Wikipedia:Reference desk/Archives/Science/2013 September 27

= September 27 =

SF - Some kind of anatomic pathology?
Anyone of you guys heard about a pathology shortly named "SF" ? (Common in runners), Can someone tell me it's actual name? thanks. Ben-Natan (talk) 00:00, 27 September 2013 (UTC)
 * I'm betting stress fracture. Looie496 (talk) 00:18, 27 September 2013 (UTC)
 * Agree. I've added it to the SF disambiguation page.--Shantavira|feed me 09:32, 27 September 2013 (UTC)


 * ...and it's been reverted. Oh, well...--Shantavira|feed me 16:12, 27 September 2013 (UTC)


 * Google search failed to find ANY cases where people had used the abbreviation "SF" in conjunction with running except in the context of San Francisco and Science Fiction. So, yeah - I support that revert - I don't think it belonged in the dab page for SF. SteveBaker (talk) 19:10, 27 September 2013 (UTC)


 * You probably used the wrong search terms. I got many relevant hits mentioning runners and stress fracture in which "SF" is abbreviation for the latter. Here're some snippets from the hits:


 * "Medial tibial stress syndrome (MTSS) and tibial stress fracture (SF) are common lower leg disorders in runners."
 * "We assessed CDR in female runners (> or = 20 km/wk) with a recent stress fracture (SF) and with no stress fracture history (NSF)."
 * "Anyone who's experienced a stress fracture, please comment. Is the pain from a SF constant or only when it's bearing weight?"
 * "Subjects consisted of 10 females with a history of at least one lower extremity stress fracture (SF) .... All subjects were between ages 18-35 and ran between 30-80 miles per week."
 * --98.114.146.6 (talk) 02:01, 28 September 2013 (UTC)

Waterfall fish kill ?
Do dead fish accumulate at the bottom of waterfalls, having died when they hit rocks at the bottom ? If not, why not ? StuRat (talk) 11:41, 27 September 2013 (UTC)


 * I'm interested to know why you think fish allow themselves to be carried over waterfalls in any but insignificant numbers. There are other issues to consider. The size of the fish, the depth of the splash pool, the volume of the flow, the speed of the flow, for example. The question is a little vague, but if we take it at face value I suspect the flow of water, local predators and natural decay prevents any accumulation. Richard Avery (talk) 14:01, 27 September 2013 (UTC)


 * I should think that waterfalls high enough to kill fish would be sufficiently rare that they wouldn't have developed an evolutionary response, that is, an avoidance strategy. Indeed, going over short waterfalls would be in the interest of the species, allowing it to spread to new areas.  And can a fish tell how high a waterfall is, from the top side ?  Maybe, if they can detect the low frequency sounds generated when the water hits, hundreds of feet below ?  StuRat (talk) 14:50, 27 September 2013 (UTC)


 * I would expect that fish do get carried over waterfalls every so often, but really nothing except rock can accumulate there, because it is constantly scoured by a blast of water. Looie496 (talk) 14:57, 27 September 2013 (UTC)


 * I don't think that's true, there are often eddies there holding floating objects right against/behind the falls. StuRat (talk) 15:29, 27 September 2013 (UTC)


 * You mean a recirculating stopper or hole, which can pin a kayak full of buoyancy to the riverbed for a considerable length of time. An eddy (in whitewater terms) is an area of placid water formed at the side of the main current. Alansplodge (talk) 21:59, 27 September 2013 (UTC)


 * I disagree on "placid". At my link it says "...in very powerful water, eddies can have powerful, swirling currents which can flip boats and from which escape can be very difficult." StuRat (talk) 11:31, 28 September 2013 (UTC)
 * Yes, but that's prefixed by "However, in very powerful water...". The flipping action is generally on the "eddy line" which divides the eddy from the main flow. I've never seen or heard of an eddy which is dangerous once you get into it. If such a thing exists, then it is a serious exception to the rule. In general terms, eddies are places that paddlers seek out to get their breath back. Trust me, I'm a kayak coach. See Basic Whitewater Kayaking River Features, "Eddys (sic) are usually calm spots that kayaks, rafts, and canoes can sit in while the rest of the river flows downstream." or Whitewater Paddling - Eddy Use & Etiquette, "Eddies are places where paddlers can stop for a while or relax before they start paddling again. Paddlers can rest, eat, or mingle around with other paddlers in eddies, which generally vary in size." Alansplodge (talk) 15:45, 29 September 2013 (UTC)


