I haven't studied physics and I know most of you haven't either. Then, I'll try to tell that as simply as I could.As you might know, if you are driving your car at 80km/h and there's another car that overtakes you at 120km/h, his driver will be fined if he's near Barcelona and, anyway, you will see him as he was driving at 40km/h. Then, we might conclude Vb/a = Vb - Va. It might seem a logical conclusion and it's the conclusion Newtonian mechanics would take out. But let's see it's not as easy as it seems.
In non-relativistic mechanics, the velocities are simply added. But using the relativistic theory we can conclude that:
If the velocities we are talking about are both small compared to the speed of the light c, then the answer is approximately the same as the non-relativistic theory. But if, for example, we are moving (let's suppose it's possible) at 0.25c and we are overtaken by an object moving at 0.75c, we will see this object as it was moving at 0.61c (and not 0.50c).Notice that in the limit where the velocity of the object is equal to c, it doesn't matter what's our velocity: the sum gives c. This confirms that anything going at the speed of light does so in all reference frames.
