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Physics Speed of Light and car Question

  1. Mar 29, 2003 #1
    If a car was travelling at the speed of light, and it turned on its headlights, would light project from the car[?]
  2. jcsd
  3. Mar 29, 2003 #2
    Probably since the photons weren't generated until the headlights were turned on. But I think the car woul have been long incinerated if you accelerated close to the speed of light so there would be no headlights to turn on.
    Last edited by a moderator: Mar 29, 2003
  4. Mar 29, 2003 #3


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    Greetings !

    Welcome to PF 22-16(are you a Borg unit ? ) !
    To the passengers in the car everything
    will "look" normal - the light will move
    from them at c. To the outside relativly
    stationary observer everything will also
    "look" normal - he/she'll see the light
    moving slowly ahead - just a bit faster
    than the car (the car can not reach c, it
    can only come close to it).

    I said "look" because the only way for you
    to know that the light is there is for some
    of it to get reflected towards you by
    particles. (In the vacuum of space, that's
    a problem.)

    Live long and prosper.
  5. Mar 29, 2003 #4


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    Gold Member

    First of all, it is important to note that no material object can travel at the speed of light in a vacuum) It will always travel at some speed just slighty less. In which case, the answer Drag gave is correct.

    It also is useless to ask, "But what if the car could travel at the speed of life".

    Because that would mean that the rules which we need to use to answer the question have been broken and no longer apply to the question. (All bets are off in that case, and anything could happen.)
  6. Apr 1, 2003 #5
    This is actualy an interesting question when broken down. Light doesn't always travel at c, in some materials it moves much slower than c, but in this same situation another object could in theory not only move at the same speed of the light, but even faster than the speed of the light. So if the car was moving just slightly slower than the light, then the light would slowely move away from the car, but beyond that I don't know what would happen. You would assume that the same thing that happens to sound abouve the sound barrier, only you would have to work out the equations for the "Chandrikara"<spelling is off, can't think of how to spell it right now> would have to be applied, but I would assume the passangers would see some sort of strange doppler lighing effect, would be cool to see, and mabey even more interesting to think more on.
  7. Apr 1, 2003 #6
    ^^^ The trick is "if the car was moving just slightly slower than the light." According to who?... some second observer; they would see light just barely getting away from the car. The people in the car would see their light moving away at normal speeds, and the outside world squished and sped-up.
  8. Apr 1, 2003 #7
    Yes .... the fundamental postulate to special relativity is that light always travels at the same speed in a vacuum ..... no matter who you are (providing all individuals concerned are travelling at a constant velocity). When the equations are worked through you find that the car can not travel at a velocity of c because the equations reach a singularity which is not something you want.

    A variant on this theme was proposed by Einstein himself .... if myself and a mirror are travelling (in a vacuum) at a the speed of light would I see my reflection in the mirror? :smile:
  9. Apr 1, 2003 #8
    As per SR, if we assume u r able to get velocity slightly slower than c, then according to ur dilated time of 1 sec the relative velocity of light is c with respect to u ( c is constant in any frame of reference ).
  10. Apr 1, 2003 #9


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    If black was white what color would it be?

    Such questions are utterly meaningless.
  11. Apr 1, 2003 #10
    I think your missing the point, the car can't move at the speed of light in a vacum, but a car could move at the speed of light in another medium, and that is what poses the interesting question.
  12. Apr 2, 2003 #11

    i think the answer is yes
    this question make Einstein create the theory of relativity
    for the theory
    the light will never change it speed....
    no matter the source of the light moving with wat speed...
  13. Apr 2, 2003 #12


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    Not correct. The speed of light through a medium is APPARENT speed, not actual speed. It only appears to be traveling faster because it stops every now and then. Photons ALWAYS travel at C.
  14. Apr 4, 2003 #13
    but, charges moving through water (say) at less than c_water(~c/1.3?) produce cherenkov radiation, a neat blue glow that is used to detect all sorts of things (famously neutrinos, recently).

    per SR,

    imagine fast car travelling through medium akin to water.
    to ourside observer, car is nearly keeping up with it's beams - neat

    to inside observer, length contraction should make the water seem much much denser - which will make the beam in the car frame travel very slowly.

    this, coupled with time dilation should make things work out consistently.

    (an interesting thought - beams (as apposed to light - the distinction between "classical beams" and photons is important) can be slowed to ~0m/s (in BEC's at harvard and elsewhere?) so one could almost preform this experiment!)

  15. Apr 6, 2003 #14
    This is true, but your missing the point of the question, in another medium the car would be able to "out run" its headlights.
  16. Apr 7, 2003 #15


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    Indeed, fascinating isn't it...:wink:
  17. Apr 8, 2003 #16
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