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In nature, we observe the distance between two galaxies increasing faster than light, the intervening space increasing in size without their speed increasing.

• tux3 4y ago
  Since the distance between two galaxies isn't made of stuff, it's allowed to 'expand faster than light'. There's other examples of the same thing, and we know how to do it. But it can't be used for anything useful. If you point a light at the moon, and slide your hand in front of the light, the shadow can also "move" faster than light accross the surface of the moon. But the shadow isn't an object that moves to the side, it's really many lightwaves moving forwards (or their absence), so again there's no real thing moving faster than c that could transmit information. Some materials can be made to have an index of refraction below 1, so that light's phase velocity becomes 'faster than light' (while the group velocity, which is the speed of light, remains the speed of light). So you can end up with speed values in nature above c, but in all those cases it's not matter or information, so you can't move anything tangible faster than light. You can see that as an outside-the-box where things move faster than light, and that's fine, but these can never be used to go back inside the box with some real thing that has moved faster than light. That'd break the box, and so any outside-the-box you create can't be usedul inside