If you travel on car with nearly the speed of light and turn on the car headlights: will it shine in gamma light instead of visible light?

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Solution 1

It is a relativistic effect so it depends on the relative velocity with respect to the light source.

Imagine that your car is moving close to the speed of light relative to some road (let us forget about the physics of your car and the road for this question). If you are inside the car, for you the lights are in the visible spectrum. For somebody still with respect to the road, let us call this person P, there is what is called a Doppler shift. P will not measure the electromagnetic radiation with the same frequency as you.

The Doppler shift depends on the relative velocity (direction included). If the car is moving towards P at relativistic speeds, then P may detect gamma radiation (frequency goes up, blueshift). If the car is moving away from P, the shift is in the other direction and P may detect low frequency radio waves instead (frequency goes down, redshift).

Solution 2

Only to someone outside the car; if they are in your path they will firstly be irradiated to death, and anything left will be vaporized in the ensuing collision.

Of course, if they are behind the car, they will see all the lights red-shifted, and they will survive!

Solution 3

The key point to bear in mind if you are perplexed by questions like this is that all motion is relative, and all the effects of SR apply symmetrically between two inertial reference frames.

When you sit in your car and turn on the headlights, they produce visible light. To a particle passing the Earth at 0.99999999999c the light from your headlamps appears to be gamma rays. If you were able to drive your car at 0.999999999999c past the Earth then when you turned on your headlights they would produce visible light, but that visible light in your frame would appear to be gamma radiation to people on Earth. The key point is that your headlamps don't change, and nor does their output, but how that appears to others does change as a result of the Doppler effect.

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Robotex
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Robotex

Updated on February 11, 2022

Comments

  • Robotex
    Robotex over 1 year

    If you travel on car with nearly the speed of light and turn on the car headlights: will it shine in gamma light instead of visible light?

    • fishinear
      fishinear almost 2 years
      "...with nearly the speed of light" - with respect to what/whom? "will it shine in gamma light instead of visible light" - according to whom?
    • Solomon Slow
      Solomon Slow almost 2 years
      Are you in the car? or are you standing by the side of the road, watching the car flash past you?
    • Jason P Sallinger
      Jason P Sallinger almost 2 years
      George, you know I was wondering, like if you were traveling through outer space, I mean like you're going real fast, like the speed of light, you know... hoooohhhhh... and all of a sudden you started screaming... aaaahhhhh aaaaahhhhh... Do you think your brain would blow up?
  • Solomon Slow
    Solomon Slow almost 2 years
    ...except, if the Doppler shift is enough to make the headlight beams look like gamma rays, then the receding tail lights will "look like" radio waves. You're not really going to "see" that except in a metaphorical sense.
  • Robotex
    Robotex almost 2 years
    So, people it front of car will see the huge gamma radiation and behind the car only a radio waves?
  • PM 2Ring
    PM 2Ring almost 2 years
    Of course, a car with a Lorentz gamma factor of a million cannot travel through an atmosphere. And if it's on an Earth-sized planet without an atmosohere, it's also going to have difficulties with the extreme centripetal force required to maintain its circular motion as it circumnavigates the planet 7 times per second. ;)
  • Robotex
    Robotex almost 2 years
    "all motion is relative" - even motion with speed of light? If two objects moving with speed of light but without acceleration, will they see no movement and no relativistic effects?
  • Marco Ocram
    Marco Ocram almost 2 years
    Firstly, no object can move at the speed of light. Otherwise, all inertial motion is entirely relative. My point is that you are NOW as you read this moving at nearly the speed of light relative to passing muons- do you feel any different as a result? Does the world around you look any different? No, of course not. However, you would observe relativistic affects applying to the muons who are moving relative to you.
  • Mark Ransom
    Mark Ransom almost 2 years
    @PM2Ring if you can get something up to a significant fraction of the speed of light, keeping it in a circular orbit should be child's play by comparison. Good luck surviving the necessary forces though.
  • Acccumulation
    Acccumulation almost 2 years
    "Of course, if they are behind the car, they will see all the lights red-shifted, and they will survive!" No, the car will constantly collide with air molecules at relativistic velocities, causing a shower of radiation in all directions.
  • Luaan
    Luaan almost 2 years
    Of course, you probably wouldn't see the light from your car, since it needs to reflect from a surface to return to your eyes - that might be tricky with the frequencies involved :D
  • Abdur Rakib
    Abdur Rakib almost 2 years
    Just a dumb thought of mine- what if OP attaches a reflector in front of his car's headlight? What frequency would he find?
  • Mauricio
    Mauricio almost 2 years
    @AbdurRakib visible too. as long as the source is in the same frame of reference as the driver, the driver will see the frequency of the source (in that frame).
  • Aron
    Aron almost 2 years
    @AbdurRakib However, if the road has cateyes on it. Those would be illuminated by the car's headlights, the reflected light would be double blue shifted...
  • Abdur Rakib
    Abdur Rakib almost 2 years
    @Aron, Okay, that was unexpected. Could you explain a little bit more why cat-eyes would be double blue shifted?
  • m4r35n357
    m4r35n357 almost 2 years
    It is a nice change to see some "realism" and "practicality" applied to these discussions ;) We have come a long way since Mr Tompkins!
  • Mauricio
    Mauricio almost 2 years
    @AbdurRakib when the light hits the mirror in the street, the mirror is in a different frame compared to the car. It sees the frequency to be blueshifted as the car is moving towards it. When it reflects and strikes back your car, your car is again in a different frame and the light from the mirror is coming towards it so it gets blueshifted again.
  • Robotex
    Robotex over 1 year
    I'm calling "light" for all the EM spectre
  • Ion Freeman
    Ion Freeman over 1 year
    @Robotex go ahead and redefine terms all you want, but it's going to be a barrier to communication