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What is my time?

  1. Nov 3, 2013 #1
    If I am traveling at the speed of light, I see a beam of light travelling in the same direction not moving. In other words, at the speed of light my time stops. But I would see another beam of light passing in the opposite direction travelling at the speed that is double the speed of light (?), or I would have to think that my time is flowing in the negative direction. Which is my time? Do I have two different times, one for each beam of light?
  2. jcsd
  3. Nov 3, 2013 #2


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    Instead of travelling at the speed of light (which you can't), try thinking about travelling close to the speed of light.
  4. Nov 3, 2013 #3


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    No observer can travel at the speed of light.

    This is the Einstein thought-experiment from his early years - he realized that it violates Faraday's Law of Induction, and hence is not possible.

    But you can have an observer at rest, and another travelling at 0.99999c; they would both agree that the light passing in each direction is traveling at the same speed: they both are going at c.

    And looking at each other each of you would see the other as running slow ... but looking at your own clock, everything is working fine. And the other guy (whichever one he is) will say that his clock is running fine as well.
  5. Nov 3, 2013 #4
    It does not matter. Let us say I am travelling at half the speed of light. So my clock is slower by a factor of 0.5. And as per that that clock the speed of the first beam, which is travelling in the same direction, would calculate to c. But, if I you use the same clock for the other beam, which is travelling in the opposite direction, the speed would be 1.5 times c (?)
  6. Nov 3, 2013 #5


    Staff: Mentor

    You are missing length contraction and the relativity of simultaneity, all of which have to be considered. Relativity is not just time dilation.

    What you need to look at is the full Lorentz transform. It is easy to show that the Lorentz transform leaves c unchanged.
  7. Nov 5, 2013 #6
    You seem to assume that lightspeed in different directions is some how related to your speed, like automobiles velocity addition and subtraction. That's not what happens.

    Everybody observes the local speed of light at c regardless of their direction or the direction of the light.

    Velocity addition and subtraction is explained here:

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