Wikipedia:Reference desk/Archives/Science/2018 February 27

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February 27

Isn't the anthropic principle a tautology?

Isn't the anthropic principle a tautology? Doesn't it basically say that we contemplate our existence, well, because we can? Or does it relate to the old cogito ergo sum (with which I incidentally disagree)? I feel I'm missing something important here. 93.142.74.255 (talk) 09:11, 27 February 2018 (UTC)

I don't see how that would make it a tautology? Have you read Anthropic principle?--Shantavira|^{feed me} 09:20, 27 February 2018 (UTC)

• The observation behind the "anthropic principle" is by itself a fairly mundane observation (though not strictly a tautology). Our article mentions that Others[ref] have criticised the word "principle" as being too grandiose to describe straightforward applications of selection effects. Its point is to explain apparent paradoxes by selection bias.

For instance, a classic creationist argument is to say that humans have unique characteristics among animals, Earth has unique climatic conditions and is the only place to host life, physical constants of the universe are tuned extremely precisely to allow matter to exist as we know it, etc.; all this would have had a ridiculous chance of happening by random chance, and a single deviation would have caused sentient life not to exist; hence, there must be some deity who fine-tuned all of this. The AP is in effect saying that we wouldn't be here to observe it if it was not the case, which invalidates the "hence" (because in both the deity-created and the random-dice universes, only the outcomes with a sentient species to observe it are sampled).

Of course, only the precise formulation is new, not the idea itself - the very similar "shipwrecked crew make no offerings" existed in the Antiquity. Tigraan^{Click here to contact me} 09:53, 27 February 2018 (UTC)

The anthropic principle is notable mostly in the use, and mostly in the use to dispel "it won't happen because it hasn't happened before" type arguments. For example, the Earth didn't collide with another planet a million years ago because then we wouldn't be here -- which means that we don't know (at least from Earth alone) whether such collisions happen all the time. Or, more practically, we know the Earth didn't overheat and form a Venus-like greenhouse because we're alive to look, but that means we don't know whether the odds of that happening are low in the future. In the extreme case, I recall a Nature paper from some decades ago arguing that because the position of a human today is random within the order of all humans ever to exist, humans probably will not produce more offspring than that in the future; hence an end of the world can be predicted within a few generations. But I didn't track it down just now. Wnt (talk) 13:01, 27 February 2018 (UTC)

@Wnt: You are describing the Doomsday argument. Tigraan^{Click here to contact me} 19:05, 27 February 2018 (UTC)

• The anthropic principle is tautological in the week sense (of course we have to live in a universe that supports our sort of life in order to support our sort of life) and is entirely arbitrary and unfalsifiable in the strong sense. In the strong sense, the claim is that the universe had to be the ways it is (or very close) because otherwise there would be no intelligent life to observe it.

We have no way of knowing that there being intelligent life to observe it is a necessary property of existence, and we have no way of knowing that in other universes without matter as we know it there might not be some sort of non-material life as we don't know it. We cannot enumerate these possible universes, we have no idea of the unknown unknowns or any way of actually modelling these existences other than in very crude parochial terms.

"week sense": is that when you sense how weeks pass? --Hofhof (talk) 18:37, 27 February 2018 (UTC)

Seems like they pass more quickly every year. --47.146.60.177 (talk) 19:00, 27 February 2018 (UTC)

Linking relevant article... Hofhof (talk) 19:25, 27 February 2018 (UTC)

To New Yorkers or isolated Pacific islanders, life is only possible in Manhattan or on a coral atoll. See the famous parochial New Yorker Magazine cover, and translate that into a cosmological model if you will. μηδείς (talk) 16:52, 27 February 2018 (UTC)

Said cover being titled View of the World from 9th Avenue. --47.146.60.177 (talk) 18:59, 27 February 2018 (UTC)

And you can make your own! —Tamfang (talk) 06:37, 4 March 2018 (UTC)

• Thnaks to IP 47's image link and key nigh. I flame WP for having an antic spell-checker with know glamour funk shin. As for the numerology Wnt alluded to, there was also an argument before 2001 that, since statistically, any entity is statistically 5% likely to be in its first %5 of lifespan (this assumes a very unlikely linear relation) and 5% likely to be in its last 5% of it's existence, we can be sure that the Catholic church, if assumed to be a round 2,000 years old, has existed that long, there is a 90% chance that it won't live fewer than another hundred and five years or longer than another 38,000 years. This is, of course, yet more meaningless gibberish, funded at taxpayer dollars. μηδείς (talk) 03:31, 28 February 2018 (UTC)

