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November 11

Could you collect all the CO2 that a vehicle emits?

How much volume would that have? Can you transform it into a solid form? Comploose (talk) 00:48, 11 November 2012 (UTC)

Yes, it could be transformed into a solid, but the question is whether that would be cost-effective (one method would be to lower the temperature to where it forms dry ice). It could also be stored as a compressed gas. If left as an uncompressed gas, it would take up too much space. Also consider that carbon dioxide tanks pose a risk, as leaking ones could knock the driver out and cause an accident, and you can't smell or see it. StuRat (talk) 00:59, 11 November 2012 (UTC)

But, if you remove the 02 and keep the C, the amount won't be bigger than the gasoline. Comploose (talk) 01:35, 11 November 2012 (UTC)

The amount of energy that would take would be prohibitive. StuRat (talk) 01:38, 11 November 2012 (UTC)

((edit conflict) and network timeouts) Removing the O₂ from CO₂ takes a huge amount of energy--you would have to burn probably several times the amount of gas in order to drive that process as well as the car itself. Both of your proposed products are reasonably high-energy (coal is a good fuel and oxygen strong oxidizer) and your proposed starting material is low energy (already-burned), so it takes even more energy to undo that reaction than you get by burning it. Essentially you are proposing an overall process of that gets economically viable energy from "gas + oxygen to coal + water" conversion. It is thermodynamically correct if I understand the numbers correctly, so at least you would not be violating the laws of nature, but not by a lot, so your overall car efficiency (miles per gallon) will suffer greatly. Coal still has a lot of energy, so you are only getting a small amount of the energy density of the gasoline. Energy density is one of the major problems in various alt-energy vehicles. DMacks (talk) 01:56, 11 November 2012 (UTC)

Hypothetically, you could sequester the CO2 from car emissions chemically, that's what Carbon dioxide scrubbers do on an industrial scale. There are several ways to sequester CO2 in this way, most commonly as carbonates. The problem is how to do that efficiently on car exhaust; the chemistry however is simple and used in other applications for exactly this purpose. --Jayron32 02:23, 11 November 2012 (UTC)

Yes: Top Gear (2002 TV series) has demonstrated this. See http://youtube.com/watch?v=nz-Q-4RUd28. The bit about the Range Rover and the greenhouse is complete hogwash, but from about 2m30s there is some actual science. They show that you can use chemicals to sequester the CO2, like Jayron mentioned. They say that they're using Lime crystals; I'm not sure how accurate that is, but it seems to work. See also Carbon dioxide scrubber. - Cucumber Mike (talk) 12:28, 11 November 2012 (UTC)

If something "increases dopamine receptor sensitivity", is that good or bad?

From a fast research, I could find that morphine, fasting (sexual and food-wise), hypothyroidism, and Electroconvulsive shock do that. But is that a desirable effect or a harm done? Is it reversible? Comploose (talk) 00:55, 11 November 2012 (UTC)

I think it all depends on the context. Consider an analogy with increasing light sensitivity in your eyes. That's good in the dark, but not when it's bright out. StuRat (talk) 00:57, 11 November 2012 (UTC)

There are several dopamine receptors with subtly different effects. It also depends on how and where in the brain you are altering dopamine receptor activity. Looking at some real classes of drugs, MAOIs enhance increase the levels of dopamine at certain synapses, which may treat symptoms of depression. Antipsychotics, on the other hand, inhibit dopamine receptor activity, which may treat symptoms of schizophrenia. So the answer to your question is that whether it is desirable, whether harm is done, and how easily it is reversed is entirely contextual. Someguy1221 (talk) 01:05, 11 November 2012 (UTC)

OK, I see that people can have too sensitive dopamine receptors, in the case of schizophrenia, or not sensitive enough dopamine receptors, in some mood disorders. What happens to two normal cases where one mentally healthy person has more sensitive dopamine receptors than the other? The sensitivity of dopamine receptors is also inversely related to the BMI. Could you say that the brain of thin people works differently than the brain of fat people? If yes, how would a brain with more sensitive dopamine receptors differ? Comploose (talk) 01:21, 11 November 2012 (UTC)

As far as I know, none of those manipulations actually increases dopamine receptor sensitivity. They enhance the effects of dopamine, but that isn't quite the same thing. There are drugs that increase the sensitivity of receptors -- for example, benzodiazepine tranquilizers increase the sensitivity of GABA receptors. But I'm not aware of any such effect occurring at dopamine receptors. Looie496 (talk) 02:54, 11 November 2012 (UTC)

There's a general rule in evolutionary biology (Fisher's geometric model) that very small mutations have a 50% chance of being beneficial. The idea is that biological pathways aren't absolutely perfectly tuned, so if you jiggle them just a little, your odds are 50-50 of jiggling them the right way. However, larger mutations are more commonly bad, and extremely large mutations almost invariably so. The same should usually apply to chemical perturbations (provided that they are strictly regulatory rather than say nutritional in nature). I think. So these various drugs, taken in very low doses, should be very slightly good 50% of the time. Combined with even a vague knowledge of when such a modification is a good thing, the effect of many drugs chosen in this way intermingled, at low doses, should on the average be beneficial quite often. Come to think of it, I would speculate (though I've never seen it expressed this way) that this might be the basis of traditional herbal medicine, at least in cases where there is less specific indication for one particular herb. Wnt (talk) 07:53, 12 November 2012 (UTC)

Liquifying francium

I know that francium has a huge heat of decay. However, I'm wondering that if we could prepare a macroscopic sample, keep it renewed by constant synthesis of new atoms, and have a cooling system powerful enough to ensure that the atoms remain liquid, i.e. conducting away all the heat from radioactivity.--Jasper Deng (talk) 02:07, 11 November 2012 (UTC)

There's information on speculative states of matter of Francium at the Wikipedia article titled Francium. Otherwise, I'm not sure the reference desk's mission would be such as to allow us to sit around and invent things out of whole cloth to speculate about what liquid Francium would look like or behave like if we could collect enough of a sample, which we have not yet. --Jayron32 02:18, 11 November 2012 (UTC)

I don't know how to answer this, but it's answerable. Our article shows an image of 300,000 francium atoms in a magneto-optical trap, as a gas. I think it's possible to cool atoms in traps like this, but I don't know how quickly. I assume it's not easy or we'd have a picture of liquid francium there too. Wnt (talk) 16:15, 11 November 2012 (UTC)

It's worth bearing in mind that 300,000 atoms is not a lot—collected into a droplet, you would have a sphere about 80 atoms (probably around 15 nanometers) across. Under visible light, you're not going to see much of anything, and I'm not sure if a particle that size would even be properly 'macroscopic' in its behavior (you get weird things like melting-point depression happening at these sizes, for instance). TenOfAllTrades(talk) 21:41, 11 November 2012 (UTC)

Now there's a general science question I never thought of. What's the smallest possible quantity of a liquid? (or of a solid?) The article doesn't really say, and melting point depression per se shouldn't prevent you from having one. Wnt (talk) 03:53, 12 November 2012 (UTC)

My question is purely theoretical; what if we got at least nanogram quantities (at least ~2.7 billion atoms) and were able to maintain such a sample? The central question is not what the liquid would look like, but whether we would be able to cool such a sample to keep it liquid.--Jasper Deng (talk) 02:56, 12 November 2012 (UTC)

In that case, it is unanswerable by the reference desk. We provide references here so you can know what people already know about things. This isn't the place for people to speculate about unknowns. --Jayron32 00:06, 13 November 2012 (UTC)

Our choice of 25 °C and 1 atm as standard conditions is rather problematic for Fr: the melting point is predicted to be close to 25 °C, but has a large uncertainty, so it's not entirely clear whether we should label Fr as a liquid or a solid. It's worth noting that a simple extrapolation of melting points would result in Fr being a liquid, though. If the 27 °C value from the Francium article is experimental, then it raises questions like whether the experimenters corrected for the decay heat of Fr, since this value is very close to that of Cs (28.44 °C). Double sharp (talk) 05:51, 15 November 2012 (UTC)

Would dogfighting spacecraft need to bank when turning?

