Wikipedia:Reference desk/Archives/Science/2017 September 19

= September 19 =

Black hole vs. white hole
A thought occurred to me: if a black hole is a region of spacetime from which nothing may leave, and a (wholly theoretical) white hole is one in which nothing may enter, what does the math say about what would happen if one of each of these objects were to meet? Thanks! – ClockworkSoul 01:01, 19 September 2017 (UTC)


 * For as simple an answer as possible... Any object approaching a white hole will fall towards the event horizon around the white hole, but not enter the event horizon until it is a black hole. Because "any object" includes black holes, black holes falling towards a white hole won't reach the white hole event horizon while it is a white hole. 209.149.113.5 (talk) 12:54, 19 September 2017 (UTC)


 * I've just about given up on this stuff. One of the better? sources for our article  says that white holes spew out matter, yet the matter they spew out will self-collapse from gravitation.  So why doesn't it collapse before it comes out...?  Hell, I still don't really understand Kruskal-Szekeres coordinates or why it looks like the black hole event horizon is moving out at the speed of light endlessly so that (apparently) everything will fall into it. Wnt (talk) 20:13, 19 September 2017 (UTC)
 * Every normal, active Sun is a "white hole" if you like. Our Sun has a surface temperature of 5.000.000 degree Kelvin btw. I dare you to propose anything (exept another Sun) that could manage to fly near or even past aka enter that. --Kharon (talk) 21:17, 19 September 2017 (UTC)
 * I'm not sure what "5.000.000" is supposed to mean. The actual, referenced value from our article Sun is 5,772 °K. The term White hole has a meaning in Cosmology entirely different from a Star. {The poster formerly known as 87.81.230.195} 90.197.25.72 (talk) 15:44, 20 September 2017 (UTC)
 * This might be slightly pedantic of me, but the unit of Kelvin is K, not °K. Rmvandijk (talk) 14:43, 21 September 2017 (UTC)
 * You're quite correct, and I am aware of it, but decided to compromise as I was answering Kharon, who used "degree Kelvin" and appears not to be a native English speaker: I didn't want to confuse him or other less scientifically trained readers. {The poster formerly known as 87.81.230.193}


 * The white hole should have a mass. The black hole will absorb most of it, with some escaping tidally and some as radiation.  Once the white hole is absorbed, the black hole will long after evaporate, becoming its ow white hole. μηδείς (talk) 06:23, 21 September 2017 (UTC)
 * Ahh, in Cosmology!! In Teletubbology a white hole is a psychedelic babyface!
 * Unfortunately both are missplaced here, since this is a reference desk for Science. --Kharon (talk) 06:50, 21 September 2017 (UTC)


 * This is on course to archive as the single worst answer given in the history of the Science Refdesk, so although I still don't really understand the topic, let me at least dump some refs from searching: White holes might emerge from mini black holes around the time of the big bang, randomly? dumping supernova-like explosions all over the place, black holes might "rebound" into white holes after a long time, it is really easy to model a black hole with no mass, white holes are like "IOUs", purely hypothetical, white holes are too unstable to exist because there's no inverse of Hawking radiation. That last one smells promising, but what do I know?  I used to have clearer conversations back when my modem would get carrier the wrong way and spit out screens of random characters. "VERY hypothetical" is the way NASA puts it.  Figuring out what such a thing would do in contact with another thing we know not very well is really out there. Wnt (talk) 17:48, 23 September 2017 (UTC)

Caffeine allergy
Can a caffeine allergy really kill someone who consumes even a small amount of caffeine? (Question inspired by the 4th case in Criminal Case: Pacific Gay.) 2601:646:8E01:7E0B:EDA1:6DA2:2F4C:8E0A (talk) 05:47, 19 September 2017 (UTC)


 * It's possible. The medical consensus is that anaphylaxis and death can be caused by very small amounts of any antigen. That said, caffeine allergies are rare, and I can't find any reports of deaths due to a caffeine allergy. C0617470r (talk) 07:15, 19 September 2017 (UTC)


