Does the train example show relative simultaneity?

[swansont]

Relativity of one event to different times of different frames is issue of simultaneity. Don't make axiom without math.

 

In relativity examples, simultaneity refers to two events being measured by different observers.

 

 

From the 1905 paper (italics in original, bold added)

So we see that we cannot attach any absolute signification to the concept of simultaneity, but that two events which, viewed from a system of co-ordinates, are simultaneous, can no longer be looked upon as simultaneous events when envisaged from a system which is in motion relatively to that system.

 

Since we're discussing a version of Einstein's train example, I think we should defer to his explanation of simultaneity. It is the simultaneity of two events that is in question. He discusses the issue of two observers not agreeing on the time a signal arrives as a synchronization of clocks issue; IOW, the light pulse being sent at noon in one frame can't be used to synchronize clocks in another.

[DimaMazin]

 

But you can only measure the times that the event is observed in different frame with respect to some other event. Then it becomes a matter of relativity of simultaneity (between the two events).

Yes and observation with measurement is an event.

[swansont]

Yes and observation with measurement is an event.

 

Not according to Einstein, and probably most physicists.

[DimaMazin]

 

Not according to Einstein, and probably most physicists.

Then start of relative motion of O1 and O2 from co-location and start of the photons motion similarly are two events according to Einstein.

[Strange]

Yes and observation with measurement is an event.

 

IF that is true, then you are dealing with three events: the flash of light, detection by one observer, detection by the other observer. I have no idea how you expect to define or test the simultaneity of any of these.

Then start of relative motion of O1 and O2 from co-location and start of the photons motion similarly are two events according to Einstein.

 

They don't start in relative motion. They are always in relative motion.

 

I'm not sure why you are going on this weird diversion as the concept simultaneity is clearly defined (as quoted by swansont). Why make up you own version (which is not obviously useful or even meaningful)?

[md65536]

Not according to Einstein, and probably most physicists.

But it could be. Absorption of a photon is an event. Then it's not a single light signal observed twice, but two signals sent to each observer.

 

Anyway events coincident with the observations could easily be defined, or unambiguously implied by the thought experiment.

 

I don't see how this is an issue. How does defining the events that yknot is implying change anything? The issue is that delay of light has to be considered, not that the number of events is wrong.

Edited November 14, 2013 by md65536

[Strange]

The point about Einstein's example is that you have two events which are very obviously and unambiguously simultaneous in one frame of reference.

 

Trying to identify a frame of reference where DimaMazin's two observer events are seen to be simultaneous is possible but not as simple. Why complicate things by introducing more events which do not form such a simple example?

[Janus]

 

Yes, of course it's because O2 is closer to the source, but how did he get closer? My answer is simply that he moved relative to the source or, more specifically, rel. to light's emission point, or even more specifically, he moved closer or toward the tip of the approaching ray. In other words, it's observer motion relative to the ray's tip that matters here, and not observer motions relative to each other. We can see this clearly by using a single observer and a light source. The closer this obs. is the the source, the quicker he will see the light that is emitted (toward him). Yes, this is too simple, but it is also critical to understanding E's train ex. which purportedly showed rel. simult. whereas all it really showed was the very simple fact that observer motion rel. to light makes diff. obs's see the light differently. We can say more: It is wrong to use light rays from events + differently-moving obs's to judge the events' occurrence times if you refuse to take into account the fact that the obs's move diff'ly wrt to the sources. (The best way to correctly time events is by placing correctly synch'd clocks at the events.)

But here's the point. It is only when viewed from the embankment frame that O2 sees the light from one flash before the other because he is closer to that flash.

 

In his own inertial frame (that of the train), he is not closer to the source of one flash than he is to the other. The lightning strikes the ends of the train and he is halfway between the ends. And here is where the invariance of the speed of light comes in. The light coming from one end of the train cannot travel towards him any faster than the light from the other, thus light leaving one end must take an equal amount of time to reach him as light coming from the other. The flashes of light do arrive at different times(this is something that both frames agree on.), so the lightning strikes that cause them must have occured at different times in the train frame.

 

Putting clocks that are synchronized to each other in the Train frame at the ends of the train does not change this conclusion, the clocks will record different times for the strikes.

[yknot]

>You don't seem to understand that the relativity of simultaneity is caused by the effects of motion that you are talking about (combined with the constant speed of light for both observers).