Someone told me recently re: "soft science epic Star Wars, that the space craft bank when they make sudden turns (as in dogfights), and they would not need to do this in a gravity-free vaccuum. It made me wonder tho, coz banking would mean the inertial force was felt by the pilot as one pushing him down on his seat, rather than to the left or right.

So where you need to turn suddenly left and right a number of times in quick succession, a banking pilot would feel each of these as a force in the same direction, not as ones zapping him suddenly now to the right, and then to the left. And the force downwards is one very famiiliar to all terrestial beings experiencing gravity, whereas chiral-specific forces are not.

Perhaps banking in extreme turns would also be less stressful on the craft's structure. Myles325a (talk) 07:24, 11 November 2012 (UTC)

Yes, inertia still exists without gravity, so banking would help the pilot. In fact, you'd want to bank more, a full 90 degrees, while on Earth you want somewhat less than 90, because gravity is also pulling you down. As for stresses on the ship, that would all depend on how it was designed. Of course, if they figure it will bank in turns to protect the pilot, then they will design with that in mind. StuRat (talk) 07:32, 11 November 2012 (UTC)

Of course, a ship that does zero to lightspeed in five seconds might need inertial dampeners, in which case it probably wouldn't matter. (Given that maybe the inertial dampeners aren't 100%, the ship might not "really" accelerate to go up and beyond lightspeed, etc.) Wnt (talk) 16:12, 11 November 2012 (UTC)

As far as human tolerances go, in the absence of aerodynamic considerations it would make more sense in a turn to yaw than to roll; humans can tolerate much higher g-forces perpendicular to the spine than along it.

That said, the whole thing is a little bit silly. Standard dogfighting tactics and doctrine (heck, even movie dogfighting tactics and doctrine) have to be rethought once you're working in no air and no gravity. There are an awful lot of what-if and why-didn't-they games one can play (why are there no missiles in the Star Wars universe? why do such otherwise-advanced craft have to rely on manually-aimed guns? etc. etc.) but I'll leave off with a brief nod to the recent Battlestar Galactica remake. In the new BSG series, pilots facing a bad guy 'on their six' didn't panic; they just flipped end-over-end with their manoeuvring thrusters so they could bring their guns to bear while coasting 'backwards'. TenOfAllTrades(talk) 16:42, 11 November 2012 (UTC)

If you would like to read about why aircraft bank during turns (and they don't always!) - I recommend the vintage Aerodynamics for Navy Aviators (1965), available for free online from the FAA. Combining a bank and a yaw maneuver is called a coordinated turn, and helps keep the aircraft stable. During a dogfight, stable flight is not always desirable, especially with very high-performance aircraft. However, in modern air doctrine, dogfighting is totally nonexistant: a combat pilot should never be anywhere near his opponent. Direct air-to-air engagement is risky, ineffective, and expensive, compared to alternatives. This would be even more prominent if current technology were projected forward to guide military doctrine in space: risking manned vehicles in remote areas for minimal strategic benefit would be hard to justify. Nimur (talk) 19:12, 11 November 2012 (UTC)

However, in modern air doctrine, dogfighting is totally nonexistant: a combat pilot should never be anywhere near his opponent. Direct air-to-air engagement is risky, ineffective, and expensive, compared to alternatives. What are you talking about? How else is a pilot supposed to kill his opponent? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 21:32, 11 November 2012 (UTC)

I think Nimur is talking about using long-range AA missiles. A8875 (talk) 22:12, 11 November 2012 (UTC)

Which are easily avoidable/distractable/interceptable etc. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 23:43, 11 November 2012 (UTC)

I assume it must depend on your point of view. If one military is telling its pilots not to engage, surely the other must be saying "Get as close as you can, and engage those Star Destroyers at point-blank range." Wnt (talk) 03:47, 12 November 2012 (UTC)

"However, in modern air doctrine, dogfighting is totally nonexistant: a combat pilot should never be anywhere near his opponent. Direct air-to-air engagement is risky, ineffective, and expensive, compared to alternatives." -- Wasn't this doctrine discredited during the Vietnam War? 24.23.196.85 (talk) 05:40, 12 November 2012 (UTC)

It's a cost benefit analysis. A modern American fighter costs about $100 million. A modern medium-range (20 km) anti-air missile has a cost of only about $100,000. Against older planes, missiles have demonstrated a kill rate over 50% (Falklands War). Against modern aircraft the kill rate is expected to be much lower (e.g. 10-20%), but it is still perceived as more cost effective to launch 5, 10, or even 20 missiles at a target than risk a much more expensive fighter. Of course, there will be tactical situations where your aircraft runs out of missiles and is far from any surface-to-air support, and needs to be able to dogfight, but modern American tactics encourage fighters to avoid those situations whenever possible. Better to have the expensive fighter run away, and wait for cruise missiles or other weapons to destroy the enemy's airbase. Of course it helps that America has recently focused on opponents that generally have much older technology, which makes them easier to kill and easier to avoid. Dragons flight (talk) 20:39, 12 November 2012 (UTC)

"Better to have the expensive fighter run away, and wait for cruise missiles or other weapons to destroy the enemy's airbase." What are you going to do after enemy fighters, missiles, and AAA shoot down those cruise missiles? Besides, as 24.23.196.85 says, the shoot-missiles-at-the-enemy doctrine was totally discredited during the Vietnam War. The only reason air-to-air missiles are any use at all against defended aircraft is that since Vietnam they have only been used against obsolete aircraft. Every air force nowadays knows that cannon and Genies are the only effective method for fighter jets to down up-to-date enemy aircraft. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 23:08, 14 November 2012 (UTC)

Playing the vintage arcade game Asteroids will give you a very accurate intuitive grasp for the mechanics of dogfighting in space. μηδείς (talk) 17:09, 11 November 2012 (UTC)

The most damage resistant configuration would be to have the main engines fixed in place with several fixed reaction jets for turning. (Think Apollo Command/Service Module.) The pilots would be strapped down to resist the push of the main engines, and wouldn't worry about any other forces. Hcobb (talk) 17:26, 11 November 2012 (UTC)

Great swooping turns in space to face an enemy rather than just rotating with thrusters or reaction wheels would would be be very wasteful of fuel. Lots of sci-fi writers either don't know this or don't care, and maker of space-opera like Star Wars just want pretty dogfight scenes like in World War 1, where biplanes made Immelmann turns and other aerobatic maneuvers, which are pointless in the absence of air. If one is worried about stress on the pilot, he could be in a sphere in the center of the craft which rotates independently of the warcraft, to keep him stable while it spins, and in an optimal attitude for translations. Edison (talk) 21:14, 11 November 2012 (UTC)

Dogfighting in space? Surely if the enemy ship was close enough for it to register on your ship's sensors, it would be close enough to hit with a computer-targeted laser beam? Considering that it's a vacuum, I'd guess that you could probably shoot him from the other side of the solar system if your machine could correctly predict his trajectory, provided that there was nothing in the way. As he could also do to you. --Kurt Shaped Box (talk) 23:56, 11 November 2012 (UTC)

When you come right down to it, there's no reason the pilot has to be actually in the craft; without the extra mass required for life support, a drone space fighter would have a lot better performance, and no worries about the pilot's reaction to inertial loads. Gzuckier (talk) 01:50, 12 November 2012 (UTC)

It may help to have a human nearby, to make decisions with short communication lag. But of course space battles, if any, are more likely to be fought by computers. It's absurd (but dramatic) that the Enterprise waits for the Captain's order to fire ... Now!. —Tamfang (talk) 03:09, 12 November 2012 (UTC)

Particularly when the captain is James Kirk who has this tendency to................. pause............... randomly. Gzuckier (talk) 04:00, 12 November 2012 (UTC)

"[Long-range AA missiles] are easily avoidable/distractable/interceptable etc.": In space at very long ranges, there would probably be some dedicated interceptor missiles which would only have one purpose: to slam head-on into incoming anti-ship missiles or any "fighter" foolish enough to get close. There would be dedicated anti-missile guns, both ship-mounted and in drone spacecraft (which would not be crippled as easily as ship-mounted defenses, should one torpedo actually strike home, and thus have a better chance to ward off follow-up strikes) orbiting the capital vessels. And then, there would be some smaller long-range missiles, not intended to destroy, but to fool and to saturate enemy point defense.

