Is the speed of light always 299 792 458 m / s

[Bjarne]

The speed of light is measured to 299 792 458 m / s and 1 meter defined to 1/299 792 458 of the speed of light.

Let us assume that A live at the top of a skyscraper and B in the cellar the past 10 billion years.
After 10 billion years B’s clock have “lost” 10 second due to different gravitational influence, compared to A’s clock.

10 billion years ago 2 photons was leaving a star 10 billion light years away and hit A and B at the same moment 10 billion year after.

B would now say that he measured the time it took for the photon to reach earth, to 10 second less than A measured.

B would therefore claim that either the speed of light must have been traveling faster than 299 792 458 m / s, or local distance (where B is) must be stretching or contracting proportional with the stretch of time.

Which option is correct, and what proves it ?

 

[russ_watters]

Length contraction.

Proof? It can't be proven with that experiment, each would have to locally measure the speed of light and measure that it is C. Then they would know the distance has to be different.

Please don't make this about the recursiveness of the definitions. That is not a confounding factor.

 

[Nugatory]

Which option is correct, and what proves it ?

Neither, really (B, in the cellar is still subject to some gravitational effects so will get a different result from a hypothetical observer in a gravity-free vacuum). But as you've stated the problem, there's no way of "proving" that any of them are correct or not - we just have a bunch of very slightly different measurements, all multiplied by 10 billion to exaggerate the differences.

To completely understand their observations, they must both measure the local speed of light; they'll both get the same result. Then they can assume that that value is also the speed of the light traveling from the distant galaxy, see whether that assumption leads to an internally consistent theory that also matches experimental evidence. Only then can they accept the explanation that A and B experienced different amounts of gravitational time dilation.

(And you are still free to try to construct a completely different theory if you want to... but given the experience of the past century with relativity, this is unlikely to be a good use of your time).

 

[Dale]

B would now say that he measured the time it took for the photon to reach earth, to 10 second less than A measured.

B would therefore claim that either the speed of light must have been traveling faster than 299 792 458 m / s, or local distance (where B is) must be stretching or contracting proportional with the stretch of time.

Which option is correct, and what proves it ?

Why would he make either claim? Depending on personal inclination and unspecified details B could also attribute the difference to:
A different simultaneity convention
Different paths of the photons through spacetime
Different paths of A and B through spacetime
A's clock is wrong
B's clock is wrong
Etc.

The math can be interpreted many ways, but the important thing is that GR makes correct predictions about experimentally measured results.

 

[Bjarne]

Length contraction.
.

B is counting 10 second less, and the explanation is "length contraction" of the distance to the source that 10 billion years ago emitted the photon, according to GR.

I guess that according to GR this must mean that 1 local meter by B (in the cellar) is longer, compared to 1 meter at the top of the skyscraper. And this is why the distance B would measure is shorter (contracted).
I mean in reality the distance to the source (10 billion km away) is not changing, - it would be illogical if it was, - but what instead happens is that B experience some kind of optical change due to change of space time ( deformation of space)..

Or how can we be more specific to understand “length contraction” in this case?

 

[russ_watters]

Saying "optical change" implies to me that you think it is an illusion. It isn't. Length is just different for different observers, just like time and speed...and even color.

 

[Bjarne]

Saying "optical change" implies to me that you think it is an illusion. It isn't. Length is just different for different observers, just like time and speed...and even color.

Right, and as I wrote this must mean that 1 meter is not the comparable same for A and B (?)

 

[Dale]

Depends how you do the comparison. This is something that you didn't clearly specify in your previous thread either.

 

[Bjarne]

Depends how you do the comparison. This is something that you didn't clearly specify in your previous thread either.

This is off course not possible to confirm because A and B cannot be both at the top of the building and in the cellar of the skyscraper to compare whether 1 meter is the comparable same, - in these 2 different space time realities. Anyway that should not mean this is a dead end. What is wrong by chosen the most logical conclusion..(?)

We know speed of light is the "same" for both A and B.

We also know speed for both A and B

Speed * Time = Distance.

The result is that the time it took the 2 photons to reach the skyscraper is 10 less second seen from B’s perspective compared to A’s observation..

The first simple mathematical conclusion is therefore that length cannot be the same.

