How can nothing be faster than light when speed is only relative?

Question

You always hear this phrase if you watch something about astrophysics ‘Nothing can move faster than light’. But speed is only relative. How can this be true if speed can only be experienced/measured relative to something else?

In: Physics

Answer 1

It is hard to ELI5, but here goes:

You are standing beside a train, you shine your flashlight at someone else standing beside their train, they shine their flashlight when they see yours. You measure the time for the round trip, you get the speed of light.

You both get on your trains, heading toward each other. You shine your flashies, you measure the round trip.. wow, same speed of light.

You both go real fast, say leaving the station at half light speed each toward each other. You shine your flashies, you measure the round trip.. same speed of light.

Now, the reason for this complicated, but essentially your point of view, your perspective, is where all the distances and times are measured from. And those numbers don’t work by adding up, when they get closer to the speed of light, the figuring starts to distort.

For example if you hop on a spaceship and head toward a distant planet and start accelerating. The rest of the universe, including the distance to that planet, will seem to get shorter (not just because you are moving that way). Essentially you turn everything into pancakes. But, people on that other planet see that happening to you.

The takeaway, is that is actually how movement works, our ability to add and subtract distances and speeds is actually the weird little simplified version of reality that we get to live in.