Is there a physical explanation for the relationship between light and space?

[PeterDonis]

what I don't understand is why measuring these stresses is sufficient as being called a cause of these stresses, which it seems to me is what you are trying to say

No. What I'm saying is that the stresses are the physics. All this talk about "is it rotating relative to spacetime" is not physics, it's just words.

There must be a reason for this measurement to show these stresses. What is the reason?

According to GR, the reason is that in the first case (where the ring shows stresses), the worldlines of the particles in the ring have nonzero path curvature (which means they feel nonzero proper acceleration), whereas in the second case, the worldlines have zero path curvature (which means they feel zero proper acceleration). In other words, it's how the worldlines "sit" in the geometry of spacetime.

If you want to use the words "the ring is rotating relative to spacetime" to describe this physics (the stresses and GR's explanation of them), that's fine as long as you understand that those words are just words. They're not the physics; they're not the machinery you use to actually make predictions. The machinery you use to make predictions is the theory of GR; it's expressed in math, not words. So if you actually want to reason about physics, instead of just using words to label things, you have to learn the actual model.

 

[Buckethead]

No. What I'm saying is that the stresses are the physics. All this talk about "is it rotating relative to spacetime" is not physics, it's just words.
According to GR, the reason is that in the first case (where the ring shows stresses), the worldlines of the particles in the ring have nonzero path curvature (which means they feel nonzero proper acceleration), whereas in the second case, the worldlines have zero path curvature (which means they feel zero proper acceleration). In other words, it's how the worldlines "sit" in the geometry of spacetime.

If you want to use the words "the ring is rotating relative to spacetime" to describe this physics (the stresses and GR's explanation of them), that's fine as long as you understand that those words are just words. They're not the physics; they're not the machinery you use to actually make predictions. The machinery you use to make predictions is the theory of GR; it's expressed in math, not words. So if you actually want to reason about physics, instead of just using words to label things, you have to learn the actual model.

OK, that makes sense. So although I can say the ring is rotating relative to spacetime, I can't just say we have that simple relationship, the relationship is more complex and must be described using a mathematical relationship between the object and spacetime. All good! In the case of two rings, should the relationship between one ring and the other be first and foremost a relationship between one ring and spacetime and then that spacetime and the second ring? In other words I suppose it would be impossible to have a relationship just between 2 rings and not between 2 rings and spacetime as just two rings without spacetime gets us back to square one where we can't say if one ring is spinning and the other isn't anymore than we can say one inertial object is moving and another isn't. In other words, we need spacetime to build the relationship between one ring and the other and to say which one is spinning and which one isn't. Is that correct?

 

[Buzz Bloom]

Hi @Buckethead:

I strongly recommend you take a careful look at the following old thread.

https://www.physicsforums.com/threads/effort-to-get-us-all-on-the-same-page-balloon-analogy.261161​

The creator of this thread, Marcus, presented an excellent and simple non-mathematical explanation of the concepts of space, distances, light speeds, etc., using the expanding balloon analogy.

I hope you find it helpful.

Regards,
Buzz

 

[PAllen]

Look,if you have an isolated ring with stress, you can deduce a state without stress that the isolated ring rotates relative to. Thus, even though the ring is isolated, it physically defines its own nonrotating reference.

 

[Dale]

So you can't have a reference frame move through space because that would change the laws of physics as described by Maxwell's equations and because it would also imply a change in c.

Essentially, yes. Except that I would say that it could happen in principle, but we have measured it and found that it doesn't happen.

 

[Dale]

n other words I suppose it would be impossible to have a relationship just between 2 rings and not between 2 rings and spacetime as just two rings without spacetime

This doesn't make any sense. How could you even have two rings without spacetime? If there is no geometry then in what sense is there a ring, let alone two rings? Doesn't what you are describing as a "two rings" presuppose geometry?

 

[PeterDonis]

we need spacetime to build the relationship between one ring and the other and to say which one is spinning and which one isn't. Is that correct?

