Talk:Relativity of simultaneity

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Position of observers[edit]

I find the statements on the position of the observers to be unhelpful in this article. For the thought experiment to be valid, the observer on the train does not need to be in the centre of the carriage, nor does the the flash need to be emitted the moment the train passes the observer on the platform. Observations of time differences between two events depends on relative motion of observers, not on their exact position at any given time.

I would remove these statements about observers positions. They tempt the reader to start thinking about visual perception of the events (thinking about time it takes photons to get from events to them, which is _not_ part of the argument) rather than the _observation_ of events which is independent of location. I made this edit, but it was reverted - "Undid revision 447223181 by (talk) undid removal of essential information) (undo)" — Preceding unsigned comment added by (talkcontribs)

The text says "The observer onboard the train sees the front and back of the traincar at fixed distances from the source of light and as such, according to this observer, the light will reach the front and back of the traincar at the same time." That can only happen if this observer is midway inside a speeding traincar, which is a word that you removed in your edit. The other part that you removed ("... just as the two observers pass each other") is also essential in the setup. This thought experment is set up, precisely to show what happens when the two observers assume that light speed is isotropic for both of them. As such you had removed essential information. Note that this setup is extensively described in the literature (see, for instance here), and changing something to that setup would not only amount to inducing erroneous information to the article, but also, and perhaps even more importantly, to wp:original research. DVdm (talk) 12:12, 31 October 2011 (UTC)
I believe you are making a common misunderstanding about the meaning of an observers perception of events. The observer on the train 'sees' the two ends of the train at fixed distance from the light source wherever the observer stands. These distances are of course not changed if the observer stands in a different place in the car. In your statements you seem to considering the visual perception of the scene by the observer (involving photons coming to his eyes from events - these consideration play no part in this argument (nor any other dilation/contraction experiments) as can be seen by the fact that in the train diagrams that depict the two frames, there is no indication of where the observers are, only an understanding of their relative motion that sets the frame in which light travels isotropically out at c for each case. In this article there are several unfortunate reference to observers 'seeing' things. In better texts there is a more careful language of 'observations' that try to avoid making people think of photons travelling from events to the observer. "This thought experment is set up, precisely to show what happens when the two observers assume that light speed is isotropic for both of them" - this is correct, but still not dependent on the 'location' of the observers, just their relative velocity.
The page you refer to here is a slightly different thought experiment in which the specified location of the observers is used to simplify the argument (but is not essential to the physics).
I'll make another argument to support this view. Think of there being several observers on the train, at different places on the train. They are all in the same inertial frame, and so they all agree that the light hits the ends of the train at the same time, as they agree on the time of all events. Just consider the Lorentz transformation with v=0 if you disagree with this statement. If you still disagree, then I think you are confusing time of visual perception of events including light travel time from the events to the observer, with the 'observed' time of events, for which this is accounted for. (Observers at different distances from a lightening strike see and hear the lightening and thunder at different times, but they all agree that the events themselves were simultaneous.) So, all observers on the train agree the events are simultaneous wherever they are standing. All observers on the platform agree that the events are not simultaneous wherever they are standing, and they agree on the delta-t between the events. Thus is cannot matter which specific observers we choose for our thought experiment, provided one is on the train, and one on the platform. Therefore, we need not specify the observers' locations.
A related argument from Lorentz transformations. Take two events, and use the Lorentz transforms to transform the measured time interval between the events from one inertial frame to another to get
\delta t' &= \gamma \left( \delta t - v \delta x/c^{2} \right)  \\ 
Note that the transformation of the time interval between the frames depends on the separation of the events in the unprimed frame, but not on the absolute positions of the events which is the thing that changes when we change the position of the unprimed observer. Only the relative velocity of the two observers and the separation of the events determines the disagreement in elapsed time between event. Relative position of the observers is not a factor.