 

Question: Would sim. be rel. given truly synch'd clocks* AT the events? If so, then rel. sim. is not given by nature and is therefore not a part of physics.

Question: Which experiment has revealed the "constant speed of light" from point A to point B in any inertial ref. frame?

 

>What do you mean by observers "see the light differently"?

 

One sees it before the other despite the fact that they started out side-by-side when the light started.

 

 

>No one is refusing to take the movement of the observers into account. That relative movement is the cause of relativity of simultaneity (and time dilation, length contraction, etc)

 

It is not the "relative motion" of the observers wrt each other that is being ignored; it is the motions of the obs's wrt the light signals that is being ignored. And it is only this motion that matters in the train ex. and in my examples. Diff. frames move differently wrt any approaching ray of light and this must be taken into account when light rays from events are used to judge the events' temporal order.

 

*Here is E's def. of abs. time:

"[in classical physics] [t]he simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all [other] inertial systems. This is what is meant when we say that the time of classical physics is absolute." [Einstein's book _Relativity_, p. 149]

 

[Strange]

 

Question: Would sim. be rel. given truly synch'd clocks* AT the events?

 

 

See Janus's answer above.

 

 

If so, then rel. sim. is not given by nature and is therefore not a part of physics.

 

Huh? Of course it is part of nature. Physics is a description of nature.

 

 

One sees it before the other despite the fact that they started out side-by-side when the light started.

 

It is not about whether one sees a single light before the other. That is obviously, and trivially, true. It says nothing.

 

The relatively of simultaneity is about how the two observers see the relationship between two different events. Of course they will see them at different times. But one will consider the two events to have happened at the same time, and the other will consider the two events to have happened at different times. This is AFTER taking into account the arrival time of the light.

 

 

It is not the "relative motion" of the observers wrt each other that is being ignored; it is the motions of the obs's wrt the light signals that is being ignored. And it is only this motion that matters in the train ex. and in my examples. Diff. frames move differently wrt any approaching ray of light and this must be taken into account when light rays from events are used to judge the events' temporal order.

 

How is this being ignored. This is fundamental to Einstein's example. (BTW I should stress that this is just a nice simple example to explain the concept; it is not a derivation or a proof.)

 

 

Here is E's def. of abs. time:

 

Which he then goes on to demonstrate is incompatible with reality. You can stick with your mistaken beliefs, but I am going to go with reality. Even if it is surprising and rather marvellous.

[swansont]

Then start of relative motion of O1 and O2 from co-location and start of the photons motion similarly are two events according to Einstein.

 

The start of the relative motion of O1 and O2 are not relevant to the problem. This isn't that hard: in this example (and similar ones), a signal is generated by an event, and observers detect the signal.

[yknot]

yknot said:

 

Question: Would sim. be rel. given truly synch'd clocks* AT the events?

 

Strange replied:

>See Janus's answer above.

 

He made no mention of truly synch'd clocks, but Einstein did, saying that simultaneity would not be relative given such clocks.

 

Quoting Einstein:

"[in classical physics] [t]he simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all [other] inertial systems. This is what is meant when we say that the time of classical physics is absolute." [Einstein's book _Relativity_, p. 149]

 

If all it takes is a pair of (truly) synchronous clocks at the events to get rid of relative simultaneity, then it is clear that the latter is not given by nature but by man via his choice to use asynchronous clocks at the events or by improperly using light rays from the events.

 

The only way that observers can properly use light signals from events to judge the simultaneity of them is if the observers know how they are actually moving in relation to these light signals. But neither observer in Einstein's train example knows this. In fact, it is utterly ignored. (More on this below.)

 

But why go to the trouble of using observers and light signals; why not just place absolutely synchronous clocks at the events. As Einstein said, this eliminates relative simultaneity and gives all observers absolute time (for judging event time differences).

 

Strange:

>The relatively of simultaneity is about how the two observers see the relationship between two different events. Of course they will see them at different times. But one will consider the two events to have happened at the same time, and the other will consider the two events to have happened at different times. This is AFTER taking into account the arrival time of the light.

 

But if the observers know that one is actually moving _toward_ one light signal and _away_ from the other, and that the other observer is _not_, then they will simply conclude (correctly) that the events happened in only one way (say truly simultaneously), and will say that they cannot properly use light signals from the events whilst they are moving helter-skelter wrt said signals.