 * Some may also confuse allergic reaction with the inability to metabolize caffeine. The latter could result in cardiovascular issues and death but without anaphylaxic shock (using a caffeine quantity that someone else could tolerate).  Some animals are also more sensitive to caffeine because of that reason, like cats and dogs.  — Paleo  Neonate  – 07:39, 19 September 2017 (UTC)
 * This is a common confusion; that people confuse the meanings of a Food allergy with a Food intolerance. Food allergies always are immune system responses to an ingested substance, such as anaphalaxis, hives, or the like.  Food intolerances can include things like excess gas, indigestion, acid reflux, feelings of bloating, or other responses which are not immune system responses.  -- Jayron 32 10:45, 19 September 2017 (UTC)
 * A well-known example being lactose intolerance, which is caused by the body being unable to digest lactose. --47.138.161.183 (talk) 05:41, 20 September 2017 (UTC)

How many pages of information for a science degree?
How many pages are needed to put all knowledge for a college degree? (for reading and as look-up material like dictionaries). Interested in computer science, mechanical engineering, physics.--Hofhof (talk) 16:06, 19 September 2017 (UTC)


 * Much of that knowledge (nay, perhaps the bulk of it) is practical knowledge which is not learned through written material, but through direct instruction, lecture, labs, practice, etc. Also, "science degree" is far too vague, there are hundreds of different kinds of science degrees.  Thirdly, I have no confidence this is the sort of thing anyone has ever bothered to study before (given my second point) which makes searching for such non-existent data fruitless.  -- Jayron 32 16:47, 19 September 2017 (UTC)


 * For an alternative link that is blue see experiential knowledge. Looie496 (talk) 21:38, 19 September 2017 (UTC)


 * I did perhaps half of an Applied Physics degree by understanding four books (I bought maybe just six textbooks over the course, then I read those until the ink wore out). The rest was dipping into books and journal papers in the library. Those four books though (Bleaney & Bleaney?, Solymar & Walsh?) represented serious effort to understand them. You could also just try counting how many pages there are in a copy of Feynman.


 * I did physics partly because it involves hard reading of few books, rather than something like palaeontology where the corpus is perhaps less terrifying on a line-by-line basis, but much larger. Andy Dingley (talk) 17:04, 19 September 2017 (UTC)


 * My answer gets in the same lines as the answers above. It's not enough to do the reading, but if you read thoroughly one good book per course, it would be reasonable to read 25 to be at graduate level.B8-tome (talk) 18:00, 19 September 2017 (UTC)


 * 1000-1200 pages would do. When you memorized, understood and than can apply that much science, you earned your degree! In case its specialized Science it could be less. For example a Metallurgist could probably find everything in "Donald R. Askeland - The Science and Engineering of Materials, 2011" (only 896 pages thin). --Kharon (talk) 21:25, 19 September 2017 (UTC)


 * I suspect that this has changed, and much of the need to memorize info has been replaced by the ability to access it instantly, when needed. Of course, this doesn't apply to all info.  For example, I suspect a chemist still memorizes the atomic numbers, weights and electron configurations of the most common elements, but not the rarer elements and isotopes of each. StuRat (talk) 23:56, 19 September 2017 (UTC)


 * The majority of my physics course was taught with Tipler's Physics for Scientists and Engineers. This is 1172 pages, and it covers everything on the core syllabus. Of course, there are also elective modules which adds more (a random example: my biophysics module used Philip Nelson's Biological Physics, which is 600 pages, although we didn't cover every chapter) and there's stuff from other fields that overlapped (for instance, I needed a roughly 300 page linear algebra textbook and a C programming book), and many lecturers gave us 50-100 pages of notes. 2000 pages seems like a decent estimate. Smurrayinchester 08:36, 20 September 2017 (UTC)

