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May 7

Antidepressants

I'm looking for sources on antidepressants causing the side effect of emotional numbness. Benjamin (talk) 10:12, 7 May 2017 (UTC)[reply]

"Antidepressants" encompass a huge spectrum of very different drugs and medications. Our article links to several review articles, including Review of maintenance trials for major depressive disorder.... Papers like that one go over lots of research on a wide variety of clinically-prescribed drugs. If you're interested only in a specific category - say, the SSRI family of drugs, our article points to many references on that topic. If you need help connecting a brand-name medicine to the research articles about that drug, let us know. Nimur (talk) 12:15, 7 May 2017 (UTC)[reply]

I don't see anything in that article about emotional numbness. Benjamin (talk) 12:33, 7 May 2017 (UTC)[reply]

There are lots of things you won't find in that specific article; it is a review of many different research studies across many different drugs. Evidently, the authors did not feel, based on their literature survey, that the symptom you named was relevant or prominent enough to mention. Sometimes, reading a review article is good to give you perspective: it doesn't mean that you won't find the detail you seek - perhaps some specialist has studied that specific detail in other work - but perhaps the detail is not so important that it merits mention in a broad summary of the state-of-the-art knowledge about the entire field.

Here is another, perhaps more accessible, resource: SSRIs, from the Mayo Clinic. This website, operated by the Mayo Clinic, describes SSRI drugs that are commonly prescribed for depression and other clinical purposes. Many side effects are listed, but "emotional numbness" is not one of them - nor is it listed by any similar name in "medical-ese," such as reduced affect display, (let alone paresthesias).

This doesn't mean it couldn't happen - it does not mean it's never been studied - but it's all providing evidence for a pretty solid case that you're describing an out-of-the-ordinary side-effect, as opposed to a well-known side-effect.

Here is the forty-four page FDA-approved drug label sheet for Zoloft, one popular brand-name for one type of SSRI. On CAPS, (a standardized battery of tests used to measure, among other items, "avoidance/numbing" - this is a specific clinical term for a type of emotional disorder commonly comorbid with post traumatic stress disorder), the correctly-administered drug "was shown to be significantly more effective than placebo on change from baseline to endpoint on the CAPS." This means, in plain English, that the drug statistically reduces the effect you are describing.

These drugs are powerful and complex. They can be helpful or harmful. They should be taken only in consultation with a professional. If you have questions specific to your conditions, whether you take medications or not, you should consult a medical professional.

Nimur (talk) 12:46, 7 May 2017 (UTC)[reply]

I found this and this, though note that they're specific to SSRIs. Incidentally, searching for antidepressants emotional numbness gives you a lot of extremely questionable pseudoscientific stuff that you have to be careful to pick through. Matt Deres (talk) 13:12, 7 May 2017 (UTC)[reply]

Indeed, a search engine will find content that matches your search query, regardless of the quality of that content. This is one reason that I specifically disrecommend the use of a search engine, and try to promote methodological survey of actual scientific references... Despite the speed with which a modern search engine delivers content, I find that it is typically significantly slower at delivering information when I compare it to more traditional methods of information-gathering. I would go farther to say that search engines, compared to traditional research methods, are many orders of magnitude faster at delivering data; yet simultaneously, search engines are many more orders of magnitude slower at delivering knowledge. Nimur (talk) 13:29, 7 May 2017 (UTC) [reply]

Why is it that when I talk to people about their personal experience with side effects of antidepressants, they often mention emotional numbness, but I never see it mentioned as an official side effect of the medication? Benjamin (talk) 16:52, 7 May 2017 (UTC)[reply]

Because "emotional numbness" is not a side effect. I've talked to many people who are on antidepressants and nobody ever mentioned it. --AboutFace 22 (talk) 17:15, 7 May 2017 (UTC)[reply]

Antidepressants are supposed to put the user into a calm state of mind. Maybe the OP is confusing calmness with numbness? ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:00, 7 May 2017 (UTC)[reply]

I've seen lots of anecdotes about people who discontinued SSRIs due to emotional numbness. And Matt Deres' two links above show that "emotional blunting" or "emotional indifference" is a side effect. Loraof (talk) 19:54, 7 May 2017 (UTC)[reply]

All antidepressants are also antianxiety drugs, they make people less anxious. Perhaps this is mistaken for emotional numbness? --AboutFace 22 (talk) 23:38, 7 May 2017 (UTC)[reply]

I'd highly qualify that. After 9/11 my doctor first tried versions of zoloft and prozac. They had no effect on my anxiety and insomnia, they just made me jittery during the day and falling asleep even harder. μηδείς (talk) 01:21, 9 May 2017 (UTC)[reply]

I have no idea what emotional numbness is. But I should note that anhedonia is a symptom of depression. Question: Is it plausible that SSRIs treat some other symptoms of depression but are not as effective against anhedonia? Oh... and I see now our article says that sexual anhedonia can be a symptom of SSRIs. (I really haven't looked into this, was just thinking out loud...) Wnt (talk) 01:00, 10 May 2017 (UTC)[reply]

Letters on a pill

What do they use to write on this pill? Is there any black ink that's safe for ingestion (I think it's black)? --Dikipewia (talk) 18:43, 7 May 2017 (UTC)[reply]

There are certainly black dyes that can safely be eaten - you can get black icing paste for decorating cakes. It is E151 (Brilliant Black) or E153 (Carbon Black, or Vegetable Carbon). Wymspen (talk) 21:42, 7 May 2017 (UTC)[reply]

Does completely charcoalized vegetable lose that horrible burnt taste? Sagittarian Milky Way (talk) 22:31, 7 May 2017 (UTC)[reply]

Plain charcoal (as in Activated carbon) has no taste and is widely used for medical purposes, but also for the filtration of water (or vodka). --Stephan Schulz (talk) 23:26, 7 May 2017 (UTC)[reply]

Thanks, I have never put charcoal in my mouth so wasn't sure. Sagittarian Milky Way (talk) 23:38, 7 May 2017 (UTC)[reply]

A warning: Don't put charcoal in your mouth that's intended for bbq's. It's not the same as charcoal intended for filtration use. It may be coated with toxic chemicals and fuel. StuRat (talk) 02:32, 8 May 2017 (UTC)[reply]

"... coated with toxic chemicals" How? Why? Richard Avery (talk) 07:21, 8 May 2017 (UTC)[reply]

In some markets, some brands of charcoal for BBQ purposes are pre-soaked in liquid hydrocarbons (think charcoal lighter fluid), to make them easier to light. The theory is that the fluid will fully evaporate and burn off before you put the dead cow on top of the fire. But that does not apply if you eat it ;-). In general, charcoal for medical use is carefully prepared to avoid contamination, and I would not recommend to just pick any from a bag of BBQ supplies. --Stephan Schulz (talk) 08:26, 8 May 2017 (UTC)[reply]

These for example. --Jayron 32 14:47, 8 May 2017 (UTC)[reply]

And, as Stephen Schultz' link says, it's "harmful or fatal if swallowed". StuRat (talk) 22:40, 8 May 2017 (UTC)[reply]

Liquorice candy is typically black. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:39, 8 May 2017 (UTC)[reply]

Many commercial black food dyes are a mixture of red, blue, and yellow/green dye with cocoa powder. Perfectly edible and very black. Someguy1221 (talk) 22:46, 8 May 2017 (UTC)[reply]

I don't consider liquorice edible, but that's a matter of taste, not toxins. ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:36, 9 May 2017 (UTC)[reply]

"Eww ! This tastes like anise !" StuRat (talk) 15:42, 9 May 2017 (UTC) [reply]

May 8

Astronomy / Calendar : Thirteen Groups of (Almost) Eight

What possible astronomical meaning or calendrical interpretation, if any, can be assigned to a structure containing thirteen groups of eight objects (of which two such groups might be slightly smaller, containing only six or seven objects each) ? Of course, one probable explanation would be the number of seven-day weeks in two years; however, the civilization which build the structure is known to have had a six day week, hence the dilemma. — 82.79.178.230 (talk) 08:13, 8 May 2017 (UTC)[reply]

Your question sounds like you're confusing astronomy (a discipline of physics, a body of knowledge based on application of the scientific method to elucidate knowledge about the universe) and astrology, which is bullshit. --Jayron 32 14:45, 8 May 2017 (UTC)[reply]

Nothing of the sort. There is an ancient Carpathian sanctuary, consisting of four concentric circles. The meaning of the second and fourth circles are quite easily discernible; that of the first and third, not so much. There is also a smaller round sanctuary nearby, similar in structure (and, presumably, also in meaning) with the outer circle of the former. — 82.79.178.230 (talk) 20:57, 8 May 2017 (UTC)[reply]

Historically, most ancient civilizations conflated what we now call astronomy with what we now call astrology, and associated it/these with calendrical matters, so in a historical context the question is perfectly respectable. {The poster formerly known as 87.81.230.195} 2.122.60.183 (talk) 17:12, 8 May 2017 (UTC)[reply]

13 Venus year equals 8 Earth year. Sagittarian Milky Way (talk) 14:54, 8 May 2017 (UTC)[reply]

And at least one ancient culture has been shown to have been aware of this correspondence – see Observations and explorations of Venus, Section 1.6 Maya. Is the 6-day week civilization in question the Akan? {The poster formerly known as 87.81.230.195} 2.122.60.183 (talk) 17:00, 8 May 2017 (UTC)[reply]

No, it is Eastern European. — 82.79.178.230 (talk) 22:21, 8 May 2017 (UTC)[reply]

The lunar month is somewhere around 28 days, depending on how you measure it (see article for details). That puts about 13 in a year. StuRat (talk) 15:43, 8 May 2017 (UTC)[reply]

See pages 28 and 29 of this book: [1]. There is a list of numbers in the left hand column (i.e. on the pages for March and April - the note at the bottom of March (continued at the bottom of April) tells you what they are). Eight of these numbers generally appear in any thirteen - day period - moreover any number is 8 larger than the one above it (the numbers are not allowed to exceed 19 so you may have to add 19 to the second number to appreciate this). 195.147.104.148 (talk) 21:15, 9 May 2017 (UTC)[reply]

Whom to trust?

request for opinion on apocryphal sources

The following discussion has been closed. Please do not modify it.

