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A car travelling at the speed of lights turns its brights on

  1. Nov 3, 2003 #1
    What happens? Now I was just thinking about this before I went to bed the other night...

    I was thinking of different variables and whatnot, but the main question, what happens to the light that is "projected" by the luminous bodies that are the headlights?

    Now of course, we are in a vacuum. This also disregards the whole "time stops" thing I'd assume.

    However I am only beginning grade 11 physics, and we are just getting into the Light unit. I love it so much and I can't stop reading these forums, hopefully you physics geniuses will be guides for the rest of my physics classes! (And maybe my career if everything goes well...:))
    Last edited by a moderator: Nov 3, 2003
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  3. Nov 3, 2003 #2
    All light travels at C - the speed of light - regardless of what is causing the light or what the light is traveling through. For ample, the light from your cars headlights, travels the same speed as the light that comes from the sun.

  4. Nov 3, 2003 #3
    Not exactly. Light passes through a vacuum faster then through a medium such as water, but the reason isn't because it actually slows in speed, I believe it is because the light is being absorbed and re-emitted.
  5. Nov 4, 2003 #4


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    The car can't travel at the speed of light, so it's a moot question.

    - Warren
  6. Nov 4, 2003 #5
    chroot is right. The question (even hypothetically speaking) should be rephrased to something like the car is traveling at 3/4 of the speed of light.

    Then we can answer you that the light will not travel at 3/4 of the speed of light plus the speed of light (the speed of the car plus the speed of light), it will still only travel at the speed of light, as nautica pointed out in his post.
  7. Nov 4, 2003 #6


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    does that mean your brights would only go 1/4 as far?
  8. Nov 4, 2003 #7

    Chi Meson

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    I thought I would point out that if you were driving at the speed of light, you would instantaneously be where you are going (time dialates infinitely) so you wouldn't need your headlights!
  9. Nov 4, 2003 #8
    From my understanding - the speed of light is always the same, no matter what medium it is passing through. I am not sure what happens when it passes through a vacuum, but I do know that it travels at C whether it is passing through air, water, or steel, ect....

  10. Nov 4, 2003 #9
    Wouldn't the fact that time slows when travelling be the explanation of why the headlights are still only going at the speed of light?
  11. Nov 4, 2003 #10
    No, not exactly. The apparent speed of light through other materials is dependant on their density and index of refraction the speed of light in the material can be derived by the formula n=c/v. Here are a few values of n:

    vacuum n = 1
    air n = 1.0003
    water n - 1.33
    crown glass n = 1.52
    diamond n = 2.42
    lead n = 2.6
  12. Nov 4, 2003 #11
    The indexes you speak of are refraction indexes - not speed indexes - light will be refracted but not slowed. Although, I believe some highschool physics books do not distinquish the difference.

  13. Nov 4, 2003 #12
    Well that's not completely true. The individual photons in a medium constantly are absorbed and re-emitted by the atoms of the medium. So this makes it appear that they are being slowed down but the fact is they travel at c in between being absorbed and there is a small delay for re-emittion. In high school they teach that the speed of electromagneitc radiation just slows down in a medium for no well defined reason.I think it would be better if they just called it the "apparent" speed of light in a certain medium.

    The speed of all electromagetic radiation in a vacuum is the same constant,c(299,792,458 m/s).
    Last edited: Nov 4, 2003
  14. Nov 4, 2003 #13
    So, the speed of light is C, always, but its velocity can change depending on the medium it is passing through??

  15. Nov 4, 2003 #14
    That's what a high school physics teacher would say but the fact is it only appears to slow down(because of the constant absorbing and re-emitting of the individual photons).
  16. Nov 5, 2003 #15
    Headlights at c

    I think the best way to imagine the solution to questions like these is to consider that light is a wave like waves in a still pond or pool. When a ball is moved slowly on the surface of the water the waves propugated move ahead of the ball. When the balls motion is fast enough the waves cannot propugate ahead. Therefore, if and object were to move at the speed of light towards you, you would not be able to see it. This phenomenon is true for sound waves. A jet moving faster than the speed of sound cannot be heard by observers ahead of it until it passes overhead.
    Prior to Einstein, Classical physicist treated light in this manner, that is as a wave in a medium. They spent quite of time looking to identify the medium in which light propugated. They called this medium the Aether. However, to this date, no evidence of the existence of the aether has been found. Einstein came along and developed the relativity theory to explain how light can act as a wave without existence of an aether. This is where the time and mass stuff comes in.
    In most of physics, light is treated as a wave. However, in subatomic physics, light is treated as a particle to explain the existence of packets of energy called quantums. These are emitted when electrons change states.
  17. Nov 5, 2003 #16


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    Not really, this is the work of Maxwell. Einstein's Relativity has nothing to do with the wave nature, or even partical nature of light. He simply explores the results of the speed of light being constant to all observers.
  18. Nov 5, 2003 #17
    Well, light being constant to all observers was not something observed at first by experiment. Classical Physicist observed that light behaved like a wave such as refraction in a prism. If light is a wave then you must ask what is it waving. This led the classicist to claim the existence of an aether. If there was an aether, then the speed of light should vary depending on the velocity and direction of and observer. The Michaelson/Morley experiment failed to show this. The speed of light was invariable everywhere. Now, science was in a quandry. What is light waving??? This is the problem that Einstein was trying to solve when he came up with the general theory of relativity. Later, experiments with satelites, clocks, and Mercury's perhelion seem to have verified his theory and now has become almost universally accepted.
  19. Nov 5, 2003 #18


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    This was predicted by Maxwell in 1867, as derived from his equations c=[squ](1/ε0μ0)

    The basic nature of light is an entirely separate question from its constant veloicity.

    Perhaps you should seek out the online copy of Einsteins 1905 paper, It simply does not talk about the nature of light. It only deals with the constancy of c and its ramifcations. It does not deal with quantum or wave mechanics or the nature of space.
    Now you are confusing his later work on General Relativity. You need to seperate the various key works of A.E. They each have very unique topics. 1st was Special Relativity, dealing with the ramaficaitions of a constant c. Next was his Nobel prize winning paper on the Photoelectric effect, which formed the basis of quantum mechanics, And introduced the concept of a photon. Finally, his work in General Realitivity which dealt with gravitation. They are not the all the same bit of work to be stired into a pot pulling out various lumps like chucks out of a stew.

    Please check your believes before posting them here as facts. There appears to be much you truly do not understand.
  20. Nov 5, 2003 #19

    Chi Meson

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    Ouch! I've seen posts that were so much wronger that received nicer rebuffs.

    But it was when Einstein was pondering Maxwell's explanation of the nature of light (during the apocryphal train ride as it pulled away from the clock tower), he realized that if one is traveling with the lightwave, the electric field vector would no longer be a changing quantity. IF the electric field does not change, then no magnetic field can be created and light can not propagate. SO while this is not itself "part of" special relativity, it was this consideration that led him to begin his work on SR.
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