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Relative simultaneity

  1. Jul 13, 2018 at 12:00 PM #1
    Hello I'm having a lot of trouble understanding simultinaity and would appreciate any free book or article reccomendations.
  2. jcsd
  3. Jul 13, 2018 at 12:04 PM #2


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  4. Jul 13, 2018 at 12:26 PM #3
    I haven't looked at the wikki article because I don't trust it and don't want to be given misinformation.
    The main thing i'm having trouble with is it feels like the speed of light constant rule is being violated and I would like a bunch of examples of scenarios in order to prove to me it isn't being violated.
    For example: imagine a ship going V to the right. Someone on earth fires two lights that hit the edges of the ship simultaneously to the earth. There are three clocks on the edges and middle of the ship that are synchronized relative to the observer on the ship. What do they observe?
  5. Jul 13, 2018 at 12:59 PM #4
    I prefer the train platform that einstein originally used.

    Man standing on a platform has 2 lightning strikes equidistant from his eyes. He sees the lighting strikes occurring simultaneously.
    A person is passing by on a train that is moving at close to speed of light at the exact moment the lighting strikes. In this instant, the person is located equidistant from the 2 lighting strikes on the train, but is accelerating towards one at almost the speed of light, and away from the other at almost the speed of light. In this instance, the person on the train observes the events to occur one after the other, with the light beam that they are travelling towards seen first, and the light beam they are traveling away form second. This is due to the fact that events occurring "simultaneously" is a matter of perspective, or it is relative the the observer. This is the basis of special relativity, that there is no absolute time anywhere.

    When einstein then realized that gravity was an acceleration or a force, he was able to generalize this theory into the theory of general relativity.

    Atleast that's my understanding. There is a great NOVA on einsteins general relativity and all of the thought experiments that led him to realize it. It was a lot more to do with the fact that gravity is a force than it does with simultaneity in my mind. But maybe that's just what i'm getting from it.
    Last edited: Jul 13, 2018 at 1:08 PM
  6. Jul 13, 2018 at 1:10 PM #5


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    Look up Minkowski diagrams. They're basically displacement-time graphs, although time is conventionally drawn up the page. The key point to realise is that each choice of frame is simply a different choice of axes on the graph - axes slanted (it's a hyperbolic rotation, to be exact) with respect to the others. Because the axes aren't parallel, the idea of "at the same time" is different.

    Then go and have a look at http://ibises.org.uk/Minkowski.html, which is a little Javascript I put together that let's you draw Minkowski diagrams and boost smoothly from frame to frame. Instructions are on the page. Down the bottom are some buttons that automatically draw a few standard scenarios. I'm afraid I wrote this before I had a touch-screen device, so it assumes you can right- and left-click. I should get round to adding a touch-screen friendly interface one of these days.
  7. Jul 13, 2018 at 2:04 PM #6


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    This is confusing because the whole point is that "exact moment" is not a well-defined term - the two frames disagree on it. There are two resolutions to this. One is to specify which frame's simultaneity convention you are using. The other is to have the two observers pass at the instant they see the flashes, rather than when they occur. You seem to have used option one, but not specified the frame.
    There is no acceleration going on, and you can't measure acceleration in terms of speed anyway. It's like saying you are 2mph tall - it makes no sense.
    This phrasing obscures a key point - the person on the train is not moving according to them. And the speed of light is constant and the strikes occurred at equal distances. And the speed of light is constant. So the strikes cannot be simultaneous in this frame, as you go on to say.
    Gravity is neither an acceleration nor a force in general relativity.
    As noted, gravity is not a force in general relativity. Furthermore, the OP is asking about relative simultaneity, which is a special relativity phenomenon. General relativity is much more complex and, although the point that "simultaneous" is conventional remains true, concepts like "relative simultaneity" don't translate across very well since global inertial frames don't exist in curved spacetime.
  8. Jul 13, 2018 at 3:52 PM #7


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    Usually this happens when you've confused yourself about which frame you're using, and are trying to reason about one frame using some observations from another.
    We can straight up tell you the answer, but it might be more useful for you to tell us what you think happens and why it's problematic for you.
  9. Jul 13, 2018 at 5:46 PM #8


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    Far from contradicting the constant speed of light, it's exactly the relativity of simultaneity that allows different reference frames to measure the speed of light as identical (in a way that is understandable to all reference frames).
    Last edited: Jul 13, 2018 at 5:58 PM
  10. Jul 14, 2018 at 12:55 AM #9
    Wikipedia articles on established science topics are generally of good quality and good places to start if you want to know more about a science topic, as long as it isn't too deep into a particular discipline.
  11. Jul 14, 2018 at 5:23 AM #10
    Is there any experimental verification for simultaneity
  12. Jul 14, 2018 at 5:33 AM #11


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    For relativistic simultaneity? Yes. The GPS system was designed with corrections for relativistic effects. If these effects didn't exist, with the exact numerical values that relativity predicts, then the GPS system wouldn't work.
  13. Jul 14, 2018 at 5:55 AM #12
    Yes, but you can conclude it simply from the experimental results that show that time is relative. Can't have absolute simultaneity if you don't have absolute time.
  14. Jul 14, 2018 at 5:59 AM #13
    That's true. Some of them have contributions by PF advisors, like @DrGreg.
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