• Here[1] are some old Scott Aaronson lecture notes with more conundrums like the ones Medeis mentioned. Aaronson mentions Nick Bostrom a few times and likes Bostrom's book[2] that I haven't looked at. 173.228.123.121 (talk) 08:52, 1 March 2018 (UTC)

I don't agree with this. There's way too many assumptions, e.g. that (ignoring our personal existence) Doom Soon and Doom Late scenarios have similar probabilities, or even that we're equally likely to be born as any being in the same world (I see no reasons against, but I see no reasons for, either). And the Dice Room paradox can well be explained by the fact that the expected number of people put in the room as well as the expected number of victims is unbounded. When dealing with infinities a lot of paradoxes arise in the probability theory, and since there's no evidence yet for either an infinite amount of people or infinite amount of universes/multiverses, this doesn't open up a new line of reasoning for me. Although, dealing on a daily basis with, essentially, applied mathematics, I might be biased here... --OP 93.139.80.133 (talk) 23:32, 1 March 2018 (UTC)

• OP here, thanks for the answers, folks. I kinda understand now that the principle is useful for dealing with theological fine tuning arguments, although it reminds me of Pascal's wager, which rests on the same problem as the Dice Room paradox mentioned in the Scott Aaronson link. I think this is probably why I'm so uncomfortable with it. 93.139.80.133 (talk) 23:32, 1 March 2018 (UTC)

Also: Medeis, your calculation about the Catholic Church's expected longevity is reminiscent of Laplace's solution to the sunrise problem. 173.228.123.121 (talk) 00:40, 2 March 2018 (UTC)

• Um, the fact that I mentioned a bullshit numerological "principle" don't make it mine! Bejeezus! μηδείς (talk) 05:54, 2 March 2018 (UTC)

Deflating tyres to drive over snow or sand

When you deflate the tyres to drive in the desert or over snow, how much bigger does the surface in contact with the ground get? How meaningful is that (let's suppose that you could inflate the tyres as soon as you drive over a different terrain)? How much more friction can be expected? --Hofhof (talk) 18:20, 27 February 2018 (UTC)

From the basic definition of pressure, (pressure equals force times area): if the entire weight of a vehicle is supported by its pneumatic tires, then the area in contact with the ground is inversely related to the tire-pressure. Of course, the simple model assumes a perfect, nonrigid wheel. Automobile tire dynamics constitutes an entire field of advanced science and engineering.

The extent to which contact patch size affects friction and traction and rolling resistance is nontrivial, because the wheels are nonrigid and the surface contact dynamics are complicated. (For starters, read Slip (vehicle dynamics)). So, you cannot accurately use a simple model derived from first principles; there are so many confounding variables. Instead, empirical laws are derived for automobiles; for specific modes of operation, on specific road surfaces, and for specific vehicle models, you can obtain a table of tire performance.

Our article on SAE J2452 - an automotive industry standard for tire performance - has links to more detailed technical resources.

Nimur (talk) 19:24, 27 February 2018 (UTC)

• This is complicated, and in general don't do it.

It used to work, back in the days of crossply tyres. Tyres were also of a taller cross section. As practically all modern tyres are radials, they're a different matter: there aren't many modern tyres where you can deflate them sufficiently to make a difference to their shape where this isn't also going to cause so much bending in the sidewall as to damage the tyre.

In principle, the idea is to let the tyre flex more, thus increasing the size of the contact patch. Ideally this should increase lengthwise, but not sideways - if it increases much sideways, you're killing those sidewalls. Another problem is that reducing the pressure reduces the load carried pneumatically as a balloon tyre and increases the proportion being carried by the sidewalls - which are also getting bent out of their ideal shape.

There's no increase in friction as such - the "grip" of tyres on soft ground is far more complicated than such a simple term. In this case, it's more about the structural integrity of the surface layer of the ground and nothing to do with the friction to the tyre. A hard tyre with a small patch will have a greater loading on the surface (force/area) and this tends to break up the integrity of the mud or snow - the limit for the unbroken surface is higher than the force available once it starts to crumble.

