Wikipedia:Reference desk/Archives/Science/2020 January 29

= January 29 =

Acupuncture injuries
Can acupuncture result in permanent damage or injury to the person who undergoes it? Freeknowledgecreator (talk) 03:34, 29 January 2020 (UTC)
 * Of course it can. Google "can acupuncture cause permanent damage" for more info. ←Baseball Bugs What's up, Doc? carrots→ 03:54, 29 January 2020 (UTC)


 * See Acupuncture. --Lambiam 08:21, 29 January 2020 (UTC)
 * Pretty much any medical intervention can. What we care about is: is the intervention superior to placebo, and what is the risk–benefit ratio? Related concepts: number needed to treat, number needed to harm. --47.146.63.87 (talk) 02:59, 30 January 2020 (UTC)

Do travel restrictions stop virus outbreaks spreading? Comment Reply Suggestion Reply
E.g. the 2020 Wuhan lockdown. An article in Vox by Julia Belluz and Steven Hoffman suggests it's just "political theater". -- Jeandré, 2020-01-29t07:55z
 * The sourced answer to that question might be worth including in our article(s) quarantine or/and cordon sanitaire, which do not discuss it at all. (Notice also that both "quarantine can work in general" and "the Wuhan lockdown in Chinese government theater" could be true.) Tigraan Click here to contact me 08:18, 29 January 2020 (UTC)
 * The article in Vox states that travel restrictions may "delay the spread of disease but don’t impact the number of people who eventually get sick". It does not consider possible beneficial effects of such delay, in particular reducing the overload on medical support systems caused by massive amounts of people falling ill at the same time. --Lambiam 08:28, 29 January 2020 (UTC)
 * Indeed. One of the problems is in how "success" is defined here.  If "success" is "no one else in the world will ever get sick ever", then we have an unrealistic measure of success.  If "success" is some other standard, such as what you note, being the slowing of the spread to allow response systems to be better prepared for when it does eventually get there, then it can be successful.  And, as noted, just because it is political theater doesn't mean it isn't also effective.  Propaganda doesn't have to be untrue to be propaganda.  -- Jayron 32 13:01, 29 January 2020 (UTC)
 * I'm reminded of the leadup to the Mount Saint Helens eruption 30 years ago. The governor ordered the area evacuated, and some citizens griped about it. They didn't use the term "political theater", but they were skeptical that there was a real threat, calling it an overreaction, etc. Then the eruption came, killing everyone and everything for miles around, and the skeptics were never heard from again. ←Baseball Bugs What's up, Doc? carrots→ 13:17, 29 January 2020 (UTC)
 * Ex post.
 * One of those skeptics was Haroun Tazieff who had predicted that the 1976 Grande Soufrière eruption (yes, that's a red link - if you can read French, this is as good a wrap-up of the event/polemic as you will ever find; La_Grande_Soufrière contains a few sentences too) would be a mild event (there was some seismic activity but no dangerous magmatic phenomenon). Yet tens of thousands were evacuated for three months.
 * It does not mean either evacuation was wrong or right. It means that such judgement should be formed by analyzing what information was available/obtainable at the time of decision-taking, rather than the how the event actually turned out.
 * Hence: whether certain quarantine practices are efficient at achieving certain objectives should be evaluated from how the use of such practices turned out in the past, but whether the Wuhan lockdown is or is not a good idea could be evaluated now already (if one had perfect knowledge of internal government deliberations, that is). Tigraan Click here to contact me 15:51, 29 January 2020 (UTC)

The idea is to prevent a pandemic. If you consider it unwise to travel to Wuhan right now, you might also want to arrange that Wuhan not travel to you. It doesn't make sense that the number of people infected somehow would stay constant. Also, there is a good chance of having vaccines available pretty soon, so slowing the spread helps. 173.228.123.190 (talk) 19:42, 29 January 2020 (UTC)
 * You appear to assume that the travel restrictions will prevent a pandemic. Is that assumption justified? --Lambiam 04:43, 30 January 2020 (UTC)
 * Compared with doing nothing? ←Baseball Bugs What's up, Doc? carrots→ 05:29, 30 January 2020 (UTC)
 * Everyone putting plugs in their noses beats doing nothing. Still, that does not warrant the assumption that it will stop a pandemic. --Lambiam 06:52, 30 January 2020 (UTC)
 * It's called containment. ←Baseball Bugs What's up, Doc? carrots→ 12:42, 30 January 2020 (UTC)
 * If all forms of travel are effectively prevented, it will stop the further spreading of the disease. Such containment is only possible by draconic measures and may have side effects (e.g. worldwide famines) that are worse than the problem they are meant to solve. But no one has proposed or is going to implement this. The thesis of the referenced publication is that (lesser) travel restrictions merely slow down the speed at which the disease spreads, like a film in slow motion. One may disagree with this thesis, but it is not absurd and does make sense. Plugging the larger holes of a leaky container will not keep it from eventually being emptied; it just takes longer, but in the end it is just as empty. --Lambiam 20:48, 30 January 2020 (UTC)
 * On a side note, an amazing film is "La Soufrière" by Werner Herzog. Bus stop (talk) 01:37, 30 January 2020 (UTC)

