Wikipedia:Reference desk/Archives/Science/2012 July 10

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July 10

What is an extended structure?

What is the precise meaning of an "extended structure" in chemistry?

I want to understand "the design and construction of extended structures"[1] in the context of adsorbents. Also "extended solids" seems to be a synonym (used in the title of a paper listed there).

From Googling, it seems to be often used in the context of large complex molecules such as DNA and melanin.

Thanks! --Chriswaterguy talk 00:47, 10 July 2012 (UTC)

Okay, no takers yet...

As far as I can work out (guess, really) it's where a material doesn't have molecules in the usual sense, but a more or less consistent, repeating structure. But that sounds like a crystal: "A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly, repeating pattern extending in all three spatial dimensions." That article states that there are crystals with covalent bonds, so it's not that either.

Is it a synonym for crystal, but emphasizing the importance of the structural details? With research into adsorbents, the structure is critically important; with rock salt, the internal structure isn't of so much interest for most people dealing with it. (I suspect there's more to it than this, though.) --Chriswaterguy talk 06:58, 11 July 2012 (UTC)

As far as I can see from the use of the term here and here, it refers to the structure above the unit cell level i.e. how the unit cells stack together. It looks like it would be important when there are more than one unit cell type present, since the way unit cells stack together is generally pretty obvious. From my understanding of what a unit cell is, if there are supposedly different unit cells alternating throughout the structure, then the unit cell has been incorrectly identified, and should incorporate both of the other "unit cells" into one which does simply repeat without interuption. It also seems to have a special meaning in biochemistry. 203.27.72.5 (talk) 07:27, 11 July 2012 (UTC)

Actually, look at the section "Description of crystal structures" here. It basically says that understanding the unit cells as I said above is occasionally insufficient to describe the structure, so then they talk about extended solid-state structures that have multiple unit cell types. 203.27.72.5 (talk) 07:38, 11 July 2012 (UTC)

In the context of surfaces (which I think is what you're looking at since you mention absorbents) it might mean the structure at the surface according to this paper. I suppose in that sense it's the structure "extended all the way to the edge". Here's yet another paper that defines the term in a completely different way - using it to refer to the structure incorporating charge density. From this FAQ I think it just means any rendition of the structure that shows more than one unit cell, even if they are identical and repeating. So, in summary, it means a lot of things to a lot of people even just within the field of chemistry. 203.27.72.5 (talk) 08:07, 11 July 2012 (UTC)

Thank you, that's very helpful! --Chriswaterguy talk 03:08, 20 March 2013 (UTC)

How do ants detect rain long before it arrives?

Topic says it all. ScienceApe (talk) 02:10, 10 July 2012 (UTC)

They can't. No wait, they can. Or can they? 203.27.72.5 (talk) 02:21, 10 July 2012 (UTC)

Phase IV: http://www.youtube.com/watch?v=gcjBIkS-QC4 . μηδείς (talk) 02:34, 10 July 2012 (UTC)

The simplest way, I imagine, would be to detect falling barometric pressure, like a barometer does, to predict bad weather. (This could be done with an air sac exposed to the atmosphere, which would shrink when pressure rises, and grow when it falls. I wonder if they have anything like that.) StuRat (talk) 02:39, 10 July 2012 (UTC)

Who says they can? What would you expect them to do? The black ants on my lawn can't. On the few fine days we've had this summer in the UK they build little towers of fine soil among blades of grass in which they place their pupae only for these to be washed out by a sudden shower. I know it's OR but no less reliable than some of the sites cited above. Caesar's Daddy (talk) 07:27, 10 July 2012 (UTC)

It's true that Lasius niger in the UK don't move their colonies before heavy rain but here in Thailand where the rainy season can be a serious problem for ants, it's commonplace to see entire colonies of what I assume are Paratrechina longicornis with multiple queens emigrate hours before heavy rain, often into your house, or at least off the ground. They neatly arrange the larvae and pupae into piles on higher ground or hold onto them all until the rains have finished. Then they leave. Sean.hoyland - talk 10:05, 10 July 2012 (UTC)

Fascinating, I've never heard of such a behaviour in an ant species that is not generally nomadic by nature. As to the sensory mechanism they use to perceive these oncoming events, I agree it is likely to be barometric in nature, but would not discount some form of hydrometeor perception (specifically, a level of precipitation which is noticeable to the ant well in advance of the rain that we perceive) or even temperature. I do not think an exposed air sac is likely as it is not consistent with any precursor organ or physiological structure I am familiar with in ants and many animals are believed to have barometric perception without the need for such an external feature. It's worth noting that barometric pressure would be an inconsistent mechanism for most species of ant to detect rain on the horizon since most drops in pressure do not herald rain, but in a region like Thailand the correspondence is higher than on average, meaning the mechanism would be more dependable for species living in such a context. Add in the amount of rain involved and it makes sense why these particular ants might have the capability where most others lack it. Snow (talk) 13:00, 10 July 2012 (UTC)

The sac wouldn't need to be external, just have access to external air, say via a pore. Could be like the human ear, with a membrane covering a hole. StuRat (talk) 17:10, 10 July 2012 (UTC)

How can air conditioning units be measured in BTUs?

