Wikipedia:Reference desk/Archives/Science/2011 April 12

= April 12 =

DNA samples and testing time
Two sets of multi-part questions on the subject of DNA testing:

—SeekingAnswers (reply) 02:44, 12 April 2011 (UTC)

(1) How easy is it to get DNA samples?
In some films and television shows, the police find DNA everywhere. They can get it by dabbing the inside of a person's mouth with a swab, or from a cup that a person has drunk out of. I think I've even seen cases of police getting it from surfaces that a person has touched with the tip of his or her finger.

On other films and television shows, getting DNA samples seems to be really hard: the police have to draw blood from the person they want to test, and samples seem to get corrupted and become unusable all the time. (In fact, the difficulty of getting a DNA sample seems to be most correlated to whether the plot demands a successful or unsuccessful DNA test...)

Are dead skin cells scraped off with casual contact with any surface sufficent for a DNA sample, or are bodily fluids necessary? Which bodily fluids? (That is: saliva? urine? blood? semen? vaginal lubrication?)

If getting DNA samples is very easy, then in cases of rape, even if the rapist used a condom so that semen could not be collected for DNA testing, it should be easy for the police to get DNA samples from skin cells left on the body of the victim, correct?

—SeekingAnswers (reply) 02:44, 12 April 2011 (UTC)
 * You can DNA samples from any of the above; however the chance of recovering enough usable DNA is slimmer the smaller and deader the sample. DNA tends to degrade over time; however since the advent of PCR, even the most minute amount of DNA can be replicated and magnified into as much as you need to do tests.  Very recently, there have been a few studies on extracting DNA from samples as small as fingerprints, and work is being done to reliably extract DNA from a single hair, but the work has not been expanded into clinical viability (i.e. tested and retested to the level where the method is reliable enough to stand up in a court of law).  However, DNA has been extracted from cigarette butts and coffee cups before.  If you've touched it, you've left your DNA on it; the question becomes whether you've left enough to be useful.  We're very close to that point; it may be likely that within the next decade or so we'll have perfected the "DNA from fingerprints" technique, making actual fingerprinting moot. -- Jayron  32  04:07, 12 April 2011 (UTC)


 * I give a thumbs-up to Jayron32's response above, with a few twists. Actual fingerprints will remain useful in situations where DNA would be poorly preserved; some circumstances will favor the long-term survival of the prints, others may better protect DNA.  I suspect that databases of fingerprints are larger and more comprehensive than databases of DNA, though the difference is probably eroding.  Collecting and processing fingerprints still generally requires less-costly equipment and can be done faster than DNA typing.  Finally, DNA plus fingerprint may perform better in court than either test alone (particularly if someone ever successfully mounts a Daubert challenge against fingerprint evidence).
 * While it is technically possible to extract utterly miniscule amounts of DNA from samples – with modern PCR techniques, a single cell's worth from a suspect should in principle suffice – the risk of error or cross-contamination also rises as one becomes able to amplify smaller and smaller amounts of DNA. Consider a hypothetical case where a cigarette butt is recovered from the crime scene.  DNA on the butt matches one of the suspects, but there is also DNA from a second unidentified individual.  Defence attorneys get the evidence excluded as unreliable, or insist that the mysterious second person is the actual murderer.  What happened?  A single skin cell was shed by a cigarette plant worker; the cell settled on the cigarette packaging and fell onto the outside of the cigarette filter, from which it was eventually recovered by forensic investigators.
 * Sound far-fetched? Something similar has already happened in Germany: story.  DNA at forty different crime scenes over a span of more than a decade was linked to the same woman, dubbed 'The Phantom of Heilbronn' by the press.  The Phantom's DNA never matched any of the suspects in any of the cases; police were stumped. Investigators eventually came to believe that The Phantom was just a careless worker in a factory that manufactured cotton swabs used by forensic investigators. Sloppy manufacturing processes allowed traces of the employee's DNA to contaminate the swabs, causing her DNA to 'appear' at crime scenes across Germany.
 * Biochemists and molecular biologists who do high-sensitivity proteomics experiments are familiar with this problem. Labs routinely ignore any hits for keratin as irrelevant in their mass spec results, because that protein is abundant in skin, hair, and nails and contamination with it is virtually impossible (or horrifically inconvenient and expensive) to avoid.  TenOfAllTrades(talk) 16:18, 12 April 2011 (UTC)
 * For extra credit, you can visit Murder of Meredith Kercher and associated talk archives... Wnt (talk) 21:10, 12 April 2011 (UTC)

(2) How long does DNA testing take, and why?
On television shows like CSI: Crime Scene Investigation, DNA tests take just a few hours, or sometimes just minutes.

In the real world, I've seen news articles about police, lawyers, and/or suspects waiting for months for DNA tests. Are these cases unusual?

How long do DNA tests usually take in real life? And why would they take so long? The chemical reactions involved surely can't be so slow that they take months to run their course.