 * Sometimes, a little research can be helpful. Bielle (talk) 15:44, 27 September 2013 (UTC)


 * Interesting. Apparently the falls pose more of a danger to ducks and such.  Wouldn't you think they'd just fly away when they went over the top ? StuRat (talk) 17:28, 27 September 2013 (UTC)
 * I've often see birds hanging around waterfalls. They probably help in collecting whatever edible debris might go over. ←Baseball Bugs What's up, Doc? carrots→ 19:39, 27 September 2013 (UTC)

Algebra
How does n=e/(1+e) become e=n/(1-n). If you rearrange it just becomes n(1+e) = e. am I missing a rule of algebra here? Clover345 (talk) 12:02, 27 September 2013 (UTC)


 * $$n=\frac{e}{1+e}$$


 * $$\Rightarrow n(1+e)=e$$


 * $$\Rightarrow n+ne=e$$


 * $$\Rightarrow e-ne=n$$


 * $$\Rightarrow e(1-n)=n$$


 * $$\Rightarrow e=\frac{n}{1-n}$$
 * Gandalf61 (talk) 12:09, 27 September 2013 (UTC)


 * And don't forget:


 * Algebra = DeskMath
 * $$\Rightarrow$$ Algebra <> DeskScience


 * StuRat (talk) 12:56, 27 September 2013 (UTC)

Way to get Oils or Oil remnants of the hair?
If regular shampoo's doesn't help, how do you think you would take off (In a generally safe way), oils out of your hair? (Oils that might have got there by oily shampoos or other ways). Thanks. Ben-Natan (talk) 16:47, 27 September 2013 (UTC)


 * A detergent is what you need. Detergents for hand washing dishes are often strong, yet not too toxic, since they are for cleaning out greasy pots, but also must be safe to touch your hands.  Of course, a shampoo designed for oily hair will also have some detergent in it.  As always, be careful not to get either in your eyes.  Heat will also help, so turn the water temperature up as high as you can stand it.  And, after you remove the (presumably dirty) oil, you may want to replace it with clean oil, which is what hair conditioner does.  The only exception is if your scalp produces so much oil that your hair will soon be oily again, in any case. StuRat (talk) 17:13, 27 September 2013 (UTC)


 * Wash, rinse. Repeat. and Repeat. --208.185.21.102 (talk) 17:55, 27 September 2013 (UTC)


 * A Google search suggests that clarifying shampoos are specially formulated to remove a buildup of product residues and/or oils from hair, where ordinary shampoos may fail to do so. - Ka renjc (talk) 18:24, 27 September 2013 (UTC)

Touching a touchscreen
If you were to put a human standing on the top of a huge capacitive touchscreen (the human won't be touching anything else, but has the option of stepping on an isolating material or touching the screen with his skin), could the human activate the touchscreen?̉ OsmanRF34 (talk) 17:37, 27 September 2013 (UTC)
 * Capacitive sensing does not sound dependent on the user being grounded or having any other specific electrical properties beyond making contact. Could you explain the reason you suspect the person in your scenario would not be able to activate it? DMacks (talk) 17:44, 27 September 2013 (UTC)


 * I thought that in this scenario the human would acquire the same charge as the capacitive touchscreen, and the touchscreen won't be able to register any contrast. OsmanRF34 (talk) 17:56, 27 September 2013 (UTC)
 * You're thinking DC. But an AC current can continuously flow to an object (such as the human body) which doesn't appear to be a part of a closed circuit, due to self-capacitance.  See also body capacitance.   Red Act (talk) 20:45, 27 September 2013 (UTC)
 * The points on the display are scanned, a pulse is applied to one electrode, and the response in the other electrode is measured. Doesn't matter at what voltage the person is, because it's the change in voltage that is measured (the amplitude of the output signal compared to the amplitude of the input signal), and that will be smaller due to the added capacitance. Ssscienccce (talk) 18:42, 28 September 2013 (UTC)

