Wikipedia:Reference desk/Archives/Science/2021 February 22

= February 22 =

50 cal gun and air
I posted this on the Humanities desk! My bad. The other day, I was watching a Youtube vid (I could not find the link) of someone taking multiple shots with a 50 cal gun. The host showed how the air around the gun got cold enough to see one's breath. What is the science behind this? 70.26.18.103 (talk) 01:45, 22 February 2021 (UTC)
 * "50 cal gun" only tells us its ammunition size: it could be a Handgun, a Light or Heavy machine gun, or conceivably even a pistol, rifle or other type of Air or Gas gun. Without more detail, I doubt if anyone can begin to offer an explanation: I myself can point to a possible cause if it were an air or gas gun. {The poster formerly known as 87.81.230.195} 2.125.75.168 (talk) 02:12, 22 February 2021 (UTC)

My apologies. As I am not a gun person, I was not aware of the different ways to fire a 50 cal bullet! The gun I am referring to was a long barrel rifle fired while on a tripod on a table. It was semi-automatic but the host of the vid showed how one could see one's breath after a single shot. I hope this helps. 209.91.188.70 (talk) 17:40, 22 February 2021 (UTC)


 * I'm not a gun person (I'm European) and I don't see how a gun could cool the air. I do see a different option. The gun puts additional condensation cores in the air, which may trigger condensation if the air in the breath plume was already more or less saturated with water vapour. PiusImpavidus (talk) 09:40, 23 February 2021 (UTC)
 * I've had a quick look on YouTube, but couldn't find a video that mentioned that. If it was an airgun like this one here, then it could be due to the adiabatic cooling of the comressed air as it expands. But if its a firearm (especially a big one like one of these shooting this sort of ammo, then I can't imagine how it would cause cooling.  Iapetus (talk) 22:47, 23 February 2021 (UTC)

Blood vessels regenerating
What is the maximum length of a blood vessel being removed and then fully regenerating? 10 centimeters? More than that? Futurist110 (talk) 06:59, 22 February 2021 (UTC)

RBMK reactor
A few questions about that clunker called the RBMK:

(1) Did the 24 short lower control rods have graphite tips like the ones inserted from above?

(2) Were these 24 rods under the control of the RCPS and/or EPS systems, and were they automatically inserted by pressing AZ-5?

(3) Were there any safety valves and/or pressure relief valves on the steam drums and/or on the steam lines to the turbines (I wouldn't be too surprised if there weren't any), and if so, where did they discharge -- into the condensers, or into some kind of surge tank, or simply into the turbine hall to the peril of anyone who happens to be nearby at the time? [Additional question: other than this, was there any other way to bypass the turbines and dump the steam from the steam drums directly into the condensers if the need arose?]

(4) Why use the graphite tips at all -- was this some kind of tweak to correct an early design flaw (in the process introducing another, arguably worse one)?

(5) Considering how unstable this clunker was, and how laggy the controls were, had there ever been problems with these things introducing current/voltage oscillations into parts of the Soviet power grid (for example, did the lights in places which were heavily dependent on nuclear power, such as Pripyat, flicker more than normal)? And

(6) the million-dollar question: if they had known the finer points of operating this reactor (a partially moot point since if they had, they wouldn't have run the test as they did), could they have averted the Chernobyl disaster if, as soon as the reactivity started increasing, they kicked the main coolant pumps to maximum RPM before (or instead of) pressing AZ-5? 2601:646:8A01:B180:8C91:5C9F:935B:96B1 (talk) 08:12, 22 February 2021 (UTC)
 * [I have reformatted your post slightly to make it easier for others to read and respond to. {The poster formerly known as 87.81.230.195} 2.125.75.168 (talk) 14:11, 22 February 2021 (UTC)]


 * [OP] I take back question #3 -- I'm reading the INSAG-1 report right now, and it expressly mentions some kind of pressure-relief valve which closed in response to the reduction of the steam pressure in the #8 steam drum, so the answer to my own question #3 is "yes, and it discharged the steam into the condenser as is proper for any high-pressure steam device". (Nice to know that at least something in that design was done right!) 2601:646:8A01:B180:4080:C0D:3AC8:DA35 (talk) 09:09, 23 February 2021 (UTC)