A last point. This wiki page is not alone in making misleading statements that I am arguing against. Many web sites, and many text books (mostly at high school level) also make these errors, probably from a misguided attempt to simplify what is a hard subject to grasp. Higher level texts such as at university level are mostly more precise, and I'd suggest that such texts are the only place to check our understanding, rather than other webpages or wikis. I'd recommend the text book Griffiths, Introduction to Electrodynamics. For the record, I'm a university lecturer teaching Special Relativity at second year undergraduate level. — Preceding unsigned comment added by (talkcontribs)
Please sign your talk page messages with four tildes (~~~~). Thanks.
Sure, indeed in Griffiths' intro to electrodynamics "the lamp is equidistant from the two ends", and the "observers" could be anywhere, one somewhere on the train and riding along with the lamp, another standing on the ground, or if you like, looking at figures 12.4 and 12.5 on page 484 (in my edition). In Einstein's quoted text, which lies at the basis of the text in this article, the train observer (M') coincides with the lamp, whereas the ground observer (M) is a person standing at the place where the light flash is emitted. Here you see the difference between the modern "observer" concept as a reference frame, and the common day-to-day "observer" as a person making observations — see also our article Observer (special relativity). The day-to-day way is how the original thought experiment was described, and also the way it was done in this text, to, as you say, "simplify the argument". It is after all, as the article says (with proper and relevant references), "a popular picture", and as such I don't see anything wrong with that. DVdm (talk) 23:18, 5 November 2011 (UTC)
I actually said that in the slightly-different Einstein thought experiment here the observers are put centred to simplify the example. But that is a different thought experiment to the one presented on this page, a fact acknowledged in the History section of the present article, and so we should not be blindly including constraints from that experiment here if they are not required. In the thought experiment on the present page, which is a valid variation of Einstein's original, observer locations are not part of the logic, and it is a mistaken attempt to simplify the experiment by introducing unnecessary constraints that actually encourages people to misunderstand the nature of the simultaneity issue - which is a physical reality, and not just an issue of visual perception. As you say, Griffiths, in his wisdom, in his presentation of this thought experiment correctly does not specify locations for the observers, just frames; I'm not sure why the fact that Griffiths does this does not convince you that Wiki can and should omit it too.
Look at the spacetime diagrams for the thought experiment adjacent to the Lorentz Transformation section. These show the light flash occurring at an arbitrary positive x (and an arbitrary positive t for that matter), showing that the x-position of the flash in each frame (i.e. the location of the flash relative to each observer) is of no consequence. It then seems strange to specify a specific observer location in the preceeding section when a different and arbitrary location is used in these diagrams.
I accept that there is a distinction between the modern 'reference frame' observer, and the day-to-day concept. However, it is implicitly the reference frame observer that is invoked in this version of the thought experiment (e.g. no observer, or photons travelling from events to observers, are drawn in the train diagrams; spacetime diagrams are used that are implicitly reference frame tools), whereas it is the day-to-day observer that was used in Einstein's original version (with explicit consideration of light travel time from events to observers). This is exactly why the Einstein version requires the locations of the observers to be specified, and this version does not. Since the locations are not required here, it can only confuse to include them.
Thanks for chatting, and apologies for neglecting to sign the previous posts. If I haven't convinced you (or others?) yet, I think I will never succeed, so I will leave it at that. Congrats anyhow on an (otherwise) excellent treatment of SR in general. (talk) 03:10, 6 November 2011 (UTC)
I agree. The thought experminent in the text differs from the diagrams'. I objected to your edit because it changed one version to the other while leaving the referenced sources untouched, which is what we call wp:original research. I wouldn't mind if you would rework the text (and/or the diagrams), but the sources should reflect the changes — or rather, the new text (and/or diagrams) should reflect the content of the new sources. Cheers - DVdm (talk) 11:00, 6 November 2011 (UTC)
OK. It's just a referencing problem. There are actually no sources quoted in the text right now for this thought experiment, just refs 1, 2, and 3 that are stated to be similar experiments (but not identical and so we might expect differences). I suggest that the solution is to also reference a text book with this exact thought experiment (e.g. Griffiths), as well as these current references. This should ensure that everything is suitably verifiable. (talk) 21:14, 7 November 2011 (UTC)

