The Physical Meaning of the Relatvity of Simultaneity

[YellowTaxi]

How can one sphere morph into two separate spheres yet still physically be the one unique original sphere? We started with one sphere at the instant of generation, as the two frames moved relative to each other, the one sphere became two separate independent spheres as proven by the Lorentz transformations.

The question is, how is this **physically** possible? What does it mean in terms of what we perceive of our physical world? Two observers, moving independently relative to each other, see a spherical wavefront centered at one and the same unique physical location, but no matter how the two observers have moved apart, each observer sees identically the same spherical wavefront centered on that observer's own origin.

Pete B

Actually you're right, according to Einstein's theory they won't see an identical sphere of light.

If you assume the light source was at rest in S, but moving past at speed v in S'. Then the observer in S' sees the front of the light sphere blue shifted and its rear end red shifted ( ie the end closest to O would be red shifted).

Apart from that I agree, that both observers would see a perfect circle centred on their respective origins is not logical by any stretch of the imagination. Hence Einstein's fame. But the really strange thing about einstein's theory is that it fits the data.

 

[Al68]

So you have two observers with different states of motion and a light sphere is created at that very instant. After some time, when the observers are thousands of miles away from each other, both of them continuously hold that they are right in the centre of the sphere. In other words, S holds that the two wavefronts are always simultaneously touching some space points that are equidistant from him; instead S’ maintains that the wavefronts simultaneously touch points equidistant from her…

So what? What is the problem with this discrepancy? I still do not see any. To stop here and start asking “who is really in the centre of the sphere, S or S’?” would be a mistake. That would be philosophy

I don't think that's philosophy at all, if we use your (correct) definition of being in the center of the sphere, ie the wavefronts simultaneously touch equidistant points. By that definition, there is one light sphere, and both observers remain in the center of it relative to their rest frame.

Physical reality is that in each frame, the light will reach equidistant synchronized clocks simultaneously. That must be true in both frames, because that's exactly how the clocks are synchronized in each frame.

 

[heldervelez]

in this treath https://www.physicsforums.com/showthread.php?t=363853"
I expressed the viewpoint of Lorentz, as a real physical phenomena. It brings more compreension to Relativity and the real world.
When we see only equations it is more difficult to understand, and evade from paradoxes.
Instead of read textbooks I do preffer the reading of Einstein 1920 text for laymen (+-30 pages) freely available at http://bartleby.com/173
http://bartleby.com/173/9.html" the words on 'The Relativity of Simultaneity'.

http://en.wikipedia.org/wiki/Relativity_of_simultaneity" shows that the order of 'simultaneous' events will be perceived differently depending on the motion of the observer. It brings to my mind 'and causalty?'.

Einstein used rods and clocks and I must say that we must think of them as constructed with real atoms, and theese react to motion adapting its spatial configuration, modifying lengths (and rate of clocks because c=Length/Time) as Mr Lorentz showed.
We must look for 'physics', then math, and not the opposite.

It is important to say that NO observer can measure its own 'modification'. To him, his rulers are always constant and his atoms are undeformed. A wrong perception because we see ourselves at the center of the world (a gift from physics and Einstein), but in reality things are different!
We will see more when we start to evade from this 'observer centric' position.

 

[matheinste]

We will see more when we start to evade from this 'observer centric' position.

An observer is nothing personal. An obsever is the totality of all conceivable measuring apparatus. To observe things we have to measure them. To measure them we need apparatus. The observer is the apparatus.

Matheinste.

 

[Saw]

Physical reality is that in each frame, the light will reach equidistant synchronized clocks simultaneously. That must be true in both frames, because that's exactly how the clocks are synchronized in each frame.

Agreed. In all these discussions about what is "phsyically real" and what is not, the problem is the ambiguity of language. There are several candidate meanings for the expresion:

(a) "Events of life", like "does John die or not?" - that is the basic meaning of "physical" that people have in mind, I think, when they pose questions like the OP.

(b) "Events in the measurement instruments", like "will John's clock have ticked 2 seconds when the two wavefronts return simultaneously to him after bouncing back somewhere?". That is also "phsyical", of course.

