Are laws of nature really the same in all reference frames?

[darkhorror]

I have not invented how to calculate the orbit speed of the MW
So how was it done then ?

Observations and calculations.

How it was measured has no bearing on what I was talking about.

I am trying to figure out what you mean to say or what you are thinking. Here are a couple questions for you.

Does it matter what frame of reference you are talking about when you say Earth is moving at 250km/s? Or are we simply moving at that velocity no matter what frame of reference we take?

 

[Bjarne]

Observations and calculations.

How it was measured has no bearing on what I was talking about.

So how would A and B measure the distance of the Milky way ?
We can say A is you and B have a slower ticking clock, due to gravity, we could say that A live in a skyscraper, and B in a cellar etc. Or we can use the example above.

I am trying to figure out what you mean to say or what you are thinking. Here are a couple questions for you.

Does it matter what frame of reference you are talking about when you say Earth is moving at 250km/s? Or are we simply moving at that velocity no matter what frame of reference we take?

The orbit speed round the Milkyway is the same for both observes (A and B).
Hence it doesn’t matter

 

[Bjarne]

In GR, there is really no such thing as a global inertial frame of reference.

I have heard that before, but it is irrelevant according to the example, since this is not what the example conclude. All questions are open. So far there are no mathematical / logical explanations.

You are actually trying to selectively use SR for one purpose and GR for another.

No, - I am only distinguishing between the cause of time-difference between the 2 observers, and whether the cause is SR or GR.

The scenarios I have shown SR do not apply since speed of both observers is the same.
Both clocks orbit the MW in the same radius.
The cause of time difference between the 2 observers is in this case is only caused due to gravity (GR) (General Relativity).

Specifically, in a situation where you can detect time difference due to gravity, and where inertial paths in spacetime are observed over a whole spiral (that's how an orbit looks in spacetime), you really can't model this in terms of a global inertial frame centered on an orbiting body.

I am not suggesting "a global inertial frame" but only asking simple questions.

We can assume that A’s reality is exact the same as ours.
Hence we know everything about A’s reality, as well how A has come to all his conclusions, - for example the circumstance of the Milkyway etc..

But when a clock is ticking slower due to the gravity of the Sun, for another observer (we call that observer B,) - then it is simple calculation that A’s reality cannot be valid for B.

The answer must be that A‘s reality must be exactly so real as B’s, and therefore we also must know how is comparable distances speed etc or B.

For example;
How is speed for B ?
How is comparable speed between A and B ?
What is the distance of the MW-orbit for B ?
How is the comparable distance between of the MW orbit for A and B ?
Is B's meter stick comparable smaller than A's – or longer – or the same length ?
Is a meter stick comparable smaller – or longer – or the same length, at the surface of the Sun compared to 100 billion km away, - du to gravity (GR) ?
seen from B’s reality

Further to, maintain the paths you describe, none of your bodies (except the sun) are following inertial paths.

I do not understand , above I wrote that we assume these clock's does, we assume these have small rocket engines on board and therefore counteracts the gravity of the Sun, - (but not counteract the time difference). and therefore orbits the MW without getting attracted to the Sun.

Hence 2 clocks orbit the Milkyway exactly as the Sun does.

The difference is that one clock (B) is closer to the sun and therefore ticking slower than the other clock (A).

This certainly , logically and mathematical MUST have SIMPLE consequence(s), since 1 complete orbit round the Milkyway take less time for B as it does for A.

Time multiplied with speed = distance.

Try instead exactly to answer some or all the questions.
There must certainly be “consequences” of time dilation.

There are no reasons to make the simple scenario more difficult or impossible as necessary.

In a nutshell, every aspect of your scenario is ill conceived.

Then the Universe must also be such ill place, since time really is ticking slower due to gravity and that must have consequences .

“If you can't explain it simply, you don't understand it well enough”.
Albert Einstein

 

[DrGreg]

Asking for an answer in terms of GR-without-SR is meaningless. No such theory exists. The only theories available are either SR alone (without gravity), or else GR+SR (with gravity).

The scenarios I have shown SR do not apply since speed of both observers is the same.

This is not something that all observers agree to.

According to each of your two observers, the velocity of the other observer is zero. But according to a third observer who is falling freely directly towards the Sun, the two observers are not a fixed distance apart. According to this third observer, who within GR is an inertial observer (and the other two are not inertial), Lorentz contraction causes their distance to keep changing, i.e. they are not both moving at the same speed according to the third observer. According to this 3rd observer, this relative motion is the cause of the time dilation between the first two observers.

