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Light and time

  1. Feb 28, 2009 #1
    I have read several books on relativity and find the subject fascinating. But there are a couple of things that I hope someone can help me understand. The big question on my mind is "why is light always associated with time"? It seems to my simple mind that light just portrays a visual representation of events and that the peculiarities of light just alter our perception of the timing of events but not of the actual time that the event occurred at d=0 Why pick light over sound for instance? I know that we can view a cosmic event and say that it occurred 93 million years ago and we acknowledge that the event didn't just occur now but much earlier in time. So why do we treat things differently when talking about events happening on a train for instance? I am not disputing the theories and experimental results discovered about light to date although I sometimes wonder if using an atomic clock, which works on electromechanical principles, can be misleading us.
  2. jcsd
  3. Feb 28, 2009 #2

    Doc Al

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    Staff: Mentor

    It's a lot easier to work with, since the speed of light is the same in all reference frames. The speed of sound is not.
    Why do you think things are different for events on a train? You always take into account the light travel time (as needed) when interpreting observations. Do you have a specific example that you find puzzling?
  4. Feb 28, 2009 #3


    Staff: Mentor

    I think this can be a common misunderstanding, an idea that relativity is an optical illusion or that it describes how things "look". Relativistic effects are what happens after accounting for the travel time of light, whether we are talking about interstellar distances or "train car" distances. I think the usual "thought experiments" help promote this misunderstanding.
  5. Feb 28, 2009 #4
    Hi moppie...thats the name of a famous make of boat by Bertram, the first "deep vee" design .

    The basic reason is that light (electromagnetic radiation) is the fastest that information can travel....nothing can travel from the cosmological horizon, for example, to us faster than electromagnetic waves. So the cosmic background radiation gets here as fast as anything can, yet it is a fixed finite speed, not instantaneous which is assumed in Newtonian physics.
    But the actual time of travel is NOT obvious because while the light is traveling toward us space it traverses is expanding and so the light takes longer to arrive than one might otherwise think. See the Cosmology forum here for more discussion.

    When two observers in relative motion "see" an event, even accounting for the difference in the time it might take for light to reach them over different distances, they still will not in general agree on when the event occurred. this is called the relativity of simultaneity. Two people in relative motion measure the same light speed, but see the other person via length contraction and time dilation (shorter distance, slower time)...time and distance vary in such a way, via lorentz transformations, that they compensate for each other reflecting a fixed speed of light (in the absence of gravity). Try reading about the pole and barn paradox as an example...
    Last edited: Mar 1, 2009
  6. Mar 1, 2009 #5
    I think you missed my point here. It is absurd to use sound to determine how we should set our watches for obvious reasons. It "seems" to me that light is equally as absurd for the same reasons.

    My thoughts are that although the events are perceived to happen at different times, why does anyone have to change their watch? Can't they just acknowledge that when they meet again that they saw events occur at different times and that their watches still have the same time?

    You are correct and I am the proud owner of one made in 1963.

    It is the fastest that we know of. I still don't see how it relates to time.

    Understood (I think) but consider this thought experiment.

    We have our train rolling through the station, flashlight on the floor, target on the ceiling. We also have a sensor built into the target on the ceiling and a connection down to the rails of the track which our observer on the platform can connect to to monitor the event on the train. The person on the train will "see" the light leave the flashlight at t1 and hit the target/sensor at t2. The person on the platform will "see" the event start at the same time(he is right beside the train at the instant the flashlight is turned on) but sees the light hit the target at t3. He looks down at his monitoring equipment, however, and sees that the events actually occurred from t1 to t2. Why does the person on the train need to adjust his watch?
  7. Mar 1, 2009 #6

    Doc Al

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    There's nothing "absurd" about using sound to set your watch. (Of course, it would be quite impractical.) Say you are across a field and we want to synchronize watches. You shout out at the exact moment that your watch reads 1:00 pm. When I hear the shout I set my watch to read 1:00 pm plus the time it took for the sound to travel from you to me.

    No one "changes" their watch. But if you want to correctly predict what happens, then you'd better know how to translate measurements made on a moving clock to measurements made on your clocks.

    Why do you think that the platform observer's equipment would measure the same time interval as the train observers? Why do you think train observers have to "adjust" their watches?
  8. Mar 1, 2009 #7
    Time passes differently for different observers in relative motion, and the difference is related to c. It's not related to the speed of sound.

    One thing that should be mentioned is that the invariant speed c isn't special because light travels at that speed, it's the other way around.
  9. Mar 1, 2009 #8
    Correct, they saw events occur at different times; BUT if one observer is stationary in a train station and the other observer leaves, takes the train for experiments, and returns, their respective watches will NOT read the same time when they again compare elapsed time!!!!....The traveler's watch will reflect less elapsed time...
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