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January 12

Tooth enamel

If some bit of tooth enamel has been scratched away, is it possible for it to grow back or is it made of a non-living substance? NOTE: This is not a question asking for medical advice. — Melab±1 ☎ 02:30, 12 January 2014 (UTC)

Not surprisingly, we have a Tooth enamel article, which says "Remineralisation of teeth can repair damage to the tooth to a certain degree..." The linked article, Remineralisation of teeth, doesn't help much. HiLo48 (talk) 02:39, 12 January 2014 (UTC)

Heh. The linked to article reads like an advertisement. — Melab±1 ☎ 03:35, 12 January 2014 (UTC)

I am hardly an expert on the subject, but I found this source from the Mayo Clinic [[1]] that says that tooth enamel is inanimate and cannot grow back. Cogito-Ergo-Sum (14) (talk) 03:08, 12 January 2014 (UTC)

It say here "Because enamel has no living regenerative cells, the body cannot repair chipped or cracked enamel on its own under normal circumstances. However, research is now giving us a peak [sic] into the fact that tooth enamel can, in fact, be repaired – given the right environment". Richerman (talk) 14:36, 12 January 2014 (UTC)

Rodents solve the problem by having some teeth which grow continuously. This does cause other problems, though, like the need to constantly wear them down to prevent them from getting too long (similar to our hair and fingernails). It would be wonderful if we had teeth like that, then we could just go to the dentist to have them shortened, whenever they got too long.

Sharks produce an assembly line of teeth, and the old ones fall out. We could also just grow a new set of teeth every decade, that would solve the problem, too, except that we would often look funny with several missing teeth. StuRat (talk) 17:15, 12 January 2014 (UTC)

But if missing (adult) teeth were a normal and frequent circumstance, we wouldn't consider it funny looking. {The [gap-toothed] poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 14:04, 13 January 2014 (UTC)

Well, it's common enough in the elderly, yet we consider it funny looking unless they cover them with dentures. StuRat (talk) 03:01, 14 January 2014 (UTC)

Speed of an electron's rotation

Hi, My friends and I were having a conversation about physics and the topic of the speed of an electron's rotation came up. He argued that an electron orbits the nucleus of the atom faster than the speed of light; I said that such a speed was physically impossible. Which of us was right, and if I was, how quickly does the electron orbit? Thanks!

Cogito-Ergo-Sum (14) (talk) 03:03, 12 January 2014 (UTC)

You are correct that an electron traveling faster than the speed of light would be physically impossible, and there is no exception for an electron in an atomic orbital. However, electrons in inner orbitals of heavy atoms do attain speeds that are a significant fraction of the speed of light; see Relativistic quantum chemistry for details. Red Act (talk) 03:44, 12 January 2014 (UTC)

A fun aspect of this is that the fine-structure constant is the dimensionless ratio between the speed of an electron "orbiting" hydrogen in the Bohr model and the speed of light, about 1/137, which means that roughly around 137 (the article gives an unsourced refined estimate to be 173) you hit the "end of the periodic table". Though apparently it doesn't really end... I doubt you can banish a stray electron all that far from a positive ion even if it can't wrap its little head around it, but I don't know the specifics. Wnt (talk) 04:54, 12 January 2014 (UTC)

Beyond that limit the binding energy exceeds 2 m c^2, which means that the 1s shell will be filled spontaneously by pair creation, leading to an electron moving into that shell and a positron appearing at infinity. Count Iblis (talk) 10:42, 12 January 2014 (UTC)

Actually I was thinking of neutral atoms, and I'm sure you meant to say the positron appears next to the electron but being next to a +172 charge discovers a strong desire to be elsewhere. :) Wnt (talk) 19:40, 12 January 2014 (UTC)

Thank you very much! Cogito-Ergo-Sum (14) (talk) 05:11, 14 January 2014 (UTC)

Why does BETA stands for?

I see beta HCG test, and I know what is HCG stands for, but I don't know the meaning of BETA, what does it mean? Why is it called BETA? (the econd letter of greece) I would like to understand it. Thank you.194.114.146.227 (talk) 09:08, 12 January 2014 (UTC)

See Human chorionic gonadotropin. In general, if you purify out some kind of protein complex from a cell under gentle conditions, you may get several polypeptides that are tightly stuck together. Put them in harsh detergents and separate them (SDS-PAGE, for example) and they can be resolved into multiple bands, which you might call alpha, beta, etc. You then might find out that you've seen alpha somewhere before and know it under a different protein name, or that it matches a sequence named by sequence homology already, etc., so you tend to come up with multiple names, which is why you can have alpha and beta subunits each with their own names in addition. Wnt (talk) 09:41, 12 January 2014 (UTC)

Foam rollers and floors

When using a foam roller on a floor, the part where the cylinder touches the floor becomes incredibly slippery especially with socks. Why is that? The last floor I tested it on was plastic and grainy, but I've heard it happens on other types of floors. Th4n3r (talk) 18:13, 12 January 2014 (UTC)

Not exactly answering your question, but foam rollers are best used with non-slip mats.--Shantavira|^{feed me} 18:26, 12 January 2014 (UTC)

Probably due to a small contact area with the floor. First off, the cylindrical shape means, if everything was perfectly rigid, it would only contact the floor on a line segment, with no surface area at all. Of course, everything isn't perfectly rigid, so you do get a narrow strip of contact area. However, within that area, there are bumps and such, so that only a small portion of that is in contact. Then when you get down to the molecular level, even less of the two surfaces in is contact. So, all this makes slipping more likely, since the friction needed to prevent it only happens over a very limited area.

