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Practical meaning of simultaneity calculations?

  1. Apr 24, 2007 #1
    Is there any practical reason for calculating simultaneity of distant events?

    Two events either are causally connected or not, and there is nothing relative about this. If a cause-effect link doesn't exist, does it matter if the events are calculated to be simultaneous from a certain point of view?
  2. jcsd
  3. Apr 24, 2007 #2

    I think you should be more specific explaining what you mean by calculating simultaneity. We say that two events are simultaneous in a given inertial reference frame if they are characterized there by the same time coordinate and by different space coordinates. The two events could be or not in a causal relationship and we have a criteria for doing that.
    Relativity becomes involved if we consider the same events form another inertial reference frame where they could be simultaneous or not.
    Are there more questions?
  4. Apr 26, 2007 #3


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    In one regard it is very important conceptually. If you accept the predictions of SR and consider a device capable of transmitting information (or moving) faster that speed c, then in the frame where the two events (transmission and reception) are simultaneous the transmission occurs at infinite speed. Furthermore there must then be another inertial frame where the signal or object is actually traveling backward in time.

    So either time-travel is possible or FTL causality is Impossible. They are qualitatively one and the same thing in SR.

    The practicality of it all is that through correct understanding of empirically validated theory one can better predict the behavior of physical systems.
    Relativity of simultaneity is one inseparable component of an imminently useful theory. Accurate GPS navigation is one imminently useful practical application of that theory.

  5. Apr 26, 2007 #4


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    Note that there are many distant events
    which are not causally connected to an event O on your worldline
    but can influence events in the future of O.

    With that knowledge, one might wish to somehow prepare those events to influence a particular event P (or set of events) in the future of O... i.e. prepare "initial data" for an "initial value problem".
  6. Apr 26, 2007 #5
    bernhar.rothenstein, were these words of yours contradicting Einstein's?

    In Section 9 of the late Dr. Albert Einstein’s book Relativity, Eintein said: "People travelling in this train will with advantage view the train as a rigid reference-body (co-ordinate system); they regard all events in reference to the train."

    Einstein's words meant to me: observers in their own inertia reference frame consider events in their own frame, not events from another frame.
  7. Apr 26, 2007 #6


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    Suppose you have a robotic spacecraft that you need to make a course correction burn. How would you make that happen without simultaneity calculations?
  8. Apr 26, 2007 #7


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    Yes. If you want to use Maxwell's equations for electromagnetism, for instance, you have to set up a coordinate system to define the value of "coordinate time" of an arbitrary event.

    Furthermore, you have to use some sort of isotropic (Einsteinian) clock synchronization for Maxwell's equations to have their usual form - i.e. you can't assign time arbitrarily to events if you want Maxwell's equations to have their usual simple form, you have to assign them isotropically. Hopefully you can see why from a simple plausibility arguement - Maxwell's equations give a constant speed of light in all directions, so if you don't chose your coordinates isotropically, they can't possibly work.

    In general, coordinate systems are just too useful to get rid of, so one winds up with a notion of "coordinate time". However, I think you are correct to downplay the physical significance of coordinate time - to get philosophical for a bit, it's a human creation, not part of an observer-independent "reality".
    Last edited: Apr 26, 2007
  9. Apr 26, 2007 #8

    In any case. What I say is: Consider events E'(1)[x'(1),y'(1),t'(1)] and E'(2)[x'(2),y'(2),t'(2)] detected in I i.e in one and the same reference frame! Detected from I the two events are separated by a time interval
    an equation which enables us to find out the conditions under which the two avents are or are not simultaneous in I. That is all. Is there some thing in conflict with Einstein?:rolleyes:
  10. Apr 26, 2007 #9

    Please explain me in what consists the calculation we should make? In the reference frame attached to the spacecraft or in the reference frame relative to which it moves?
  11. Apr 27, 2007 #10
    I am sorry, I cannot understand your meth. I am a hobby reader, not a physics student. Let me show you what was my understanding of the simultaniety problem. To me, it is a simple Eucliden geometric problem.

    Events E and E' consitute two points. Two points can always be joined by a straight line like this (Euclidean):


    If something simultaneously departs from either end at a constant speed, say c, then they will always reach the midpont simultaneously.

    Such was my understanding of the simultaniety probem, regardless from what reference frame you consider it. That is, simultaniety is absolute, not relative.

    Please tell me is there anything wrong with my understanding.
  12. Apr 27, 2007 #11
    This is a different situation--the observer is at the midpoint.

    ('midpoint' of what?)
    Last edited: Apr 27, 2007
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