Wikipedia:Reference desk/Archives/Science/2021 November 28

= November 28 =

Many world interpretation
Is the MWI an actual fleshed out theory, at least mathematically? I mean does it really say that the whole universe evolves by the Schrödinger equation with a single time parameter t that reaches everywhere? Asking because I just saw this claiming time could flow forwards in some places while backwards in others. What I'm trying to understand is whether there is really supposed to be something like "branching" or if that is just a metaphor. Thanks. 2601:648:8202:350:0:0:0:69F6 (talk) 00:07, 28 November 2021 (UTC)
 * According to a section in our article, Many-worlds interpretation, some physicists see it as "real" and others "unreal". I guess that most people without a deep understanding of quantum physics are in a third "don't care" category! Mike Turnbull (talk) 19:02, 28 November 2021 (UTC)
 * I did read the article, but I'm asking less about the reality of the alternate worlds, than whether there is a reasonably complete theory that describes them (let's say it's ok to ignore general and maybe special relativity) even if they are fictional. I guess it is a question about mathematical physics (whether someone has worked out certain QM equations even if they don't describe reality) than scientific physics (how one describes the real universe(s)).  2601:648:8202:350:0:0:0:69F6 (talk) 23:21, 28 November 2021 (UTC)
 * Without getting into the question of "real" (whatever that means), the best way to get a good grip on the completeness and reasonableness of the Many Worlds theory is to look into the work of Sean Carroll. Besides being a well-known science communicator (he's up there with people like Neil DrGrasse Tyson, Brian Cox, and Michio Kaku), he's also very good at explaining the Many Worlds interpretation, of which he is an unabashed booster.  Whether you end up agreeing with him is up for debate, but if you watch some of his talks on the matter, you'll at least understand it much better.  See, for example, This video, which is a pretty good lecture on the topic, he has many more though.  His book Something Deeply Hidden is also a good exploration of the topic, written for a non-expert.  -- Jayron 32 13:19, 1 December 2021 (UTC)
 * Thanks, Sean Carroll's book has been on my want to read list for a while. I'm not too concerned about whether the MWI is right or wrong (i.e. does or doesn't describe physical reality).  I'm asking whether it describes a hypothetical multiverse in a reasonably fleshed out and consistent way (that might be wrong), as opposed to omitting enough specifics that it's not even wrong.  I'll take a look at that video if I get a chance.  2601:648:8202:350:0:0:0:4F12 (talk) 07:04, 2 December 2021 (UTC)
 * It's not just a bunch of people sitting around and thinking and making proposals based on pure logic. It isn't philosophy if that is what you are asking.  It's based on real, actual theoretical work involving mathematics and equations and guys with funny hair writing on chalkboards like all good physics is.  The primary theoretical work was worked out originally by three people: Hugh Everett III, his mentor John Archibald Wheeler, and Bryce DeWitt.  Wheeler himself is a giant in the field, in the generation after Einstein, Wheeler was arguably (perhaps alongside Feynman) the most influential figure in theoretical physics.  -- Jayron 32 12:01, 3 December 2021 (UTC)
 * In this video, they interview a professor who authored a book about it. https://www.youtube.com/watch?v=kTXTPe3wahc 67.165.185.178 (talk) 04:10, 3 December 2021 (UTC).
 * That professor would be Sean Carroll, and that book would be Something Deeply Hidden as noted above. -- Jayron 32 11:55, 3 December 2021 (UTC)
 * Ah, woops, yes. My video is a much shorter version of it. Interesting if it's only 1 main professor that specializes in this field. 67.165.185.178 (talk) 15:06, 3 December 2021 (UTC).
 * No, there are a LOT of such professors. Sean Carroll has nice hair and looks good in front of a camera, though.  -- Jayron 32 15:19, 3 December 2021 (UTC)