As for the socks, maybe some fluff from the socks sticks to the roller, and acts as a mechanical lubricant, much in the same way as armpit hairs do, to reduce friction further. There the hairs roll between two surfaces, in a manner similar to how the Egyptians used logs under pyramid stones to roll them along the ground. StuRat (talk) 21:14, 12 January 2014 (UTC)

Acceleration and speed bumps

When going over a speed bump in a car, does acceleration (positive or negative) matter at all? Both in terms of potential damage to the vehicle as well as the perceived bump for passengers. Or will only the velocity have an effect? Basically I'm wondering if you're suddenly surprised by a speed bump if it's better to decelerate quickly or to go over it at a constant speed. 68.0.144.214 (talk) 21:15, 12 January 2014 (UTC)

In a rear-wheel-drive vehicle, accel causes the car to pitch up a bit, so you get less weight on the front wheels and more on the rear. So, in that case, you might want to give it a little gas right before you hit the front wheels, then brake. Of course, the amount is probably not significant enough to be worth the wasted gas. In front-wheel-drive, I expect it to make even less difference.

What I would recommend, though, is that you protect yourself. Biting the tongue can happen if you are eating or talking, so pause either of those activities. StuRat (talk) 21:24, 12 January 2014 (UTC)

Personal experience shows that a steady rate is less jolting; presumably due to the additional force added from acceleration/deceleration (break before bump).~:71.20.250.51 (talk) 23:13, 12 January 2014 (UTC)

Yes brake before the bump, to avoid breaking on the bump. StuRat (talk) 23:55, 12 January 2014 (UTC)

Note incidentally that the pitch-up on acceleration and pitch-down on braking is independent of which wheels are involved, up to the point where those wheels lose traction. It's simply an equal and opposite reaction to the torque applied to the wheels by the engine or brakes. --50.100.193.107 (talk) 05:23, 13 January 2014 (UTC)

I disagree. The drive wheels act as the fulcrum around which the car rotates, and if the drive wheels are in front, the only part of the car to pitch-up would be that in front of the wheels, which wouldn't matter here. No matter how powerful the engine is, a front-wheel-drive vehicle won't be able to do a wheelie without the assistance of ramps. StuRat (talk) 15:47, 13 January 2014 (UTC)

This is a complex question. For example:

1. When braking, the brakes are tending to stop the wheels from rotating - which means that they'll resist not only horizontal motion - but also any vertical component caused by the speed bump.
2. Once you're across the peak of the bump, the amount of pressure on the tire drops - perhaps even to zero - so you'll lose traction. If there is power or brake force going to the wheel between the peak of the speed bump and when it impacts the ground again, then the wheels will have changed speed and that means that its rotation rate won't match the speed of the road. That would result in a jerk of some kind that wouldn't be there if the car was just rolling over the bump.
3. Braking will cause the car to nose down a little bit - which means that the mass of the body is accelerating downwards, compressing the springs - which would make it much harder for the springs to compress to take up the vertical motion due to the speed bump....so braking hard seems like a bad idea. Accelerating will raise the nose of the car a little bit - and should have the opposite effect - so for that reason, maybe you get less of a bump. However, that's just for the front wheels. The suspension on the back wheels will be doing the reverse of that. So accelerating until the front wheels are over the top of the bump, then braking until the back wheels hit it would seem to be a good strategy.
4. Another issue is the natural resonant frequency of the car suspension. Suppose you drive super-slowly over the bump...the suspension won't compress at all because the vertical acceleration will be very low and the car body will follow the shape of the speed bump exactly. On the other hand, if you drive over the bump too fast then the dampening effect of the shock absorbers will prevent the suspension from reacting fast enough to allow the springs to compress - and again, the body of the car will track the shape of the speed bump quite closely. However, at some speed between those two extremes, there will be enough vertical acceleration of the wheels to cause the springs to compress but not so much as to cause the shocks to prevent that - and the body will not rise up by the same height as the speed bump. What speed that is will depend critically on the kind of car you have.

But car suspension systems are complicated dynamic systems with complex geometry and all sorts of springy and dampening components. Tires will take up some of the bump - depending on what the pressure is inside of them.

I think this is a question that's too difficult to answer without a lot more data. SteveBaker (talk) 19:13, 13 January 2014 (UTC

The director of a place where I once worked decided people drove down the entrance road too quickly so he got speed trenches installed – quite deep ones. The best strategy was to go as fast as possible. Thincat (talk) 19:33, 13 January 2014 (UTC)

Antonym of Derived (phylogenetics)

I've tried to find the closest phylogenetical analogue to the concept of conservativity in historical linguistics. The usual antonym of conservative in this sense is innovative. It seems to me that derived is the analogical concept in biology. While primitive is a possible antonym, it is notoriously prone to misunderstanding.

Which is the best replacement for primitive, then? Basal, ancestral, plesiomorphic? Ideally, the term would be applicable both to individual traits and to entire species.

