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Based on dimensional analysis and order of magnitude estimation one can understand why for example no warm blooded animal of small size such as rats can live in the north pole, only big animals like bears can. This is because the heat loss (which goes like the surface area of the animal) per mass (which goes like volume, assuming animals have same density) goes like 1/L. Hence heat loss per mass for small sized animal is so high.

Another example is that, warm blooded animals have body temperature on average that does not exceed 40 (I can understand this as an order of magnitude in the following way, this temperature is close to the average environmental temperature on earth which itself is set by radiation received from the sun..etc)

In a similar manner I am trying to understand the following numbers

  1. the length of pregnancy of humans is about 10 months (why not 1 month or 100 months?)

  2. humans live up to 100 years (why not 1 or 10 or 1000 years?)

  3. human eye is sensitive only to the visible band of the EM spectrum

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Could some moderators, or anybody tell me why my question is voted negatively? I thought trying to understand numbers in nature based on scaling arguments is the heart of understanding physics. I am just following Victor Wiesskopf's advice who said if one cannot give a simple explanation to something then one did not understand it. This is depressing! – Revo Oct 30 '11 at 14:25
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And I think there's some trickiness with this problem, as humans are actually born a bit premature relative to other animals, thanks to our relatively large heads. Horses come out of the womb able to walk, for example. Calculating human gestation period without considering anatomy/phisiology is probably destined to fail. – Jerry Schirmer Oct 30 '11 at 16:49
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I think there is a valid topic we can investigate which is the physical basis for certain biological aspects of ourselves. #3 for instance, has a simple and clear answer. There is a very expansive story that follows from these questions. Unfortunately, SE is about questions, not stories. I think good questions come from a "notion" about something. So say you thought of some argument for why 10 months versus 100 years might be meaningful. That could be something to ask about. It's not that your question is wrong, it's just a shotgun approach that doesn't fit the format well. – AlanSE Oct 30 '11 at 16:52
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Zassounotsukushi is correct, this question really doesn't lend itself to any sort of physical explanation. However, if you had some physical reason in mind as to why the quantities you're talking about have the values they do, it should be fine to ask whether it's plausible. Your question #3 is different, though; it is somewhat physically motivated (having to do with the shape of the sun's spectrum) but it would be better to ask it separately. Check and see if it's already been handled on the site first. – David Zaslavsky♦ Oct 30 '11 at 16:56
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@Revo: nobody ever said they were anthropic questions, and even so that wouldn't necessarily be a problem. The issue is that they are biology questions, not physics questions. – David Zaslavsky♦ Oct 30 '11 at 18:36
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closed as off topic by David Zaslavsky♦ Oct 30 '11 at 16:53

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1 Answer

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if you think about the human eye, it's size is about .02 meters roughly. Also, it can tell the difference between roughly 10,000 different colors. ( How do I get 10,000, well think about a very slowly changing shades of color from purple to red, the eye can "see" slight changes along this varying color picture... maybe 10,000 is rough estimate... i'm just ballparkin' that number I could be wrong).

so the eye has to be able to tell the difference between 10,000 colors and it's got about .02meters to work with .02/10,000 is roughly 10^-7 meters which is ballpark on target 400nm-700nm region... given this is a gross apprx to real physiology...

I would suggest choosing 1 of these... actually taking some time to formulate some rough assumptions see what you can find by solving it yourself, and THEN when you run into difficulties ask a question... you're less likely to get downvoted

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10,000 is an integer number of colors that need to be distinguished (divisions in which the .02cm eye needs to be broken up into in a sense). that has nothing to do with length. I love your attitude though : ) – Bozostein Oct 30 '11 at 14:22
another argument I've heard from physicists is that the ratio of the mass of an atom to the mass of a human is similar to the ratio of the mass of a human to the mass of the solar system. The logic would be that those scales are ideal for natural selection. – AlanSE Oct 30 '11 at 14:23
@Bozostein My sincere apologies. – Revo Oct 30 '11 at 14:26
np, this question isn't so bad... not sure why it's getting voted down... one problem is that these questions are almost certainly availble just by googling as they are pretty standard questions. though 2 is actually a very intrigueing problem i'd like to see someone tackle that one. – Bozostein Oct 30 '11 at 14:28
@Bozostein - that is absolutely not how an eye works. – Richard Terrett Oct 31 '11 at 1:19
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