Why are the babies in need of sleep more than adults?
Why are the babies in need of sleep more than adults (almost twice as adults)? 95.35.246.240 (talk) 19:58, 27 September 2013 (UTC)


 * We don't understand why humans (or other animals) need sleep - so it's unlikely that there is solid information about why babies need more of it. Our article on sleep says: "Children need more sleep per day in order to develop and function properly" - which doesn't really tell us much.
 * IMHO, it's evolutionary - if babies didn't give their parents enough time to recover from 6 hours of solid screaming and fussing - they'd never make it past 6 months! SteveBaker (talk) 20:13, 27 September 2013 (UTC)
 * An interesting fact, and probably an important clue, is that babies spend a much larger fraction of their sleep in the REM (dreaming) state than adults do. Looie496 (talk) 22:15, 27 September 2013 (UTC)


 * We need sleep because we get sleepy. Babies probably get more sleepy than adults, therefore they sleep more. OsmanRF34 (talk) 22:39, 27 September 2013 (UTC)


 * Seriously, it is as Steve says, nobody knows exactly. But we know something about sleep: all species do it (=> it has to be pretty old, in evolutionary terms), in one form or the other. Although not all sleep the same. :Other species need more sleep than us. Some bats need 20 hours, elephants 3-4, giraffes need just 2 hours. If you deprive a rat of sleep, it will die in 2 weeks (=> it is biologically necessary).


 * Human babies need to sleep a lot for proper development. Growth hormones are secreted by the pituitary gland at a much faster rate while babies sleep. However, other baby mammals, like baby dolphins and baby orcas, don't seem to sleep at all, (=> evolution found other ways of making it work). OsmanRF34 (talk) 23:08, 27 September 2013 (UTC)


 * One valuable clue of the importance of sleep is what happens as a consequence of lack of sleep, as with fatal familial insomnia. Those suffering from that disorder are literally unable to sleep - and it's a death sentence. They eventually suffer dementia and finally they die. ←Baseball Bugs What's up, Doc? carrots→ 00:21, 28 September 2013 (UTC)


 * It seems to me that this is fundamentally a game theoretical issue. Suppose you start with two identical systems that compete with each other and they also need to repair themselves. Initially they function in a uniform way, they go about their business and do the repairs they need to do to themselves at a constant rate. But then one system can outperform the other by working at a faster rate that on the long run is unsustainable and then having a downtime later to do extra repairs. As a bonus those repairs can be perfomed more efficiently during the downtime. Count Iblis (talk) 01:47, 28 September 2013 (UTC)


 * And there people who have survived for decades without any sleep at all, e.g. Thai Ngoc, Al Herpin, Paul Kern. Count Iblis (talk) 02:24, 28 September 2013 (UTC)


 * That's probably a myth. There have been many claims about people who never sleep, but whenever such people have been closely monitored, they did in fact sleep to some degree.  There are no documented cases of people who could live with no sleep whatsoever. Looie496 (talk) 06:47, 28 September 2013 (UTC)


 * Here are my thoughts:


 * 1) The reason for sleep in the first place is it allows us to run our bodies at a faster, unsustainable rate while awake, and make repairs while asleep. Combine this with the rotation of the Earth, which means we get a few hours of light followed by a few hours of darkness, and we get organisms adapted for either the day or night, and sleeping for the period to which they are not well adapted.  Humans seem to have a sleep cycle which roughly corresponds with the length of the shortest nights, ensuring we can always be awake during daylight, when we could do all the things needed to ensure our survival.  (Of course those living near the poles are an exception.) It would also seem to make sense for us to sleep longer in winter, but our bodies don't appear to have adapted to do this (other species do, however, fully or partially hibernate).  And, of course, the introduction of artificial lights mean the evolutionary pressures to avoid activity at night are now gone.