The concept of living fossils strikes me as the closest analogue to languages which are described as conservative in toto – which is sometimes criticised as an oversimplification, not unlike the metaphor of living fossils: just as species cannot escape changing over the span of many millions of years, languages do not stay the same over the course of the centuries, either, even if the changes are relatively subtle and the overall structure or Bauplan respectively remains remarkably constant, creating the superficial impression that the species or language respectively has not significantly changed at all. It may be more appropriate to say that a language remains conservative only in certain respects, but not in others, and there is no language which remains particularly conservative in truly every respect; similarly, I suspect that species may change very little in some respects, but not in others, and do not stay the same in truly every respect. Conversely, I suspect that even supposedly highly innovative languages and highly derived species retain certain aspects with little modification opposed to their relatives. At least it is my experience that when you look closely, you do find traits that go against the apparent trend. --Florian Blaschke (talk) 21:20, 12 January 2014 (UTC)

• The proper contrasting term is an ancestral trait. "Primitive" doesn't work when you compare things like acquired flightlessness. Basal applies more to clades than traits. μηδείς (talk) 00:32, 13 January 2014 (UTC)

So you'd use ancestral for individual traits, and basal for entire taxons or clades? Then ancestral trait corresponds to conservative trait, derived trait to innovative trait and basal clade to conservative branch/unit?

The analogue of clade (i. e., a monolyphetic taxonomic unit or taxon) in historical linguistics would be genetic unit then. While "language" and "species" are broadly analogous concepts ("mutual intelligibility", as usually understood, and "ability to interbreed" being somewhat comparable criteria), it is true that using only this level was too narrow. "Conservative" can also apply to larger genetic units or branches, comparable to clades being more basal or derived.

The reason for my question was that I'm not completely sure if basal is really what I'm looking for, seeing that Basal (phylogenetics) defines basal clades as outgroups. Outgroups correspond to early splits in language family trees.

For example, Gothic is an outgroup within Germanic, but it was not necessarily a conservative branch of Germanic at the time of attestation, as contemporary Proto-Norse (from what little is attested) appears more similar to Proto-Germanic. Icelandic is a notoriously conservative North Germanic language, but it does not form an outgroup within North Germanic: It diverged from an already differentiated western dialect of Old Norse. (Granted, the dialect differences within Old Norse were minute at the time in question.) Finnish is a conservative Finnic language, but does not form an outgroup within Finnic, so it cannot be described as "basal". Livonian is an outgroup within Finnic, but it is notoriously innovative (although it does have a few archaic traits, these are not obvious at first comparison with its relatives). Tamil is a conservative Dravidian language, but not an outgroup. Maori is conservative within Eastern Polynesian, but not an outgroup: the outgroup is Rapa Nui, which is rather less conservative than Maori. Svan is an innovative Kartvelian language, but an outgroup within Kartvelian. East Semitic is an outgroup within Semitic, but not particularly conservative considering its early attestation, and the traits distinguishing it from the rest of Semitic are mainly innovations.

Conservativity has nothing to do with the place of a branch in a tree. It only describes the overall impression of resemblance to the proto-stage (MRCA), or amount of change from the proto-stage compared to other descendants. The divergence of the outgroup may in fact have come about through innovations (analogous to derived traits) from the start, and the outgroup can give a more innovative impression than the rest of the family, which may have undergone fewer or less salient innovations (some innovations have important ramifications, many are rather inconsequential). So I don't think basal is a fitting analogue. --Florian Blaschke (talk) 16:20, 13 January 2014 (UTC)

I hope you don't mind, I added a space between your lines to increase readability. I am a little confused now on your focus. Are you looking for an antonym of derived feature, or an antonym of something like a crown group? The point I was trying to make clear is that derived itself needs to be used in a context in order to find its opposite. You have to specify whether you are talking about traits alone, or about organisms or clades. Your points on languages are well taken. You might enjoy Old English and its Closest Relatives if you haven't already. μηδείς (talk) 17:54, 13 January 2014 (UTC)

Marketing and healthy foods psychology

People often think of mcdonalds as unhealthy food but isn't it true that in reality, its no worse than many other fast food chains and restaurants? — Preceding unsigned comment added by Clover345 (talk • contribs) 22:45, 12 January 2014 (UTC)

Point of logic. It's possible for it to be "no worse than" and "unhealthy". HiLo48 (talk) 23:00, 12 January 2014 (UTC)

It can be healthful, but it requires research. There's a book or a series of books called Eat This Not That, which contains optimally healthful suggestions for various eating places. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:09, 12 January 2014 (UTC)

That comes across as a good example of a logical fallacy. Example, lets rephrase: People often think of the AK as firing an unhealthy 7.62 slug but isn't it true that in reality, its no worse than many other rounds from similar caliber rifles?'--Aspro (talk) 23:11, 12 January 2014 (UTC)

I've noticed that the biggest fast food chains, like McDonald's, Burger King, KFC, and Wendy's, offer both healthy and unhealthy foods. They don't seem to much push the healthy foods, though. The healthy food rarely goes on sale, and sometimes isn't even listed on the menu. At my local Wendy's, if I order a Kids Meal, it will come with fries unless I ask for apple slices, and even then there's a good chance I will get fries anyway. Or, if I order unsalted fries, they will have salt on them anyway (possibly a bit less). So I'd like to say "Apple slices with that Kid's Meal please, but when you screw up again and give me fries instead, make them unsalted fries, and when you screw that up, at least put less salt on them, please".