But the most logical conclusion must be the 2 photons must be assumed to have hit the same skyscraper “in the same moment” – I mean if both A and B would see the same supernova 10 billion LY away, - the light B would receive would not be delayed 10 seconds compared to A.
So fare I believe we are on safe ground.

Furthermore the most logical conclusion must also be that the “real distance” (doesn’t matter how it is measured and by who) – is NOT changing. I mean if I would take the elevator to the top of a building, - this is not changing the “real distance” to the Sun etc...

Based on such simple logical conclusions, the only way left, whereby distances in this case possible can be different, - is if the way they are measured, - which off course must mean 1 meter is not comparable the same, - for both A and B. (or ?)

 

[Dale]

This is off course not possible to confirm because A and B cannot be both at the top of the building and in the cellar of the skyscraper to compare whether 1 meter is the comparable same, - in these 2 different space time realities. Anyway that should not mean this is a dead end.

Leaving aside the nonsense about different realities, you have it completely backwards. The problem is not that it is impossible to make this comparison, it is the opposite. There are many ways to make the comparison, so you have to specify exactly how you plan on doing it. I suggested several different ways in our previous conversation.

The first simple mathematical conclusion is therefore that length cannot be the same.

I already proposed several alternative conclusions.

Based on such simple logical conclusions, the only way left, whereby distances in this case possible can be different, - is if the way they are measured, - which off course must mean 1 meter is not comparable the same, - for both A and B. (or ?)

The problem is that you think that this question is meaningful as posed, and it isn't. You must specify how you propose to compare the meter at A to the meter at B.

Btw, the whole discussion about the light from the star has little to do with the comparison of meters local at A and meters local at B since the bulk of the path of the light is not local to either A or B. It is a "red herring".

 

[Bjarne]

The problem is not that it is impossible to make this comparison, it is the opposite. There are many ways to make the comparison, so you have to specify exactly how you plan on doing it. I suggested several different ways in our previous conversation.

I am not sure whether I understand what you mean.
Regarding whether a deeper understanding of the “length contraction phenomena” - possible could be caused due to that 1 meter not is the comparable same for A and B (at the top of the skyscraper compared and at the cellar) ? - I cannot see any method to either confirm or reject this. Can you (really) ?
I think it is impossible to measure / prove, since the device to measure it cannot be in 2 different space-time realities at the same time, and this is as I see it necessary.

I suggested several different ways in our previous conversation.

Yes that’s right, a different thread. – But the thought-experiments was much too “complicated”
SR was overlapping GR , and I was more confused after the discussion as before.
The thought experiment in this threat is far more simple, - at least to my opinion.

I already proposed several alternative conclusions.

Right, you suggested…

1. A different simultaneity convention
2. Different paths of the photons through space-time
3. Different paths of A and B through space-time
4. A's clock is wrong
5. B's clock is wrong
6. Etc.

Sorry, by in my ears none of these sounds, - like the most logical conclusion.

Option 1 - A different simultaneity convention.
If you mean this should be responsible for the whole time-difference between A and B (?)
If so that would also mean that B would see photons from a supernova (billion LY away) several seconds true delayed compared to A.
Will A and B see see the 2 photon's in the same moment?, - I believe so. Disagreement must be relative insignificant compared to the measured time difference (by A and B)..
So, to a certain degree I guess YES, - but insignificant compared to the relative large time difference measured by A and B during a period of 10 billion years.

Option 2. - Different paths of the photons through space-time
To a certain degree YES, but insignificant compared to the relative large time difference measured by A and B during a period of 10 billion years.
I mean imagine a supernova not only was 10 billion LY away but for example 1.000.000.000LY (and the Universe is 'bit' older than now) .
2 photons leaving a supernova that long distance away, would not only cause few second difference but cause many hours’ time measurement difference (seen from A and B's perspective) .
I assume such 2 photons anyways would hit the skyscraper (top and cellar) at the same moment .
We are not only talking about split second differences, - but much more than that.
I cannot imagine that A can see the supernova from first floor, after that go to the top and here have to wait 1 hour before he can see it here too, - due to something serious had happen to the photon's path.

Option 3 - Different paths of A and B through space-time
To a certain degree YES, but insignificant compared to the relative large time difference measured by A and B.

