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Alternative way of looking at simultaneity is 1

  1. May 13, 2009 #1
    I dont know if this is new or not but an alternative way of looking at simultenaity is

    1. if you have two events
    2. draw a straight line joining the two events
    3 find the mid point of that line
    4 draw a plane perpendicular to that line at the midpoint

    any number of observers moving in any direction, either accelerating or not, on that plane will percieve the events as simultaneous

    The events can be moving wrt the plane as long as both events move parrallell to the plane
     
  2. jcsd
  3. May 13, 2009 #2

    dx

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    Re: simultenaity

    Straight line joining the events in spacetime?
     
  4. May 13, 2009 #3

    Mentz114

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    Re: simultenaity

    It's a common definition, but replace 'events' with 'spatial locations'. The spatial plane that lies symmetrically between the points is the 'plane of simultaneity'.

    Now forget all about simultaneity because it has no physical significance for spatially separated events.
     
  5. May 13, 2009 #4

    JesseM

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    Re: simultenaity

    Dreads wasn't talking about the plane containing both events, the idea was something about a plane perpendicular to the line joining the events. As such the argument doesn't work, if I'm interpreting it right. If you take a plane perpendicular to the line in spacetime joining the two events, and look at an inertial observer whose worldline lies in this plane, that observer would not view the events as simultaneous. Instead, if the blue x' line in this diagram is viewed as a line joining two events, then you'd need to take a plane which contains the blue ct' line (which obviously isn't perpendicular to the x' line) in order to ensure that an observer with a worldline in that plane would view the events as simultaneous in their own frame.
     
    Last edited: May 13, 2009
  6. May 13, 2009 #5

    Mentz114

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    Re: simultenaity

    Hi Jesse,

    Nor was I ( not intentionally, anyway ). On re-reading the first post I don't know what dreads means now. If the two events are completely arbitrary, all we can say is that there is a plane of simultaneity. If the events both happen at the same time on synchronised clocks, that plane is symmetrically placed.

    M
     
  7. May 13, 2009 #6
    Re: simultenaity

    Completely arbitrary doesn't quite work, because it doesn't exclude two events which are causally related and which are never simultaneous. Say I pick two events arbitrarily, then I do some research and find those events are where-when Fred was born and where-when Fred died. Even arbitrarily chosen, those events will never lie on any plane of simultaneity.

    cheers,

    neopolitan

    (Still away)
     
  8. May 13, 2009 #7
    Re: simultenaity

    Hello all.

    I assume the events referred to are spatially separated because timelike, causally connected events cannot be simultaneous by the usual (perhaps any) definition.

    Matheinste
     
  9. May 13, 2009 #8

    Mentz114

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    Re: simultaneity

    You're right, arbitrary does not work. But I have found an observer for whom the events are simultaneous. We'll exclude the case where Fred lives for zero time, because then the events are simultaneous in everyone's frame.

    Suppose Fred lived for n years, and dies in a different place to the one he was born. An observer who is located n+k light years from the birthplace, and k light years from the hospice, will see both events simultaneously through his telescope.

    But I'm not sure if I can make a plane out of these conditions, so you could be right on that.

    I think. It's late here and this is my last post today.
     
    Last edited: May 13, 2009
  10. May 13, 2009 #9

    JesseM

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    Re: simultaneity

    Simultaneity in SR is a totally separate concept from seeing events simultaneously though...the events at the beginning and end of Fred's life won't have the same t-coordinate in any inertial frame.
     
  11. May 13, 2009 #10
    Re: simultenaity

    if you have a cartesian coord system
    observer A and observer B's eyes are located on the xy plane
    Observer A and Observr B can be moving in any direction, at any velocity and any accelleration as long as the direction of travel is parrallell to the plane
    lightening strikes on the z axis at 10 and -10 ie equidistant from the origin on the z axis
    anypoint on the xy plane will be equidistant from -10 and 10 and so the light will reach any point on the xy plane simultaneously
     
  12. May 13, 2009 #11
    Re: simultenaity

    the events will appear simultaneous to both observer A and observer B
     
  13. May 13, 2009 #12

    JesseM

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    Re: simultenaity

    Again, "simultaneity" in SR has nothing to do with seeing events simultaneously. For example, if I see the light from an event in 2005 according to my clock which occurred 5 light-years away according to my length measurement, and in 2010 I see the light from an event 10 light-years away, then both these events happened "simultaneously" in 2000 in my frame.
     
