[nate908]

What's up with this image caption?

"The inside of the Petten test reactor where the thorium salt is being tested is shining due to charged particles traveling faster than the speed of light in water."

As I understand it, nothing travels faster than the speed of light. The author is mistaken, right?

[logfromblammo]

Nothing travels faster than the speed of light through vacuum.

But the index of refraction of water is 1.33 . So the speed of light through water is much slower than through vacuum. And electrons can travel through water faster than light through water.

The effect is similar to a massive object traveling faster than the speed of sound through air.

https://en.wikipedia.org/wiki/Cherenkov_radiation

Edit: I love that HN is so smart that a dogpile can form from all the people rushing in to explain Cherenkov radiation.

[lutorm]

No, it's correct. The key is the "... in water" part.

[mikeash]

For details, see: https://en.wikipedia.org/wiki/Cherenkov_radiation

[xyzzyz]

Nothing travels faster than the speed of light in vacuum. Light is slower in other media, such as water, where its speed is only 3/4 of its speed in vacuum. This difference in speed is responsible for the light refraction, for example.

[DanBC]

Cherenkov radiation: https://en.wikipedia.org/wiki/Cherenkov_radiation

"Speed of light" seems simple, but is a bit more complicated than that.