 

[PAllen]

No, - I am only distinguishing between the cause of time-difference between the 2 observers, and whether the cause is SR or GR.

This is not possible.

I am not suggesting "a global inertial frame" but only asking simple questions.

Yes, you are asking about global coordinates but you don't realize it.

You are asking about analyzing the motion of the MW center from A or B point of view. This requires two global coordinate systems. Each of these coordinate systems is built from a non-inertial frame because thrust would be required to hold A and B in position against the Sun's gravity. They are different non-inertial frames because each would require a different amount of thrust.

Further, from A and B point of view the laws to be applied would be determining the elliptical motion of MW center around each of them. For each, this would be a complex application of GR field equations in a rather complex coordinate system. They would still find that their application GR would be successful.

You can pretend the problem is simple only by ignoring its essential features, and misapplying relativity.

Einstein also said: be as simple as possible, but no simpler.

 

[Bjarne]

Yes, you are asking about global coordinates but you don't realize it.

You are asking about analyzing the motion of the MW center from A or B point of view. This requires two global coordinate systems. Each of these coordinate systems is built from a non-inertial frame because thrust would be required to hold A and B in position against the Sun's gravity. They are different non-inertial frames because each would require a different amount of thrust.
.

Right, - I remember
We will call the third observer C
The problem that C also cannot agree how "comparable distances" (between A and B) is doesn’t make the problem smaller, does it?
Still we have time rate difference, that must mean 2 (real) and different realities.
The many questions (concerning A and B) mentioned above is still unanswered.
Answering these must be possible also even though SR and GR are "connected"

 

[PAllen]

Well, the reality is roughly as follows.

We don't have to verify physical predictions in A coordinates and B coordinates would come out the same because this follows purely from mathematical definitions and theorems. However, doing physical computations in these coordinates would be a bummer (the metric tensor would have a very complex form). So, confident of pure math, A and B would each translate their local measurements to a convenient coordinate system (MW central frame, presumably). This translation would be based on relativity. They would have to translate locally measured times, distances, angles, and red/blueshift. Once they translated their local measurements, they would compute in the MW center frame.

And no, I have no interest in carrying this out with your numbers.

[Addendum: A and B can determine how to transform their local measurements with local measurements plus GR. They measure their proper acceleration with an accelerometer. This along with a bunch of local redshift and other astronomic measurements give enough information. ]

 

[Bjarne]

Asking for an answer in terms of GR-without-SR is meaningless. No such theory exists. The only theories available are either SR alone (without gravity), or else GR+SR (with gravity).This is not something that all observers agree to.

According to each of your two observers, the velocity of the other observer is zero. But according to a third observer who is falling freely directly towards the Sun, the two observers are not a fixed distance apart. According to this third observer, who within GR is an inertial observer (and the other two are not inertial), Lorentz contraction causes their distance to keep changing, i.e. they are not both moving at the same speed according to the third observer. .

We will call the third observer C.

When C is falling towards the sun and first passing A and next B he would off course accelerate faster (due to acceleration of the Sun) when passing B as he would when passing A.

So for C it would look like B is moving faster away from C than A.

But in fact C is moving relative to A and B, - and not A and B relative to C.

I mean any other observer as C (on the Earth or other places in the Universe) would not see that A or B is moving away from the sun, (or away from each other) but only that C is moving towards the sun.

According to this 3rd observer, this relative motion is the cause of the time dilation between the first two observers

I can’t see there really is "relative motion" between A and B ?

How can the reality (an illusion) of the third observer C have anything to do with the time-rate for A and B ?

 

[zonde]

No, - I am only distinguishing between the cause of time-difference between the 2 observers, and whether the cause is SR or GR.

This is not possible.

It is possible to take gravitational time dilation as experimentally verified physical fact without any reference to SR or GR.

You are asking about analyzing the motion of the MW center from A or B point of view. This requires two global coordinate systems. Each of these coordinate systems is built from a non-inertial frame because thrust would be required to hold A and B in position against the Sun's gravity. They are different non-inertial frames because each would require a different amount of thrust.

Further, from A and B point of view the laws to be applied would be determining the elliptical motion of MW center around each of them. For each, this would be a complex application of GR field equations in a rather complex coordinate system. They would still find that their application GR would be successful.

Nobody is doing it like you describe. This is simply crap.
Orbital speed can be calculated by simple formula when ellipticity is zero (circular orbit).