—SeekingAnswers (reply) 02:44, 12 April 2011 (UTC)


 * It is strictly a matter of economics and priorities (or lack of them). Some jurisdictions are requiring DNA testing for shoplifting; others set very low priorities on testing of rape specimens when no immediate suspect is known.  Evidence can sit around for years untested.  I'd love to see these statistics broken down by the race and income of the victim. Wnt (talk) 03:24, 12 April 2011 (UTC)


 * The actual DNA test can be done in hours. It consists of a) isolation b) PCR c) DNA fingerprinting.  The entire sequence can be completed by a technician within a few hours; the average DNA analyst can, by staggering procedures, probably churn through several in a day.  The reason that real cops and prosecutors wait weeks or months for results is that there is a backlog of submitted evidence.  Occasionally, a high-profile case will get rushed, but generally it is first come-first served, and everyone waits in line.  It just takes time for the DNA analyst to get through all of the cases that got to his/her office before yours did.  It doesn't matter how much you want your case done; the 50 other prosecutors and sherrifs and whatnot that submitted evidence before you did want their evidence just as much, and you're not any more important than them... -- Jayron  32  04:12, 12 April 2011 (UTC)


 * So...why aren't jurisdictions hiring/training more technicians and buying more equipment? That DNA testing can take months for a process that should only take hours and thereby clogs up the entire criminal justice system shows that DNA testing facilities are grossly understaffed, to an extreme degree, worse than any understaffing of police, prosecutors, prison guards, etc. No doubt criminal justice systems have many inefficiencies, but this has to be one of the worst. If police and prosecutors are all waiting months for DNA tests, then it would seem that DNA testing is the obvious limiting factor in the criminal justice system, and therefore one of the most pressing problems that needs more budgeting. —SeekingAnswers (reply) 05:55, 12 April 2011 (UTC)


 * They would love to. I'm am certain that if you have several million dollars, and would like to donate it to start a trust fund with which to provide both the capital investment in new equipment, and ongoing funding for both its maintenance, and for more technicians to do this work, your local crime lab would LOVE to work with you.  Most places in the world, and especially in the U.S., are so strapped for cash that they are actually furloughing or outright laying off workers in these areas just to avoid having to file bankruptcy.  In North Carolina, where my wife works as a forensic chemist, there's been a hiring freeze; they can't even replace the analysts who they've lost through deaths, retirement, or quitting, never mind expanding capacity... -- Jayron  32  06:07, 12 April 2011 (UTC)


 * Yeah, but what I don't get is this. In the news, I constantly hear about plans to "add a hundred more officers to the streets next year" (for mid-size cities) or "add a thousand more officers to the streets next year" (for large cities). It would seem that some of the money going to those hundred/thousand extra officers for next year would surely be better served hiring more technicians and buying more equipment. There's no way that it's cheaper to hire and arm a thousand police officers than it is to hire and equip ten more technicians, so clearly, money can be found from somewhere. The problem doesn't seem to be (or at least, not just) a lack of money; it's that the money is going into the wrong places and being inefficiently allocated. —SeekingAnswers (reply) 06:16, 12 April 2011 (UTC)
 * Putting more officers on the streets is often more about politics. Politicians in the UK are always boasting about how they are going to put more police on the "frontlines" (instead of doing office/paperwork) because when people are asked what they what from the police force, it's always more police on the streets. Recent suggestions that budget cuts will lead to reductions in the number of police have led to a furore in local press. No-one ever says, "But what about the evidence processing?!" Technical work is not high-profile (unless something goes very wrong), and tends not to get funding. In fact, the UK's government-owned Forensic Science Service is being shut down, and the work contracted out to private companies, which (sorry, personal opinion here) is going to be an absolute disaster.-- Kateshortforbob talk  09:23, 12 April 2011 (UTC)
 * As I hinted above, even if the police have somewhat sufficient capacity, it's likely to be wasted. It takes only a stroke of the pen to demand that all shoplifters be tested, that people be tested on arrest simply to fill a database, etc.  I suppose there's a feeling that if you actually do budget for increased capacity, it will only be wasted anyway.  (In fact, I think that by collecting too much DNA information, countries greatly harm themselves, because in future generations it may be impossible for them to send a spy to another country without the target finding out who they sent, who the agent's family is, etc.) Wnt (talk) 00:59, 13 April 2011 (UTC)

Efficiency of fixed wings vs. rotors
Long-distance aircraft tends to be fixed-wing aircraft rather than rotary. Is this because fixed wings are inherently more efficient than rotors?

—SeekingAnswers (reply) 04:09, 12 April 2011 (UTC)


 * Fixed-wing aircraft can run on jet engines, which are orders of magnitude more efficient than propellers are. Helicopters are propeller planes, by necessity, which is why they aren't as efficient.  -- Jayron  32  04:18, 12 April 2011 (UTC)


 * On a fundamental level, how does a jet engine differ from a propeller? Both are big fans that generate thrust by spinning rapidly and pushing air behind them, so what exactly is the basic difference between them?


 * Would it be possible to attach a jet engine to the back of rotary aircraft to generate thrust and counter torque from the main rotor while that main rotor generates lift?