By contrast, the smaller fast food chains often seem to lack any healthy foods at all, and may not even provide nutritional info on their food. I'm guessing they consider their customers to be the niche market of unhealthy eaters, so aren't concerned with having options for everyone. StuRat (talk) 23:54, 12 January 2014 (UTC)

“Ironically, the fast food giant recently ended up with a PR nightmare after suggesting its own employees forgo fast food fare for healthier options like salad and water. As reported by Business Insider:”

"Several excerpts from the posts, which were created from a third-party vendor, warned against the negative effects of fast food, even going so far as labeling a cheeseburger and fries, core items on its menu, as an 'unhealthy choice. articles.mercola.com/sites/articles/archive/2014/01/08/mcdonalds-fast-food.aspx --Aspro (talk) 00:44, 13 January 2014 (UTC)

I've never had a major concern about the healthiness of the food of McD, BK, Wendy's, Subway or similar. The sheer simplicity, ubiquity and cheapness of the food is what makes them a major health hazard. Richard Avery (talk) 07:37, 13 January 2014 (UTC)

Criticism of fast food is the relevant article.--Shantavira|^{feed me} 08:29, 13 January 2014 (UTC)

I disagree. Many fast food items contain more sodium, saturated fat, and trans fats than you should have in an entire day, much less one meal. I suppose if you only ate such a thing once a month, and ate healthy the rest of the time, you might be OK, but very few people do that. StuRat (talk) 15:44, 13 January 2014 (UTC)

Sources please. After a quick scan of McDonalds' nutrition fact sheet (pdf) I find two menu items that contain more than 100% of recommended daily sodium, both of them chicken in amounts somewhat impractical for a single eater. One of them also barely exceeds 100% of recommended saturated fat. These figures are for a 2000 calorie diet, which is pretty low. Sure it is possible to eat too much hamburgers and such but let's not FUD too much on the science reference desk. 88.112.50.121 (talk) 18:52, 13 January 2014 (UTC)

I would have been more impressed if you came back with sources that show fast-food is a healthy diet.--Aspro (talk) 21:55, 13 January 2014 (UTC)

Check out some of the worst foods on this list, which exceed the sodium limit for the day: [2]. And this list seems to have used a 2500 calorie diet for it's percentages, and presumably they are using the company's own info, which tends to underestimate the unhealthiness, by quoting foods without sauces, etc. (Subway is one of the worst in this regard, quoting nutrition stats based on a half sub, with no cheese or sauces, and the most healthy bread.)

Since that list didn't cover trans fats, here's a separate list for that: [3]. StuRat (talk) 03:16, 14 January 2014 (UTC)

January 13

Internal bleeding results

Let's say that you have substantial internal bleeding: it's far bigger than a bruise (you have to have a transfusion in order to recover comparatively quickly), but you survive. What happens to all the blood? Does it just accumulate somewhere and gradually get absorbed? Does it somehow rot inside the body? Presumably people survived major internal bleeding in the past, before it was practical to perform surgery on this kind of thing — note that my scenario requires a transfusion for rapid recovery, not for survival — so I assume that the body has some way of dealing with it, and that it can be resolved without surgery to drain it from wherever it goes. Nyttend (talk) 05:32, 13 January 2014 (UTC)

Bruise doesn't seem to explain just what happens to the blood in the tissue, but as far as I know it's somehow cleaned out and broken down by the body's systems. As to whether that happens with substantial but non-lethal loss of blood internally, the internal bleeding article likewise does not seem to indicate. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:17, 13 January 2014 (UTC)

Didn't think to check bruise (thank you), but I came here only after reading internal bleeding, and attempting to read haemorrhage, only to find out that it's a redirect to bleeding. Nyttend (talk) 06:51, 13 January 2014 (UTC)

The result of internal bleeding is a hematoma -- that's not a very good article though. The mass of blood is gradually broken down and absorbed, but the process can take a long time. In any case, people usually did not survive major internal bleeding in the past. It was usually fatal. Looie496 (talk) 06:54, 13 January 2014 (UTC)

Thanks for the link; it appears in internal bleeding, but its presentation made me think that it was some kind of illness. I just figured that people sometimes survived substantial internal bleeding without modern medicine — "when it does not stop spontaneously" is part of the internal bleeding article, so I figured that it sometimes would stop spontaneously; and if that's possible, it might well be possible before life-threatening amounts are lost but after health- and strength-threatening amounts are lost. Nyttend (talk) 07:02, 13 January 2014 (UTC)

The thing is, simple bruising usually involves capillaries, which usually heal quickly. Massive internal bleeding would suggest the severing of a major vessel. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:38, 13 January 2014 (UTC)

True, but I assumed that capillary bleeding wasn't likely to "compress organs and cause their dysfunction", which comes immediately before my previous quote. Nyttend (talk) 14:21, 13 January 2014 (UTC)

Right. It comes down to how "substantial" it is, and probably also where it is. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:48, 13 January 2014 (UTC)

What is the difference between VDRL test to treponoma test?

194.114.146.227 (talk) 08:37, 13 January 2014 (UTC)

According to Venereal Disease Research Laboratory test#Mechanism, the VDRL test is a nontreponemal test. The article Nontreponemal tests for syphilis says "Nontreponemal tests refer to a class of syphilis diagnostics that detect infection by indirect markers of infection. Nontreponemal Tests detect biomarkers that are released during cellular damage that occurs from the syphilis spirochete. In contrast treponemal tests look for antibodies that are a direct result of the infection thus, anti-treponeme IgG, IgM and to a lesser degree IgA." Red Act (talk) 09:27, 13 January 2014 (UTC)

Hyperkalemia in case of putting surgical tourniquet strongly

What is the biochemistry process that causes hyperkalemia when the surgical tourniquet is putted on the arm (of course it's just example, but I think that it's true for every place like legs etc.) strongly.I see it in taking blood tests, especially. Thank you for the help. 194.114.146.227 (talk) 10:05, 13 January 2014 (UTC)