It makes no sense to me. That's a bit like measuring a car's engine in miles. BTU is a unit of energy. Surely when one decides on an air conditioning unit / heat pump one chooses one with a certain power rating, like watts or BTU per hour or something like that. How does BTU hold any meaning? I can state that a one watt air conditioning unit can produce 100,000 BTU (provided you run it for about 30,000 hours - about three and a half years). 41.164.7.242 (talk) 07:33, 10 July 2012 (UTC) Eon

British thermal unit indicates that "BTU" in that context is used as shorthand for "BTU per hour". ←Baseball Bugs ^{What's up, Doc?} carrots→ 09:07, 10 July 2012 (UTC)

In the developed world we use kW .. :-) Electron9 (talk) 22:46, 12 July 2012 (UTC)

More is needed to evaluate an air conditioner than kw input. A good air conditioner moves more BTU/h than a poor one, with the same kw consumed. Joules output would be ok.those in the heating and cooling trade in the US have long used "tons" to describe cooling units. Edison (talk) 17:03, 13 July 2012 (UTC)

And a car's lifetime, including its engine, is typically measured in miles, at least in the United States. In the same way, aircraft engines are measured in hours of running time. Miles or hours determine the maintenance and replacement intervals. ~Amatulić (talk) 22:58, 10 July 2012 (UTC)

HIV transmission by insect

Are there any proven cases of HIV being transmitted by mosquito bite? Or by any other insect bite? I'm wondering if I'm being told duff information. -- 84.12.35.122 (talk) 08:23, 10 July 2012 (UTC)

Our article suggests that HIV cannot in theory be transmitted by a mosquito bite, because mosquitoes inject only saliva into their victims, not regurgitated blood, and HIV is not present in the insect's saliva. It suggests one potential transmission route: if a mosquito containing HIV-infected blood were to be swatted against someone's skin, the infected blood could enter the victim's body via a scratch or open wound, but it cites no known examples. Wikipedia is not itself a reliable source, but the article does cite sources. The results of a Google search on the question appear to agree that there have been no reliably documented cases of HIV infection via mosquito bite. - Karenjc 08:50, 10 July 2012 (UTC).

On any other insect bite it looks less certain to me. Bubonic plague seems to transmit with regurgitated blood from flea into host for example. Simultaneous infection with Bubonic plague (to cause the regurg) and HIV looks technically plausible? --BozMo talk 10:55, 10 July 2012 (UTC)

Unlikely, given the prevalence of fleas and not a single documented case of this happening. Mind you diseases which display such a pathology/epidemiology tend to be those which have evolved for a long time to exploit exactly that route of transfer. Bear in mind also that HIV does not survive for long outside of the human body, even when it's not being attack by the immune system of another host creature. Also note that flea bites do not usually re-introduce blood when they feed multiple times -- Y. pestis has a particular mechanism for causing the flea to regurgitate the blood during a second feeding, a mechanism HIV does not share. Snow (talk) 13:17, 10 July 2012 (UTC)

How can you document such a thing? There are probably many HIV infected people who cannot spot the origin of their infection and most of us are mostly not aware when we were beaten up by a mosquito, and even, I dare to say, some people die of AIDS without realizing it what is killed them. — Preceding unsigned comment added by Plusanother (talk • contribs) 22:01, 10 July 2012 (UTC)

Well, I wouldn't exactly say that it's outright impossible, just that such a case has never been empirically documented (or even suggested as likely in serious medical literature that I know of, though the potential of the mechanism has been explored nonethless), and that it seem unlikely given what we know about the disease's pathology and the physiology of the two host organisms. Keep in mind that HIV research is a prolific field; if this was happening it's almost certain we'd know about it (and any researcher who proved the existence of the mechanism would be propelled to life-long acclaim). Snow (talk) 00:37, 11 July 2012 (UTC)

I strongly doubt it's documentable, because given the social aspects of HIV transmission modes, there are doubtless many people who will strongly deny the usual routes of infection, but who didn't get it by mosquito. The only way we could find out would be to play with a captive population and HIV-fed mosquitoes, and I doubt we've done that even at Diego Garcia, though you never know any more. Wnt (talk) 21:28, 11 July 2012 (UTC)

Hmm, mosquito bites can transmit other viral diseases, such as dengue fever and malaria, so I'm wondering why it's so implausible that HIV can't possibly be present in mosquito saliva. ~Amatulić (talk) 23:04, 10 July 2012 (UTC)

Dengue fever and malaria are caused by protozoans which actively move into the saliva. μηδείς (talk) 23:42, 10 July 2012 (UTC)

Wrong about dengue, which is viral, was thinking of Chagas' disease. μηδείς (talk) 00:43, 11 July 2012 (UTC)

There are two layers to answering that question. First there's the proximal explanation of the mechanisms (as referenced above). But to address the issue more broadly and in terms of pragmatics, there's the explanation that the virus already has a perfectly suitable (and, unfortunately, highly efficient) mechanism for propagating itself. While technically not a living organism (depending on your definition), a virus does have a kind of evolutionary course, and each has adapted to operate in highly specific context. The balance the HIV virus has struck does not (at present) allow it to survive outside the human body for long, nor to exploit routes of infection which do not involve human-to-human contact. Developing such traits would likely require significant trade-offs for the disease which, given it's current virulence and tenaciousness, are not necessarily beneficial to it. Snow (talk) 00:48, 11 July 2012 (UTC)

So why is it so easily transmitted through sharing used needles and blood transfusions? 203.27.72.5 (talk) 00:55, 11 July 2012 (UTC)

Those are very intimate routes of transmission protected from exposure to oxygen which rips apart the viral coat of HIV almost immediately. Mosquito saliva also has anti-coagulants and digestive enzymes which destroy the virus. μηδείς (talk) 01:06, 11 July 2012 (UTC)

If HIV is destroyed on contact with oxygen, shouldn't it be destroyed in vivo as your blood passes through the lungs? 203.27.72.5 (talk) 01:22, 11 July 2012 (UTC)

