User talk:Brews ohare/Sandbox12

Comments on draft article possibly called length (measurement)

Your distinction between time-of-flight and interferometric measurement is no longer applicable. The light used in interferometric measurement has a frequency that is very accurately known according the the caesium frequency standard. Length is measured in an exact number of wavelengths, this the time of flight is known. Martin Hogbin (talk) 01:31, 27 November 2011 (UTC)

Martin: I do thank you for looking at this piece. Discussion generally leads to improvements. I hope you can take some time to explain your comment further, because I do not understand it.

On the off-chance that the problem here is simply a misconception about my objectives in this piece, let me explain its purpose. My aim here is not to discuss such things as the speed of light, or whatever we disputed in the past. The goal is to discuss different methods for measuring lengths that use different instrumentation. The methods should agree where two or more methods are available, but in some applications only one method can access the small scale involved. Then the notion of metrological traceability arranges to calibrate the methods using lengths where both can be used. The established method establishes the length, and the use of both measurements on this length calibrates the second method. The newly calibrated method is then used where it works and the original method does not.

Here is my understanding of the time-of-flight method compared to the interferometer method:

The time-of-flight measurement of a length, as you know, is a measurement method, with its own instruments and techniques. It does not depend at all on knowledge of the frequency of the light used. It is very useful for longer distances, where the interferometer is quite impractical. Its instrumentation issues depend upon just how it is implemented, but one example method is to send a pulse to a receiver that sends back the signal upon arrival. There are some issues here with measuring the exact emission and reception times, as both the pulse gun and the detector have some response time uncertainties.

The interferometric measurement of length, as you know, is based upon counting how many wavelengths fit into the length. This count has its own instrumentation, one of which is the Michelson interferometer with corner cubes shown in the figure. Once you know how many wavelengths long your length is, you can quit and just say the length is so many wavelengths of the such-and-such spectral transition, but as you also know, you can convert this measure to metres if you want to by swapping the wavelength for c₀/f. Of course, then you have to know the frequency. This method is accurate to quite short distances where the time-of-flight method runs into difficulties due to the instrumentation issues associated with response times. The interferometric approach is a steady-state method, and doesn't involve pulse generation and detection.

So, given the above understandings, there is a very clear difference between these two methods, although they should agree with each other where either can be used, apart from the experimental errors involved in the two methods, which errors will be different inasmuch as the instrumentation is different.

In my opinion, then, the two methods are quite distinct from one another as regards length measurements, and so I suspect your comment is based upon a different idea of what this piece is about.

Please do continue this conversation and please suggest how the exposition can be clarified. Brews ohare (talk) 03:21, 27 November 2011 (UTC)

Thanks for reassuring me that you are not still trying to make some oblique point about the SI definition of the metre. I do not see any reason why you should not start an article on the measurement of length, although I think it would be better named Length measurement or Measurement (length).

If you do start such an article it might be an idea to start with some kind of article plan. Would it be better to cover the subject primarily in historical order or should it be mainly organised by scale? How would the different units of length be discussed? Martin Hogbin (talk) 12:31, 27 November 2011 (UTC)

Hi Martin: As the piece now stands, the organization begins with longer lengths and proceeds toward shorter lengths. It is really only an introduction to the various methods in that it doesn't contain details of instrumentation and measurement errors. Some articles are linked that provide more detail, such as the transmission electron microscope, and some topics still lack an article like the helium ion microscope. I hope the article will be useful in organizing such topics. If you are aware of other methods, or other articles that could be linked, please suggest them. Brews ohare (talk) 17:03, 27 November 2011 (UTC)

If you are going to organise the article by scale then perhaps you should include something on astronomical measurements. Martin Hogbin (talk) 00:34, 28 November 2011 (UTC)