I would say we need spacetime in the GR model in order to explain the observations that tell us that one ring is rotating and the other isn't (as well as other observations that tell us, e.g., how far apart the rings are from each other, what size each ring is, etc.). But we don't need spacetime to tell us which one is rotating and which one isn't; we can just measure the stresses (or lack thereof) in the rings directly (and the same for other measurements). In other words, spacetime is a model that we use in order to tie together lots of different observations and give a compact explanation for all of them. But the model is not the observations. It's a model.

 

[nitsuj]

There would be a Doppler effect with the observation, I'll use my own choice of light. Even if the ring was luminescent, and uniformly, or at least consistently so, this would be seen.

 

[CarlM]

When viewed in easy terms what you are proposing is not too far from the "ether" theory, that light was transferred through an ether that permeated space. That was disproven in the Michelson-Morley experiment in the late 1800s. That disproof was one of the sticky points that lead to Einstein's thought about relativity.

The problem is that this physics, like quantum physics, is in physical terms but outside the boundaries of our day to day human experience. It is described in mathematics because everyday terms like door and chair and teddy bear really do not apply. Our everyday experience is pretty well described by Newton's physics for 90 percent of it and the other ten percent we gloss over till someone starts measuring and observing closely finding discrepancies. Then it gets weird and the Newtonian space of our daily experience cannot be reconciled. Common sense, another term for experience, does not apply in the depth of space or in a black hole or inside and atom because we cannot experience and survive these places.

 

[Buckethead]

Hi @Buckethead:

I strongly recommend you take a careful look at the following old thread.

https://www.physicsforums.com/threads/effort-to-get-us-all-on-the-same-page-balloon-analogy.261161​

The creator of this thread, Marcus, presented an excellent and simple non-mathematical explanation of the concepts of space, distances, light speeds, etc., using the expanding balloon analogy.

I hope you find it helpful.

Regards,
Buzz

Thanks Buzz. I actually read through the whole thread a couple years ago and it was very interesting. I think it might be time for a re-read.

 

[Buckethead]

Look,if you have an isolated ring with stress, you can deduce a state without stress that the isolated ring rotates relative to. Thus, even though the ring is isolated, it physically defines its own nonrotating reference.

Interesting. I can accept that this is true, but it sure is abstract! It just seems impossible that it can be this way.

 

[Buckethead]

This doesn't make any sense. How could you even have two rings without spacetime? If there is no geometry then in what sense is there a ring, let alone two rings? Doesn't what you are describing as a "two rings" presuppose geometry?

I should have been more careful with my wording. What I meant to ask was if it was necessary to first compare the ring with spacetime, then compare spacetime to the second ring to determine if one ring is rotating relative to the other, or would it be possible to just compare one ring to the other directly to get the same result. But I'm not sure this is a valid question anymore. I'm getting the sense that trying to determine whether a ring is feeling forces due to spinning or not simply by math alone is not possible. That only a measurement (by accelerameters) can be used to determine this. I suppose this is just one of those mysteries up there with why light always travels at c.

 

[Buckethead]

In other words, spacetime is a model that we use in order to tie together lots of different observations and give a compact explanation for all of them. But the model is not the observations. It's a model.

Right, again, we can know spacetime by its properties, but that does not give us the ability to know spacetime as anything other than a math model.

 

[Buckethead]

There would be a Doppler effect with the observation, I'll use my own choice of light. Even if the ring was luminescent, and uniformly, or at least consistently so, this would be seen.

Interesting, although if the object were not rotating and instead was being orbited by an observer the Doppler shift would still be seen.

 

[Buckethead]

Common sense, another term for experience, does not apply in the depth of space or in a black hole or inside and atom because we cannot experience and survive these places.

This seems to be true, but we have to be careful to not use that excuse as another type of god of the gaps where if we stumble upon a paradoxical situation such as the mystery of spacetime, that we don't just throw our hands up and say, "Welp, looks like another one of natures mysteries that we are not allowed to make sense of".