You can pretend the problem is simple only by ignoring its essential features, and misapplying relativity.

Why are you inventing problems that are not there? Are you trying to confuse Bjarne? Why?

 

[PAllen]

It is possible to take gravitational time dilation as experimentally verified physical fact without any reference to SR or GR.

But that's not relevant to how I understand the Bjarne's confusion. In fact, he understands that and that is source of his confusion: "How come A and B, using their raw measurements, come up with different results? Aren't they supposed to be the same? " He is questioning in what sense there is 'relativity' between A and B, where each can directly use their measurements and find equivalent results.

Nobody is doing it like you describe. This is simply crap.
Orbital speed can be calculated by simple formula when ellipticity is zero (circular orbit).

I agree nobody would actually do it like that (as I described in another post). However, this is the only sense in which one can talk about applying the same laws to the raw measurements by A and B. I was trying to get across that in going from 'free falling enclosed labs' to global measurements by non-inertial observers, the statement the 'laws of physics are the same for all observers' takes on a more complex, less useful form. The same laws apply only if expressed in general tensor form. Otherwise, in practice, you correct measurements to do computations in a convenient coordinate system where the expression of the laws is simplest.

Why are you inventing problems that are not there? Are you trying to confuse Bjarne? Why?

No, I am trying to directly address where I think his confusion is leading to incorrect expectations.

 

[zonde]

But that's not relevant to how I understand the Bjarne's confusion. In fact, he understands that and that is source of his confusion: "How come A and B, using their raw measurements, come up with different results? Aren't they supposed to be the same? " He is questioning in what sense there is 'relativity' between A and B, where each can directly use their measurements and find equivalent results.

I agree nobody would actually do it like that (as I described in another post). However, this is the only sense in which one can talk about applying the same laws to the raw measurements by A and B. I was trying to get across that in going from 'free falling enclosed labs' to global measurements by non-inertial observers, the statement the 'laws of physics are the same for all observers' takes on a more complex, less useful form. The same laws apply only if expressed in general tensor form. Otherwise, in practice, you correct measurements to do computations in a convenient coordinate system where the expression of the laws is simplest.


No, I am trying to directly address where I think his confusion is leading to incorrect expectations.

Ok, how the question went?

First, the setup for consideration is such that we can investigate gravitational time dilation with other things unchanged.
Yes we can do that in physically meaningful way exactly as Bjarne described. And that's the right approach to understand something. Isolate that one factor as much as possible. That is exactly the thing that you do in real experiments.

Second, observers make astronomical observations about their movement relative to the center of MW and the distance to the center of MW.
Again, yes we can do that and we don't have to factor out anything related to our gravitational acceleration.
Astronomers on surface of Earth (gravitationally accelerated frame) perform observations all the time and the only thing they factor out is aberration but that's velocity not acceleration effect.

Third, we compare results for two observers.


So far question (actually background for question) is formulated in physically meaningful way. Do you agree with that?

 

[PAllen]

Ok, how the question went?

First, the setup for consideration is such that we can investigate gravitational time dilation with other things unchanged.
Yes we can do that in physically meaningful way exactly as Bjarne described. And that's the right approach to understand something. Isolate that one factor as much as possible. That is exactly the thing that you do in real experiments.

Second, observers make astronomical observations about their movement relative to the center of MW and the distance to the center of MW.
Again, yes we can do that and we don't have to factor out anything related to our gravitational acceleration.
Astronomers on surface of Earth (gravitationally accelerated frame) perform observations all the time and the only thing they factor out is aberration but that's velocity not acceleration effect.

Third, we compare results for two observers.


So far question (actually background for question) is formulated in physically meaningful way. Do you agree with that?

No, not completely. My understanding of Bjarne's issue is that time measurements will be different (they set up the idea that they were sensitive to the time difference between their A and B). So measurements sensitive to time will be different. At this level of sensitivity, doppler measurements will also be affected. Then, of course, aberration will apply (angles, as I called it in one of my posts). My understanding of Bjarne's thesis was that all of this violated the idea of laws being the same for different observers. If you used these measurements to directly compute a distant velocity, it would come out different.

I wanted to focus on this being a false expectation. That it is expected that different observer's measurements may differ. And that for arbitrary observer's, the only form 'laws being the same' that you can expect is if they are expressed in general tensor form.

 

[Bjarne]

All what we can be 100 % sure about is;

1. That the 2 clock will tick different, - experience shows this.
2. Both clocks will complete the orbit of the Milkyway in the exact same period (according to the scenario mention above).
3. And because of that either speed or distance must be different from the perspective of A as it is for B.

