# If a car was able to travel at the speed of light what would happen if you turned on the headlights?

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I can see a lot of people answering this with an "its impossible!"

We know it is, thats why you said "If".

Its actually quite simple.

Light is an incredible phenomena, in that regardless of where it is observed, whether it is moving toward or away, light travels at 300,000,000 meters per second in a vacuum.

Its something that can be hard to warp your mind around, but its plain and simple fact.

So here how it works.

Light travels as a wave.

It has peaks and troughs.

If a particular beam of light has a wavelength of 1 meter, and I am standing still, the light will be seen by me as light with a wavelength of one meter.

If another particular beam of light has a wavelength of one meter, and I am moving toward it at the speed of light, the light will be seen by me as having a wavelength of 0.5 meters.

And if yet another particular beam of light has a wavelength of one meter, and I am moving away from it at the speed of light, the light will be seen by me as having a wavelength of 2 meters.

Rediculous as that sounds, it is true.

So what will happen with your car, is, if you were in it, travelling at the speed of light, and turned our lights on, (and because it is white light, it emits several different wavelengths at once, appearing as white) to people observing;

all the red would become blue, all the green would become low frequency Ultraviolet, and all the blue would become mid frequency Ultra Violet.

• The light from the headlight will still travel at the speed of light and not at twice the speed of light as you might expect. In Einstein's theory of relativity, the addition of velocity takes the form, s = (v + u)/(1 + (vu/c^2)) where v and u are the two velocities to be added and c is the speed of light. At velocities we normally associate with our daily experience, a car's speed for example, say 76mi/hr and 90 mi/hr, the equation for s essentially boils down to s = u + v --> since the denominator is approximately equal to 1 Once you approach relativistic velocities, the denominator becomes significant. In your question both v and u equals c, and so the denominator term is equal to 2 s = (c+c)/2 = 2c/2 s = c

• If you were sitting in a car travelling at the speed of light do you think you'd give a flying fork about the headlights? I'd want to be damn sure my windscreen wipers were working though.. Have you seen the mess bugs make at sixty MPH?

That is a point of speculation as well.

• Theoretically, you would have a car travelling at the speed of light with it's headlights on!

• Not this question again. Absolutely nothing would happen. First of all it's impossible to travel at the speed of light. If we could, when the headlights were turned on you would not see the beam of light because you are within it's speed region.

• You can't go at the speed of light. Changing your question slightly to "how fast does the light come out of a car travelling at speed (s)"...

Well you'd expect the speed to be s+c without relativity (c=speed of light), but relativity predicts that it would measure c only. I have no idea why, relativity is a bastard! Guess I'll learn it next term when we do general relativity!

p.s. The light would have it's wavelength or "colour" changed due to the doppler effect.

• You can't travel at exactly the speed of light but if you got as close to it as you wanted, when you turned on your headlights you would measure the light going away at the speed of light.

• At the speed of light relative to what? Depending on the position and speed at which the observer is travelling the result is different. That's Einstein's theory of (special) relativity.

• If a body would travel at the speed of light its mass woul become infinite.No material body can travel at the speed of light

• Which speed of light are you talking about? Light's speed varies depending upon what it is passing through.

Through a vacuum, light travels at just over 186,000 miles per second. Through sodium at -272 degrees c, it's 38 miles per hour! Through a Bose-Einstein condensate of rubidium light can be brought to a complete standstill! So it depends, which speed of light you're using in your example. I think you know what happens when you switch on your lights while driving at 38 miles per hour...

I'm sure you really mean the speed of light in a vacuum, or in whichever atmosphere the car is in at the time. In this case, the light would not show you anything.