Well, individual virions are constantly being destroyed by the regular metabolic processes of the body (as well as by the immune system, even in the case of AIDS), but their numbers are such that they can weather these storms, sometimes even laying relatively "dormant" in certain tissues (nervous tissue is particularly susceptible to certain viruses, for example) and then re-amassing themselves as the opportunity presents itself. Snow (talk) 01:34, 11 July 2012 (UTC)

And the transmission of infected blood of one individual into the bloodstream of another tends to be more immediate and consist of markedly larger amounts. On top of all of this, mosquitos rarely need more blood than they can consume in a single feeding; in most cases they use it to drive egg fertilization and engage in such feeding only as necessary (though some more aggressive species do use it as a more consistent means of sustenance). Snow (talk) 01:29, 11 July 2012 (UTC)

Large amounts of the virus are actually destroyed in vivo, but the blood is a buffered, supportive environment (ignoring the immune response) and keep in mind that oxygen in the blood is largely bound by hemoglobin. Also, as I said elsewhere, biology is not math, and there are exceptions. It is entirely possible HIV may have been transmitted by insect bite, just unlikely and impossible to prove after the fact. Would anyone like to volunteer as a test subject? μηδείς (talk) 01:36, 11 July 2012 (UTC)

Is it so unlikely that it's not worth doing an experiment with simian immunodeficiency virus , a couple of chimps and several hundred mosquitos? There's also this link for anyone who's interested. 203.27.72.5 (talk) 02:14, 11 July 2012 (UTC)

There is an infinite difference of scale between the yes-or-no possibility and the almost-zero likelihood of something happening. You pay me enough, I will indeed get a mosquito to infect a chimp with SIV. But if the prospect of dying of AIDS from a bugbite actually frightens you you might as well go on benzodiazepines to prevent yourself from dying of a meteor strike. μηδείς (talk) 02:33, 11 July 2012 (UTC)

Nah, the word of the CDC is good enough for me. I was just interested in the whys and hows of it all. 203.27.72.5 (talk) 04:43, 11 July 2012 (UTC)

There is no obvious reason why it would be absolutely impossible for someone to get a bit of HIV infected blood from a mosquito or tick into a scratch and catch the disease. Mosquitoes and ticks obviously have large dollops of blood in them, which could include various pathogens. The CDC says that one unit of HIV might get transmitted in 10 million mosquito bites. It isn't clear whether the "1 unit" is enough to cause an infection, but how many millions of mosquito bites take place each year? Maybe a billion in the US, if each person gets bit 3 times. But few of the mosquitoes would have had a previous partial meal on an infected person. Insect-born pathogens were shown to be the cause of numerous diseases of people and animals, but typically in models where the pathogen survives and fluorishes in the mosquito, unlike HIV. West Nile is transmitted by skeeters who have previously bitten an infected animal, and I would be much more concerned about getting West Nile from a mosquito bite. There might be a reason for governmental authorities to poo-pooh any possibility of HIV infected blood getting carried to infect a victim who gets bitten by the insect, or who swats a blood-filled insect and it enters the bite or some other scratch, since if the small possibility were shown to exist, people might demand that HIV infected persons have to dwell far away from uninfected persons. Combine a small likelihood of insect transmission, and the societal harm from restricting HIV infected persons to something like leper colonies on a remote island, and you could get unwarranted denials."Trust me, I'm from the Government and I'm here to help you" just does not resonate with credibility for many people. Edison (talk) 20:30, 11 July 2012 (UTC)

For extremely good reason. The Government, being run by big business, does not help anyone other than its own constituent bourgeoisie. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 21:15, 11 July 2012 (UTC)

While they might not be always 100% truthful, they are here to help. Edison even pointed out that the only reason for downplaying the truth is to strike a balance between the "likelihood of insect transmission, and the societal harm from restricting HIV infected persons to something like leper colonies on a remote island". If the CDC says it's not worth worrying about, I'm not going to worry about it. But for interests sake I wanted to know just how unlikely it was. 203.27.72.5 (talk) 22:03, 11 July 2012 (UTC)

There probably is a risk of insect being a vector transmission path. The problem is how to scientifically document a transmission mechanism with low probability, long (10y) incubation time, and where the victim may not even beaware of the insect bite in question, nor would any experiment be ethical. As for the truth factor in authorities, just consider that they have other priorities than yourself! Take rather a look at the mechanism involved rather than any "we know better" from authorities. Kitchen knifes and salads is another path. Heard of any restaurants with guaranteed HIV free staff lately? So: does the blood sucked up by the mosquito present an infection risk (for HIV)?, can it be pushed into a person be smacking it? can viruses migrate from the blood into the saliva? Any data where the number of viable viral particles is measured would be interesting. And can't be THAT hard to establish. Then probabilities can be calculated. Electron9 (talk) 00:25, 12 July 2012 (UTC)