I've read two books (In French, by French people, printed in France), each one discusses a topic from a scientific point of view. The first one (talks about "Sleep") was against the use of alcoholic drinks and tobacco since they are harmful. The other one (about "Pleasure") encourages readers to drink and smoke and stuff, claiming that they are good for the human health! My question is: whom should we believe if both claim to be based on scientific evidence? (I don't mean only these books, but in general.) — Preceding unsigned comment added by 105.110.188.195 (talk) 09:33, 8 May 2017 (UTC)[reply]

Never trust anything that speaks in very general terms about "health". Health is far too complex a topic for any one factor to be "good" or "bad". Instead, look to things which have specific outcomes (be they good or bad, which is up to you). For example, Wikipedia has an article titled Alcohol and health which discusses specific outcomes of consumption of alcohol, including health risks (such as alcohol poisoning and alcoholic liver disease). Then you have things noted at health effects of wine, which notes some potential health benefits from small amounts of drinking wine (an alcoholic beverage), including the French paradox, whereby some types of alcoholic drinks (including red wine) may be linked to better health outcomes. It's FAR too simplistic to say that "alcohol is good for health" or "alcohol is bad for health" in such broad terms. --Jayron 32 10:24, 8 May 2017 (UTC)[reply]

If you find a source claiming that smoking is good for you, you can assume it was written by someone in the tobacco industry. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:20, 8 May 2017 (UTC)[reply]

I can think of a lot of drugs that are more pleasurable and less dangerous than tobacco! One of them is even legal in a few states... Wnt (talk) 13:01, 8 May 2017 (UTC)[reply]

This medscape article[2] says: "any amount of alcohol increases the risk of getting a range of cancers." This is very true. Alcohol is poison. The title of the paper: "New UK Guideline" --AboutFace 22 (talk) 14:35, 8 May 2017 (UTC)[reply]

And there are documented health benefits of moderate alcohol consumption: [3]. Certain negative health outcomes show small decreases in prevalence among people who consume small amounts of alcoholic beverages. That is different than saying that alcohol is "healthy" which is far too broad of a statement to make. There are risks, there are benefits, and it is up to the individual to make decisions after weighing them all. Also "Alcohol is a poison" is only true in the sense of the dose makes the poison. Water is also a poison (see water intoxication), and yet water is also OK in low enough amounts (vital even!). --Jayron 32 14:44, 8 May 2017 (UTC)[reply]

Also, in any such Q, we need to discuss the "versus what" part. For example, if their argument in favor of smoking is that it reduces psychological stress, then we should point out that there are many other stress reduction methods which don't involve inhaling toxins. Try meditation, for example. If their argument is that smoking keeps weight down, well then there are healthier ways to do that, like improved diet and exercise. StuRat (talk) 15:38, 8 May 2017 (UTC)[reply]

Closing, we can't comment on unspecified material. μηδείς (talk) 01:24, 10 May 2017 (UTC)[reply]

Magnetism

Since magnetism is electromagnetism, what is the wavelength and frequency of the photons around e.g. two bar magnets next to each other? Th4n3r (talk) 12:24, 8 May 2017 (UTC)[reply]

I'm not familiar with the most advanced theories of sub-atomic physics, which have all kinds of strange particles not usually mentioned in introductory university physics courses (with a target audience of science and engineering students). If we confine ourselves to electromagnetism as explained by Maxwell's equations, just because there are electric and magnetic fields, there isn't any electromagnetic radiation unless the electric and magnetic fields are changing. So in the situation you describe, supposing the bar magnets are static, there aren't any photons. Jc3s5h (talk) 12:42, 8 May 2017 (UTC)[reply]

This is a field of physics known as Quantum electrodynamics (QED). The math involved here is way over my head, but your question can be answered by QED. --Jayron 32 14:39, 8 May 2017 (UTC)[reply]

Without going into very difficult mathematics, we can give you this answer: the magnetic field around a bar magnet is, essentially, static - it does not change with time. So it is in steady state, which corresponds to zero frequency; hence, infinite wavelength. This is why the photon model is not very useful when we describe steady-state ferromagnetism: if you want to describe a static magnetic field and you choose to write it down with the math that describes photons, you have to deal with a lot of zeros and infinities in inconvenient places.

It might frustrate you to see a real scientist talk to you about photons with wavelength-infinity. But, in photon physics, there are a lot of non-intuitive things you'll have to wrap your mind around. Here is a brief anecdote that may help you to accept that this answer is actually correct and valid: in answering this question, I pulled out my copy of Griffiths' Electrodynamics.... After spending 500 pages building up the details of mathematical treatments for electromagnetism, the author introduces the photon in the very last chapter, by presenting a conundrum about conservation of momentum; and, in answering, he writes: "personally, I would regard this argument as a joke, were it not for the fact that at least one massless particle is known to exist in nature: the photon."

So - don't get too hung up on the conundrum: you can have wavelength-infinity waves, and you can even think of them as particles: they're just not behaving in a very particle-like way. They are not moving, they are not conveying energy and momentum. If you write the math very carefully, these conclusions follow directly.

Nimur (talk) 14:54, 8 May 2017 (UTC)[reply]

Making sense out of North Korea's assassination plot suggestion

I don't doubt the North Koreans make stuff up or that they're nuts... nonetheless, killing Kim Jung Il sounds like a capital idea to most Americans. And I generally go by the rule of thumb that almost every nasty thing either side says about the other in any war is true, and this is a nasty thing. So can we make sense out of it? Now obviously it would help if someone can track down the original Korean and translate this without the "crazy" filter setting (I mean, I suspect it's pretty nuts in that language but I bet the BBC translators were hamming it up) but here is what I have to work with now: [4]

A hideous terrorists' group, which the CIA and the IS infiltrated into the DPRK on the basis of covert and meticulous preparations to commit state-sponsored terrorism against the supreme leadership of the DPRK by use of bio-chemical substance, has been recently detected...
They hatched a plot of letting human scum Kim commit bomb terrorism targeting the supreme leadership during events at the Kumsusan Palace of the Sun and at military parade and public procession after his return home.
They told him that assassination by use of biochemical substances including radioactive substance and nano poisonous substance is the best method that does not require access to the target, their lethal results will appear after six or twelve months, bio-chemical substance can be added in cooperation with the CIA if one single correct information is obtained, the component of terrorism-purposed bio-chemical substance is the know-how of the CIA and it is only the CIA that can produce such substance, and that hardware, supplies and funds needed for committing terrorism against the supreme leadership will completely be borne by the IS. Then they handed him over 20 000 U.S. dollars on two occasions and a satellite transmitter-receiver and let him get versed in it...
In January, May, August and September of 2016 IS agents had satellite contacts with Kim who resided in Pyongyang. The IS agents gave instructions to choose and report the most effective and safe method with high probability of success while presenting him various terrorist methods using biochemical substances along with operational code of terrorism against the supreme leadership, the ways of bribing an object who would directly carry out the terrorist act and ways of entering the grounds of events.
On August 12, 2016 they gave him an instruction to collect and send as much information as possible concerning the surrounding environment of event ground where celebrations are held frequently, guard situation there and orders observed at times of events, saying that once concrete and detailed data are given, they would study the most reasonable way in cooperation with the CIA...
In March and April last an IS agent [name omitted] and his secret agent [name omitted], met the terrorist accomplice in Dandong of China and handed him over new satellite transmitter-receiver and 50 000 U.S. dollars. They signed a "contract" on setting up the overseas liaison center and let the necessary equipment and materials be introduced as the first instalment in early May.
On April 7 a guy surnamed [name omitted], chief of the IS team, taught Kim the way of bribing terrorist accomplices, saying that "even the U.S. CIA uses gradual engagement with due consideration given to the greed and mentality of persons depending on which class and strata they belong" and told him to use it as a reference in engaging terrorist accomplices to be infiltrated to the event ground.
On Nov. 4, 2016 and on April 13, 17 and 20 this year they let Kim know that they officially confirmed the types of bio-chemical substance and hardware to be used for committing terrorist act against the supreme leadership and requested it to the CIA, and instructed him to restudy the "creed" of the terrorist executor and reconfirm the state of "brainwashing" of him and report about them. They also repeatedly instructed him to take the best measure for the examination and preparations for the terrorist operation, as there can be such catastrophic incident as a war once the fact about terrorist means and funds provided by the IS is known.
The chief of the south Korean puppet Intelligence Service [name omitted] praised the terrorist as a "very valuable existence for the nation and 'IS'" and directly organized the terrorist operation and let the chief of the IS team [name omitted] and agent [name omitted] take the lead in executing it. The puppet forces gave the terrorist more than 80 instructions for the execution of the operation.

The remarkable claim here is that it is a radioactive substance and a biochemical substance. This implies a level of sophistication in the attack comparable to something like a targeted cancer therapy that brings a radioactive isotope to a specific (cancer) cell to kill it. The North Koreans have obviously thought up the notion of finding some cell type to target (what???) that is a lot less dispensable than a cancer cell. Whether they have thought that up by interrogating a U.S. agent or have purely invented it in their own laboratories to use on us, it is still of interest.

