Talk:Speed of light - Misplaced Pages

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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:

"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)

A diagram of the Fizeau apparatus would fill the requirement, and might be useful in explaining that experiment. Dandrake 08:25, Aug 6, 2004 (UTC)

In Two New Sciences Galileo claimed to have performed his experiment, not just proposed it (though in a more ambiguous way than I'd realized); he also said explicitly, long before Hooke, that his experiment couldn't prove that the speed was infinite. Assuming he did perform it, this is the first known case of anyone's trying to make an actual measurement of the speed. The criticism by Descartes is interesting and looks as if it would be valid; does anyone know the argument in enough detail to say? Dandrake 08:25, Aug 6, 2004 (UTC)

begging the question

According to the theory of special relativity, all observers will measure the speed of light as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light. A simple three-step analysis is sufficient to show that this is the case:

the speed of light in vacuum can be derived from Maxwell's equations;
special relativity requires the laws of physics, such as Maxwell's equations, to be identical in all unaccelerated frames, and so
observers in all such frames must observe the same speed of light.

The constant speed of light derives from the Lorentz transforms, which distort distances and times for observers travelling at large velocities in the same way.

Isn't 3 precisely what we are trying to show is the case? "all observers will measure the speed of light as being the same" because "observers in all such frames must observe the same speed of light"? Also, as the speed of light in a vaacum is a defined value, the fact that it can be derived from Maxwell's equations needs to be explained. anthony (see warning) 19:00, 12 Aug 2004 (UTC)

Er - to précis - "ToSR says SoL is constant. How? Because (i) SoL comes directly from Maxwell, (ii) Maxwell is a law of physics, (iii) laws of physics must be the same in all inertial frames; hence, (iv) SoL same in all inertial frames. Plus Lorentz gives you a way to work out how it all works." I don't think this is begging the question, is it? -- ALoan (Talk) 19:14, 12 Aug 2004 (UTC)

I guess I see what is being said. 3 is the conclusion from 1 and 2. This seems to be worded strangely, though. anthony (see warning) 19:41, 12 Aug 2004 (UTC)

Feel free to copyedit. :) -- ALoan (Talk) 19:48, 12 Aug 2004 (UTC)

As you've perhaps seen I've already made a number of copyedits. This one I'm not sure how to reword, though, in part because I'm not 100% sure what the point is. anthony (see warning) 20:12, 12 Aug 2004 (UTC)

I've tweaked slightly - is that better? -- ALoan (Talk) 02:05, 15 Aug 2004 (UTC)

I guess. I'm still confused as to what we are showing to be the case. Are we showing that "all observers will measure the speed of light as being the same" or are we showing that this is a result of the theory of relativity? Because it's not really a result of the theory or relativity, it's a hypothesis of the theory or relativity (I think, I'll double-check that one). anthony (see warning) 02:21, 15 Aug 2004 (UTC)

Some doubts and/or contradictions

The article looks great! But... (ther's always a but):
(a) in 1: «The constant speed of light derives from the Lorentz transforms, which distort distances and times for observers travelling at large velocities in the same way» - I think it's the other way around. The Lorentz transforms derives from Maxwell's equations and Einstein's constant speed of light postulate.

(b) in 4.1: «Einstein took result as a given fact» contradicted by «It is uncertain whether Albert Einstein knew the results of the Michelson-Morley experiment» (in 5.4). - I don't know wich is true.
(c) I failed to understand the concept of "light slowing". Can someone elaborate on that?--Nabla 00:12, 2004 Aug 13 (UTC)

re (a): Yes, that's the same point I argued. In a way, the very notion of "distance" requires some means of measuring it. Einstein's reasoning was to require everything to be done locally, at one point. So, he defined distance as the quantity that you can measure by sending a light pulse to a mirror and wait for it to come back, than halve the time it took and devide it by c. You need this definition to derive the Lorentz transform. But the definition is only compatible to the usual notion of distance if you assume c to be constant.

re (b) I remember having read that Einstein said, that he was unaware of the works of Lorentz and Fitzgerald, who had found the Lorentz transformation and the relativistic length contraction before Einstein. (However, they only postulated them heuristically in order to explain the Michelson-Morley experiment and the anomalies of the Thompson parabolas, without being able to give a derivation.) But I doubt that Einstein was unaware of the whole aether discussion as otherwise he surely must have slept well during his university studies.

But let's check his origibnal 1905 paper which is translated here: "Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the "light medium," suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest." So what else could he have had in mind when referring to these "unsuccesful attempts" other than the Michelson-Morley experiment? => The second sentence is wrong.

re (c): It's not well written. First one should mention that of course light is always slower than c when travelling through media. But, usually, the light speed is perhaps 3/4 of it (in water), or maybe half of it (Flint glass) - but recently, people managed to reduce the speed down to only a few meters per second, using additional laser fields which modify the optical properties of the medium. In my opinion this should go in a seperate article - or maybe we have one already.

Simon A. 08:57, 13 Aug 2004 (UTC)

Thanks.

Re (a): of course, Lorentz formulated his transforms before Einstein thought up relativity, but I see what you mean. The intention here was simply to explain how a person travelling at 0.9c could still measure a speed of light of c because distances and times are dilated. If you can think of a better way of doing it, please be my guest.

Re (b): One of the sentences must be wrong. I've seen it reported a couple times that Einstein was not aware (or said he was not aware) of the Michelson-Morley result in 1905, so I've fixed the first passage (in any event, I think there were other, also unsuccessful, attempts to measure the velocity of the aether, apart from Michelson-Morley).

Re (c): I've reworded substantially. -- ALoan (Talk) 02:05, 15 Aug 2004 (UTC)