 

[Dale]

What I meant to ask was if it was necessary to first compare the ring with spacetime, then compare spacetime to the second ring to determine if one ring is rotating relative to the other, or would it be possible to just compare one ring to the other directly to get the same result.

The comparison to the other ring is pretty much irrelevant. You can determine the rotation of one ring (without reference to the other) either by comparing it to spacetime (using accelerometers) or directly (measuring the stress in the ring).

I'm getting the sense that trying to determine whether a ring is feeling forces due to spinning or not simply by math alone is not possible. That only a measurement (by accelerameters) can be used to determine this.

This is true, but I am not understanding what you feel is at all surprising about that. I mean, you cannot determine anything about an object through math alone. Why should spinning be determined through math alone? Indeed, to me that would be far more surprising.

 

[nitsuj]

The problem is that this physics, like quantum physics, is in physical terms but outside the boundaries of our day to day human experience. It is described in mathematics because everyday terms like door and chair and teddy bear really do not apply. Our everyday experience is pretty well described by Newton's physics for 90 percent of it and the other ten percent we gloss over till someone starts measuring and observing closely finding discrepancies. Then it gets weird and the Newtonian space of our daily experience cannot be reconciled. Common sense, another term for experience, does not apply in the depth of space or in a black hole or inside and atom because we cannot experience and survive these places.

Are you talking about measurement accuracy, our ability to perceive comparatively microscopic physical effects or our ability to understand them? The logic between the theories is vastly different, contradictory and as easy to spot as the difference between a chair and a teddy bear, or to speak your language 0≠1

fyi Newton physics describes 0% of our, or anythings daily experience. It does make very accurate predictions, but lacks the SUPER axiom that, as seen in this thread, changes everything.

I haven't seen anything in this thread that suggests they're applying "everyday experience" [Newtonian physics] to this imaginary physics scenario

The question was simply put, simply answered , rinse repeat, and maybe is better understood now.

 

[pervect]

If someone asked in a post "Is this frame rotating relative to space-time", I'd ask them what they meant. It'd be unclear at best. I'd also not word my answer in those terms, I'd pick some different ones I felt was less ambiguous as well, as I wouldn't want my words to be interpreted differently later and by other people.

In the one problem I worked where it made a difference, I'd talk about whether something was rotating "relative to the fixed stars" or "relative to a gyroscope". This is a bit clearer, though it may possible to improve it further. Both make more sense to me than talking about "rotating relative to space-time".

A problem I was working on a while ago where it made a difference -might provide some insight. Consider a frame on block sliding on the floor of Einstein's elevator. It - it seemed to be generally better understood when I said that a gyroscope attached to the sliding block would precess. It's logically equivalent to say that the block rotates relative to a gyroscope, but it seemed to cause less confusion when I assumed what I'd call "the fixed star frame".

In this example in particular, if someone asked "is the block rotating relative to space-time", the answer is not very clear. It's clearer IMO to say that the block is not rotating relative to the fixed stars, but a gyroscope attached to the block would precess.

 

[Mister T]

If someone asked in a post "Is this frame rotating relative to space-time", I'd ask them what they meant. It'd be unclear at best.

I agree. It would make me think they might be referring to space-time as a substance of some sort. What you could say is that it's rotating in some specified reference frame. That reference frame is used to define the space and time coordinates. Is space-time, a, some naturally-occurring thing whose creation we attribute to Nature; or is it, b, a model created by the human intellect? Or is there some third option? Perhaps we use the term space-time in more than one way, so that in one way we mean a but in the other we mean b?

 

[PeterDonis]

I'd talk about whether something was rotating "relative to the fixed stars" or "relative to a gyroscope".

The second is better, IMO, since it's local. When you make "relative to a gyroscope" rigorous, you end up talking about the vorticity of the congruence of worldlines that describes the object.