The Relativity of Simultaneity: A Fundamental Concept in Special Relativity

[Dale]

I think it is worth making the distinction, because, as mentioned above, an invariant measurement of c does not imply RoS; it is an invariant measurement of c plus RoS which results in RoS; with the latter being circular in nature.

Well, then you should find someone else who thinks it is a distinction worth making if you wish to pursue that topic further, that person isn't me. However, I should point out, that if you do make the distinction between measured and actual values of one thing (e.g. c) then you can make a similar distinction between measured and actual values of something else (e.g. RoS) and thereby un-circularize anything you run into, even if you consider the measured/actual distinction worthwhile.

I should, more precisely, have said, I don't see the relevance of using a transformation that is not the Lorentz transformation.

Consider the attached Venn diagram representing the set of all linear transformations on spacetime. The Lorentz transformation has $\text{LC} \cap \text{TD} \cap \text{RoS}$ so it is in subset 4. When you make a statement like $(\text{LC} \cap \text{TD}) \rightarrow \text{RoS}$ you are saying that subset 1 is empty. You cannot determine the emptiness of subset 1 by considering only transforms in subset 4.

If you assume the Lorentz transform then all you can say is $\text{LT} \rightarrow (\text{LC} \cap \text{TD} \cap \text{RoS})$ and you cannot make any claims about whether or not any of them are implied by any of the others. I am certainly happy to do that, but it basically ends the entire discussion after my original point that you had neglected RoS.

 

[mangaroosh]

For example a change of clock rate in a train due to a change of velocity can be detected with clocks along the railway, but depending on clock synchronisation one can measure an increase or a decrease. Thus that the detection of the effect is independent of synchronisation, but not the description of the effect.

You'll have to forgive me for being very slow on the uptake, but I'm having trouble tying this into the context of the discussion.

Sorry, I don't know what you mean with "the actual speed of light": what do you mean with "actual", and do you mean the 2-way speed of light?

The distinction being drawn is between the measurement of the speed of light and the actual speed of the light.

For example, let's say that you measure the speed of light to be ca. 300,000 km/s, using your instruments; then I measure the speed of light to be ca. 300,000 km/s using my instruments, but my metre stick is contracted such that "my meter" is shorter than "your metre", and "my second" (measured by my slower clock) is longer than "your second", then it means that the actual speeds represented by those measurements are different.

In reality, the light in my reference frame took a little longer than a second to travel a distance shorter than 300,000 km/s - although our units of measurement are the same, the actual speeds represented by those measurements are different.

Time is not a physical object, but it can be measured. Do you have a problem with that?

I do have trouble with how time is actually measured. I just can't seem to see how it is possible to measure what is supposed to be a physical property, even if it isn't considered an object.

?? Not at all! As measured with every inertial reference system, everyone's present corresponds to a certain present event (x, t) in that system...
Perhaps you mean that an observer who is using a certain reference system can attribute certain distant events that he/she has not yet seen, to the past or future while using another reference system, the contrary would be attributed to those events.

If we take two relatively moving observers for example, where the relative velocity is something like 0.6c; there will be events in the present of one observers reference frame, that are in the past of the other (and vice versa). This suggests that the events which are in the past for one observer continue to exist.

To accept this as true, however, would require that observer to assume that, not only the past events continue to exist, but also their "past self"; the same can be said of "future" events and "future self"; each and every observer would have to make this assumption, despite the fact that this would be contrary to the empirical evidence (with regard to "past" and "future").

 

[mangaroosh]

The best way to understand SR is geometrically. Take a sheet of paper and draw a simple spacetime diagram: put two dots on the sheet horizontally aligned to represent two events which are simultaneous in the frame of the drawing and spatially separated. Now get some transparent film, draw graph lines on it and place it over the paper so one grid line passes though the two events. Rotate the film a few degrees to represent a different frame and the events can no longer lie on the same gridline, they are not simultaneous in the new frame. What special relativity says is that there is no intrinsically preferred frame in reality, any choices of frame is equally valid.

Note that this is jut an analogy, to get an accurate picture you have to rotate the vertical and horizontal lines in opposite directions (e.g clockwise and anti-clockwise).

Length contraction and time dilation are just names for the effect of changing coordinate separations resulting from frame rotation.

Thanks George; the issue isn't so much understanding what Einsteinian relativity says about RoS, it is more understanding the assumptions which such an interpretation requires.