Further Reading[edit]

I'd like to suggest adding a "Further Reading" section to this entry. One proposed reading would be the following:

  • Craig, William Lane and Smith, Quentin (Editors) (2008): Einstein, Relativity and Absolute Simultaneity. Routledge, London and New York. ISBN13: 978-0-415-70174-7.

However, being a relatively inexperienced editor, I'm not sure of the correct way to do this. Any help from more experienced hands would be appreciated. Thanks. JCNSmith (talk) 02:45, 8 March 2012 (UTC)

I don't think this is a good candidate for further reading, as it is mainly philosophical, and rather fringy. For instance, on page 5 it says: "Apart from leaving unaddressed the epistemolocial and semantic presuppositions of STR, there is an even stronger factor behind physicists' unwillingness to abandon the Special Theory. [...] In fact, there is a theory that is not merely observationally equivalent to the Special Theory, but also observationally superior to it, namely Lorentzian or neo-Lorentzian theory..." On page 6: "It may be concluded that the main reasons why many physicists still hold to Special Relativity are (1) an insuficient awareness of the epistemological and other philosophical problems with Special Relativity; [...]" - etc... There's also an essay by fringe scientist Tom Van Flandern. - DVdm (talk) 08:22, 8 March 2012 (UTC)

Thanks for your comments. I agree that the quality of essays in the book I recommended is uneven, but I found enough of interest in it to think it might be worthwhile for those interested in the topic. Regardless of whether the book I mentioned is included in a "Further Reading" section or not, however, I certainly think the topic merits having a Further Reading section of its own! Perhaps you could recommend some other, more worthy readings to populate such a section? Thanks. JCNSmith (talk) 20:21, 8 March 2012 (UTC)

I had a look at the other related articles on consequences of special relativity (Time dilation, Length contraction, Velocity-addition formula, Mass–energy equivalence) to see how it's done overthere. It looks like none of them has a further reading section. I guess the reason is that in each case the references section is pretty rich already and provides ample reading material. Having had a look at the guideline Wikipedia:Further reading, I think the reason is indeed clear. As these subjects are pretty narrow, it's hard to find book material that is sufficiently topical (per subsection 1.1). The Craig collection doesn't qualify on topicality either, as there's more on quantum stuff than on this article's subject. Furthermore, as becomes quite clear from the introduction of the book, the authors have an agenda based on a fringe view on special relativity, which certainly makes it insufficiently balanced (per subsection 1.3). I guess, if you're looking for good reading, there's some really good pointers in the ref list. - DVdm (talk) 21:32, 8 March 2012 (UTC)

Thanks for the guidance. As noted, I'm relatively inexperienced at this, so appreciate your insight and pointers to Wiki policy/guidance on the use of Further Reading sections. JCNSmith (talk) 03:33, 9 March 2012 (UTC)

2nd Paragraph Illustration[edit]

Something is wrong with this sentence (from 2nd para. of article): "Where an event occurs in a single place–for example, a car crash–all observers will agree that both cars arrived at the point of impact at the same time." First of all, it is postulated that "an event occurs," and since you can't attribute the property of simultaneity to a single event (at least two events are required for the property of simultaneity to come into being), we're off on the wrong foot. We then are meant to visualize a car crash, which again is a single event, but which *by definition* requires two things in the same place at the same time, i.e., the fact that there was an impact *requires* there to have been the simultaneous arrival of two things at the same place and time (overlapping space and time: hmm, yep, requires simultaneity already). It is thus a tautological endorsement because the postulation of a wreck requires there to have been two things in the same place and time for all reference frames. In other words, it begs the question about simultaneity being relative, and therefore shouldn't be used to illustrate relative simultaneity.

It seems to me one would somehow have to illustrate two different things happening (in a physics sense involving states of particles and fields, etc.) simultaneously at the same point to evoke the desired comparison to the space-separated case, but I'm not sure anyone can come up with a proper example of such a thing. Perhaps there is a quantum mechanical angle here. I'm out of my depth. I do hold to the above (logical, not physics) objection, though. Chafe66 (talk) 07:41, 14 May 2012 (UTC)

