VandD

Due to relativity of simultaneity, relative moving observers don't measure between same set of simultaneous events. Hence different distance, length.

Keep in mind that in 4D spacetime the 'object' is not a 3D item: a measured 3D object is only part of the full object, being 4D. An observer doesn't view a 3D object from a new frame. A new (different) frame measures a different 3D section through the 4D object, hence a different 3D unit is the result. All 3D sections are equally "physically real".

To be honnest, I completely lost track of what the real issue is. Or worse, I probably never understood what it is all about. That's not the first time when I read Tim posts... Mea culpa.

Obviously the experimental evidence -in the lauch frame- will be the same, for the simple reason the exercise wants to keep distance to be the same. But that's not the full story. With your Bell/Poincaré contraction only applying to objects, there IS empirical difference between Einstein and Bell/Poincaré, ... in a rocket frame, after launch: In such a frame, for Einstein the distance (space) between rockets increases. For Bell/Poincaré it doesn't. If Bell/Poincaré only applies length contraction to objects, then he only applies the Lorentz Tranformations to objects (?). If he wants to teach special relativity that way, then he is wrong. Because special relativity is not about only objects contracting. The 'contraction' refers to changing of length. Doesn't make a difference whether it's about objects or distance between objects (space).

Are you now telling me that "length contraction" only refers to dynamics as measured in a single reference system ?

You said: Is doesn't lead to self contradictions such as Bell's spacehip paradox. For Einstein "space itself" does contract, but in Bell's case study one prevents that empty space to contract. In Bell's case study, the space does not contract. Einstein agrees. BUT... not because "only objects Lorentz contract", but because Bell's set up is such that one prevents -per launch frame- the empty space contraction between the rods. Don't you understand the difference? Now tell me what Poincarés explanation is for the space between the rods NOT contracting. Is it same explanation as Einstein's?

You wrote: For a space traveller the distance, empty space, between earth and a star contracts because of Lorentz contraction. Hence not only objects Lorentz contract.

Tim, to help you understand special relativity and Bell's paradox: In Bell's spaceship paradox it's a given to keep distance (length) constant between the rockets in launch platform frame. Then wonder what happens to the rope. It breaks. Why? Let's start the easy way. If the rockets would accellerate in such a way that in their common reference frame the distance between rockets remains constant (if they are at rest relative to each other they share same ref system), then for us on launch platform, the rope and the distance (or 'empty space' around the rope if you like) between rockets, would contract. Or do you think by cutting the contracting rope all of a sudden the rockets would seperate because the space between the rockets can not be contracted? Do yo think the contraction of the rope pulls both rockets together?? In Bell's spaceship paradox we keep the distance between rockets constant by accellerating the rockets equally and simultaneously, per launch platform frame. This - and not because "only objects can Lorentz contract" - actually prevents the rockets -still according to launch platform frame- from getting closer to each other ('empty space' contracting between rockets). But in a rocket frame the other rocket will seperate (both rockets will not share same ref frame) and stretches rope until it breaks. One can say that per launch plaform frame the string wants to contract but is prevented doing so (there will be relativistic stress in the rope) because we (the set up of the paradox) don't allow the distance (of empty space) between the rockets to contract. That's a total different story than pretending only objects can Lorentz contract

This doesn't make sense. Length contraction occurs because of relativity of simultaneity. Whether there is 'material' between the events or only empty space doesn't make any difference. For a space traveler the distance of empty space to a star gets shorter. Period.

Indeed. And you forgot once more to read what followed: << But.... the lights from the bolts DO NOT reach me simultanously, hence the two events DID NOT occur simultaneously for me>> You are wrong. The whole gedanken experiment is the find out whether for the embankment observer AND train observer the two events occured simultaneousy WHEN THE OBSERVERS WERE TOGETHER. The embankment observer says that when they were together both flashes occurred simultaneously for him (embankment observer), the train observer says both flashes didn't occur simultaneously when they were together. That's the relativity of simultaneity to be proved. Both observers are at the same spot, but disagree on simultaneity of spacelike like events. Yes, and WHY is that important? TO FIND OUT WHETHER THE EVENTS OCCURRED SIMULTANEOUSLY FOR THE TRAIN OBSERVER WHEN THEY WERE TOGETHER, AND WHETHER THE EVENTS OCCURRED SIMULTANEOUSLY FOR THE TRAIN OBSERVER WHEN THEY WERE TOGETHER! I give up, pzk, I see it's hopeless continue discussing this with you. Good luck.