 

[mangaroosh]

The Lorentz transformation incorporates RoS and no interpretation can alter that. Perhaps that's the cause of the confusion?

Not necessarily though, do they? Lorentzian relativity uses the same transform, but doesn't incorporate RoS, no?

 

[mangaroosh]

Well, then you should find someone else who thinks it is a distinction worth making if you wish to pursue that topic further, that person isn't me. However, I should point out, that if you do make the distinction between measured and actual values of one thing (e.g. c) then you can make a similar distinction between measured and actual values of something else (e.g. RoS) and thereby un-circularize anything you run into, even if you consider the measured/actual distinction worthwhile.

But RoS is a consequence of the actual speed of light remaining invariant. If the distinction between measured and actual values is made, then RoS doesn't arise.

Consider the attached Venn diagram representing the set of all linear transformations on spacetime. The Lorentz transformation has $\text{LC} \cap \text{TD} \cap \text{RoS}$ so it is in subset 4. When you make a statement like $(\text{LC} \cap \text{TD}) \rightarrow \text{RoS}$ you are saying that subset 1 is empty. You cannot determine the emptiness of subset 1 by considering only transforms in subset 4.

If you assume the Lorentz transform then all you can say is $\text{LT} \rightarrow (\text{LC} \cap \text{TD} \cap \text{RoS})$ and you cannot make any claims about whether or not any of them are implied by any of the others. I am certainly happy to do that, but it basically ends the entire discussion after my original point that you had neglected RoS.

It doesn't have RoS as a necessity though, does it? Again, RoS is not a part of Lorentzian relativity which uses the same transform, no?

 

[harrylin]

Not necessarily though, do they? Lorentzian relativity uses the same transform, but doesn't incorporate RoS, no?

Sure it does, as we have told you several times. You can't use the LT (which incorporate RoS) and not incorporate RoS.

 

[GeorgeDishman]

Thanks George; the issue isn't so much understanding what Einsteinian relativity says about RoS, it is more understanding the assumptions which such an interpretation requires.

SR was derived from the observation of the independence of the speed of light from the speed of the source so as such it doesn't need any assumptions. Rather, the old aether model and SR's geometric model are two diametrically opposed philosophical interpretations of the same mathematical theory, the Lorentz Transforms. While it is useful to compare and contrast them, you can't mix them.

For example you say "my metre stick is contracted such that 'my meter' is shorter than 'your metre'" but that only applies in the aether interpretation. In SR, the metre stick is unchanged and the shortening is due to coordinate projection, a purely geometric effect. Mixing the models will usually create confusion simply because of their different interpretations and I butted in because I think that may be part of the cause of the difference of opinions.

 

[harrylin]

You'll have to forgive me for being very slow on the uptake, but I'm having trouble tying this into the context of the discussion.

It was just a precision in my comment on you saying "physical effect"; however that is not the topic here, and that is why I did not elaborate on it.

The distinction being drawn is between the measurement of the speed of light and the actual speed of the light.

For example, let's say that you measure the speed of light to be ca. 300,000 km/s, using your instruments; then I measure the speed of light to be ca. 300,000 km/s using my instruments, but my metre stick is contracted such that "my meter" is shorter than "your metre", and "my second" (measured by my slower clock) is longer than "your second", then it means that the actual speeds represented by those measurements are different.
In reality, the light in my reference frame took a little longer than a second to travel a distance shorter than 300,000 km/s - although our units of measurement are the same, the actual speeds represented by those measurements are different.

No, here is at least one, but likely two errors in one sentence (not regarding a glitch on top of it):

- you mix up reality with a point of view: what I measure with my inertial frame cannot be claimed more "reality" than what you measure with your inertial frame. That is even the basis of SR.

- With my reference frame, the same light ray took - if for example your lab is moving in the same direction as the light ray - a little longer than a second to travel a distance greater than 300,000 km. That is because I measure the speed to be 300,000 km/s.

If we take two relatively moving observers for example, where the relative velocity is something like 0.6c; there will be events in the present of one observers reference frame, that are in the past of the other (and vice versa). This suggests that the events which are in the past for one observer continue to exist.

If you like to suggest that to yourself, then it will look that way; what it suggests to me is what I replied to you earlier.

To accept this as true, however, would require that observer to assume that, not only the past events continue to exist, but also their "past self"; the same can be said of "future" events and "future self"; each and every observer would have to make this assumption, despite the fact that this would be contrary to the empirical evidence (with regard to "past" and "future").

