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Thread: Two-way relativity?

  1. #1 Two-way relativity? 
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    In a book I read when I was younger (I'm 14 now, so excuse me for annoying ignorance) the way relativity explained space being squished by speed could be explained using a train with some kids in it with a laser guns and some really accurate timers/laser detectors. The guy at the front of the train shot the laser at the guy at the back of the train, who recorded a time that, according to the speed of light, showed that the train was the same length as it was while it was stationary. Another kid outside, however, was timing it as well, and it was explained that while the light was moving from the gun to the back, the back had, obviously, moved forward, and light-speed doesn't change depending on how fast anyone's moving (since everything's relative). This would result in the reader reaching the light beam sooner, resulting in a smaller size of recorded train size. This was all well and good, and I accepted the explanation for a while, but then I realised: what if the gun was at the back of the train and the reader was at the front? With the same explanation, we could deduce that the train had become longer, rather than shorter!
    A book that I have just recently read is Ender's Game (anticipating getting the DVD when it comes out! ), and (spoiler alert) Mazer Rackham uses relativistic speed to come back 80 years later while he's only aged 8 years. This theory also has a similar conundrum. According to relativity and the people on Eros, Mazer's moving at light speed, causing his time to move slower. When he gets back, everyone sees him as younger than he should be and he helps save the world and everyone's happy. But considering it from his point of view, Eros is the one moving at light speed, and their time is moving slower. When he gets back, he has aged 8 years, but everyone else has just aged an eighth of a year. This sounds like it could almost be simple: everyone on Eros conceives that he has aged less, while he sees everyone else as aged less. But for everyone to see him as aged less, there must be a Mazer Rackham that has aged less than everyone else to be seen. But this makes it seem as if he has to age both less and more, just like the train has to both stretch and shrink. The corrolation between them would also make sense, seen as both space and time are connected and basically the same. Firstly (and probably most relevantly), how ignorant am I? Secondly, if I'm not being ignorant, who's thought this up before? Thirdly, if I'm not being ignorant, and no-one's thought this up before, how revolutionary is it? Thanks in advance. FYI, the book where I got the squished space theory from was 'Dead Famous: Albert Einstein and his Inflatable Universe'.


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    Brassica oleracea Strange's Avatar
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    This is an example of the "Twins Paradox" (which isn't a paradox).

    In the first case of a train moving at a constant speed, there is no way of telling if the train is moving or you are moving and the train is stationary. Yoo can only detect/measure speed relative to something else so it is symmetrical.

    Because it involves one person going somewhere and coming back, there is acceleration. Acceleration is absolute: you can feel it so you know who is accelerating and who isn't, so the situation is no longer symmetrical.

    How does relativity theory resolve the Twin Paradox?: Scientific American
    Twin paradox - Wikipedia, the free encyclopedia


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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by duncrob99 View Post
    In a book I read when I was younger (I'm 14 now, so excuse me for annoying ignorance) the way relativity explained space being squished by speed could be explained using a train with some kids in it with a laser guns and some really accurate timers/laser detectors. The guy at the front of the train shot the laser at the guy at the back of the train, who recorded a time that, according to the speed of light, showed that the train was the same length as it was while it was stationary. Another kid outside, however, was timing it as well, and it was explained that while the light was moving from the gun to the back, the back had, obviously, moved forward, and light-speed doesn't change depending on how fast anyone's moving (since everything's relative). This would result in the reader reaching the light beam sooner, resulting in a smaller size of recorded train size. This was all well and good, and I accepted the explanation for a while, but then I realised: what if the gun was at the back of the train and the reader was at the front? With the same explanation, we could deduce that the train had become longer, rather than shorter!
    What this set up actually describes is the concept of the Relativity of Simultaneity. Imagine that there were guns at both the back and front of the train. Also imagine that you have an observer in the train at the midpoint. The guns are fired such that the lasers beams pass him going in opposite directions at the same time. Since the lasers travel an equal distance to reach him, according to him (or anyone else on train, the guns were fired simultaneously.

    Now consider things from the perspective of the kid standing on the embankment. He agrees that the lasers passed the midpoint of the train at the same time. But, he also will measure that the laser coming from the front of the train takes less time to reach the midpoint than it does for the laser coming from the back of the train. Therefore, according to this Kid, in order for the laser to meet at the midpoint of the train, one gun has to have fired before the other.

    This leads to the conclusion that simultaneity is not absolute but relative.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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