Well, it's not as if the issuehasn't been studied. (Those are just a few amongst scores that treat the issue that I found in a cursory search). You're quite right that incubation periods often muddle the issue some for any disease, but that doesn't researchers can't determine a given vector as impossible for a given virus based solely on what they do know about the pathophysiology involved. Or even on strong epidemiological evidence; one of those examples I included in this post points out that if mosquito were capable of transmitting the disease then you wouldn't see as striking a trend on the age-range of new infections -- barring infections by transfusion and mother-to-child, almost all new cases excluded the very young and the very old, for obvious reasons, but if mosquitos were transmitting the disease, their dispersed selection of targets would not allow this to be the case. Now, this might not be satisfying to those who might argue that it could still have happened just a handful of times, but puttign together statistical evidence (bearing in mind the massive numbers involved) and a number of different findings as regards the pathology of the disease and the physiology of the carriers, it's about as certain as anything gets in medical science. As to the disease being transmitted by someone prepping a salad, that's obviously unrealistic and if there are indeed kitchens that go out of their way to not hire personnel with HIV, and then are foolish enough to advertise the fact, you can probably expect to see some discrimination lawsuits on the matter in the near future (and the plaintiffs will almost certainly win too, if we're talking about any country in the English speaking west here). Even if you meant that the HIV-infected individual cut themselves then I still don't see it happening. First off, consuming anything less than ounces of infected blood is probably not likely to cause infection (unless you had a large open cut in the mouth or somewhere else in the upper digestive tract). Second, its unlikely a salad with any amount of blood on it is going out to a customer and three -- don't eat your salad if it comes with blood on it! Snow (talk) 09:28, 12 July 2012 (UTC)

What about a HIV positive teppanyaki chef? 101.173.170.146 (talk) 10:47, 12 July 2012 (UTC)

Hah! Well, if your teppanyaki chef is stabbing himself, you're taking your life into your own hands being so close to him regardless. Snow (talk) 12:11, 12 July 2012 (UTC)

Actually, those references you cite have a case of mumble. "Extremely low or nonexistent", "extremely unlikely", aren't actually answers as to whether one case has ever occurred. They're practical answers for everyday living; still, if you run into someone who claims to have experienced an Immaculate Infection, they deserve to be given the benefit of the doubt. Because it's biology, and you never really know. Speaking of which, if a chef serves up his own testicles, [2] even if properly cooked, what are the odds that a piece of viral RNA or integrated DNA can become transfected into a vulnerable epithelial cell. Hmmm.... (but then again, he wasn't in one of the risk groups! Wnt (talk) 13:26, 12 July 2012 (UTC)

Hard facts would be for each mg of blood sucked up by the mosquito there is N number of infectious viruses. And the same number for the saliva. Then a probability for N number of viruses to cause an infection. No need for transmission experiments, except for letting one mosquito to suck. The rest just lab work (BPL-4?). Electron9 (talk) 22:43, 12 July 2012 (UTC)

Things with vanishingly small probability do happen from time to time, like someone a few years ago surviving a rabies infection, or a full term baby delivered healthy from an ectopic pregnancy, or a train derailment recently in Illinois causing a train and railroad bridge to fall on a couple driving under the bridge in their car, killing them, or someone winning the lottery twice, or a girl getting HIV from a dentist's drill, or a 5 year old girl, Lina Medina, giving birth to a healthy baby. In the case of the OP's question, it would likely take experiments with simians and infected mosquitoes, similar to the experiments proving mosquito transmission of Yellow Fever, but public policy issues as outlined above would likely prevent any researcher getting government funding for such iron clad research, and we are left with probability calculations and poo-poohing by the government. If someone claimed they got HIV from an insect bite, no one would believe them. It would be like a girl claiming she must had gotten pregnant from sperm in the public swimming pool, or got VD from a toilet seat. Edison (talk) 20:19, 12 July 2012 (UTC)

Oh Japan, what are we ever going to do with you? The best part of that article has got to be the reporter wearing a surgical mask and a dazed expression that seems to say "Where did my life take this turn? I was supposed to be a journalist..." or maybe "Really!? we're doing this now? I've gotta get that work visa." Heh. Snow (talk) 20:41, 12 July 2012 (UTC)

Evolution of the four fundamental forces

I seem to recall there was a section in my physics book that gave a timeline of when the four forces evolved. Which came first, and when, etc. Is there any article here that gives a timeline? ScienceApe (talk) 11:44, 10 July 2012 (UTC)

Chronology_of_the_universe#Very_early_universe ? --BozMo talk 12:10, 10 July 2012 (UTC)

The "orthodox" sequence is that gravity separated first, followed by the strong force, and finally the weak force separated from the electromagnetic force when the Higgs mechanism gave rest mass to W and Z bosons but not to photons - see this timeline. But as of now only the final one of these three phase transitions is understood in any detail - different sources give different answers about the timing and mechanism of the first two, with varying amounts of speculation. And if you add supersymmetry breaking to the mix, that complicates the picture even more. Gandalf61 (talk) 12:15, 10 July 2012 (UTC)

Infant eye glasses

I know that with toddlers who can at least talk, they do vision tests with symbols of things like boats and houses, but I've seen infants barely able to crawl with glasses. How can doctors ascertain the visual acuity of someone who can't tell you what they're seeing because they haven't acquired any language higher than "bbbbb whaa whaa abababa"? 20.137.18.53 (talk) 12:16, 10 July 2012 (UTC)

The glasses could be used to correct a squint: NHS link. Brammers (talk/c) 12:51, 10 July 2012 (UTC)

It is possible to measure the roundness of the eyeball using modern equipment. This will indicate astigmatism, which can bee corrected using glasses. --TammyMoet (talk) 13:53, 10 July 2012 (UTC)

There is an instrument called a “Retinoscope” which we use to shine a light into the toddler’s eye from a distance of about 17 inches. We can interpret the reflection that comes back from the back of the toddlers retina into the pupil area. As we move the instrument from side to side, the reflected light we are watching will move either in the same direction as our instrument or in the opposite direction. This tells us whether we are dealing with nearsightedness or farsightedness. The speed of motion of the reflected light is an indication of how much prescription power will be required. In a seasoned practitioner’s hands we are able to very precisely determine how much and what power is required without asking the toddler a single question. We do the same with adults who can’t communicate or are illiterate or can’t speak the same language, as in third world countries where in some instances on field trips no communication is possible at all. It’s pretty neat. Dr. Dhavid Cooper DriveByWire (talk) 14:18, 10 July 2012 (UTC)

