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Speed of light and universal expansion.

  1. Nov 12, 2006 #1
    The biggest chunk of evidence for expansion of the universe is the whole red-shift/Doppler effect deal, saying that as something that emits light while traveling away from the observer, the wavelength of the light will be "stretched" making it appear red. There is one thing I'm wondering. Could the red shift be caused by something else, like long distances perhaps. Maybe over the course of hundreds of thousands of light years, the wavelength of the light lengthens, due to maybe extremely long distances, or maybe a long term effect of the pull of gravity on the light. Does this sound reasonable, or are there any other ways it could happen?

    Also, it is theorized that the speed of light is constant. I'm not doubting this yet, but can anyone tell me what this is based on? If it turned out to be false, I'm sure that many theories of science in general would have to change, and that fact itself might uphold its truth, but what is it based on? Is there any experimental evidence on this?

    My last question is: why is the speed of light constant, why is it at that speed, and why is that the maximum speed of an object? Surely, there would be some geometrical way to exceed that speed, even if the methods seem unreasonable. For example, if one were to take a pole, say one light-year long, and swing it around at a speed in which it would complete one radian in one year, then the end of the pole would be traveling at the speed of light, so what if it was swung faster? Theoretically, anyway, I'm hoping not to get responses on how that is impossible, I'm just asking what would happen? What would happen to the end of the pole, where the speed of light is exceeded?

    Thank You.
  2. jcsd
  3. Nov 13, 2006 #2


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    Broadly the idea you refer to is 'tired light', the concept that light loses energy by a myriad of proposed mechanisms as it travels. This has shown to be inconsistent with observations, particularly the fact that it does not explain cosmic time dilation.

    Relativity (both special and general) a founded on the idea that light travels at a constant speed for all observers. This has been verified countless times by many different experiments. If you've ever used a GPS you are relying on correction terms that depend on SR, and hence the constancy of light, to be correct. Why do you doubt that the speed of light is constant?

    The speed of light is determined by the inverse square root of two other constants relating to how electric and magnetic fields propagate through a vaccum. This comes about from Maxwells equations of which light is a wave like solution. Why these constants have their specific values is not known. At present we can determine them by experiment only, but perhaps in the future we may have a theory that predicts they should have the values that they do for some other more fundamental reason. Perhaps they are just those values because that's what they happen to be in our universe.

    Unfortunately if your question makes no sense then the answers cannot either :frown: Since our current theories do not permit faster than light travel it is impossible to meaningfully say what they predict would happen if you did break that speed... It's like asking how 1+1=3, but not allowing the response that it simply does not!

    If you did find a 1 light year long pole and attempted to swing it faster than 1 radian per year you would find that you would need to supply an infinite amount of force in order to do so.
  4. Nov 13, 2006 #3

    I wasn't doubting that the speed of light is constant, I actually said I don't yet, meaning that if no one had a reason for it then I would.

    Otherwise, it was very helpul. Thank you.
  5. Nov 13, 2006 #4


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    Ah yes I misread you post slightly, sorry! I hope you continue not to doubt it (within the healthy level of doubt everyone should have about every idea).

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