I see no reason to complicate that more than this.

I believe we first at all only need to look at this purely / simple mathematical.

Speed multiplied with time = distance, - this must be true both for A as well for B, - since there is no reason to believe that only our ( or A’s) reality is more true like others.

This leaves us with 2 simple mathematical options;

Option 1.
The orbit speed for the 2 clocks (and the Sun) must seen from the perspective of observer B be faster as for observer A. - (I don’t believe in that option, - since A and B do not change distance between them).

Option 2
B's meter stick is longer and therefore distances shorter. - This is the only explanation I can accept so far.

As I understand relativity; - there are no certain reality (also not ours), since time (and probably also “size / distance”) not is comparable the same.
I think before accelerating the speculation into too much complexity we should try to look at the must simple level, as just shown, and finish here first.

 

[PAllen]

All what we can be 100 % sure about is;

1. That the 2 clock will tick different, - experience shows this.
2. Both clocks will complete the orbit of the Milkyway in the exact same period (according to the scenario mention above).
3. And because of that either speed or distance must be different from the perspective of A as it is for B.

I see no reason to complicate that more than this.

I believe we first at all only need to look at this purely / simple mathematical.

Speed multiplied with time = distance, - this must be true both for A as well for B, - since there is no reason to believe that only our ( or A’s) reality is more true like others.

This leaves us with 2 simple mathematical options;

Option 1.
The orbit speed for the 2 clocks (and the Sun) must seen from the perspective of observer B be faster as for observer A. - (I don’t believe in that option, - since A and B do not change distance between them).

Option 2
B's meter stick is longer and therefore distances shorter. - This is the only explanation I can accept so far.

As I understand relativity; - there are no certain reality (also not ours), since time (and probably also “size / distance”) not is comparable the same.
I think before accelerating the speculation into too much complexity we should try to look at the must simple level, as just shown, and finish here first.

It is easy to see that the most direct interpretation is that speed of distant objects will be greater for the one closer to the sun. This simply follows from direct interpretation of doppler - they will be slightly more blue shifted. Thus, if interpreted without adjustment, distant objects should be viewed as going slightly faster than the 'further from sun' observer would conclude. This is consistent with the slower time, obviously. Measurements by other methods would be expected to generally agree, but not necessarily be exactly the same. One key point is that it is only in flat spacetime, for inertial frames, that all reasonable ways of measuring a distant velocity or large distance will agree. For your observers (non inertial, curvature present), if they interpret their measurements as if they were inertial, Minkowski, observers, different measurement methods for distant observations will disagree.

 

[Bjarne]

It is easy to see that the most direct interpretation is that speed of distant objects will be greater for the one closer to the sun.

No
According to the example the 2 clock counteracts gravity from the Sun, - (they have small racket engine on board).

This simply follows from direct interpretation of Doppler - they will be slightly more blue shifted.

No there will be no Doppler effect due to speed difference of the 2 clocks; both clocks orbit with the exact same orbit-speed as the Sun, and with the exact same radius to the center of the Milkyway

This is consistent with the slower time, obviously.

You may have misunderstood that both clocks follows the orbit of the Sun, and is not approaching the Sun due to the gravity of the Sun, - because that gravity attraction is counteracted (by small rockets on board on the 2 clocks) . Therefore both clocks and the sun move in the exact same orbit around the Milky Way.

Measurements by other methods would be expected to generally agree, but not necessarily be exactly the same.

The 2 clocks (observers) are certainly meassuring different time-rate and can't therefore agree how distance (or speed) around the Milkyway.

All other observers (in the Universe) will observe that the Sun and the 2 clocks are completing 1 orbit of the Milkyway in the exact same period of (their) time.
This will not affect the motion (or time-rate) of the 2 clocks..

 

[zonde]

No, not completely. My understanding of Bjarne's issue is that time measurements will be different (they set up the idea that they were sensitive to the time difference between their A and B). So measurements sensitive to time will be different. At this level of sensitivity, doppler measurements will also be affected. Then, of course, aberration will apply (angles, as I called it in one of my posts).

Certainly different time rate will affect measurements. And that's the point of example.
I think that aberration can be factored out if both observers translate their observations to rest frame of MW mass center. After they do that they should point in the same direction as where is MW mass center.

My understanding of Bjarne's thesis was that all of this violated the idea of laws being the same for different observers. If you used these measurements to directly compute a distant velocity, it would come out different.