Anyway, some topic of interest here: is there any record of people considering "targeted assassination therapy", and what would they target? Is there something that is easier to inhale and get into a bloodstream than an antibody that they could link to the radioactive isotope? Or would they just be hoping to make a lot of shrapnel and something would hit, or do they have a way to make inhaled antibody cross from lung to bloodstream? There are so many options here I don't even know where to begin - that's obviously a really vague source text. What can we think of? Wnt (talk) 14:05, 8 May 2017 (UTC)[reply]

Oh, my --- I just looked up Kim Hak-Song on PubMed and found several results, including this cancer therapy targeting paper. (other results - I have no idea whether that is the same person - the address for author information was in South Korea (and also is in the latest publication even though NCBI somehow left it out, sorry) If the North Koreans are writing a novel here, they certainly did their background research. Wnt (talk) 14:40, 8 May 2017 (UTC)[reply]

Possible relevance: "repebody" molecules that are bacterially produced antibodies with multiple leucine-rich repeat modules -- earlier paper by Kim. IF the North Koreans can be taken at their word, one of these might currently be under selection by someone working with the CIA to recognize a target on critical cells. (It is quite possible that the researcher involved is unaware he is helping to build a weapon). This patent covers repebody inhalation, but it might be purely boilerplate. Ditto this one, which also talks about inhaled aerosols. Actually, it looks like the repebody term is almost unique to this researcher, and is based on an understanding of the lamprey immune system - it can benefit, however, from rational design using LRRs from known human proteins. It's introduced here - when I get time I'll try to understand how it actually works. This seems like an interesting, multi-purpose technology. Wnt (talk) 15:03, 8 May 2017 (UTC)[reply]

Going back the the OP's question. Doubt if there is any plot to assassinate Kim Jung. It would not achieve anything, because in this type of regimen there is always someone else that can step in. So, there is no chance of creating a 'power vacuum' which could destabilize the regimen. Things would just carry on as normal. Secondly, the DPRK are not nuts. The 'family' are brought up learning gamesmanship. They are very astute and very ruthless towards anyone who descents. Third: biochemical substances and nano poisonous substance may be metaphors. Most NK's are peasants. The DPRK is able to maintain control with the help of small middle-class elite. Being well educated they are naturaly very curious about the outside World to which they have no Internet nor any other access. Therefore: Nano poisonous substances may refer to some plans which include simple things like launching balloon from South Korea to drop pen-dives and DVD's on the middle-class city in NK just across the river. The data in them could even included copies of WP. The NK finder may well feel too frighted to tell their friends at first -as to what the have discovery and read – but then one must take into account human nature. If the drops of data are intensive enough, some will realize from normal conversational topics, that their friends have also read these drops, and a bond of mutual understanding and trust forms as they discus what they have learnt. A similar thing is thought to have helped to destabilize the former Soviet Union. The privileged elite, could get not only Betamax video machines but films. Glimpsing the outside World made them discontented with the then Soviet regimen. Result was that the regimen lost the 'blind' support of the elite and the rest is now history. Kim Jung was born in Russia and he may be using the metaphor biochemical substances to refer to the elite of the former Soviet Union which turned on their suppressors once they eyes had been opened. Unfortunate, the military elite of all or most counties like a blitzkrieg. They just want to storm in, leaving long term problems in their wake, rather than taking the gently, gently approach. Those that forget history are doomed to relive it. This is not an issue to be left up to the politicians in the 21st Century – it can lead to an ever more dangerous situations, which as ever, the politicians & military will make a mess of – if left to them. We (the World) need to become (or already are) the biochemical substances to bomb any repressive regimen with Nano liberating substances. Aspro (talk) 00:12, 9 May 2017 (UTC)[reply]

Does the Milky Way rotate around anything?

For a haywire SF plot, I'm stepping through rotational frames of reference and summing up angular velocities relative to a given gravitational center (Earth rotation, Earth orbit, orbit of solar system around galaxy center...). Now I'm trying to figure out whether I can go one farther and add an angular velocity for the movement of the Milky Way. However, IIUC, there is no indication of actual rotation of the Milky Way around, e.g., the gravitational center of the Local Group, or anything at larger scales. Once we leave the galaxy, movements are linear and/or omnidirectional (Hubble flow, microwave background). Is that correct? --Elmidae (talk · contribs) 15:17, 8 May 2017 (UTC)[reply]

See: Supermassive black hole — 107.15.152.93 (talk) 15:25, 8 May 2017 (UTC)[reply]

That contributes to rotation within the galaxy, while the OP was asking about rotation of galaxies relative to each other. StuRat (talk) 15:28, 8 May 2017 (UTC)[reply]

Note also rotation vs. revolution; this is actually about the latter. Wnt (talk) 01:08, 9 May 2017 (UTC)[reply]

I always learned that the movements on those scales - larger than the local group - were virial (thermal). Nowadays, every cosmologist you talk to has a different idea - there is less consensus than there was many years ago. I would attribute that development to the rise of powerful computers that can simulate general relativity - so lots of people can try out various theories and make predictions that are consistent with observational data. Nimur (talk) 15:26, 8 May 2017 (UTC)[reply]

That some galaxies move towards each other and eventually collide, rather than moving away, to me suggests there are eddies in the large-scale movements of galaxies. This also implies rotation within those currents. However, at human timescales, I'm not sure if those can be measured. That is, the time for a significant change in direction to occur may be longer than we have been measuring. StuRat (talk) 15:27, 8 May 2017 (UTC)[reply]

See the Local Group and the Great Attractor. Most bodies in motion towards each other tend to be off center and hence have some rotational component (think of the Theia hypothesis but the space scale may be so vast we have not had time to observe it directly. μηδείς (talk) 17:22, 8 May 2017 (UTC)[reply]

[Edit Conflict] "Eddies" and "currents" are deceptive terms in this context, as galaxies are not (so far as we know) embedded within any fluid that could apply external positive pressures to them – force fields such as gravity are not fluids and do not have eddies and currents in this sense (despite Eddy currents!).

It is however fairly trivial that in a gravitationally bound group of galaxies such as the Local Group, all members must be orbiting the group's common barycentre, just as is the case with larger Galaxy clusters, and heirarchically upwards with Superclusters. (One also needs to take account of the dark matter associated with the galaxies individually and perhaps the Group as a whole.) However, orbits are not just circular, they are elliptical and can be of any eccentricity, even approaching linearity. Moreover, on the huge duration scales involved in movements of galaxies, it would be very difficult for us to observe any curvature of their orbits directly – we can only take effectively instantaneous measurements of their velocities, which will appear linear until we take more measurements over many millions of years.

The Milky Way and the Andromeda Galaxy are currently nearing each other and are clearly gravitationally bound; all other members of the Local Group are by comparison much less massive, and most if not all of them will likely also be bound (though interactions could conceivably cause some satellite galaxies, globular clusters or individual stars to exceed escape velocity). {The poster formerly known as 87.81.230.195} 2.122.60.183 (talk) 17:45, 8 May 2017 (UTC)[reply]

Re: "galaxies are not (so far as we know) embedded within any fluid that could apply external positive pressures to them". See dark fluid for one possible theory that proposes just that. StuRat (talk) 22:02, 8 May 2017 (UTC)[reply]

If by "one possible theory" you mean "one dude with a crazy idea" (he's basically the only sources for that poorly cited article). --Jayron 32 00:45, 9 May 2017 (UTC)[reply]

I found a 1999 article about the Sun's motion relative to the Local Group here. It says the Sun is moving at 300 km/s "relative to all LG members" and gives a heading in Galactic coordinates that I should admit I don't necessarily understand. There is discussion about a local standard of rest that refers to the barycenter of the Local Group and/or the point where cosmic background radiation is equally shifted from all directions, but I didn't follow what they did there. I assume someone reasonably skilled in the art is supposed to know the Sun's motion relative to the Milky Way and can mentally delete that from its Local Group motion... in any case, there must be updated numbers if you search for what cites that. But the bottom line is that the Local Group is defined by mutual gravitation and is relevant. As opposed to the situation with Great Attractor, where they start talking about "peculiar motions" because everything is being pulled apart by expanding space! There really is a scale beyond which there are no orbits; it's just bigger than the Local Group. Wnt (talk) 01:30, 9 May 2017 (UTC)[reply]

@Wnt: l is galactic longitude. b is galactic latitude. Those coordinates are about in Cepheus or so towards the plane of the Milky Way. a is altitude and A is azimuth. Alpha is right ascension because they ran out of English A's and delta is declination. Lambda is ecliptic longitude and beta is ecliptic latitude because they both can't be lambda and alpha's already taken. Ecliptic longitude is more important so it gets to be lambda. Galactic coordinates use the English versions of the ecliptic Greek letters and supergalactic latitude and longitude are just SGB and SGL because they're new and they've run out of good options. Their use of apex is likely analogous to the "apex of the Sun's way" which is it's motion relative to the galaxy. If you're wondering if there's a left ascension or wrong ascension then no. The opposite of right ascension is oblique ascension which is what's ascending (rising) at any given time from where the equator is at oblique angle to the horizon instead of a right angle (that is, not the terrestrial equator (or poles where horizon and equator are parallel)). It is much more obscure than right ascension. Sagittarian Milky Way (talk) 02:37, 11 May 2017 (UTC)[reply]

Alright, thanks guys. This strikes me as "it's complicated, but for practical purposes there is no orbiting motion with readily discernable characteristics". That should be enough to not make me break my intended Level 4 position on the SF Mohs Scale :) --Elmidae (talk · contribs) 08:01, 9 May 2017 (UTC)[reply]

@Elmidae: That wasn't the message I meant to send with my comment about the paper above. It actually did define a region based on the distance where the gravity from the Local Group can retain galaxies within a region ... I think. So AFAICT the Milky Way is orbiting ... it's just that the nature of the "center" in a group of galaxies, when there are a few stray galaxies mixed in that aren't expected to be gravitationally bound but nonetheless affect the whole, and all the data is incomplete anyway, is a little iffy. Wnt (talk) 16:15, 9 May 2017 (UTC)[reply]

Appreciate it! And if I was writing an article or a paper, I'd have to dive into that. But really, fact-checking for an SF story is equal parts mining for ideas and ass-covering. As long as I'm not committing a blatant faux-pas here ("Everyone knows that the Milky Way orbits the Manichaean Gap!") and not overlooking some obvious cool feature to exploit, I'm good. - Leaning far enough out of the window in some other aspects, admittedly %) --Elmidae (talk · contribs) 17:05, 9 May 2017 (UTC)[reply]

Blackest black fabric that is reasonable to achieve?

