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The question Reliable length scales in nature has been received very poorly, for reasons I don't really understand. In the help center, it is stated that questions concerning "Experimental designs and results" and "Experimental technology used in physics or astronomy" are on topic. I can reasonably interpret a question about how to make a ruler from objects in nature as falling under both of these headings.

The spirit of the question I asked was about how to make a measurement. I suspect that if I had instead asked how to determine a length scale from, say, observations of astronomical objects, it would have been much better received. (The question of length scales in astronomy is in fact very important for physics--see the cosmic distance ladder.) The differences I can identify between a question about astronomical objects and about terrestrial objects are the following:

  1. We have sophisticated ways of measuring distances on earth that render length scales of nature unnecessary.
  2. Plants and animals which provide reliable length scales are also within the purview of biology, and thus are not exclusively of interest to physicists.
  3. Why would you need to use natural objects for measuring things anyway?

My question to the stackexchange community is: For which of these reasons (or for what other reasons) was the question dismissed?

My responses to the above points are the following:

  1. Not everyone has access to laser range finders all the time. It is still of interest to be able to measure things in a primitive way. (This is also why "back of the envelope physics" is still an important skill.)
  2. Science is not mutually exclusive.
  3. See response 1.

And by the way, there are good answers (in my opinion) to the question in question. E.g. atmospheric pressure and a lake can furnish a length scale: You could fill up an airtight bag and sink it into water until its volume was half what it was at the surface. The depth of the bag is then 33 feet.

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    $\begingroup$ Also, I note that in the comments for the question under consideration Kyle Kanos remarked that he thought a similar question had been asked in the past. He was not able to find such a question, but the fact that he thought it had already been asked suggests that it is in fact a natural question to think about. $\endgroup$ – Yly Apr 10 '16 at 22:55
  • $\begingroup$ As a practical matter the way the Boy Scouts taught me to deal with "If I am hiking in the woods (not carrying any sort of ruler with me) and I decide I want to measure something [...]" was to know the actual size of various human measures on my body (my cubit is almost exactly 18", for instance) and to practice stepping off a reliable pace (I can still get $\pm 5\%$ over tens of yards that way). You've ruled that out, but I don't understand why: it is what people do. My suspicion is that what you request is either non-existent or very rare. $\endgroup$ – dmckee Apr 10 '16 at 23:15
  • $\begingroup$ I'd also note that the cosmic distance ladder isn't built on a scale that is reliable in your meaning: it's build starting from a singular exemplar. Imagine explaining our astronomical distance scales to another race by long distance signal: US Well, a parsec is the distance at which you see an angular paralax of 1/(360*3600) of a circle from a 1 AU baseline. THEM What's a "AU"? US Oh, that's the mean radius of Earth's orbit? THEM ...? You'd have to go back to something like an identifiable transition line and counting wavelengths. $\endgroup$ – dmckee Apr 11 '16 at 0:09
  • $\begingroup$ @dmckee The analogy is not 1 to 1, but the point is that there is interesting physics in identifying standard rulers in nature. (See also the wiki article on standard rulers in cosmology: en.wikipedia.org/wiki/Standard_ruler) $\endgroup$ – Yly Apr 11 '16 at 4:07
  • $\begingroup$ If I remember correctly, the question began its life differently and involved some weird scenarios with a precision metrologist with peculiarly selective memory on a lonely island. That version had little to do with the final version and it was suggestive that physics was somehow dependent on a particular choice of units, which, as I did explain in my comment quite clearly, is a 100% false lesson that we do not want to send. I suggested to edit it, which took it into a completely different direction (into the woods). How we get to cosmic scales from there... the heavens only know. $\endgroup$ – CuriousOne Apr 16 '16 at 0:23
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I don't know exactly why the question was not well received; for that you'd have to get responses from the people who voted to close it, or downvoted it (if they care to identify themselves). That being said, my guess is that your reason #1 is pretty close. We have better ways to measure terrestrial quantities than comparing to natural objects. Sure, not everyone has laser range finders and electronic timers. But not everyone does physics. And when people are doing physics, they use laser range finders and electronic timers, or more generally, whatever measuring tools are most appropriate for the job. That renders questions about natural measurement standards likely off topic.

In astrophysics, the situation is different because astrophysicists actually do use things like pulsars and spectral lines for measurement. So the preceding reasoning wouldn't apply to questions about natural measurements used in astrophysics.

Of course, I wouldn't say that questions about measuring tools are automatically on topic. One of the principles that determines the scope of this site is that it should be about the physics and not about the tools used to do physics. This is why questions about using Mathematica, for example, are off topic (and have always been off topic, even before there was a dedicated site for such questions). We can also extend the same reasoning to say that questions about math itself are off topic. That being said, we've softened our stance on such questions over the years, so it's possible that a good question on using, say, a photogate timer would be considered on topic now. But it would be a huge stretch to extend that to something like a bag of air in a lake.

One other reason the question would be considered off topic is that it's an open-ended list question. The SE model doesn't handle such questions well; we prefer specific questions that have a right answer. So in addition to any issues your question may have with fitting into the site's topical scope, it's also too broad, if you ask me.

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