It is the same kind of problem as option 2, - we are dealing with a relative large time difference, increasing during billions of years. - This should make it possible to ignore all insignificant factors.

Option 4+5
Wrong Clock’s ?
This aspect of GR / SR have past plenty test / GPS etc..

We can even simplify the thought-experiment further
Imagine one neutrino, traveling the same distance, first hitting the skyscraper from right above, (and detected at the upper floor,) and a split second later detected in the cellar.
Now relative motion of A and B, - or – “traveling by different paths” - cannot be significant options or at least very insignificant.

And still A and B would far from agree about the distance the neutrino had traveled since it started 10 (or 100.000.000) billion LY away.

So we are back where we started; - is it possible to reach a logical and deeper difination of ; what is length contraction ?
How many true options are there?
I mean several options must be possible, - logical – to shut out.

 

[nitsuj]

So we are back where we started; - is it possible to reach a logical and deeper difination of ; what is length contraction ?

Oh that's easy, it's width widening

A meter stick as measured by you (comparably at rest) rockets off a boomerang trajectory at a significant chunk of c.

on it's return towards you in impales your body; sorry just trying to bring these separate physical realities together.

how much length passes through you; a full meter?

 

[russ_watters]

Furthermore the most logical conclusion must also be that the “real distance” (doesn’t matter how it is measured and by who) – is NOT changing. I mean if I would take the elevator to the top of a building, - this is not changing the “real distance” to the Sun etc...

There is no such thing as one singular "real distance" just as there is no such thing as "absolute time". If I measure the Earth's distance to the sun and get 90 million miles and you measure it and get 80 million miles using a similar method but experiencing relativistic effects, both are "real".

 

[arindamsinha]

Would it be valid to explain it in this way?

• B is in higher gravity, so his 'seconds' are slightly longer than A's (say x%)
• For the same reason, B's 'meter' is also slightly longer than A's, and by the same proportion (x%)

Therefore when B computes the speed of light, which has units of 'meters' per 'second', the effects cancel out, and he gets the same numerical answer as A?

 

[Bill_K]

Spacetime is locally Minkowskian at every point. The local speed of light is the same everywhere. The diameter of a hydrogen atom is the same, and the frequency of its spectral lines are the same. So from B's local point of view, nothing has contracted, neither time nor space. And c is c.

The only question then is how A views B's experiments. He does so using his own Schwarzschild coordinates, r, t, θ and φ. A interprets the velocity of a light ray moving in the radial direction to be its coordinate velocity, dr/dt. In the azimuthal direction he interprets it as r dφ/dt. Neither of these is equal to c. Furthermore they are not equal to each other. A believes the speed of light down in the well is not c, and not isotropic either.

 

[Dale]

I cannot see any method to either confirm or reject this. Can you (really) ?
I think it is impossible to measure / prove, since the device to measure it cannot be in 2 different space-time realities at the same time, and this is as I see it necessary.

This is nonsense. In GR there are not two separate realities, there is only one. There is only one interpretation of one theory (the many worlds interpretation of QM) which contemplates multiple realities, and then the device is automatically in an infinite multitude of realities, not just one.

Yes that’s right, a different thread. – But the thought-experiments was much too “complicated”
SR was overlapping GR , and I was more confused after the discussion as before.
The thought experiment in this threat is far more simple, - at least to my opinion.

It is a complicated topic. I am sorry that you wound up confused, but I think that is largely because you are deliberately attempting to impose a non-mainstream viewpoint and are resistant to learning anything contrary to your preconceived notions.

Right, you suggested…

1. A different simultaneity convention
2. Different paths of the photons through space-time
3. Different paths of A and B through space-time
4. A's clock is wrong
5. B's clock is wrong
6. Etc.

Sorry, by in my ears none of these sounds, - like the most logical conclusion.

Nevertheless, they are valid and logically possible which means that we are not forced to make the conclusion you suggest.

Option 1 - A different simultaneity convention.
If you mean this should be responsible for the whole time-difference between A and B (?)
If so that would also mean that B would see photons from a supernova (billion LY away) several seconds true delayed compared to A.
Will A and B see see the 2 photon's in the same moment?, - I believe so. Disagreement must be relative insignificant compared to the measured time difference (by A and B)..
So, to a certain degree I guess YES, - but insignificant compared to the relative large time difference measured by A and B during a period of 10 billion years.