 * Oh wow, these are very interesting questions. And I'm not sure a lot of people could seriously answer them. I know the physics behind all this but your questions are very specific to this reactor. My recommendations would be for you to try to reformulate the more RBMK specific ones into more general questions about reactors so I can give a shot at answering them. Or you could try to leave a message on the talk page of one of the main editors of the RBMK page. I can do the second one for you if you want. Feynstein (talk) 18:42, 26 February 2021 (UTC)
 * Actually I can answer #4. Since you open up a reactor like that from the bottom up, the first things neutrons will see are those tips. And they will displace the water. It's written somewhere on that page. So it's conceivable that when starting up the reactor they needed a moderator gradient to get the reaction going properly. Feynstein (talk) 19:30, 26 February 2021 (UTC)


 * For question #1: here is a possibly useful source (already cited in RBMK). The 24 lower control rods seem to be the ones described as "shortened absorbing rods (SAR) for regulation of the axial neutron distribution". The makeup of each type (p. 15): "The absorbers of the type SAR have only three absorbing elements. Their length is 3,050 mm [7]. Other absorbing rods are assembled from 5 absorbing elements. Their length is 5,120 mm [7]. There are another feature in absorbing rods of the RBMK reactor. The absorbing rods of the type SAR, MR and ER have special graphite displacers that are assembled from 5 graphite elements." See also the very helpful Fig. 3 (p. 16) in the linked article. This confirms that the 24 SARs do have the graphite portion, in opposite position relative to the manual and emergency absorber rods, but that the auto control rods do not have the graphite portion. --Amble (talk) 19:01, 26 February 2021 (UTC)


 * The following sentences (p. 15) also provide an answer to question #4: "These displacers remain in the core by full withdrawal of absorbing fractions of rods. The use of graphite displacers improves significantly the neutron economy of the RBMK reactor because graphite absorbs neutrons much less than the light water." --Amble (talk) 19:08, 26 February 2021 (UTC)


 * The linked article has some hints to question #2, but I can't find an unambiguous answer. It says in one place (p. 11) "all control rods"; in a second place (p. 19) "all control and scram rods"; in another place (p. 23) "all absorbers for the manual regulation and all emergency absorbers". --Amble (talk) 19:18, 26 February 2021 (UTC)


 * The linked article has considerable discussion that's relevant to question #6, although not directly posing the question you asked. See p. 23-24 and conclusions. It seems there's a difference in expert opinion as to whether pressing AZ-5 initiated the disaster, made it worse, or only failed to stop it. It depends on how much of the problem was caused by boiling of water in the reactor core. Given that disagreement, it's probably not possible to say with any certainty whether another specific course of action would have prevented the disaster. It does suggest that at least some experts believe it would have been better to not press AZ-5 and do something else. There's also discussion on p. 24 about a possible role of circulating pump motors that gets very close to your question, but here also I don't find certainty about what actually happened or what could have been done. --Amble (talk) 19:30, 26 February 2021 (UTC)


 * In p. 24-25 you can find a list of prior incidents in Soviet nuclear power plants. They don't mention any effects for power consumers as in your question #5, but they do describe dangerous failures at the Leningrad and Chernobyl plants in previous years that were closely related to the eventual Chernobyl disaster. --Amble (talk) 19:36, 26 February 2021 (UTC)


 * [un-indent] Thanks, Amble and Feynstein! 2601:646:8A01:B180:B4DA:DC4F:C5F3:9C50 (talk) 08:05, 1 March 2021 (UTC)

Plant identification requested
These flowers have appeared in our garden (southern Scotland, mid-February). Can anyone identify them, please? --rossb (talk) 16:26, 22 February 2021 (UTC)


 * Eranthis hyemalis or winter aconite(s). Mikenorton (talk) 16:45, 22 February 2021 (UTC)


 * Thank you very much. rossb (talk) 17:02, 22 February 2021 (UTC)


 * They are toxic. Richard Avery (talk) 17:13, 26 February 2021 (UTC)