The illiterate part doesn't make sense. At the eye doctor, they never actually ask me to read the letters, in any case, they just ask which is clearer. Now, when I get a driver's license, then I have to read the letters, because there they want proof that I can see clearly, but even there, they could substitute symbols like houses and dogs and ice cream cones. As far as people who don't speak the language, all they need to communicate is "better" or "worse", which could be a thumbs up and thumbs down. StuRat (talk) 16:55, 10 July 2012 (UTC)

Going back over 20 years when I was an adult literacy tutor, I taught a young man who wore bottle-end glasses but who still couldn't see you unless he stood right up close to you. He was more or less unable to read, and over about 18 months we worked on that. One day he walked in to class with new glasses, which were much thinner, and he didn't stand right next to me. What had happened was that he'd finally been able to read the letters and texts the optician had been giving him during his eye test, and the prescription was exactly what he'd needed - for the first time ever. --TammyMoet (talk) 19:01, 10 July 2012 (UTC)

If his eye doctor had simply asked when it looked clearer, rather than ask him to read the letters, the whole problem could have been avoided. It seems that some eye doctors need to be retrained. I suspect that, to avoid embarrassment, the student had faked being able to read, and, once he guessed the letters correctly, continued to say them that way, even as they got blurrier with stronger lenses. The eye doctor should also switch the letters used at each increment, if asking people to read them, to avoid such deception. StuRat (talk) 19:10, 10 July 2012 (UTC)

Tammy has pulled your leg, Stu. Quite apart from the fact that anybody with even half a brain would sooner or later, and most likely very much sooner, discover for himself whether the prescribed specs were right (if like me, you need a different prescription for reading than for distance, you probably do what I do - if I am reading and want to look at a clock some distance away, I just hold my reading specs a couple of cm forward off my nose - that changes the effective power by about 2 diopters and matches my distance prescription), the optometrist would have had to be a complete moron. Usually they check by homing in on the correct prescription from different directions. If you give wrong answers (as naughty children sometimes do) they soon detect it. Whenever I have had my eyes checked, the optometrist has first used a retinoscope to quickly get a rough idea, then uses the big-E chart to fine-tune.

I must correct some misconceptions however. It is not sufficient for the optometrist to ask "which is clearer?" The preception of what is clearer can differ from that which is best read. The patient may have the impression that the letters on the chart are their clearest, yet a small change in lens power of cylinder axis may improve his accuracy in reading the letters, particularly if he is getting specs for the first time. Usually what they do is ask "Which is better - first or second? (changes setting, pauses, changes setting back again) second or first?". then as it gets pretty good, they change to asking "Please read the 2nd last line." or similar. Then they distract you, change setting again, - and what do you know, one of those fine print letters seems to have changed!

Also, just because someone you know suddenly rocks up with lenses noticeably thicker or thinner, it DOES NOT necessarily mean they have radically changed prescription. It could be that he/she has changed to lense material of different refractive index. Spectacle shops these days tend to push the newer high-index lenses for cosmetic reasons, but the high-index materials have poorer colour dispertion. Wickwack60.230.208.161 (talk) 02:37, 11 July 2012 (UTC)

When you talk about "eye doctors", are they optometrists or opthamologists? 203.27.72.5 (talk) 22:10, 10 July 2012 (UTC)

Presumably an optometrist, if his only vision issue was that he needed glasses. StuRat (talk) 02:15, 11 July 2012 (UTC)

Infants are resolute about throwing off eyeglasses that do not help their vision. DriveByWire (talk) 14:20, 10 July 2012 (UTC)

I'm betting an infant would also throw off a pair of glasses frames with no lenses at all just because the plastic is touching their temples. 20.137.18.53 (talk) 17:57, 10 July 2012 (UTC)

They don't throw off glasses that let them see properly, when they couldn't previously, in my experience. It's kind of amazing to watch, because you'd expect them to not tolerate something on their face like that, but clearly if their vision is bad enough they are just pleased to be able to see properly.

Also, Wickwack, opticians aren't always so good at spotting the naughty children. I knew a child who lied on their eye test to get glasses (this soon came out after the novelty wore off). She was a particularly bright child, so maybe she gamed the eye test particularly well? But then I also know a naughty child who deliberately showed up to a school eye test without their (powerful) glasses on, so that the optician got very excited about how this child had slipped through the net, and the child's parents got a very strange letter home. Perhaps I know unusually determined naughty children? 86.164.62.161 (talk) 02:31, 14 July 2012 (UTC)

You can just compare their reactions without needing them to speak. If they ignore a toy, they probably can't see it clearly enough to realise it is something interesting. So you change the lenses until they show interest in the toy, at which point you know they can see it. --Tango (talk) 15:03, 10 July 2012 (UTC)

What's the street value of testogel?

Testogel, which I will not link as it is merely a redirect, is a form of testosterone that comes as sachets of gel, the idea being that someone with low testosterone can apply it to themselves. It is prescription medication meant either for hypogonadism or for FtM transexuals.