You acknowledge that the effect is present. That it is insignificant is a quantitative argument that needs to be supported mathematically.

Option 2. - Different paths of the photons through space-time
To a certain degree YES, but insignificant compared to the relative large time difference measured by A and B during a period of 10 billion years.
I mean imagine a supernova not only was 10 billion LY away but for example 1.000.000.000LY (and the Universe is 'bit' older than now) .
2 photons leaving a supernova that long distance away, would not only cause few second difference but cause many hours’ time measurement difference (seen from A and B's perspective) .
I assume such 2 photons anyways would hit the skyscraper (top and cellar) at the same moment .
We are not only talking about split second differences, - but much more than that.
I cannot imagine that A can see the supernova from first floor, after that go to the top and here have to wait 1 hour before he can see it here too, - due to something serious had happen to the photon's path.

Option 3 - Different paths of A and B through space-time
To a certain degree YES, but insignificant compared to the relative large time difference measured by A and B.

It is the same kind of problem as option 2, - we are dealing with a relative large time difference, increasing during billions of years. - This should make it possible to ignore all insignificant factors.

Again, you agree that the effects are present. To claim that they are insignificant is a quantitative argument requiring math.

Option 4+5
Wrong Clock’s ?
This aspect of GR / SR have past plenty test / GPS etc..

Yes. GPS in fact treats the orbiting clocks as wrong and corrects them.

So we are back where we started; - is it possible to reach a logical and deeper difination of ; what is length contraction ?
How many true options are there?
I mean several options must be possible, - logical – to shut out.

I agree, several are possible, which is why you must always specify the details.

Again, the light from the star is irrelevant to the comparison of the meters at A and B.

 

[Bjarne]

There is no such thing as one singular "real distance" just as there is no such thing as "absolute time".

Corrrect
I was just trying to explain that seen from any overall perspective - "the distance" - "it self" - (between the Earth and the distant star) is not changing, - (regardless which overall observer that would measure it), - the disagreement between A and B, is rather due to a "local" different space time deformation / reality perception.

If I measure the Earth's distance to the sun and get 90 million miles and you measure it and get 80 million miles using a similar method but experiencing relativistic effects, both are "real".

I fully agree to that.
But still the point is what happens to 1 meter, - I guess the same thing.

Let say A would use a lot of effort putting a measurement tape between the Sun and the Earth. Everybody could now see that the distance was exactly 1,5E11 meter, so as A predicted.

But B is skeptical, he would say, seen from his cellar perspective, - (due to length contraction), - the distance must be 10 meter less. Something very basic must be wrong with A’s measurement tape.

It must be crystal clear that B never would be able accept that A's total measurement to the Sun is correct.
If A's total measurement not is correct (seen from B's perspective), - he must most likely also have failed to measure 10% of the path to the Sun, - and also failed to measure 1% of the path.

In the end of the day it comes down to that B would say to A, - I don't trust your ruler is exactly 1 meter. Prove it to me.

A's definition of 1 meter is the time interval of 1/299,792,458 of a second, - So what is exactly wrong with that?

I think the definition of one meter is always correct = 1/299,792,458 of a second
BUT it must be relative to the local time-dilation.
Which mean a short second = a short meter, - and a relative longer second = a proportional longer meter. That would be the most simple and logical way to exactly settle the disagreement.

Such conclution should not come as a surprise, because space is curving, - illustrated by the stretching rubber sheet.
So when space is ‘stretching’ towards a field of gravity, and time also, - I cannot see what prevent such logical conclusion, - There should as I see it not be any conflict to that

 

[nitsuj]

This is nonsense. In GR there are not two separate realities, there is only one.

Does this point have any purpose? It seems to me to be of an opinion of interpretation, for example with russ' one measures distance as 80 million miles an other measures the same points as 90 million miles. That is two separate physical realities. I can even do the math 90-80 = 10 see they're not the same length.