Shower physics
I have some questions about the everyday stuff I've noticed regarding water flow from my shower. I'll supply the details in case they're relevant, but this is a fairly basic shower stall setup. The bathroom is on the top floor, so the water is coming up through the floor. About one meter up, the cold and warm water tubes meet at the control mechanism (turn for temp, push/pull for volume). Since it's a shower stall there's no other faucet, as you'd find in most bathtub setups. A further meter up, the single pipe pokes out into the shower area and I've attached a standard hand-held shower attachment. The hose portion is almost exactly the same length as the bracket for it is high; when it hangs loose it just barely misses striking the floor. So, call the hose about 2 meters. In the normal position, the wand portion sits in a bracket so that the functional face of it is a bit higher than the pipe. We have decent pressure. Pretty common stuff, I think. So, here are my questions: I'm assuming the short version of the answer is going to be capillary action, but I'm curious as to how it's getting applied. Also, just to be clear, I have no reason to suspect a leak; the behaviours have been the same for many years and I don't think it's unique or anything. Matt Deres (talk) 16:50, 22 February 2021 (UTC)
 * 1) When we shut off the water, it stops pretty immediately, apart from a drop or two. However, about 10-15 seconds later there may be a bit of discharge, which I assume is actually caused by the occupant jostling the hose. Then, some larger amount of time later, like 20 minutes or more, there will be a sudden... ejaculation of more water. Not a lot, but enough to make noise and it's a long enough time away from the shower itself that it might wake us up if we showered before bed. What causes that?
 * 2) In part due to the aforementioned noise, we sometimes take to removing the wand from its bracket after showering and just letting it hang. Problem solved, but when I go to hang up the shower head the next day, water will discharge as I'm raising the wand. Now, I'm obviously disturbing the system when I do that, but I'm mostly just adding potential energy: stuff is being raised, not lowered or flexed. What's supplying the energy to make the residual water in the hose squirt out when the wand is raised? For that matter, why doesn't it empty out while the wand is hanging loose for several hours?
 * I suspect that the various ejections of water from the shower head are due to variations in water pressure, perhaps due to heating/cooling of water combined with some sort of siphon effect. -- Jayron 32 17:06, 22 February 2021 (UTC)
 * I'm not sure about that. Wouldn't pressure only affect the water on the other side of the tap? After I shut the water off, there would still be water in the pipe up and in the hose and in the handset, but none of it would be under pressure, would it? I guess I'm assuming everything downstream from the tap would be free flow and only subject to atmospheric pressure. Or is that what you meant? Please don't hesitate to dumb it down. Matt Deres (talk) 18:46, 22 February 2021 (UTC)
 * Perhaps water is getting stuck behind a backflow prevention device and slowly leaking through until there's enough to overcome surface tension and come out in a small spurt. --Amble (talk) 18:32, 22 February 2021 (UTC)
 * I hadn't considered that. I'm quite in the dark when it comes to plumbing, but I normally associate backflow traps with waste water. For example, various bends in the pipes below my sink/toilet to prevent gas from escaping upwards. They're not typically added to the inbound water, are they? Matt Deres (talk) 18:49, 22 February 2021 (UTC)
 * A backflow preventer might be build into, or attached to, a hand-held shower head on a flexible line: . This isn't a trap; it's a little valve that lets water through in one direction, but snaps shut to stop water from flowing in the other direction. It can work by gravity or a spring. It hadn't occurred to me earlier, but it could also happen that a backflow preventer is stuck in the open position after your shower, and when it eventually snaps shut, it jostles some water free to drain from the shower head. --Amble (talk) 19:01, 22 February 2021 (UTC)
 * I did not know that; as the commenter on that link says, it seems pretty unnecessary. I water starts crawling two meters up a hose, I think we'd have bigger problems than water getting into the water supply. But I guess it's just standard. That sounds like a plausible explanation for the first question - thank you! Any thoughts on the second one? Matt Deres (talk) 03:25, 23 February 2021 (UTC)
 * I checked my shower head, which happens to be made of transparent plastic. After showering, it still had a lot of water inside, which stayed inside when I let the shower head hang down instead of putting it back on the hook. The water was inside the head by the nozzles, past anything that could be a backflow preventer. This makes me think any backflow preventer is not relevant here, and PiusImpavidus’s idea is a much better match to what I saw happening. —Amble (talk) 16:08, 23 February 2021 (UTC)

