Wikipedia:Reference desk/Archives/Science/2017 April 29

= April 29 =

Perseid meteor shower and other meteor showers, could it be from the Earth/Moon breakup?
Hi, scientists studying the Earth/Moon breakup recently have favored a collision as the cause. It seems that would create lots of debris that not all of which would be gathered into either the Moon or Earth, and since it originated on some point along the earth's orbit, there would be a chance the debris could continue to intersect the Earth's orbit for billions of years. Is this possible/likely? thanks.Rich (talk) 02:54, 29 April 2017 (UTC)


 * Billions of years seems unlikely. Over that period is should all be cleared out, if it intercepted Earth's orbit.  Also,  wouldn't the particles which escaped head off in random directions, rather than all the same direction ? StuRat (talk) 03:23, 29 April 2017 (UTC)


 * As that article implies, and Comet Swift–Tuttle confirms (with a reference), the Perseids come from that comet. Matt Deres (talk) 03:35, 29 April 2017 (UTC)
 * Yes, but I hope you realize that in itself doesn't mean the Swift-Tuttle comet couldn't be made up from the debris of the impact. Rich (talk) 23:44, 1 May 2017 (UTC)
 * The earth/moon breakup is discussed at Giant-impact hypothesis and it mentions that the time of the impact can be dated from "stony meteorites" and it is backed by a reference, but it doesn't specify which shower (if any) the meteorites came from. Unfortunately, the reference is behind a paywall. The abstract implies that the meteorites involved were from long ago. Matt Deres (talk) 03:41, 29 April 2017 (UTC)
 * The source you're referring to is available on one of the authors sites here. I only skimmed through it and only understand half the terminology anyway, but as far as I can tell, they aren't referring to meteorites hitting the moon (or earth), but rather meteorites from the moon/earth breakup hitting other things, particularly 4 Vesta without linking them to any particular shower. It does say at the end:
 * "although the importance of GI ejecta returning to strike the Moon has yet to be quantitatively evaluated, the values computed here suggest that it could play an intriguing role in the earliest phase of lunar bombardment."


 * Nil Einne (talk) 04:06, 29 April 2017 (UTC)


 * It's a cool source. It also mentions in the supplement that the debris created by the impact was expected to be cleared relatively quickly, with 99% of the ejected fragments either landing on the moon, a planet, sun, or ejected from the solar system by 250 million years afterwards. The supplement also lists the sources of the meteors referenced for the study, which come from all over the place, and in no case is it immediately obvious when the meteor was deposited on Earth except for the Chelyabinsk meteor. Perhaps if you really dug through the references you could find out when these meteors arrived on Earth (on first pass, even the articles they reference don't mention that, so you'd have to go for references of references at least). Anyway, seems like the authors are interested only in the age of the meteors themselves (since last molten), and not how long they've been on Earth. They do mention that the only fragments of collision expected to survive more than a few hundred million years would be those in highly inclined orbits of the sun, which would fit the perseids, except that those come from a comet. Someguy1221 (talk) 04:07, 29 April 2017 (UTC)


 * BTW, since it's been more than a year, the article even on the Science (journal) is no longer behind a paywall. It's available here with free registration. You can also access the supplements if you register. Nil Einne (talk) 04:45, 29 April 2017 (UTC)