I appreciate the perspective that the interval is the same, and stands as a "true" measure of spacetime, in turn one physical reality. But i don't think that is your point with calling different measures of length time of the same physical reality. To reword my point of view, the Twins are different ages, at one point they experienced different physical realities...comparably.

 

[Dale]

Corrrect
I was just trying to explain that seen from any overall perspective - "the distance" - "it self" - (between the Earth and the distant star) is not changing, - (regardless which overall observer that would measure it),

This is not correct. There is no such thing as the distance "itself". There is only the distance according to a specified reference frame/coordinate system. And that does change wrt different systems.

But still the point is what happens to 1 meter, - I guess the same thing.
...
In the end of the day it comes down to that B would say to A, - I don't trust your ruler is exactly 1 meter. Prove it to me.

Again, the answer depends on the method of comparison. What experiment would B accept as a method of comparison?

I don't know why this concept is so hard for you to understand. I thought you understood it after our previous conversation on the same topic.

 

[Dale]

Does this point have any purpose? It seems to me to be of an opinion of interpretation, for example with russ' one measures distance as 80 million miles an other measures the same points as 90 million miles. That is two separate physical realities. I can even do the math 90-80 = 10 see they're not the same length.

I appreciate the perspective that the interval is the same, and stands as a "true" measure of spacetime, in turn one physical reality. But i don't think that is your point with calling different measures of length time of the same physical reality. To reword my point of view, the Twins are different ages, at one point they experienced different physical realities...comparably.

No, they are not different realities, they are merely different perspectives on the same reality. The point is that there is no such thing as distance by itself. Only distance relative to some coordinate system. So the fact that distance relative to A is not equal to distance according to B does not imply multiple realities.

Even in non relativistic physics this happens with some quantities. Would you say Newtons universe implies a different reality for each Newtonian reference frame?

 

[nitsuj]

The point is that there is no such thing as distance by itself. Only distance relative to some coordinate system.

Even in non relativistic physics this happens with some quantities. Would you say Newtons universe implies a different reality for each Newtonian reference frame?

I wouldn't, and that's a good point Dalespam regarding lengths. Now I can't say two separate physical realities

 

[Bjarne]

No, they are not different realities, they are merely different perspectives on the same reality. The point is that there is no such thing as distance by itself. Only distance relative to some coordinate system. So the fact that distance relative to A is not equal to distance according to B does not imply multiple realities.

Even in non relativistic physics this happens with some quantities. Would you say Newtons universe implies a different reality for each Newtonian reference frame?

I both agree and disagree.
On the one hand we can easy end up to have unlimited "space-time realities", that could be a confused problem to deal with.
On the other hand, - if you say, “they are merely different perspectives on the same reality” – which “space-time reality” is the basic one then ? - that too could be confused to answer , right ?

This is not correct. There is no such thing as the distance "itself".

Right, this is why I wrote "itself" and not itself. - implying there are several (overall) perspectives possible.

There is only the distance according to a specified reference frame/coordinate system. And that does change wrt different systems.

Right.

Again, the answer depends on the method of comparison.
What experiment would B accept as a method of comparison?

I don't think any direct method exists. If so, - I would be happy to hear about it. Left are only one (or several) logical conclusion(s).

I don't know why this concept is so hard for you to understand.
I thought you understood it after our previous conversation on the same topic.

The discussion, whether there was comparable differences between a ruler in different space-time, - ended unsettle, - because how would it finally be possible to prove what is right or wrong.
Well I agree to that, but that shouldn’t prevent us from asking; - what is the most logical conclusion?.
So long we haven’t fully understood such important consequence of the theory of relativity, - we cannot say that anyone fully have understood the full range of that theory.

 

[HomogenousCow]

Light moves on geodesics with null magnitudes, that is all that needs to be said

 

[nitsuj]

I both agree and disagree.
On the one hand we can easy end up to have unlimited "space-time realities", that could be a confused problem to deal with.
On the other hand, - if you say, “they are merely different perspectives on the same reality” – which “space-time reality” is the basic one then ? - that too could be confused to answer , right ?

I see where you coming from,

The interval is the same, that's invariant and is a "measure" of the two "different realities".

we can have different perspectives of that same interval, such as length. A different perspective doesn't necessitate a different reality.

such as orientation of a pencil in 3D, is it shorter if I look at it from a different angle than you? Is that then a separate reality.