(c) Man-made concepts like simultaneity or equidistance of two DISTANT events, like John's statement that the two wavefronts bounced back "simultaneously" at points 1 light-second away from him. We can call that "physical" as well. In fact, science is the art of creating and using properly this sort of concepts.

What I wanted to highlight, after all my words, is just that whereas (a) and (b) are final truths, (c) instead is an INSTRUMENTAL truth: you create it on the basis of (b) events in order to predict (a) events. So the disagreement that takes place at the level of (c) must always be provisional, during reasoning process, but the outcome of the reasoning in terms of (a) or even (b) events should always be full agreement between frames. And if ever the system did not work that way (for example, FTL travel were found out to be possible) what we should do is changing the concepts, not fearing that two conflicting realities may emerge.

 

[atyy]

Agreed. In all these discussions about what is "phsyically real" and what is not, the problem is the ambiguity of language. There are several candidate meanings for the expresion:

(a) "Events of life", like "does John die or not?" - that is the basic meaning of "physical" that people have in mind, I think, when they pose questions like the OP.

(b) "Events in the measurement instruments", like "will John's clock have ticked 2 seconds when the two wavefronts return simultaneously to him after bouncing back somewhere?". That is also "phsyical", of course.

(c) Man-made concepts like simultaneity or equidistance of two DISTANT events, like John's statement that the two wavefronts bounced back "simultaneously" at points 1 light-second away from him. We can call that "physical" as well. In fact, science is the art of creating and using properly this sort of concepts.

What I wanted to highlight, after all my words, is just that whereas (a) and (b) are final truths, (c) instead is an INSTRUMENTAL truth: you create it on the basis of (b) events in order to predict (a) events. So the disagreement that takes place at the level of (c) must always be provisional, during reasoning process, but the outcome of the reasoning in terms of (a) or even (b) events should always be full agreement between frames. And if ever the system did not work that way (for example, FTL travel were found out to be possible) what we should do is changing the concepts, not fearing that two conflicting realities may emerge.

Ok, let me give you a hard time, since you are being so nice and clear about this. Since it's Christmas, let's avoid having John die. Let's ask, does John get kissed under the mistletoe or not? I see, "under" is a bit problematic. But my real question is, John is an extended entity, so doesn't his dying involve simultaneity? Kissing seems more localised, so maybe that is more real.

 

[sweet springs]

Hi.

I have about fifty different physics books in my library that have no problem with what I said about clocks in two different inertial frames being synchronized to t = t' =0. So I am not going to discuss this particular issue any further.
Pete B

Do not worry about it. You are right, t' =0 when t=0 at x=0 is included in the relation ct'=γ(ct - v/c x) , Lorentz transformation. Near by the synchronized clock rest in S showing time t at place x , the synchronized clock rest in S' showing time t' the value of which is given by the above formula, is passing through.

Synchronization is not universal. Each inertia system has each synchronization. Even if you would like to state t=t'(=0) for all the synchronized clock pairs passing by everywhere, the relativity theory reveals that it is impossible.

Regards.

PS
x'=γ(x - v/c ct) means "the later, the farther", moving far a way.
ct'=γ(ct - v/c x) means "the farther, the later".

 

[heldervelez]

An observer is nothing personal... The observer is the apparatus.
Matheinste.

The observer has a MIND to interpret.
If we relly only on measures we can be wrong.
Lorentz suspected, and said: Hey, the apparatus is changing ! and he said why it happens, and why we can not sense the observer modification.
If everything change we must presume that we are also changing.

 

[matheinste]

The observer has a MIND to interpret.
If we relly only on measures we can be wrong.
Lorentz suspected, and said: Hey, the apparatus is changing ! and he said why it happens, and why we can not sense the observer modification.
If everything change we must presume that we are also changing.

Of course humans have minds which interpret observations.

I was enlarging on Schutz's definition of an observer.

"It is important to realize that an 'observer' is in fact a huge information gathering system, not simply one man with binoculars. In fact, we shall remove the human element entirely from our definition, and say that an inertial observer is simply a coordinate system for spacetime, which makes an observation simply by recording the location and time of any event"

The point I was trying to make is that reality is what is measured directly or indirectly as lengths or times, and we as humans of course interpret this to try and describe how the world works. If the "information gathering system" cannot measure or observe it, then it is of no relevance to non-philosophers.

