# Turning on headlights near light speed [duplicate]

If i turn headlights on in a car at v = 0.99*c, will the headlight speed be 0.01*c? I know speed of light is always the same but it really confuses me? My question is, does space contract so much at that speed that even light at just the speed of 0.01*c will appear to be 299792 km/s? It seems to me space contraction and time dilation make the slow light appear to be normal light at normal light speed. Can someone explain to me please?

## marked as duplicate by Bill N, John Rennie, Kyle Kanos, ZeroTheHero, Jon CusterJan 28 at 17:17

• Are you wanting information, or are you expressing uncertainty about the validity of special relativity? For information see simple.wikipedia.org/wiki/Speed_of_light BUT then there is this: arxiv.org/abs/1411.3987 Has there been a response to this paper? – Bill N Jan 26 at 19:27
• If you are riding in a car, then from your point of view, the car's headlights are not moving. If the car is not accelerating, then for any physics problem you choose to solve, you should get the same answer regardless of whether you say that the car and everything attached to it is moving and the scenery is standing still or, whether you say it's the scenery that is moving and the car and everything attached to it are standing still. – Solomon Slow Jan 26 at 19:29

No.

How would time dilation and space contraction work? If you are at rest in your car and you emit a flash, you see it propagating out at $$c$$ in a spherical shell.

What if you are moving at $$v=0.99c$$? Well you already are in the reference of some cosmic ray. Did you notice the time dilation and space contraction? No.

You still see a shell expanding at $$c$$. Now an observer riding with the cosmic ray would also see a shell of light expanding spherically at $$c$$. The only way time dilation and space contraction enter the picture is if the cosmic ray riding observer used a Lorentz transformation to confirm that you also see a shell expanding at $$c$$ in your reference frame, but of course, your observations are completely independent of an imaginary (or real) observer looking at you from a boosted frame.

The reason you both see a spherical shell of light expanding at $$c$$ in each of your own reference frames (or any frame) is because the position of the light-surface is space-like separated from the observer, so that simultenaety is velocity dependent. "Where" the light is now depends on "when" is now, and that is boost (magnitude and direction) dependent in such a was that all observers see light propagating isotropically at the speed of light.

From your frame of reference, the speed of light will always be c. It doesn't matter what speed you are going or what direction the light is coming from.

• Is the speed of light always the same because there is time dilation and space contraction? – as997 Jan 26 at 19:34
• @as997 May I suggest you read about the. Michelson-Morley experiment – Bones Jan 26 at 19:59