I've found that that often occurs in my shower head, and I'm fairly sure that it's caused by the water being held in the nozzle holes by surface tension, and maybe the slight suction effect of the water in the shower head and hose cooling and shrinking. I assumed the release was caused by air entering the uppermost of the shower nozzle holes, so hanging the hose down with the shower nozzle plate more vertical makes this release happen immediately. Well that's my theory! 49.197.246.104 (talk) 06:55, 23 February 2021 (UTC)
 * 1: After showering, there's still water in the showerhead. Because of surface tension, the air has difficulty breaking through the water surface in the tiny holes of the showerhead, so it can't enter. As no air enters, the water can't leave.
 * After a while, 3 things may happen:
 * Water slowly leaks out of the showerhead, governed by surface tension, gravity and viscosity.
 * The water in the standpipe, hose and showerhead contracts as it cools down.
 * The hose itself expands as it cools down. A peculiar property of rubber and other polymers is that, if held under constant stress, it shrinks with increasing temperature.
 * These three effects cause the pressure in the showerhead to drop and eventually it gets low enough to suck air in, against surface tension. When this has happened, the highest holes in the showerhead are free of water, so surface tension no longer blocks the entry of air, and the entire showerhead drops its water.
 * 2: With the showerhead hanging down, any air leaking into the showerhead will climb through the hose, keeping the showerhead full of water and keeping the holes mostly blocked by surface tension. When moving the showerhead up again, you bend the hose, which flattens the hose and reduces its volume. This forces some water out. PiusImpavidus (talk) 10:36, 23 February 2021 (UTC)
 * Cool! Thank you! Matt Deres (talk) 15:36, 23 February 2021 (UTC)

The fraternal birth order effect for male homosexuality: Why didn't evolution select against and weed out this effect?
I found this Wikipedia article to be extremely informative: Fraternal birth order and male sexual orientation. Apparently, the more elder brothers (by the same gestational mother, I'm presuming) a male has, the more likely this male is to be gay (or bisexual?). This effect appears to have been confirmed in research and studies numerous times and thus appears to be true. The hypothesis that a maternal immune reaction is responsible for this also likely appears to be true due to its plausibility and the implausibility of any alternative explanations. However, why exactly did evolution not select against and weed out this effect over the centuries and millennia? After all, one would think that women whose immune systems are LESS likely to have an immune reaction to additional male fetuses are going to have a reproductive advantage–a reproductive premium, if you will–due to them having more descendants in the long(er)-run (since gay men are less likely to reproduce than straight men are), no? Futurist110 (talk) 18:52, 22 February 2021 (UTC)


 * It depends. Community survivability, especially in social species like humans, is very much a thing. Remember, it is populations, and not individuals at "evolve." Let's say we have two communities, each with 10 mothers. Each mother as 4 male offspring (we are assuming, for the sake of this discussion, that every individual is able to find a mate for reproduction, even though I'm not stating them). So, population A has 10 mothers and 40 male offspring, and population B has 10 mothers and 40 male offspring. Population A has a genetic trait that results in the fourth male offspring always being homosexual, and population B lacks this trait. Let's say that each heterosexual male offspring has 4 children of their own. All things being equal, in your scenario, population A has 144 grandchildren to the original 10 mothers, and population B has 160 grandchildren to the original 10 mothers, and this is where you are having the impression that the trait should be selected against, since mothers without the trait had more grandchildren (we will say all grandchildren are female and heterosexual, to simplify the numbers). However, humans are a social species. We live in communities and different people can play different roles in that community. Perhaps, without the motivation of personal reproduction, the homosexual male children of the population A mothers take on other roles, such as helping with child rearing, helping gather food, or even being lookouts for potential threats. Let's that this has a big impact. In population A, 75% of the grandchildren with homosexual siblings survive to sexual maturity and have children, since they are all heterosexual females. That means that of the original 144 children in the Population A that has the trait for homosexuality, 108 grandchildren successfully reach sexual maturity and have great grandchildren. Population B, which doesn't have homosexual uncles serving their social role, only has 50% of the grandchildren surviving to reach sexual maturity (half dying of starvation or attacks on the community). Of the original 160 grandchildren, only 80 survive to sexual maturity. So, having the trait of some homosexual offspring conferred the community survivability of Population A an advantage over Population B, 108 grandchildren surviving to sexual maturity vs 80, even though only 75% of Population A children were themselves reproductively viable. The genetic trait for some homosexuality will, in short order, dominate. --OuroborosCobra (talk) 19:11, 22 February 2021 (UTC)


 * (E/C) Gay men may reproduce less than straight men, but it could be that having a non-mating relative is an overall reproductive advantage. This is seen often in birds, where the term helpers at the nest has been coined to cover the situation. Humans of course take much more time and energy to mature than birds, perhaps making the advantages even more pronounced. Matt Deres (talk) 19:18, 22 February 2021 (UTC)