I want a black backdrop for making some project videos for YouTube and photographers seem to recommend velvet but it seems to me that even velvet can have some sheen and reflect light, appearing grey. Would it make sense to buy a fabric and then attempt to dye it further? It's possible that all the locations in the fibres that can be occupied by the dye are already occupied. I know photographers can make a white background look black in a suitable environment but that's not possible in the confinement of my living room. How to make a surface not reflect visible light and absorb more? --78.148.99.149 (talk) 16:26, 8 May 2017 (UTC)[reply]

Maybe get ahold of some super black? Or maybe use a more solid background, such as a piece of plywood painted in matte or flat black? Just some ideas. --Jayron 32 16:42, 8 May 2017 (UTC)[reply]

Velvet is the right idea, but most velvets are designed to shine, so they're made from a fabric with fibres that are individually shiny. It doesn't usually look grey, more of a pearlescent sheen mixing black and quite light shades. A cotton velvet might do it, or even a moleskin. With a budget, you can simply buy backcloth fabrics that are made to be an invisible black. In the fabric shop though, you're probably looking at dull non-sheen cottons, rather than velvets. Once you're far enough away to not see the weave, it matters less what the fabric is. Andy Dingley (talk) 17:05, 8 May 2017 (UTC)[reply]

Velvet also shines more when seen at an angle. You can get a small piece and experiment before you buy enough for an entire set. μηδείς (talk) 17:16, 8 May 2017 (UTC)[reply]

Some photographers use the lining from blackout curtains. This may require 2 layers if the weave is more open than you wanted. DrChrissy ^(talk) 17:49, 8 May 2017 (UTC)[reply]

The lining of my blackout curtains seems to be a shiny silver color. It is opaque but not at all nonreflective. CodeTalker (talk) 21:50, 8 May 2017 (UTC)[reply]

When I've done black backdrops for photographs (IANAPP so take this advice with what it is worth), I've just used off-the-shelf black linen sheets from a big box store, something like thus. No idea if that serves your purposes or not. You may want to actually go to the store to look at it rather than order online to see if it meets your needs. --Jayron 32 17:52, 8 May 2017 (UTC)[reply]

Want the ultimate non-reflective coating (price is no object)? See: Vantablack & [5] — Preceding unsigned comment added by 107.15.152.93 (talk) 21:12, 8 May 2017 (UTC)[reply]

The professionals simply use a green or blue screen aka "colour-separation overlay" for such purposes. --Kharon (talk) 02:37, 9 May 2017 (UTC)[reply]

The OP does not explain the purpose of the black backdrop. An analog video signal can never represent absolute black, it has only a systematic "black level" that is limited by Signal-to-noise ratio and Lens flare. Chroma key as Kharon says is an effective way to replace a background with any desired color (or a background image) but depends on having en evenly lit rear wall in a uniform color that does not occur in the main subject. A Test card of physical, not digitally generated type, for TV cameras was made by Marconi to calibrate a wide range of brightness; the black segment of the grey scale was actually a porthole to a black velvet lined box constructed behind the card. Blooteuth (talk) 11:34, 9 May 2017 (UTC)[reply]

Personality types.

This article [6] explores an interesting concept that people can broadly be put into 4 different categories. However, can it be argued that this is too broad even for managing relationships in the workplace? Because this could differ based on different cultures (whether organisational, related to a specific profession) or even depending on a person's mood or period of their life. In addition, I can think of many personality types which don't fit into these 4 categories. For example, where would you put the competitive type who likes to play strategic mind games with others, often prevalent in macho organisations? 2A02:C7D:B953:4700:4F9:B85D:C8AB:CF4F (talk) 17:31, 8 May 2017 (UTC)[reply]

Wikipedia has an article on Personality. Perhaps you would like to look a that first and then come back with a more specific query. Dmcq (talk) 17:36, 8 May 2017 (UTC)[reply]

Alfred Adler would have strongly disagree with such rough categories while Sigmund Freud always seemed to like (intellectually) sorting individuals into just a few big boxes. The resulting two main- and some other, newer, different "schools of Psychology" are the base for Science till today. So if you would ask your question at two different (real) Experts you likely would end up with 3 opinions nevertheless. --Kharon (talk) 02:26, 9 May 2017 (UTC)[reply]

Jung said 16 types. Five factor model would say 32 black or white types and more gray types. Sagittarian Milky Way (talk) 02:57, 9 May 2017 (UTC)[reply]

US energy efficiency drop

In both 2011[7] and 2016[8], US used approximately 97 quads of primary energy. In 2011, 41.7 quads went into energy services, a 43% efficiency. While in 2016 only 30.8 quads went into energy services, resulting in an efficiency of only 32%.

With all the talk about electric cars and energy conservation, I expected a slight improvement in overall energy efficiency. But this is a eleven percentage points drop in energy efficiency. In just five years! What's going on here? ECS LIVA Z (talk) 18:37, 8 May 2017 (UTC)[reply]

The change to energy-efficient light bulbs in homes may be a big factor, if not the biggest. ref - Akld guy (talk) 20:07, 8 May 2017 (UTC)[reply]

So energy efficiency should be going up then, right? I'm asking why instead it's dropping like a rock. ECS LIVA Z (talk) 20:11, 8 May 2017 (UTC)[reply]

I don't know the answer, but I notice that the biggest difference between those two charts seems to be the efficiency of the "industrial" block.

Perhaps something changed there. (Obvious question : Is this a real effect, or a change in how those numbers are reported?)ApLundell (talk) 21:37, 8 May 2017 (UTC)[reply]

You do realize that electric cars are highly inefficient. The batteries are hugely expensive, the charging networks the same, and the electricity still comes from burning hydrocarbons. This is simply hid from the end-use consumer, since he doesn't see the huge subsidies the car makers get and the coal plants and transmission wires losing energy to heat as they carry the wattage of huge distances. But think of how efficient they are at virtue signalling!

The description under the charts points out that these numbers are estimated. Also such "global" numbers actually reflect the efficiency of the energy market, not of the technology used. However energy services are generally becoming less efficient (economically!) because a service network always needed to be balanced, or more specifically power supply always needs to be regulated according the power demand, and since the socalled renewable energy production produces energy independent of demand and unpredictable, yet has (politically given) priority to be used, the conventional electric power production is shut off more frequent the more renewable energy takes its increasing share of the total. This is one possible but not necessary the main reason for these numbers. Another could simply be that, besides renewable, the main production switches to whatever natural resource is the cheapest. Given the changes on the market for crude oil, gas and coal this can change these numbers too. And last not least allot of energy is "wasted" because it needs to be transfered. Even the advanced 800 kilovolt High-voltage direct current lines have a loss rate of 4%/1000km. Ofcourse local low voltage nets have a much higher loss rate and on top this all adds up in total. --Kharon (talk) 01:44, 9 May 2017 (UTC)[reply]

Fixed your link. --76.71.6.254 (talk) 05:30, 9 May 2017 (UTC)[reply]

Looking at the flowcharts, it's clear that electricity wastage is less of a factor - the rejected energy directly from electricity generation goes down, and the rise in rejected energy from industry is greater than the input to industry from the electricity generation. So a large chunk of the efficiency drop off is probably due to the direct industrial uses of natural gas and petroleum. For residential use, the change in efficiency might be related to electricity wastage, but that's a smaller contribution to the total waste.