Matheinste.

 

[Dale]

Matheinste is correct, in modern SR usage "observer at X" is shorthand for "coordinate system with the origin located at X". In modern QM usage "observation" is any kind of measurement. In neither case is it intended to have any psychological or mental implications.

@ heldervelez. You are certainly free to prefer Lorentz's Aether Theory over Einstein's Special Relativity, but since Lorentz's aether is, by design, completely undetectable experimentally you are simply not going to get a lot of enthusiastic support for the extra and unnecessary complication. Go ahead and use it, and we will go ahead and not use it, and we will still all agree on everything measurable.

 

[Al68]

Einstein used rods and clocks and I must say that we must think of them as constructed with real atoms, and theese react to motion adapting its spatial configuration, modifying lengths (and rate of clocks because c=Length/Time) as Mr Lorentz showed.

The Lorentz transformations work both ways. Rods are shorter and clocks run slower in a reference frame in which they are in motion, whether the velocity of the rod or clock changed or not.

Length contraction and time dilation can't be attributed to a change in motion of the rod or clock when there is no such change of motion, only a change of reference frames.

 

[matheinste]

@ heldervelez. You are certainly free to prefer Lorentz's Aether Theory over Einstein's Special Relativity, but since Lorentz's aether is, by design, completely undetectable experimentally you are simply not going to get a lot of enthusiastic support for the extra and unnecessary complication. Go ahead and use it, and we will go ahead and not use it, and we will still all agree on everything measurable.

While I completely agree with the above remarks it is interesting to read a quote from Tolman-Relativity, Thermodynamics and Cosmo;ogy page 13 in the 1949 reprint of 1934 first edition.

Speaking about the null results of the Michelson-Morley and Kennedy-Thorndike experiments:-

“To account for these experiments on the basis of a fixed ether it would be necessary to introduce ingenious assumptions as to a change in length or Lorentz-Fitzgerald contraction just sufficient to give a null effect in the Michelson experiment, and as to a change in period or time dilation just sufficient to give a null effect in the Kennedy experiment-all to the end of retaining a fixed ether so devilishly constructed that its existence could never be detected.”

No doubt about where he stood as regards the question of the ether.

Matheinste.

 

[Dale]

I agree also, it is a rather absurd and cumbersome assumption, and it even goes further than that. Since the other known forces also exhibit Lorentz symmetry you have to posit that this same "devilishly constructed" aether is somehow the medium for EM and also the medium for the weak force and also the medium for the strong force. And in addition you have to posit that it affects all of these other forces in the same way that it affects EM. That is a minimum of 9 postulates compared to Einstein's 2.

 

[heldervelez]

The last posts were fine and I must, and will, write some words about each one, but in Christmas time I only have a few moments to spare out of the family.
These posts were better in the thread https://www.physicsforums.com/showthread.php?t=363853". I will reread the OP to see if the answers will go along with the intentions of the author.
Let it be here or there.
To all of you a happy season.

 

[Dale]

Merry Christmas to you also heldervelez!

 

[matheinste]

With regards the two spheres question, purely by chance, while looking at ether theories I cam across the following from Whittaker-a History of Theories of the Ether and Electricity Volume 2. 1953. Page 36.

-------Some of the consequences of the new theory seemed to contemporary physicists very strange. Suppose, for example, that two inertial sets of axes A and B are in motion relative to each other, and that at a certain instant their origins coincide: and suppose that at this instant a flash of light is generated at the common origin. Then, by what has been said in the subsequent propagation, the wave-fronts of the light, as observed in A and in B, are spheres whose centres are the origins of A and B respectively, and therefore DIFFERENT spheres. How can this be?

The paradox is explained when it is remembered that a wave-front is defined to be the locus of points which are SIMULTANEOUSLY in the same phase of disturbance. Now events taking place at different points, which are simultaneous according to A’s way of measuring time, are not in general simultaneous according to B’s way of measuring: and therefore what A calls a wave-front is not the same thing as what B calls a wave-front. Moreover, since the system of measuring space is different in the two inertial systems, what A calls a sphere is not the same thing as what B calls a sphere. Thus there is no contradiction in the statement that the wave-fronts for A are spheres with A’s origin as centre, while the wave-fronts for B are spheres with B’s origin as centre.