 * 2) In the special case of babies, they have very little they need to do each day which requires that they be awake, just eat and learn (they don't even need to be awake to poop). On the other hand, they are doing a great deal of growing, and this requires a lot of work which can be done in their sleep, like digesting, forming neural connections, etc.  So, it makes sense for babies to be asleep more than adults.  And the workload on the parents might figure into it, too, as babies which are awake and in need of watching constantly make it hard for their parents to make a living, and they might have tended to not survive, as a result. StuRat (talk) 11:25, 28 September 2013 (UTC)


 * Interesting fact: hibernation in animals is periodically interrupted, and it seems that they do so in order to be able to sleep. This suggests that sleep has nothing to do with reducing energy expenditure. Ssscienccce (talk) 14:55, 28 September 2013 (UTC)

How does a laser cut glass?
It seems counterintuitive that a laser could cut something that is clear. How is this possible? Specifically, laser cut sapphire glass. --208.185.21.102 (talk) 21:27, 27 September 2013 (UTC)


 * Are you looking for a technical or a non-technical answer? This absolutely is counter-intuitive; commodity laser cutters can't do it (reliably).  Our article describes several distinct physical mechanisms that allow a laser to cut a material.  In the case of glass, the procedure is difficult and dangerous; a powerful laser is shined onto a surface that's simultaneously specular and transparent... it takes a lot of engineering to make sure the laser energy gets absorbed, and not wasted, or reflected back into the laser, or accidentally reflected to somewhere else undesirable.  Nimur (talk) 21:47, 27 September 2013 (UTC)


 * Was that the technical or non-technical answer? It seems you didn't answer the question at all except to say, "With difficulty man!  It's hard!!  In fact commodity lasers can't do it.  It takes a lot of engineering."  I guess "with a lot of engineering" counts as a nontechnical answer for how does a laser cut glass! :) 178.48.114.143 (talk) 23:07, 27 September 2013 (UTC)
 * My response wasn't really meant as an answer... it was just a request for clarification. Shall we deep dive into optical physics and evanescent waves now, or should we wait for Steve Baker (who has operated his own laser cutter nearly 24/7 for the last year!) to bring a more pragmatic hands-on style explanation?  I have a sort of suspicion that his lasersaur does not cut sapphire glass, but I'd be curious to hear his input.  Nimur (talk) 23:15, 27 September 2013 (UTC)


 * If Steve Baker has operated his own laser cutter nearly 24/7 for the last year, 1) he's probably the world's least qualified person to answer this question 2) give up man, just give up already... 178.48.114.143 (talk) 23:29, 27 September 2013 (UTC)
 * I've been working with laser optics for over a decade, for many different purposes, ranging from laser cutting to optical probes of chemical species, materials and surface properties; and Steve's one of the reference-desk's best and most well-respected contributors; but if you're unhappy with the quality of our free information and would like me to "give up," perhaps you can find better information by purchasing a textbook on laser physics. My class used an unpublished textbook, and its course webpage is not currently available; but you can purchase Laser Materials Processing at Amazon for the low price of just $350.  Nimur (talk) 00:56, 28 September 2013 (UTC)
 * It's just a joke, Nimur. If Steve's been running a laser non-stop for a year, he's not making very much progress (unless he's trying to break into Fort Knox). Clarityfiend (talk) 01:23, 28 September 2013 (UTC)
 * Steve's running a small business manufacturing miniature laser-cut figures, and operates out of his living room (or so it appears). His laser cutter, according to his website and his comments, has been highly utilized.  He'll probably be happy to discuss it in more detail.  Nimur (talk) 01:36, 28 September 2013 (UTC)
 * I'm far from an expert on lasers, but I presume that at least one consideration is that sapphire glass is only transparent in the wavelength range of about .15-5.5 µm, so it's opaque to the light from lasers outside of that range. There exist commercially available lasers with wavelengths up to 699 µm; see here.  Red Act (talk) 21:59, 27 September 2013 (UTC)
 * A CO2 laser has a wavelength of between 9.4 and 10.6 µm and was the first (and still is the main) laser used for cutting purposes (highest continuous power output, high efficiency). It uses germanium or zinc selenide windows (first windows were made of salt crystals). A popular laser with hobyists because it's easy to make and cheap. Lots of examples on youtube.
 * For sapphire (like semiconductor wafers), one would probably use a UV pulse laser; the pulses deliver more power, limiting the heat transfer to the rest of the wafer; in effect, a small dot is ablated (evaporated) without putting thermal stresses on the rest of the material. (Dicing sapphire wafers) Ssscienccce (talk) 12:44, 28 September 2013 (UTC)