From Hyperkalemia#Pseudohyperkalemia: "Pseudohyperkalemia is a rise in the amount of potassium that occurs due to excessive leakage of potassium from cells, during or after blood is drawn. It is a laboratory artifact rather than a biological abnormality and can be misleading to caregivers. Pseudohyperkalemia is typically caused by hemolysis during venipuncture (by either excessive vacuum of the blood draw or by a collection needle that is of too fine a gauge); excessive tourniquet time or fist clenching during phlebotomy (which presumably leads to efflux of potassium from the muscle cells into the bloodstream); or by a delay in the processing of the blood specimen. It can also occur in specimens from patients with abnormally high numbers of platelets (>500,000/mm³), leukocytes (> 70 000/mm³), or erythrocytes (hematocrit > 55%). People with "leakier" cell membranes have been found, whose blood must be separated immediately to avoid pseudohyperkalemia." Red Act (talk) 16:16, 13 January 2014 (UTC)

First of all thank you for your comment. But still I don't understand the biochemistry process that causes hyperkalemia in case of putting surgical tourniquet strongly. Did you mean to say that when it's putted on the arm strongly, it causes to hemolysis? (and homolysis causes hyperkalemia). In addition, I understand that we are talking about hemolysis that occurs in the time of taking blood and rather than about the time after taking blood, because we both know the rule of conservation of mass that explains us which the potassium can not rise by itself after taking blood, it must be occurred in the time of the taking blood and rather than after. 5.28.147.222 (talk) 19:51, 13 January 2014 (UTC)

The Hyperkalemia article says that hyperkalemia is an elevated concentration of potassium "in the blood", and MedlinePlus, whose target audience appears to be non-experts, also uses that phrase.[4] However, I think that's intentionally using sloppy wording in order to be more accessible to non-experts. Medscape, whose target audience appears to be medical professionals, instead defines hyperkalemia in terms of potassium concentration in serum,[5] which I presume is more accurate wording. The Hyperkalemia#Pseudohyperkalemia section then makes sense if the concentration of potassium within blood cells is higher than in serum, so that hemolysis increases the potassium concentration in the serum. Red Act (talk) 02:01, 14 January 2014 (UTC)

sub machine gun

what does the sub mean in sub machine gun, what's the difference between machine gun and sub machine gun? — Preceding unsigned comment added by Potholepete (talk • contribs) 21:30, 13 January 2014 (UTC)

The difference is in the type of ammunition - see Machine gun and Submachine gun. A submachine gun uses the same ammunition as a pistol, while a machine gun will use rifle ammunition or rounds specifically designed for the gun in question. Tevildo (talk) 22:13, 13 January 2014 (UTC)

In general, a submachine gun is a light, portable weapon capable of firing more than one round per pull of the trigger. THAT definition covers such weapons as the M2 carbine variant of the M1 carbine, which was never issued in a pistol chambering by the US Army, but in .30 caliber carbine, but can fire either full or semi-automatic mode, all the shorter (barrel less than 16" long) models in various chamberings of the Colt Automatic Rifle system (the XM-177 variant of which is considered a submachine gun by the US Army and by Colt, its manufacturer despite being chambered in 5.56mm NATO, a rifle round), the various variants of the Avtomat Kalashnikova, the "AK", and "machine pistols" such as the IMI "Uzi," and the MAC-10/MAC-11, Škorpion vz. 61, and Stechkin automatic pistols, which were provided with full-auto capacity to give added firepower to tank crews and special forces troops. Troops don't have to be unusually physically strong to fire submachine guns effectively, and even quite slight women soldiers have routinely carried them.

In contrast, machine guns are all heavily-built fully-automatic weapons chambered either in assault rifle loadings or their own specific ammunition (such as .50 BMG) to provide sustained automatic rifle fire to infantry units or fixed positions in aircraft or tanks. Machine guns are typically neither very light nor very portable (the FN Minimi and M249 machine guns, the MG42, the Bren and the original M1918 Browning Automatic Rifle being the notable exceptions to that rule). They generally require either a husky "squad automatic rifleman" or a pair of infantrymen to be used in an infantry unit afoot.

That, then, is the practical difference between a "machine gun" and a "submachine gun." loupgarous (talk) 05:37, 14 January 2014 (UTC)

Agree with the above, but I believe that the term was coined at the end of World War I when the ingenious Germans came up with the MP 18. At that time, the lightest Allied machine gun was the Lewis gun which was considerably heavier and required more than one man to keep it in operation. Therefore a sub-machine gun was a new class of weapon below or "sub" a full blown machine gun. The term light machine gun was already in use to describe the Lewis and similar weapons, the Vickers machine gun was described as "medium" and a "heavy" machine gun fired bigger ammunition. Hence the need for a new way of describing the German weapon. Alansplodge (talk) 08:52, 14 January 2014 (UTC)

The Thompson submachine gun was developed by the U.S. at the end of WWI and was apparently the first weapon called a "submachine gun" by 1919. Rmhermen (talk) 19:01, 14 January 2014 (UTC)

What may be of some interest is that the German term is "Maschinenpistole", literally "machine pistol". --Stephan Schulz (talk) 21:42, 14 January 2014 (UTC)

January 14

Spacetime or Timespace?