 * Thank you for that clarification. I think what's confusing me is that they seem to be using the aforesaid meteorites to date the proposed impact using a technique I know nothing about called "40 Ar- 39 Ar shock degassing". We have an article on degasification, but it doesn't seem to be the same thing. In any case, it implies that the meteorites were available to study. We've been to 4 Vesta, but that article doesn't say anything about "argon". There's probably something in the PDF, but I'm unfamiliar with a lot of the terminology, making it difficult to quickly parse the text. Matt Deres (talk) 13:48, 29 April 2017 (UTC)
 * It's some kind of argon–argon dating. I presume "shock degassing" has something to do with how the argon got into the sample. --47.138.161.183 (talk) 08:23, 30 April 2017 (UTC)
 * The short answer: over time most of the stuff gets pulled onto a collision course with something. The gravity of the Solar System's planets and moons perturbs stuff. Very small debris also loses orbital energy due to the Poynting–Robertson effect. This is why you don't see stuff randomly distributed around the solar system. It's why there's an asteroid belt. I'm sure there's the odd bit of leftover stuff out there, but most of the debris from the Theia impact never even reached escape velocity (which we know because it fell back to Earth or coalesced into the Moon). --47.138.161.183 (talk) 05:20, 29 April 2017 (UTC)
 * Your short answer says most of the debris would "fall back" or coalesce into the Moon. The article others referenced above estimated the amount of some types of debris that didn't fall back or coalesce into the Moon as several percent of the mass involved. So although most of the debris fell back or coalesced as you say, the escaped part could have been several percent of a Marssized or even Earthsized object. That's a hefty amount of debris.Rich (talk) 23:53, 1 May 2017 (UTC)

Is there a name for this mental defect ?
Normally when we interpret speech, we must decide between different possible meanings. For example, if someone said "Be careful, it's hot" after handing you food, then that would apply to the food. But if they said that before you went outside on a hot day, it would apply to the air temperature. If somebody is unable to pick up on the context, is there a name for that disorder ? StuRat (talk) 03:36, 29 April 2017 (UTC)


 * "Difficulties understanding what is not explicitly stated (e.g., making inferences) and nonliteral or ambiguous meanings of language (e.g., idioms, humor, metaphors, multiple meanings that depend on the context for interpretation)." (emphasis mine) This is an aspect of Pragmatic_language_impairment. SemanticMantis (talk) 15:46, 29 April 2017 (UTC)


 * Inability to use Telepathy to detect whether someone is talking about the day or the food is not classed a mental disorder. The onus is on the speaker to ensure they are understood by appropriate choice of language towards the listener. It can be difficult to whisper discreetly something that should have already been realized. Blooteuth (talk) 17:23, 29 April 2017 (UTC)


 * If you hand the average person food and say "Be careful, it's hot", the average person will know you mean the food, or perhaps the dish, but definitely not the outside temp. Actually saying everything in a manner which removes all ambiguity is extraordinarily difficult.  See the Jarada for a fictional treatment. StuRat (talk) 16:54, 30 April 2017 (UTC)


 * "This coffee tastes like mud!" "Well, it was ground this morning!" ←Baseball Bugs What's up, Doc? carrots→ 19:01, 29 April 2017 (UTC)


 * I lava volcano. Sagittarian Milky Way (talk) 06:03, 30 April 2017 (UTC)
 * Yes blootoooth you are super duper smart, thanks for making a point to bring up something entirely unrelated from months ago. If you can't understand contextual cues in language, that's your problem, not mine, please keep your personal problems off the reference desk. SemanticMantis (talk) 16:49, 30 April 2017 (UTC)
 * For those of us who aren't so super duper smart, what are you talking about? ←Baseball Bugs What's up, Doc? carrots→ 18:37, 30 April 2017 (UTC)

Cold bouncy balls


In this video (link), a bouncy ball is placed in liquid nitrogen. Once it's good and cold, it's thrown against the ground, and it shatters with a sound similar to that of glass. I'm not surprised that the coldness affects the ball's Elasticity (physics) somehow, but I would have guessed that it would act like a piece of gravel, simply not bouncing much at all. Why does it become brittle and shatter? Nyttend (talk) 13:39, 29 April 2017 (UTC)


 * The polymer has been cooled below its glass transition temperature, causing it to cease to deform elastically, and to fracture like a brittle material. Here's an interactive online demonstration from Cambridge University: The Glass Transition in Polymers.  That page goes over the theory, using a level of presentation complexity appropriate for a high-school or early university science class.  Nimur (talk) 16:10, 29 April 2017 (UTC)