Well, yes, there are problems Bjarne's position but my point was that you where adding confusion to the problem and not making it clearer.

I wanted to focus on this being a false expectation. That it is expected that different observer's measurements may differ. And that for arbitrary observer's, the only form 'laws being the same' that you can expect is if they are expressed in general tensor form.

Generalized form might not be the best starting point for clearing confusions. Special simplified cases where you can replace tensors with simple specific transformations might be better.

 

[zonde]

Option 1.
The orbit speed for the 2 clocks (and the Sun) must seen from the perspective of observer B be faster as for observer A. - (I don’t believe in that option, - since A and B do not change distance between them).

It is a good idea to check your beliefs against observations in real world.
So it might be good to check out about http://en.wikipedia.org/wiki/Shapiro_delay"

 

[Bjarne]

It is a good idea to check your beliefs against observations in real world.
So it might be good to check out about http://en.wikipedia.org/wiki/Shapiro_delay"

I agree gravitational time delay is real.

 

[PAllen]

No



No there will be no Doppler effect due to speed difference of the 2 clocks; both clocks orbit with the exact same orbit-speed as the Sun, and with the exact same radius to the center of the Milkyway

Doppler may not have been the best word choice. I meant gravitational blue shift (sloppily, I sometimes use doppler for all kinds of frequency shift). All distant objects will appear slightly more blue shifted to the observer closer to the sun.

 

[PAllen]

Option 2
B's meter stick is longer and therefore distances shorter. - This is the only explanation I can accept so far.

.

Actually, along with gravitational time dilation, there is also gravitational length contraction. According to the 'further from sun' observer, the closer observer's rulers are slightly short, rather than long.

Be that as it may, there is straightforward way the two can observers agree on their speed relative to the milky way center. Suppose each adopts as their distance standard (converting other ways of measuring distance to far away object to match this standard) c times light round trip time to object as they measure it. Then the closer to sun observer thinks the MW center is closer (less time for the round trip). They then figure a smaller circumference for the orbit. They divide the smaller circumference by the shorter time, and come up with the same speed as the 'further from sun' observer.

 

[PAllen]

We will call the third observer C.

When C is falling towards the sun and first passing A and next B he would off course accelerate faster (due to acceleration of the Sun) when passing B as he would when passing A.

So for C it would look like B is moving faster away from C than A.

But in fact C is moving relative to A and B, - and not A and B relative to C.

I mean any other observer as C (on the Earth or other places in the Universe) would not see that A or B is moving away from the sun, (or away from each other) but only that C is moving towards the sun.



I can’t see there really is "relative motion" between A and B ?

How can the reality (an illusion) of the third observer C have anything to do with the time-rate for A and B ?

In GR, C is not accelerating at all. C is the inertial observer. A and B are accelerating at slightly different rates, as seen by C, their distance is shrinking over time (per C), they have a relative speed (per C). These are facts computable in special relativity alone (treating C as inertial, as required, and treating A and B as accelerating so as to keep distance constant per A. You can read all about this under the Bell spaceship 'paradox'. That A and B are the non-inertial observers is an objective, invariant fact - they experience a force that can be measured by an accelerometer, locally. C feels no force, therefore is inertial.

This concretely explains the idea that, within GR, there is no objective meaning to an SR effect versus a GR effect. Almost always, you can validly treat some effect as different mix of SR vs. gravitation effect by choosing different observers or coordinates.

There is yet another way to choose to treat gravitational time dilation as kinematic rather than gravitational (involving parallel transport of 4-vectors). However, I don't think you have the background for that.

 

[zonde]

I agree gravitational time delay is real.

Shapiro delay does not measure time delay. It measures changes in coordinate speed of light.
To make some statements about time delay in context of Shapiro delay you have to make some assumption about distance measurements. And this assumption is that distances stay the same. When you assume this then Shapiro delay agrees with expected time delay.

Or looking at this from another side. From Shapiro delay we find out that coordinate speed of light is different at different gravitational potentials.
Now if local speed standard (c) is different for two observers then speed measurements for the same (global) physical situation should be different for two observers. Just like it is with time.

You can try to make prediction for coordinate speed of light using your Option 2 (B's meter stick is longer and therefore distances shorter). What it will be?

 

[Bjarne]

Doppler may not have been the best word choice. I meant gravitational blue shift (sloppily, I sometimes use doppler for all kinds of frequency shift). All distant objects will appear slightly more blue shifted to the observer closer to the sun.