Common cold and temperature.
It is often said that being in the cold has nothing to do with catching a common cold but people do catch a cold after being in the cold. Furthermore staying out in the cold seems to make it worse. I understand that viruses cause a cold but there must be an indirect way in which the low temperatures increase the chances of catching a cold. The old wives tale can't have survived this long if it's not true. 90.198.254.50 (talk) 17:17, 19 September 2017 (UTC)


 * Yep, cold, dry air causes chapped skin, including in the respiratory tract, and breaks in the skin allow microbes in. There could also be effects on people spending more time inside, close together, causing more spread of disease.  Vitamin D production from sunlight may also be down, but I don't know that this vitamin has an effect on colds.StuRat (talk) 17:23, 19 September 2017 (UTC)


 * Scientists have done experiments to prove that being cold is not a direct cause of a cold (because a cold is always caused by a virus), but that doesn't mean it is not an indirect cause, as explained above. Often, being cold (a drop in core temperature) just allows existing viruses to gain a hold when they had previously been inhibited by the immune system.    D b f i r s   18:17, 19 September 2017 (UTC)


 * The problem is caused by being afraid of the cold. There are many people who turn on the heating when it gets colder than 20 C in their homes and who dress for winter when the outside temperature dips below 15 C. Their bodies stay acclimatized for tropical conditions all year round even in winter when it's -5 C outside. When they go outside in winter, their blood flow to their body extremities, their noses etc. shuts down. The immune system then becomes less active in their airways, making them susceptible to catching the cold. Another contributing effect here is that these people tend to not be outside for long during the cold periods. If they do regular exercise they prefer to do that indoors rather than outdoors, they hate the idea of running outside at, say 2 C. Weeks before many people get hit by the cold, the cold virus will already be there in Nature, if you run outside every day for an hour, you're likely to breath in that virus but in low concentrations that will lead to immunity without making you ill. Also, it may be a less virulent version of the virus that will later cause severe cold symptoms in many people. Count Iblis (talk) 20:43, 19 September 2017 (UTC)


 * It's true, those who cringe at the thought of acclimating to the discomforts of environmental changes often seem to be the most susceptible to illness. Reminds me how I was once dared to dive into an ice-cold swimming pool situated next to a hot-tub. Several people there advised against it on the grounds that I might catch a cold, have a heart attack, etc. I did it anyway (several times) and exhausting as it was I actually felt quite invigorated for days after that. Another friend made a half-hearted attempt but quickly retreated out of discomfort and swears to this day that it only resulted in stiff muscles for him. I've always wondered though if that would have been the case had he truly been willing to "take the plunge"? 73.232.241.1 (talk) 05:06, 20 September 2017 (UTC)
 * Hence the concept of a Sauna, popular in some cultures, where one alternates hot and cold phases. {The poster formerly known as 87.81.230.195} 90.197.25.72 (talk) 15:50, 20 September 2017 (UTC)


 * A cold happens when rhinoviruses implant in the nasal mucosa and manage to reproduce fast enough to outrun the immune response for a while. There is a good bit of evidence that extended exposure to cold air can cool the nasal tissues enough to weaken or delay the immune response there.  However many doctors are skeptical that this mechanism plays more than a minor role in seasonality. Looie496 (talk) 21:33, 19 September 2017 (UTC)


 * The low temperatures not only make the immune response less effective. The low temperatures also allow the viruses to remain virulent for a longer time outside the body (see e.g. ), facilitating the transmission. Dr Dima (talk) 22:19, 19 September 2017 (UTC)


 * See also There is A Scientific Reason That Cold Weather Could Cause Colds from the Smithsonian Institution and Why common cold may thrive at low temperatures from the UK National Health Service. Alansplodge (talk) 16:59, 20 September 2017 (UTC)


 * Nonsense. Eveyone has heard of Ben Franklin's demonstration that keeping the windows shut in winter helps spread colds.  In NYC, steam heat is standard, mandatory, and excessive in most buildings.  I keep the window open with a box fan blowing out all winter, and I haven't had a cold since the 90's, if not before. μηδείς (talk) 10:04, 22 September 2017 (UTC)