This is not just speculation on my part: many of these exist today in the wet navies of the world: Phalanx / Kashtan CIWS , and manned interceptors guard the capital vessels against both air and surface attack.

"If one is worried about stress on the pilot, he could be in a sphere in the center of the craft which rotates independently of the warcraft": just what I would think but for different reasons. If there is only one fixed "main" thruster, that thruster can be more powerful than a vectoring thruster or different thrusters for different directions. Turning before firing the main thruster could save a lot of mass (and thus, fuel).

"a drone space fighter would have a lot better performance" but from time to time, it has to receive new orders. If the enemy manages to jam the communication, its performance goes out the window. Humans on-board may be more like today's AWACS crews rather than "pilots" as depicted in the movies, but they safeguard against both, runaway AI and obsolete programming.

"Considering that it's a vacuum, I'd guess that you could probably shoot him from the other side of the solar system if your machine could correctly predict his trajectory" which is a big IF. One light-second is ~300,000km, an AU is 500 times that, and that's still a fraction of the solar system. Of course, there are only two options: maneuver from time to time erratically but give your position away each time you do so, or put Newton in control (drift) and remain stealthy but predictable if the enemy did spot you before. Stealth will probably play a very important role in space. Huge vessels are not even close to the optimum in that case.

I liked the Honorverse novels the most. They have some plot devices, including inertial dampeners and gravitic drives, but they are few and far in-between. Weapons include lasers, gamma ray amplifiers(basicaly a more badass kind of laser), and anti-ship missiles carrying disposable X-ray lasers. Defensively, they use small point-defense weapons , ECM and banking maneuvers (they "roll ship" to expose the well-protected top and bottom rather than the more vulnerable sides). And I'm not sure if a Honor Harrington movie could not be as epic as another Trek or Wars movie. - ¡Ouch! (^{hurt me} / _{more pain}) 14:27, 12 November 2012 (UTC)

Weber, of course, devised the technologies of the Honorverse so that his ship-to-ship battles would recapitulate the sense and style of ship-to-ship combat of the Napoleonic era. (Honor Harrington's universe owes a great deal to the world of Horatio Hornblower—right down to her initials.) Weber invented a shielding system that protected his spaceships from attacks coming from above and below, because seagoing ships of the early nineteenth century essentially could not be attacked from these directions by the technology of the day. Weber's fictional spaceships lack shielding fore and aft, to mimic the vulnerability of ships of the line to raking fire and emphasize the strategic value of Crossing the T. Weber's 'ships of the wall' were named after, and deployed in a manner akin to, the ships of the line. His more recent work in the Honorverse has moved into the development of carrier warfare and tactics. While it's all fun, and shiny, and exciting, underneath the scifi gloss it's the Battle of Trafalgar. TenOfAllTrades(talk) 21:15, 12 November 2012 (UTC)

Indeed. I find the battles, strategies, and technology in The Lost Fleet a bit less contrived. But it still is not a bulletin from real life... --Stephan Schulz (talk) 11:22, 13 November 2012 (UTC)

OP myles325a back live. I take it that there is a general consensus that, GIVEN a dogfight situation, yawing and banking would make sense. Whether there need be dogfights at all is outside of this question. We could imagine that these dogfights were arranged as games, and the contestants had to fire guns without the aid of computers, and they fired paintballs, or whatever. The Federation thanks the Space Cadets who beamed in their msgs. Myles325a (talk) 09:26, 13 November 2012 (UTC)

The bottom line is not to worry about the structure as much as about the crew. That's where I think that Weber was wrong too, see below. The article about crossing the T seems to concentrate on steamers rather than Ships of the Line, though. While it does mention that battleships moving "horizontally" can use their full armament while the column moving "vertically" can use only about half their firepower, it does not mention that sailing ships used to be handicapped even more. They could bring hardly any guns to bear on the enemy when they get crossed. - ¡Ouch! (^{hurt me} / _{more pain}) 11:44, 14 November 2012 (UTC)

Side topic: Honorverse

In The Short Victorious War, there is one paragraph where he writes that should the inertial compensators fail, there would be not enough left to analyze the circumstances of the failure. I beg to differ. At a quite tame 500g (compared to Trek and Wars), the crew will surely be destroyed beyond visual recognition, but not entirely homogenized into one puddle. The structure will fail horribly, but given that we can analyze planes which crashed head-on into solid rock, (OR) the quality of the debris will be less of a problem than finding it in the first place.

"Weber invented a shielding system that protected his spaceships from attacks coming from above and below, because seagoing ships of the early nineteenth century essentially could not be attacked from these directions by the technology of the day." To an extent. It allowed for the "wall of battle", which is nothing more than a stack of X "lines of battle." It does allow for "rolling ship" (banking) too, i.e. exposing only the unarmed but well-protected top and bottom to direct fire. To merely mimic the effects of raking fire, it would have sufficed to increase the penetration of the disposable X-ray lasers to a degree where they destroy every component in the line of fire.

Weber postulated ("invented") only very few plot devices. No Alien-of-the-week syndrome (although there are nigh-implausible treecats and insectids), no Particle-of-the-week or Force-field-of-the-week, only gravitic drives, macroscopic wormholes (not as abundant as in Stargate either, more like DS9 on a rarity scale), and limited FTL capability. The most amazing achievement is to write an entertaining sci-fi with ships battles between near-Newtonian ships which basically suck dick at maneuvering compared to other space operas... - ¡Ouch! (^{hurt me} / _{more pain}) 11:44, 14 November 2012 (UTC)

Bicycle wheel question

Hi. A spoke from my rear bike wheel broke the other day and I had to buy a new wheel because the rim had cracked. I was chatting to the guy in my LBS and he suggested that the problem arose because I use a single pannier bag, always on the left. My bag weighs 4kg or so. I smell BS, but what do you guys think? Robinh (talk) 09:05, 11 November 2012 (UTC)

I carry a single bag only on the right for about 4900 km per year (I have two on for the remaining 100 ;-). I do sometimes have broken spokes, but very rarely. I've never had a cracked rim. Normally, spokes break because the wheel is not well-build. I had one wheel where the rim was unsuitable for the angle of the spokes caused by the hub, with spokes literally breaking at the the rim. But the most frequent problem is spokes breaking at the knee due to metal fatigue if there is to little tension on the spokes so that they go through a load-unload cycle on each rotation. So yes, I agree with your suspicion. --Stephan Schulz (talk) 09:14, 11 November 2012 (UTC)

If this guy is in your local bicycle shop why not push him a bit further with his explanation. I won't ask why you didn't ask for a specific explanation at the time ;-). Stephan seems as close to any expert we'll see on here. Richard Avery (talk) 14:20, 11 November 2012 (UTC)

We would also need to know how many miles you put on these wheels. It might be completely normal after all. OsmanRF34 (talk) 18:55, 11 November 2012 (UTC)

(OP) rough figures, 20km per working day for about 2 years, thus about 5000km, comparable with Stephan. Follow-on question, the *new* wheel has developed three very very loose spokes (which seem to be in compression) and I've taken it back to the LBS. The guy said that sometimes new-built wheels take some time to "settle out" (I think that was his phrase) and need retensioning after a few weeks. Is this also BS? Hey, I appreciate your advice here guys, Robinh (talk) 20:11, 11 November 2012 (UTC)

With standard wheels, for a standard city-bike, things should be working much better than that. Unless you are going through a rough terrain with a mountain bike, I would expect less incidents and less strange explanations. OsmanRF34 (talk) 00:25, 12 November 2012 (UTC)