 * Evolution isn't magic pixie dust, it works by what has been called "satisficing" in other contexts (i.e. perform the task good enough in order to allow some species to survive and evolve; good enough means far from perfect). Tgeorgescu (talk) 19:36, 22 February 2021 (UTC)


 * This BBC article outlines a few theories. This article in Current Anthropology looks into three theories, most favouring the hypothesis that "Same‐sex alliances have reproductive advantages, and sexual behavior at times maintains these alliances. [...] Homosexual emotion and behavior are, in part, emergent qualities of the human propensity for same‐sex affiliation". PaleCloudedWhite (talk) 19:50, 22 February 2021 (UTC)


 * Perhaps making babies the usual way is not the favorite pastime for homosexuals, but in general they are and were quite capable of fathering children. Tgeorgescu (talk) 13:14, 23 February 2021 (UTC)


 * To exactly the same extent, on average, as heterosexual men were capable of doing? Futurist110 (talk) 22:17, 23 February 2021 (UTC)


 * According to the Kinsey scale 100% homosexuals are as rare as 100% heterosexuals. The large majority of people who ever lived fall somewhere in-between. Tgeorgescu (talk) 23:47, 23 February 2021 (UTC)


 * "women whose immune systems are LESS likely to have an immune reaction to additional male fetuses are going to have a reproductive advantage" What if having less immune reaction to male fetuses is correlated with having less immune reaction in general? Maybe "weeding out" male homosexuality would come at the expense of increased vulnerability to disease for women in general and pregnant women in particular. --Khajidha (talk) 21:01, 22 February 2021 (UTC)


 * Shouldn't there have then been selection for women with strong immune systems in regards to everything else other than this specific thing, though? Futurist110 (talk) 22:16, 23 February 2021 (UTC)
 * Pregnancy in general might then become harder. Evolution isn't magic and traits aren't perfectly segregated. --Khajidha (talk) 21:35, 28 February 2021 (UTC)
 * Some people seem to be analyzing the issue from a modern perspective, where individuals are able to choose whether to stay single or not, to have children or not, and even to choose whether they prefer a hetero or homo relationship. Let's not forget that along evolution people (gay or straight) were supposed to marry people of the opposite sex and engender children, whether they liked it or not. Evolution could not have selected against reproductive successful homosexuals. Bumptump (talk) 13:32, 23 February 2021 (UTC)
 * Even if those gay cavemen couldn't make lot of kids, they could make the cave look nice with scatter cushions and some lovely flowers. 86.187.233.59 (talk) 09:47, 26 February 2021 (UTC)


 * Unless a male homosexual views heterosexual sex as so repulsive that he's unwilling to actually perform the penis-in-vagina sex act, that is! After all, even a 1% hit in fitness per one generation will become MUCH more severe over an extremely huge number of generations! Futurist110 (talk) 22:16, 23 February 2021 (UTC)


 * This entire line of thinking demonstrates the classic misunderstanding of evolution "If such-and-such a trait is less reproductively advantageous, why does it exist at all", as though every trait that isn't the "best" trait (for whatever meaning of "best" is) should somehow instantly disappear from a population. Evolution doesn't have goals, and it doesn't select anything in that way.  Traits which don't kill an individual before they reproduce will generally get preserved in a population for a very long time.  There's of course the other issue that human sexuality is somehow a single trait displaying simple Mendelian inheritance like pea pod colors or something like that.  Then there's the other other issue that sexuality is a binary trait, and that people are either homosexual or not homosexual, and not on some continuum.  Then there's the other, other, other issue that all people should have a stable and unchanging sexuality.  Then there's the other other other other other issue that because something has a heritable component, it has no environmental component, or that one can suss out the differences simply.  The lines of demonstrably bad thinking in this entire thread are so mindbogglingly numerous, I can't even begin to figure out how to make sense of starting to correct for it.  -- Jayron 32 19:24, 24 February 2021 (UTC)


 * You seem to have lots of issues. Bumptump (talk) 16:21, 28 February 2021 (UTC)
 * It's the OP's premise that has issues. ←Baseball Bugs What's up, Doc? carrots→ 17:37, 28 February 2021 (UTC)