 * An anecdote is not evidence. --Phil Holmes (talk) 11:30, 22 September 2017 (UTC)

Do we have an article on chapped skin ?
If so, I'd like to add a link to whatever name it is under. StuRat (talk) 00:01, 23 September 2017 (UTC)


 * Perhaps Xeroderma. -- ToE 09:12, 23 September 2017 (UTC)


 * Yes, that's it, and includes other non-weather causes, too. I've added a redirect.  Chapped lips, on the other hand, redirects to cheilitis, which also list multiple causes.  You'd think there would be a common term for both, as lip skin is just a type of skin, after all, but two articles works, too. StuRat (talk) 21:22, 23 September 2017 (UTC)

What exactly is "Thermally activated delayed fluorescence"?
I've seen the term used many times, typically with regard to materials for OLEDs, and I'm not quite sure what it means. OrganoMetallurgy (talk) 17:31, 19 September 2017 (UTC)


 * Delayed fluorescence sounds like phosphorescence, but the "thermally activated" bit sounds like it only emits light at certain temperature ranges. StuRat (talk) 17:37, 19 September 2017 (UTC)


 * This paper has a relatively decent intro, depending on your ability to read scientific papers, as well as keywords to feed to your bibliographic search. For instance, I suspect this paywalled chapter would be interesting to read (but I cannot go through the paywall). Tigraan Click here to contact me 17:48, 19 September 2017 (UTC)


 * I'm not sure myself! Years since I did anything like this. Efficient blue organic light-emitting diodes employing thermally activated delayed fluorescence (2014) is a reasonable start (I think it's one of the first and canonical papers on it). In OLEDs, a cause of inefficiency is that some of the excited states are singlet states, from which they can transition and emit a useful photon, but others are triplet states which don't. Excitation to triplet states is a wasted, inefficient use of energy for the OLED.
 * There are three generations of OLED tech. The first were based on fluorescence, then the second on phosphorescence and now a third generation using this TADF. Fluorescent were inefficient, as they only put 25% of their energy into the fluorescent singlet states (I think that 25%'s a pretty fundamental limit from the 1:3 ratio). The phosphorescent ones managed to be efficient emitters by adding noble metal dopants which made the triplet states into practical emitters too, but these were expensive to make. The new generation doesn't need these metals, but potentially offers similar efficiencies. It overlaps with here: Intersystem crossing. They work by eliminating the triplet states, by converting them to useful singlets. This up-conversion obtains its energy thermally. Current SotA is here:
 * (And why am I telling anyone with a username like that how to suck eggs?) Andy Dingley (talk) 18:10, 19 September 2017 (UTC)


 * As an aside: Thermoluminescence dating is a form of thermally activated delayed fluorescence. Aspro (talk) 20:35, 20 September 2017 (UTC)

strange equation in physics class
My daughter is taking physics II in college. She has the equation $$ C = \frac{k \cdot \epsilon_0 \cdot A} {d} $$

(Sorry, I'm not too familiar with TeX), where C is capacitance, epsilon_0 is the permativity constant, A is the surface area and d is the distance. Usually k is Coulomb's constant, but she is wondering if k is something else here. I say that the equation doesn't make sense because in $$ k \cdot \epsilon_0 $$ the physical units cancel out, leaving Farads on the left and meters on the right. Does she have something wrong in this equation? Bubba73 You talkin' to me? 23:44, 19 September 2017 (UTC)


 * May we see the units you are using for each term ? StuRat (talk) 23:52, 19 September 2017 (UTC)