Is it at all reasonable to say that each instant in time exhibits no change at all until the collapse of some Coloumb barrier which-instantaneously-puts us into the next time instant? Is this a reasonable consideration? — Preceding unsigned comment added by 99.182.38.146 (talk) 01:04, 14 January 2014 (UTC)

I humbly suggest you think carefully about your question, and how to better communicate it to a group of diverse volunteers. You seem to be leaping across several different topics in math/physics, without ever really explaining what you mean. Additionally, your terms are unclear. I am familiar with reaction diffusion systems and advection-diffusion equations (and even Reaction–diffusion–advection_equations), but I have no idea what you mean by "adjunct-diffusion equation." A quick google search indicates that that is not standard nomenclature. Also, please consider asking a single question, instead of several at once. SemanticMantis (talk) 01:26, 14 January 2014 (UTC)

NB: the OP has apparently cut out large portions of the question in response to my comment. See history/diffs if you are curious. I only note so that I don't look crazy :) SemanticMantis (talk) 01:42, 14 January 2014 (UTC)

What is the resolution of such reaction-diffusion-advection equations? considered infinitesimal? but how exactly?

• Ignoring the stuff about Coulomb barriers, the question seems to be asking whether it is possible that time is discrete. The answer is yes, it is possible. But if it is, the quantum of time is likely to be extremely small, as the presumed quantum of space is thought to be (the Planck scale). Looie496 (talk) 03:11, 14 January 2014 (UTC)

This is the proposed quantum of time: http://en.wikipedia.org/wiki/Chronon 217.158.236.14 (talk) 14:21, 14 January 2014 (UTC)

Postulation of Even F-number

There exists one even fibonacci pseudoprime. Can we 'calculate' this number? — Preceding unsigned comment added by 99.182.38.146 (talk) 01:21, 14 January 2014 (UTC)

8539786 is an even Fibonacci pseudoprime.[6] Red Act (talk) 02:39, 14 January 2014 (UTC)

It looks like the answer to this question depends on what you mean by "Fibonacci pseudoprime". Lucas pseudoprime#Fibonacci pseudoprimes lists three references of authors who define a Fibonacci pseudoprime as being a Lucas pseudoprime with parameters P=1 and Q=-1, and Di Portio proved in 1993 that no even Fibonacci pseudoprimes exist using that definition.[7] However, from that link it looks like some authors consider "Fibonacci pseudoprime" to be synonymous with "Lucas Pseudoprime", and with that alternative definition André-Jeannin proved in 1996 that even Fibonacci pseudoprimes exist for all P>0 and Q=±1 except if P=1[8]. As far as whether an even Fibonacci pseudoprime using that definition can be "calculated", it seem like the answer would have to be "yes", because even though André-Jeannin's proof isn't a constructive proof, even a brute-force search for an even Fibonacci pseudoprime for given P and Q would count as a "calculation". Red Act (talk) 06:04, 14 January 2014 (UTC)

Shouldn't this be on the math desk? SteveBaker (talk) 13:07, 14 January 2014 (UTC)

Yes. Red Act (talk) 17:12, 14 January 2014 (UTC)

Pulse vs heart rate

Why do people measure their pulse at the wrist or neck rather than just measure the heart rate and pattern at the chest, wouldn't this be more accurate? Is it because it can be harder to feel the heart rate for some people? Clover345 (talk) 12:50, 14 January 2014 (UTC)

I think there's a number of things at play. For one, it can be difficult to pick up the heart pattern at the chest. Clothing, muscles, breasts, fat deposits, etc. all get in the way of getting a clear reading. On top of that, you have the lungs working, meaning you're trying to take a reading on a moving medium. Further, the chest is definitely within most people's personal space and they may find it a bit upsetting to have someone grope them in that way (even leaving aside the issue of a man taking a woman's pulse). If you take a first aid course, they'll tell you not to take the pulse at the carotid for exactly the same reason - it unnerves people and may freak them out if they're already hurt / in distress. Wrists are not ideal (the pulse is often weak there), but at least there's usually not much tissue or clothing in the way and "holding hands" is much more publicly acceptable. Matt Deres (talk) 14:28, 14 January 2014 (UTC)

This source also mentions convience as a reason for the radial pulse (which effectively covers a bit of what you've mentioned and more) [9]. As I understand it, per that source and others like auscultation, the apical pulse is generally determined by auscultation i.e. listening rather than by tactile feedback or palpation although I'm not totally sure if this applies if no stethoscope is available i.e. you'd need to do immediate auscultation. That source does mention the apical pulse can be palpated in about 50% of adults which suggests it's not a particularly reliable method. And [10], while not a RS, also mentions problems attempting to palpate, and not surprisingly given the reasons outlined by Matt Deres, that it's easier in children and smaller adults. As the first source also mentions, the apical pulse is preferred in a few cases like in infants or where the radial pulse appears weak or non existent. It's also needed to determine if there is a pulse deficit. Nil Einne (talk) 16:33, 14 January 2014 (UTC)

Don't doctors also get a clue on some circulatory issues from the wrist pulse, or am I imagining that? μηδείς (talk) 19:45, 14 January 2014 (UTC)

As for accuracy, if you are only interested in the heart rate the wrist or the carotid is just as good as the chest, as long as you can feel the pulse there. What you feel is the increase in pressure from the heart beats, so the heart rate is the same wherever you measure. Sjö (talk) 20:45, 14 January 2014 (UTC)

