Talk:Speed of light/Definition of the metre

This is a page to discuss the relationship between the speed of light and the definition of the metre in relation to what is said inthe 'Speed of light' article.

Initial discussion

John: An approach using both length (metre) and time (second) units is not "incorrect". It is the system in use prior to 1983. It requires a speed to be established by measuring a distance traveled and the time of travel, and as both of these measurements are indeed measurements, the speed has an experimental error bar. An approach using a standard speed also is perfectly viable. It could be the speed of sound in a specified medium, for example. Then a distance is determined as a "time-of-flight", that is, how long it takes the sound wave to travel the length. Such a system requires only a time unit, the second, and all lengths are determined in terms of times-of-flight in seconds. In choosing between these two systems, it is not their "correctness" that is at stake. What is involved is practical matters, most notably, how much trouble it is to insure that the standard speed has actually been realized. In the case of a standard speed in terms of the propagation of sound, the issue is how readily and accurately one can ascertain that the standard medium has been realized. It is when considering this matter that the speed of light becomes so highly recommended, because experiment has shown the speed of light to be readily realized in a great variety of circumstances, available to all observers without undue concern and preparation. Brews ohare (talk) 17:23, 17 July 2010 (UTC)

I agree with you Brews but I do not see your point. The current method of defining the metre is philosophically sound and practically and experimentally superior. Martin Hogbin (talk) 18:54, 17 July 2010 (UTC)

It is incorrect in that it's out of date: if e.g. a teacher asked a student what's the best way to measure a metre the correct answer is 1/299,792,458 the distance travelled by light in one second. Any other answer is wrong. The pre-1983 answer might once have been right, and for most purposes is accurate enough, but it's not the correct modern approach. And it's only the most recent such – go back further and other methods were used, accurate enough for their time, but all incorrect by modern standards. And a direct consequence of the modern definition of a metre is that the speed of light is an exact constant.--JohnBlackburne^words_deeds 19:21, 17 July 2010 (UTC)

It depends on what you mean by "best": certainly it isn't the cheapest way. :-) (The most accurate, yes.) A. di M. (formerly Army1987) (talk) 19:26, 17 July 2010 (UTC)

Hi Martin: I am glad that what I say seems to make sense to you. My original point was simply that in the subsection on Measurement values for c with error bars appear, and I felt that was a source of confusion for those entertaining the notion that c had an exact value. My proposal was simply that a pre-amble be put there along the lines of:

“Before the adoption of ‘time-of-flight’ for lengths that brought in teh defined value of c as an exact value, the speed of light was measured in a system using using individual standards for length and time, with the result that c was a measured quantity, accordingly with an experimental error bar. Some of these measurement results are outlined next.”

Brews ohare (talk) 19:32, 17 July 2010 (UTC)

I see your point about the 'Measurement' section. It ends with the Boulder measurement, thus giving the impression that the SoL can still be measured. I suggest all that it needs is a sentence at the end stating that after the Boulder measurement the SoL was fixed by definition and referring to the more detailed discussion below. Does anyone object to doing this. Martin Hogbin (talk) 21:12, 17 July 2010 (UTC)

Response to Blackburne: “When I use a word," Humpty Dumpty said, in rather ... a scornful tone, "it means just what I choose it to mean – neither more nor less.” So out-of-date = incorrect, eh? The use of ‘time-of-flight’ is the present choice in the SI units. That is not to say that other choices cannot be made, or that actual measurement of the speed of light is not to be attempted anymore. And its present adoption in SI units certainly is not an argument against presenting the rationale behind different methodologies. Brews ohare (talk) 19:44, 17 July 2010 (UTC)

Well, what Blackburne had originally written was: "Again there is no "difference in logical basis", there is only the current correct and universally accepted approach. Everything else is wrong and/or historical, and is already covered at length in the article." (Italics mine.) A. di M. (formerly Army1987) (talk) 20:11, 17 July 2010 (UTC)

"Again there is no difference in logical basis, there is only the current correct and universally accepted approach. Everything else is wrong and/or historical, and is already covered at length in the article." (Italics mine.) This viewpoint is unsupportable. Brews ohare (talk) 20:51, 17 July 2010 (UTC)

To some people, yes, that and other older methods are incorrect. As for other measurement systems the other one in common use, imperial measures, has an identical second and has lengths defined in terms of SI units, so a similarly exact light speed (though it would be a fraction).

