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Speed of light and length contraction

  1. Jul 28, 2004 #1
    Hey, I am new to this site, i'm not entirely sure where i should post this question but can anyone here help?
    I need to know Einsteins assumption of the speed of light as a constant by referring to length contraction, mass and time dialation and how it affects people traveeling in space at high speeds...
  2. jcsd
  3. Jul 28, 2004 #2
    when traveling at very high speeds:

    To = T(sqrt(1 - v^2/c^2)) [To is stationary time, and T is the travellers time]
    Lo = L(sqrt(1 - v^2/c^2)) [Lo is stationary length, and T is the length experienced
    by the traveller]
    Mo = M/(sqrt(1 - v^2/c^2)) [Mo is rest mass, and M is the travellers mass at high speed]

    to do these calculations it is simple is you use units in c, (ex. 0.8c, 0.9c) and not in m/c. If you use c (speed of light) units, then the units cance out nicely and your answer can be obtained in 30 seconds.
  4. Jul 29, 2004 #3
    The answer that im about to give you is not my own but brian greenes from the fabric of the cosmos .
    he states that when you are stationary you are travelling only through time but when you move you are travelling through both space and time.
    now when you a stationary you are travelling through time at the spped of light but when you move some of this speed is transfered to travelling through space. this means that because the sum of travelling through space and time must always =c then the faster you travell through space the slower you must travel through time. hence time dilation.
    hope this helps.
  5. Jul 29, 2004 #4
    Both Nenad and jamie's answers were correct, however your looking at the problem backwards. Einstein didn't assume the speed of light is constant because of the previously mentioned relativistic effects, he discovered these effects because he assumed the speed of light is constant. This assumption comes from James Clerk Maxwell's equations for electromagnetism, which don't provide a frame of reference for the calculated speed of light, c. The conclusion (which was drawn by many before Einstein too) was that therefore light must travel at c relative to everything, contradictory to Newton's laws of motion. Therefore Einstein developed his special theory of relativity to show how it's possible for light to travel at c relative to everything, the explanation being drawn from such things as time dilation and length contraction.
    Welcome to PF!
  6. Aug 5, 2004 #5


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    Science Advisor

    Moving at high speeds per se doesn't have any effect. The laws of physics are independent of one's velocity. So if one is in a closed spaceship, or a closed train, one can't tell from local observations whether or not one is even moving.

    Of course, there are some practical issues, like the importance of avoiding collisions. Running into even a very small dust speck at relativistic velocities would be Bad. Without some more details, it's hard to say which of these practical issues would be important to you.
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