporate background independence.

 

[lugita15]

No, no grammar mistake, I don't think so; not in it's current formulation where it fails to (fully) incorporate background independence.

But doesn't string theory say that background independence is only approximately true at the scales that GR operates in, but that it fails in other length/energy scales?

 

[exponent137]

No, no grammar mistake, I don't think so; not in it's current formulation where it fails to (fully) incorporate background independence.

I also believe that quantum gravity should be background independent. But, quantum mechanics is not background independent - do you so think that there are problems with quantum gravity background independence.

What if we said that space inside of black hole does not exist similarly as tahions (the most probably) does not exists. This eases same problems at fusion of GR and QM.

 

[tom.stoer]

I also believe that quantum gravity should be background independent ... do you so think that there are problems with quantum gravity background independence.

Yes, I think so.

String theory (from which gravity does emerge) is usually formulated in terms of a classical background (10 dim. spacetime with a certain geometry, e.g. flat 3+1 Minkowski space + compactified 6-dim. Calaby-Yau as a simple example). There are different possibilities and backgrounds which are all 'present' in string theory but except for rare cases I do not see that the theory can be formulated w/o first fixing a specific background.

In loop quantum gravity one starts with a 3+1 globally hyperbolic spacetime which allows for foliations, i.e. one fixes at least the topology to R*M³.
[It is interesting that the final theory of spin networks seems to allow for a larger class of "geometries", i.e. the spin networks need no longer be dual to triangulations and therefore need not be equivalent to PL manifolds; in addition a graph (a spin network) does not have a fixed, integer dimension; last but not least in the deep quantum regime there need not be an emergent manifold at all.]

So these two approaches seem to depend on certain background structures which look somehow artificial and should be removed frommthe final theory.

 

[harrylin]

If I understand correctly, SR & GR describe with accuracy (ultra-relativistic) motion, acceleration and gravity but do not explain the underlying mechanisms that may be causing it.

What is the present state of theorising regarding the mechanisms which may underly SR/GR?

To take the speed of light for one, has anyone proposed a mechanism which would explain "why" its speed is invariant in all frames of reference?

IH

Yes, such a mechanism was proposed right from the start; however it cannot be directly verified, as we cannot "see" at a deeper level than the phenomena described by those theories and QM. What kind of explanation are you looking for?

 

[Neandethal00]

If I understand correctly, SR & GR describe with accuracy (ultra-relativistic) motion, acceleration and gravity but do not explain the underlying mechanisms that may be causing it.

What is the present state of theorising regarding the mechanisms which may underly SR/GR?

To take the speed of light for one, has anyone proposed a mechanism which would explain "why" its speed is invariant in all frames of reference?

IH

This has been one of my gripes for a long time.

Both SR and GR describe the effects, but not the causes.
That is one reason I say, some may agree with me, SR and GR are incomplete theories. They are tips of some massive strange icebergs.

Because all inertial systems are equivalent. Exponent

Suppose a few inertial systems are moving around with different speeds. A high speed spaceship and a lightbeam zoom past them. Each inertial system will record different speeds for the spaceship but the same speed for the light beam. Right?

 

[morningstar]

space itself is the underlying mechanism.

'space is the substance, matter is the unsubstantial dream' -einstein

all the equations describe how different forms of energy react to being in space.

 

[bobc2]

... A high speed spaceship and a lightbeam zoom past them. Each inertial system will record different speeds for the spaceship but the same speed for the light beam. Right?

Right.

 

[bobc2]

'space is the substance, matter is the unsubstantial dream' -einstein

Please give a reference so we can check the context of that statement.

 

[exponent137]

Yes, I think so.

String theory (from which gravity does emerge) is usually formulated in terms of a classical background (10 dim. spacetime with a certain geometry, e.g. flat 3+1 Minkowski space + compactified 6-dim. Calaby-Yau as a simple example). There are different possibilities and backgrounds which are all 'present' in string theory but except for rare cases I do not see that the theory can be formulated w/o first fixing a specific background.

In loop quantum gravity one starts with a 3+1 globally hyperbolic spacetime which allows for foliations, i.e. one fixes at least the topology to R*M³.
[It is interesting that the final theory of spin networks seems to allow for a larger class of "geometries", i.e. the spin networks need no longer be dual to triangulations and therefore need not be equivalent to PL manifolds; in addition a graph (a spin network) does not have a fixed, integer dimension; last but not least in the deep quantum regime there need not be an emergent manifold at all.]

So these two approaches seem to depend on certain background structures which look somehow artificial and should be removed frommthe final theory.

What about theory of Fotini Markopoulou? Where it belongs (close to LQG?) and how it is with its background independence? I think that there is not a problem.

 

[ParticleGrl]

Here is a question- I have a decoration shaped like an M on my wall. Yet, when I stand on my head I see a W. What is the underlying mechanism?

Obviously, there isn't one- its just a question of reference. I can describe my room equally well from the ceiling down or the floor up.

How does this relate to the original question? SR is the same way. There isn't a causal mechanism- the situation is the same, it doesn't make sense to ask for one. Just like standing on your head doesn't change the room your in, change frames doesn't change anything about the situation.