 * —SeekingAnswers (reply) 04:37, 12 April 2011 (UTC)
 * A jet engine is a Reaction engine it is fundamentally completely different to a propeller engine. The fans inside certain types of jet engine are NOT what actually cause the thrust, they are the intake, compression and exhaust stages; they move the fuel and gas around but it is the combustion gas that creates the actual thrust. There are several jet engine designs which completely lack any kind of propellers or fans. Vespine (talk) 04:53, 12 April 2011 (UTC)
 * As for rotary aircraft, the main problem is the rotating wing has to travel "backwards" at some point. When the aircraft begins to approach a speed comparable to the speed of the blades, in essence when the blade is traveling backwards it has a zero airspeed and generates no lift. If you strapped a jet on for speed, you'd also need to use it for lift beyond na certain velocity, at which point, your whole "rotor" mechanism just becomes dead weight to a conventional jet aircraft. We have tilt rotor aircraft which combines the benefits of rotary aircraft and fixed wing aircraft, but if you haven't noticed, there aren't many around in civilian use because they have their own whole set of challenges and difficulties. Vespine (talk) 05:02, 12 April 2011 (UTC)


 * Fundamentally different? Large airliners universally use turbofan engines, essentially a ducted many-bladed propeller driven by a gas turbine in the middle. The fundamental difference between that and a turboprop is just that the latter has a gearbox between the turbine and the propeller and no duct. Modern helicopters also, as far as I understand, tend to be driven by gas turbines. –Henning Makholm (talk) 12:16, 12 April 2011 (UTC)
 * Hmm, well I'm not an expert but I think you are wrong. In a turbofan engine, it's NOT the "fan" that creates the thrust but the exhaust gasses. The exhaust gasses are NOT created by the fan but by fuel combustion, which is what creates the thrust AND also powers the fan. The fan is there to provide compression and exhaust stages to the engine. Read the turboprop article again: The engine's exhaust gases contain little energy compared to a jet engine and play a minor role in the propulsion of the aircraft. A turboprop is powered by a jet turbine, that's like a jet engine except the primary role of a turbine is to turn a shaft (like in a helicopter), not create thrust like a jet engine. Fundamentally different methods of propulsion. Vespine (talk) 23:00, 13 April 2011 (UTC)
 * Have another look at Reaction engine. Jet engines are reaction engines whether they have fans in them or not. Propeller engines are NOT reaction engines whether they are driven by internal combustion or a turbine. Vespine (talk) 02:09, 14 April 2011 (UTC)


 * In a turbojet engine (or pure jet engine) there is only the exhaust gas leaving the engine, and it is responsible for all the thrust. In a turbofan engine there are two streams of gas leaving the engine - firstly, the hot stream which has passed through the fan, the compressor, the combustion chambers, the turbines and finally the nozzle, and is analogous to the exhaust gas in the turbojet engine; and secondly, the cold stream which has passed through the fan but not through the compressor or the combustion chambers.  In a low by-pass turbofan engine the thrust from the hot stream is greater than that from the cold stream, but in a high by-pass turbofan engine the thrust from the hot stream is less than that from the cold stream.  The cold stream has a lower speed than the hot stream and it causes a turbofan engine to achieve higher propulsive efficiency than a turbojet.  In a propeller engine, the air leaves the propeller at a slower speed than even the cold stream of a turbofan so at slow speeds a propeller-driven aircraft has higher propulsive efficiency than a turbofan.  The downwash from the rotor of a helicopter is even slower than the flow through the propeller of a propeller-driven aircraft, so the lift generated by a helicopter is done fairly efficiently but nowhere near as efficiently as the lift generated by a fixed-wing. Also, helicopters are condemned to be much slower aircraft than fixed-wing aircraft so the fuel consumed per unit of horizontal distance will always be higher for a helicopter than for a fixed-wing aircraft of similar payload capacity.   Dolphin  ( t ) 07:08, 16 April 2011 (UTC)

Maximum carrying capacity of aircraft
Our article on the Airbus A380 says that it is certified to carry up to 853 passengers, almost a thousand people. That got me wondering, just how big can we theoretically make aircraft, assuming that cost-benefit-risk ratios were not much of a factor? (What sort of actual physical limits would there be on the size of aircraft?) What would be a good estimate for the upper limit on the number of passengers a long-distance airplane could theoretically carry (again, assuming cost-benefit-risk ratios were not much of a factor)? Could an airplane for 2,000 people be built? 5,000? 10,000? 20,000? 50,000? 100,000? Would such an enormous airplane fly more efficiently with fixed wings or with rotors?

—SeekingAnswers (reply) 04:09, 12 April 2011 (UTC)