This is really getting ridiculous. Stop inventing things I didn't say. I NEVER said an observer meets the other observer when the strikes are seen. I said that IF the two events would ALSO occur simultaneously for the train observer, THEN the beams should reach the train observer simultaneously. BUT the train observer experiences they DON'T reach him (the train observer) simultaneously, hence the two flash events DID NOT occur simultanelously for the train observer. It's all about whether the two flashes at the ends of the train occur WHEN THE OBSERVERS ARE TOGETHER. The embankment observer says that when they were together both flashes occurred simultaneously for him (embankment observer), the train observer says both flashes didn't occur simultaneously when they were together (which is correct because for the train observer NONE of the flash events occurred when they were together, but that's beyond the scope of the experiment which is only to know whether they happened simultaneously or not for the train observer. And the train observer gets the answer: the flashes didn't occur simultaneouslyfor him. Period.) Very weird you don't understand what I write. It was not a 'correction'. Did I say it was wrong what you stated? I said to be careful when stating <<2. Both observers, given the initial setup, are entitled to consider themselves as located between the two events>> because readin your sentence an unititiated reader might think that from the train observer's point of view he (train observer) is halfway the events WHEN BOTH EVENTS OCCUR (which is wrong, see above) . THAT is the reason why I should not phrase it like you did. But I never said it your senence was wrong. Your analysis of what I said is wrong. And you keep on telling me it's wrong because you keep on reading what I didn't write. Very weird.

Never. I said that if the events also occur simultaneously for the train observer, then ...etc. Read my lips: I said if... I never said they actually do. I don't see where you get this. See my post 21: The train observer says: << IF both flashes also occur simultaneously for me when (at the moment in time) I meet the platform observer, THEN both light beams HAVE TO reach me simultaneously. But.... the lights from the bolts DO NOT reach me simultanously, hence the two events DID NOT occur simultaneously for me.>> Maybe I had better said: <<If both flashes WOULD also occur simultaneously for me ... >> ? Sorry, english is my third language....

Pathetic.

No, I talked about the train observer. yes The OP asks for the point of view of the train observer. He considers himself at rest and the platform moving. That's exactly what I do. It doesn't answer how the train observer has to reason. From the point of view of the train observer he (the train observer) DOES NOT move towards the beam of light. I.o.w. your <<Now in reality (considered with .... etc >> is not a reality for the train observer. He cannot use that. That's the problem with focussing on location. People might think M1 moves from the midpoint towards (the location of) the first flash. It doesn't. The train observer doesn't consider himself moving toward the front beam, nor flash location. It's not silly, because you also have to refer to what the embankment observer experiences if both events occur simultaneously for the embankment observer. If you don't, why would you need an embankment observer in the thought experiment? If I read you correctly you would simply state: there are two events at same distance from an observer, the beams do not reach observers simultaneously, hence events didn't happen simultaneously for him. The whole purpose of the experiment is to find out whether the events occur simultaneously when both observers are together. From the train observer's perspective he (train observber) does not move toward the front beam, nor to location of flash.Hence what is left for the train oberver to know whether the events did happen simultaneously or not? All train obsever can think of is: <<IF both flash events also occur simultaneously for me when I and embankment observer are together, then both lightbeams should also reach me simultaneously. But I experiences they don't reach me simultaneously, hence flash events didn't happen simultaneously for me.>>

