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Question on Light speed

  1. Jul 28, 2009 #1
    Hi there folks,

    Physics isn't my speciality, i'm in medical studies, so i don't really understand a few things.

    One thing i don't understand is how the velocity of light remains the same independant of the observer's frame of reference, but everything else is relative. What is it about light that makes it different from everything else.

    I mean, the example of the train and car. The train travelling at 65mph, and the car at 60mph...the Train passenger would see the car moving back at 5mph...so why is it different with light?

    Can someone explain this to me in a way that's easy to understand, as possible?

    Thanks :)
  2. jcsd
  3. Jul 28, 2009 #2


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    Actually the observer would see the car moving back at slightly less than 5mph.
    The simple addition of velociities is wrong by a factor of;
    But until v gets close to c (the speed of light) you don't really notice the effect, 5mph is only 2m/s so the factor is only one part in a billion
  4. Jul 28, 2009 #3
    Ok, i didn't know that...and don't understand why..lol,
    but still, the velocity of the car is relative to the point of observation, why is that not the same with light? or is it?
  5. Jul 28, 2009 #4
    You can visualize things like if there is an observer on the ground and a train traveling in one direction when someone throws a ball or anything in the same direction... the addition of velocities in special relativity for two objects moving towards same direction (for simplicity) with velocities u and v respect to an observer is no longer u+v, but
    [tex]\frac{u+v}{1+\frac{uv}{c^{2}}}[/tex], but the effect is very small for velocities of ball and/or train small compared to c. If you sustitute there c in place of u or v, you obtain c, that is for the traveler is the same velocity measured for the light as for the people in the ground. Actually for this is "relativity" since any frame of reference (here inertial frames) is equally valid to describe the systems, and exactly for this was constructed the theory, to have c to al observers moving at constant velocity.
  6. Jul 28, 2009 #5


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    Well, one reasoning typically offered is that the speed of light appears in the equations of electromagnetism. So if the speed of light depended on the observer, you'd have to have different versions of the EM equations depending on how fast you were moving. The experiments that people did in the mid-1800's to establish the electromagnetic laws didn't show any variation like that; they suggested that the laws, including the fundamental constants which determine the speed of light, were the same no matter how you were moving. So it was natural to assume that something in the universe has to "adjust" so that the speed of light would be invariant for all observers, unless something was seriously wrong with the whole idea of light as an electromagnetic wave.

    Of course, there's no fundamental physical equation that includes the speed of a car as a constant, so no reason that the car's speed can't be relative to the observer's.
  7. Jul 28, 2009 #6
    What makes light different is that it is massless.

    But that avoids the real question here. Although the speed c is often called "the speed of light," it's really only incidental that light travels at that speed. It's not the light that is special (anything massless travels at c), it's the speed itself.

    It turns out that there is one speed, c, that is absolute. You are correct: it doesn't depend on the observer. Anything observed traveling at that invariant speed is observed at that speed by everyone. Why is the universe that way? *shrug* That is the way it is observed to be. Why shouldn't it be that way?
  8. Jul 29, 2009 #7
    maxwell discovered that speed of light's an constant , but he didn't know "why?"

    einstein came in speical relativity to explain that why speed of lights never change ever !?

    in relativity speed (v≈c) γ (gamma) or lorentz factor not different make the time dilation . when car is too solwer than light (v<<c) time won't dilation for car .

    that's the different between light and other .

    If I in wrong please someone correct it

    bye bye
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