  14. May 13, 2009 #13
    Re: simultaneity

    I think that you are implying that Fred and the observer have a relative speed of c, which doesn't seem right. (Think of a photon spawned by the birth of Fred, which then passes Fred at the time of his death, then goes on to be observed by the observer - either that's what is being implied or you have photons from the birth catching up with photons from the death.)

    In any event, observing two events together does not make them simultaneous.

    cheers,

    neopolitan
     
  15. May 13, 2009 #14
    Re: simultaneity

    How about Fred living and dying on earth at age 60, and an observer on a ship is moving toward earth at 0.8c, then after Fred is dead (in the ship's frame), he decelerates to come to rest with earth at a distance of 75 light years. Now, Fred's a baby again.

    It's not an inertial frame, and the ship observer can't know that Fred is alive or dead until later, but it looks like according to the clock on the ship, lots of people on earth who were dead "before" are alive "after".

    Is this a case where LET would be a better (philosophical) description of reality?
     
  16. May 13, 2009 #15

    JesseM

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    Re: simultaneity

    There are an infinite number of ways to construct a non-inertial frame where the ship is at rest, none of them preferred over any other. You appear to be trying to construct one where the ship's definition of simultaneity always matches that of the ship's current inertial frame at that instant, but there's no need to build it that way. You could equally well construct a non-inertial frame where Fred is exactly the same age before and after the ship decelerates, for example.
    The standard interpretation of SR doesn't imply that the non-inertial coordinate system you constructed is a "description of reality", it's just an arbitrary way of labeling points in spacetime.
     
  17. May 13, 2009 #16
    Re: simultaneity

    I agree with your entire post. That's why I asked, "Is this a case where LET would be a better (philosophical) description of reality?"
     
  18. May 13, 2009 #17

    JesseM

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    Re: simultaneity

    Why would adopting a (probably fictitious) belief in a preferred frame be "better" here? Are you saying that the fact that there's no "proper" way to construct a coordinate system for a non-inertial observer is somehow philosophically problematic for the standard SR view? I don't consider it philosophically problematic that there's no standard way to construct a non-cartesian spatial coordinate system on the surface of the Earth where the non-straight coastline of Florida has a constant x-coordinate, for example, so why should it be any more problem that there's no standard way to construct a non-inertial spacetime coordinate system where the non-straight worldline of an accelerating observer has a constant position coordinate?
     
  19. May 13, 2009 #18
    Re: simultaneity

    I meant to say Lorentz Relativity Theory, not LET.

    The philosophical issue is that according to the ship observer, a person who is dead "now" according to the standard SR simultaneity convention will be alive "later" according to the same convention. The "now" and "later" are each in an inertial frame. The ship observer experiences a moment simultaneous with the person being alive after experiencing a moment simultaneous with the same person being dead.

    This seems very similar to how the earth clock "jumps ahead" according to the ship's twin in the standard twins paradox resolutions. Except in this case, earth's clock "jumps backward".
     
  20. May 14, 2009 #19
    Re: simultenaity

    reality doesnt change. only your perspective changes.
     
  21. May 14, 2009 #20

    Dale

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    Re: simultenaity

    Hi Dreads,

    This is definitely your best post so far. As long as you restrict the two events to being spacelike separated and as long as your observers use radar time and distance then you are correct.

    Another way to describe that plane is as the plane that contains the intersection of the light cones of the two events. If an observer emits a radar pulse at any event on the "past light cone" intersection region then they will recieve the radar echo from both events at the same time on the "future light cone" intersection region.

    Events don't move. It is like talking about the slope of a point. Points don't have slopes, lines do.
     
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