Gravitionel blue shift yes, but it has nothing with speed to do.

Actually, along with gravitational time dilation, there is also gravitational length contraction. According to the 'further from sun' observer, the closer observer's rulers are slightly short, rather than long.

Actually, along with gravitational time dilation, there is also gravitational length contraction. According to the 'further from sun' observer, the closer observer's rulers are slightly short, rather than long.

I can only understand it like that; if time is ticking slower, the meter stick must be proportional longer (for B) and distances hence seen from the perspective of B – shorter. But seen from a “outsider” distances is the same. Remember both observers complete the “same distance” seen from the perspective of observer C.

Be that as it may, there is straightforward way the two can observers agree on their speed relative to the milky way center. Suppose each adopts as their distance standard (converting other ways of measuring distance to far away object to match this standard) c times light round trip time to object as they measure it. Then the closer to sun observer thinks the MW center is closer (less time for the round trip). They then figure a smaller circumference for the orbit. They divide the smaller circumference by the shorter time, and come up with the same speed as the 'further from sun' observer.

Right but keep in mind that reality by your feet is not the same as by your head.
The meter stick cannot be the same comparable length both places, - can it ?

In GR, C is not accelerating at all.

Notice C is the third observer “invented” by DrGreg ( it is not “c” )
https://www.physicsforums.com/showpost.php?p=3543384&postcount=64
C is; “a third observer who is falling freely directly towards the Sun”. Off course C is then accelerating, due to acceleration due to gravity.

C is the inertial observer. A and B are accelerating at slightly different rates, as seen by C, their distance is shrinking over time (per C), they have a relative speed (per C). These are facts computable in special relativity alone (treating C as inertial, as required, and treating A and B as accelerating so as to keep distance constant per A. You can read all about this under the Bell spaceship 'paradox'. That A and B are the non-inertial observers is an objective, invariant fact - they experience a force that can be measured by an accelerometer, locally. C feels no force, therefore is inertial.

This concretely explains the idea that, within GR, there is no objective meaning to an SR effect versus a GR effect. Almost always, you can validly treat some effect as different mix of SR vs. gravitation effect by choosing different observers or coordinates.
There is yet another way to choose to treat gravitational time dilation as kinematic rather than gravitational (involving parallel transport of 4-vectors). However, I don't think you have the background for that.

As I see it and hopefully any other observer in the Universe, - C is really acceleration towards the Sun.
A and B is not affected due to the fact that C can have the illusion that it is A and B that is moving opposite.
I don’t understand the point.
C’s reality and the illusion that A and B is moving opposite, is not real for anyone else than C.
Why make a big point out of what only is an illusion. ?

You can try to make prediction for coordinate speed of light using your Option 2 (B's meter stick is longer and therefore distances shorter). What it will be?

Good exercise.
Let us now say that B’s clock tick half so fast like A’s (for simplicity reasons) - (still according to the example above) .
A and B would send a light beam to the same planet .
The light beam would reflect and return.
After the exact same period of time (seen by any external third observer “EX”) the light beam would return to both A and B.
Observer A would now say it took 1 (earth)-year, (31536000 s.)
But B would say it took half so much time.

Seen from observer EX perspective the distance the light was traveling to A and B is the exactly same.

The ONLY way both A and B can agree that the light was traveling with the “same” speed, is when B’s meter-stick is comparable double so long as A’s meterstick.
So simple is that.

This mean that speed is really “c” (300,000 km/h) seen from both the perspective of observer A , as well as from B’s reality.

BUT when you would compare the speed it would be a different history.

The only solution to that (as I can see) is that we cannot mix realities, but are forces to separate these.

And as I wrote this must mean a different comparable meter stick – that’s all, and the only simple mathematical solution.

Why not keep things simple, when they are simple?

 

[zonde]

Good exercise.
Let us now say that B’s clock tick half so fast like A’s (for simplicity reasons) - (still according to the example above) .
A and B would send a light beam to the same planet .
The light beam would reflect and return.
After the exact same period of time (seen by any external third observer “EX”) the light beam would return to both A and B.
Observer A would now say it took 1 (earth)-year, (31536000 s.)
But B would say it took double so much time.

Seen from observer EX perspective the distance the light was traveling to A and B is the exactly same.

The ONLY way both A and B can agree that the light was traveling with the “same” speed, is when B’s meter-stick is comparable double so long as A’s meterstick.
So simple is that.