Yes, it looks like this is caused by a tendency to identify an event with its definition. If one defines event_A with definition_A ("car_A arrives at crossroads X") and event_B with definition_B ("car_B arrives at crossroads X"), then we have in fact two different definitions for one event, and we have that event_A = event_B, although definition_A ≠ definition_B. We can even add a third definition: "car_A and car_B crash at crossroads X". Tricky. Perhaps we could add one word ("distinct") in the preceding sentence: ... whether two distinct events occur at the same time..., and then remove the car crash sentence altogether. I also think there's a style problem with the sentence: the car crash is a dashed example, but the main sentence uses the cars from that example. - DVdm (talk) 08:52, 14 May 2012 (UTC)
Sounds good. But I don't really think the issue has to do with definitions and reference. Consider thinking of "car_A arrives at crossroads X" as simply naming an event rather than defining it. Now from philosophy of language, we know that the quoted phrase has a sense and a reference. The reference is the event itself, and the sense (the "meaning") of the phrase has to do with an automotive type object appearing in reference to an intersection of paths, etc. These are different things (one involves mass and energy, etc., the other is a linguistic entity) and aren't really being confused here (in my opinion). To use your terminology, my objection is this: the meaning of "Car_A arrived at CRs X at the same time as car_B arrived at CRs X" logically entails that there was a crash and the meaning of "there was a car crash between car_A and car_B at CRs X" logically entails the former phrase. They have the same reference, but, as in your example, different meanings. So either 1) if one event happened then two more events happened or 2) if two events happened then a third event happened. I'm sure the list goes on. "Metal was bent in such and such a way" could be added to probably an infinite list. But they all describe a single event--in a certain sense. And if we pick the single event as the canonical representation of "what happened" then it begs the question about simultaneity at a single location. But that has me wondering if one can even conceive of examples where two distinct things (there's your word) could happen at the same place and time. What kind of example would it be if not something like the car crash example?Chafe66 (talk) 09:38, 14 May 2012 (UTC)
I don't really care about the difference between, like you say, "naming an event rather than defining it". In this context the difference is not important. Let's just get rid of the sentence, unless of course we have a proper source for it. Lacking such a source we can dump it as someone's original research. I'd go ahead and wp:boldly do it. - DVdm (talk) 10:13, 14 May 2012 (UTC)
I would, but I honestly wouldn't know what to change it to. I can't think of an example involving a single location that makes sense as an illustration of possible non-simultaneity! I.e., in order for the example to work, the events described at a single location would have to be conceivably not simultaneous, and events happening successively at a particular location already assumes they're not simultaneous...sigh.Chafe66 (talk) 18:22, 14 May 2012 (UTC)
I don't think we have to change it. Let's just dump it. Ok, I went ahead and dit it. We'll see where it gets us. Good catch, by the way! - DVdm (talk) 18:33, 14 May 2012 (UTC)
 :) Cheers. Thanks for all your work on this article, and being so open to discussion. Chafe66 (talk) 18:46, 14 May 2012 (UTC)
"Where an event occurs in a single place–for example, a car crash–all observers will agree that both cars arrived at the point of impact at the same time." ~ There are two distinct kinds of simultaneity of position: 1) causal, and 2) observational. A crash is causal simultaneity. Observational simultaneity requires separation of events that are observed to be simultaneous from some locations but not from others. Location, relative velocity, and relative acceleration of the observer determine what is (actually! if you really think about it, is) simultaneous. BlueMist (talk) 08:59, 14 May 2012 (UTC)
But this example has to do with, first, the fact that an event happened *at* a single location, and the notion of where observers are is invoked afterward. What is first postulated is observerless. Something happened independently of whether anyone observed it at all, let alone their relative locations. My point has to do with the logical entailing of each other of "crash" and "car_A arriving at X at the same time as car_B." The postulation of a crash requires (unless one thinks a crash can have occurred wrt one reference frame and not another, which breaks some accepted law I'm sure) that this alleged 'simultaneous' event of two things arriving at the same place and time also occurred. So it can't have been otherwise if there was a crash; so I think "crash" should be left out of it altogether. But that raises another issue. See above...Chafe66 (talk) 09:38, 14 May 2012 (UTC)

Here's another way of putting it: the very notion of simultaneity requires the notion of sameness of time and place, particularly now that relativity has shown how we need to think about temporality when one event happens in location A and another event happens in distinct location B. If we can't talk about being in the same place at a given time, then we don't even know what "two events being simultaneous" could possibly mean. Therefore, we can't use any single event at a given place as an example of simultaneity (either causal or observational). Why not? Because if a given thing happening at a given time means something else happened at that same time and place, then those things could not have been unsimultaneous, by the definition of whatever that event is. And that doesn't help in terms of what's needed in order to contrast with the coming example (in the article) explaining simultaneity's being relative.Chafe66 (talk) 10:11, 14 May 2012 (UTC)