Be careful stating it this way. Both observers are indeed together when for the platform observer both events simultaneously happen. But that's a statement that is not valid for the train observer. Because for the train observer when he (train observer) passes the platform passenger NONE of the two events occur for the train passenger! Hence stating the train passeger was in the middel of the two events is very miseading statement. Actually for the train passenger first the front flash occurs when the train passenger is not yet at the platform observer. And the second flash occurs when the train passenger has already passed the platform observer. The reference text states that M1 moves relative to platform observer only to show/explain that the lightbeams definitely do not REACH the train observer. It is never stated that the two beams left the flashes (= the two events) when M1 is at platform observer. Why? Because for the simplle reason it's the purpose of the experiment to find out what actually happened (whether the two events occur simultaeously or not) for the train observer when he is at the platform observer! The train observer has to reason as follows: << IF both flashes also occur simultaneously for me when (at the moment in time) I meet the platform observer, THEN both light beams HAVE TO reach me simultaneously. But.... the lights from the bolts DO NOT reach me simultanously, hence the two events DID NOT occur simultaneously for me.>> Actually in the train thought experiment the train observer doesn't know when exactly the flashes occur. But that's not important. What is important for the experiment was to find out whether for the train observer the flashes happened simultanously or not. And the experiment gives the answer: they don't happen simultaneously for the train observer. Actually I do agree with you that the train thought experiment is not so easy to interpret correctly ... for the unitiated. I seems piece of cake, but actually it takes some time to understand the analysis correcty. Get to the bottom of it and you will love Einstein's genius thought experiment for the rest of your life

Sorry for typo (I also reshaped the text of analysis in new post original thread. )

You probably missed (or didn't fully understand) my post #5

You mean a lot of CORRECT posts suffer being moved to the WRONG thread because of moving a 'block of posts', collateral damage so to speak? Is that the correct way to manage a forum? This forum needs urgently some computer expert to sort out the "block of posts" problem. If my post suffered collateral damage I don't see why I should not copy paste my post back in the correct thread. I wait moderation motivation.

Hi Studiot I don't see why I should go private to discuss this. It's of everybody's interest to know why my post is considered 'speculations'. I deal with Eistein's thought experiment for over 30 years now. It's the first time in my life the analysis is considered 'speculations'. Of course I'm emotional about it.

My post http://www.scienceforums.net/topic/102777-madmac-surprise-hijack-from-two-bolts-strike-train/#entry969577 had nothing to do with any hijack or 'speculations'. It's totally unacceptable to move my post to speculations section.

Why has my post been moved to the 'speculation' section????? This is unacceptable! My post has NOTHING to do with speculation. It's the correct down down to earth analysis of Einstein's thought experiment. @Modarator: Please explain why my post falls under speculations.

Be carefull here. When the train passenger passes the platform passenger NONE of the two events occur for the train passenger! Hence stating the train passeger was in the middel of the two events sounds a bit awkward. For the train passenger first the front flash occurs when the train passenger is NOT YET at the platform observer. And the second flash occurs when the train passenger has already passed the platform observer. But the platform observer doesn't know this at the start of the experiment. The train observer can only reason as follows: << If both flashes also occur simultaneously for me when I meet embankment observer, then I am in the middle of the flashes and they happen at same distance from me, hence both lights from bolts HAVE TO reach me simultaneously. But.... the lights from the bolts do NOT reach me simultanously. It can only be because for me (in my frame), It can not be correct that lighting bolts occurred simultaneously when I and platform observer>> Actually the platform observer doesn't know when exactly the flashes occur. But that's not important. What is important for the experiment was to find out whether for the train observer the flashes happened simultanously or not. And the experiment gives the answer: they don't happen simultaneously for the train observer.

Two events separated by zero space and zero time are a contradiction in terms. Two events mean space separation and/or time separation. You cannot melt two events into one.