That kind of conclusions from the suggestion that you fell for suggest to me that it is likely a wrong one.

 

[bobc2]

...If we take two relatively moving observers for example, where the relative velocity is something like 0.6c; there will be events in the present of one observers reference frame, that are in the past of the other (and vice versa). This suggests that the events which are in the past for one observer continue to exist.

To accept this as true, however, would require that observer to assume that, not only the past events continue to exist, but also their "past self"; the same can be said of "future" events and "future self"; each and every observer would have to make this assumption,...

Your logic looks good to me, mangaroosh, but I do not want to hijack this into another block universe discussion.

...despite the fact that this would be contrary to the empirical evidence (with regard to "past" and "future").

I don't think that is necessarily contrary to emprical evidence, but again, I don't want to get into this any further.

 

[mangaroosh]

Sure it does, as we have told you several times. You can't use the LT (which incorporate RoS) and not incorporate RoS.

Lorentzian relativity used the LT though, and doesn't incorporate RoS; no?

 

[harrylin]

Your logic looks good to me, mangaroosh, but I do not want to hijack this into another block universe discussion.

Ah yes, I forgot to mention that - ok then, I do that now!
Mangaroush you may be interested in this topic:
https://www.physicsforums.com/showthread.php?t=583606

Lorentzian relativity used the LT though, and doesn't incorporate RoS; no?

The LT certainly incorporates RoS, and Lorentz was perhaps the first to introduce the "local time" concept (note: it was probably Poincare who first understood what that means for clocks).

What Lorentzian relativity doesn't incorporate is the "block universe" interpretation, about which a still open discussion exists in the other thread.

 

[mangaroosh]

SR was derived from the observation of the independence of the speed of light from the speed of the source so as such it doesn't need any assumptions. Rather, the old aether model and SR's geometric model are two diametrically opposed philosophical interpretations of the same mathematical theory, the Lorentz Transforms. While it is useful to compare and contrast them, you can't mix them.

For example you say "my metre stick is contracted such that 'my meter' is shorter than 'your metre'" but that only applies in the aether interpretation. In SR, the metre stick is unchanged and the shortening is due to coordinate projection, a purely geometric effect. Mixing the models will usually create confusion simply because of their different interpretations and I butted in because I think that may be part of the cause of the difference of opinions.

The point being made though, is that all we ever have is a measurement of c; however, an ivariant measurement of c, by itself, doesn't imply RoS; only if RoS is assumed along with the measurement of c, can we conclude that RoS prevails - but this would be circular reasoning. That is, we have to assume RoS to arrive at the Eisteinian interpretation, but RoS is one of the conclusions of the Einsteinian interpretation, and so it appears to be circular reasoning.

Lorentz aether theory, or rather neo-Lorentzian relativity, has supposedly been divested of the concept of the aether

 

[Dale]

But RoS is a consequence of the actual speed of light remaining invariant. If the distinction between measured and actual values is made, then RoS doesn't arise.

Again, the distinction between actual and measured values is unimportant. Scientifically, all that matters is the measurements, so all of my statements here refer to measurements. If there is a hidden actuality that cannot be measured then it is irrelevant to physics.

It doesn't have RoS as a necessity though, does it? Again, RoS is not a part of Lorentzian relativity which uses the same transform, no?

No. The RoS falls directly from the Lorentz transform. It is a testable part of any theory which uses the Lorentz transform for experimental predictions. As you have been told multiple times by multiple people now.

 

[mangaroosh]

It was just a precision in my comment on you saying "physical effect"; however that is not the topic here, and that is why I did not elaborate on it.

ah OK, I'm still none the wiser though

No, here is at least one, but likely two errors in one sentence (not regarding a glitch on top of it):

- you mix up reality with a point of view: what I measure with my inertial frame cannot be claimed more "reality" than what you measure with your inertial frame. That is even the basis of SR.

But it isn't being claimed that one measurement is more real than another, it is simply being stated that the same figure of measurement, using different length instruments, means a different actual measurement.

- With my reference frame, the same light ray took - if for example your lab is moving in the same direction as the light ray - a little longer than a second to travel a distance greater than 300,000 km. That is because I measure the speed to be 300,000 km/s.

But if my clock is ticking slower, then it means that "my second" is longer than your second; and if "my metre" is contracted, it means that "my metre" is shorter than "your metre"; so, if I measure the speed of light to be 300,00 km/s and you measure it to be the same, but our measuring instruments are of different lengths, then the actual measurement represented by those units are different - unless the contractions don't acutally occur, and are only illusory.

