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Speed of light equal for all observers!

  1. Mar 6, 2004 #1
    In this paper :
    Light is Heavy
    the following is claimed:
    "the speed of light appears to be equal for all observers"

    How can this be true if astronomers rely on the doppler shift to determine if an electromagnetic source is moving away or closer to the earth?

    How does this tie in with velocity = frequency * wavelength?

    Is it not because the velocity 'aka speed in a straight line' changes that the wavelength, or frequency changes? hence, the velocity is changed if you are travelling toward or away from a source, resulting in a change of wavelength?

    Or is it that the velocity remains constant, but the wavelength changes?
  2. jcsd
  3. Mar 6, 2004 #2


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    The speed of light remains constant, the wavelength and frequency appear to vary with relative motion of the source.
  4. Apr 13, 2004 #3
    what if the moon was one light-minute away and you had person "A" on the moon and person "B" on the earth, and person "B" shinned a lazer to the moon. Person "A" woild say it took two minutes to get there because It would take the light a minute to get there and another minute to before it could be seen from earth. Now the man on the moon would see it as soon as it got to the moon. Thus person "B'" would say it took 2 minutes to get there and person "A" would say it only took a minute to get there. If both men said speed = time * distances they woild come up with diffrents speeds, this is relativity right, but I don't see where the speed of light is constant because relativity say the speed of light is to be the same for both observers.
  5. Apr 13, 2004 #4


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    There is more to it than that. In your experiment, both A and B can agree on the times and distances perfectly (since they both can take into account the fact that one will need the light to return).

    The constancy of the speed of light refers to the speed that two observers in relative motion will measure for the same light ray.

    Imagine you are driving a car at, say, 35 mph, and your friend's car passes yours quite fast, say at 70 mph (with respect to the asphalt, which means he is going 35 mph with respect to you). If you both see a third car passing by at 90 mph, you and your friend will measure its speed as 55 and 20 mph, respectively. I.e., the speed of the third car will be measured differently by you two.

    However, if instead of a third car, you both decided to measure the speed of a light ray passing by, you both would get the same exact result. It is not something you are supposed to "understand" the first time you learn it. It actually came as a big surprise when pofessional physicists first discovered it (by doing precise experiments), but it is the way nature works. We need to build our models from there and, when applicable, adequate our intuition to it.
  6. Apr 13, 2004 #5
    I see what you are saying and I agree. I use a cad program and plotted out a graph of this. What I see if the light is passing bettween the 30 and 90 mph cars and that all meet at a common referance point then keep traveling as the light travels in repect to the cars the distance is from the light and cars are changes and the distance from the cars from the referance point changes too. However if your in the 90mph car from your point of view you could say that you were stationary and then It would look as if the light did not travel as far in a give time frame and this would proprosional. If you would consider the 30 mph car as sationary then you would say that the light travel further than the 90mph would have said but it took a longer time and all this would be proprosional to each other because as the light source is tavel away from you at a set speed the light traveling back to you (that you see) is travel at the same speed as the source is traveling away from you. With all this said I still can not see how travel faster the light would cause time travel.
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