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What's exactly simultaneity?

  1. Mar 14, 2007 #1

    First of all, I’d like to tell you that I’m not fluent in English (I think it’s important to say) and that’s a reason why I’m asking my questions here. Sorry for all the mistakes in this text.

    When I was in my bedroom, looking at the light bulbs of the ceiling, I wondered about the speed of light, simultaneity, etc. Then comes a question to which I have no answer. Here it goes :

    Let’s say two light emitters are at rest in my inertail frame of reference. They are at two different places. Of course, the distances between me and each one of them aren’t the same. So, the time taken for a beam of light to travel the distance from an emitter to me depends on which light bulb emits the light beam.

    Well, I wonder in which circumstance I, the observer, can tell that both emitters have emitted their ray of light simultanuously. If the farther light bulb emits a light beam and than, when this light beam passes by the second emitter, this emitter emits another light beam, both light beams would reach me at the same time. On the other hand, if the emitters emits a light beam at the same time, wouldn’t I receive both ray simultanuously. However, which circumstance reveals simultaneity?

    Thanks for you help
  2. jcsd
  3. Mar 14, 2007 #2
    I am a little confused of which light beams are you referring to. You said about a light bean that goes to another light bulb and one that goes to your body. My head is spinning. :rofl:

    Can you draw a picture and upload it? Like the old saying says: 'A picture is worth thousand words'.

    Edit: If it is the word 'simultaneous' that is confusing you, it means: 'at the same time.'

    If your question is that:
    -two light bulbs that have different distance from your body
    -you want to know if two light beams that was emitted at the same time can reach your body at the same time too.

    it depends on the position of the light bulbs and your body, and anything between the light that is being transmitted. to give you an specific answer, draw the picture.
    Last edited: Mar 14, 2007
  4. Mar 14, 2007 #3
    If for example the emitters (light source are on a line that is colinear - the further light source will emit a photon that travels some distance and then reaches the second light source - at which time the second light source will also emit a photon - then both photons will reach you at the same time - SR is based upon the notion that simultaneity depends upon arrival at the same time.
  5. Mar 14, 2007 #4
    Normally, however, the relativity of simultaneity depends upon signals arriving at a point midway between sources at the same time - so observes in frames having relative motion will have a different view of when two events are simultaneous - When no motion is involved, one can determine the order in which events take place in their own frame by the time taken for the light to arrive.
  6. Mar 15, 2007 #5
    It is helpful to pose the problem in terms of light both emitted and detected at the location of the observer. Let the observer send out light pulses to two mirrors placed at different, but unknown, distances from him. He then measures when they arrive back at his position. In general, they will arrive at different times, but he can adjust the emission times to make them arrive simultaneously. He knows that the mirrors are at different distances if he sent the pulses at different times. However, if he finds that he needs to send the two pulses simultaneously in order to receive them simultaneously, then he knows that the mirrors are at equal distances from him.

    If the emitters are at two distant locations, the clocks at those locations could have been previously synchronized. Following the simultaneous arrival, the emission times at the two locations would then be sent to the observer to let him determine which pulse had been emitted first and thus which mirror was more distant.

    Of course, if the observer has no knowledge of the emission times he cannot tell how far the two pulses have traveled.
  7. Mar 15, 2007 #6
    Let restate your problem (English is not my first language to). I (you) know the location of the two light sources by theirs position vectors r(1) and r(2). The two light sources emit light of different colours at the times t(1) and t(2) respectively i.e. not simultaneously. You will receive them at the times t(r,1) and t(r,2) respectively, related by
    t(r,2)-t(r,1)=t(2)-t(1)+[r(2)-r(1)]/c. If the emissions are simultaneous t(1)=t(2) then and only then t(r,2)-t(r,1)=[r(2)-r(1)]/c.
    I consider that the solving of your problem is a good exercise in handling the readings of distant syhnchronized clocks, as country boy mentioned above.
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