Where deflation is used today it's mostly with CTIS and tyres designed specifically to cope. They might also need a beadlock system to stop the tyres pulling off the rims when there's not enough pressure to hold them otherwise. Tyres can be re-inflated as soon as you hit tarmac, which is important for allowing higher road speeds and avoiding damage. Andy Dingley (talk) 20:49, 27 February 2018 (UTC)

Tires are much more abused in high speed curves. Todays sidewalls or tires in general can take an awesome lot of "abuse", so its a minor problem to run them partly deflated for some hours to give them more contact on sand, mud and alike where there is danger of sinking in to a degree you get stuck. This does not work on snow and ice tho because you actually will get more grip by having your given weight on a very small contact area. --Kharon (talk) 21:42, 27 February 2018 (UTC)

Tyres "in high speed curves" aren't abused, they're run at proper pressures, so that the sidewalls aren't excessively flexed as they are when the same tyre is run under-inflated. If they were run like this, then they'd fail very rapidly. Tyres in high speed curves are heavily loaded, but that's not the same as abuse - it's what they're designed for -and their sidewalls are in the correct shape. Andy Dingley (talk) 22:24, 27 February 2018 (UTC)

(irony on) Ah, is that why "under-inflated" tires always squeal so loud compared to the silence of tires in sharp curves at high speed?(irony off) I guess the logic dictates. --Kharon (talk) 09:09, 28 February 2018 (UTC)

Instead of speculating wildly on topics I don't even know about, I've been reading a book this month called Race Car Vehicle Dynamics (Milliken, 1995); you can even buy it on Amazon. It has a whole chapter on tire dynamics, including proprietary data from Goodyear for tires at various pressure points.

If you like vehicle physics, it is something of a great read, albeit extremely technical.

My insight for this morning, after reviewing the tire physics chapter, is that aerodynamic drag on the tire is sufficiently large that it cannot be ignored in a discussion of net rolling resistance. Automotive engineers have models to estimate that - just like how aerospace engineers measure the induced drag on an airfoil!

Nimur (talk) 16:55, 28 February 2018 (UTC)

Tyrrell P34 Andy Dingley (talk) 11:59, 1 March 2018 (UTC)

Probably does not matter in some years when all vehicles drive on Audi RSQ-style ballon tires. --Kharon (talk) 22:16, 1 March 2018 (UTC)

Bison fur pattern

Why American bison has only that partial fur covering his upper parts, neck and head instead of the entire body? Perhaps some other animals have the same. 212.180.235.46 (talk) 18:34, 27 February 2018 (UTC)

Compared with the European bison, for example? And note that they both have plenty of fur. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:13, 27 February 2018 (UTC)

I don't know about the southern states, but in the northern states and in Canada, the fur over the shoulder hump grows long and thick as it gets colder. In the winter, the fur is very thick and helps them handle the cold (as cold as -40°). When it warms, the fur molts in large clumps and the difference is mainly in texture during the summer rather than length. 209.149.113.5 (talk) 20:46, 27 February 2018 (UTC)

This is the science desk. Please be more precise, since we can't tell whether you meant -40 F or C. μηδείς (talk) 19:58, 2 March 2018 (UTC)

Compared with more or less uniform fur distribution in other animals. Also, it it the same mechanism involved in uneven hair distribution in humans compared to apes who have more uniform fur? Thanks. 212.180.235.46 (talk) 21:23, 27 February 2018 (UTC)

It is hard to say if their hair has changed from behavior or if their behavior is based on their hair. Bison are known for facing into cold winds. Growing up on a reservation in Montana, we had a saying that you should stand and face harsh conditions the way the bison stand and face the cold bitter winter winds. They don't need thick hair on their rears because they take the wind to their face. 156.143.240.137 (talk) 23:44, 27 February 2018 (UTC)

• Horses, lions, and humans all have manes, see also sexual selection. μηδείς (talk) 03:17, 28 February 2018 (UTC)
  • As do some other cats, such as Maine Coons. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:39, 28 February 2018 (UTC)
    • I apologize for having given an exclusive-seeming list. For goshsakes, see sexual dimorphy at bird. μηδείς (talk) 04:56, 2 March 2018 (UTC)

See reference: "The Bison of Yellowstone National Park NPS Scientific Monograph No. 1". www.nps.gov. National Park Service. —2606:A000:4C0C:E200:2C25:60CC:6BF7:403B (talk) 00:52, 5 March 2018 (UTC)