Why didn't bones evolve with carbon ?
Carbon makes any object sustain more tensile and yield strength. So Why can't bones evolve with carbon instead of calcium? Rizosome (talk) 02:38, 28 November 2021 (UTC)
 * Carbon in any old form or (allotrope) doesn't make objects stronger, only certain ones like diamond and carbon fibre. Natural Diamond is made under immense temperatures and pressures deep in the Earth (and though very hard, is also quite brittle); carbon fibre is constructed artificially, and as far as I'm aware could not be made by natural biological processes, or at least ones that are likely to be achievable through evolution. {The poster formerly known as 87.81.230.195} 90.205.225.31 (talk) 07:27, 28 November 2021 (UTC)
 * Graphite is the form of carbon that is stable under normal conditions and it is a very soft and weak material. Carbon, as part of compounds, forms part of the hard supporting structure in many types of animal - see chitin, calcite and aragonite. Mikenorton (talk) 11:31, 28 November 2021 (UTC)

This sentence answered my question: Carbon, as part of compounds, forms part of the hard supporting structure in many types of animal - see chitin, calcite and aragonite. Rizosome (talk) 01:45, 29 November 2021 (UTC)
 * Also relevant; your bones DO contain carbon. Bone is about 30% collagen and other proteins, which is mostly carbon. -- Jayron 32 02:14, 29 November 2021 (UTC)
 * And without that protein, the bones would probably not be resilient enough to function properly. There's an old demonstration of this where you soak one chicken bone in vinegar for a significant time (removing the calcium, phosphorus, and other mineral components) and bake another in an oven (burning off the protein portion). The first becomes rubbery, the second becomes brittle. --Khajidha (talk) 23:52, 29 November 2021 (UTC)
 * Also note that the mineralogical components of bone also contain carbon in the form of carbonates. --Khajidha (talk) 23:56, 29 November 2021 (UTC)

S2 / Sagittarius A* acceleration
I'm reading about S2 (star) and I noted the line that it experiences "acceleration of about 1.5 m/s2 (almost one-sixth of Earth's surface gravity)." I was wondering, disregarding all the horrible radiation, the fact that you'd in orbit around a black hole which would tear us to bits, and stuff what would happen if you were somehow replace S2 with Earth. Would you feel it? Like, would sitting on the side of the planet facing in the direction of the acceleration feel heavier than sitting on the opposite side? Or at least, would it be measurable? Essentially I'm wondering if our entire planet was accelerated suddenly would we notice or because we're small and our planet is big we wouldn't? Would any of this be different if we were in orbit around S2 instead of simply taking the place of it? 194.72.22.84 (talk) 10:17, 28 November 2021 (UTC)

[Corrected 11:58, 29 November 2021 (UTC); the earlier value of $$\Delta r$$ was off by a factor of $$1000$$. --.L]
 * I'll assume we can model the gravitational effect of the black hole as that of a hugely massive body in Newtonian spacetime. An observer in free fall near the Earth, whether falling down or circling it, does not directly experience the acceleration in their reference frame, but they may be able to measure the minute differences related to the distance to the Earth's centre. The acceleration in a stationary frame of a small body in orbit around a much more massive body is given by $$a=cr^{{-}2},$$ in which $$r$$ is the distance to the centre of gravity, which we may put at the centre of the larger body. (The value of $$c=GM,$$ in which $$M$$ is the mass of the larger body, is further irrelevant.) For a difference $$\Delta r$$ in distance, the difference in acceleration is given, to a first approximation, by $$\Delta a=\tfrac{da}{dr}\Delta r=-2ar^{{-}1}\Delta r.$$ This can also be found by integration of the tidal force across that distance. Using $$a=1.5\,\mathrm{m},$$ $$r=18\times10^{12}\,\mathrm{m}$$ (the pericentre distance of S2) and $$\Delta r = 12.756\times10^{6}\,\mathrm{m}$$ (twice the Earth's equatorial radius), we find $$\Delta a\approx-2.1\times10^{{-}6}\,\mathrm{m}\mathrm{s}^{{-}2}\approx-\tfrac{1}{4{,}600{,}000}g.$$ Perhaps measurable with precision instruments, but not something an observer might feel. --Lambiam 11:58, 28 November 2021 (UTC)

Why are humans so bad at climbing/walking downwards?
I was on a walk with some snow-covered steep paths and it occurred to me how much better were are at walking/scrambling up than down. Going up I rarely had to worry about whether I had a firm foothold. I could feel when it was ok to transfer weight to the front foot, and if I slipped at all a quick scrabble and I was ok. When I have previously slipped on very steep slopes I end up on all fours with little impact and can easily grab something and continue.