 * Roughly speaking, larger aircrafts need larger engines and more fuel to operate; and they must be able to lift their own engines and their own fuel. What this means is that the larger an aircraft is, all other things being equal, the larger the proportion of its weight will be taken up fuel and engines, and thus the smaller the proportion of its weight will be availible for passengers and cargo.  Given a set of materials and general construction mechanisms, there is an upper limit for any aircraft design, in terms of size.  This probably varies depending on the exact aircraft type, but there is a limit.  -- Jayron  32  04:16, 12 April 2011 (UTC)
 * The above comment needs sources. While the first part – that larger aircraft will require larger engines – is reasonable, the second assumption – that larger aircraft must use proportionately more mass for engines and fuel – doesn't (ahem) fly.  Fuel efficiency in transportation notes that a 747 uses about one-sixth the fuel per passenger mile of a Gulfstream business jet.  I strongly suspect – though I would welcome a search for valid numbers – the thrust-to-weight ratio of turbofan engines generally improves slightly as the engines get more powerful, and not the other way around.  Upper limits on aircraft size are governed by the maximum capacity of airport facilities (including gate and taxiway spacings) and the amount of time it takes to load and unload passengers and cargo from a hypothetical extremely large passenger aircraft, rather than physical limitations on aircraft efficiency.  Few routes carry enough traffic to justify aircraft carrying more than five hundred passengers, and even on high-traffic shorter-haul routes an airline my prefer to use two fast-loading jets instead of one big craft that spends a long time at the gate.  Aircraft manufacturers have no incentive to design aircraft that won't be able to land at existing airports and which airlines won't want or need to buy. TenOfAllTrades(talk) 16:41, 12 April 2011 (UTC)


 * Airports preparing for the A380 had to extend and widen runways, and modify terminal facilities so people could get on and off the plane in a sensible time. The same would apply for bigger planes, only more so. HiLo48 (talk) 06:01, 12 April 2011 (UTC)

So, assuming you have unlimited budget and no concern for profit, does anyone have any actual maximum passenger estimates? My thinking is that at some size, even with unlimited budget and no concern for profit, you would still encounter insurmountable problems because of fuselage stress issues (as in, compressive stress, tensile stress, etc., would make aluminum and titanium no longer viable options for construction), and you would have to turn to increasingly exotic construction materials. —SeekingAnswers (reply) 06:27, 12 April 2011 (UTC)


 * I'm not sure that there is an upper limit, if we ignore costs/benefit analysis. After all, you could just chain multiple airplanes together and call it a single airplane.  As for a large, single fuselage plane, perhaps a ground effects airplane (no article ?) that launches and lands in water would work best, since that would eliminate the need for runways, and would limit fuel req's.   See Spruce Goose (if we made this out of aluminum or composites it would be far more practical). StuRat (talk) 06:55, 12 April 2011 (UTC)


 * There is an article at ground effect vehicle, and I've now created a redirect from "ground effects airplane". But yes, my question regarding an estimate for the upper bounds of passengers is for a single-fuselage plane. I'd also be interested in any comments people have for the type of construction materials to use for this theoretical maximum-size aircraft. —SeekingAnswers (reply) 09:53, 12 April 2011 (UTC)


 * According to this there is a worldwide limit at air terminal gates of 80 meters wing span. The A380 fits at 79.8 meter wing span but the "Spruce Goose" is too large at 97 meters wing span. Cuddlyable3 (talk) 13:07, 12 April 2011 (UTC)
 * Also, the Spruce Goose would need to have the runway flooded! Alansplodge (talk) 20:34, 14 April 2011 (UTC)


 * I've seen a documentary about 10 years ago in which it was pointed out that flying wing designs become more efficient for very large planes and that an optimal design for a plane capable of carrying many thousands of passengers would look like a giant flying saucer. Count Iblis (talk) 14:24, 12 April 2011 (UTC)


 * You might want to read about the Antonov An-225 which is the world′s heaviest aircraft and also longer than the Spruce Goose (although not as wide). Our article says it can carry 640 tonnes, so I guess you could equip it to carry more people than the A380 can if you wanted to. SmartSE (talk) 15:15, 12 April 2011 (UTC)


 * A regulatory limitation, as opposed to a physical limitation, is the requirement that the airplane be capable of evacuation in 90 seconds, limiting travel distance and requiring increasing numbers of exits.  Acroterion   (talk)   17:05, 12 April 2011 (UTC)

Giant Sequoia
How long do Giant Sequoias live up to? 174.114.236.41 (talk) 04:19, 12 April 2011 (UTC)


 * According to our Giant Sequoia article, the "oldest known Giant Sequoia based on ring count is 3,500 years old." —SeekingAnswers (reply) 04:38, 12 April 2011 (UTC)

Taxonomy/Etymology question...


I understand that the Australian Possum and the North American Opossum are only very distantly related; they belong to different orders. Still, I have a question about the etymology of the Australian Possum name specifically. It seems that the North American Opossum gets its name from an Algonquin word. That makes sense; they were here before the English speakers, and lots of native New World animals and plants have derived their English name from Native American names. I am quite sure that is also true about Australian animal and plant names; many of them derive from Aboriginal names of the same critters. Still, I am thus confronted with an interesting conundrum: How is it that these two marsupials should have similar names at such a geographic distance. I am left with some unsatisfying possibilities: So, to sum up this insanely long question: What is the actual etymology of the Australian Possum's name? -- Jayron  32  05:28, 12 April 2011 (UTC)
 * That the Australians took the name of the American animals and applied it their own. This doesn't seem to make much sense; other than being marsupials, Possums and Opossums don't look all that much alike; Opossums are more rodent-like in appearence, Possums more resemble primates like Lemurs or Bushbabys.  Furthermore, the entire idea that Australians would appropriate a Native American name, to describe a native Australian species seems a bit weird, if you ask me, and any path that would take the name from North America to Australia seems quite convoluted.
 * The Australian Possum is derived from an Aboriginal name after all. This seems equally weird; Aboriginal languages and Native American languages are linguisticly so far apart, I can't imagine one influencing the other, and yet this would be an example of the most bizarrely close False cognates I;ve heard, how random that both Aborigines and Native Americans would have given similar names to small marsupials.