 

yknot said:

Here is E's def. of abs. time:

Strange replied:

>Which he then goes on to demonstrate is incompatible with reality

 

In exactly what way are truly synchronous clocks "incompatible with reality"? It seems to me that Einstein's clocks are the ones that are incompatible because they wrongly say that two events can be said to occur in many different ways (temporally) whereas anyone knows that events are independent of observer observations and events occur in only one way in reality. (Ironically, Einstein's above agrees.)

[Strange]

 

He made no mention of truly synch'd clocks, but Einstein did, saying that simultaneity would not be relative given such clocks.

 

Quoting Einstein:

"[in classical physics] [t]he simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all [other] inertial systems. This is what is meant when we say that the time of classical physics is absolute." [Einstein's book _Relativity_, p. 149]

 

You seem to be selectively reading and/or quoting from Einstein's work (I'm not going to try searching for an online copy to check the context. And anyway, there are far better sources to learn relativity from.)

 

The whole point of his work is to show that such absolute synchronization is not possible; there is no absolute time. If you haven't understood that, then clearly you haven't understood the theory of relativity.

 

There is little point you arguing against it from this position of relative (see what I did there) ignorance. You need to study the theory first, then ask for help with parts you have a problem with. Starting out with a closed mind is not going to get you very far.

 

 

whereas anyone knows that events are independent of observer observations and events occur in only one way in reality

 

I don't know how to respond to this. It appears to be a case of thinking that your intuition trumps objective evidence. I'm afraid that the timing of events is observer dependent (as are many other things such as length, energy, ...) whether you like it or not. The universe apparently doesn't care about your opinion.

Edited November 15, 2013 by Strange

[swansont]

Einstein said classical physics (i.e. with Galilean transforms) has absolute time. He then goes on to show that Galilean transforms do not describe reality, thus absolute time doesn't exist in reality. The "truly synchronous clocks" you want do not exist. That's not a choice, that's how nature actually behaves.

[DimaMazin]

 

 

 

They don't start in relative motion. They are always in relative motion.

 

 

Well, but event of change of color of co-located traveler exists.

 

The start of the relative motion of O1 and O2 are not relevant to the problem. This isn't that hard: in this example (and similar ones), a signal is generated by an event, and observers detect the signal.

If the observers don't observe each other then how are they defined as co-located at the same time of flash event?

[swansont]

Well, but event of change of color of co-located traveler exists.

I don't know what that's supposed to mean. What the hell does the color of the traveler have to do with anything?

 

 

If the observers don't observe each other then how are they defined as co-located at the same time of flash event?

Is that how the example is presented? (hint: no. There is no such thing as "the time of the flash")

 

 

But it could be. Absorption of a photon is an event. Then it's not a single light signal observed twice, but two signals sent to each observer.

An event, in this context, is something observable by all. When your detector absorbs a photon is only observable by you.

[DimaMazin]

I don't know what that's supposed to mean. What the hell does the color of the traveler have to do with anything?

 

Change of motion direction of traveler changes wave frequency of photons which are reflected from the traveler.

 

Is that how the example is presented? (hint: no. There is no such thing as "the time of the flash")

 

 

 

Flash event and any events of co-located observers are simultaneous.

 

[swansont]

Change of motion direction of traveler changes wave frequency of photons which are reflected from the traveler.

Which has nothing to do with the example.

[Strange]

Flash event and any events of co-located observers are simultaneous.

 

 

I'm not sure what that means. There are no co-located observers in the example and there are no observers co-located with the flash(es).

 

You seem to be making almost random statements in order to defend an untenable position. The train example is exceedingly simple. Why not use it?

[DimaMazin]

 

I'm not sure what that means. There are no co-located observers in the example and there are no observers co-located with the flash(es).

 

You seem to be making almost random statements in order to defend an untenable position. The train example is exceedingly simple. Why not use it?

You even can't mathematically check it !

[yknot]

No, this won't work. One of the ramifications of relativity is that it's not possible to synchronize clocks in different reference frames.

 

Since a negative statement cannot be proved, I ask for evidence.

 

You seem to be selectively reading and/or quoting from Einstein's work (I'm not going to try searching for an online copy to check the context. And anyway, there are far better sources to learn relativity from.)

 

The whole point of his work is to show that such absolute synchronization is not possible; there is no absolute time. If you haven't understood that, then clearly you haven't understood the theory of relativity.

