I have a little paradoxonal joke regarding this:
"If a car is moving with lightspeed, what happens if we turn on the headlights?"
In my opinion, the anwer is simple:
Assume we are in vacuum. The car travels at lightspeed relative to the light source (Sun or whatever), so the car and the light, as a ray projected from the source, travels at equal speed. The law says that nothing can travel faster than light. Now, if we turn on the headlight, that creates another light source, wich will emit light rays. Now as I see, two things can happen.
First, if we see the whole thing relative to the orig light source, and the light travells with 300,00 km/s in vacuum no more no less, the light emitted from the secondary source(the headlights) should remain the speed of the other (original) light ray, due to the lightspeed law (300,00 km/s). If this is true, the secondary light rays shall remain stationary in the source(the headlight), wich is a bit unbeliavable to me
The second theory is that we turn on the headlights, the secondary light rays shall travel with the same speed too(300,00 km/s) wich will add up. So 300,00 km/s + 300,00 km/s = 600,00 km/s is the speed wich the secondary ligh(emitted from the headlights) will travel relative to the original source. This breaks the lightspeed law.
So what now?