Dr. David Heddle from the blog, He Lives, has an excellent post on The Antrhopic Principle. While the post as a whole is a great read, this part drew my engineering interest:
In its proper place the Anthropic Principle can be quite useful. But it must be remembered: it can be a guide—but it is never an explanation.
Dr. Heddle goes on to demonstrate how the Anthropic Principle (AP) can be used as a guide, but it got me thinking about a thing engineers like to use: rule-of-thumbs. Like the AP, rule-of-thumbs are not explanations (they have no basis in theory or empirical design), but are based on engineering tradition and/or aesthetics. To give an example, when sizing a beam for a specific span, L, structural engineers limit the beam depth (d) to a minimum of L/20 or L/30*. Now it is possible that a more shallow and robust beam would be just as adequate (if not more so) to resist applied loads (strength, stability and deflection). However, the more shallow beam would not “look” right. Doing a rough cost-benefit analysis, a deeper beam** is a “cheaper” alternative to client uneasiness. Using this rule-of-thumb as a starting point for design can also a time saver for the engineer.
However, there is generally no theoretical nor experimental (empirical) basis for the L/20 beam depth rule-of-thumb. It also is not an explanation for the specific beam chosen (i.e. how the beam responds to applied loads). The L/20 rule-of-thumb is just a design guide to assist the engineer in the design process.
So that gets me thinking, based on linked post, is the AP a physicist’s rule-of-thumb? If Dr. Heddle’s example is accurate, IMO, it would appear to be so. Ain’t it cool how concepts can transcend disciplines? 
*For example, if L = 9 metres (29.5 ft), then using L/20, minimum beam depth = 450 mm (approximately 18 inches).
**In most circumstances, the deeper beam would weigh less than the more shallow and robust beam, thus saving money on material. That said, the price of steel these days is relatively cheap.
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