In common language we speak of events which happen at different points of space as happening ‘at the same instant of time,’ and we also speak of events which happen at different instants of time as happening ‘at the same point of space.’ We now see that such expressions can have a meaning only by virtue of artificial conventions; they do not correspond to any essential physical realities.-------


Matheinste.

 

[Austin0]

It is perhaps worth pointing out that, in relativity, the concept of simultaneity is a convention rather than an experimentally meaningful idea. As nothing can be transmitted instantaneously from A to B, nature doesn't care about "simultaneity" at all. It's a man-made concept which eases our mathematical analysis within a frame of reference, but has no real "physical" significance.

In pre-relativistic physics, all observers agreed on what was simultaneous, which is why we intuitively feel simultaneity is important. In relativity, nobody agrees on simultaneity, but they all agree on the speed of light; it's the notion of being able to send light from event A to event B which is the important relation in connecting events (rather than simultaneity).

Hi DrGreg While agreeing completely with all you have stated here I would like to clarify a point.

simultaneity is a convention rather than an experimentally meaningful idea

WOuld you agree with the idea that this is a result of our lack of technilogical ability to accelerate systems of clocks and rulers to significant velocities rather than a limitation of meaningful conceptions?
That if we were able to do so, we would expect to experimentally confirm the relative desynchronization of clocks between systems. Which, lacking any ,even hypothetical means of determining instantaneous simultaneity, would demonstrate and confirm SR concepts regarding the matter.??
To my mind this still leaves open an even more intriguing question : Is the desynchronization purely the result of the conventional procedure for synchronization or is it the result of and represents an actual temporal displacement of some incomprehensible kind??
Specifically ; in this case if we assume two systems with clocks synched with the convention, even if this method is actually arbitrary the simple mechanical desynchronization produced by the convention would seem to explain all the observed phenomena with regard to the centered light spheres in both systems. AS well as the contraction resulting from the difference of ideas of distance derived from measurements taken at the "same" time in different systems.
On the other hand the desynchronization or shift of simultaneity may not be a result of a synch method but may be an intrinsic resuly of relative velocity , an actual temporal/spatial shift which the convention also conveniently ensures.
AN experimental test of these alternatives might be ; If a system in inertial motion has its clocks synched by converntion and is then accelerated to a new significant velocity before resuming inertial motion there would seem to be two possibilities:
The clocks need to be resynched through convention. Light measurement would not automatically be correct.
Or
The acceleration and new relative velocity has in itself shifted the clocks into a new relative synchronization so the measured speed of light would remain isotropic and constant without requiring adjustment to the clocks.
It may be the answer must await empirical determination but I am drawn to number two in spite of the obvious logical reasons why it shouldn't apply.
ANy comments appreciated thanks

 

[Al68]

AN experimental test of these alternatives might be ; If a system in inertial motion has its clocks synched by converntion and is then accelerated to a new significant velocity before resuming inertial motion there would seem to be two possibilities:
The clocks need to be resynched through convention. Light measurement would not automatically be correct.
Or
The acceleration and new relative velocity has in itself shifted the clocks into a new relative synchronization so the measured speed of light would remain isotropic and constant without requiring adjustment to the clocks.

It seems that each choice is half right.

The clocks would be out of synch, but light measurement would still automatically be correct, since it does not rely on clocks being in synch, only that they run at the same rate.

The second choice is also right that light would remain isotropic and constant without requiring adjustment to the clocks, although the clocks are out of synch.

Clocks in an inertial frame that are out of synch will still show equal elapsed time for equal distances traveled by light.

For example, if light is emitted equidistant between two clocks, one reading zero and the other reading t1 when the light is emitted, the first will read t and the second will read t + t1 when the light is received, both showing an elapsed time of t.

 

[YellowTaxi]

@ heldervelez. You are certainly free to prefer Lorentz's Aether Theory over Einstein's Special Relativity, but since Lorentz's aether is, by design, completely undetectable experimentally you are simply not going to get a lot of enthusiastic support for the extra and unnecessary complication. Go ahead and use it, and we will go ahead and not use it, and we will still all agree on everything measurable.