Your LBS chap is bullshitting in all respects. I also have biked to work & shops and back every day, about 15 to 18 km every day for years, mostly carrying laptop, personal stuff, shopping, etc often in unbalanced loads. My rims are aluminium alloy and the bike is a fairly heavy general use bike (ie not racing or mountain). I usually did in about one spoke per one to 2 years. New built wheels never showed any problem. I agree with Stephan that the cause of broken spokes appears to be metal fatigue as early failures don't generally occur. Riding over potholes and curb-jumping increases the probability of spokes failing. If you have broken spokes soon after a wheel rebuild/adjustment, I suggest that you LBS chap is not doing the job right. I take it you do sensible weekly checks on your bike, and immediately after shop servicing? As well as checking brake travel, bearing slop, gearchange action, etc, while you have your bike upside down, give each wheel a spin, and watch the rim pass a frame member. There should be NO visible side-to-side movement in the rim. If there is, spokes have either failed or are just about to, or have been incorrectly fitted. Also, at the shop, while the bike is upside down, or just held off the ground, give each wheel a spin and hold your thumbnail so it lightly hits the spokes. They should all make the same "ping" sound. If they don't, they need attention. I bet if you do this when you go to pick up your bike at the shop, the guy will do one of 4 things: a) Say, "sorry I need to give your bike more attention" (ok), or get his more skilled mate to fix it (ok), get bad tempered (find another shop), or tell you more bullshit, such as "the rim needs replacement" (which may or may not be true, but if it is, why did he not say before?). Wickwack 121.215.11.232 (talk) 00:37, 12 November 2012 (UTC)

In this case I disagree with the "in all respects". Newly build wheels, especially if machine-build (which most wheels are nowadays) will indeed tend to "settle", and will need re-tensioning after a reasonable amount of use (2-4 weeks, if you ride as much or as little as you or I ;-). After that, or if build a master wheel builder to begin with, it should be good for a year or three before it needs attention again. I've changed my back rim after 6 years (~30000km/~20000miles) all-weather riding, but it was not strictly necessary. I just like to have everything about my bike shipshape, and the spokes needed replacement anyways. --Stephan Schulz (talk) 08:36, 12 November 2012 (UTC)

(OP) thanks for this. My daily route is all road/cyclepath, no rough ground at all. Just to be clear, the old wheel's rim cracked after two years' use. The new wheel has not suffered any broken spokes, it was perfectly true when bought a couple of weeks ago (ie no side-to-side motion, I checked when I adjusted the brakes). Wickwack, you say you break a spoke every two years or so, but you don't need to replace the rims as often? Stephan's rims last indefinitely, I think. I guess the old wheel's rim was a lemon (lasting only two years), and the new wheel had poorly tensioned spokes(lasting only two weeks before needing retensioning). How long should a rim last? How often should I retension my spokes? Robinh (talk) 01:06, 12 November 2012 (UTC)

I don't know how long a rim should last. I purchased my bike about 20 years ago. The rims have never been replaced, and they are still perfect. However, I only rode it every day to work for about 10 years staring about 12 years ago, due to changing work address. I still use it for shopping. When a wheel has had service attention, use the thumbnail trick to make sure it's perfect. However, after some use you will get some variation in spoke ping. Repalce/retension spokes if there is visible side-to-side movement of the rims, which in my experience will ocurr roughly once a year or two as I said above. If there is no visible side-to-side movement, nothing needs to be done. That's the rule I've followed anyway. Check each week, but do nothing unless it needs it. Wickwack 121.215.11.232 (talk) 01:54, 12 November 2012 (UTC)

I can't prove it mathematically, but off-center loading, particularly 4 kg at the short distance from the center a pannier is at, seems very unlikely to bust a spoke and crack a rim. Particularly since you are one of probably millions doing this, and no epidemic has surfaced. Thinking as I write, I'd say, cracked rim is likely to be result of rim flexing and metal fatigue. From too much tension in spokes? Too little? Can't decide right this second. Somebody probably will know. How did it crack; parallel to the radius, I presume, as if flexing in and out towards the center in the plane of the rim? Note (of course) that the rear wheel is asymmetrical, the right side spokes being shallower and under higher tension, trigonometry being what it is. If it cracked from flexing side to side, that's another thing entirely. There are various kluges around to construct wheels where the rim is less asymmetric, but I can't think of them right now. Yeah, sometimes new wheels need retruing after like 50 miles or something on that order, but various practices like relaxing twist in the spoke nipple after adjustment, etc. reduce that. Also, of course, one can build a metastable wheel which has the spokes all out of whack but is kept in true by the rim for a little while. Oh yeah, another item, are the spokes 3 cross, 4 cross, or something odd? 36 spokes? My current ride has radial spokes in the front, and radial on the left side of the rear. Which is just me screwing around, since I've never really broken a spoke or rim before or since, other than catastrophically with a pothole or automobile or such. Anyway, this is like the Bible of wheelbuilding. Gzuckier (talk) 02:15, 12 November 2012 (UTC)

• If the weight load on your bike is asymmetrical, in order to balance you will have to have your wheel at an angle to the ground, which is not good. But I doubt that 4kg is enough to matter. Looie496 (talk) 03:27, 12 November 2012 (UTC)

Timeline of chemical elements discoveries

Why it took so long to confirm elements 113, 115, 117, 118? 3 of them has been discovered since 2002 and 2003. God, that is like 10 years! Some scientists (especially if they're old) may be dead after 10 years and never know if their discovery has been confirmed. When I think about this, it just doesn't make sense! Did they already confirm it and the article simply is out of date or is it actually not confirmed? Seriously 10 years is a long long time to me. And if it hasn't been confirmed then how come the article Joint Institute for Nuclear Research, it says "2010 – successful synthesis of element 117"? How could it be a successful synthesis if it wasn't being confirmed?174.20.101.190 (talk) 09:21, 11 November 2012 (UTC)

I think its because almost no one have an equipment to repeat the experiment. Only a few big labs have it. --Ewigekrieg (talk) 11:43, 11 November 2012 (UTC)

Some of the elements have been created in one place and nobody ever tried the confirm it. Why should I spent millions worth of equipment time to verify a experiment? You can not publish such a result, because it is nothing new. If you have the equipment everybody would aim for 119 a leave confirmation of 118 to others. After 10 or 20 years the equimpent becomes better and you can do what the others had to do for the discovery of an element for calibration of your machinery in a few days, weeks or months. --Stone (talk) 12:59, 11 November 2012 (UTC)

Put another way, there is "not a lot of interest" in the scientific confirmation of Element 113, 115, and so on; in this case, I have arbitrarily defined "a lot of interest" to mean "enough interest to justify funding immediate confirmation." These elements have no currently-known practical use; their discovery did not refute any well-established theory of nuclear physics, or enlighten us about subtleties that we did not already know. The production of these very heavy transuranic elements can be described as a sort of incremental advance to the knowledge of nuclear physics. Compare this with the history of the semiconductor, or even the discovery of uranium fission; for many decades, the cutting-edge of pure research into the physics of these materials immediately translated into commercialization and productization. Nimur (talk) 20:02, 11 November 2012 (UTC)

I guess Stone brought a good point. People are naturally thirsty for fame and famous but being the first one to confirm it is something right? It is much harder to try to create a "new element" than to try to repeated an early successful experiment to confirm it. I guess the one who discovered it will gain more fame but being the first one who confirmed it is not too bad either. At least your name will be somewhere in science's history or physic's history for being the first one to confirm it, maybe it is just a brief mention of your name if it is there! Eventually someone got stand up to confirm it.

@Nimur: it can said that that while those discoveries of new elements have no significant impact on science but look at it this way, I'm sure somewhere along the line it will become a crucial tool for us, humans, to understand the universe and perhaps dark matter. It may take until element 119 or 125 or even 150 so on... Great discovery doesn't come easy but we're building up to it. I believe the discovery of any new element now will be another step closer to something BIG, but unfortunately we don't know what it is yet! And yes this is comparable to getting man to the Moon. What is it really benefiting us about getting man to the Moon really? That's mostly the result of the "arm race" between Soviet and US and the pride it gives for being the first to get man on the Moon. I'm sure the cost of getting man to the Moon is significant higher. I wish there is something like "element race" between all the countries in the world to find like the ultimate element. If that happened, I think within the next 10 or 20 years, plenty of new elements will be discover. It is sad that people are losing interest in science compare to the 1940's to 1960's (look at all the feats we have accomplished). That being said in the 1940's and 1960's science discoveries are mainly for war purpose and not for the sake of science. 174.20.101.190 (talk) 20:42, 11 November 2012 (UTC)

And one of question above didn't receive any answer, can anyone answer it? how come the article Joint Institute for Nuclear Research, it says "2010 – successful synthesis of element 117"? How could it be a successful synthesis if it wasn't being confirmed??174.20.101.190 (talk) 20:42, 11 November 2012 (UTC)

Maybe it means that they believe they successfully synthesised it, but it hasn't been confirmed yet. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 21:28, 11 November 2012 (UTC)

I believe there is something wrong with that assumption. They can't just assume that they actually synthesized it without confirmation. If it is considered as successful then there is no reason to be confirm. What if they were wrong? Who knows? To me that is a false statement.174.20.101.190 (talk) 06:01, 12 November 2012 (UTC)

No, that makes sense. As far as their evidence is concerned, they've synthesised it, but there has been no independent conformation by a different lab. Also remember that with these elements it's not like you get a nugget of them, we're talking a few atoms here, requiring careful analysis of data and statistics to find them. Fgf10 (talk) 07:59, 12 November 2012 (UTC)

Here's a quote from IUPAC about this:

"Discovery of a chemical element is the experimental demonstration, beyond reasonable doubt, of the existence of a nuclide...

