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Basic conceptual description of lightspeed having a limit

  1. Dec 5, 2009 #1
    To be able to explain in a nutshell to my daughter why you can't go faster than the speed of light does this work:

    The universe exists now for us based on the expansion from a point. assume this expansion is occurring at a fixed rate. This runs the risk of assuming the answer I know.

    If you could go faster than this expansion rate, you could go where space doesn't exist, therefore it can't happen. If she asks, well why can't you go SOME distance into space faster than light until you reach the edge and then proceed at the limit, is the proper response that the "edge" is right here...proceeding into space faster than light would immediately blow through into nothingness and that the space you see is actually back in time (in the past)
  2. jcsd
  3. Dec 5, 2009 #2
    The size of the universe is not the cause to the speed of light, as far as we know. If anything, it's probably the other way around. It is an intrinsic rate at which disturbances/changes/information (all essentially the same thing) can travel in space. Everything we know (matter/energy etc) are defined this way, so they cannot travel faster than light either.
  4. Dec 5, 2009 #3


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    A much simpler explanation is use special relativity. When something goes near the speed of light it gets more massive, so that it it takes a lot more energy to speed it up further. It would take an infinite amount of energy to make it reach light speed.
  5. Dec 5, 2009 #4
    Special relativity is based on the principle of relativity. You can't talk about mass until you know the speed of light is constant.
  6. Dec 5, 2009 #5
    I just saw this on a TV show about Einstein. While contemplating the speed of light, he was riding a train away from the face of a large clock tower. He imagined the train going faster and faster until it reaches, then exceeds, the speed of light. If the train could go faster than light, the light from the clock would not reach the train.

    The speed of light is measured as length divided by time. To measure time we use a clock. What Einstein discovered was that all observers measure the speed of light as the same, so the rate at which clocks operate is relative to the motion of the clock.

    In other words think of a meter stick as made of rubber, and a pedulum clock that speeds up or slows down its "tic toc" frequency, but the two travel around the universe always making speed of light c a constant!

    If this is logically incorrect, someone please provide a compelling response. I am fairly certain it captures the logic of the theory in non-rigorous terms.
  7. Dec 5, 2009 #6
    I think that's right. But I don't think the meter stick should be rubbery. The idea is that if a length measurement and time measurement of c always agree locally, then really the length and time are equivalent measurements, and c is just a scaling factor. I carry a nanostick in my backpack which gives the distance light travels in 1 nanosecond. It doesn't matter if I have that with me here on earth or traveling near the speed of light it will still make the same measurement.
  8. Dec 5, 2009 #7
    I see your point as far as the observer travelling with his own meter stick is concerned. However Appendix V-5 of Physics Part I&II, Halliday & Resnick concerns Special Relativity and has the Title, Time Dilation and Length Contraction. I was recalling this concept vaguely when I said the meter stick (rod) is made of rubber.

    It is a bit complicated to sort out the concept, but from this appendix, it appears that observers looking at a clock or rod in the other's frame of reference compared to one's own, would measure different lengths, or different times between events. To explain it better I'd have to think through the specific examples discussed in the reference.
  9. Dec 5, 2009 #8
    regor60 (everyone else),

    This is an award winning animation of Einstein's theories. Works great on high speed internet with Flash player plug-in:


    The interface can be distracting. Follow the links on the right for a quick run through, or drill down on topics.
  10. Dec 6, 2009 #9
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