- including the constancy of any observer's value of c

But the difference between the 2 versions IMHO is just an argument of semantics. Neither is superior or more logical than the other. Until somebody can give a logical reason for the constancy of c. Or show that an ether does exist.

but since Lorentz's aether is, by design, completely undetectable experimentally

Are you certain of this Dale? Most our experiments to detect the aether have been carried out on Earth. (not much use really). Like trying to show that everybody in the world speaks English because we talked to everyone in england and they all spoke English.

And secondly can't it be argued that einstien's GR is just an aether theory ? The Aether just distorts in a region of large mass density.

 

[matheinste]

Don't advocates of both theories agree that it is, even in theory, impossible to decide beteween the two formulations. That is, no experiment can be designed to decide between the two. Both formulations predict exactly the same observable outcomes.

Matheinste.

 

[Dale]

But the difference between the 2 versions IMHO is just an argument of semantics.

I tend to agree.

Are you certain of this Dale? Most our experiments to detect the aether have been carried out on Earth.

Yes, I am certain of it. Both theories are simply interpretations of the Lorentz transforms. So they make the same predictions in all circumstances. An extra-terrestrial experiment which detected aether would disprove both SR and Lorentz's Aether Theory.

 

[yuiop]

...
“To account for these experiments on the basis of a fixed ether it would be necessary to introduce ingenious assumptions as to a change in length or Lorentz-Fitzgerald contraction just sufficient to give a null effect in the Michelson experiment, and as to a change in period or time dilation just sufficient to give a null effect in the Kennedy experiment-all to the end of retaining a fixed ether so devilishly constructed that its existence could never be detected.”
...

The above attack on the Lorentz Ether theory suggests that Special Relativity does not treat time dilation or length contraction as a physical manifestation, but just a difference of opinion between observers where all such measurements are simply "relative".

But...

AN experimental test of these alternatives might be ; If a system in inertial motion has its clocks synched by convention and is then accelerated to a new significant velocity before resuming inertial motion there would seem to be two possibilities:
The clocks need to be re-synched through convention. Light measurement would not automatically be correct.
Or
The acceleration and new relative velocity has in itself shifted the clocks into a new relative synchronization so the measured speed of light would remain isotropic and constant without requiring adjustment to the clocks.

The first option is the prediction of SR and the Lorentz transformations. After a synchronised system has been accelerated, its clocks will no longer be in sync when it acquires a new constant velocity. The one-way measured speed of light will no longer be correct or directionally symmetric until the clocks are resynchronised. This also poses the question as to how the clocks got out of sync, if we hold to the interpretation of Special Relativity that time dilation is not actually a physical occurrence. If we interpret the time dilation and length contraction of Special Relativity as physical, then it is quite ingenious how they are just sufficient to ensure that the speed of light is constant for all inertial observers. Maybe nature is devilishly constructed whichever way you look at it.

 

[matheinste]

The above attack on the Lorentz Ether theory suggests that Special Relativity does not treat time dilation or length contraction as a physical manifestation, but just a difference of opinion between observers where all such measurements are simply "relative".

Because there is no differnce in predictions between the two formulations I am OK with either of them. But Einstein's seems simpler and, to me, more aesthetically pleasing. I have no personal desire to attack followers of LET or their beliefs as the differences of viewpoint are irrelevant to physics if not to philosophy.

As regards how SR treats the reality of length contraction I personally take the view of Rindler and most other textbook writers when they say "Length contraction is 'real' in every sense of the word", and is, in theory, experimentally demonstrable.

Matheinste

 

[Austin0]

It seems that each choice is half right.

The clocks would be out of synch, but light measurement would still automatically be correct, since it does not rely on clocks being in synch, only that they run at the same rate.

The second choice is also right that light would remain isotropic and constant without requiring adjustment to the clocks, although the clocks are out of synch.

Clocks in an inertial frame that are out of synch will still show equal elapsed time for equal distances traveled by light.

For example, if light is emitted equidistant between two clocks, one reading zero and the other reading t1 when the light is emitted, the first will read t and the second will read t + t1 when the light is received, both showing an elapsed time of t.