On the other hand when descending I found I was deliberately having to test whether my front (lower) foot would lose grip. If it did I was in a very precarious position with no instinctive scrabble to recover. When I have previously had bad slips I have ended up falling heavily on my backside, and my hands have not been in a position to grab anything until I'm slipping faster. I have ended up with bruises and cut hands before now. In fact this is so obviously a difficult way that my instinct is to descend backwards if anything is very steep and treacherous, where I gain the advantages of moving forwards but go much slower.

My dog, on the other hand, appears to be as sure footed ascending and descending head first. My first thought was that this is a side effect from when we moved to two legged walking. However I would have thought evolutionary pressure would have led to safe descent as well as ascent. I also wonder whether it is part of a much older evolutionary legacy, because as far as I know there are no primates which will descend trees head first, though many other animals can. - Q Chris (talk) 16:42, 28 November 2021 (UTC)


 * The easiest explanation is that it is a trade-off. For short bursts of speed, many land animals outdo humans, but in endurance running Homo sapiens outdoes many of its prey animals. Evolution may have favoured this trait over handling steep descents quickly. Also, I suppose that someone who is used to barefoot walking may be more confident while descending, as the grip of their unshod sole may be better and they also get a more informative feedback. --Lambiam 18:03, 28 November 2021 (UTC)


 * Our bipedalism does put us at a disadvantage. A dog's centre of gravity is lower than ours, and they have four potential points of ground contact. Also our feet are flat with no claws, and almost all of a human foot is forward of the leg and ankle (which results in the foot mechanics of going uphill being different to going down). The reason why we haven't evolved to be good at descending slippery slopes is presumably because 1) much of human evolution occurred in parts of the world with little or no snow, and 2) descending slippery slopes isn't an important attribute for us to have - as Lambiam states, we evolved as endurance hunters for whom running quickly isn't important (and we have good brains and can invent arrows and poison darts etc. that, at least for hunting purposes, circumvent the need to run down slopes quickly). PaleCloudedWhite (talk) 18:47, 28 November 2021 (UTC)


 * How often did bipedal savannans hunt and gather on slopes? Sagittarian Milky Way (talk) 00:57, 29 November 2021 (UTC)


 * The same reason why it's easier to back down a ladder than to go forward down a ladder: our knees only bend in one direction. This gives us a stable pose where the forward foot is higher than the rear foot. There's no similar pose where the forward foot is lower than the rear foot, because that would require the knee to bend in the opposite direction. --Amble (talk) 18:55, 28 November 2021 (UTC)
 * In some circumstances it helps to descend an uneven slope sideways with the upper knee more bent and the body leaning upslope, which reduces the discrepancy of feet placements and helps with balance: it's tiring to perform for an extended period, however. {The poster formerly known as 87.81.230.195} 90.205.225.31 (talk) 00:24, 29 November 2021 (UTC)


 * This explains why descending facing forward is harder with the current, actual build of the species, but not why evolution did not take a path towards faster secure forward descent. --Lambiam 11:38, 29 November 2021 (UTC)
 * As you yourself implied above, the results of evolution by natural selection are often compromises between several competing 'pressures', so what alternative body plan evolvable in only a few million years could a primate, moving from a predominently arborial lifestyle to a predominently ambulatory one, have arrived at that better facilitated forward downhill movement without detracting from the more common requirement of out-running prey over long distances on relatively level savannah? {The poster formerly known as 87.81.230.195} 90.205.225.31 (talk) 23:36, 29 November 2021 (UTC)


 * Sure. I thought that part was already well covered. One other interesting point is that I always find it much more efficient to go down an uneven trail with a galloping motion than walking. It turns out there's an interesting paper on the mechanics of this motion: . --Amble (talk) 17:06, 29 November 2021 (UTC)