> How can something travel faster than light?

> Nothing can travel faster than the speed of light in a vacuum. However, in other mediums, particles can potentially move faster than light. For instance, while in water, light would instantly slow down to 75% of its normal speed, but there are other particles that don’t slow down as much and end up moving faster than light. Whenever that happens, a blue or violet glow occurs.

After reading this answer, I was not any wiser.

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hdgvhicv an hour ago | parent | next [-]

In water photons travel at say 200,000km a second. Neutrinos travel at nearly 300,000km a second. That’s causes a blue glow. Which is how neutrino detectors work.

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pfdietz 8 minutes ago | parent | next [-]

> In water photons travel at say 200,000km a second.

It really depends on the energy of the photons. There is "dispersion". It's the same effect that causes a prism to split white light into different wavelengths.

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chinathrow 34 minutes ago | parent | prev [-]

Thanks - but I fell over this sentence:

> but there are other particles that don’t slow down as much and end up moving faster than light.

Not slowing down as much I can understand but shouldn't it read as

"but there are other particles that don’t slow down as much OR EVEN end up moving faster than light."

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chinathrow 33 minutes ago | parent [-]

EDIT:

Got it, faster than light IN THAT MEDIUM.

	▲	Betelbuddy 27 minutes ago \| parent [-]
		Yes ...it deserves to be flagged...This is the type of article we would never waste time with at the Vulcan Academy of Science. But you guys there at the Star Trek Academy, always had looser standards...

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cbolton an hour ago | parent | prev | next [-]

Yeah I didn't find that helpful. What I remember from Feynman's lectures is that photons still travel at "full speed" c between atoms, but if you look at the global progression of light as photons get absorbed then emitted it progresses slower than c.

	▲	yayachiken 37 minutes ago \| parent [-]
		You cannot treat light as particles in that scenario. The primary wave gets absolutely and completely delayed, with no part getting ahead. It's not some photons doing something with a certain probability and then causing a macroscopic effect once the probability goes towards 1 once you passed sufficient matter. What Feynman does (where this confusion comes from) is that you can look at discrete wave packets (i.e. photons) and the math comes out the right way for the primary wave if you assume that only some of these wave packets get phase-shifted, and add all elementary waves together afterwards. But still, it's photons as "wave packets" that influence the whole system, not photons as independent particles that either bounce on something or don't.

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adaml_623 an hour ago | parent | prev | next [-]

How can something travel faster than light?

Answer: Light slows down when going through water or air or gas. It's only in a vacuum that light travels at 'c' (from Einstein's equation). And it's that speed c that is a limit due to relativity.

But the exciting thing is that when you're not in a vacuum particles can be traveling faster than the local speed of light (maybe 75% c). And that process of a particle zipping along gives off Cherenkov radiation.

I think of it as the light equivalent of a supersonic shockwave and sonic boom. Faster than sound gives noise. Faster than light gives light (or other electro magnetic radiation)

(People with more knowledge might say the sonic boom analogy is very inaccurate but not sure)

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georgemcbay an hour ago | parent | prev [-]

They didn't word that very well.

Would have been clearer if they said "However, in other mediums (like water), particles can potentially move faster than light does in that same medium."

	▲	zhivota 36 minutes ago \| parent [-]
		Ah, so it's not faster than c, it's faster than light's speed inside the medium. This makes a lot more sense.