Reading the notes gives a better picture - the 2016 flowchart is calculated assuming 65% energy efficiency for residential and commercial, and 40% for industrial (based on the DOE's estimate of manufacturing in 2017). For 2011, those three sectors are calculated assuming 80% efficiency. So the efficiency on the chart has gone down because the estimated efficiencies used to calculate it have been changed - and to see the reasons for that, you'd need to find which DOE report the revised estimates are based on. MChesterMC (talk) 09:38, 9 May 2017 (UTC)[reply]

As MChesterMC says, it's really a bookkeeping issue. The primary inputs to the users are easily measured while the user efficiency is estimated, not measured. For sure, LEDs use roughly 1/10 the power of incandescents for the same amount of light, but was electricity wasted by incandescent bulbs counted as "rejected" or not? -Arch dude (talk) 19:45, 9 May 2017 (UTC)[reply]

Agree with MChester - note that it says the estimates were "updated" in 2017, and the 2016 estimates were all 4/5 or 1/4, which means they were probably really handwavey numbers. So I'm not sure anything actually changed at all, and if it did, it probably changed over much more than one year, and if it did, it might be something like that light bulb case mentioned. Wnt (talk) 01:07, 10 May 2017 (UTC)[reply]

Do we use electricity to produce gasoline from CO2 or hydrocarbons to produce electricity? μηδείς (talk) 01:18, 10 May 2017 (UTC)[reply]

Not yet, but science or to be more precise Industry is very, very interested in developing such processes because then they could keep using cheap (or to be more precise almost "free"(when they already payed for the licence to exploit an available source)) fossil fuels without the increasingly threatening perspective of paying more and more carbon taxes or for carbon pollution certificates. --Kharon (talk) 01:54, 10 May 2017 (UTC)[reply]

- 1) We use hydrocarbons (methane, i.e., natural gas) and just carbon (coal) to produce electricity. These charts show that we are shifting from coal to methane, which produces less CO₂ for the same amount of electricity. All the other electricity numbers change only a little.
- 2) There is no economic incentive to produce liquid fuel from electricity in general. There is one major exception. The US Navy would really like to produce jet fuel and ship fuel from electricity and/or steam generated by the nuclear reactors aboard aircraft carriers. The carrier's time on station is limited primarily by the fuel needed by its aircraft and escorts, so they are actively researching this. -Arch dude (talk) 22:48, 10 May 2017 (UTC)[reply]

Animals swallowing live prey

I saw a video of an African Bullfrog swallowing a mouse whole without chewing or anything. The mouse was still alive without even a scratch as it was swallowed. How come the mouse can't attack the insides of the Bullfrog and escape? Isn't swallowing live prey that can attack your insides very dangerous? How long would the mouse be alive down there and what would be the cause of its death?--83.136.45.50 (talk) 21:50, 8 May 2017 (UTC)[reply]

I once drowned a mouse, and it took 12 seconds for it to stop moving. (I wasn't trying to be cruel, but it was caught in a glue trap and this seemed to be the quickest way to stop it's pain.) But I've wondered myself why animals which are prey to such animals that swallow them whole don't evolve a defense against being swallowed whole, like porcupine spikes, pointed in all directions. StuRat (talk) 22:19, 8 May 2017 (UTC)[reply]

Put the mouse in a plastic bad and step on its head. Death in under a second, and then discard the bag. Drowning is much crueler, μηδείς (talk) 23:04, 8 May 2017 (UTC)[reply]

I was right by the sink, and it would have taken longer to get a plastic bag. StuRat (talk) 23:31, 8 May 2017 (UTC)[reply]

In small apartments in NY it's common practice to hang the day's small plastic shopping bag from the doorknob for use as a teporary trash depositso it can be taken out to the building's garbage when next you leave. In any case, I would discount the stuck time from the equation as irrelevant, and prefer One hour and 12 seconds of being glued, then an instant, imperceptible death, to an hour of being glued then 12 seconds of being drownt. μηδείς (talk) 01:14, 9 May 2017 (UTC)[reply]

And here I thought a trash bag on the doorknob meant "Don't come in, I have a trashy woman in here". StuRat (talk) 16:53, 9 May 2017 (UTC) [reply]

Animals do develop defences against being swallowed whole. There are species of catfish that have sharp spines that evolved to get stuck in the predator's throat (I'll find a link in a minute, I forgot the binomial for it but it'll come back to me soon enough). There are rough-skinned newt species that have TTX toxin in their skin secretions: they've been documented to kill a toad and climb back out from toad's mouth intact. Finally, there was a paper recently about a typhlops surviving a trip through a toad and coming out alive from the other end (with pictures - NSFW). Dr Dima (talk) 22:51, 8 May 2017 (UTC)[reply]

Here: "‘Fantastic Voyage’: a live blindsnake ... journeys through the gastrointestinal system of a toad" [9] Dr Dima (talk) 22:55, 8 May 2017 (UTC)[reply]

Oh, and also check out porcupinefish, of course. Dr Dima (talk) 23:04, 8 May 2017 (UTC)[reply]

The mouse is effectively crushed, stopping its lungs and perhaps its heart, while the other prey are smaller and cold blooded. This goes along with recent research that says constrictors kill by stopping the heart, not suffocation. Notice the bullfrog even closes its eyes to help force down the mouse. http://news.nationalgeographic.com/2015/07/150722-boa-constrictors-snakes-animals-science-kill/ μηδείς (talk) 23:02, 8 May 2017 (UTC)[reply]

Humans swallow live prey, sometimes without much chewing. See raw bar. --Jayron 32 00:42, 9 May 2017 (UTC)[reply]

That's where "live raw shellfish are shucked and served". The shucking part is what many animals which eat their prey live might have trouble with, lacking the tools for the job. (Those which have the tools general tear their prey into bite-sized bits before swallowing.) StuRat (talk) 00:45, 9 May 2017 (UTC)[reply]

Animals have their means of shucking. Aeschylus#Death is insightful. Also, krill are eaten live by baleen whales. No shucking involved, just scoop them up by the thousands. --Jayron 32 00:49, 9 May 2017 (UTC)[reply]

See Jonah and [10]. 195.147.104.148 (talk) 20:45, 9 May 2017 (UTC)[reply]

May 9

DIY teleprompter glass plate angle

Online many sources say that 45° is the appropriate angle to position the glass used to reflect a screen [in a teleprompter] but I want to know if that's really true and why? Does it make a difference if plastic is used instead of glass (i.e. does the refractive index matter)? Will a different angle reduce the brightness of the reflected light? --78.148.99.149 (talk) 14:05, 9 May 2017 (UTC)[reply]

Wouldn't the text on the teleprompter be elongated or foreshortened if the angle was changed ? You could always compensate for this with the display device, but why not keep it as simple as possible ? StuRat (talk) 15:39, 9 May 2017 (UTC)[reply]

The refractive index does not have a direct impact, per the laws of reflection. However, the coefficient of specular reflection, which you want to maximize, will depend both on the material and the angle.

Total internal reflection may also be of academic interest (but since the "outside" medium is air, glass/plastic have a higher refractive index, it cannot happen in that case). Tigraan^{Click here to contact me} 15:47, 9 May 2017 (UTC)[reply]

Yes, 45° is generally best, because of the law of reflection. See teleprompter, and look at the arrows in the diagram. If you angle the glass or beam splitter more than a few degrees from 45°, then the blue arrow will shift angle up or down, and the image of the prompted words will go above or below the speaker's head. SemanticMantis (talk) 17:40, 9 May 2017 (UTC)[reply]

It wouldn't have to be 45 degrees if that screen weren't flat - you could make a sextant out of the thing and shoot the speaker with it (albeit, alas, only navigationally). Wnt (talk) 01:09, 10 May 2017 (UTC)[reply]

The refractive index does have a direct impact on the intensity of the image - consider what would happen if the refractive index of the glass was reduced to unity. Also, the image will be polarized - see Brewster's angle. --catslash (talk) 23:36, 10 May 2017 (UTC)[reply]

Feynman Lectures. Exercises PDF. Exercise 1-8 JPG

...
1-8. Explain qualitatively why and how friction in a moving machine produces heat. Explain also, if you can, why heat cannot produce useful motion by the reverse process.

— R. B. Leighton , Feynman Lectures on Physics. Exercises

...
1.8. No matter how polished the rubbing surfaces of the machine, they will never be completely smooth. In reality, these surfaces can be imagined to be covered (albeit very small) by tubercles, steps, etc. When such surfaces move relative to each other, many microscopic impacts occur, the unevenness crumples one another. As a result of such random impacts, the velocity of chaotic motion of molecules eventually increases (in Ch. 2 it is shown that when a molecule strikes a surface moving against it, the velocity of the molecule increases). And this means that rubbing bodies heat up.

— MEPhI , Solutions (Google Translate)

MEPhI has given solution only for the 1st part of the exercise. Is there a way to solve 2nd part? In Lecture 44 Feynman says with no proof that Second law of thermodynamics forbids converting heat to work. I wonder if there is a way to understand it assuming all we know is the atomic theory from Lecture 1?
I assume the direct process: a steel body (mass = 1 kg; speed = 1 m/sec) glides by horizontal steel surface until stops in 1 sec, heating itself and the surface by 0.5 J.
Reverse process: the body consumes the heat and accelerates.
I have only one idea - the atoms' motion becomes chaotic and can't be extracted anymore. But at the first moments atoms' motion is more or less ordered in the trajectory direction. Is it correct?
Username160611000000 (talk) 17:26, 9 May 2017 (UTC)[reply]

This goes back to things like Carnot's theorem (thermodynamics) and the Carnot cycle; statement 2 is actual only correct if properly qualified: you CAN use heat to do work, that's what a heat engine does, and there are probably several such devices within walking distance of you right now. You just can't use heat to do work with 100% efficiency; that is the joules of heat energy input into any heat engine will always exceed the joules of work you can get out. Also, the key word is reversible above. Friction is an irreversible process: you can't re-order a system which has become more disordered without expending energy, energy so lost is one perspective on what is meant by entropy, and since systems can become disordered without any input of energy, but require energy to become reordered, that's the basis of the second law of thermodynamics. --Jayron 32 18:41, 9 May 2017 (UTC)[reply]

What Jayron said. An easy way to visualize this is two clock gears rotating in one direction in a plane. They create heat, the vibration of the molecules in the cogs as the rub while rotating in one direction. The heated molecules then vibrate back and forth in three dimensions. This random motion can not be converted back into one-directional linear motion. See also Maxwell's demon. μηδείς (talk) 19:32, 9 May 2017 (UTC)[reply]

The gear is complicated system. Let's better consider body on horizontal surface. When the body passes it hits atoms of the surface in direction of motion. Atoms vibrate. Since the lattice is periodic these vibrations must repeat. So if we push a body backwards it can be driven by atoms like by a wave. Username160611000000 (talk) 20:27, 9 May 2017 (UTC)[reply]

