Hacker News Reader

Comment by scarygliders | Hacker News Reader

The concept I have in my head if I can describe it well enough, is that the speed of Causality is different in water, than it is in a vacuum.

An underwater explosion travels slower in water than it does in a vacuum. That's not just to do with the coefficient of friction of the water. The light from the explosion will take longer to reach you than it would if you and the explosion were in a vacuum.

Which leads me to another thought - does that mean that the matter of water is basically just another form of spacetime, in which the speed of Causality is different from the speed of Causality in spacetime in a vacuum?

View on HN

Nah.

The speed of light is slower in water, yes, which is why Cherenkov radiation exists -- that's basically the photon equivalent of a supersonic shockwave. The speed of causality doesn't change, and it's easy to demonstrate that by throwing particles through it at superluminal speeds; nuclear reactors do it all day long.

Wait. Isn't that entirely explained by the light being repeatedly absorbed and re-emitted after a short delay? But otherwise travelling at c?

Good Fermilab video explaining how the slowdown works: https://youtu.be/CUjt36SD3h8

Absorption and re-emission are probabilistic, so you'd expect experiments to show a spread distribution of propagation times, deflection angles, etc that aren't actually observed. The explanation gets into looking at light as a wave.

https://www.youtube.com/watch?v=CiHN0ZWE5bk

https://www.youtube.com/watch?v=YW8KuMtVpug

I'm not convinced by that.

Cherenkov radiation is still light/electromagnetic energy, and it still must obey the speed of light (Causality) through the water, which means it will still be slower to arrive from source to observer at a slower speed than it would have had it been travelling through a vacuum.

You're mixing up what Cherenkov radiation is. Cherenkov radiation is light that is emitted by particles going faster than the local speed of light.

You're not supposed to measure the speed of the Cherenkov radiation. You're supposed to measure the particles that are causing Cherenkov radiation.

If you measure those fast particles, you'll see that they will cross a tank of water just as fast as they will cross a tank of vacuum. They can cross faster than a beam of light will.

> it's easy to demonstrate that by throwing particles through it at superluminal speeds

I am confused, what particles could reach superluminal speeds?

Gravity travels at the speed of Causality, and that isn't changed in water

Gravity is the bending of spacetime though.

> matter of water

A.k.a. hydrogen oxide.

Reply on news.ycombinator.com