Yes, that's correct.

The only problem is that if we would make prediction for Shapiro time delay it would be zero because coordinate speed of light does not change in your case.
And yet we observe slowing down of coordinate speed of light when signal passes gravitating object at close distance. So your model does not agree with observations.

So what we do next?

 

[Bjarne]

Yes, that's correct.
The only problem is that if we would make prediction for Shapiro time delay it would be zero because coordinate speed of light does not change in your case.

Right
Now we assume the meter stick always is comparable the exact same for both A and B.
Observer A and B will now in a certain period measure a photon traveling a certain distance (300,000 km).
Both observers agrees that this is what really happen.

Based on this observer A would say that the speed of light is exactly 300,000 km in one (of his) second.

But observer B would say OK I agree the distance the photon was traveling is 300,000 km ...
BUT I do not agree it took one second, - my clock shows it only took ½ second, so here the speed of light is 600,00km/s
Do you prefer that solution?
Hmmm… So what we do next?

And yet we observe slowing down of coordinate speed of light when signal passes gravitating object at close distance. So your model does not agree with observations.

I have never heard about Shapiro time delay. If it really is certain and confirmed knowledge, and not something only at a test level, - yes we have a one more problem/challenge..

So what we do next?

Hmmm speculate, but not too loud, suggestion could be wrong, and we would look stupid.
What do you think the answer is (except that distances / the meter stick always are comparable the same lenght) ?

 

[PAllen]

Gravitionel blue shift yes, but it has nothing with speed to do.

The context was, suppose you don't know you're in a gravitational field, or what consequences that would have. What laws do you apply to your observations? If you apply either pure SR or Galilean physics (even accounting that you know you are accelerating), you would conclude different relative velocity for distant objects than you would if you were not subject to gravity (or subject to less gravity).

I can only understand it like that; if time is ticking slower, the meter stick must be proportional longer (for B) and distances hence seen from the perspective of B – shorter. But seen from a “outsider” distances is the same. Remember both observers complete the “same distance” seen from the perspective of observer C.

You claim to accept gravitation time dilation, per se. The same derivations that lead to it, also lead to gravitational length contraction. The thing that allows all of it to be consistent is that the observer that sees your clock slow and your rulers short also sees light going slower for you. When you put in the actual numbers, this observer 'understands' why you still measure the same value for light speed. (this comment was unrelated to observer C. It was in reference to how the 'outer' of A and B would view the inner. Observer C would be more complex, because they have relative motion to account for).

Right but keep in mind that reality by your feet is not the same as by your head.
The meter stick cannot be the same comparable length both places, - can it ?

The point is ultimately related to the fact that only for inertial observers in flat spacetime do you have the nice property that any 'reasonable' way of doing measurements comes out the same. For inertial observers in flat spacetime, radar ranging, parallax distance, luminosity distance, etc. all yield an equivalent distance scale. For either curved spacetime or non-inertial observers (even in flat spacetime), they disagree with each other. You can choose which to favor, getting different answers for where your results are 'unexpected'. My example shows, if you choose to favor radar ranging, you get shorter distances to remote objects, but the same speeds (well, there would be higher order differences, but let's not worry about that) for A and B.

There truly is no unique, preferred answer to large distances in GR (short of choosing a preferred global coordinate system). Actually, there isn't in SR either - distances are observer dependent.

Notice C is the third observer “invented” by DrGreg ( it is not “c” )
https://www.physicsforums.com/showpost.php?p=3543384&postcount=64
C is; “a third observer who is falling freely directly towards the Sun”. Off course C is then accelerating, due to acceleration due to gravity.


As I see it and hopefully any other observer in the Universe, - C is really acceleration towards the Sun.
A and B is not affected due to the fact that C can have the illusion that it is A and B that is moving opposite.
I don’t understand the point.
C’s reality and the illusion that A and B is moving opposite, is not real for anyone else than C.
Why make a big point out of what only is an illusion. ?

You are simply wrong here. In GR, all observers agree C is the inertial observer and A and B are the non-inertial observers. ('Accelerating', on the other hand, has very little relevance in GR if it is referring to coordinate acceleration, as you are; proper acceleration, computed in any coordinates, by any observer, says A and B are accelerating and C is not accelerting). On this, there is no 'relativity'. What is describe here is not an illusion at all, but the essence of relativity (note that C sees the distance between A and B shrinking over time; not sure you got that). The point (initially by Dr. Greg) is that the how much of an effect is related to gravity or SR effects is observer dependent. This is fundamental in GR, not an illusion to be ignored. And, in particular, for C, difference between A and B would be primarily the same effect as the SR bell spaceship 'paradox'.