I seriously can't believe that you think two cog wheels is so complicated, but the principle is the same: WORK UNIDIRECTIONAL. HEAT RANDOM. μηδείς (talk) 01:13, 10 May 2017 (UTC)[reply]

@Medeis: You use the trick to prove the random motions -- many changing contact surfaces. Consider one atom within the upper layer of the lattice. Atom is kicked in some direction (e.g. normal to the surface, in z-axis direction). Adjacent atoms are kicked in same z-direction. How will the atom obtain motion in other 2 axes (x, y)? Username160611000000 (talk) 05:01, 10 May 2017 (UTC)[reply]

Yes, but every collision will not be orthogonal. If we establish the "z axis" along the line of striking of atom "A" to atom "B", and assume that is the overall motion of the larger objects "A" and "B" are part of, then relative to that axis, yes, that one collision will not (by definition) move atoms in the x or y dimensions. Now, without moving that axis system, look at the positions and relative motions of every other atom in the system. Almost surely none of them will lie on that "z" axis . SO every other collision results in particles moving in some other direction, even though the overall motion of the larger objects remains unidirectional. That's because the position and motion vectors of individual atoms was stochastic to begin with; so the interaction of randomly moving particles (the individual atoms) produces random motions themselves. Even though the objects rubbing together are moving in a defined direction. From a statistical thermodynamics point of view, the interactions between two random systems MUST produce more randomness (i.e. there are more states for the system to exist in, because now we have new states introduced by the new interactions). More states = more entropy (S=k log W) Q.E.D. --Jayron 32 14:38, 10 May 2017 (UTC)[reply]

Thanks, Jayron. I would not have had the patience to respond to a user who says I am using a "trick" when this is material covered in high school chemistry and physics. μηδείς (talk) 17:29, 10 May 2017 (UTC)[reply]

Yes, we also have articles on friction and irreversible process that may help OP. SemanticMantis (talk) 20:31, 9 May 2017 (UTC)[reply]

Thermodynamics is only right in general. While friction causes microscopical Tension (physics) and Compression (physics) and therefor vibration and therefor heat there are infact machines or mechanical sets that dont cause any friction at all. The Industry offers Linear actuators that work by High-temperature superconductors for some time now for example. Additionally the assumption that energy (force) can not be recuperated from friction is wrong for the field of Piezoelectricity. Btw. to be precise a carnot engine is not really a "heat engine" because it actually just transforms mechanical motion from the expansion and contraction of the medium that is heated on one and cooled on the other side. Just like a steam engine gets its motion from the steam pressure, not the steam heat. --Kharon (talk) 01:39, 10 May 2017 (UTC)[reply]

Well, you're just playing games with semantics. Heat is not work, but heat can cause changes to the world that are themselves work. Saying "Steam pressure is not steam heat" is misleading, yes, it's the pressure that moves the pistons, but the pressure increased because the molecules of water increased in temperature, and that increase in temperature is due to the heat transferred to the water molecules by burning coal. You're rationale is like saying "John F. Kennedy wasn't killed by a bullet, he was killed by blood loss and brain damage." No shit, blood loss and brain damage caused by a bullet. Likewise, steam pressure caused by heating water. Heat generally is understood to take three forms: conduction, convection, and radiation. All three of these forms can be used to cause an object to move, the basic definition of Work (physics). Consider the following three scenarios:

Heat is conducted into a metal, and the metal expands. The expanding metal can be used to push an object. That's Work
Two rooms with a doorway between them have air at different temperatures. A fan is placed in the doorway, and convection currents turn the fan blade. That's Work
Radiant light energy strikes a photovoltaic cell which provides electricity to run a motor. That's Work

Every one of those examples shows heat causing work to be done. It doesn't matter that it takes one, or two, or three additional steps; energy is changing forms, and it's in the form of heat at an earlier point in the chain of changes than it is in the form of work in a later stage. That's all that matters. The reason the second law of thermodynamics works is that heat is always stochastic whereas work is always directional, indeed fundementally that's why when heat is generated, some free energy is always lost, if you're trying to move an object along the X-axis, only particles with some forward motion along the X-axis will strike it in such a way to move it; particles in motion in other directions don't strike the object and move it forward. That's what Medeis is trying to explain; the process is not reversible because you cannot get back to your original conditions without any input of outside energy. --Jayron 32 13:45, 10 May 2017 (UTC)[reply]

Where's the highest railway? (above the surface, not sea level)

Where's the highest that doesn't kindof cheat by crossing a valley/canyon that's narrower than it's deep or not much wider? Or like a viaduct that's not the highest in the world except for that one bit where there's steep crack or canyon. Or a railroad that happens to go over a cave, seabed or other surface below the surface. Sagittarian Milky Way (talk) 21:31, 9 May 2017 (UTC)[reply]

List of highest bridges (height to bridge deck) and List of tallest bridges (height to top) may have the answer, but not sure if they list which are for trains. StuRat (talk) 21:47, 9 May 2017 (UTC)[reply]

The list does indicate whether road or rail, making the Najiehe Railway Bridge the one to look at. Wymspen (talk) 11:27, 10 May 2017 (UTC)[reply]

https://www.google.com/search?num=100&newwindow=1&site=&source=hp&q=highest+railway+bridge+of+world&oq=highest+railway+br&gs_l=hp.3.5.0l10.3883.9316.0.14377.19.15.0.2.2.0.841.3766.2-6j3j0j1j1.11.0....0...1c.1.64.hp..6.12.3534.0..0i131k1.w_GXR7_xCB0#spf=1. μηδείς (talk) 01:08, 10 May 2017 (UTC)[reply]

May 10

Several siblings, all male or female

The Boy or Girl paradox article led me off onto a vaguely related train of thought. Imagine that a couple has had six or seven children together, and all of them are girls or all of them are boys. If they have a seventh-or-eighth child, is the baby more likely than not to be of the same sex as its siblings? I'm wondering whether in such cases certain biological factors could come into play, since this isn't merely a coin flip: for example, the father's sperm production is unevenly balanced one way or the other (so it's not a 50-50 chance at conception), or the mother's somehow physically more likely to miscarry one sex than the other (so sex distribution at conception doesn't match the sex distribution at live birth). Or are such families (assuming no intentional abortions, post-birth deaths, etc.) basically just the result of chance, since approximately 1/32 of all six-children families can be expected to have children of just one sex? Nyttend (talk) 04:59, 10 May 2017 (UTC)[reply]

There are at least three mechanisms at play that may lead to deviation from an exactly 50-50 chance of either sex. First, human sex ratio is not 50-50, but rather the chance of a boy being born is slightly higher than 50%. This may be because the Y-sperm is slightly lighter (and therefore more mobile) than X-sperm, or due to a number of other mechanisms some of which are discussed in the human sex ratio article. Second, families may "want a boy" or "want a girl", in which case they are more likely to stop an the minority sex (n boys 1 girl or n girls 1 boy) rather than at the majority sex (n boys 0 girls or n girls 0 boys). Third, as you say in the question, there are genetic mechanisms that may cause substantial deviations from 50-50 in affected families. I'll see if I can find some more-or-less accurate numbers to quantify these effects. Dr Dima (talk) 07:47, 10 May 2017 (UTC)[reply]

To the extent that sex at birth is random, Dima's second point is fallacious. Say that every day you start flipping a fair coin and continue until it comes up tails. You will see sequences of T, HT, HHT, HHHT, etc., but it's still true that most probably 1/2 of the total flips will be heads. The longer the run of heads, the less probable it is. Well, similarly with births, if they are random. However, this is really irrelevant to the original poster's question, which is asking if the births are random. --76.71.6.254 (talk) 10:17, 10 May 2017 (UTC)[reply]

I appreciate your comment, but I'd like to point out that you misunderstood my statement. Consider a process where coin is tossed until both heads and tails occur at least one. For such a process, all 6-toss sequences will have either 5 heads and 1 tail or 5 tails and 1 head; indeed, having 6 heads will result in a more than 6-toss sequence, until a tail occurs. I interpreted the OP question "... since approximately 1/32 of all six-children families can be expected to have children of just one sex?" to refer to exactly 6-children families, in which case my statement above is valid, and not fallacious. Dr Dima (talk) 16:03, 10 May 2017 (UTC)[reply]

Agreed, by that interpretation it's valid. --76.71.6.254 (talk) 19:04, 10 May 2017 (UTC)[reply]

If we do a Fermi calculation, and ignore the fact that it isn't really 50-50 as noted above, let's assume it is 50-50. The answer to your question " is the baby more likely than not to be of the same sex as its siblings?" the answer is that the 8th baby has a 50/50 chance of being the same sex as its prior siblings. If there were a 9th baby, it would have a 50/50 chance of being the same sex as its prior siblings. Odds don't have a memory, so each child has a 50/50 chance of being one gender or the other. Now, the probability of one mother having, say, nine boys (again, assuming a true 50/50 chance rather than the reality which is slightly different) is 1 in 2⁹ or 1 in 512, which is only a 0.195% chance. Though not impossible; my grandmother's sister had nine children, all boys. --Jayron 32 10:33, 10 May 2017 (UTC)[reply]

Whilst I applaud the rationality, I have a sneaking suspicion that a given womb-host-system may be slightly more allergic to one sex than the other. So it isn't a pure coin flip. Greglocock (talk) 10:55, 10 May 2017 (UTC)[reply]

But as the OP says, the question is whether it is 50-50 and by how much it can devy. It is a Bayesian inference problem, but we only have the observation that most children are of either sex, and the prior probability distribution (is "50-50" a good model?) is hidden in a giant heap of biological and environmental data.

