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{{FAC}} should be substituted at the top of the article talk page

The following needs to be reworked to make it fit in the context of an encyclopedia article. As it is it is a bit too chatty.

=== How Fast is the Speed of light ===

I like to use the vacation analogy to give people a feel for how fast the speed of light is. It goes like this. Let’s say I wanted to take a vacation on the moon. Fortunately there is a highway called Pretend that connects the earth to the moon. The speed limit on highway Pretend is 100 mph and I can only drive 10 hours a day. I had better pack a big trailer with plenty of food and pull it behind my SUV because under these conditions it is going to take me about 250 days to get from the earth to the moon. Light can travel the same distance in one and one forth seconds or about 5 beats of the drum at one-quarter time.

As a layman, I don't understand how it's possible for something to travel faster than c but not carry information faster than c. Could someone explain this? -- User:Evercat

A very rough explanation is that the "something' that travel faster than light doesn't carry energy. -- looxix 00:43 Apr 19, 2003 (UTC)

Also, it is only in a vacuum than nothing (no information) can travel faster than light; in a medium thing can travel faster than light (see Cherenkov effect) -- looxix 00:49 Apr 19, 2003 (UTC)

Irrelevant. The electrons involved in the Cherenkov effect are still going slower than c. (It is true that light travels even slower than those electrons in that medium).

The fact that Group velocity is greater than c doesn't mean that there is really a particle moving (travelling) at that speed but rather that something is changing at this speed, in this case the waveform. (See also Phase velocity) -- ReiVaX 16:44, 18 Jul 2004 (UTC)

Sure. Take a laser pointer. Make a spot on the moon. Then turn your wrist to make a spot on the earth. It takes over a second for light to travel from the moon to the earth, yet I can move that little spot the same distance in far less than one second. --68.229.240.25 03:54, 20 Jul 2004 (UTC)

Similar, probably better, explanations:

• http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3
• http://www.phy.duke.edu/research/photon/qelectron/proj/infv/fast_tut.ptml
• http://www2.abc.net.au/science/k2/stn-archive1/posts/topic42526.shtm
• http://answers.google.com/answers/threadview?id=73321
• http://math.ucr.edu/home/baez/physics/Relativity/SR/scissors.html

"It is a solution to the wave equation"

How is the speed of light be a solution to a vector equation? Κσυπ Cyp 08:08, 13 Nov 2003 (UTC)

As my electromagnetics professor explained it (and you'll have to bear with me - it's been almost 2 years), it's not that the speed is the solution, per se. It just doesn't have a solution for any other speed besides C. --Raul654 08:28, 13 Nov 2003 (UTC)

One more thing. Here is the exact derivation you are looking for: http://people.ccmr.cornell.edu/~muchomas/P214/Notes/OtherWaves/node18.html --Raul654 08:36, 13 Nov 2003 (UTC)

If I understood correctly. <- This sentence was written by me. Κσυπ Cyp 21:30, 16 Nov 2003 (UTC) The rest wasn't. -> Faster than light transmission of information follows some uncertainty principals, it also sidesteps a couple rules. When information is transmitted at such speeds, it can never be proven that the light recieved is the light that was transmitted. Photons subjected to this process have their frequency changed, their overall energy content is different due to the processes that caused this feat. However, if those people in line were to shout in sequence, the information would have to be previously known, this caused it's own speculation. As with the noted experiment of 300c, the photons arrived faster than light accounts for, the arrival of the photons is information, it arrived at it's destination faster than C, there IS NO explanation. - GouRou

Wile, why are you removing the scientific notation approximation? In 99% of cases when I'm performing a calculation involving the speed of light, the number I'm looking for is 3 × 10. I'm pretty sure this approximation, in this format, is useful to other people as well. Fredrik (talk) 18:49, 8 Jun 2004 (UTC)

Well, it is cluttering, as it doesn't serve any obvious purpose. Anyone who is actually making use of c in computations is surely capable of approximating it as 3 times 10^8 or 0.2998 times 10^9 or whatever they please. The vast majority of the remainder of the readers will be much more at home with "thousands of somethings" instead of scientific notation. -- I feel pretty strongly that the introductory sentences of an article must get right straight to the point. Naturally it is quite possible that the introduction still isn't getting there, so let's work in that direction. Regards, Wile E. Heresiarch 20:45, 8 Jun 2004 (UTC)

How would you know that anyone actually making use of c is capable of approximating it? Every high school student? It isn't immediately obvious to everybody that 100,000 kilometers equals 10 meters. And sure, they might be able to figure it out, but our job is to make this information available as conveniently as possible. Perhaps the note should be placed elsewhere, but there's no reason not to provide it. Fredrik (talk) 21:02, 8 Jun 2004 (UTC)

We don't seem to be discussing anything very fundamental here; the article is about the speed of light, not scientific notation. That 299,792,458 is close to 3 x 10^8 seems to be a footnote that is useful in some contexts. Maybe there is a place for it somewhere in the article (or maybe not) but it seems far from central. Wile E. Heresiarch 21:24, 8 Jun 2004 (UTC)

This is not an article about scientific notation, but it is neither an article about obsolete imperial units (the mile). The question is indeed whether the information is central (useful to someone), and I hold that it is. It would be nice to get input from a few other people with regard to this matter. Fredrik (talk) 07:36, 9 Jun 2004 (UTC)

I've put the 3 × 10^8 m/s just before 30 cm/ns (under the heading "Overview"). It is appropriate since both are convenient approximations. Wile E. Heresiarch 16:21, 9 Jun 2004 (UTC)

Thank you. Fredrik (talk) 16:30, 9 Jun 2004 (UTC)

I'd like to support Fredrik on this point. It's far simpler to get a quick idea of the scale of c without having to count the number of digits in 300,000,000. Cederal 16:52, 9 Jun 2004 (UTC)

I think that perhaps it should be clearly stated that by adding two velocities with the Einstein velocity addition formula we cannot get a speed greater than c. (Because the interval (-c,c) with that operation is an Abelian group.) I know that its said that c is a "speed limit" but what its not said is that the formula is consistent with that. -- ReiVaX 18:09, 18 Jul 2004 (UTC)

The article currently says:

Definition of the Metre

Since the speed of light in vacuum is constant, it is convenient to measure both time and distance in terms of c . Both the SI unit of length and SI unit of time have been defined in terms of wavelengths and cycles of light. In 1983 the metre was redefined in terms of c .

I'm fairly certain that is correct for "distance", but incorrect for "time". If you read http://www.bldrdoc.gov/timefreq/general/precision.htm#Anchor-60273 carefully, you see that "transitions of the cesium atom" are the reference for time, not the speed of that light.

Suggested replacement:

Definition of the Metre

Since the speed of light in vacuum is constant, it is convenient to measure distance in terms of c and time. In 1983, the SI unit of length (the meter) was redefined in terms of c and time.

Sounds good to me. Go ahead and change it if you want, I'll do it if you don't get around to it. Wile E. Heresiarch 03:15, 21 Jul 2004 (UTC)

For a Featured Article this should have some graphics. Yes, I know it it is hard to find a good graphic for speed of light, but perhapse Google Image search will have something we can use? --Piotr Konieczny aka Prokonsul Piotrus 21:37, 31 Jul 2004 (UTC)