If you like to suggest that to yourself, then it will look that way; what it suggests to me is what I replied to you earlier.

That isn't so much what I would like to suggest to myself, as opposed to what has been suggested to me by others. I'm not really clear how your formulation differs though.

The example that has been used to demonstrate this, is the lightning rod thought experiment, where the observers disagree about the simultaneity of the events; the observer on the train sees the lightning at the "forward" rod before the "rear" rod, while the observer on the platform sees them as simultaneous; where t=0 is the time when the lightning strikes the rods in the platform observers reference frame, and the time when the train observer is in-line with the platform observer.

The train observer calculates that the flash at B occurred at a time before t=0, while the platform observer calculates that they both occurred at t=0. In order for the platform observer to accept this, he must make an assumption about the existence of the lightning event before t=0, as well as the event after t=0. The train observer must make similar assumptions.

That kind of conclusions from the suggestion that you fell for suggest to me that it is likely a wrong one.

As Bobc2 mentiosn above, it appears to be necessary to accept the concept of "the block universe", or even RoS.

 

[mangaroosh]

Your logic looks good to me, mangaroosh, but I do not want to hijack this into another block universe discussion.



I don't think that is necessarily contrary to emprical evidence, but again, I don't want to get into this any further.

Thanks bob.

I'm not sure if it makes a difference, as the thread starter, to say that I don't think you'd be hijacking it; it seems to be a related concept.

 

[mangaroosh]

Again, the distinction between actual and measured values is unimportant. Scientifically, all that matters is the measurements, so all of my statements here refer to measurements. If there is a hidden actuality that cannot be measured then it is irrelevant to physics.

The point above demonstrates that it is an important distinction, because an invariant measurement of c doesn't necessarily imply RoS.

No. The RoS falls directly from the Lorentz transform. It is a testable part of any theory which uses the Lorentz transform for experimental predictions. As you have been told multiple times by multiple people now.

This conflicts with what has been said about Lorentzian relativity though, by multiple people, multiple times; that Lorentzian relativity doesn't include RoS, but yet it does use the Lorentz transform. If this is true, then it is a non-sequitir to suggest that RoS is necessarily a consequence of the LT.

As has also been stated by miltiple people, multiple times, RoS is a consequence of an invariant speed of light.

Again, as has been demonstrated, an invariant measurement of c does not imply an invariant actual speed of c.


That it may be a hidden actuality isn't relevant, it's that one interpretation appears to rely on circular reasoning, while the other doesn't.

 

[GeorgeDishman]

The point being made though, is that all we ever have is a measurement of c; however, an ivariant measurement of c, by itself, doesn't imply RoS; only if RoS is assumed along with the measurement of c, can we conclude that RoS prevails - but this would be circular reasoning. That is, we have to assume RoS to arrive at the Eisteinian interpretation, but RoS is one of the conclusions of the Einsteinian interpretation, and so it appears to be circular reasoning.

If you look at his paper though, you find he didn't assume RoS, his two postulates were only that light moved at a speed which was independent of the speed of the source and that the laws of electrodynamics and optics apply in any inertial frame. The RoS is then derived from those.

http://www.fourmilab.ch/etexts/einstein/specrel/www/

Lorentz aether theory, or rather neo-Lorentzian relativity, has supposedly been divested of the concept of the aether

Really? Lorentz's interpretation was that objects shrank by an amount determined by their speed relative to the aether in the direction of motion. How can that speed be defined without an aether? It not only must exist, it must also provide a reference against which motion can be measured (which is a much stricter requirement).

 

[harrylin]

[..]
But it isn't being claimed that one measurement is more real than another, it is simply being stated that the same figure of measurement, using different length instruments, means a different actual measurement.
[..]
so, if I measure the speed of light to be 300,00 km/s and you measure it to be the same, but our measuring instruments are of different lengths, then the actual measurement represented by those units are different [..]

Yes i can agree with that.
[..]

The example that has been used to demonstrate this, is the lightning rod thought experiment, where the observers disagree about the simultaneity of the events; the observer on the train sees the lightning at the "forward" rod before the "rear" rod, while the observer on the platform sees them as simultaneous; where t=0 is the time when the lightning strikes the rods in the platform observers reference frame, and the time when the train observer is in-line with the platform observer.