Note that per the sources I mentioned earlier, technically this isn't totally correct since a pulse deficit can exist. See [11] for more (which despite the blog nature and poor formating seems and okay source and I don't think is a copyvio). Of course being aware it exists is generally important so if you suspect it may be the case, I suspect you should take both the apical and radial (or similar?) simultaenously to test for it as per the sources. In other words, you're somewhat correct in that you can generally assume they are the same, because when they aren't you have more serious concerns than simply having the a lower pulse than the heart rate. (This is all presuming you take the pulse correctly.) As a minor aside, I was reminded by Talk:Pulse of something I may or may not have heard before. For 15 months Dick Cheney had no pulse. Although to ward of jokers, this only happened after he left the Vice Presidency [12] (and our article). Nil Einne (talk) 23:34, 14 January 2014 (UTC)

i have a few questions but ifirst need someone to talk to me about dreams or dreaming

wel first of all i think that the time it takes to fall as sleep thats the same thing about dreaming but like i said i need someone talk to first — Preceding unsigned comment added by 67.225.110.19 (talk) 23:15, 14 January 2014 (UTC)

I don't understand what you are saying, but I can't see any way to interpret it that would make it true. Dreams usually don't come until after several hours of sleep. The transition from waking to sleep is known as hypnagogia -- it is not the same thing as a dream. Looie496 (talk) 23:26, 14 January 2014 (UTC)

Hypnopompic being the opposite situation, i.e. the transition from sleeping to waking. These are times when hallucinations can happen, i.e. the feeling that one is being attacked by a ghost or some such. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:32, 15 January 2014 (UTC)

College student behaviour.

Is there a scientific reason so many college students only seem to be able to do things in extreme and not in moderation? For example many drink extreme amounts of alcohol, study fr extreme hours in the library, eat extreme amounts of unhealthy foods. If all of this is done in moderation, surely they can understand it would be healthier and may even help reduce stress levels. Clover345 (talk) 23:44, 14 January 2014 (UTC)

There are reasons however those reasons are not scientific. 202.177.218.59 (talk) 23:56, 14 January 2014 (UTC)

The term "reckless youth" comes to mind, and has to do with a truism I've often heard, that the young think they are immortal. Hence the typical young person might not worry about the issues that the OP is talking about. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:27, 15 January 2014 (UTC)

One related idea on the matter is the Cultural Theory of risk which asserts that one's perception of risk is primarily influenced by one's culture. The culture of college campuses enforces a certain perception of risk (that is, what behaviors are considered "risky" and what behaviors are considered "normal" or "safe") is influenced by the environment, which feeds a "status quo" of risk behaviors that would seem reckless in other contexts. --Jayron32 01:50, 15 January 2014 (UTC)

One scientific reason is that college students are in the prime of their life, and can tolerate drinking extreme amounts of alcohol, studying for an extreme number of hours (presumably because they didn't begin studying earlier), etc. A 70-year-old doesn't engage in such behaviors because his body can't handle it, not necessarily because he doesn't want to. --Bowlhover (talk) 03:43, 15 January 2014 (UTC)

Is this true or just a stereotype? To which country is the question referring? Also, should the question be more about the stereotype that all young people exhibit these types of behaviours and not just students. CambridgeBayWeather (talk) 04:26, 15 January 2014 (UTC)

People between the ages of 12 and 25 behave differently to fully matured adults because their brains are in the process of being rewired. For instance, it says at THE ADOLESCENT BRAIN -- WHY TEENAGERS THINK AND ACT DIFFERENTLY "The frontal lobes help put the brakes on a desire for thrills and taking risk -- a building block of adolescence; but, they're also one of the last areas of the brain to develop fully". So they do things to excess because the part of the brain that should tell them when to stop is not fully developed. Googling "the teenage brain" will reveal a lot more on this subject. Of course, this is also a very creative period and people of this age have had a great influence on the development of human society. As it says here "Teenagers can be a pain in the ass. But they are quite possibly the most fully, crucially adaptive human beings around. Without them, humanity might not have so readily spread across the globe.” Richerman (talk) 17:42, 15 January 2014 (UTC)

January 15

Is there any antibodies or antigens which be inherited by genes?

194.114.146.227 (talk) 07:06, 15 January 2014 (UTC)

When people refer to "antibodies" they are typically referring specifically to proteins affected by V(D)J recombination, which cannot be inherited genetically (although a mother can pass some antibodies directly to her infant child). However, there are other components of the immune system, such as the complement system, which are heritable. In brief, the complement system is a set of genes that are pre-encoded to recognize and combat certain pathogenic microorganisms, although they still work better if accompanied by a typical antibody response. See also Pattern recognition receptor and antigen. Someguy1221 (talk) 08:38, 15 January 2014 (UTC)

תודה על התשובה (Thank you for the answer)5.28.161.105 (talk) 11:31, 15 January 2014 (UTC)

Another interesting feature, which is heritable but prone to variation, is human leukocyte antigen (which is the human version of the major histocompatibility complex). --—Cyclonenim | Chat  10:37, 16 January 2014 (UTC)

Ampere vs. Coulomb

Why does the Systeme International use the Ampere, the unit of Current, as a base unit instead of the Coulomb, the unit of Charge? I would think that charge would be the basic fundamental quantity, comparable to length, time and mass, while current is simply charge/time.Inkan1969 (talk) 16:33, 15 January 2014 (UTC)

Arguably, electric charge is a more derived property and is more difficult to directly measure than electric current. Besides, you can have electric current even if no charge moves. S.I. defines electric current in terms of its mechanical effect, not in terms of its constituent motion of charge. Nimur (talk) 16:45, 15 January 2014 (UTC)

Thank you for the answer.Inkan1969 (talk) 17:08, 15 January 2014 (UTC)

