Warning: special relativity hurts your brain. First, there's no such thing as objective space. There is no objective Frame of Reference. You can pick a frame o

Warning: special relativity hurts your brain. First, there's no such thing as objective space. There is no objective Frame of Reference. You can pick a frame of reference, and it should probably be an inertial (non-accelerating) frame of reference, but it's just as valid as any other frame of reference. Inertial frames of references move with respect to each other at constant, less-than-C velocity. Ok, so you picked a frame of reference, and in that frame of reference you have two objects A and B moving away from each other, each with a velocity of 0.5c, for a total relative velocity of c. What does this look like to either of the objects? Well, to answer that question, we have to jump into their frames of reference, which are moving at 0.5c with respect to ours. In this new frame of reference, one of the objects A is stationary. Object C that was stationary in the previous frame of reference is now moving at 0.5c. And the other object B, that in the previous frame of reference was moving away at c? In this frame of reference, it is moving away at less than c. Why? A combination of time dilation and space compression, which you probably know happens in special relativity. In the original frame of reference, object A was moving at 0.5c because it took 1 year to travel from one point to another than are half a light year apart. In the new frame of reference, the object A is standing still and the two points are moving, and they're more than half a light-year apart, and it takes less than 1 year for the new one to get here. Similarly, time & distance separation between points that the other object B is passing have altered in the new frame of reference. There's some math which isn't that complicated but is a bit onerous, but the punchline is that it appears to be traveling less than the speed of light in the new frame of reference. Why does space and time contort like this? So that a light particle traveling will appear to be moving at the speed of light in any inertial reference frame, regardless of the relative velocities of those reference frames.