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Time is relative

  1. Nov 14, 2005 #1

    Let’s say one observes three events on earth while looking up at the night sky. One being right next to an observer on Earth (a flash light turning on), a second being on the moon (a large flash of light of some sort), and a third on Proxima Centauri, (perhaps a massive solar flare or the star going supernova).

    Let’s say these events are observed simultaneously on Earth.
    They are all observed at (t1)

    If the Earth event happened at t1, then the moon event must have happened at t1 – 1.3 seconds wrt Earth, and the Proxima Centauri event must have happened at t1 – 4.22 years wrt Earth.

    Still they all appeared to happen simultaneously at our observation point on earth. However, another observer on the moon or a third near Proxima Centauri will state definitively that the events happen in a different order. So no absolute times or absolute sequences of these events can be established.

    It seems only relative times and sequences can be established, and only after one chooses a reference frame. (one of the three locations)

    Is this a workable way of looking at the relative nature of time and temporal events?



    EDIT: It may be better to replace Proxima Centauri with another planet in our solar system like Mars because the time it takes light to reach Mars is known more precisely. Cheers. E.
    Last edited: Nov 14, 2005
  2. jcsd
  3. Nov 14, 2005 #2
    yes your right, The view points on the moon and on the earth (about which light went on first) is equal; that is to say they are both just as vaild as the next. This is the reason you should not think of time in special relitvity as "flowing" from one second to the next. time is relitive, So relitive to an observer on the moon he will see that light go on first but an observer on the earth will see the light on the earth go on first.
    p.s. i can't spell
  4. Nov 14, 2005 #3
    Thanks for your thoughts Simon.


    I don't understand what you mean by this.

    I think you are right.

    Einstein says the following about time.

    To me, The above is 100 percent correct.

    I was kind of trying to determine if there is a type of relativity of simultaneity that depends on just location without reference to the state of motion of an object (body).

    Or if I was conflating two different ideas and had my wires crossed somewhere. :smile:

    best and thanks for the reply,

    Last edited: Nov 14, 2005
  5. Nov 15, 2005 #4


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    Actually, this is not what the "relativity of simultaneity" is all about. After all, this sort of thing (perceiving events in different orders) would be just as true with sound waves in newtonian physics, but there is no disagreement over simultaneity there. Simultaneity deals with the time-coordinate you retroactively assign an event after taking into account the signal delay--for example, if I see a flash happen right next to me, then 1.3 seconds later I see a flash on the moon, then 4.22 years later I see a flash on alpha centauri, I will say that all three events happened at the same time-coordinate in my frame, even though I didn't see them at the same time. An observer who is at rest in my frame and who is sitting on alpha centauri will see them in a different order, but when she takes into account the light-speed delays of the signals in reaching her, she will also conclude all three events happened at the same time.

    Where the relativity of simultaneity comes in is when different observers are in motion relative to each other, and they each retroactively calculate the times of different events based on the assumption that light signals travel at c in their own frame. So, for example, if I'm in the center of a train car and the light from two flashes at opposite ends of the car reach me at the same time, I'll retroactively conclude both flashes took place at the same time, since the light from each flash had an equal distance to travel to reach me. But now say that at the moment the light flashes reach me, I pass right next to you standing on the side of the tracks, and you also see both flashes at that moment. From your point of view, at the moment the light reaches you both ends of the train must be at equal distances from you (because at that moment you're passing next to me at the center of the train), but since the train is moving, this must mean that in the past, when the flashes were set off, the front of the train was closer to you then the back of the train. So, if you assume the light from each flash travels at the same speed relative to you, then for the light from each flash to reach you at a single moment, you must conclude the flash at the back of the train was set off before the light at the front of the train. So in your frame, the two flashes cannot have happened simultaneously once you take into account the signal delays, even though the light from each one reached you at the same moment.
  6. Nov 15, 2005 #5
    Looks like I was mixing up apples and oranges.
    Thanks for the clarification. :biggrin:

    Your example above makes it clear. I think remembering the standard embankment and the train example is the best way to remember the concept of simultanaety. It works well.


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