If light is relative in speed...

Darg · 12 Jul 2007 at 17:03

Okay, another one of these threads.

Imagine I'm driving in my car past the earth at half the speed of light c/2.

I turn on the headlights. Now since the speed of light is relative to the observer I would see them shining out in front of me like normal.

But to a static observer on the earth they would appear to be going at only half the speed of light relative to my car.

Time for some bad ASCII art to explain better:

According to me in my car (C = Car, O = Earth):

Code:

                    C----------

                    O

According to observer on Earth:

Code:

                    C-----

                    O

The --- indicating the distance the light travels in 1 second in front of the car according to each observer. This is ignoring the time taken for the light from the headlights to reach the static observer. I don't think that would make any difference other then to add confusion.

So what is the actual speed of the light? I cannot understand how it could be going at two different velocities just because it is being observed from two different speeds.

Darg · 12 Jul 2007 at 17:07

Ex-RoNiN said:
There is no "actual" speed of light, it depends on frame of reference.

Just like there is no "outside" reality, it is all a social and mental construct.

Don't do that to me when I'm this tired dude..

Darg · 12 Jul 2007 at 17:14

So how long would my headlights look to an observer on earth?

Darg · 12 Jul 2007 at 17:25

*but* if the light appears the same length to the observer on the planet as it does to the person in the car then the light would appear to the observer on earth to be going at 1.5X the speed of light as you have the speed of the car plus the speed of the light. How is this possible?

Darg · 12 Jul 2007 at 17:37

I should have done astro physics.. you'd have to be stoned to understand all this stuff!

Not that I wasn't anyway.. :rolleyes:

Darg · 12 Jul 2007 at 17:47

Slam62 said:
ooh so light can never go above the speed of light, sounds logical but hmm

I suppose light waves are different ie not matter as such but energy and therefore would not gain the additional speed of the car.

But if that were the case then the light would appear to be travelling at half the speed of light to the observer in the car.