Taste of food
Why is it that food which is good for you tastes bad, and vice versa? — Preceding unsigned comment added by 2600:1000:B160:9CC8:2D34:DDFE:8A95:EFCD (talk) 16:49, 29 January 2020 (UTC)
 * This article has some good information which may help drive your research in this area. -- Jayron 32 16:56, 29 January 2020 (UTC)


 * It depends on the food. Liver is supposed to be good for you, and apparently some people actually like it. ←Baseball Bugs What's up, Doc? carrots→ 17:23, 29 January 2020 (UTC)
 * It is generally agreed that fruit is a healthy food and most fruit is delicious. Richard Avery (talk) 19:39, 29 January 2020 (UTC)
 * I think the OP might be suggesting that fruit, delicious as it is, still doesn't compare to, err, bacon double cheeseburgers or chicken chow mein :)   ——  SN  54129  14:11, 30 January 2020 (UTC)


 * Because you wouldn't eat bad food otherwise. So if food is bad (sausages made of the infamous 'all lips and arseholes' recipe), you add salt or sugar or MSG or herbs or something to make it tasty. Or at least, crudely tasty (sweet or salty) to an unsophisticated palate.
 * There's also the problem of vanished scarcity. At one time, sugar was rare. So a dense source of energy from it (wild fruit, maybe honey) would be appreciated by a forager and consumed. It was good for you (stopped you starving). Now we've learned to produce enough sugar to kill us, but we still like it, and will (mostly) keep on choosing to eat it. Andy Dingley (talk) 16:55, 30 January 2020 (UTC)
 * There is a broad misunderstanding behind the question and many of the replies. When speak of foods as "bad" that typically means that they're full of fat, salt, and/or sugar. However, those things are all vital to our health. Without salt we would die and sugar and fat supply the calories we need to walk/talk/move/etc. So, we evolved in such a way that those things taste freaking great to us. Taste is an ongoing area of study, but we basically only recognize a handful of tastes directly (as opposed to flavour, which is mixed with aroma/smell). Saltiness and sweetness are signs we should eat a thing, while sourness and bitterness are signs we should not, as they indicate unripeness and poison, respectively. That's how important saltiness and sweetness are to us. So what's wrong? Well, our desires for those tastes have been exploited by food producers - what started as a keen enjoyment of some sweet fruit has become weaponized into an addiction to refined sugar. Our desire to eat the fatty parts of animals to extract the much-needed calories has been co-opted into eating meat that is much more saturated in fats than what we'd ever be exposed to "naturally". That is to say, the things we desperately required thousands of years ago are now in a superabundance at the local quick service restaurant and that abundance has developed much faster than our tastes possibly could, so we still keenly desire then as if they were still rarities. Matt Deres (talk) 19:41, 31 January 2020 (UTC)