The train observer calculates that the flash at B occurred at a time before t=0, while the platform observer calculates that they both occurred at t=0. In order for the platform observer to accept this, he must make an assumption about the existence of the lightning event before t=0, as well as the event after t=0. The train observer must make similar assumptions.

As Bobc2 mentiosn above, it appears to be necessary to accept the concept of "the block universe", or even RoS.

Now assume that by chance (just for simplicity of discussion) that the train is at rest in the ether (Lorentz interpretation). Then everything will be measured as Einstein described. And then there is nothing mind-boggling going on at all.
Historically you can choose between the Lorentz interpretation (stationary ether) and the Minkowski interpretation (block universe), and in recent years a few more subtle variants have appeared (I think we had a recent topic on that). But all those interpretations of RoS should not be confounded with the RoS itself, which as Dalespam mentioned is a feature of the equations that we call Lorentz Transformations, and which apply to certain reference systems that are set up in a certain way.

 

[Dale]

an invariant measurement of c doesn't necessarily imply RoS.

I agree. That is why I said $(\text{C} \cap \text{PoR})\rightarrow \text{RoS}$ and not $\text{C}\rightarrow \text{RoS}$

This conflicts with what has been said about Lorentzian relativity though, by multiple people, multiple times; that Lorentzian relativity doesn't include RoS, but yet it does use the Lorentz transform. If this is true, then it is a non-sequitir to suggest that RoS is necessarily a consequence of the LT.

As has also been stated by miltiple people, multiple times, RoS is a consequence of an invariant speed of light.

OK, point well made, we cannot appeal to numbers of sources for a correct analysis. That is why math derivations are so important. I can and have backed up all of my claims mathematically.

RoS is an unavoidable and testable consequence of any theory which uses the LT for its experimental predictions. Any assertion to that any Lorentz-transform based theory does not have RoS is either wrong or deliberately referring to something untestable and therefore scientifically irrelevant.

 

[mangaroosh]

If you look at his paper though, you find he didn't assume RoS, his two postulates were only that light moved at a speed which was independent of the speed of the source and that the laws of electrodynamics and optics apply in any inertial frame. The RoS is then derived from those.

http://www.fourmilab.ch/etexts/einstein/specrel/www/

The point being made, though, is that an invariant measurement of c is different to the actual speed of c remaining invariant; where only an invariant actual speed of c results in RoS, and a measurement of c doesn't necessarily.

Given that we only ever have a measurement of c, RoS would have to be assumed; although the points being raised by DaleSpam and Harrylin, below, might offer a different interpretation.

Really? Lorentz's interpretation was that objects shrank by an amount determined by their speed relative to the aether in the direction of motion. How can that speed be defined without an aether? It not only must exist, it must also provide a reference against which motion can be measured (which is a much stricter requirement).

I'm not so sure that the alternative that time and space are affected by the motion of an object is any less fantastical, particularly when the effects can only be measured using the objects that Lorentzian relativity claims would shrink. Also, I think neo-Lorentzian relativity does away with the concept of the aether, and instead uses the concept of an absolute rest frame.

Without the ether the speed could probably be defined by defining a "rest frame" for measurements, as opposed to an ether.

 

[mangaroosh]

Yes i can agree with that.
[..]

Now assume that by chance (just for simplicity of discussion) that the train is at rest in the ether (Lorentz interpretation). Then everything will be measured as Einstein described. And then there is nothing mind-boggling going on at all.
Historically you can choose between the Lorentz interpretation (stationary ether) and the Minkowski interpretation (block universe), and in recent years a few more subtle variants have appeared (I think we had a recent topic on that). But all those interpretations of RoS should not be confounded with the RoS itself, which as Dalespam mentioned is a feature of the equations that we call Lorentz Transformations, and which apply to certain reference systems that are set up in a certain way.

I think you've mentioned the difference between the two kinds of RoS before, but I'm not entirely clear on what is meant. The issue gets somewhat muddied for me when you mention the alternatives as being the block universe and a stationary ether, because I've read that the concept of the ether has been effectively removed from neo-Lorentzian relativity, with the concept of an "absolute rest" frame being retained.

When I think of RoS I think of it as contrasted with absolute simultaneity, or the idea that only the present moment exists.


--------------------------------------------------------------------------------
Have to run, will try and finish this at home

 

[Dale]

When I think of RoS I think of it as contrasted with absolute simultaneity, or the idea that only the present moment exists.

