Relativity of Simultaneity

mangaroosh

Apologies har, I don't fully understand the point being made.

That's not really what I was trying to get at. As outlined above, an invariant measurement of c does not necessarily imply RoS; RoS is a consequence of the actual speed of light remaining unchanged. This requires us to assume RoS, which is the conclusion being drawn.

I think that saying that no absolute simultaneity can be measured is somewhat of a category mistake, because absolute simultaneity is not a physical object. However, a consequence of RoS, I believe, is that for each individual observer, their "past" state continues to exist in some reference frame, and their "future" state also "exists" in some reference frame; this requires each observer to make unverifiable assumptions about the existence of "past" and "future"; absolute simultaneity, however, doesn't require such assumptions, it simply requires us to accept the empirical evidence that an ever changing present moment is all that exists - this is because, I'm pretty sure, no observation of "past" or "future" can be made by an observer.

harrylin

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

GeorgeDishman

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.

harrylin

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

DaleSpam

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.

Consider the attached Venn diagram representing the set of all linear transformations on spacetime. The Lorentz transformation has [itex]\text{LC} \cap \text{TD} \cap \text{RoS}[/itex] so it is in subset 4. When you make a statement like [itex](\text{LC} \cap \text{TD}) \rightarrow \text{RoS}[/itex] 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 [itex]\text{LT} \rightarrow (\text{LC} \cap \text{TD} \cap \text{RoS})[/itex] 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.

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mangaroosh

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.

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.


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.

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

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

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

mangaroosh

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.

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

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

GeorgeDishman

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

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

bobc2

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.

mangaroosh

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

harrylin

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

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

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

DaleSpam

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.

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.