 

[zonde]

Right
Now we assume the meter stick always is comparable the exact same for both A and B.
Observer A and B will now in a certain period measure a photon traveling a certain distance (300,000 km).
Both observers agrees that this is what really happen.

Based on this observer A would say that the speed of light is exactly 300,000 km in one (of his) second.

But observer B would say OK I agree the distance the photon was traveling is 300,000 km ...
BUT I do not agree it took one second, - my clock shows it only took ½ second, so here the speed of light is 600,00km/s
Do you prefer that solution?
Hmmm… So what we do next?

This is different from how Shapiro experiment was performed.
There is only one observer who is sending radar signals so that sometimes they are passing close to the Sun and sometimes far from the Sun. When you make a correction for time delay depending on signal's closest passing distance from the Sun you can consistently describe orbit of observed object (Venus).
In your case speed of light is always the same because proportion "m/s" does not change.

I have never heard about Shapiro time delay. If it really is certain and confirmed knowledge, and not something only at a test level, - yes we have a one more problem/challenge..

From Wikipedia about http://en.wikipedia.org/wiki/Shapiro_delay" :
"The time delay effect was first noticed in 1964, by Irwin I. Shapiro. Shapiro proposed an observational test of his prediction: bounce radar beams off the surface of Venus and Mercury, and measure the round trip travel time. When the Earth, Sun, and Venus are most favorably aligned, Shapiro showed that the expected time delay, due to the presence of the Sun, of a radar signal traveling from the Earth to Venus and back, would be about 200 microseconds,[1] well within the limitations of 1960s era technology.

The first tests, performed in 1966 and 1967 using the MIT Haystack radar antenna, were successful, matching the predicted amount of time delay.[2] The experiments have been repeated many times since then, with increasing accuracy."

Hmmm speculate, but not too loud, suggestion could be wrong, and we would look stupid.
What do you think the answer is (except that distances / the meter stick always are comparable the same lenght) ?

First of all speed of light globally is not the same everywhere.
Statement that "laws of physics are the same in all inertial reference frames" means that local experiments will give the same results. But global observations can be different.

 

[PAllen]

Right but keep in mind that reality by your feet is not the same as by your head.
The meter stick cannot be the same comparable length both places, - can it ?

I realize I didn't directly answer this question. In theory, a meter stick by your feet would be slight shorter than one by your head (not longer as you have argued several times). Light would be slower at your feet compared to your head. Clocks would be slower at your feet than your head. The only one of these that has been experimentally verified is the clocks, because they have reached the precision to detect differences over 6 feet in the Earth's field. The others will not be observable in the foreseeable future (of course, unforeseeable future could be only a few years away; never know when there is a breakthrough).

None of this is relates at all to the issue I was presenting (measuring distance over tens of thousands of light years using radar ranging distance as your definition, with other measuring methods calibrated to match). Especially because your own scenario had these measurements being done from lab held stationary (by thrust) with respect to the sun. Also, of course, there are no astronomic measurement that could be made at a precision where it mattered whether they were done at your head or your feet.

 

[Passionflower]

In theory, a meter stick by your feet would be slight shorter than one by your head (not longer as you have argued several times).

Your claim that meter sticks are shorter closer to the EH, could you back it up with some math or at least a reference? And shorter tangentially or radially, or perhaps both?

What I can show you mathematically is that both the volume and radial distance between two shells is more than we would suspect if we would calculate it based on their areas. And the discrepancy increases for lower r-values closer to the EH.

 

[PAllen]

Your claim that meter sticks are shorter closer to the EH, could you back it up with some math or at least a reference? And shorter tangentially or radially, or perhaps both?

What I can show you mathematically is that both the volume and radial distance between two shells is more than we would suspect if we would calculate it based on their areas. And the discrepancy increases for lower r-values closer to the EH.

Ah, but if delta r represents distance as perceived by an observer at infinity, and a local, stationary observer computes a proper distance (with their t=0 simultaneity) of something greater, that implies the local rulers look short to the observer at infinity (in the radial direction).

I've only seen this contraction discussed radially. Two references validating its existence (but not deriving it) are (search for contraction on of these pages):

http://www.upscale.utoronto.ca/PVB/Harrison/GenRel/GenRel.html

http://www.mathpages.com/rr/s6-01/6-01.htm