If you play dice and observe that certain numbers come up more often than others, external clues can give you more confidence that the dice are loaded - the same sequence could be attributed to chance in a high-security Monaco casino, and to loaded dice in a Chicago suburb mafia-run pub. The question is how strong these external clues are in the case of pregnancy - by how much can genetic changes improve odds of either sex and how likely those genetic changes are. Tigraan^{Click here to contact me} 11:07, 10 May 2017 (UTC)[reply]

I agree that the OP is correct that the are some circumstances when the odds aren't really 50-50 (as with others, let's ignore they are never actually 50-50), but the probabilities of these are likely so low that even with 8 children of one sex these cases will still be lost in the noise i.e. nearly all such cases will just have been due to complete random chance. Nil Einne (talk) 11:29, 10 May 2017 (UTC)[reply]

Further complicating these calculations is that estimates range from 25 to 40 percent of pregnancies that result in miscarriage. (Google the subject and you'll see.) So unless the sex of the miscarried embryos is known, you can't really be certain that an apparent trend of all one sex is really a trend. It could be like XmmmXmmmXm for the case of seven successful pregnancies yielding males. If the X's were all female, that might suggest a sex-related problem, but it could also be coincidental. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:00, 10 May 2017 (UTC)[reply]

Empirical evidence: About 30 years ago I collected extant data (sorry I can't remember where) on sex of a child as a function of sex distribution of a couple's previous children. I found that there is a mild tendency for couples with more children of one sex than of the other to have their next child be of the majority sex. If I recall correctly, having several children all of one sex increased by several percent the probability that the next child will be of that sex. The context was the question of what effect perfect advance selectability of children's sex would have on number of children per couple. Loraof (talk) 14:01, 10 May 2017 (UTC)[reply]

The article Sex ratio has a good discussion, including the passage sex ratios at birth may be considerably skewed by factors such as the age of mother at birth[3]. Also, the article mentioned by Dr. Dima above, Human sex ratio, has a good section called "Natural factors", which says These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress. Loraof (talk) 14:12, 10 May 2017 (UTC)[reply]

Many decades ago I came across (in the New Scientist etc.,) of couples being able slew the chances of getting a boy or girl -by diet alone. Hans Eysenck noticed that airline crew had more daughters. Well back then, little girls did not grow up to be airline pilots -like dad, so dad wanted a little boy ( or two tow) follow in dads footsteps. And hey what?!!! They fathered more little boys when their wives followed the diet. Didn’t think much of it at the time but now as the question has arisen- have googled it. Lots of references that this belief continues. (i.e., https://www.everydayfamily.com/gender-selection-what-is-your-personal-ph/) It makes some biological sense as well. Aspro (talk) 17:33, 10 May 2017 (UTC)[reply]

What does it mean "Negative logarithm" of the concentration of hydrogen ions.

I've read on this site about the definition of pH as follow: "pH stands for potential hydrogen, and is defined as the Negative logarithm of the concentration of hydrogen ions." I understood whole the sentence except of "negative logarithm". what does it mean? 93.126.88.30 (talk) 17:05, 10 May 2017 (UTC)[reply]

Wikipedia has an article on pH which explains this. The logarithm usually turns out negative. The pH value doesn't include the minus sign, so an alternative definition is "the logarithm of the reciprocal of the concentration of hydrogen ions". Ask again if you need more help. Dbfirs 17:30, 10 May 2017 (UTC)[reply]

Thank you. Iv'e checked this article and honestly I still don't understand. There is a sentence there which says: "For example, a solution with a hydrogen ion activity of 5×10−6 = 1/(2×105)", then I don't understand how existence can be represented by minus (=no existence). 93.126.88.30 (talk) 01:16, 11 May 2017 (UTC)[reply]

Negative numbers do not mean "nonexistent". If you have a sample of pure water at STP, autoionisation will ensure [H⁺] = 10^–7 mol·dm⁻³. By the definition of the common logarithm, log [H⁺] is the exponent 10 must be raised to to give 10⁻⁷, which is clearly −7. pH is defined to flip the sign and make it +7, because most common acid solutions have nowhere near a concentration of protons of 1 mole per litre. That's why it's not the logarithm, but rather the negative (additive inverse) of the logarithm. Double sharp (talk) 01:28, 11 May 2017 (UTC)[reply]

For negative powers of ten, our article Scientific notation might help. Dbfirs 08:27, 11 May 2017 (UTC)[reply]

I'll try this from scratch. Positive logarithms are used for increasing quantities of substances: 10 balls is 1 log unit, 10000 balls is 4 log units. Negative logarithms are convenient when the quantities are really small: a concentration of 1 in 10 is -1 log units, 1 in 100 is -2 log units, 1 in 10,000,000 is -7 log units. That's because these concentration numbers would be written as fractions (1/10,000,000 = 0.00000001 in the last example, which is typical for neutral pure water.) For pH, we also assume specific units - in general, a concentration recipe could be something like 1 teaspoon in 10 cups of porridge, but for pH we're defining it to be mole (unit) per liter, i.e. molarity, more or less. (There are unimaginable levels of pedantry available about exactly how pH is defined, pH vs. p[H+], activity (chemistry) etc. - not even most scientists have any desire to know these details, but fortunately their effect is relatively small most of the time. A basic college chem course will use the definition I just gave and stay mum about the rest.) Wnt (talk) 12:49, 11 May 2017 (UTC)[reply]

As more and more Hydrogen ions exist the more acidic it is?

I've read the information about pH here, then can I conclude that as more and more Hydrogen ions exist the more acidic it is? the more acidic it is, or it is better to say that as more and more the cations exist the more acidic it is while the more and more anions exist the more basic it is? 93.126.88.30 (talk) 17:21, 10 May 2017 (UTC)[reply]

Read Ocean acidification. --Kharon (talk) 20:50, 10 May 2017 (UTC)[reply]

Thermodynamics of a permanent magnet

Does a permanent magnet's field weaken every time it is used to pick up a paperclip from my desk? It is after all doing work in lifting a mass, thus there must be waste heat. Roger (Dodger67) (talk) 18:50, 10 May 2017 (UTC)[reply]

No. Why is it, you think, they called it "permanent"? --Kharon (talk) 20:49, 10 May 2017 (UTC)[reply]

Proton decay. Sagittarian Milky Way (talk) 21:36, 10 May 2017 (UTC)[reply]

This is a red herring. ApLundell (talk)

Nothing with such a high percent of protons in it may be permanent. Of course that is irrelevant to whether the work wears out the magnet or just come from the person lifting the magnet and proton decay takes longer than human timescales to possibly happen. Sagittarian Milky Way (talk) 22:33, 10 May 2017 (UTC)[reply]

No. This question is a based on a very common misunderstanding of how magnets work. They do not expend energy that must be replenished.

They are simply converting potential energy to kinetic energy and back. Similar to moving a weight up a heavy hill and letting it roll down. The Earth does not wear out its gravity field when things move up and down.

This common misunderstanding often leads people to believe they have an idea for a perpetual motion machine. (They don't.)

As a practical matter, some magnets do weaken over time, but that's unrelated to the 'energy' they seem to expend. (But don't really.)

ApLundell (talk) 21:58, 10 May 2017 (UTC)[reply]

Was the comment way up above about proton decay intended to answer the very reasonable question, or was it an attempt at humor? And the claim that the magnets are "permanent" is also untrue, since they weaken over time. Horseshoe magnets in science demonstration sets came with an iron "keeper" with instructions to leave the keeper across the poles to reduce the loss of magnet strength over time. If I use a permanent magnet to an iron weight some distance, as in when the object flies up from a desk to the magnet, travelling a small distance through the air, work has certainly been done by the magnet, and the energy has to come from somewhere. Edison (talk) 00:17, 11 May 2017 (UTC)[reply]

Proton decay is mostly humor. Maybe insufficient coercivity or other things like orbiting the galaxy till hitting a black hole would demagnetize anything before the protons decay so if that's proven would only be a weak disproof of permanence by contradiction. Sagittarian Milky Way (talk) 01:43, 11 May 2017 (UTC)[reply]

Please do not make jokes or other pointless statements that seem like they might be answers to the question, but definitely are not.

It is very confusing to people asking questions. ApLundell (talk) 18:57, 11 May 2017 (UTC)[reply]

The question here covers a lot of different things which are peripherally related.

A magnet does work in picking up a paperclip, but the amount of work done is quite small, and the energy must be put back into the system from an external source if you expect to get your paperclip back.
In theory, a magnet might not need to produce much waste heat. For example, you might set up a little plastic track near a magnet and roll a ball along it as if you were rolling it in a dish under the effect of gravity. In such a situation the ball might go back and forth quite a few times, dissipating relatively little energy in rolling friction on the track, before losing all its kinetic energy and coming to the lowest potential energy state at the closest point on the track.
That said, a conductor such as a steel ball moving through a magnetic field is going to experience some kind of eddy current, which will produce a sort of "friction" not seen with a ball in a dish. The current will tend to oppose the motion and slow the ball sooner than it would have otherwise, I think.
There is energy in the magnet that can be lost if it loses its magnetization. To illustrate this, consider what happens if you have a magnet you're screwing around with and you break it. I suppose if you break it perpendicular to the N-S line then the north pole of one piece will stick to the south pole of the other, but definitely if you break it the other way you'll have two N-S magnets next to each other that want to end up flipped relative to each other. Well, magnets are made of a whole lot of little paramagnetic atoms (or in theory molecules, I think) which in ferromagnetism form up into large domains where they are all lined up the same way for various reasons; nonetheless, beyond a certain scale the force of the opposing field will flip them if you simply heat up the magnet and cool it again, leaving you with an ordinary piece of metal or ceramic. You can also ruin a magnet by bashing on it hard enough, I think.
Which brings me to the interesting center of your question: I'm not entirely sure if the small magnetic field induced in an object approaching the magnet due to eddy currents will have any incremental effect on the alignment of domains in the magnet. After all, these are very small regions which, by random chance, will tend to lose their alignment anyway over time. I suspect the effect of that and even the vibration of the paperclip hitting the magnet might truly be insignificant relative to other effects, but I don't know that. Wnt (talk) 13:06, 11 May 2017 (UTC)[reply]

Wnt's language may make someone believe that you can break the "north" and "south" poles of a magnet. You can't. Tigraan^{Click here to contact me} 16:45, 11 May 2017 (UTC)[reply]

Why do fat people have a lower pain threshold?