The other argument for the current system is philosophical, as Martin Hogbin suggests. Relativity unifies time and space into one 4D space, spacetime, where space and time are interchangeable: Lorenz boosts are just rotations in this 4D space made of three space and one time axis. In this context it makes sense to set c to 1, so time and space are measured the same way and c (or c²) drops out of the formulae for relativity. For non-reletavisitc applications the current units are much better, so c becomes a scale factor conversion between the units on the space and time axes, which is also the speed of light. There's a lot of solid theory and good experimental evidence behind this: relativity is well established science, and the definition of the metre of c from it makes perfect sense.--JohnBlackburne^words_deeds 20:14, 17 July 2010 (UTC)

John, wouldn't you agree that the question isn't whether the “time of flight” method is the best choice at present, but simply that the “time of flight” method is different from the “length-and-time” method used in the recent past? Quoting things like c =299,710±22 km/s makes sense only in a “length-and-time” method, while c is exactly 299,792,458 m/s makes sense only in the other method. A clear explanation of why that is so is useful to the reader. Brews ohare (talk) 20:48, 17 July 2010 (UTC) .

I don't see that this is needed, in that I don't see anything either missing from the article on this or unclear that's currently there.--JohnBlackburne^words_deeds 21:02, 17 July 2010 (UTC)

The 'Measurement' section

As it currently stands the 'Measurement' section ends with the Boulder measurement thus implying that the SoL can still be measured. The rest of the article, in particular the 'Increased accuracy and redefinition of the metre' section makes clear that this is not the case but I think the current state of affairs is a little confusing. What is the opinion of the following options?

Rename the 'Measurement' section something like 'Historical measurements'

The problem with this is that the title becomes a little obscure. What about current measurement? Martin Hogbin (talk) 14:34, 18 July 2010 (UTC)

Also, the astronomical time-of-flight measurements in astronomical units per day are not only historical (though since 1983 they are equivalent to measuring the astronomical unit in metres). A. di M. (formerly Army1987) (talk) 15:49, 18 July 2010 (UTC)

They're not quite equivalent (although as near as makes no practical difference) – to make the calculation for AU/day, you have to agree how you're going to take account of GR, on which SI is silent. As far as I can see, the figure for the AU in metres that the article quotes is not a proper length (which a figure in metres should be). More an issue for the astronomical unit article than here. Physchim62 (talk) 17:16, 20 July 2010 (UTC)

What I meant is that renaming the section to "Historical measurements" would be a Bad Thing as it also mentions ongoing measurements. A. di M. (formerly Army1987) (talk) 13:53, 21 July 2010 (UTC)

Include 'Measurement' in the 'History' section

I do not like this as it involves a major rearrangement. Martin Hogbin (talk) 14:34, 18 July 2010 (UTC)

This was the way the article was before the part about modern astronomical measurements in AU/day was added, but several people didn't like it. A. di M. (formerly Army1987) (talk) 15:49, 18 July 2010 (UTC)

Add a sentence to the Measurement section

For example:

In 1983 the meter was redefined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second" making the speed of light fixed by definition at 299,792,458, and its measurement no longer possible, see 'Increased accuracy and redefinition of the metre'.

That sentence closes the measurement section with and up-to-date description of the subject. Martin Hogbin (talk) 14:34, 18 July 2010 (UTC)

I like this option the best. Only, it is 299,792,458 metres per second, not the dimensionless number 299,792,458; and it is not the case that its measurement is no longer possible (but only that if you're using metres per second – or any unit of speed defined as an exact multiple of the metre per second – the only thing such a measurement achieves is calibrating your measurement devices, as you already know the result you're supposed to get), so I would say:

In 1983 the metre was redefined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second", making the speed of light fixed by definition at 299,792,458 m/s; see 'Increased accuracy and redefinition of the metre'. A. di M. (formerly Army1987) (talk) 15:49, 18 July 2010 (UTC)

Yes, I forgot the units. I am happy with your wording, does anyone else have a problem with it? Brews? Martin Hogbin (talk) 17:09, 18 July 2010 (UTC)

I had no serious problems with it but saw a couple of ways to improve it and then added a sentence to relate it to the rest of the section and make it more paragraph-like, which I've done.--JohnBlackburne^words_deeds 16:43, 20 July 2010 (UTC)