 * Dictionaries that I consulted are unanimous in stating that "possum" is derived from an abbreviation of "opossum." I don't find this puzzling at all, actually. The Americas were settled by Europeans much earlier (by centuries) than Australia was (late 18th century), so the word "opossum", and its informal abbreviation of "possum", would already have been well-established in the English language. Thus, while it would be strange for Australians to appropriate a Native American name, it would not be strange for them to appropriate an American or a common English language name, which is probably what they thought it was. I doubt the first Australian settlers were even aware that the word "opossum" was of Native American origin; they probably just thought of it as another regular word in the English language. They're both marsupials. And to the layman, opossums and possums look very much alike. Just look at the photos (File:Opossum_2.jpg and File:Trichosurus vulpecula 1.jpg, which I also posted to the right) in the infoboxes at the top of the articles (as of this current writing) for the two respective animals; those photos look a lot alike at a casual glance. —SeekingAnswers (reply) 06:08, 12 April 2011 (UTC)


 * My Australian Macquarie Dictionary suggests that the two words are connected and interchangeable, so yes, Australia did borrow the American word. HiLo48 (talk) 06:11, 12 April 2011 (UTC)


 * Just by the way, hardly anyone actually says opossum in the States, at least outside of a scholarly context. In normal colloquial speech they are called possum.  Using the two words distinctively is likely to lead to confusion.  --Trovatore (talk) 19:07, 12 April 2011 (UTC)


 * According to Wiktionary, the phrase "playing possum" is documented back to 1822. I think the Australian possum does the same thing but I'm not sure.  Actually, the Australian Possum doesn't look like an opossum to me - because of its face it makes me thing more of kangaroos and koalas than the opossums I've seen - I suspect it's the creature's behavior that has given it its name. Wnt (talk) 21:25, 12 April 2011 (UTC)

Note that although only one species of opossum is found in the United States, several dozen species can be found in South America. (A few extend as far north as Mexico.) Some of the South America species, such as the Gray Short-tailed Opossum, look quite a bit more like an Australian possum than the Virginia Opossum does. Looie496 (talk) 21:35, 12 April 2011 (UTC)

The power of noise
What proportion of the power of a jet engine or car engine is wasted in producing noise? My notion is that the most efficient engine would be silent. In other words, by how much would the ratio of output/input power increase if all noise energy was redirected to the output power? Do high efficiency modern engines tend to be quieter? Thanks 92.15.21.224 (talk) 13:23, 12 April 2011 (UTC)


 * Only a very small portion of the energy goes into sound itself. However, a loud engine may be indicative of other inefficiencies, such as friction, turbulence, cavitation (on prop attached to boat engine), an unbalanced engine, or missing cylinders (internal combustion engine), which tend to waste most energy as wear, heat, and vibration.  So, a silent engine likely would be more efficient, yes.  (Note that this logic doesn't apply to noise which is produced by the engine but then muffled in some way; this actually reduces engine efficiency.) StuRat (talk) 17:26, 12 April 2011 (UTC)


 * Before starting, I should say that so far as I've seen, measurements regarding sound are nearly always dishonest or actively misleading. Either a manufacturer wants you to believe something is more powerful than it is, or a polluter wants you to think it is weaker.  Numbers are usually given in "decibels", but the decibel is a relative unit, i.e. meaningless on its own, tied to one of many different arbitrary reference levels - the article gives some of the many sub-definitions used.  A particularly obnoxious trick is the usage of a "filter" (dBA) which supposedly reproduces the characteristics of human hearing - by means of a simple mathematical curve which underestimates the impact of low frequencies.  And even that is relative to a curve generated by psychoacoustic measurements...  I take particular exception to the low-frequency business because people do hear infrasound and suffer physiological effects from it, but it is claimed, not "consciously" (i.e. they don't call the vibration/sensation they feel a sound); for those deemed unable to hear things which are plainly audible, psychology rapidly wears out its welcome.  Another use of psychoacoustics is frequently seen when a polluter takes data that people surveyed on a form said that a 10 dB increase is only a 2-fold increase (even though it's more), and uses it to say that their newest proposal will only "sound" twice as loud.
 * Now that hopefully I've emphasized the need for you to take such data with a truckload of salt, I'll point you to sound power, which gives some energy figures. Though I am quite suspicious that an excavator is quieter than a heavy truck, for example.  To convert these to a few other sound measures, see ; probably there are others.  But the bottom line is that if the figures in the article can be trusted, vehicles generally emit less energy, perhaps much less, by sound than from their headlights. Wnt (talk) 22:05, 12 April 2011 (UTC)


 * Yea, that 1/100 of a watt for helicopter noise seems particularly low. That article needs some work. StuRat (talk) 22:12, 12 April 2011 (UTC)