Yest, but I think Tir21 wants to know how the thought experiment runs from the train passenger point of view. That's a different story. I'll give it a try: @Tir21 Very good question, Tir21. Let's first start with Einstein's setup before jumping to your question. . -The embankment observer sees both bolts simultaneously. For him, in his frame of reference, both lightning bolts happened simultaneously( lights from the bolts travel same length). -The train passenger does not see both bolts simultaneously. Why does this mean for him both bolts didn't happen simultaneously? If one would state (as is often the case) it is because he moves toward the front bolt, we could throw in maybe also for the train observer both bolts happened simultaneously, but he only SEES the light from the events at a different moment in time because he moves towards the light coming fron the front bolt and thus sees that one first... No relativity of simultaneity in that scenario! But in fact he does't see them simultanously because when he meets the embankment observer no lighting bolt occurs: 1. first the front bolt occurs (before the train passenger meets the embankment observer), 2. then both observers meet. No lighting bolt occurs. 3. Finally the rear bolt occurs, after both observers separated a split second ago. (Note: in train frame both light will travel dame distance to train passenger) Look at it this way: when train passenger and embankment passenger meet, both bolts happen simultaneously for the embankent observer. Now, IF both bolts would ALSO happen simultaneously for the train passener when both observers meet, then the train passenger HAS to SEE both bolts also simultaneously, because in his frame the lights travelling from the simultaneous events also travel same distance (not necessarily same distance as in embankment frame, but that is irrellevent because only the front and rear distance in the train frame is important for the train observer analysis!), and hence would reach him simultaneously. But.... because the train observer sees both bolts not simultaneously he concludes: "For me (in my frame), lighting bolts occuring simultaneously when I and embankment observer meet can not be correct". We know the train passenger indeed does not see the light from both bolts simultaneously, because that's what's experimentally observed bij the embankment observer. Receiving light from one bolt after recieving light from the other bolt are two different events. And that cannot change for the train passenger. Now we get to your question: a thought experiment from the train passenger's point of view for simultaneous lighting bolts in the embakment frame. Both bolts occur simultaneously in the embankment frame. Lights from the bolts do not reach the train passenger simultaneously (=two events). That's what he will physically experience and observe. Your question now is: how could the train passenger EXPLAIN seeing the front bolt first? Train passenger knows the set up: in embankment frame both bolts occur when both observers meet. And lights hit embankment observer simultaneously (=one event). Now train passenger he reasons as follows. "IF they ALSO occur simultaneously for me when I meet embankment observer, then they are also at same distance from me, hence both lights from bolts HAVE TO reach me simultaneously. But the lights from the bolts do NOT reach me simultanously. It can only be because for me (in my frame), lighting bolts occuring simultaneously when I and embankment observer meet can not be correct" Is there another option? The train observer might think for him both bolts DID occur simultaneously, but both lighning bolts were not at the same distance from him... hence both lights do not reach him simultaneously... But that's not a valid option. Because it would mean the three events 1/ front lightning bolt 2/ both observers meet 3/ rear lightning bolt occur simultaneously in train AND embankment frame, but the embankment observer considers both observers halfway between the two simultaneous lightning bolts, whereas the train observer considers both observer NOT halfway between simultaneous bolts... You might think it's a valid option, but there is no theory to explain it. ------- Your question might be: if we consider the train at rest end the embankment moving, why does the train passenger knows he will not NOT see both bolts simultaneously (lights from the bolts not reaching him simultaneously)? Or better formulated, why HAS IT to be that way (not seeing them simultaneously)? The only 'thought experiment' reasoning we can make is: he and the embankment observer know it happens that way, because in the embankment frame the bolts reaching the train observer are two different events and they can not melt together into one event in the other frame. And the best way to find out is doing the experiment. Both lights from bolts will and do not reach the train observer. But as a thought experiment one has also to wonder why ;-).

That's indeed your personal guess. I know what Einstein wrote: << Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence. >> (Albert Einstein, "Relativity", 1952). << From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world". >> (Albert Einstein. "Relativity: The Special and the General Theory." 1916. Appendix II Minkowski's Four-Dimensional Space ("World") (supplementary to section 17 - last section of part 1 - Minkowski's Four-Dimensional Space). <<...for us convinced physicists the distinction between past, present, and future is only an illusion, although a persistent one." >> ( Letter to Michele Besso family, March 21, 1955. Einstein Archives 7-245). In case you doubt; Karl Popper about his encounter with Einstein: << The main topic of our conversation was indeterminism. I tried to persuade him to give up his determinism, which amounted to the view that the world was a four-dimensional Parmenidean block universe in which change was a human illusion, or very nearly so. He agreed that this had been his view, and while discussing it I called him "Parmenides".... >> (Karl Popper, Unended Quest: An Intellectual Autobiography.Routledge Classics. Routledge. pp.148–150). And Einstein believed in physics when he said his famous quote: <<People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion." Einstein died one month and 3 days after his friend, on 18 April 1955.>> https://en.wikipedia.org/wiki/Michele_Besso