The TWG realizes that the term ‘reasonable doubt’ is necessarily somewhat vague... Confirmation demands reproducibility... In the case of the new elements the TWG attaches considerable importance to reproducibility and would indeed like to be able to suggest that no new element should be recognized officially until the data upon which the claim is based have been reproduced, preferably in another laboratory and preferably by a different technique. However, ...it would appear unreasonable to apply such a demand of demonstrated reproducibility in all rigidity. We do not believe that recognition of the discovery of a new element should always be held up until the experiment or its equivalent have been repeated, desirable in principle as this may be. However, we would waive this requirement only in cases where the data are of such a nature that no reasonable doubt is possible (for instance for data with a high degree of internal redundancy and of the highest quality), and under circumstances where a repetition of the experiment would imply an unreasonable burden."

Double sharp (talk) 05:43, 15 November 2012 (UTC)

The TWG is the Transfermium Working Group from the Transfermium Wars. Double sharp (talk) 05:46, 15 November 2012 (UTC)

Good physics simulation sandbox?

Could you recommend me a good physics simulation educational software? Something powerful enough to recreate most of school physics experiments? --Ewigekrieg (talk) 11:56, 11 November 2012 (UTC)

Garry's Mod is a very good physics sandbox based on the Half-Life engine. You might also be interested in Learning physics with the Unreal Tournament engine. - Cucumber Mike (talk) 13:18, 11 November 2012 (UTC)

Visual Python makes it very easy to quickly write a 3D visualization for simple mechanics and electrodynamics equations, like simulating an orbiting planet-sun system, in just a few lines of code. Visual Python is free and open source software; you can find installation instructions at the VPython.org website, including some demo videos. Here is a video demonstrating a for-loop and elementary kinematics. If you want to learn physics and learn simulation programming, a little bit more "structured" format will be very helpful. Here is a (not-free) introductory physics textbook, Chabay & Sherwood's Matter & Interactions (available from the publisher), which is used at many universities to guide the freshman-level university physics course with programming examples. Owners of the textbook have access to online versions of the source-code. Nimur (talk) 18:44, 11 November 2012 (UTC)

Always glad to see free publicity for the family! —Tamfang (talk) 03:02, 12 November 2012 (UTC)

Psychology

What's the difference between social anxiety disorder and avoidant personality disorder? --168.7.231.4 (talk) 18:47, 11 November 2012 (UTC)

social anxiety disorder and avoidant personality disorder has the answer. More specifically in Avoidant_personality_disorder#Differential_diagnosis OsmanRF34 (talk) 18:58, 11 November 2012 (UTC)

Self-help books

What research have psychologists done into the effectiveness of self-help books? 65.92.7.202 (talk) 20:44, 11 November 2012 (UTC)

[[1]] OsmanRF34 (talk) 21:29, 11 November 2012 (UTC)

Richard Wiseman, a psychologist, has written a self help book about self help books called 59 Seconds. I'm a big fan of his. Vespine (talk) 21:43, 11 November 2012 (UTC)

Dogbert would say they are highly effective - provided they sell enough copies. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:06, 11 November 2012 (UTC)

Behavior while drunk/under the influence

To what extent does results expectancy affect one's behavior while under the influence of a substance? e.g Are the impairing effects of alcohol naturally induced, or do they occur as a result of our expectations? Same goes for the reduction in inhibitions associated with particular substances. Ankh.Morpork 22:18, 11 November 2012 (UTC)

That's as broad a question as the Milky Way. Can you specify at least a specific drug? μηδείς (talk) 22:25, 11 November 2012 (UTC)

Alcohol. I live on a desert island and eat some fermenting fruit, having never watched Geordie Shore, what happens next? Ankh.Morpork 22:32, 11 November 2012 (UTC)

(ec) To start with alcohol: Testa et al. (2006) "Understanding Alcohol Expectancy Effects: Revisiting the Placebo Condition" Alcohol Clin Exp Res. 30(2):339–348. As Medeis says, this is a very broad and complex question, even if you limit it to one well-studied compound like alcohol; that article provides a lot of references to get you started. See also Short-term effects of alcohol. TenOfAllTrades(talk) 22:36, 11 November 2012 (UTC)

Given you live on an island with no people, don't worry, none of your symptoms will be socially constructed. Enjoy. μηδείς (talk) 23:31, 11 November 2012 (UTC)

Didn't some philosopher once wonder how many people would fall in love if they hadn't been made familiar with the concept via society? Gzuckier (talk) 02:18, 12 November 2012 (UTC)

Related question... The effect of alcohol placebos is very interesting, though I wonder if it is definitely known to be placebo as opposed to some other (unknown) mechanism of contact high. I remember that there have been instances, falling in with people who were drunk for 15 minutes or more, when I've not merely felt intoxicated and felt less socially constrained, but literally felt unsteady on my feet and observed apparent motion of stationary lights when turning my head due to poor tracking, just as with actual alcohol use. I've wondered if there is some natural transition between internal social/mental states of sobriety and drunkenness that preexisted before alcohol was discovered, and if alcohol is only the most common means by which it is forced. Wnt (talk) 19:44, 12 November 2012 (UTC)

I had that same feeling at a party...turned out it was just a bunch of frat guys kept staggering into me and the lamp was shaking because some drunk girl was dancing with it. DMacks (talk) 03:33, 13 November 2012 (UTC)

Colour black

Hello, is it true, that colour black does not exist in nature? And what about white? Bennielove (talk) 22:20, 11 November 2012 (UTC)Bennie

Are you suggesting those colors are supernatural? Can you clarify your question? μηδείς (talk) 22:22, 11 November 2012 (UTC)

I wouldn't say supernatural. You can not find colour black in nature; I've heard sth like that, but now I'm not sure anymore ... Bennie — Preceding unsigned comment added by Bennielove (talk • contribs) 22:37, 11 November 2012 (UTC)

Black is actually when light is not being reflected, so, it doesn't exist in the sense that it's the absence of light being reflected. OsmanRF34 (talk) 22:43, 11 November 2012 (UTC)

So how is black different to colourless? Ankh.Morpork 22:50, 11 November 2012 (UTC)

There are many caves where, if you go down a certain way, no light penetrates. Is not what we see pure blackness? Also, many animals are black - some species of panther, for example; and cats, dogs, cattle, snakes, you name it. As for white, pick from thousands of different white flowers. -- Jack of Oz ^[Talk] 22:59, 11 November 2012 (UTC)

Those animals still reflect some light, so they are not pure black. StuRat (talk) 07:00, 12 November 2012 (UTC)

Re how black differs from colorless: Wiktionary says only that colorless means lacking color, and one of the definitions it gives for color is Hue as opposed to achromatic colours (black, white and greys). [For example] 'He referred to the white flag as one "drained of all colour".' In line with this, in my experience when people say colorless they mean lacking any hue -- that is, being only white, gray, or black. Duoduoduo (talk) 23:13, 11 November 2012 (UTC)

When my black cat is strongly backlit, I can see that his coat is really dark brown. —Tamfang (talk) 03:01, 12 November 2012 (UTC)

