# Speed of light and headlight [duplicate]

What will happen if you drive a car at the speed of light and keep headlights on?

Will it be behind the car or just work like normal?

## marked as duplicate by Alfred Centauri, ACuriousMind♦, Norbert Schuch, Cort Ammon, David HammenSep 9 '16 at 0:42

• question is unclear. Are you talking about the observation with respect to yourself or with respect to someone in front? – Kosala Sep 8 '16 at 20:41
• Headlights do not travel at the speed of light. – WillO Sep 8 '16 at 20:43

This was a famous thought experiment Einstein himself claimed to have considered when he was a child that helped him work out special relativity later in life. The question is somewhat paradoxical in that you are using the notion of Galilean relativity to intuit that the light should never be able to stay in front of the car if the car is moving at the speed of light.

The solution to this paradox is that nothing with mass can attain the speed of light in vaccum, and in the reference frame of the car, light is still moving at $c$. However, in a material with some index of refraction, objects that emit light can move faster than light in the medium.

When this happens, a sort of shockwave forms and the phenomena of Cherenkov radiation occurs. This effect is used by neutrino detectors such as IceCube and Super-K to observe 'decaying' neutrinos.

Your question does not obey with the Einstein's special theory of relativity. An object cannot exceed the speed of light as its mass becomes infinite when it gets the speed of light.

• It's not that the mass becomes infinite when it gets to the speed of light, it's that there is no inertial reference frame (IRF) in which it has speed c since an object with speed c in any IRF has speed c in all IRFs, i.e., such an object has no rest frame of reference. – Alfred Centauri Sep 8 '16 at 22:33

Hope you have heard of Doppler's effect and know how to add velocities in Special relativity.

So basically, c+c doesn't make 2c(the speed of headlight will still be c with respect to you), as the maximum possible speed in nature is c (the speed of light).
• I do see a problem with your answer. It's not valid to apply the relativistic velocity addition formula for $c + c$ since one of the velocities must be less than $c$. This is because one of the velocities in the formula is the velocity of an inertial reference frame relative to another inertial reference frame. But there are no inertial reference frames with relative velocity of $c$. – Alfred Centauri Sep 8 '16 at 22:31