Speed of light times 1.4

The speed of light is constant when the definition is:

"The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,212 miles/second)."

When light passes through a medium such as fiber the light is refracted. The definition of the index of refraction is:

"The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium."

As you can the speed of light is "variable" to us but is not variable by definition.

There are several different things here:

It's the "phase velocity" that can be faster than c. The phase velocity basically tells you how fast a pure sine wave moves. But there's no such thing as a pure sine wave, because it extends infinitely far ahead and behind.

Realistic signals are made by summing sine waves of lots of frequencies, so that after some finite number of wavelengths they interfere destructively and you get a signal of finite width and duration. When you figure out how fast a real signal (group of sine waves) goes, that's the "group velocity." And that's always less than c.

And then there's the fact that light slows down in matter (as compared to vacuum). So if the speed of light in some medium is 0.5c, you can make matter particles go 0.6c, and if you want you can call them superluminal. You do get neat effects like Cerenkov radiation (that lovely nuclear reactor glow), but you're not violating causality because that's tied to the vacuum speed c. Same for the "searchlight effect" and others.

Nine times out of ten, when you hear a "faster than light" story it's the confusion about phase velocity and group velocity. The tenth time it's one of the other things.

There is a very human effect here: Reporters want people to read their stories, so they write headlines that grab attention. When they read or (more rarely) listen to the scientist's explanation, they pick out certain things that sound sensational. Even if the scientist says no, the point is something else entirely. I've seen my own work exaggerated pretty dramatically, and that was from a reporter I had a lot of contact with. It's not necessarily deliberate, just the effect of people being busy and not listening to everything. The combination of them wanting attention grabbers and us not explaining things very well is volatile.

I'll be interested to see the headlines when LIGO bags a pulsar. "Little Green Men emit gravity waves?" That's what they said about the first radio pulsar, after all.

And then everyone rushes to give SETI all their resource share instead of us....

Ben

Gary Roberts wrote:

Glad to hear it!

An enormous amount of time and effort (not to mention money) is going into improving the instruments and the data analysis techniques. Constantly. We'll have a fighting chance of detecting something within the next year. By the early 2010s, the instruments will be so sensitive and have so much data that if we don't detect anything, that will actually be a bigger "discovery" than if we do. More likely, we'll be doing a pulsar census at that point.

But it doesn't happen without you folks running the screen saver, and people are more enthused about doing it when they know what's going on.

We do appreciate that-
Ben

Question: What is the advantage of running the screen saver? My thinking is - letting the program run of itself is sufficient. Please advise -

Light travelling through a medium ( non-vacuum ) is a modelled as a process of absorbtion followed by re-radiation, primarily by interaction with the electrons in ordinary matter. There is vacuum propagation between each interaction, matter being mostly 'empty' space. This predicts well for the states of matter found hereabouts anyway. The 'c' constant appears as a factor in the mathematical 'propagator' of quantum electrodynamics, where the overall calculation involves Feynman's sum over histories or path integral to get the amplitudes, phases and then probabilies which yield the final prediction of behaviour. In the absence of matter this method predicts the classical straight line movement with classical speed c.
So yes, the distance that light travels, divided by the time it takes to do it is going to vary depending on what it passes through. I think the merit in claiming 'c' as a constant of nature is to separate out light's intrinsic behaviour ( when left alone, so to speak ) from it's behaviour during interactions by passing through matter.
One thing I've wondered about is whether relativistics needs to take into account indices of refraction etc. After all Einstein grounded alot of his conclusions by considering light signals between separated points in space. Now if the light is slowed because it has to 'stop and talk to the electrons' on the way, then that is going to have an effect. I can't imagine a GPS system not having to account for the timing change because the signal from the satellites is passing through the atmosphere, to achieve the required accuracy.
Also the propagation of other forces are based on 'c' too. The weak and strong nuclear forces are too short range to really worry about over everyday distances, but gravity isn't. I'd assumed that matter would affect gravitational radiation via it's mass/energy content, however small, but that we could ignore electric charge etc. Maybe this is the area which is not yet unified, but I'm not sure.