 

There is little point you arguing against it from this position of relative (see what I did there) ignorance. You need to study the theory first, then ask for help with parts you have a problem with. Starting out with a closed mind is not going to get you very far.

 

 

I don't know how to respond to this. It appears to be a case of thinking that your intuition trumps objective evidence. I'm afraid that the timing of events is observer dependent (as are many other things such as length, energy, ...) whether you like it or not. The universe apparently doesn't care about your opinion.

You have a lot of nerve accusing someone of selective quoting without having even looked at the source. I think that you owe me an apology. And who better to learn SR from than the originator?

 

And the whole point of E's work cannot be to show that absolute synch is impossible because such a negative cannot be proved. If you know of such a proof, then state it.

 

I further resent being labeled close-minded this early in the game. See my last post for this "session."

 

As for my statement that events are observer-independent, here is someone else who stated that:

 

[Tom Roberts wrote:]

This is a foundation of modern physics: all physical phenomena are

completely independent of coordinates, because Nature quite clearly

doesn't need or use coordinates. Coordinates are merely tools of

description, and artifacts related to the choice of coordinates cannot

affect physical phenomena, only the _description_ of them. There's no

reasonable definition of "real" that makes _descriptions_ be "real".

http://groups.google.com/group/sci.physics/browse_frm/thread/d4ac4d13086ac07e/a29b0e07db6f1f8b#a29b0e07db6f1f8b

 

Maybe you would like to argue with Tom?

Einstein said classical physics (i.e. with Galilean transforms) has absolute time. He then goes on to show that Galilean transforms do not describe reality, thus absolute time doesn't exist in reality. The "truly synchronous clocks" you want do not exist. That's not a choice, that's how nature actually behaves.

What, specifically, is "wrong" with the Galilean trans. claim that light's speed from point A to point B varies per inertial frame?

 

OTOH, what is "right" about relativity's claim that light's speed from point A to point B is c in all inertial frames?

 

To whom it may concern:

 

Einstein's embankment observer says that he was centered on the events when they occurred, and he also says that since he need not take into account any movement toward either light ray, he can safely conclude that the events occurred simultaneously simply because the rays from the events reached him simultaneously.

 

Similarly, Einstein's train observer says that he also was centered on the events when they occurred, and he also says that since he need not take into account any movement toward either light ray, he can safely conclude that he events did not occur simultaneously simply because the rays from the events reached him at different times.

 

After being adjacent (or side-by-side), the two observers separate. Since they are in different locations when the light rays arrive, they _cannot_ see the rays in the same way. (This was even hinted at by Einstein when he said "If an observer sitting in the position M’ in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated.")

 

Clearly, this has nothing to do with how the lightning events occurred, but everything to do with the spatial separation of the two observers.

 

Just as clearly, it is improper to use light signals from events to judge their time order if observer differences (such as spatial separation during the observation) are completely ignored.

 

One way for Einstein's observers to validly use light rays from the events to judge their occurrence order would be if each observer knows how he moves in relation to the rays. For example, if the embankment observer knows that he remains centered on the approaching rays, then he can he validly claim that the events occurred simultaneously.

 

Of course, the best way for observers to determine event order is simply by placing truly synchronous clocks at the events in question. Here is how Einstein described such clocks:

 

"[in classical physics] [t]he simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all [other] inertial systems. This is what is meant when we say that the time of classical physics is absolute." [Einstein's book _Relativity_, p. 149]

 

Yes, Einstein decided to discard such clocks (because he wrongly believed that they violated the principle of relativity), but he of course could not prove the negative that such clocks cannot exist. All that is needed is some method for truly synchronizing clocks.

 

And after discarding truly synchronous clocks, Einstein replaced them with absolutely asynchronous clocks, but such clocks cannot possibly make correct measurements. As we all know, events are independent of coordinate systems, and two events can occur in only one way, and clocks should reflect this simple fact. For example, two people born on opposite sides of Earth will reflect the fact that they were born truly simultaneously by remaining practically the same age, so clocks should also reflect this.

 

Edited November 17, 2013 by yknot

[swansont]

Since a negative statement cannot be proved, I ask for evidence.

Synchronization requires clocks to run at the same rate. Clock frequencies vary by gamma from frame to frame, so clocks in different frames run at different rates. Q.E.D.