However it could also be used by body builders as it is an anabolic steroid. What is the street value of a pack of 30 x 50mg sachets of it? Egg Centric 13:37, 10 July 2012 (UTC)

Where are you? Is the purchase you ask about illegal? DriveByWire (talk) 14:26, 10 July 2012 (UTC)

I'm not interested in making any purchase - or indeed making any sale. I am legally prescribed testogel and being on the isle of man it costs me £3.65 a month (as any prescribed drug does). I was just interested in what the street value of it is as I enjoy boasting about that sort of thing in pubs (I am also prescribed another drug that has a much higher street value, incidentally, but that is irrelevant to this question). Egg Centric 17:38, 10 July 2012 (UTC)

You can order steroids legally via US webshops. Count Iblis (talk) 15:29, 10 July 2012 (UTC)

It's a bit hard to accurately assign a street value, even for a given location. A crack head that wants it might not be able to pay much of anything, or might offer a stolen stereo, whereas the token party-girl rich lawyer's daughter might pay a few quid, not realising that it can be so easily obtained legally. Oh, and purely out of interest, is yours prescribed for hypogonadism or FtM transexualism? 203.27.72.5 (talk) 22:02, 10 July 2012 (UTC)

It's just me or do other people also find it funny that the OP is on the Isle of Man? Plusanother (talk) 00:04, 11 July 2012 (UTC)

Surely 84,000 dooinneyyn(?) all have to be somewhere. That's not including Moddey Dhoo who can chase a greater number of cats than available tails. DriveByWire (talk) 15:13, 11 July 2012 (UTC)

Mick Jagger (a popular singer m'lud) has called for all drugs to be legalised on the Isle of Man. It seems that Narcotics Anonymous meetings are held at St. Joseph's Catholic Church, Douglas, Isle of Man. DriveByWire (talk) 22:29, 10 July 2012 (UTC)

Domestic wiring

I formatted this as a new question. DriveByWire (talk) 14:24, 10 July 2012 (UTC)

Which wire is used in domestic wiring? — Preceding unsigned comment added by Nikhilbdvg (talk • contribs) 14:17, 10 July 2012 (UTC)

Depends on the load and the local electrical code. In the U.S. most common is 3-wire 14 gauge for 15 amp circuits. You also see 12 gauge for 20 amp circuits for heavier loads (large appliances, a/c, etc.) Rmhermen (talk) 14:34, 10 July 2012 (UTC)

There are various kinds of domestic wiring. Suggest you read the article.--Shantavira|^{feed me} 15:40, 10 July 2012 (UTC)

'Non-metallic-dry' or NMD is the common wire used for woodframe in Canada. Most only count the insulated conductors though so 14/2 NMD would be 14 guage, (15 amp), black, white and bare ground. 14/3 has a red. Don't use heat trace cable though. I heard about a guy that stole a bunch of wire from work and wondered why is walls got warm and none of his stuff worked very well.--Canoe1967 (talk) 18:29, 10 July 2012 (UTC)

Why wasn't there fusion in the early evolution of the universe?

The strong nuclear force existed only a few fractions of a second after the Big Bang, and only a few fractions of a second later protons formed. So what was stopping the universe from undergoing fusion at around this time since presumably the required heat and pressure was present? 148.168.40.4 (talk) 16:01, 10 July 2012 (UTC)

According to the article, most helium-4 in the universe is thought to trace back to fusion "shortly after the Big Bang". See Big Bang nucleosynthesis. From 3 to 20 minutes after the Big Bang, protons and neutrons were stable, but fusion was still possible. Wnt (talk) 16:11, 10 July 2012 (UTC)

Too much heat and pressure was present. It's basically the same reason why million-degree-Celsius steam does not condense into ice. Looie496 (talk) 16:13, 10 July 2012 (UTC)

Wnt is right. There was fusion during the Big Bang nucleosynthesis. 203.27.72.5 (talk) 21:57, 10 July 2012 (UTC)

Yes, but, as Wnt said, not until around the 3 minute mark. The OP was asking about fractions of a second after time zero. The answer is that it was simply too hot for fusion then -- if anything fused together for an instant, something would slam into it and break it apart an instant later. Looie496 (talk) 22:58, 10 July 2012 (UTC)

It's an interesting question, and I don't really know enough to answer. But my understanding from the article is that first the quark-gluon plasma cooled down, but even then proton-antiproton pairs were being produced spontaneously at 10^-6 second, maybe later. So you don't have the situation where a cooling quark-gluon plasma would spit out intact iron nuclei because they're the most stable degradation product; they'd get hit instantly by antiparticles (indeed, almost always would contain antiparticles...) and be blown apart. At the latest point, right before the pairs stop being produced, there should only have been enough energy to make single nucleons, not alpha and anti-alpha pairs and such. After that, Looie's answer applies. I assume three minutes is when there was too little energy left to knock helium atoms apart, and twenty was when there was too little left to knock them together. (For comparison, consider how high temperatures favor production of nitrogen oxides, and lower but not too low temperatures resolve them to nitrogen and oxygen in a catalytic converter) Wnt (talk) 13:17, 12 July 2012 (UTC)

Electromotive force in an unclosed circuit

Even if an electric circuit is unclosed, the source of voltage continues to work — and the energy that would move the charges in the circuit, had the circuit been closed, is still there.

What resists the emf in an unclosed circuit? Intuitively, the gap in the circuit can be viewed as a resistor with infinite resistance that would imply infinite energy loss, had the current not been zero. But Ohm's and Joule's laws are abstractions. What's their physical sense at the ends of the gap, in terms of individial charge carriers (e.g. electrons)?