Black and color are both psychological terms. They have to do with human perception, not nature in and of itself. Black is what one perceives when there is no even semi-saturated hue. This is so basic as to be childish. Please read hue and color. μηδείς (talk) 23:29, 11 November 2012 (UTC)

However, "a colourless liquid or gas" is transparent, and lacks all hue. Bielle (talk) 23:31, 11 November 2012 (UTC)

The word black is ambiguous, in that it can either mean the absence of light, or the complete absorption of light. But even within those two definitions, the word light is also ambiguous, in that it can either mean electromagnetic radiation of any frequency, or merely electromagnetic radiation in the visible spectrum. There are no substances, whether found in nature or not, that absorb light perfectly; the blackest material yet created has a total reflective index of 0.045%, which is small but nonzero.[2] Using the "absence of light" meaning of "black", the example of the inside of a deep cave would be a good example of the existence of black in nature, if you take "light" to mean just the visible frequencies of electromagnetic radiation, but in such a situation there would still be infrared radiation, so it wouldn't count as black if you're considering all frequencies of electromagnetic radiation. The sky, as seen from space in a direction other than in the direction of a star, would be a purer absence of electromagnetic radiation, although even then there would at least be the cosmic microwave background radiation. But the blackness of the sky is beat hands down by the blackness of a black hole, which is indeed extremely black. The only electromagnetic radiation coming from a black hole is Hawking radiation, but a reasonable-sized black hole emits far less power per area in the form of Hawking radiation than even the cosmic microwave background radiation. Red Act (talk) 00:27, 12 November 2012 (UTC)

There's no such thing known to science as truly black light, but undoubtedly there is black pigment, especially a blackbody. To confuse you further, blackbodies can glow red hot or white hot --- indeed, to within a small approximation, the Sun is black! (And with that, everybody in Colorado or Washington has to take a toke) Wnt (talk) 04:17, 12 November 2012 (UTC)

My reading of black body is that this is a very useful theoretical approximation: an idealized description that does not occur in physical objects on earth. It even opens with "A black body is an idealized physical body." SemanticMantis (talk) 13:48, 12 November 2012 (UTC)

Well, the taste of maple syrup and the sound of an oboe also lack all hue, but we don't therefore call them white or black or grey. There are different ways to perceive color: from reflective surfaces like coal, from luminous surfaces like computer monitors, through transparent object like rose-tinted glasses, or even without focused dimension, like the red one sees through closed eyelids, or the utter black of an unlit cave. Developmentally we first conceptualize colors as perceived on reflective surfaces, fundamentally the colors of a crayon box, including ROYGBIV as well as white, black, grey, brown, silver, gold, etc., all equally as "colors" and all falling within certain ranges where two slightly different blacks still count as black.

It's only when we gain some scientific or artistic sophistication that we learn there is no way to make black light, that grey and brown lights don't exist in isolation, but only in contrast to brighter or more saturated hues, that the colors gold and silver are identical in hue to yellow and white--that their properties are based on the coherent reflectivity of their surfaces, and painters have to use the trick of painting highlights to portray them. All this requires making finer distinctions and expanding our vocabulary. Simply saying black is not a color, rather than, say, explaining in detail how black is different from green, doesn't achieve that. μηδείς (talk) 18:15, 12 November 2012 (UTC)

See here. Count Iblis (talk) 18:48, 12 November 2012 (UTC)

As it happens, on my laptop the change in display color according to change in viewing angle is just sufficient to counteract the optical illusion so that they actually do look the same color. Wnt (talk) 19:48, 12 November 2012 (UTC)

Regarding lightless caves being perfectly black, see eigengrau. Matt Deres (talk) 17:44, 13 November 2012 (UTC)

Now that's a great link - thanks! But I don't necessarily fully understand. I know that there's a distinction between a "dull black" which I suppose is eigengrau, and a "living black" which perhaps is the darker color the article mentions. But when I'm in a darkroom the "dull black" breaks up into "living black" and light blue speckles, which form various sorts of pulsing wave patterns and phantasms of various sorts (including rehashes of recent visually distinctive and unfamiliar stimuli which take on their own original colors, and simple dreams that remain in blue colors), occasionally tinging into yellow. And then there's a "blacker than black" color which I only very rarely see, usually at right around this time of year, on wet reflective surfaces where contrast is present. So I feel like there's some variety of subjective states... but how to study them with any meaning? Wnt (talk) 05:35, 14 November 2012 (UTC)

November 12

Size of Eliminated Dendritic Spines

Hello. Can an eliminated dendritic spine have a size? If so, why can such spines classified as "eliminated"? --Mayfare (talk) 01:32, 12 November 2012 (UTC)

If it has been eliminated, it no longer exists. It is possible that at least in some cases spines are actually reduced to a nub rather than eliminated, but such a spine could not properly be said to be eliminated. Was there some statement somewhere that created this confusion for you? Looie496 (talk) 03:21, 12 November 2012 (UTC)

The only occurrence of this phrase coming up on straight Google was at [3] - see figure 1H, in which the yellow square (lower row of panels) marks the eYFP signal corresponding to a single dendritic spine that goes away. As you see, the spine itself disappears, but it still occupies space in terms of the length of dendrite that is now unmarked by a spine. I assume this is what is meant by that, but could easily be wrong. Wnt (talk) 04:13, 12 November 2012 (UTC)

The last full sentence of the second page in http://www.nature.com/neuro/journal/v15/n7/abs/nn.3134.html --Mayfare (talk) 05:56, 12 November 2012 (UTC)

I guess you mean this sentence: "Measures of the relative size of eliminated spines—a measure of synaptic strength—showed no differences across groups (Fig. 2c), indicating that our stimulation paradigm was not causing the loss of a specific subgroup of spines.". My take on that is that they mean the size of a spine before it was eliminated. Looie496 (talk) 07:18, 12 November 2012 (UTC)

IQ and artists

I know that there's a correlation between IQ and performance in professional careers, like medicine and law. Have any studies explored the correlation between IQ and artists? 65.92.7.202 (talk) 03:45, 12 November 2012 (UTC)

We have a section, IQ#Social_outcomes. I looked at some of the studies linked there in, but the best ones are paywalled so you probably won't have access to them. The gist is that IQ is a better-than-random predictor for performance at virtually every entry-level job, and usually the best. This is assuming you have controlled for variables such as education, age, and socioeconomic origin. As far as music goes, the research discussed in our article looks at it from the other direction, asking the question "Does learning to play music make you smarter?" The answer seems to be yes. Someguy1221 (talk) 04:15, 12 November 2012 (UTC)

Do we have any substances or pills that are the opposite of anti-depressants?

Specifically, I would mean depressants in the mood or emotional sense, not in the CNS or respiratory sense. Do we have a pill that can make you sad, or less happy? If anti-depressants are SSRI (Seratonin reuptake inhibitors), do we know of any seratonin reuptake promoter, or dopamine reuptake inhibitor?

I ask out of curiosity, because it seems like an interesting field of research as to its possibility and effects, and possible treatments for extreme cases of narcissistic personality disorder or mania fits (or insert other potential uses here). Is this even possible? Is there research into it? Ehryk (talk) 06:12, 12 November 2012 (UTC)

See depressants. StuRat (talk) 06:57, 12 November 2012 (UTC)

No no no - I don't mean CNS depressants, as I said, and I don't mean depressants as opposed to stimulants. I'm talking about 'a pill that makes you feel sad', the opposite of 'a pill that makes you feel happy.' Ehryk (talk) 07:02, 12 November 2012 (UTC)

(ec) That's the wrong article -- depressogenic is the one that refers to mood. Adding to it: (1) There is a drug called tianeptine that works as a selective serotonin uptake enhancer, but strangely, it seems to have antidepressant effects rather than prodepressant effects. (2) Dopamine reuptake inhibitors do not have depressive effects, rather the opposite -- in fact, cocaine is a dopamine reuptake inhibitor. Looie496 (talk) 07:05, 12 November 2012 (UTC)

The way tianeptine works doesn't seem completely strange, the brain tries to keep the homeostasis, so a push in one direction can have an effect in the opposite direction. Philoknow (talk) 18:23, 12 November 2012 (UTC)