Hi Al68
There may be some miscommunication here as to the meaning of out of synch.
It seems to me fairly sure that unsynched clocks cannot measure the elapsed transit time of anything at all. They must be synched by some rational method and operate on the assumption of simultaneity/synchronicity even if in the case of SR this is an operational assumption with no implcation of actuality outside the frame.

For example, if light is emitted equidistant between two clocks, one reading zero and the other reading t1 when the light is emitted, the first will read t and the second will read t + t1 when the light is received, both showing an elapsed time of t

In your example here you omitted the time of emission at the midpoint. Consequently the observed proper times at the two receivers have no basis for evaluating the elapsed time.
For them to have a time for the emission requires, both that they know the observed time at the midpoint and that that clock is not only operating at the same rate but is synchronized with their clocks.

In my example the assumption was that after attaining a new velocity that simple , normal one way light tests or measurements would be conducted, on the naive assumption that the clocks were still in synch. And would then either be correct or not.
If the clocks are no longer in synch they will not directly return isotropic measuremnts at c. I agree that asynchronous clocks can be used to correctly make those measurements but only if the degree or interval of desynchronization is known beforehand. This effectively synchronizes them through calculation.
If the distance is known, this discrepancy can of course be determined as the tests become one way synchronization procedures; as the proper time at the midpoint is transmitted and the recipient clocks can determine the correct time of reception from this [Emission T + D/c = Reception T ] and then correct their clocks.
It is a given that the measured speed of light will be isotropic and constant if correctly measured in any frame but in my setup the question was really an either or ,,,,with a clearcut A or B outcome regarding purely the clocks. Thanks

 

[Dale]

After a synchronised system has been accelerated, its clocks will no longer be in sync when it acquires a new constant velocity. ... This also poses the question as to how the clocks got out of sync, if we hold to the interpretation of Special Relativity that time dilation is not actually a physical occurrence.

Time dilation is a comparison between two different inertial frames at the same event, not a comparison from a single inertial frame before and after acceleration.

 

[Dale]

Because there is no differnce in predictions between the two formulations I am OK with either of them. But Einstein's seems simpler and, to me, more aesthetically pleasing. I have no personal desire to attack followers of LET or their beliefs as the differences of viewpoint are irrelevant to physics if not to philosophy.

I agree completely.

As regards how SR treats the reality of length contraction I personally take the view of Rindler and most other textbook writers when they say "Length contraction is 'real' in every sense of the word", and is, in theory, experimentally demonstrable.

I just don't like using the word "real" as it always leads to semantic arguments about the definition of "real". So instead I describe length contraction as "coordinate dependent and measurable". That way people who think that "real" -> "coordinate independent" can draw the conclusion that length contraction is "unreal" and people who think that "measurable" -> "real" can draw the conclusion that length contraction is "real", both without drawing me into the semantic argument.

 

[Austin0]

The above attack on the Lorentz Ether theory suggests that Special Relativity does not treat time dilation or length contraction as a physical manifestation, but just a difference of opinion between observers where all such measurements are simply "relative".

But...



The first option is the prediction of SR and the Lorentz transformations. After a synchronised system has been accelerated, its clocks will no longer be in sync when it acquires a new constant velocity. The one-way measured speed of light will no longer be correct or directionally symmetric until the clocks are resynchronised. This also poses the question as to how the clocks got out of sync, if we hold to the interpretation of Special Relativity that time dilation is not actually a physical occurrence. If we interpret the time dilation and length contraction of Special Relativity as physical, then it is quite ingenious how they are just sufficient to ensure that the speed of light is constant for all inertial observers. Maybe nature is devilishly constructed whichever way you look at it.

Does either SR or the Lorentz math really make any predictions in this regard?
For all the factors: time dilation, contraction and desynchronization the math provides a quantitative prediction for observed relationships but as far as I can see does not go into any mechanism behind these effects and therefore neither provides nor leads to any real expectations outside of inertial conditions.
As you question here,,, how would the clocks get out of synch?
Although in this example it seems to me more a case of them retaining the original synchronization which is no longer appropriate to the new velocity rather than positing a change in the clocks.
The second alternative, where the clocks directly return correct readings at a new velocity would seem to suggest a change in the clocks themselves through acceleration or relative velocity. Which is not so different from a change in length is it? In which case as is discussed right here, it is somewhat unclear as to the meaning or physicallity of these "changes" and certainly no rational physics to explain a mechanism effectuating this in any of the cases.
To my mind SR does provide a rational explanation for the logically challenging isotropic constancy of c but time dialtion and length contraction by themselves are insufficient. It seems to demand clock desynchronization as an essential element as well.
In any case it appears to be a fiendishly clever arrangment of physical factors , no argument there.