 * A jet engine delivers "noisy" i.e. irregular thrust. Longitudinal pressure variation in the output stream is just the way it delivers useful power and all the jetstream is good, though not for your ears if you stand behind a jet aeroplane. The only wasted sound power is that emitted radially (and, at subsonic speed, forwards) from the engine. A car's reciprocating combustion engine cannot be silent because of its pulsing exhaust. By dimensioning the intake and exhaust tracts to resonate at particular engine speeds, high efficiency (which can be variously defined) is obtainable with the inconvenience of loud noise. (Formula One racers are very noisy .) Two-stroke engines can only work with a resonant exhaust so they are always noisy, while four-stroke car engines are usually muffled in a compromise with efficiency but will not be completely silent. Cuddlyable3 (talk) 22:14, 13 April 2011 (UTC)

What will happen if bicycle tyre will be filled using Helium gas?
What will happen if bicycle tyre will be filled using Helium gas? — Preceding unsigned comment added by Mayank3 (talk • contribs) 13:37, 12 April 2011 (UTC)
 * It's an interesting question! What sorts of effects do you expect might happen? DMacks (talk) 14:05, 12 April 2011 (UTC)


 * The compressed helium in the tire would weigh slightly less than the compressed air when the tire was inflated to the same pressure. One would have to know the volume of gas contained in the inflated tire to calculate the exact weight difference. The weight of the gas in the tire might be very insignificant compared to the weight of the tire and the bicycle: helium in the tires would not make the bike lighter than air, for instance. The helium would likely leak out of the tire quicker than air leaks out, since it is monatomic compared to the larger and heavier molecules of the component gases in air. It might have a lower viscosity, and circulate more freely inside the tire. It would have greater thermal conductivity than air. Being an inert gas, it would not react chemically with the rubber of the tube or tire as would the oxygen in air. Edison (talk) 14:10, 12 April 2011 (UTC)
 * To expand a bit on Edison's response, the helium in the tire will weigh proportionally quite bit less than the same volume and pressure of air: about 85% less, in fact.  In absolute terms, however, the mass of air in the tires makes a negligible contribution to the bicycle's – or even the tire's – weight; there's only a few grams of air in each tire, accounting for something like 0.1% of the total weight of the bike. TenOfAllTrades(talk) 15:17, 12 April 2011 (UTC)
 * Furthermore, the tire would go flatter much faster; Edison lights on this but fails to name the effect, which is called Graham's law of diffusion. A mathematical treatment of Graham's law is covered in that article if you are interested.  -- Jayron  32  15:34, 12 April 2011 (UTC)

Filling it with hydrogen gas instead may give spectacular effects in case of an accidental tire puncture :) Count Iblis (talk) 14:16, 12 April 2011 (UTC)
 * Only if punctured with something that is also on fire. Hydrogen is not spontaneously combustable; perhaps you are thinking of the Hindenburg disaster, whose explosion cause was unknown, however given the properties of hydrogen all of the serious hypothesis revolve around a source of either fire or electricity as the initial cause.  Hydrogen itself will not explode without sufficient cause, and merely puncturing the tire is not sufficent cause.  -- Jayron  32  15:34, 12 April 2011 (UTC)
 * Yes, but then the so-called "inversion temperature" of hydrogen is lower than room temperature. This means that when under pressure and then expanding into the environment, it will heat up, not cool down like most other gasses, due to the Joule-Thomson effect (the temperature of an ideal gas will not change. While due to exansion it would cool down, the energy goes into kinetic energy of the gas; it shoots out of the puncture, that kinetic energy is subsequently dissipated and heats up the gas again). This is why hydrogen is never stored under pressure at room temperature (which complicates implementation of green energy technologies that depend on hydrogen to store energy, e.g. generated from wind energy). Count Iblis (talk) 15:48, 12 April 2011 (UTC)
 * Er, hydrogen is frequently stored (and readily available) as a room-temperature compressed gas; see for example hydrogen tank for some specs. Google offers lots of additional pictures: .  And a bicycle tire – starting at perhaps six atmospheres of pressure – is never going to reach the autoignition temperature of hydrogen through simple expansion.  TenOfAllTrades(talk) 17:00, 12 April 2011 (UTC)


 * I see! I blame my old Prof. for giving a misleading homework problem on this issue a long time ago, although I should have been able to see that you don't get problems with tires. Count Iblis (talk) 01:45, 13 April 2011 (UTC)

What eventually happens to a beam of light?
Does it keep getting red-shifted for eternity? Imagine Reason (talk) 16:09, 12 April 2011 (UTC)
 * If the red shift doesn't get it, extinction will. --Sean 17:19, 12 April 2011 (UTC)
 * How could the photon change? Where would the energy go? Metric-expansion red shift never really made sense to me, so I don't really understand how it happens. My best guess is that it's only the relative difference in velocity between the objects in space and the photon that changes (so it only redshifts relative to you), but I'm not clear on how the energy budget is worked out. Ariel. (talk) 01:53, 13 April 2011 (UTC)
 * Do the articles Red shift and Photon help? Cuddlyable3 (talk) 14:07, 13 April 2011 (UTC)
 * Your guess is right. The redshift happens because the source of light and the detector are moving away from each other. The energy at emission will not be the same as the energy at detection because they are measured in two different inertial frames. Dauto (talk) 18:35, 13 April 2011 (UTC)