The idea that only the present moment exists is called presentism, and is contrasted with the idea that past present and future all exist on equal footing, which is called eternalism. They are both philosophical concepts that do not have any scientific meaning (i.e. they do not lead to testable predictions).

Simultaneity is the concept of whether or not two different events happened at the same time. Two things may be simultaneous regardless of if you accept eternalism or presentism.

The relativity of simultaneity means that two observers moving relative to one another will disagree about whether or not two events are simultaneous. Any theory which uses the Lorentz transform for its experimental predictions automatically incorporates the relativity of simultaneity into its measurements.

 

[GeorgeDishman]

The point being made, though, is that an invariant measurement of c is different to the actual speed of c remaining invariant;

You need to explain to me what you mean by "actual speed" if it isn't what is measured.

where only an invariant actual speed of c results in RoS, and a measurement of c doesn't necessarily.

RoS will always occur if the universe is Lorentz Invariant irrespective of interpretation as 'DaleSpam' has said.

Given that we only ever have a measurement of c, RoS would have to be assumed;

No, it follows as a consequence. You can define a method for synchronising clocks by sending a light signal from A to B and back to A for example. If you apply that method in two frames, you get different decisions as to whether two events are simultaneous. It turns out that all other physically possible methods are equivalent.

I'm not so sure that the alternative that time and space are affected by the motion of an object is any less fantastical

I would probably agree but that's not an interpretation I've ever heard of, the only ones I know are LET and SR.

Also, I think neo-Lorentzian relativity does away with the concept of the aether, and instead uses the concept of an absolute rest frame. Without the ether the speed could probably be defined by defining a "rest frame" for measurements, as opposed to an ether.

You will see that suggestion made by people who don't understand LET (or basic physics for that matter) but it has never been a theory. In LET, the mechanism that causes the various effects is an interaction with the aether hence the velocity that goes into the transforms is that of the object relative to the aether. Without some physical substance against which to measure velocity, there is no way to define the quantity. Note you can't even measure speed relative to something which is continuous, a uniform electric potential for example.

 

[GeorgeDishman]

They are both philosophical concepts that do not have any scientific meaning (i.e. they do not lead to testable predictions).

I'm not sure that is true, doesn't presentism require an aether-like approach to avoid temporal displacement in the twins scenario?

This is probably a topic better discussed in the philosophy forum though.

 

[harrylin]

I think you've mentioned the difference between the two kinds of RoS before, but I'm not entirely clear on what is meant. The issue gets somewhat muddied for me when you mention the alternatives as being the block universe and a stationary ether, because I've read that the concept of the ether has been effectively removed from neo-Lorentzian relativity, with the concept of an "absolute rest" frame being retained.

What is the difference, except for words?

When I think of RoS I think of it as contrasted with absolute simultaneity, or the idea that only the present moment exists. [..]

That sounds like the block universe interpretation to me.

A good start for such philosophical issues may be Newton's mechanics, which defines "absolute" and "relative" velocities. Newton's mechanics compares to classical mechanics as Lorentzian mechanics to special relativity.

Here's another one: in England people live in another time than in Germany. Do you think that such time definitions have anything to do with existence of a present moment or not?

Cheers,
Harald

 

[mangaroosh]

I agree. That is why I said $(\text{C} \cap \text{PoR})\rightarrow \text{RoS}$ and not $\text{C}\rightarrow \text{RoS}$

That depends on the interpretation of the PoR. If the measurement of c remains invariant, then an observer cannot conduct an experiment to distinguish their reference frame from any other, but this still wouldn't necessarily imply RoS.

OK, point well made, we cannot appeal to numbers of sources for a correct analysis. That is why math derivations are so important. I can and have backed up all of my claims mathematically.

RoS is an unavoidable and testable consequence of any theory which uses the LT for its experimental predictions. Any assertion to that any Lorentz-transform based theory does not have RoS is either wrong or deliberately referring to something untestable and therefore scientifically irrelevant.

RoS isn't testable either though, is it?

Do you, by any chance, know what people might be referring to when they say that Lorentzian relativity incorporates absolute simultaneity, and not RoS?

 

[Dale]

That depends on the interpretation of the PoR. If the measurement of c remains invariant, then an observer cannot conduct an experiment to distinguish their reference frame from any other, but this still wouldn't necessarily imply RoS.

Yes, it does necessarily imply RoS. That was one of the points of sections 2 and 3 of Einstein's paper "On the Electrodynamics of Moving Bodies".