 

[Bjarne]

I see where you coming from,

The interval is the same, that's invariant and is a "measure" of the two "different realities".

we can have different perspectives of that same interval, such as length. A different perspective doesn't necessitate a different reality.

such as orientation of a pencil in 3D, is it shorter if I look at it from a different angle than you? Is that then a separate reality.

We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly.
Quote, - Niels Bohr

 

[nitsuj]

We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly.
Quote, - Niels Bohr

...

 

[pervect]

That's far from my favorite Bohr quote. My favorite would be "Never express yourself more clearly than you can think."

Going back to the original question, the modern definition of the meter is, in fact, 1/299,792,458 of a second.

With this modern defintion, the speed of light is no longer measured, it's defined to be a constant. Anyone who is using the modern defintion of the meter rigorously would talk about measuring the length of the prototype meter bar, rather than measuring the speed of light.

As far as being "relative to local time dilation", there's nothing in the standard about that, and it seems slightly "off" to me as written.

If instead one says "the meter is measured by local clocks" I would completely agree, and not have lingering reservations.

Time dilation is something that enters only when you define a coordinate system. One can (and should) think of distance as something that's independent of coordinates, therefore it's in my opijnion not good to include a coordinate correction factor into one's notion of distance when the notion of distance is defined independently of coordinates.

 

[Dale]

I both agree and disagree.
On the one hand we can easy end up to have unlimited "space-time realities", that could be a confused problem to deal with.
On the other hand, - if you say, “they are merely different perspectives on the same reality” – which “space-time reality” is the basic one then ? - that too could be confused to answer , right ?

It's a good question, and there is no confusion at all to the answer. According to relativity they are all equally valid and none is singled out as more basic than the others.

I don't think any direct method exists. If so, - I would be happy to hear about it. Left are only one (or several) logical conclusion(s).

Several methods exist, but I don't know which you might characterize as direct.

The discussion, whether there was comparable differences between a ruler in different space-time, - ended unsettle, - because how would it finally be possible to prove what is right or wrong.

Easy, pitck a method for comparison and do the comparison.

Well I agree to that, but that shouldn’t prevent us from asking; - what is the most logical conclusion?.
So long we haven’t fully understood such important consequence of the theory of relativity, - we cannot say that anyone fully have understood the full range of that theory.

At this point it isn't a flaw in the theory, just a failure by one individual, yourself, to clarify which of many length comparison methods he will use.

 

[nanosiborg]

Have enjoyed reading this thread so far. I've mulled over similar considerations as Bjarne's.

Especially liked pervect's quoting of Bohr: "Never express yourself more clearly than you can think." Hadn't heard that one before. I consider Bohr to be sort of the Yogi Berra of physics.

 

[Bjarne]

It's a good question, and there is no confusion at all to the answer. According to relativity they are all equally valid and none is singled out as more basic than the others.

Yes this is true…
But on the other hand, our reality begins (or end, - if you prefer) where the photons reality begin. From the perspective of the photon there can (mathematically) not exist distances, time, and I guess even not space, - it must be nothing.
To be extreme “reality” as a whole includes nothing and everything between nothing and everything, depending on space-time 'perspective'
It is at least attempting to talk about “different space-time realities” – because "they" can be so diffrent, - but is as you point out also not perfect to do so, - because "these" are not clear separated but rather “one” reality - ("seperated" by time, - at least photons, neutrinos etc. seems to be) and as you see now I contradict what I Just wrote. .
"Never express yourself more clearly than you can think."
Hmmmm... Maybe Bohr had Einstein in his mind.

"We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly. "
Hmmmmm

We can measure a photon existence, and the speed of it, - but seen from the photons own perspective, it doesn’t exist (in space and time) because by the speed c, space-time doesn’t exist.

What is the ultimate relality ?

Who is not confused ?

At this point it isn't a flaw in the theory, just a failure by one individual, yourself, to clarify which of many length comparison methods he will use.

As I see it, I cannot see any direct experiment is possible, to reveal that.
As I wrote; only logical indirect conclusion.
How can A prove to B that his definition of one meter is "correct" (meaning universal).
The problem is A is still in top of the skyscraper and B in the cellar.
Where must the comparison take place?
I simply have no idea, and don’t think such experiments are done already.
Should I be wrong I would like to hear about it.