And what happens in the circuit, in the immediate vicinity of the gap, in the first moments after it is closed (e.g. by connecting wires or flipping a switch), enabling the flow of current? How do the charges flow and how does the information that the circuit was closed propagate from the gap to the circuit as a whole? - Sikon (talk) 16:53, 10 July 2012 (UTC)

Close circuit: electrons bounce back. Open circuit: electrons stop bouncing back. 188.76.173.109 (talk) 18:16, 10 July 2012 (UTC)

The free electrons have nowhere to go, so stay in the wire, until connected to another wire, then they flow into it. The charge in the wire (modeled by the percentage of free electrons) is equal to that of the source, say the battery terminal, so all flow stops. StuRat (talk) 18:25, 10 July 2012 (UTC)

At the level of the electrons, its best to analyse the situation using electric potentials and the "lines of force" along the gradient of the potential. The electric potential at a point is a measure of the energy at that point due to electrons and protons around it attracting and repelling any charge placed there. In a conductor like a metal, if no current is flowing, the potential all over its surface is the same; the electrons which are free to move jostle around so that no point is more crowded than another.

When a switch is open, the two metal contacts are at slightly different potentials - one is more crowded with electrons than the other. However, the electrons are bound to the surface of the metal (because of the metal nuclei) and it takes some energy for them to escape -either as free electrons or by jumping onto the air molecules and forming ions. If the voltage (electron overcrowding/shortage) is low enough, no electrons can escape from one surface to another. If the voltage is sufficiently high some of them will break out of their bonds and move between the metal and the air molecules. These air ions will zoom away from the metal surface, gaining speed as they are repelled and smash into more air molecules as they go to the other metal surface. In short order you will have a white hot stream of ions and electrons tearing across the gap. This is a spark.

As soon as the gap is closed, electrons from the more crowded side flow into the less crowded metal. (The electric field from these new electrons travels to all corners of the universe at the speed of light. But this field of course dies of as 1/distance-squared with the constant of decay depending on dielectric shielding by things in the way.) These electrons push the electrons around them, who in turn push the electrons around them and so on, letting the electrons at the far end of the wire flow out. In most wires, the electrons are packed so tightly and can move so rapidly that the effect is instantaneous. The time taken for the bunched-electron wave of information to travel down a line can be significant if you turn the switch on and off fast enough- for example in radio transmission.Staticd (talk) 19:01, 10 July 2012 (UTC)

We answered a similar question about Surface charges on an open circuit recently. DriveByWire (talk) 21:45, 10 July 2012 (UTC)

What causes a material to reflect light and why does light travel slower through matter than vacuum?

• What causes some materials to reflect light (metals for example)?
• What causes different materials to have different refractive index (i.e. light waves travel at different speed through it)?

85.230.137.182 (talk) 18:46, 10 July 2012 (UTC)

The article Refractive index gives a microscopic explanation of the slowing of light. DriveByWire (talk) 21:52, 10 July 2012 (UTC)

Light speed is around 186,000 miles per second in a vaccuum, or some such. I wonder, what is the slowest speed that light has ever been measured - and whether it's possible to slow it down even more. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:59, 10 July 2012 (UTC)

See slow light. 203.27.72.5 (talk) 23:07, 10 July 2012 (UTC)

That's a different kind of slowness than the refractive-index aspect. DMacks (talk) 03:09, 11 July 2012 (UTC)

Not being a scientist, I didn't totally follow the article, but it basically answered my question. ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:50, 11 July 2012 (UTC)

There is a touching story about a scientist who invented a slow glass through which light takes over a year to pass. He tested panes of the glass in his house and afterwards spent many days in the garden, occasionally looking up and waving. The explanation is that his wife had died the previous year. DriveByWire (talk) 14:33, 11 July 2012 (UTC)

That sounds like Light of Other Days by Bob Shaw. Gandalf61 (talk) 14:49, 11 July 2012 (UTC)

The reflection in a metal is basically because it conducts electricity. The electromagnetic wave in the light causes electric current to run on the surface of the metal. An electric field cannot exist parallel to the metal surface, and yet the light wave would normally have one. The net result of this is another light wave that bounces off in just the right direction for a reflection with the phase altered to cancel out that electric field. Graeme Bartlett (talk) 08:26, 11 July 2012 (UTC)

That sounds utterly wrong to me. I'm pretty sure that EMR does not induce a current in metal that it is incident on. This would also imply that all conductive materials are reflective, which they aren't. And what's so special about the visible spectrum in all of this anyway? Why should x-rays not induce a current in metals and reflect throught that mechanism if visible light does? 101.173.85.81 (talk) 09:26, 11 July 2012 (UTC)

Certainly currents are induced (in a perfect conductor, they are only surface currents); see also penetration depth. The visible spectrum is special because at higher frequencies the electric currents induced fail to precisely match the phase of the incident radiation, so absorption results instead; see also plasma frequency. --Tardis (talk) 13:20, 11 July 2012 (UTC)

Specular reflection#Explanation and Fresnel equations. DriveByWire (talk) 14:40, 11 July 2012 (UTC)

See the article on Plasmons--78.150.236.161 (talk) 19:52, 11 July 2012 (UTC)

Mysterious behavior of high-speed projectiles

Is it possible that a bullet (gun-shot) fired from a high-velocity rifle (say M-16 etc.) instead of entering the human head, orbit around it, breaking it's move-in-a-straight line rule and enter the skull from back side (opposite side , not available to gunner ) i.e. changing it's course of movement. If yes, then what laws of physics make it possible ? 124.253.91.183 (talk) —Preceding undated comment added 21:27, 10 July 2012 (UTC)

Unless the gun is actually firing very small, maneuverable rockets, no. What made you ask the question? Someguy1221 (talk) 21:32, 10 July 2012 (UTC)

I know it sounds idiotically strange ! I am asking this question because it has happened... — Preceding unsigned comment added by 124.253.91.183 (talk) 21:38, 10 July 2012 (UTC)

