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Doubts in special theory of relativity

  1. Apr 1, 2013 #1
    Hello friends,

    From special theory of relativity,it is understood that if two events are simultaneous to a stationary reference frame,it is not simultaneous to a reference frame that is moving uniformly in straight direction. Is opposite possible? That is are two events are simultaneous to a reference frame that is moving uniformly simultaneous to a reference frame that is not moving?
  2. jcsd
  3. Apr 1, 2013 #2
    actually i meant the two events that are simultaneous to a reference frame which is moving uniformly simultaneous to a reference frame that is not moving?
  4. Apr 1, 2013 #3


    Staff: Mentor

    Yes, that is possible. If two reference frames are in the standard configuration (origins coincide at t=0, all spatial axes parallel, motion along x axis) then two events separated in the x direction which are simultaneous in one frame will not be simultaneous in the other frame, but two events separated in the y or z directions which are simultaneous in one frame will be simultaneous in the other frame.
  5. Apr 1, 2013 #4
    Those questions are really hard to understand but let's put it this way.
    You are at point 0 now one car comes from point -1 the other from point +1, if the both cars happen to travel at the same speed they will arrive at point 0 together and you standing in point 0 will be able to say that both cars started of simultaneously.
    The same goes for universe if we receive light at the same time from events that are 180 apart we can say that they both happened simultaneously.Ofcourse in universe there are forces like gravitational redshift and etc which bends light also slows it down so you have to account for that but for a simple picture I hope you got it.

    Now if you are moving and not standing in the point 0 then you will see the one car approaching you faster than the other , so a reference frame that is moving will not see two events starting at the same time in opposite directions because the observer itself is moving with respect to those events.
    The way you perceive events happening in relativity is bound to the position you have either a stationary or accelerating.
    But stationary positions can also be traveling at different speeds if compared to one another.
  6. Apr 1, 2013 #5
    so what is the answer to my quaetion?
  7. Apr 1, 2013 #6


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    Both DaleSpam and Crazymechanic gave you the same answer- yes, it is possible.
  8. Apr 1, 2013 #7
    i mean in x-direction itself. Well,i found out that it cannot happen. Events that are simultaneous to observer who is moving should be simultaneous to observer who is stationary. It is because time dilation takes place only to observer who is moving. The reason why time dilation takes place in moving frame becuase simultaneous events in stationary reference frame were not simulatneous to to moving frame.as a result they won't agree on time of the event. But opposite is not possible because stationary observer is not moving.
  9. Apr 1, 2013 #8
    well,i got the conclusion that it cannot happen.but one change will happen.in both frames,it will happen simultaneously but duration of the event may not be same.
  10. Apr 1, 2013 #9

    Vanadium 50

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    Well, you're wrong.

    Sorry, but there's no more polite way to say that. You asked a question, got an answer, and decided you don't like the answer. This isn't like "I hate broccoli", where any opinion is equally valid.
  11. Apr 1, 2013 #10
    Let me explain why it can't happen.I just want reviews about it.I want you guys t help me correct my conclusions as you guys are senior members!!

    According to me time can be simply explained as a series of all events happening in between the preferred two main events which explain the rate at which the time ticks. For example,i will make two ticks of a clock as the two main events.We can find that all the clocks which can make the two main events simultaneous go at the same rate. Now in between those two events,many events are taking place.some events are simultaneous.some events take place after that. So in between two main events,there are finite number of events that describe the time itself.So according to me time can be traced as events. And when events also have duration.(in this take this as according to stationary observer)

    So since i have given definition of time,let me bring out Einstein's thought experiment. All of you guys must have heard,so no need to explain it. In this experiment, Events that were simultaneous to the observer who is at rest was not simultaneous to the observer in the train. Based on this,i can say that clocks of observer at rest and observer in motion do not go at the same rate. let me bring two ticks of clock as main events. Events happening to stationary observer happens to observer who is moving. And also a particular duration of time is explained by not a single event,that duration of time is explained by many events also. That many events is what we call as simultaneous events. now these events are not simultaneous to observer who is moving.i.e:events which explained same duration of time explained different duration in the observer who is moving.Since events that take place in stationary observer do take place in all observers irrespective of their motion(as they are not travelling at the speed of light), Two ticks which are main events that describe same rate are delayed in the case of moving observer.That is why observer moving time slows down.