 

[Dale]

Yes this is true…
But on the other hand, our reality begins (or end, - if you prefer) where the photons reality begin. From the perspective of the photon there can (mathematically) not exist distances, time, and I guess even not space, - it must be nothing.
To be extreme “reality” as a whole includes nothing and everything between nothing and everything, depending on space-time 'perspective'
It is at least attempting to talk about “different space-time realities” – because "they" can be so diffrent, - but is as you point out also not perfect to do so, - because "these" are not clear separated but rather “one” reality - ("seperated" by time, - at least photons, neutrinos etc. seems to be) and as you see now I contradict what I Just wrote. .

This is so self contradictory that it doesn't even rise to the level of philosophy, let alone science.

We can measure a photon existence, and the speed of it, - but seen from the photons own perspective, it doesn’t exist (in space and time) because by the speed c, space-time doesn’t exist.

https://www.physicsforums.com/showthread.php?t=511170

As I see it, I cannot see any direct experiment is possible, to reveal that.
As I wrote; only logical indirect conclusion.
How can A prove to B that his definition of one meter is "correct" (meaning universal).
The problem is A is still in top of the skyscraper and B in the cellar.
Where must the comparison take place?
I simply have no idea, and don’t think such experiments are done already.
Should I be wrong I would like to hear about it.

Here I post, for reference, a relevant answer for comparing clocks from our previous conversation. With a small amount of thought you should be able to turn those into methods for comparing distances instead.

http://74.86.200.109/showpost.php?p=3783933&postcount=151

The result you get, as I have said many times, depends on the method of comparison, and GR can correctly predict the result for each method.

 

[Bjarne]

This is so self contradictory that it doesn't even rise to the level of philosophy, let alone science.

It was intended. There was no conclusion, - it should not make sense.

https://www.physicsforums.com/showthread.php?t=511170

Which mean a mathematical "reality" contradict logical rational thinking, - and doesn’t make logical sense (?). I can only agree to that.

Here I post, for reference, a relevant answer for comparing clocks from our previous conversation. With a small amount of thought you should be able to turn those into methods for comparing distances instead.
https://www.physicsforums.com/showthread.php?t=511170... (copy past)...>>
Sure, I can see at least 4 ways to compare two distant clocks:
A) Broadcast a reference signal, measure the frequency of the signal locally at each clock
B) Take a reference clock, physically transport it from one clock to the other and measure the rate of the reference locally at each clock
C) Agree on a standard physics experiment as a reference, perform it locally at each clock and measure the time for the experiment
D) Agree on an astronomical reference and measure the time for the astronomical reference locally

The beauty of GR is that it is a single law of physics which explains A, B, C, and D all together

I have read it long ago.
These experiments are already (more or less) done, and with reference to the context of this thread to my opinion only “indirect scientific evidence”.
I don’t think this is enough to finally determinate, whether the definition of 1 meter is universal or not? – What do you think?

The result you get, as I have said many times, depends on the method of comparison, and GR can correctly predict the result for each method.

Right
But what about the question; is the definition of 1 meter (based on the current definition of 1 second) universal or not? – Or must the definition be “floating” and only true when it is be based on a relative local stretching or contracting “second” ? – What exactly do you think.

 

[HomogenousCow]

This thread is very lacking in any quantitative discussion, and most of it is talk of meta-physical ideas such as "reality" and what not.

 

[Drakkith]

But what about the question; is the definition of 1 meter (based on the current definition of 1 second) universal or not? – Or must the definition be “floating” and only true when it is be based on a relative local stretching or contracting “second” ? – What exactly do you think.

I think every time you measure a meter it will be the same length. An objects physical dimensions may change from the point of view of your frame of reference due to relativistic factors, but you will never measure a meter as being different.

 

[Bjarne]

This thread is very lacking in any quantitative discussion, and most of it is talk of meta-physical ideas such as "reality" and what not.

The right brain sees the forest, but not the (quantitative) trees (it is not important), - the left brain see the trees very detailed, but not the forest. The confused brain that try to see the forest cannot because there are too many (quantitative) trees.