No, it doesn't. A crash is a single event without separation, so it looks the same from everywhere. But two separated events may or may not, seemingly and actually, *be* simultaneous depending on the location, velocity, and acceleration of the observational frame of reference. This concept is extremely counter-intuitive. Space-time is not a fixed Newtonian construct. Simultaneity of two events is relative, see [1]. ~ BlueMist (talk) 00:13, 16 May 2012 (UTC)
We're talking about different things. My subject has to do with logic and the example of a single event as being helpful or meaningful. You are talking about principles of physics. You're taking the example as given, and moving from there. I'm not taking the example as given, i.e., not accepting it as saying anything other than what is required by logic (it's true "by definition"), hence it (example of crash) is not a case in point of a physical principle. It's required definitionally, and nothing can be concluded from such statements.Chafe66 (talk) 07:34, 16 May 2012 (UTC)
Aristotelian logic of either-or cannot be applied to relativity of simultaneity. The same pair of events separated in space are both simultaneous and not simultaneous. Even at the same time, if time is properly adjusted by the speed of light. This is not an illusion, but a demonstrable reality. Again, see wikibooks for a more detailed explanation[2]. The point is that Aristotelian logic fails in all cases of physical relativity.
The single event example is used in contrast. But if a pair of events are gradually merged, then the single event becomes just a special case of a more general law. As far as I am concerned, you can take it or leave it. ~ BlueMist (talk) 11:20, 16 May 2012 (UTC)
Yes, let's leave it, unless there's something notable, interesting and properly sourced to add to the article. Otherwise this is just off-topic chat per our wp:talk page guidelines. - DVdm (talk) 11:31, 16 May 2012 (UTC)

Lead promotes a philosophical bias[edit]

"In physics, the relativity of simultaneity is the concept that distant simultaneity – whether two spatially separated events occur at the same time – is not absolute, but depends on the observer's reference frame. According to the special theory of relativity, it is impossible to say in an absolute sense whether two distinct events occur at the same time if those events are separated in space, such as a car crash in London and another in New York. The question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in other frames (in a different state of motion relative to the events) the crash in London may occur first, and in still other frames the New York crash may occur first."

The bias is that simultaneity "depends on the observer's reference frame." I will give an example where that is clearly not the case. A split-beam laser is fired at two targets on the Moon, separated by whatever distance. The beams leave the laser at the same instant when it is fired and travel at the same velocity (c), so therefore they arrive at the two targets simultaneously. That (thought experimental)fact is established. Now introduce two observational frames, passing through the Earth/Moon system. Each will observe the targets to be struck by the beams at different times. The point is that the above assumed philosophy, that simultaneity depends on the observers' reference frames does not change the established simultaneity of strikes hitting the two targets, separated by whatever distance on the moon's surface. The assumption upon which the lead statements are based endorses a philosophy which absolutely precludes the possibility of events happening simultaneously independently from when they are observed from different frames. My example illustrates that such an assumption is false. LCcritic (talk) 21:19, 1 December 2013 (UTC)

Alas, on article talk pages we are not allowed to discuss our own philosophical, psychological, or even scientific views and musings about the subjects of our articles — see wp:talk page guidelines. This was explained to you before in the discussions at
I have now left a 3rd level warning on your talk page. DVdm (talk) 08:02, 2 December 2013 (UTC)


Since the length of an object is the distance from head to tail at one simultaneous moment, it follows that if two observers disagree about what events are simultaneous then they will also disagree on the length of objects.

If a line of clocks appear synchronized to a stationary observer and appear to be out of sync to that same observer after accelerating to a certain velocity then it follows that during the acceleration the clocks ran at different speeds. Some may even run backwards. This line of reasoning leads to general relativity.

Just granpa (talk) 21:00, 4 April 2014 (UTC)

Please note that article talk pages are for discussions about the article, not about the subject. See wp:talk page guidelines. - DVdm (talk) 09:06, 5 April 2014 (UTC)

The train-and-platform thought experiment[edit]

A naive question. This thought experiment assumes that observers in both the train car frame and the platform frame agree that the flash of light is emitted at the midpoint of the train car. But why should this be true?

For example, if in the platform frame, the light flash occurs at a point (1-β)/2 behind the front of the car (assuming the car has length 1 in the platform frame and β is the dimensionless velocity v/c) then in the platform frame the light flashes will arrive simultaneously at each end of the car, just as in the car frame.

I don't understand how the postulates of special relativity (equivalence of physical laws and light speed in all frames) prohibit this possibility. Jeffrw (talk) 07:32, 27 July 2014 (UTC)

The middlepoints are taken because it makes the calculation trivial, and to precisely demonstrate the point of ROS—see the literature. If you don't understand something about it, the place to ask is our wp:reference desk/science. Here, per wp:talk page guidelines, we are supposed to discuss the article, not the content. Good luck at the refdesk. - DVdm (talk) 09:27, 27 July 2014 (UTC)