OK, here is what I know and why. My g/f and I own and operate two lasersaur UV laser cutters. We mostly cut plywood to make model buildings for tabletop gamers. Our CO2 lasers can't cut glass - but they can etch the surface a little. However we can cut acrylic plastic (even the stuff that's optically transparent...but we can't cut polycarbonate - which looks pretty much identical to acrylic! What's going on here is that you're thinking of glass as being transparent - which it is in the visual spectrum.  However, in UV, it's totally opaque.  Knowing whether something is transparent to visible light really doesn't tell you much about IR, UV, microwaves, radio, etc - which is why acrylic and polycarbonate (which are completely indistiguishable to the naked eye) are so completely different in UV.  In fact, the lid of the lasersaur is made from polycarbonate precisely because it's impervious to the laser light and blocks it from wrecking your eyesight.  The laser safety goggles we have are also polycarbonate.

Some opaque materials (like plywood) cut nicely because the laser can start a chemical reaction that removes the material bit by bit until it's cut all the way through. Others absorb heat from the laser and melt, others undergo some other kind of reaction. But glass, pretty much just heats up on the surface and cracks a tiny bit...which lets you etch it - but not cut it.

That said, there are lasers that work in other parts of the electromagnetic spectrum that might be able to cut glass. I don't know. (And I'm using a tablet computer which makes it painful to research this for you!!)

SteveBaker (talk) 13:28, 28 September 2013 (UTC)
 * Glass sheets for flat panel displays used in mobile phones can be cut by a co2 laser. Often the lasers are used to scribe the material, after which it is broken along the scribe line, just as you do with a diamond tip glass "cutter" (the breaking can be done with a second laser reheating the scribe line). Scribing with CO2 lasers can be used on glass from 200 µm to 10 mm thickness. Full body cutting is used on 30 µm to 1 mm glass. There is a marked difference in speed, scribing can be done up to a meter per second or more, full body cutting happens at less than 20 mm per second. In both cases the glass passes under a coolant nozzle right after the laser beam, liquid cooling used for scribing, gas cooling for full body cutting. In scribing, the cooling causes small thermal stress cracks, in full body cutting, the higher energy and lower cooling has the crack propagate through the entire thickness. (Laser glass cutting in flat panel display production) Ssscienccce (talk) 13:51, 28 September 2013 (UTC)


 * Steve, do you know which of the cutting modes (chemical reaction, local heating, etc.) happens for the CO2 laser on acrylic and why polycarbonate is not affected? It sounds counter-intuitive that polycarbonate absorbs the laser light but does not get affected by it: why doesn't all that energy do...something?. DMacks (talk) 05:38, 29 September 2013 (UTC)
 * CO2 lasers can be used for cutting polycarbonate, but only with gas-assisted lasers: a nozzle providing 2-6 bar nitrogen or air blowing away the fumes. The smoke will clog up filters, the gas cannot be recycled, basically, it's just messy. See for example this company who mention "Polycarbonate (PC, Lexan) – we stopped cutting Lexan due to the fumes." It's the only material as far as I can see that they initially offered but later abandoned. Ssscienccce (talk) 14:41, 29 September 2013 (UTC)
 * Steve, I'm a bit confused by your mentioning of UV, don't you mean IR? UV exposure would be the main concern when operating the CO2 cutter, because the high temperature at the target is a significant source of UV radiation (same as with welding), but the laser beam itself is IR of about 10.6 µm. While the Ir is dangerous as well, it doesn't penetrate the eye and get focused on the retina like the UV can. IR damage you would feel when it occurs, unlike the delayed and cumulative effects of the UV, "arc eye" or welders' keratitis. So the emphasis may be on the UV when it comes to safety, but the only CO2 lasers with UV laser light I know of are the CO2/UV laser drilling machines for multi-layer boards that combine diode laser pumped UV lasers and CO2 lasers; the IR used for making 45 to 200 µm holes, the UV for small 18 µm holes. Ssscienccce (talk) 18:48, 29 September 2013 (UTC)