    Did you get my point up to here? Then i will explain the other part.
  12. Apr 1, 2013 #11
    Sorry friend,I didn't get the answer. I get conflicting answer. I posted this discussion as i didn't get the answer.Suddenly an answer came to me that it cannot happen.But here the posters said that it can happen.So don't i have the right to know what i was wrong in my conclusion?
  13. Apr 1, 2013 #12


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    ash64449, there is one point in the formation of your question I would address. You speak of "stationary frame" vs "moving frame". The "relativity" part of SR is that the observer in each frame sees himself as stationary and the other as moving. Note that there is Einstein's special relativity and its precursor, Galillean relativity which SR asymptotically approaches for low velocities relative to c and thus our experience based intuition tends toward assuming.

    [edit] My point here being that even in Galillean relativity you have no special "stationary frame" but motion is relative. (It's just that the transformation group is different.)

    I bring this up because formulating the question the way you do can introduce implicit assumptions (guided by your Galilean intuition) which are contrary to the facts in SR. You have to first understand Gallilean relativity, then unlearn those parts of it which differ in SR then work through some examples like the Twin "Paradox" doing the numbers so to speak. It takes a while to retrain your intuition. Remember that drawing a space-time diagram on paper, while a very useful tool, is embedding a non-euclidean geometry onto euclidean paper. You can't just "rotate" the paper but have to redraw different versions of a diagram for different obsever frames.
  14. Apr 1, 2013 #13
    yes.i know this friend.I am sure you know the famous Einstein's Thought Experiment:
    I know what you said above."The "relativity" part of SR is that the observer in each frame sees himself as stationary and the other as moving"

    I based on this one itself. Let me tell the problem in Galileo's theory of relativity.(though it had some points same as to SR,there is one problem. it is because of this problem that Einsteins lead to the creation of his famous thought experiment!!)

    the problem; According to both SR and Galileo's theory of relativity, the meaning of rest and uniform motion has no meaning. Everything is relative. It is because an object in rest would be in motion relative to something else.As a result we cannot say one is moving uniformly or he is at rest. You cannot identify your motion as you consider yourself as rest even though you are moving uniformly. But in actual sense he is moving. But he thinks that he is at rest. So it becomes a fact that one cannot identify who is moving who is not. But when we compare with the light,We can understand that we are moving!! This is contradicting.. Is this the thing you were trying to explain?
    OR are you telling that Maxwell's theory appear to contradict Galileo's theory of relativity?
  15. Apr 1, 2013 #14
    Sorry,Not moving frame, train moving relative to observer who is stationary.
  16. Apr 1, 2013 #15


    Staff: Mentor

    Restricting yourself to the x-direction only then two distinct events which are simultaneous to one will not be simultaneous to the other.


    Irrelevant. Time dilation is not the cause of the relativity of simultaneity.
  17. Apr 1, 2013 #16
    Who said??? i said Relativity of simultaneity is the reason for time dilation.i.e events are not simultaneous to moving train relative to observer stationary. And events are simultaneous to observer who is stationary. That is the reason why time dilation takes place.. Read the comment #10 And tell what was i wrong in that comment.
  18. Apr 1, 2013 #17
    Then what causes Time dilation?
  19. Apr 1, 2013 #18


    Staff: Mentor

    This is also false. The two postulates are the cause of time dilation, length contraction, and relativity of simultaneity. The effects do not cause each other.
  20. Apr 1, 2013 #19


    Staff: Mentor

    Two ticks of a single clock cannot be simultaneous in any reference frame. They are timelike separated. Only spacelike events can be simultaneous in any frame.

    No, there are an infinite number of events between any two events.

    Events do not have duration. They are instantaneous. I.e. they are "points" in spacetime.

    No. The relativity of simultaneity does not cause time dilation. They are both caused by the two postulates.

    Again, no, for the same reason as above.
    Last edited: Apr 1, 2013
  21. Apr 1, 2013 #20
    Read my comments carefully,i said two clocks whose pointers are simultaneous are simultaneous

    May be i was wrong on that sense

    May be i was wrong on this count also

    Actually because of those two postulates,Simultaneous events are not simultaneous as a result Time dilation can be explained.

    Same as the above i told
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