 

You're also wrong about proving a negative, because this is not a matter of data or evidence. The statement is a ramification of the theory, which means the prediction is mathematical and it is possible to prove a negative in mathematics.

 

 

What, specifically, is "wrong" with the Galilean trans. claim that light's speed from point A to point B varies per inertial frame?[/size]

It is observed not to agree with how nature behaves. It's also disagrees with electrodynamics, a very successful theory.

 

OTOH, what is "right" about relativity's claim that light's speed from point A to point B is c in all inertial frames?

It agrees with how nature behaves, to within experimental error. It's also consistent with electrodynamics, a very successful theory.

 

[yknot]

swansont, on 17 Nov 2013 - 10:11 AM, said:

>Synchronization requires clocks to run at the same rate. Clock frequencies vary by gamma from frame to frame, so clocks in different frames run at different rates. Q.E.D.

 

No, clock synchronization does not call for equal clock rates across frames; it just requires that the clocks in each frame are related in some way, preferably correctly (i.e., absolutely synchronized). (In order to determine event order, all you need is a pair of synch'd clocks no matter what rate they have as long as both have the same rate, as they will if they are in the same frame.)

 

>You're also wrong about proving a negative, because this is not a matter of data or evidence. The statement is a ramification of the theory, which means the prediction is mathematical and it is possible to prove a negative in mathematics.

 

Math is not physics. And how would you prove that two clocks cannot be absolutely synchronized? This could happen purely accidentally or by the random thumpings of a herd of monkeys, so it is indeed physically possible.

 

>It is observed not to agree with how nature behaves. It's also disagrees with electrodynamics, a very successful theory.

 

 

Einstein said otherwise. He said that given the clocks of classical physics (and he never said that such clocks cannot exist, only that we should discard them and replace them with his asynchronous clocks), light's one-way speed will certainly vary with frame velocity.

 

[Quoting Einstein:]

"w is the required velocity of light with respect to the carriage, and we have

 

w = c - v.

 

The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c.

 

But this result comes into conflict with the principle of relativity.... For, like every other general law of

nature, the law of the transmission of light in vacuo must, according to the principle of relativity, be the

same for the railway carriage as reference-body as when the rails are the body of reference."

http://www.bartleby.com/173/7.html

 

Actually, this does not come into conflict with the principle of relativity because that principle just calls for everyone to have the same general laws, and in this case the general law is w = c ± v. There was never even an apparent conflict, so there was never any need to invent a theory (SR) to "fix" it .

 

>It agrees with how nature behaves, to within experimental error. It's also consistent with electrodynamics, a very successful theory.

 

Here we have a major problem; there has never been any experiment showing light's invariance between two mutually-at-rest clocks; in fact, this cannot even be shown on paper (no matter how much math you use).

[xyzt]

swansont, on 17 Nov 2013 - 10:11 AM, said:

>Synchronization requires clocks to run at the same rate. Clock frequencies vary by gamma from frame to frame, so clocks in different frames run at different rates. Q.E.D

 

Here we have a major problem; there has never been any experiment showing light's invariance between two mutually-at-rest clocks; in fact, this cannot even be shown on paper (no matter how much math you use).

It is not "we" who have a problem, it is YOU who has a problem. Experiments contradict your claim above.

[swansont]

swansont, on 17 Nov 2013 - 10:11 AM, said:

>Synchronization requires clocks to run at the same rate. Clock frequencies vary by gamma from frame to frame, so clocks in different frames run at different rates. Q.E.D.

 

No, clock synchronization does not call for equal clock rates across frames; it just requires that the clocks in each frame are related in some way, preferably correctly (i.e., absolutely synchronized). (In order to determine event order, all you need is a pair of synch'd clocks no matter what rate they have as long as both have the same rate, as they will if they are in the same frame.)

My original statement was that that a ramification of relativity is that you can't synchronize clocks in different frames. So any discussion of synchronizing within a frame is moot. We already know we can do that.

 

Math is not physics. And how would you prove that two clocks cannot be absolutely synchronized? This could happen purely accidentally or by the random thumpings of a herd of monkeys, so it is indeed physically possible.

Math is part of physics, and the claim was purely mathematical. There is no way to synchronize clocks that run at different rates; synchronization requires equal frequencies and the same phase. Two values that are unequal can't be accidentally equal, regardless of the presence of monkeys.

 

Einstein said otherwise.

Only if you selectively quote and misunderstand him.