Eliminating sex
(sorry my English) One of the common themes in a lot of science fiction tales is a society that no longer repruce by bilogical sex. I wonder to wich extent that would be possible with current technology. For example, suposse a relatively isolate community (something like the Amish) Then imagine this community decide to apply chemical castration to all its male babys and oly reproduce by artificial insemination. Could this community efectivily erase both the desire and the need of sex in a generation or two? --83.59.47.204 (talk) 17:43, 12 April 2011 (UTC)


 * How is chemically castrating all the males (which probably would erase the desire in all males anyway, simply greatly reduce it) going to erase the desire in females? In addition, I'm doubtful chemical castration has ever been tried on male babies and even ignoring the ethical considerations I would guess given the way most forms of chemical castration work it's going to cause a great degree of feminisation of the males as they develop to the point where you will have trouble getting sperm for the artificial insemination and who knows what other problems. Nil Einne (talk) 17:53, 12 April 2011 (UTC)


 * Many science fiction tales seem to be based on the idea that you can't totally erase the human instinct to have sex and/or reproduce naturally. See the film THX 1138 for example. I don't know about eradicating human sexual desire...but I imagine that, in theory, it is possible to sustain a human population through artificial reproduction, and that over many generations, we might evolve to a point where we have no sexual desires. But that's just speculation. Quinn ☂THUNDER  18:03, 12 April 2011 (UTC)


 * I expect that removing testes and ovaries at birth (or before), then reproducing by cloning, would pretty much eliminate sexual desire. You could also have all males or all females, and thus at least eliminate any hetero sex. StuRat (talk) 19:00, 12 April 2011 (UTC)


 * That all presumes that the sexual drive is a purely reproductive one, but the sexual act itself is done for all sorts of reasons, social, emotional, etc. To eliminate the reproductive desire does not necessarily eliminate the other sorts of reasons why people have sex; its why efforts to sterilize or chemically castrate males who have committed sex crimes have limited success in stopping them; they aren't committing sex crimes because of a desire to reproduce; as often as not its about establishing power relationships or other issues which have nothing to do with reproduction.  -- Jayron  32  19:23, 12 April 2011 (UTC)


 * That's where the "at birth (or before)" part comes in. If you wait until adulthood to remove them, then the hormones will have already altered the brain in a sexual way.  You must remove them before the brain becomes sexualized.  All other sexual stimuli, like porno movies and pictures, would also need to be removed.  If your brain has no concept of sex, then it can't have any concept of rape, either.  (Of course, there are many other ways to humiliate people, like defecating on them, which might then become more prevalent.)  StuRat (talk) 19:30, 12 April 2011 (UTC)


 * Well, you hit on one big point there; sexual abuse is in many ways the symptom of other social problems; removing sex as an option would not necessarily change rapists into fine people. It may very well just change the mode of abuse without altering the underlying causes that make some peopel want to be abusive.  -- Jayron  32  19:39, 12 April 2011 (UTC)


 * Every life form currently living on Earth has excelled at one thing: reproducing. In vertebrate species, reproduction is usually sexual. If any behaviors are determined by evolution and genetics, copulation (i.e. 'having sex') is the most likely. Lastly, in order for some sort of asexuality to be 'evolved' it would have to be advantageous under selective pressure. You may be interested in the species which have evolved parthenogensis. This is common among the insects, such as aphids, and even some lizards and fish can reproduce this way. We suspect that this lack of sexual recombination may lead to an evolutionary bottleneck, and ultimately the extinction of the species. SemanticMantis (talk) 20:57, 12 April 2011 (UTC)
 * In Brave New World (1932), reproductive sex is banned but recreational sex is encouraged. Most people are infertile anyway because of the chemical treatment they receive during their pre-natal development in an Artificial uterus. Fortunately, science has not yet devised one. Alansplodge (talk) 20:25, 14 April 2011 (UTC)

UV emission from electric arcs
What is the actual mechanism of ultra violet emission from electric arcs.--92.28.38.62 (talk) 18:05, 12 April 2011 (UTC)
 * Electric arcing causes a lot of non-specific excitation of electrons, across a broad spectrum of wavelengths. Generally, certain types of wavelengths are closely associated with certain types of excitation (for example, the infrared wavelength is particularly correlated with vibrational modes, while radio waves are found in the range of nuclear magnetic resonance).  In a situation like an electric spark, you are basically exciting electrons non-specifically, so they tend to promote to higher energy levels, and re-emit photons as they relax, between nearly every allowable transition within the material.  Basically, they don't just emit in the ultraviolet, they emit in the ultraviolet along with emitting in just about every frequency from radio waves through to the UV.  Radio waves emitted by simple electric sparks was, for example, one of the cornerstone areas of research that eventually led to modern radio technology, see Heinrich Hertz.  -- Jayron  32  19:18, 12 April 2011 (UTC)

Farmer's action plan?
There is a large farm located on the Eastern shore and the farm grows corps, and to many bugs is destroying the crops and the farmer last year sprayed heavy doses of pesticides, this year the farmer was asked not to use the same chemicals and wants to control the amount of soil that is being washed into the stream. What is an action plan that will help the Bay and the farmer —Preceding unsigned comment added by 71.191.176.79 (talk) 22:07, 12 April 2011 (UTC)


 * Sounds suspiciously like homework, but, in case it's not, here's some thoughts:


 * 1) Plant crops which are more resistant to "bugs".