Inkan1969's question is a good one. I have often pondered the same thing. The formal definitions of the Ampere and the Coulomb establish that the Ampere is the fundamental unit, and the Coulomb is the derived unit. However, I imagine that it is easier for a young student to comprehend electric charge (the Coulomb) first; and then to comprehend electric current (the Ampere). I'm not a teacher, but if I were a teacher of physics to teenagers I would first introduce the concept of electronic charge and the Coulomb, and secondly introduce the concept of electric current and the Ampere and the properties associated with an electric current. Dolphin (t) 05:16, 16 January 2014 (UTC)

Which is considered the base unit may change with the proposed redefinition of SI base units. The "why" is probably more determined by measureability considerations than which could be consider more fundamental. —Quondum 07:14, 16 January 2014 (UTC)

Yes - having worked in a metrology lab at one point I can confirm that the choice of base units has very little to do with the ease with which the concepts can be taught and is only somewhat related to how fundamental they are as concepts (basically you want to reduce the interdependence of the base units as much as practical, which does lead one towards more fundamental units in general). The main consideration is indeed measurability - specifically minimising the measurement uncertainty and ensuring the measured value is stable over time and between different laboratories (which is why there is so much effort going on to redefine the kilogram as something based on fundamental constants, rather than as the mass of a block of platinum-iridium in Paris). Equisetum (talk | contributions) 14:01, 16 January 2014 (UTC)

What is the scientific basis for the claim that studying individually is better than studying in a group?

What is the scientific basis for the claim that studying individually is better than studying in a group? And under what conditions is this so? 140.254.227.120 (talk) 16:45, 15 January 2014 (UTC)

Who makes this claim, and in what context? It seems like too much of a generalisation to make any 'scientific' assertions one way or another. AndyTheGrump (talk) 16:52, 15 January 2014 (UTC)

But a study has shown that that is the case! I just can't seem to find the actual article again. I swear it's not a dream! :( 140.254.227.120 (talk) 17:08, 15 January 2014 (UTC)

I was hoping I could find some support or doubts on that study, since scientific studies are usually tentative. 140.254.227.120 (talk) 17:10, 15 January 2014 (UTC)

When an article in the media starts with "A recent study shows", it's best to take it with a giant grain of salt. p value for "significance" was set intentionally low so that studies would pick up things worthy of further examination, NOT at all to determine if they are actually real or true. To do the latter, you have to independently replicate the results a few times. Actually having a quick look, the Misunderstandings section of our p value article seems to cover it pretty well. Vespine (talk) 21:53, 15 January 2014 (UTC)

I could certainly see both advantages and disadvantages to group study sessions:

ADVANTAGES = Others can explain things to you which might be unclear from the lectures and/or book and/or labs.

DISADVANTAGES = Not tailored to your own weaknesses. Presumably you will study more where you know you are deficient.

In other cases, it might depend on the study group. One that encourages you to study when you want to stop would help, while a group that is always goofing off is not at all helpful. Also, I might apply the same rule I apply to learning chess, where I think it's best to play those who are just slightly better than you. They have something to teach you, yet are not too advanced to understand. Also, real experts often don't want to bother explaining "the obvious". StuRat (talk) 22:14, 15 January 2014 (UTC)

As a teacher I would recommend to students that they do both. Their benefits would be complimentary. HiLo48 (talk) 22:19, 15 January 2014 (UTC)

Citing the anonymous authority of a study someone might remember does not constitute a scientific approach. The result of such a study would itself be qualified by whether it was done by an individual or a group. A scientific method of studying the claim would be to design and carry out a Blind experiment that removes the influence of preconceived notions. This falls within the article about Educational research and a hypothesis that why one study method is better than another falls under Educational psychology. 84.209.89.214 (talk) 22:26, 15 January 2014 (UTC)

The trouble with this sort of study, for things that are as individual as study methods is that the study can usually only conclude that "on average" one method is better than the other for the population studied. This might just mean that method A is better for 40% of the study sample, for 30% of people it makes no detectable difference, and for 30% of people method B is better. Interpretation is further complicated by the fact that you can't guarantee, without close reading of the relevant paper, that the study is sampling a population of which you are a member - maybe the study was on humanities students, and you are a physics student. The results of these studies are potentially useful for schools and education authorities who need to find the approaches which are effective for the maximum number of people while being cheap enough to implement, but as an individual you are much better off trying different techniques and seeing which work best for you (ideally in a semi-formal manner to remove as many biases as possible). Equisetum (talk | contributions) 12:15, 16 January 2014 (UTC)

January 16

science behind makara jyothi

There lies a big mystery behind the makara jyothi(a light glown on the day of makara sankranti near the sabarimala hills,kerala,India).There is no proper scientific reason given yet.Most of the people believe it as a divine light and many criticise it as an 'artificial light created to cheat people'.Please solve the mystery and help the wikipedia users.Kindly use the following links to know more about it. http://sinosh.wordpress.com/2008/08/26/makarajyothy/ http://www.spiderkerala.net/resources/4618-Divinity-Makarajyothi-questioned-Is.aspx http://ayyappadevotionalsongs.blog.com/2010/12/01/the-science-behind-makara-jyothi-in-sabarimala/ Makara_Jyothi — Preceding unsigned comment added by Kathir kishan (talk • contribs) 12:57, 16 January 2014 (UTC)

I see no mystery. As stated in the article (and ignoring the blog sites you linked to, where anyone can write anything) Makara Jyothi is a star.--Shantavira|^{feed me} 13:33, 16 January 2014 (UTC)