Firefly question
On April 22 20, while I was out photographing something else, I took the first photo - an 8-second exposure. You can see fireflies flashing up to five times in 8 seconds. I went back to the same location five seven days later, to get more photos of the fireflies, but now a firefly didn't appear to flash the same way. I tried longer and longer exposures. The second one is a 97-second exposure, but it doesn't show any trails. What is the reason for the difference? Could it be a different species? Bubba73 You talkin' to me? 14:29, 29 April 2017 (UTC)
 * Yes, there are many species, and lightning bugs distinguish themselves by characteristic flashing patterns, the point being to get a mate of the right species. There are even cannibals who mimic the patterns of other species, then ambush and eat the males that fall for the ruse. μηδείς (talk) 15:49, 29 April 2017 (UTC)


 * So a different species could be at the same area five seven days later? Bubba73 You talkin' to me? 16:19, 29 April 2017 (UTC)


 * Yes, that's one of the points of them having different flashing patterns, so that if they are in the same place at the same time they don't interbreed. Also, having (developing) non-synchronous breeding seasons is one of the means by which speciation occurs.  So they could both breed at the same time, but stay separate by different signals, or have slightly separated breeding periods, or both. (See for example the Hawthorn fly and the effect on its breeding period by adaptation to feeding on fruit that ripen at different periods Speciation). I don't recall ever seeing a lightning bug in Manhattan, but in suburban South Jersey I am only aware of one species.  It might be helpful to google the fireflies of your area, since the further you move toward the tropics the more species there are.  You could find out their flashing patterns. μηδείς (talk) 17:24, 29 April 2017 (UTC)


 * I'd throw in a caveat: beware of long exposures on digital cameras! Sometimes they are digital composites of short exposures.  This can lead to unusual image artifacts.  Digital sensors with software computers just aren't as simple as photochemically-active gels - a "long exposure" on an advanced digital camera might not be continuous.  It may be conducting image stacking, which leads to difficult, profound, and detailed technical questions about the benefits of long exposure.  (One might actually get a better-quality image if they capture thousands of shorter-exposure frames of video or time-lapse shots, and post-process it on their home computer, with 64 bits and floating-point support and lots and lots of RAM - like the astrophotographers do - this depends entirely on your electronic rolling shutter's equivalent shutter angle!  There are no good internet articles on these topics!  I went looking, and all I found was Ken Rockell fawning over iPhone low-light performance...)
 * Here's a little more reading on bulb mode for Nikon digital cameras. Why are the bulb modes limited to two minutes on some camera models?  I can speculate!
 * If loooooong exposures are what you do, then what you really need is one of the old-fashioned CCD cameras - like Santa Barbara Imaging Group 2-megapixel monochrome sensors. CCDs are a must-have, and the reality is, you just can't buy them anymore: now that CMOS sensors are so much cheaper, nobody makes CCDs in even small volumes, except for specialized military and research applications.
 * Nimur (talk) 16:34, 29 April 2017 (UTC)


 * I don't know, where are you? (Georgia maybe?) Photinus_pyralis is by far the most common on North America, and I'd be hesitant to chalk this up to different species. While there can be a few species in an area, the dominant, high density and easily visible species would probably not change so quick. But it could! Hard to say. At least some species flash at different frequencies, depending on temperature. See here . Flashing patterns in glow beetles are extremely well studied, with very good work starting in the 60s and continuing to today. So, if you can figure out one or more of the species, chances are we can find some scholarly studies on the flash patterns of that specific thing, and how it may change based on external cues. Also I do think Nimur's caveat is apt, this is not film. If the frequency change is obvious to you eye (and maybe a stopwatch), then it probably is real. SemanticMantis (talk) 17:15, 29 April 2017 (UTC)


 * Yes, I'm in coastal Georgia. These are not composites.  The first one showing the trails was an 8-second exposure, so I started with about that the second night.  I wasn't getting the trails so I kept doing longer exposures.  This corresponds to what I saw visually.  I don't know about the temperature - it could have been a little different.  The second one was also 43 minutes earlier in the evening.  Bubba73 You talkin' to me? 17:29, 29 April 2017 (UTC)


 * PS - I like the trails so I'll try to get out there before April 22 next year and do a better job. Bubba73 You talkin' to me? 17:31, 29 April 2017 (UTC)