 

[Austin0]

Time dilation is a comparison between two different inertial frames at the same event, not a comparison from a single inertial frame before and after acceleration.

Why do you make the distinction if we are talking about the concept??
You could easily make a comparison between a frame initially at rest with a second frame , which then accelerates to a new velocity . COuldnt you then quatitatively derive a dilation factor through comparison to this refernce frame that would be equivalent to a comparison of a single frame before and after acceleration?

 

[matheinste]

I agree completely.

I just don't like using the word "real" as it always leads to semantic arguments about the definition of "real". So instead I describe length contraction as "coordinate dependent and measurable". That way people who think that "real" -> "coordinate independent" can draw the conclusion that length contraction is "unreal" and people who think that "measurable" -> "real" can draw the conclusion that length contraction is "real", both without drawing me into the semantic argument.

The inverted commas around 'real' are my addition as I am aware that the word can cause all sorts of problems. I quoted from memory but I have copied from source below. I don't know if the addition of the words "relative to a given frame" make it any more acceptable.

---It must be stressed that the phenomenon is not to be regarded as illusory, due perhaps to some peculiarity in our methods of measurement: relative to a given frame it is real in every possible sense.----

Rindler--Special Relativity. Published by Oliver and Boyd second ediition 1966 page 26.

Matheinste.

 

[Austin0]

Suppose, for example, that two inertial sets of axes A and B are in motion relative to each other, and that at a certain instant their origins coincide: and suppose that at this instant a flash of light is generated at the common origin. Then, by what has been said in the subsequent propagation, the wave-fronts of the light, as observed in A and in B, are spheres whose centres are the origins of A and B respectively, and therefore DIFFERENT spheres. How can this be?
Matheinste.

I have nothing to add to the general explanation which is well covered by this post and others in this thread.
What seems to be central to the logical confusion of the OP and others is the term observation. The actual colocated observers in both frames simultaneously observe the same singular spherical wavefront. Just at different proper times. These are the actual "observations"
There are no observers in either frame that have the perspective we enjoy looking at the situation. Within the systems it is impossible to "observe" the spheres or relation to the origin. This has to be calculated from the disparate measurements after the fact and then reconstructed. These constructions are the two spheres, which as you point out have no objective reality outside of the individual frames.

 

[Dale]

Why do you make the distinction if we are talking about the concept??

I make the distinction because there appears to be some confusion in the community as a whole about what the terms "length contraction" and "time dilation" refer to.

If you are measuring "before" and "after" some process then it is simply not length contraction or time dilation. If you are involving acceleration then it is probably also not length contraction or time dilation (unless you are being really careful with the use of non-inertial frames). Length contraction and time dilation are comparisons between measurements in different reference frames for the same event, not comparisons between measurements in a single reference frame for different events.

You could easily make a comparison between a frame initially at rest with a second frame , which then accelerates to a new velocity . COuldnt you then quatitatively derive a dilation factor through comparison to this refernce frame that would be equivalent to a comparison of a single frame before and after acceleration?

I certainly wouldn't use the word "easily" to describe it. This frame would be non-inertial so the usual SR formulas would not apply. You could do it if you were careful as I mentioned above, but you would have GR gravitational time dilation effects as well as SR velocity time dilation effects. And the results would be different depending critically on the specific details of how you arbitrarily chose to define your non-inertial coordinate system.

 

[Al68]

If the clocks are no longer in synch they will not directly return isotropic measuremnts at c. I agree that asynchronous clocks can be used to correctly make those measurements but only if the degree or interval of desynchronization is known beforehand. This effectively synchronizes them through calculation.

This is exactly what I meant. But knowing beforehand the "interval of desynchronization" is required even if that interval is zero, ie even if the clocks are in synch, that must be known beforehand.

