3 ms·

Veritasium recently published a video [1] on why the speed of light can't be measured one way, only two ways (i.e. the speed it travels from point A to point B

• cygx 6y ago
  We never discussed this in our SR lectures - instead, I stumbled upon it in a creationist paper where someone tried to finagle a young earth out of it. For example, you can use a convention where the speed of light is infinite for infalling light rays, and twice as slow than normal for outgoing ones. It's a bit weird (when you look into the mirror, light from the sun reaches you instantaneously, 'slowly' moves towards the mirror, and then jumps back to your retina instantaneously), but predictions will work out the same. Of course, it'll affect more than just travel time of light, you'll also mess with anything that inludes a gamma factor, eg making inertia anisotropic (the force you need will depend on the direction you push in). Ultimately, it doesn't help the creationists, because things like time passed since the big bang for an observer following the Hubble flow does not depend on such conventions. All you'll achive is changing the big bang from an instantaneous thing (which in itself is already a bit of a dicy concept in relativity) that happened everywhere all at one into a continuous thing that started 13.8 billion years ago and has been going on ever since.