The nearest that I find: Recreational_drugs#Depressants. Comploose (talk) 19:01, 12 November 2012 (UTC)

Thank you. Next time, carefully read the entire question as well as the previous answers, so you'll suspect that having depress in its name doesn't make it what the OP wanted. —Tamfang (talk) 06:16, 13 November 2012 (UTC)

Transmission of radioactive and electromagnetic rays through glass

Can alpha, beta and gamma rays pass through glass ? Sunny Singh (DAV) (talk) 07:40, 12 November 2012 (UTC)

Alpha: no, beta: no, gamma: generally yes, with the added bonus of often colouring the glass through prolonged exposure. Fgf10 (talk) 07:54, 12 November 2012 (UTC)

It isn't really a case of each ray being able to penetrate or not penetrate. How much gets through depends on the energy of the radiant particles and the thickness of the glass, as with any media. For a thick piece of glass, the rays suffer attenuation per unit distance. Sufficiently energetic (ie.e, accelerated) betas (ie electrons) will penetrate glass without too much attenuation if it is not too thick. If it were not so, linear accelerators and other sorts of accelerator could not work. Floda 120.145.138.192 (talk) 12:36, 12 November 2012 (UTC)

The electrons also suffer energy loss due to their ionising effect so for any given energy there is a maximum distance they will penetrate. Dmcq (talk) 14:48, 12 November 2012 (UTC)

Follow up question,
OK, gamma ray can pass through glass. In electromagnetic spectrum, ultraviolet ray lies between gamma ray and visible light. The latter two can pass through but former cannot. Why ? Sunny Singh (DAV) (talk) 16:38, 13 November 2012 (UTC)

There are numerous, distinct physical phenomena that contribute to the bulk optical density of a material, and not all effects are monotonic with respect to the wavelength. Nimur (talk) 18:25, 13 November 2012 (UTC)

square ring falling through magnetic field

Suppose you have a square loop falling through a circular loop through which a current flows. Is there an equation relating the position of the square loop to time? Widener (talk) 07:42, 12 November 2012 (UTC)

You can write a system of differential equations for this problem. However I am not sure that it has the exact analytic solution. Ruslik_Zero 18:40, 12 November 2012 (UTC)

Mil Mi-24

Is the Mil Mi-24 attack helicopter equipped with infrared camera?--Wrk678 (talk) 10:51, 12 November 2012 (UTC)

This undated article (December 2008 or later) states "Russian Army Mi-24s are being upgraded with new avionics including thermal imagers." This article, also undated, mentions an "upward hinging cover for IR sensor" for the Mi-24K. Clarityfiend (talk) 11:11, 12 November 2012 (UTC)

Although if this is connected with your previous question; Noveber 4: Syria Rebels, then that is a different matter. Alansplodge (talk) 18:20, 12 November 2012 (UTC)

Yep. Buying a helicopter is much like buying a car in that nearly any accessory can be added -- provided you're willing to pay for it. Even if there was no integrated infrared camera option for an Mi-24, a bolt-on camera and an independent controller/screen for the cockpit can't be all that difficult to add if "getting it done" is more important than "doing it right". — Lomn 23:53, 12 November 2012 (UTC)

Well, allow me to expand on my question then, is the infrared camera "standard" or is it usually a add on? Also I'm referring to a infrared targeting system for their 30 mm machine gun and rockets.--Wrk678 (talk) 10:44, 13 November 2012 (UTC)

I had a good look on the web, but Google couldn't find anything for me. I think it's a fairly safe bet that if the Russians had to fit it as an upgrade, then it wasn't supplied as standard to the Syrians. Many aircraft manufacturers have an "export version" for sale to developing countries, with much more limited electronics, to reduce both the purchase price and the maintenance costs, and further to avoid secret technology falling into the wrong hands. So I'd assume that the Syrian Mils had a very limited sensor fit unless you can find a source that says otherwise. Sorry I couldn't be more helpful. Alansplodge (talk) 13:56, 13 November 2012 (UTC)

What is "matter" according to modern physics?

In classical Physics essential feature of "matter" was "mass". Right? Now since "mass" and "energy" are same so what is the defining characterisic of "matter"? :: Vineet Chaitanya 14.139.82.7 (talk) 13:18, 12 November 2012 (UTC)

Unsurprisingly, there's an answer in our article on matter: Matter is whatever possesses mass and occupies space. This is then linked to the Pauli exclusion principle. AlexTiefling (talk) 13:32, 12 November 2012 (UTC)

Can you just have one of this features? For example, occupy space but not have mass, or have mass but not do not occupy space? Philoknow (talk) 18:10, 12 November 2012 (UTC)

Both I believe. There are particles with no rest mass, like the photon, which presumably occupy space, and massive objects which have no volume, like the gravitational singularity in a black hole. StuRat (talk) 19:25, 12 November 2012 (UTC)

Per the Simpsons, "What is mind? No matter. What is matter? Never mind." μηδείς (talk) 23:23, 12 November 2012 (UTC)

Simpsons? George Berkeley must be rolling about in his grave. Even a hundred years ago, Bertrand Russell complained about it being hackneyed. Matt Deres (talk) 02:09, 14 November 2012 (UTC)

Stu, photons do not occupy space (in the sense that if it is there nothing else can be there). They are bosons and thus are not bound by Pauli's principle, which is a fermionic statistic. On those scales the laymam's ideas of "mass" and "occupy space" as we know them are not really meaningful. 24.92.74.238 (talk) 01:45, 13 November 2012 (UTC)

Aren't photons a type of imaginary particle whose entire existence is defined purely by its history? History includes: absorption, emmision, and gravitational interaction. Plasmic Physics (talk) 02:54, 13 November 2012 (UTC)

What is imaginary? All the words and concepts we use to describe the physical world are heuristic. An atom is "imaginary" at that level. Photons are exactly as real as atoms are. They behave differently than atoms do, but that doesn't make them any less or more real. So, it isn't helpful to think of concepts like "real" and "imaginary". The concepts "useful" is far more important. Photons are useful heuristic in explaining how the world works, so we use it. Atoms are a useful heuristic as well. But using concepts like "real" and "imaginary" misses the point in the same way that concepts like "facts" and "proof" misses the concept of science. --Jayron32 04:15, 13 November 2012 (UTC)

I mean, an atom can actively be observed, a photon can never be observed, because observation destroys it. You can observe what an atom is doing, but you can only deduce what a photon did or will do. The difference being, atom is the falling tree in the forest that can be seen, whereas a photon is the falling tree that can only be heard. A photon is in this way more heuristic than atoms. Plasmic Physics (talk) 07:55, 14 November 2012 (UTC)

"Hey this mass has lost some energy, lets call it a photon." Plasmic Physics (talk) 03:09, 13 November 2012 (UTC)

I don't think you should take that article too seriously. At least read Talk:Matter (and note there are two archive pages). There's no platonic entity "matter" whose properties are discovered by physicists; it's just a word and may mean different things to different physicists. I'm not sure there is a good single definition. "Has mass and occupies space" is clearly no good. The non-baryonic dark matter doesn't occupy space, but you can't argue it isn't matter—it's right there in the name. Just "has mass" doesn't work either because, as the original poster said, everything has mass. No one would say that the energy stored in a battery is matter, for example.

In many cases "matter" is short for baryonic matter (that is, stuff made of atoms) or for other things made of quarks such as nuclear matter and strange matter. Otherwise I think it's assigned haphazardly by individual physicists based on a loose analogy with baryonic matter. -- BenRG (talk) 21:46, 13 November 2012 (UTC)

Nothing; what is matter you? Gzuckier (talk) 19:30, 15 November 2012 (UTC)

Getting a given wavelength radio signal given an antenna with real-world constraints

Wavelength is related to transverse distance between peaks, not axial displacement of charge carrier.

Supposing you are given a transmitting antenna of whatever type of metal is most commonly used these days, and supposing that you are applying an AC current with the intent of transmitting a 1 m wavelength radio signal, about how far is one electron likely to make it in one direction along the antenna conductor in one half cycle from a negative peak to a positive peak, factoring in the likelihood of the presence of antenna material atoms affecting the electron's path? Can individual charge carriers actually make it very far (on the order of meters) through metal?

