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Problem about einstein's postulate

  1. Oct 31, 2003 #1
    evaluate its validity:
    "einstein said that nothing can go faster than c but if I'm in a rocket going at 1/2c and I shine a laser out of the noes of my rocket, then the laser beam is going 1+1/2c, so einstein is wrong."

    Please help......it is so hard to understand....
    Last edited: Oct 31, 2003
  2. jcsd
  3. Oct 31, 2003 #2

    You have answered ur question... "c" is the ultimate speed. Though its value is finite it has got infinity like properties: c+c=c, c+1 = c etc.Special Relativity also says that C is constant with respect to all reference frames. Which means whether u are moving, stationary, accelerating, C is always constant.

  4. Oct 31, 2003 #3


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    If you are going to "evaluate" relativity, then you need to work in terms of relativity.

    The formula you are using: "if I'm in a rocket going at 1/2c and I shine a laser out of the noes of my rocket, then the laser beam is going 1+1/2c," or more generally, "if my speed relative to point O is u and the speed of some other object relative to me is v, then the speed of that object relative to point O is u+v" is classical mechanics, not relativity.

    The relativity formula is "(u+v)/(1+ uv/c2)".

    In the particular example where v= c this reduces to
    (u+c)/(1+ u/c)= c(u+c)/(c+u)= c. Light travels at "c" relative to ANY reference point.
  5. Oct 31, 2003 #4
    thank you very much...
    but my teacher hasn't talked too much about the relativity yet, she just wanted us to use the words to explain what happen....
    so i just used "the speed of light appeared the same to everyone,everywhere" this postulate to explain it...
    but the main thing is, i don't understand how this postulate made up...can you explain how it was made up,or suggest where could find some brief imformation about it... :smile:
  6. Nov 2, 2003 #5
    I think there's two different things being mixed up here.

    I think the basic postulate (backed directly by experiment) is:
    "Vacuum lightspeed is the same for any observer, regardless of his state of motion".

    So the answer to your original question is easy:
    "The laser beam is going at c. For any observer."

    The other postulate is "Nothing can move faster than c".
    I think this is much more sophisticated than the other one. If you make a statement containing the word "nothing", you have to check very carefully on all aspects of physics - and define very precisely what you consider a "thing". You might (or not) arrive at this statement if you carefully draw the implications of the other one.

    I think the postulate "Nothing can move faster than c" sounds spectacular, but is not a good starting point when teaching relativity. The other one, IMO, is.
  7. Nov 2, 2003 #6
    Einstein never said that. His two postulates were:

    1. "The ... laws of electrodynamics and optics will be valid for all coordinate systems in which the equations of mechanics hold ..." (Relativity Postulate)

    2. "Light always propagates in empty speed with a definite velocity [c] that is independent of the state of motion of the emitting body." (Speed of Light Postulate)

    Einstein's theory does not forbid the existence of bodies that move faster than light (tachyons). However, faster-than-light travel is problematic for causality, and in quantum theory tachyons tend to destabilize the vacuum. (This isn't necessarily fatal, see e.g. Sen's work on tachyon condensation in string theory, but it's usually regarded as a sickness of the theory.)

    HallsofIvy has already pointed out that this Galilean velocity addition law doesn't hold in relativity. There is more of a discussion here:


    Do you mean, what gave Einstein the idea to propose this postulate?

    Maxwell's theory of electromagnetism predicted a definite value of the speed of light (c). But in Newtonian mechanics says that if something travels at speed c in one reference frame, it must travel at speed c+v in a frame that's moving at a speed v with respect to it (in the other direction). Thus, people concluded that Maxwell's prediction of a speed c must hold for one particular frame -- that of the so-called "luminiferous aether", the material medium that light was supposed to travel through. So they kept Newton's mechanics, and assumed that Maxwell's theory wasn't true for all observers.

    Einstein didn't like this; he thought that Maxwell's theory was always true, and he didn't care much for this aether idea. So he simply postulated that Maxwell's theory should be true for all observers, and therefore the speed of light should be the same for all observers. This postulate turned out to be right, as experiments later showed.
  8. Nov 3, 2003 #7
    Ambitwistor: Agree. Nice post IMO.
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