 * Photinus pyralis says that they flash every 6 seconds. The ones on the first night flashed up to 6 times in 8 seconds.  On the second night, if they were flashing every 6 seconds then they might be flashing too far apart to show a trail.  Bubba73 You talkin' to me? 17:45, 29 April 2017 (UTC)

I've added a second 8-second exposure from the first night. In the center and on the left there are several trails of about 4-5 flashes in 8 seconds. I assume these are males. On the right are single flashes - I assume they are female. So perhaps there were only females on the second night? But I read that females flash in response to the male flashes. Bubba73 You talkin' to me? 18:03, 29 April 2017 (UTC)

If jetliner pilots both die soon after giving autopilot the controls on a SYD(morning)-LAX nonstop?
Who should declare mayday into the radio and who shouldn't?

1. One of the passengers is a pilot rated to fly this airplane model, he is healthy, not under the influence of drugs, fatigue, alcohol or sleepiness, not jet lagged, will only have been awake for about 17 hours when they land and they quickly let him fly: No need to declare mayday as opposed to something less urgent right? The flight's jet lag would even serve to delay any pilot sleepiness.

2. The most qualified person on the plane is the same guy except he only has the wrong (but similar) type rating (i.e. 777 vs 747)

3. The most qualified person is the same guy except he has an Airbus type rating and is on a Boeing or vice versa

4. The most qualified person is the same guy and has IFR except he's only rated to fly a much smaller aircraft. Should he declare mayday when asking for help? (He has about 10 hours to get a crash course in landing a big airliner from whoever answers the radio remember)

5. The most qualified person is the same guy except he's only rated to fly a small aircraft in visual flight rules

6. The most qualified person is the same guy except he's never touched a flight control besides X-Plane or Microsoft Flight Simulator on a PC, has a PhD in a hard science and has reliably landed big jetliners in said flight simulators

7. The most qualified person on the plane is the dude above except he only has average IQ and average physics savvy but has played Microsoft Flight Simulator enough to reliabily land a big jetliner. Should he say mayday into the radio as soon as possible? Sagittarian Milky Way (talk) 14:44, 29 April 2017 (UTC)
 * Please don't invite us to make personal judgments μηδείς (talk) 15:45, 29 April 2017 (UTC)

Here's an actual reference to help you deal with emergencies: Fly the Aircraft First. Aviate, navigate, then communicate. Nimur (talk) 16:19, 29 April 2017 (UTC)


 * Yes, in such cases, there's a pretty well-established checklist, regardless of how unlikely it is to occur. The biggest problem is e.g. the Germanwings scenario, where aircrew are locked out of the cockpit, which needs to be mitigated with crew overrides.  The Rambling Man (talk) 21:45, 29 April 2017 (UTC)
 * The article Mayday describes use of the emergency procedure word, typically by aircraft pilots. However anyone who can reach a radio transmitter can send a Distress signal, preferably on an aircraft emergency frequency such as 121.5 MHz in ordinary speech such as "Help this is an emergency, both pilots are dead. How do I steer this thing?" with equal effect. I don't see how there can be any question of disallowing an emergency call in the obvious emergency that the OP describes, nor why there is any question of delaying it or arguing who is qualified to make it. Cabin crew are most likely to respond promptly. The qualifications that the OP gives are relevant to selecting an emergency substitute pilot. A pilot in service might receive criticism for a premature mayday declaration but that will not happen to a volunteer on board, who let us hope is not a terrorist who shot the pilots. Blooteuth (talk) 00:01, 30 April 2017 (UTC)

Helios Airways Flight 522 "At 11:49, flight attendant Andreas Prodromou entered the cockpit and sat down in the captain's seat, having remained conscious by using a portable oxygen supply.[21][22] Prodromou held a UK Commercial Pilot License,[23] but was not qualified to fly the Boeing 737. Crash investigators concluded that Prodromou's experience was insufficient for him to gain control of the aircraft under the circumstances." Count Iblis (talk) 01:39, 30 April 2017 (UTC)