RoS isn't testable either though, is it?

Yes, it is. Synchronize two sets of clocks moving relative to each other, measure the time of two events, and see if they agree on the simultaneity.

 

[mangaroosh]

Yes, it does necessarily imply RoS. That was one of the points of sections 2 and 3 of Einstein's paper "On the Electrodynamics of Moving Bodies".

OK, but is there an issue with the logic, as outlined above, that it doesn't necessarily imply it, because you can have an invariant measurement of c without implying RoS; only an invariant actual measurement of c results in RoS; and we can't make such deductions from only an invariant measurement of c.

The consequence of the PoR would also remain intact.

Yes, it is. Synchronize two sets of clocks moving relative to each other, measure the time of two events, and see if they agree on the simultaneity.

There is an assumption in that which relates to a closed thread (which is under appeal) so I'm not sure how explicit I can be, without risking an infraction. The thread is in the general discussion section an pertains to the mechanics of a clock.


If, however, we take the example of two relatively moving atomic clocks, which use a laser and detector (or photon and detector) to "measure the time of two events"; the relatively moving clocks might tick at different rates because the photon in each clock has a different distance to travel to the detector. This wouldn't demonstrate RoS, rather slower ticking clocks.

 

[mangaroosh]

The idea that only the present moment exists is called presentism, and is contrasted with the idea that past present and future all exist on equal footing, which is called eternalism. They are both philosophical concepts that do not have any scientific meaning (i.e. they do not lead to testable predictions).

Simultaneity is the concept of whether or not two different events happened at the same time. Two things may be simultaneous regardless of if you accept eternalism or presentism.

The relativity of simultaneity means that two observers moving relative to one another will disagree about whether or not two events are simultaneous. Any theory which uses the Lorentz transform for its experimental predictions automatically incorporates the relativity of simultaneity into its measurements.

Is RoS compatible with presentism? My understanding is that it isn't.

My understanding is also that Lorentzian relativity includes presentism, and hence why I am of the understanding that it doesn't include RoS.

 

[mangaroosh]

You need to explain to me what you mean by "actual speed" if it isn't what is measured.

The point was that if two observers measure the speed of light to be 300,000km/s but the instruments of one of the observers are contracted, such that his metre stick is shorter than the metre stick of his counterpart, and his clock ticks slower than his counterparts clock, then the actual speeds represented by those two measurements are not the same.

RoS will always occur if the universe is Lorentz Invariant irrespective of interpretation as 'DaleSpam' has said.

OK, but people on here have mentioned that Lorentzian relativity includes absolute simultaneity (presentism) and not RoS.

No, it follows as a consequence. You can define a method for synchronising clocks by sending a light signal from A to B and back to A for example. If you apply that method in two frames, you get different decisions as to whether two events are simultaneous. It turns out that all other physically possible methods are equivalent.

This requires the assumption that it takes an equal amount of time for the light to travel in each direction, doesn't it?

I would probably agree but that's not an interpretation I've ever heard of, the only ones I know are LET and SR.

Does SR not suggest that time and space are affected by the motion of an observer, such that time slows down and space contracts?

You will see that suggestion made by people who don't understand LET (or basic physics for that matter) but it has never been a theory. In LET, the mechanism that causes the various effects is an interaction with the aether hence the velocity that goes into the transforms is that of the object relative to the aether. Without some physical substance against which to measure velocity, there is no way to define the quantity. Note you can't even measure speed relative to something which is continuous, a uniform electric potential for example.

I know that LET includes the aether, but what is termed neo-Lorentzian relativity supposedly has been divested of the concept of the aether

the last vestiges of a substantial ether had been eliminated from Lorentz's "ether" theory, and it became both empirically and deductively equivalent to special relativity. The only difference was the metaphysical[C 7] postulate of a unique absolute rest frame, which was empirically undetectable and played no role in the physical predictions of the theory. As a result, the term "Lorentz ether theory" is sometimes used today to refer to a neo-Lorentzian interpretation of special relativity

 

[mangaroosh]

What is the difference, except for words?

I'm not sure to be honest. My understanding is that neo-Lorentzian relativity includes, or is compatible with, presentism, but that presentism isn't compatible with RoS, which would lead to the conclusion that neo-Lorentzian relativity doesn't include RoS and so the Lorentz transform doesn't necessarily include RoS.

But, if RoS is included by necessity in the LT, then there must be two different interpretations of RoS - unless there is an issue with the reasoning above.

