# Light/Talk

HomePage | Light | Recent Changes | Preferences

Difference (from prior major revision) (no other diffs)

Changed: 30c30,32
 Are light-years really the prefered unit in Astronomy? I have heard that they are mostly used in popular science articles, but real astronomers use Parsecs instead. I'm not a real astronomer, so i can't vouch for this. -- Geronimo Jones.
 Are light-years really the prefered unit in Astronomy? I have heard that they are mostly used in popular science articles, but real astronomers use Parsecs instead. I'm not a real astronomer, so i can't vouch for this. -- Geronimo Jones. The star and galaxy catalogs tend to use parsecs rather than light-years, so I'd guess it's a safe bet.

I saw a show on PBS where a scientist transmitted music faster than c. He recorded it and played it back on a tape recorder. It was staticy, but recognizable.

--Alan D

See plea for help in article....

As far as I understand it, the evanescent wave coupling stuff is the same as the other group velocity > c stuff in the case of extreme absorption.
There seems to be a lot of interest and half-explainations of this topic scattered around the wikipedia, I may try to write a superluminal communication article, In My Copious Free Time.

What's really interesting is the effect that occurs between the plates of a Casimir Force experiment, that's the only legitimate v>c stuff I know about. -- DrBob

How is "velocity" incorrect? It was good enough for Einstein:
In short, let us assume that the simple law of the constancy of the velocity of light c (in vacuum) is justifiably believed by the child at school. [1]
I suppose we should make it clear that we are referring to the velocity of the propagation of light. --TheCunctator

Well, if the velocity of light was a constant, then it would always move in the same direction. (Velocity being a vector quantity.) This is not true for all observers (the direction of light propogation being different in different reference frames, e.g. the light-clock thought experiments), and not even true for a single observer (light being able to go in any direction). However, the speed of light, being the magnitute of the velocity, is the same for all observers.

Maybe I'm being nit-picky but it seems that Einstein was being loose with the terminology in the article you reference. I still think 'speed' is the most technically correct. -- DrBob

I agree, velocity is generally taken to be a vector quantity and speed is a scalar. The speed of light in vacuum is constanct and equal to c; the velocity vector of light in vacuum is not constant, because light can travel in different directions; the magnitude of the velocity vector is c. --AxelBoldt

Fine with me. I think it would be fair to note that Einstein used "velocity", and define speed. --TheCunctator

Are light-years really the prefered unit in Astronomy? I have heard that they are mostly used in popular science articles, but real astronomers use Parsecs instead. I'm not a real astronomer, so i can't vouch for this. -- Geronimo Jones.

The star and galaxy catalogs tend to use parsecs rather than light-years, so I'd guess it's a safe bet.

HomePage | Light | Recent Changes | Preferences