If so then you have already answered your own question "Is it possible....[?]". The article Orbit equation gives the physical laws that govern a gravitational orbit. I think it impossible to satisfy them with practical values for the bullet mass, head mass and bullet tangential velocity. The value of Escape velocity would surely be exceeded. Might the incident have actually been an accident with a Boomerang? DriveByWire (talk) 22:11, 10 July 2012 (UTC)

The 5.56 NATO round fired by AR-15 family rifles was known as the tumbler during the Vietnam war due its strange terminal ballistics. After striking flesh the bullet would move in counter-intuitive directions leading to exit wounds where you wouldn't expect them. A vet once told me a story about seeing a dead VC with a single entry wound near his navel, and a single exit wound near his collar bone. 203.27.72.5 (talk) 21:53, 10 July 2012 (UTC)

Intriguing. Do you happen to have a source or remember if they determined the compositional elements of the rounds that caused this uncommon ballistic profile? I'd be very interested in studying the issue since our current article on the round does not make mention of it (though it does note an unusually high degree of fragmentation for the round's mass and profile). Was the issue prevalent with regard to the M-16 or particular firing modes? Snow (talk) 00:18, 11 July 2012 (UTC)

No reliable sources, but here is a forum thread that basically confirms the same story my vet friend told me. As the posters note, having the centre of mass towards the rear increasing the chance of tumbling on contact with a medium denser than air. 7.62x39 are even worse. They often start to tumble end over end without impact with anything. You know this is happening when your bullet holes look like this. 203.27.72.5 (talk) 00:46, 11 July 2012 (UTC)

Thanks for the link (and actually there does seem to be some decent literature linked to there). And yeah, that explanation seems consistent with the yaw and accuracy issues that are noted on our 5.56 NATO article. I wonder how much the tumbling itself contributes to the fracturing or if they are better characterized as both the result of the round's center of mass. I'll peruse the literature and see if I can't find a way to note this behaviour as a brief mention in the relevant articles. Thanks again! Snow (talk) 01:03, 11 July 2012 (UTC)

I suspect that you are merely confusing the entrance and exit wounds. While, typically, the entrance wound is smaller than the exit would, that's not always the case. StuRat (talk) 22:08, 10 July 2012 (UTC)

That sounds like an element of a conspiracy theory, namely one related to the JFK assassination. Plusanother (talk) 22:07, 10 July 2012 (UTC)

A bullet found on Connally's hospital gurney, and two bullet fragments found in the presidential limousine, were ballistically matched to a 6.5 × 52 mm Italian Carcano M91/38 bolt-action rifle found on the 6th floor of the Texas Book Depository. The M16 fires a different 5.56×45mm cartridge, and there were not many M16's around on November 22, 1963. Obviously the existence of an M16 bullet that can fly backwards in space and time, and leave no trace, is exactly the kind of secret that a conspiracy of the CIA, the KGB, the American Mafia, the Israeli government, FBI director J. Edgar Hoover, sitting Vice President Lyndon B. Johnson, Cuban President Fidel Castro, anti-Castro Cuban exile groups and the Federal Reserve won't reveal. DriveByWire (talk) 22:46, 10 July 2012 (UTC)

You forgot the Rothschild and Rockefeller families and the Bilderberg Group. They are into this too, pushing the ropes from behind the scene. Plusanother (talk) 23:09, 10 July 2012 (UTC)

I've talked about gunshot wounds with a lawyer at some length, some time ago, and one point he emphasized rather emphatically is that bullets can behave extremely unusually once inside the human body. It's not weird physics, it's the fact that ammunition does not always have sufficient force to break out of the body at all points, and will hit various hard and soft things inside the body, and can do weird things like follow bones (get shot in the arm, exit wound out the leg, was one example the lawyer had seen). No weird physics there — just chaotic movement and complex systems. So, just a priori, I wouldn't rule it out, though it doesn't seem especially likely. But given enough gunshot wounds... --Mr.98 (talk) 00:23, 11 July 2012 (UTC)

There's also a simple ricochet, where the bullet could bounce off an object behind the target, say a tree, and hit the target from the rear. This could be the entire bullet, or just a fragment, if it shatters when hitting the first object (more likely if it first hit a rock). StuRat (talk) 00:29, 11 July 2012 (UTC)

Mystery solved? In all seriousness, OP, this behaviour is inconsistent with the basic physical principals we'd expect to be at work here - as noted by others above, the gravitational influence of the mass of the head upon the bullet would be miniscule compared to its forward velocity of the round -- nor could atmospheric resistance account for such a radical arc in said trajectory. Snow (talk) 01:16, 11 July 2012 (UTC)

Weirder yet was the soldier that got shot though his privates during the civil war. The musketball carried on and hit a female civilian in the abdomen and she got pregnant from it. I think they got married after the war when they figured out what happened.--Canoe1967 (talk) 15:50, 11 July 2012 (UTC)

Never happened. Someguy1221 (talk) 16:02, 11 July 2012 (UTC)

It did happen, it was Dilbert, a robot, a cow, and some aliens, though. μηδείς (talk) 16:34, 11 July 2012 (UTC)

But wasn't Dilbert getting shot at from the rear anyway? [3]--Aspro (talk) 23:03, 11 July 2012 (UTC)

I saw a murder trial in which someone killed a man with two shots to the head from a 32 caliber pistol. One round entered under the scalp on the side of the head, and tunneled around from the side of the head to the back of the skull, where it lodged in the neck muscles, but unlike the OPs incident, it did not proceed to break through the skull in the back, but at least it ended up in the right spot to do so if it still had sufficient velocity and tumbled the right way. Edison (talk) 20:18, 11 July 2012 (UTC)