That sounds like the block universe interpretation to me.

Are there other interpretations of RoS?

A good start for such philosophical issues may be Newton's mechanics, which defines "absolute" and "relative" velocities. Newton's mechanics compares to classical mechanics as Lorentzian mechanics to special relativity.

Apologies, I don't fully understand the comparison.

Here's another one: in England people live in another time than in Germany. Do you think that such time definitions have anything to do with existence of a present moment or not?

Nothing whatsoever I would say. I would say that when it is 2pm in England, it is 3pm in Germany, but both exist, simultaneously, in the present moment.

 

[mangaroosh]

Yes, it does necessarily imply RoS. That was one of the points of sections 2 and 3 of Einstein's paper "On the Electrodynamics of Moving Bodies".

OK, but is there an issue with the reasoning, outlined above, that an invariant measurement of c doesn't imply RoS, that it is only an actual measurement of c that implies it?

Given that the oft stated consequence of the the PoR would remain intact i.e. no experiment could distinguish between reference frames.

Yes, it is. Synchronize two sets of clocks moving relative to each other, measure the time of two events, and see if they agree on the simultaneity.

Lorentzian relativity would suggest that the different time measurements of the clocks would be down to the mechanical effects on the clock, wouldn't it, and not due to time slowing down; this would preserve the concept of absolute simultaneity wouldn't it?

How would we determine that time, as opposed to the mechanics of the clock, has been affected?

 

[GeorgeDishman]

The point was that if two observers measure the speed of light to be 300,000km/s but the instruments of one of the observers are contracted, such that his metre stick is shorter than the metre stick of his counterpart, and his clock ticks slower than his counterparts clock, then the actual speeds represented by those two measurements are not the same.

"Speed" is a measurement. If the length of his ruler and times on his clock comply with the physical definition of the metre and second, then the actual speed he has measured is c.

OK, but people on here have mentioned that Lorentzian relativity includes absolute simultaneity (presentism) and not RoS.

Lorentz's aether assumed Galilean Relativity and proposed ad hoc physical changes in objects due to an unknown mechanical interation between matter and the substance of the aether to explain the invariance of the speed of light so it is compatible with both presentism and eternalism. However the Relativity of Simultaneity refers to events having different separation of their time coordinates in mutually moving frames so it still occurs in LET. LET offers an alternative philosophical explanation for the RoS but since it is an observed effect, it could not discard it without being falsified.

This requires the assumption that it takes an equal amount of time for the light to travel in each direction, doesn't it?

c can be derived from the permitivity and permeability in Maxwell's Equations. Both are scalar hence the speed must be scalar and therefore it must be isotropic.

Does SR not suggest that time and space are affected by the motion of an observer, such that time slows down and space contracts?

No, SR says that spacetime has fixed Reimann geometry. Draw two dots on a plain white sheet of paper. Place a grid drawn on a transparent sheet over the top and measure the X and Y coordinates of the dots. Now rotate the transparency a little. You get different coordinate separations but the white sheet of paper has neither shrunk nor stretched and the dots haven't moved. SR says changing your coordinate scheme will change the numbers that label an event but that's all. Spacetime intervals are invariant for the same reason that Pythagoras will give the same result for the separation of the dots regardless of how you orient the transparency with the gridlines.

 

[Dale]

OK, but is there an issue with the logic, as outlined above, that it doesn't necessarily imply it, because you can have an invariant measurement of c without implying RoS; only an invariant actual measurement of c results in RoS; and we can't make such deductions from only an invariant measurement of c.

Scientifically there is no issue since, as I have mentioned several times, any unmeasurable aspects are scientifically irrelevant. This site is for discussing science, not philosophy.

If, however, we take the example of two relatively moving atomic clocks, which use a laser and detector (or photon and detector) to "measure the time of two events"; the relatively moving clocks might tick at different rates because the photon in each clock has a different distance to travel to the detector. This wouldn't demonstrate RoS, rather slower ticking clocks.

Correct, as we have discussed previously TD and RoS are two entirely different concepts.

 

[Dale]

Lorentzian relativity would suggest that the different time measurements of the clocks would be down to the mechanical effects on the clock, wouldn't it, and not due to time slowing down; this would preserve the concept of absolute simultaneity wouldn't it?

No. In LET local time exhibits RoS.

How would we determine that time, as opposed to the mechanics of the clock, has been affected?

We cannot. That is why the distinction is scientifically meaningless.