 * These are from her writeup of her notes. She has C in Farads, epsilon_0 in Farads/meter.  She has

$$ k = \frac{1} {4 \pi \epsilon_0} \frac {N \cdot m^2} {C^2} $$

and I suppose area in m^2 and distance in meters. With this equation for k, the epsilon_0 cancels out in the first equation. Bubba73 You talkin' to me? 00:04, 20 September 2017 (UTC)
 * This looks like Capacitance and Capacitor. And it looks like there is some confusion over which epsilon is used for which context (permittivity or dielectric constant of whatever the material is vs the electric constant (an actual "constant"). If she's using epsilon_0 as the dielectric constant of the material in F/m, that's already confusing, but that would mean k is the Vacuum permittivity but without the units? Or else is she using epsilon_0 as the vacuum permittivity in its usual F/m units, with k being the dielectric constant of her material? DMacks (talk) 02:07, 20 September 2017 (UTC)


 * Yes, k is a dimensionless constant here, representing the relative permittivity of the dielectric material that separates the plates. It is 1 for empty space, and larger than 1 for any other material. Looie496 (talk) 02:37, 20 September 2017 (UTC)


 * To add to this, that's the equation for the capacitance of a parallel plate. It is a notable equation and she should understand its derivation and what it means. ε0 is the permittivity of vacuum, which has the lowest possible permittivity. So the equation says that if you have a better dielectric, you will have a better capacitor. And that's because the dielectric resists the electric field, so you will have a lower voltage difference for a given amount of charge, so you can store more charge in the capacitor at a given voltage. (Tangent: another practical reason to use a dielectric instead of air is to increase the dielectric strength. Dielectrics including air will break down and become conductive at high enough voltages, and we want that upper limit as high as possible.) C0617470r (talk) 03:06, 20 September 2017 (UTC)


 * So the k in the equation at the top is NOT Coulomb's constant? In her notes, she has $$k=\frac{1} {4 \pi \epsilon_0} = 8.99*10^9$$ (and SI units), which is Coulomb's constant. Bubba73 You talkin' to me? 03:18, 20 September 2017 (UTC)

She talks about an example the teacher did "a 4.2 nF capicitor has an area of 2.8 m^2 and a separation of 12mm. What is the dielectric constant?" He used the equation at the top, solved for k, getting k=2.03. So this k is the dielectric constant, not Coulomb's constant? Bubba73 You talkin' to me? 04:42, 20 September 2017 (UTC)


 * Right, it is not Coulomb's constant. Unfortunately they share the same symbol. C0617470r (talk) 04:46, 20 September 2017 (UTC)


 * Coulomb's constant is usually written $$k_\text{e}$$ - she apparently failed to write down the subscript, leading to the confusion. Bubba73 You talkin' to me? 04:53, 20 September 2017 (UTC)


 * It is also notable that "k" stands for a bewildering number of constants in scientific equations, to the point where the best assumption about its meaning is "whatever constant is used in this particular equation". Besides the aforementioned Coulomb's constant, there's also (just think off the top of my head, I am certain this list isn't exhaustive) reaction rate constant, the spring constant in Hooke's law, the Boltzmann constant, the generic proportionality constant, etc. etc.  -- Jayron 32 10:59, 20 September 2017 (UTC)
 * Some other big ones in physics: wave number, and momentum when "p" is already in use (especially off-shell momenta in quantum field theory). And it's also very often used as a summation or integration variable. -- Link (t&bull;c&bull;m) 22:25, 20 September 2017 (UTC)


 * The dimensional analysis emphasizes that relative permittivity is a dimensionless ratio - permittivity of a substance compared to vacuum. The original equation can be written nF = ? * F/m  * m^2 / m, with ? representing k, and we see indeed it has no units.  But constants without units are aggravating because they imply you actually have to remember a formula.  Maybe it would be useful to think of the (relative permittivity * permittivity of vacuum) as a subunit here, because then you can think of it as a conversion factor relating the other data you have available. Wnt (talk) 12:08, 20 September 2017 (UTC)

Thanks everyone! Bubba73 You talkin' to me? 23:07, 20 September 2017 (UTC)