Eating a small amount of sugar and carbs while in starvation induced ketosis?
During starvation after the liver has used up all of its glycogen stores and entered ketosis, what happens if the person were to eat say one 500 calorie cake, but otherwise remain in starvation? Would the sugar and calories be stored in the liver or just immediately used by the body? Would the body remain in ketosis despite eating sugar? — Preceding unsigned comment added by treciluo (talk • contribs)
 * See Ketosis. Logically, since glucose is necessary for life, the cake will be used for energy.  How much of that goes toward reducing ketosis will depend on many factors (such as the peak blood glucose, and the individual's health) and we can't really calculate that knowing only the cake's calories. μηδείς (talk) 21:27, 29 April 2017 (UTC)

Air pressure in soccer ball, less than 1 bar
Why is it recommended by the manufacturers an internal air pressure of less than 1 bar? It's mostly 0.6 - 0.8 bar inside. I, intuitively, thought that the pressure should be higher inside the ball for it to keep its round shape. --Clipname (talk) 23:43, 29 April 2017 (UTC)
 * 1 bar is approximately the standard atmospheric pressure, about 14.5 Pounds per square inch. Soccer balls are inflated to a pressure more than 1 bar. Pressure gauges on Air compressors are often calibrated to show pressure above atmospheric. When the gauge shows 0.6 - 0.8 the actual internal pressure in an inflated ball is 1.6 - 1.8 bar. Blooteuth (talk) 00:18, 30 April 2017 (UTC)
 * Sometimes the difference between the pressure in a vessel and the outside pressure is called gauge pressure; as the link explains, in some contexts pressures are conventionally reported as gauge pressure, not absolute pressure. It's a practical choice, even if it annoys physicists and other picky people not me, of course, who, picky, me? . --Trovatore (talk) 00:28, 30 April 2017 (UTC)
 * That ball will still have the same amount of air in it in Azteca Stadium Mexico national football stadium (7,200' elev) but there'd be less pressure to resist it so the outward force should be higher. Thus relative pressure is more useful for soccer balls. Sagittarian Milky Way (talk) 01:03, 30 April 2017 (UTC)
 * Yep. That's why it's a practical choice.
 * Drilling down further, there's a distinction between "vented gauge" pressure, which is relative to the actual ambient pressure, and "sealed gauge", which is relative to a reference pressure maintained within the gauge. I believe "gauge pressure" without further qualification means vented gauge, which is the right one to use to predict how bouncy your soccer ball will be. --Trovatore (talk) 01:15, 30 April 2017 (UTC)
 * Just don't get it too cold. Nyttend (talk) 03:20, 30 April 2017 (UTC)
 * A difficulty for anyone planning to build a football stadium at Antarctic station Vostok would be the complaints from car driving visitors whose rubber tires turn brittle as the air temperature drops well below −70°C / −94°F. Blooteuth (talk) 12:17, 30 April 2017 (UTC)
 * There are no cars on Antarctica actually. There are Antarctica license plates though for motor driven equipment. The numbers are very low. Sagittarian Milky Way (talk) 16:07, 30 April 2017 (UTC)
 * There are no cars on Antarctica actually. There are Antarctica license plates though for motor driven equipment. The numbers are very low. Sagittarian Milky Way (talk) 16:07, 30 April 2017 (UTC)


 * You should at least read the WP entry on Transport in Antarctica before coming to a false conclusion.--TMCk (talk) 16:19, 30 April 2017 (UTC)


 * Then vehicle registration plate is wrong. I will add a dubious tag. Sagittarian Milky Way (talk) 16:47, 30 April 2017 (UTC)


 * Never trust WP w/o checking the source which is sparse and only talking about permanent residents/registrations, not actual existence (and seems outdated). "Antarctica has no permanent population and no vehicles are permanently registered". ;) --TMCk (talk) 17:14, 30 April 2017 (UTC)
 * http://www.driven.co.nz/news/news/hyundai-santa-fe-the-first-car-to-drive-across-antarctica/?ref=nzh_networkfooter Greglocock (talk) 02:40, 1 May 2017 (UTC)
 * Vivian Fuchs drove across Antarctica 60 years ago. DuncanHill (talk) 14:25, 1 May 2017 (UTC)


 * He used tractors with tracks, not cars.--TMCk (talk) 15:06, 1 May 2017 (UTC)