 * 2) Introduce natural predators of those "bugs".


 * 3) Utilize "no till" planting methods.


 * 4) Plant trees at the edges of fields to limit both wind and rainwater losses of soil. StuRat (talk) 23:30, 12 April 2011 (UTC)


 * Don't forget Bt corn. This would be easier to answer if we knew the crop, the bug, and the pesticide. Wnt (talk) 00:54, 13 April 2011 (UTC)
 * The farmer could also plant cover crops in the fall (like alfalfa, which, aside from simply reducing weather-based erosion, restores favorable levels of nitrogen in the soil). Juliancolton (talk) 02:14, 13 April 2011 (UTC)


 * Just for the benefit of the readers who aren't familiar with Atlantic Seaboard of the U.S. geography, the Eastern Shore refers to the Maryland and Virgina parts of the Delmarva Peninsula. The Bay referenced would then be the Chesapeake Bay.  The region is known for agriculture, large industrial chicken farming, and crab fishing.  -- Jayron  32  05:32, 13 April 2011 (UTC)

light-bulb
hows the glass attached to the metal in this   http://www.traderscity.com/board/products-1/offers-to-sell-and-export-1/mr16-gu10-halogen-light-bulb-240-volt-energy-saver-gu10-lamp-50w-189861/  — Preceding unsigned comment added by Wdk789 (talk • contribs) 23:32, 12 April 2011 (UTC)
 * Since the late 19th century a substance called "vitrite" was used to join metal of the incandesent lightbulb shell to the glass bulb. Edison (talk) 05:14, 13 April 2011 (UTC)
 * Wikipedia does have a brief article on it: Vitrite. -- Jayron  32  05:25, 13 April 2011 (UTC)
 * Note where the info about vitrite came from. Wikipedia previously called it "vitrit."

Mining of thick seams
some of the ways of mining thick seams are :

can anybody explain the methods? — Preceding unsigned comment added by Him.12.pat (talk • contribs) 23:33, 12 April 2011 (UTC)
 * blasting gallery method
 * inclined slicing method
 * horizontal slicing method
 * sublevel caving


 * I just looked on Google, there are websites which cover this. I think you could find out most of this information yourself. Your question sounds like homework.217.158.236.14 (talk) 12:24, 13 April 2011 (UTC)

wrong named structure
To my mind the structure shown on the picture should be named 3-Methyl 4-propyle-octane because you have to choose the main-chain with the most branches. I dont participate very often in the en wp so id like to ask sb from here to either correct the structures name or to (let) deleate it. Greetings --Oliver s. (talk) 23:38, 12 April 2011 (UTC)


 * Straight from the IUPAC themselves:


 * The longest chain is numbered from one end to the other by Arabic numerals, the direction being so chosen as to give the lowest numbers possible to the side chains. When series of locants containing the same number of terms are compared term by term, that series is "lowest" which contains the lowest number on the occasion of the first difference. This principle is applied irrespective of the nature of the substituents.


 * See also IUPAC_nomenclature_of_organic_chemistry. Your rule (main-chain with the most branches) is used only when there are two or more possible main chains of the same length. Buddy431 (talk) 23:51, 12 April 2011 (UTC)


 * 3-methyl 4-propyl is lower numbered than 4-(1-methylpropyl), so far as I know. When I search for "3-methyl 4-propyl octane" I get a few serious-looking mentions, whereas the other name only gets me derivatives with a lower numbered substituent, and Wikipedia hits. (including a talk page discussion about someone being wrong for naming the compound that way) Wnt (talk) 00:52, 13 April 2011 (UTC)


 * The OP is correct. I assume the molecule is being used to demonstrate how to name a molecule with a branch off of a branch, but it doesn't work because there ARE two main chains of the same length (both octanes) and in that case you choose which octane has the most branches.  So the 3-methyl-4-propyloctane is a better name for that reason.  It also works with the rule quoted above, because the alternate name, being 4-(1-methylpropyl)octane has the "occasion of the first difference" on carbon 4, whereas the 3-methyl-4-propyloctane has it on carbon 3.  Since it has the lowest number of first difference (3 vs. 4) its the better name.  Using either rule, that's still a badly named compound.  -- Jayron  32  01:23, 13 April 2011 (UTC)
 * Good catch Jayron. I was counting 7 carbons on the chain the OP wanted to be the main one.  They say organic chemists only need to be able to count to 8, but it doesn't even look like I can do that. Buddy431 (talk) 03:20, 13 April 2011 (UTC)


 * so I have to recommend to deleate the missleading picture: http://commons.wikimedia.org/wiki/Commons:Deletion_requests/File:Iupac-alkane-5.png --Oliver s. (talk) 14:19, 13 April 2011 (UTC)