 

[bobc2]

If I understand correctly, SR & GR describe with accuracy (ultra-relativistic) motion, acceleration and gravity but do not explain the underlying mechanisms that may be causing it.

What is the present state of theorising regarding the mechanisms which may underly SR/GR?

To take the speed of light for one, has anyone proposed a mechanism which would explain "why" its speed is invariant in all frames of reference?

IH

We don't have a fundamental theory of reality. We just have physics. For example your question takes on a totally different meaning depending on whether the universe is four-dimensional or whether the universe is a dynamically changing 3-dimensional universe. There does not seem to be any consensus among physicists about which model is appropriate for discussing your question.

ParticleGrl's comment is particularly relevant to the 4-dimensional universe model (there isn't a causal mechanism--it's just all there--then, you might then ask, "How did it all get here?"). But, from here we can continue with many speculations, and that's why this kind of question is not dealt with in depth on this forum.

 

[Neandethal00]

Right.

And you do not see a problem with this scenario?
OP's question was exactly about this, why in one case we have different speeds and light speed is the same for all inertial frames.

That is why I tend to think light travels by embedding itself into 'space materials', considering space is a medium. The medium we still know nothing about. This medium may also be responsible for relativistic mass increases in SR.

Theoretical and experimental investigation into empty space may seriously surprise us in the future.

 

[elfmotat]

And you do not see a problem with this scenario?
OP's question was exactly about this, why in one case we have different speeds and light speed is the same for all inertial frames.

That is why I tend to think light travels by embedding itself into 'space materials', considering space is a medium. The medium we still know nothing about. This medium may also be responsible for relativistic mass increases in SR.

Theoretical and experimental investigation into empty space may seriously surprise us in the future.

It's against the site rules to make speculative posts.

 

[bobc2]

And you do not see a problem with this scenario?
OP's question was exactly about this, why in one case we have different speeds and light speed is the same for all inertial frames.

No. I do not see a problem with this scenario, because that's the way things seem to work. Special relativity indicates this and the special relativity theory has been verified in experiment after experiment for a century now.

Below is a graphic that illustrates the situation with respect to velocities among observers and the universal speed of light, c, measured the same, regardless of the frame of reference. We have a sequence of different blue frames representing different observers moving at relativistic velocities with respect to a black rest system. Notice how the blue X1 axis (the space the observer is "living" in) and X4 axis rotate symmetrically about the 45-degree line (light photon world line). This accounts for the blue observer in each of the sketches measuring the same speed for light.

If you have trouble interpreting the sketch, google "spacetime diagram." You should really work at understanding the sketch, because once you understand it, the reason every different observer measuring the same speed for light becomes obvious. I promise you will not see a problem with your scenario.

Of course, you can then ask, "Why do the X4 and X1 coordinates rotate like that in the 4-dimensional universe." That discussion is outside the scope of this forum.

 

[Neandethal00]

Special relativity indicates this and the special relativity theory has been verified in experiment after experiment for a century now.

What does it suppose to mean? Who questioned about validity of SR/GR? Question was "why?" not "does it?"

Why does time dilate with speed?
Why does length shrink with speed?
Why does mass increase with speed?
Why does space curve by mass?

One doesn't need world-line, blue-line, black-line to find c is constant in all frames. You can also prove the same by using relativistic velocity addition.

It's against the site rules to make speculative posts. elfmotat

Without logical speculation science would have died in its infancy.

 

[bobc2]

What does it suppose to mean? Who questioned about validity of SR/GR? Question was "why?" not "does it?"

I thought you were questioning the validity of special relativity when you asked if we did not see a problem with "this scenario." And I was just indicating that I did not see a problem with this scenario.

Originally Posted by Neandethal00:
"And you do not see a problem with this scenario?
OP's question was exactly about this, why in one case we have different speeds and light speed is the same for all inertial frames."

Why does time dilate with speed?
Why does length shrink with speed?
Why does mass increase with speed?
Why does space curve by mass?

One doesn't need world-line, blue-line, black-line to find c is constant in all frames. You can also prove the same by using relativistic velocity addition.

Using the space-time diagram, I was just showing you one universe model that would answer those questions if it truly represented reality. If you accept that model, then the answers are there in the picture. In this view (Hermann Weyl's view) the universe is 4-dimensional occupied by 4-dimensional objects, including bodies and brains). Consciousness moves along the 4th dimension at the speed of light, experiencing a continuous sequence of 3-D cross-sections of the universe.

Time dilation and length contraction result from the different 3-D cross-sections of the 4-D universe within which different observers live (each one's consciousness moving at a different relativistic velocity with respect to some rest system).

So, I think it would definitely help understanding this model if you will learn how to interpret the space-time diagrams as actually representing pictures of the 4-D universe. But, again, this is all stuff that is not appropropriate on this forum.

Without logical speculation science would have died in its infancy.

But, the established purpose of this forum is to help people understand the physics that we have. There may be other forums that would accommodate speculative ideas. Perhaps the philosophy forum.