Supernova
Considering that full moon is bright enough to cast shadows in a city night, I'm wondering how close would a supernova have to be to cast shadows on a clear moonless night in a low light pollution area? 89.172.78.189 (talk) 16:51, 29 January 2020 (UTC)
 * What your asking is how to calculate the apparent magnitude of a supernova, and at what distance from earth would a typical supernova need to be in order to have an apparent magnitude equal to that of the moon. There are equations in that article which may help you work that out.  -- Jayron 32 16:58, 29 January 2020 (UTC)
 * Well - almost.... Jayron is absolutely correct that we do want to know the apparent magnitude.  This is sort of "step zero", or maybe "step one", in setting up the problem.  This number is easy-ish to compute, and it tells us how much light there will be - using units that are easy to compare to all the other sources of light.  If we want to be very approximate, we can just say "it will cast a shadow if..." the apparent magnitude is bigger than some arbitrary reference value - let's say, maybe, the brightness of the moon.  That's not very precise, but it might be all you're looking for.
 * The challenge is that "casting a shadow," used as a plain english phrase, actually has a pretty ill-defined technical meaning in the context of optical imagery. We have an article - shadow - that at least introduces the subject, and talks about its use in astronomy.  To throw some technical jargon at the problem: we want to know what brightness will provide a sufficiently large contrast between a shadowed area and an unshadowed area - which means that we need to carefully define the imaging condition.  Basically, we want to know how large the object is, and on what surface is it "casting" a shadow, and the precise geometries of those items (e.g. we need to fill out the extrinsics matrix for the object casting the light and the object casting the shadow and the object on which the shadow shall be observed)... and we need to define a statistical "confidence interval" in which the absence-of-an-observed-photon is strong evidence of the existence-of-a-shadow, for all regions where we expected to see a shadow based on geometry and the simplification of photons-that-travel-in-straight-lines... which is not actually what photons do!
 * The point is, though, the ordinary day-to-day use of the word "shadow" is a little too informal for these kinds of astronomical objects. Even in the simple common case of "sun/earth/moon" shadowing, we have to worry about the umbra and penumbra - and that's just because the relevant objects have non-zero size!
 * In the absolute simplest case, consider a two-pixel camera - a pair of photodiodes. Place the two pixels so that one is blocked by a box, and one is ... not blocked by the same box.
 * If this one of these diodes captures any light at all - in other words, if it images anything - the box is "casting a shadow" on to a very low resolution image: a 1x2 pixel digital photograph.
 * Because we really literally truthfully cannot know where the photon came from, we really literally truthfully can't tell you whether that photon came from the supernova. All we can do is present a statistical likelihood - which means we need to use lots of math and physics - and the name of this is "to solve for the imaging condition", estimate a resolution, and compute a signal-to-noise ratio.
 * Nimur (talk) 17:36, 29 January 2020 (UTC)
 * It wouldn't have to be as bright as moon but anyhow I was wondering if anyone else has done those calculations. Surely I'm not the first person to think of this. 89.172.78.189 (talk) 17:08, 29 January 2020 (UTC)
 * ... no, ... no, there is a long legacy of optical physicists whose tenure extends for centuries; and they have thought about this problem and many variations on the theme, for a long, ... long time.
 * We don't need to look much farther than the writings of Isaac Newton or Christiaan Huygens to see that they considered the problem of shadow-casting: Huygens' wave principle is a surprisingly modern take on the problem, treating every point in the illuminated area as point-propagators of a new wave of illumination; and every shadowed, un-illuminated point as ... not a point-propagator. And Kepler's Supernova was studied by none other than Johannes Kepler in 1604 - as well as many other scientific observers around the entire world; many of the recorded observations were mathematical and quantitative.  All of these old dead guys were really big into their telescopes, and they tended to point them at the interesting weird dots they spotted in the night-sky.  Then, they'd spend hours and hours inventing new kinds of mathematics to explain what they saw and figure out new ways to see things differently.
 * It's not easy to come up with a truly new idea in optical physics...
 * Nimur (talk) 17:50, 29 January 2020 (UTC)
 * I have seen a shadow from a crescent moon so low it was orange from a suburb where I couldn't see the Milky Way. It was shadow-producing as fuck when higher up and the faint shadow faded to nothing by the time the sand couldn't see the 25% phase or less crescent anymore (half a half moon or less, almost 90 degrees from the zenith). This was because I was almost in a cocoon with only a few square degrees of light polluted sky visible and almost zero other artificial light like being able to see a light bulb or a patch of ground lit by a light bulb or a patch of ground lit by a patch of ground lit by a light bulb. It couldn't have been more than magnitude minus 6 when that low. Some people say Jupiter is a shadow-producing object when it's dark enough, that would be magnitude minus 2. Sagittarian Milky Way (talk) 00:54, 30 January 2020 (UTC)
 * A supernova can be absolute magnitude minus 20 so 18 magnitudes would be "9 magnitudes further away" or 4,000 times 10 parsecs which is too close for a Magellanic Cloud and too far for the furthest part of the young star disc which you can't see anyway due to dozens of magnitudes of interstellar extinction. The sample size is very low but our list of supernovae gives some idea of what is realistic as sightlines with less galactic dust tend to have less supernovae. Sagittarian Milky Way (talk) 01:40, 30 January 2020 (UTC)
 * To explicitly state something for clarity's sake, a supernova's naked-eye brightness from Earth depends not only on its distance but its intrinsic brightness—which astronomers describe as absolute magnitude—in the visible spectrum. This is of course the case for all astronomical objects. There isn't one fixed distance. This is why apparent magnitude is used to describe an object's brightness as observed from Earth. Shadow helpfully tells you that an object has to have an apparent magnitude equal to or less than -4 to cast shadows. --47.146.63.87 (talk) 02:49, 30 January 2020 (UTC)
 * -4 seems about right (eagle eyed freaks excepted) Sagittarian Milky Way (talk) 04:24, 30 January 2020 (UTC)
 * We can ask an analogous question that does not need a definition of shadow casting: at what distance would a nova be as bright as the full moon? The apparent magnitude of Betelgeuse going supernova is estimated to be −12.40, at a distance of 700 ly. The apparent magnitude of the full moon is, under ideal circumstances, −12.90. A value of −11 may be more realistic. So a Betelgeuse supernova will be slightly dimmer than an ideal full moon, but brighter than a more typical full moon. --Lambiam 06:46, 30 January 2020 (UTC)


 * Interesting! 40,000 pc even in unrealistic conditions is quite a bit more than I expected. 89.172.78.189 (talk) 08:12, 30 January 2020 (UTC)
 * 1 ly = 0.3066 pc. 700 ly = 214.62 pc. --Lambiam 19:49, 30 January 2020 (UTC)