We can read here: "An extra layer of fat won't provide a cushion against pain -- in fact, obese people are more sensitive to pressure pain than those who are not overweight, and they are equally susceptible to extremes of hot and cold."

The explanation offered in the article didn't sound all that credible to me:

" "It could be the case that a person who is more sensitive to pain is less likely to do physical activity and therefore more likely to gain weight and become obese," says Dr Tashani."

You would then expect the result to apply to just a fraction of the obese people, if their results then affect the average of the entire group of obese people, you would end up with a significantly larger standard deviation in that group. Count Iblis (talk) 22:14, 10 May 2017 (UTC)[reply]

I think the important comment is: "The team plan to carry out further research into the factors that make people more susceptible to pain. This includes examining the chemicals secreted by fatty tissues in the body which could affect the response of pain receptors." The team's results show a possible correlation but not a cause. Dbfirs 23:47, 10 May 2017 (UTC)[reply]

You have given us the source. You ask us "why" and then argue "You would then expect..." It seems like you want us to give our opinions, or validate your conclusion. Is there something else you are looking for? If you want anecdote, I can tell you that having lost some 100lbs, I can now walk barefoot without pebbles and twigs being excruciatingly painful to step on, yet I find I still act as if I am at my heaviest weight. Weighing now what I did in the early nineties I am still very careful about motions that before might have caused injury, but which now are presumably much safer. (For example, I used to take two steps at a time going up stairs, now I put each foot on a step before feeling fror the next one, expecting a fall.) I notice no difference to my reaction to burns or splinters, though. μηδείς (talk) 00:16, 11 May 2017 (UTC)[reply]

If you want a hypothesis off the top of my head, well, POMC mutation causes obesity, and involves the loss of an endogenous opioid chopped up out of that protein. Perhaps that contributes to "runner's high" and also reduces pain sensation in other settings. The mutation I describe also causes red hair, which is not mentioned in your source article and might be useful to look at if the statistic is available. However, the effect of the opioid could also be inhibited if its receptor is less active, or something its receptor sets off is less active, and none of those mutations would have this particular effect. That said -- this is just a random hypothesis and the odds that this is what the researchers come up with is quite low. There are a lot of ways to invoke biochemistry to relate a particular cause with a particular effect - so many things interact with so many other things, it's like looking at a complicated roadmap and trying to say which route a car must have taken. Wnt (talk) 13:14, 11 May 2017 (UTC)[reply]

Universe expansion

I've tried to use Hubble's law to estimate an approximate value of the size of the observable universe and found some difficulties to reach the same value (93 billion light years). If I assume the furthest point where light can still reach us regardless expansion, I realize it must be less than 13.8 Bly, specifically 13.8/e and so, the universe should then expand by a factor of e till light reach us. If I assume an initial size of 13.8 Bly, I realize light cannot reach us due to the fact that expansion is almost the same rate as light speed at that point. Is there something I am missing?Almuhammedi (talk) —Preceding undated comment added 22:35, 10 May 2017 (UTC)[reply]

The visible universe is about 93 billion light years across. In any direction we look we see the cosmic background radiation which is now a little over 46 billion light years away, this is because not only has it been moving away from us over a little more than 13 billion years, the metric expansion of space has also been increasing the overall distance. μηδείς (talk) 00:08, 11 May 2017 (UTC)[reply]

But how will you apply Hubble's law in this case? — Preceding unsigned comment added by Almuhammedi (talk • contribs) 09:23, 11 May 2017 (UTC)[reply]

See Particle_horizon#Conformal_time_and_the_particle_horizon. Dragons flight (talk) 13:13, 11 May 2017 (UTC)[reply]

May 11

Psychology study about the perception of achievement

I know that there is a specific psychology study about scholastic achievement. Basically, there are two groups of students. One group of students is assumed to do very well. The other, no response. Well, the result is that the teacher's expectations can influence the achievement of the students. What's the name of this? 50.4.236.254 (talk) 03:54, 11 May 2017 (UTC)[reply]

This paper has an overview of many such studies. --Jayron 32 10:01, 11 May 2017 (UTC)[reply]

It's the Pygmalion effect or Rosenthal effect. Loraof (talk) 14:52, 11 May 2017 (UTC)[reply]

Convection coefficient for Heat sink Calculation

we know about heat dissipation equation derived for a fins.see.kindly tell me where can i get a data book that specifies convection coefficient values for (aluminium -air) contact at 540^m3/h.If no such kind of data book exists,where/how can we find/calculate h(convection coefficient) value. Thanks in Advance — Preceding unsigned comment added by Sameerdubey.sbp (talk • contribs) 10:13, 11 May 2017 (UTC)[reply]

Convection is a local phenomenon that depends mostly on the fluid properties and its speed (outside the boundary layer) relative to the solid. The convection coefficient itself is relatively independent on the solid (but see Biot number for the competition of convection vs. solid conduction effects). Your "540^m3/h" is not a speed, but probably a flow rate which you will need to divide by the cross-section.

A priori evaluation of the convection coefficient is deceptively hard. (For instance, this paper about a very rough approximation for it in a very specific case has been cited more than a thousand times.)

[11] gives a correlation between the convection coefficient and air speed:

h[W/m/K]=10.45-v[m/s]+10{\sqrt {v}}

(for speeds between 2 and 20 m/s). But I have no idea where it comes from and what are the conditions for validity, so... Tigraan^{Click here to contact me} 11:38, 11 May 2017 (UTC)[reply]

Elephants in a balanced ecosystem

What kept historically the population of elephants limited? What were their natural predators? --Hofhof (talk) 12:09, 11 May 2017 (UTC)[reply]

Elephant#Ecology_and_activities mentions several predators (primarily taking calves). Also note their slow reproduction. Henry ^Flower 12:34, 11 May 2017 (UTC)[reply]

Predators don't typically limit a prey population much - either the prey can sustain the predators even in lean years, or else the predators would die. (The exception is if the predators are sustained by something else and the prey is really bad at surviving - I mean, if you could let loose a flock of dodos in the woods the foxes and coyotes would figure out something to do about them; then they might be restricted, say, to a few local back yards where the predators are chased off by the owners' mutts. Or in this case, poachers who spend most of the time sticking people up on the road or something but who aren't averse to going in after an elephant if they happen to see one)

The usual problem is food availability: see [12]. The ecologists talking about culling elephants (!) are of course trying to prevent nature from taking its course by converting the ecosystem to the point where very few can survive; in former times perhaps bands of elephants roamed around looking for fresh forests to destroy. The implication from that source seems to be that the overall ecology of Africa (forest vs. grassland) has been impacted by the reduction in elephants! Wnt (talk) 13:24, 11 May 2017 (UTC)[reply]

"Predators don't typically limit a prey population much" - careful. Trophic cascades go both ways. Some systems are under top-down control. Prey switching is the real and common phenomenon where predators seek alternative prey when primary prey is scarce. What is true is that most predators are not highly specialized, and can switch among many prey species. One famous example where single-predator control of single prey species is obvious is the lynx and hare system [13].

Anyway, one class natural controls on elephant populations that is often overlooked is parasites e.g. ([14]) and disease (e.g. [15]). SemanticMantis (talk) 15:36, 11 May 2017 (UTC)[reply]

Let's not forget that elephants are considered keystone species - a species that has a disproportionately large effect on its environment relative to its abundance. DrChrissy ^(talk) 19:12, 11 May 2017 (UTC)[reply]

Why are combustion motors noisy?

Could a combustion motor be silent? Why are fires (kind of) silent, but combustion in a motor noisy? --Clipname (talk) 17:28, 11 May 2017 (UTC)[reply]

Thermoelectric generators are silent. Internal combustion engines are noisy because they have rapidly moving parts moving under relatively high pressure of the combustion products, and because the combustion itself is rapid and periodic rather than continuous. Dr Dima (talk) 17:41, 11 May 2017 (UTC). NB we also have a good article on Aircraft noise, you may want to read that one as well. Dr Dima (talk) 17:47, 11 May 2017 (UTC)[reply]

(edit conflict) Most rely on explosions to provide energy (except for gas turbines, jet engines and rockets which have different sounds). See Internal combustion engine. Dbfirs 17:52, 11 May 2017 (UTC)[reply]

Would an imaginary frictionless combustion engine make no or little noise? Clipname (talk) 18:37, 11 May 2017 (UTC)[reply]

There would still be the sound of the explosions (or turbulent flow). Dbfirs 18:41, 11 May 2017 (UTC)[reply]

Technically deflagrations but by that definition gunpowder also doesn't explode. Sagittarian Milky Way (talk) 18:55, 11 May 2017 (UTC)[reply]

Fundamentally the engines work on sudden, repeated changes of pressure as the fuel burns, pushes down the piston, and the waste flows out the exhaust.

Those pressure changes are almost certainly going to result in noise unless something is done to stop it from happening. Mufflers help, but the vibrations from all those pressure changes is going to be hard to eliminate entirely. ApLundell (talk) 19:03, 11 May 2017 (UTC)[reply]