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The old car headlights at the speed of light scenario. conceptual questions!

  1. Jan 5, 2012 #1
    Thank you in advance for reading this and hopefully answering my questions.
    I've spent about two hours or so now trying to get my questions organized in a clear way, so that they are direct and to the point.

    I've drawn an image to help explain what I'm getting at.
    I'm not much of an artist though, so don't expect too much.

    Basically - We're driving a car going nearly the speed of light and turn the headlights on. Do the headlights work like normal?

    The standard answer is yes and the reason is Einstein's postulate (assumption) of the invariance of c, giving way to SR.

    why was this assumption made and/or necessary?

    What is wrong with applying Galilean invariance/relativity/transformation to the speed of light here? Is there a reason aside from violating the postulate?

    We have directly observed phase shifting and we know it exists. We have also observed similar phenomena with all other waves. So we have a precedent for this type of behavior. Yet we do not follow it.

    What was so convincing that we discredited this 'old standard' approach/model?

    So are the waves propagating uniformly from the car at c, or are they being phase shifted and undergoing doppler like effects? According to SR it depends on the observer, which is true for observing anything.

    However there is a problem - I'm pretty sure (unless I messed up the math) that phase shifted waves from a moving source do not propagate in a spherically symmetric way.
    and Therefore they cannot be equivalent wave functions if the other observer's wave propagates at a speed c from the source(observer), independent of the source's velocity. (The bottom two images help show it)

    Yet there is only one source, and so there must be only one wave function in space and time, regardless of the frame of observation. When I say one wave function, I mean that transforming between frames by compensating for the difference - i.e. removing the speed of the observer - should yield an equivalent wave function to the other. (Seems like solid logic to me)

    They do not form the same geometric shapes - so I can dilate time as much as I like, but it will not help the wave functions to be equivalent. Unless I undo the prior effects of the invariance of c, through a transformation, I cannot equate the two wave functions. But doing and undoing the effects of the invariance of c do not help me determine where the wave function actually exists.

    So. Guess I've been over thinking this but, why is the invariance of c accepted / justified?

    any thoughts / comments ?

    thanks again for reading all this, and hopefully answering/rebutting any points or logical problems.


    http://img546.imageshack.us/img546/7633/headlights.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 5, 2012 #2


    Staff: Mentor

    It fits a very large amount of experimental evidence, and Galilean invariance does not:

    Both happen. I don't understand your use of the word "or" here, as though they were mutually exclusive effects.

    This site is not for promoting personal beliefs, it is for discussing and learning about mainstream physics. Unless you can find some mainstream scientific reference which supports your belief then it is not appropriate for this forum.
  4. Jan 5, 2012 #3
    Sorry, belief was a poor word choice. It is a technical conclusion that I have reached based on my own attempt to model the propagation of the waves.

    I am not in any way advocating any non- or anti-mainstream physics. I am simply trying to better understand the basis for SR and the development of GR.
    Last edited: Jan 5, 2012
  5. Jan 5, 2012 #4


    Staff: Mentor

    Your technical conclusion is not supported by experiment. Specifically all of the experiments regarding the speed of light in the link that I posted above.
  6. Jan 5, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    A quick comment. Thought experiments that haven't been done can be illuminating, but if you're asking for experimental support, they're not a good choice. You'd want to analyze experiments that have been done, for instance the Michelson Morley experiment, one of the many repeats of this experiment, or any of the other experiments that have been mentioned in the references in the standard FAQ or that others have posted in this thread.

    So, if you've got some intuitive and non-standard idea of how things "ought to work", and you want to compare your intuitive predictions to relativity, you need to start analyzing the actual experiments that have been done and do a checklist, seeing where each is correct.

    Maybe the experiments that have been done wouldn't be your first choice. But, if you are serious, you just have to live with the fact that the experiments measured something other than what you wanted. Usually the experiment was selected to make it as sensitive as possible with existing equipment at the time.

    Also, while we might teach you how relativity analyzes the experiments (sometimes people unfamiliar with the theory get it wrong), you're more or less on your own as to your personal "intuitive" analysis of them.

    The usual result will be that relativity will predict something correctly where your intuitive analysis doesn't. If your intuitive theory gets everything correct, and there also exists an experiment that would be different, you'd have in principle a publishable theory.

    However, we're not in the business of publishing new theories here. It's rather unlikely that your theory will make it that far, however, to be candid.

    There isn't any good way of avoiding "getting the right answer via faulty logic", alas, other than being very careful.
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