Like you said, knowing the "interval of desynchronization" is what matters, making that interval equal to zero (synchronizing) just simplifies the math.

 

[Austin0]

Originally Posted by Austin0
Why do you make the distinction if we are talking about the concept??

I make the distinction because there appears to be some confusion in the community as a whole about what the terms "length contraction" and "time dilation" refer to.

If you are measuring "before" and "after" some process then it is simply not length contraction or time dilation. If you are involving acceleration then it is probably also not length contraction or time dilation (unless you are being really careful with the use of non-inertial frames). Length contraction and time dilation are comparisons between measurements in different reference frames for the same event, not comparisons between measurements in a single reference frame for different events.

Agreed there is confusion regarding terms.
I may have been unclear in my post. There is no acceleration involved in the comparison which is assumed to take place after attaining a new inertial velocity.
I am unsure of what you mean by referring to measuremnet of the same event. In the case of contraction are you referring to two simultaneous measurements within a frame as a single event?
In the case of dilation isn't it always a case of comparing the interval between two events as measured in different frames??

Originally Posted by Austin0
You could easily make a comparison between a frame initially at rest with a second frame , which then accelerates to a new velocity . COuldnt you then quatitatively derive a dilation factor through comparison to this refernce frame that would be equivalent to a comparison of a single frame before and after acceleration?

I certainly wouldn't use the word "easily" to describe it. This frame would be non-inertial so the usual SR formulas would not apply. You could do it if you were careful as I mentioned above, but you would have GR gravitational time dilation effects as well as SR velocity time dilation effects. And the results would be different depending critically on the specific details of how you arbitrarily chose to define your non-inertial coordinate system

Simply put: Frame A and Frame B are comoving . A specific interval between events (dt) in A can be assumed to be the same as (dt') in B.
Frame B is then accelerated through whatever profile to effect a return pass by Frame A in an inertial state. The interval in A [dt ] is repeated and compared with the same interval measured in B [dt'] and the relative dilation observed and calculated .
In what way do you think this would not be logically and quantitatively equivalent to a comparison of B [accelearated] with B when it was comoving with A ?
As you can see the period of acceleration does not enter into the comparison or measurements.

 

[Austin0]

This is exactly what I meant. But knowing beforehand the "interval of desynchronization" is required even if that interval is zero, ie even if the clocks are in synch, that must be known beforehand.

Like you said, knowing the "interval of desynchronization" is what matters, making that interval equal to zero (synchronizing) just simplifies the math.

OK we have reached agreement about synchronization.
But this still, really, begs the original meaning of my question with its assumption of a normal concept of synchronization with its, as you put it, previous knowledge of zero interval of synchronization.

 

[yuiop]

Does either SR or the Lorentz math really make any predictions in this regard?

In an old thread (that I am too lazy to track down right now) I demonstrated that using the "relativistic rocket equations" http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html and the equations of Born rigid motion http://www.mathpages.com/home/kmath422/kmath422.htm, that the clocks would be be out of sync after the acceleration phase. These equations are based on SR and the Lorentz transformations and as far as I know, are not controvertial.

As you question here,,, how would the clocks get out of synch?
Although in this example it seems to me more a case of them retaining the original synchronization which is no longer appropriate to the new velocity rather than positing a change in the clocks.

That is another way of looking at it, but the end result is the same.

Consider the following experiment. Say we start with two identical rockets facing in opposite directions and at rest with respect to each other. At the nose and tail of each rocket are synchronised ideal clocks that have been sealed to make them tamper proof. At a later time, one of the rockets accelerates away and then settles down to a constant non zero velocity relative to the other rocket. An observer onboard the rocket that accelerated, would be able to determine that that it was his rocket that underwent acceleration, simply by comparing the clocks fore and aft on his own ship even if he slept through the acceleration phase and even if there were no accelerometers onboard the rocket. At the end of the experiment the observer onboard the accelerated rocket could consider himself at rest and that it was the other rocket that is moving away, but he would be aware that "something physical had happened to his own rocket". On the other hand, if each rocket only had a single clock each, then the situation would have the appearance of being symmetrical, if they missed the acceleration phase.