But if the answer is that individual charge carriers actually only make a microscopic or very small displacement in space, how could a 1 m electromagnetic wave be produced? I don't see how a large number of charge carriers each only making a very small displacement in space can add up to a 1 m wave.

This site, especially Fig. 2 there, is where I'm getting my basic understanding of how alternating current gets an EM wave transmitted. From that is where I got the feeling that charge carriers would actually need to make a displacement in space on the same order as the intended EM wave. Peter Michner (talk) 16:43, 12 November 2012 (UTC)

The individual electrons will only move a very small distance - see drift velocity. In the example on that page, the electrons only move a couple of nanometres (that's ignoring the random motions the electrons have anyway and just includes the motion induced by the electricity). It may help to think of it like a Newton's cradle. The balls in the middle hardly move at all, but they are still able to pass the energy along. One bumps into the next, which bumps into the next and so on. Electrons carrying current through a wire do roughly the same thing (it's not a great analogy, but it might help to see that lots of things moving a very small distance can add up to result in large movements). --Tango (talk) 18:25, 12 November 2012 (UTC)

I think I get it now after looking at this visualization. The radiated field wavelength doesn't have to do with the physical displacement of the charge carriers in the direction of the conductor, but is related to the transverse distance perpendicular to the conductor with the waves emanating out at the speed of light (in a vacuum, and only slightly slower in a medium such as air) as shown by this picture which I just made to try to explain it to myself. Peter Michner (talk) 19:32, 12 November 2012 (UTC)

Long time effect of drugs

Some people lose their mind after too much use of recreational drugs, and their brains never get tuned again, even if they stop to take drugs. Is the opposite case also possible? Could someone who goes through a controlled therapy change the chemistry of his brain long term but in a positive direction? Comploose (talk) 17:46, 12 November 2012 (UTC)

First of all, we need to figure out what we mean by changing the chemistry of the brain in a positive direction. We tend to consider any deviation from the norm to be a bad thing (evolution tends to do a pretty good job of finding a fairly optimal norm, so that's not a bad definition). I can imagine situations where someone has some kind of neuro-chemical disorder that is permanently cured by medication - would you count that? Or are you specifically looking for improvements above the norm? --Tango (talk) 18:29, 12 November 2012 (UTC)

Nope, no improvement above the norm. Just if, for example, a schizophrenic or depressive patient goes through a treatment with psychoactive drugs, can his brain chemistry be changed for ever, or for a long term, after he stop taking the medication. Comploose (talk) 18:45, 12 November 2012 (UTC)

• To begin with, the claim that people may lose their minds after overuse of recreational drugs is very dubious, unless they overdose to a degree that causes brain damage due to the heart or lungs shutting down and depriving the brain of oxygen. But to address the main question, ECT is a therapy that is supposed to change the chemistry of the brain long term in a positive direction, for people who are depressed. There is some recent evidence that ketamine also can cause long-term positive changes in depressed people. Looie496 (talk) 18:50, 12 November 2012 (UTC)

• "Lose their minds" might be overstating it, but many drugs can cause negative long-term brain changes. StuRat (talk) 19:27, 12 November 2012 (UTC)

• There are a lot of neurotoxic recreational drugs, that could cause Olney's lesions among other things. I don't know exactly what is meant by "lose your mind". Has someone suffering from Parkinson lost his mind? He will be able to think, but not to control completely his movements with his brain. OsmanRF34 (talk) 21:02, 12 November 2012 (UTC)

I'd be more optimistic of changing the brains of children in a positive way than adults. In adults, the basic way the brain works is already established, and only incremental improvements are normally possible. StuRat (talk) 19:29, 12 November 2012 (UTC)

Drugs are used as part of successful therapies to overcome such things as alcoholism and acute psychological traumas, PTSD and OCD with varying success rates. See librium, valium, antabuse and so forth. It's also certainly not dubious that people become psychotic or terribly damaged long-term after drug abuse and some don't recover. μηδείς (talk) 23:21, 12 November 2012 (UTC)

I agree with Medeis, from professional experience (OR, ?not valid) I have seen about 6 cases over 20 years of individuals who have lapsed into serious mental illness triggered by the use of recreational drugs - mostly cannabis. The relationship between their mental illness and the taking of RDs was not dubious. @ StuRat, so that means that antidepressant and antipsychotic medication is ineffective? Richard Avery (talk) 08:16, 13 November 2012 (UTC)

Meds can still work to treat adult mental problems, but are unlikely to cure them. That is, once adults go off their meds, you can expect their problems to return. StuRat (talk) 08:37, 13 November 2012 (UTC)

That's rather broad, StuRat. You might be better off saying congenital organic problems are rarely permanently curable, as opposed to acute psychological trauma, which can be more pliable. μηδείς (talk) 05:15, 16 November 2012 (UTC)

For an example dealing with rewiring the brains of children, emerging evidence supports the idea that if autism is successfully treated early in the preschool years, this can prevent much of the damage to the brain, which otherwise locks them into autism for the rest of their lives. StuRat (talk) 08:41, 13 November 2012 (UTC)

Evidence of filamentation of the milky way

Given that dark matter sculpts the galaxy clusters into filament structures I would assume that this is also an effect at smaller scales such as our Galaxy itself.

Is there any evidence of filamentation of the 1) Milky way galaxy and also 2) The gas in our solar system.

I can visualize the dark matter as tiny chargeless particles flying towards more massive objects and causing a sort of "gravitational current" effect which causes the filament structures . Am I on the right track? :) --Ap-uk (talk) 18:52, 12 November 2012 (UTC)

I'm not sure it's correct to say dark matter is what "sculpts" the galaxies into the formation they take. It would certainly play a part, if it even exists, which I don't doubt very much, but it's not certain yet. Whatever effect dark matter has, it's quite weak, so weak that it is difficult to measure on sub-galactic scales. That's why we didn't notice it for so long. Looking for it inside our galaxy is hard, we really notice the effect when we look at whole galaxies or on groups of galaxies. Looking for an effect of dark matter on gas in our solar system would probably be a waste of time at this stage. Also most of the current best theories don't describe dark matter as "tiny" particles. Maybe you should give our dark matter article a good read. Vespine (talk) 21:37, 12 November 2012 (UTC)

I don't think there is any reason to assume that the universe will show the same structure at different scales like that. There are other factors dominating the structures of galaxies and solar systems that would completely eliminate any kind of dark matter filament structure. I'm not sure we even have any good theories for how galaxy filaments formed - without that, there is no way to extrapolate to other scales. --Tango (talk) 22:47, 13 November 2012 (UTC)

plastic PE discoloration

hello i was asking about the plastic PE film tube discoloration to reddish or yellowish since it will be changed after putting in a warehouse or even inside the plant and this matter will happen after only few days. thank you — Preceding unsigned comment added by 77.31.104.159 (talk) 20:55, 12 November 2012 (UTC)

It's not clear what you are asking. Do you want to know what caused it, how to prevent it, how to repair it, or what ? And what's a PE film ? StuRat (talk) 20:57, 12 November 2012 (UTC)

I assume PE = Polyethylene. Perhaps if it's being stored at high temperature it could be loss of plasticizer? or some sort of side reaction? I agree though, more information would help. (+)H₃N-Protein\Chemist-CO₂(-) 21:36, 12 November 2012 (UTC)

cornstarch and water experiment

I have been looking everywhere for this. I am doing a science project and I don't have a good idea for my research draft.It has to be 1-5 pages.what I mean is that it can be 2-4 or 1-3 or 2-5.So what I need is a good research draft.Can I have help? — Preceding unsigned comment added by 99.32.182.192 (talk) 22:31, 12 November 2012 (UTC)

It is difficult for us to answer with regards to the expectations of your teacher. As a teacher myself, I wouldn't object if a student asked me for a clarification on my requirements regarding an assignment. Length requirements are a secondary issue: the teacher wants you to write enough to completely cover the topic of the paper, but to not drift into off-topic writing. Remember that longer is not better, but incomplete is also bad. Your goal should be completeness but not excess. Still, your best option is to ask your teacher directly. A smart student with initiative gets their assignment done several days in advance, and asks their teacher to look it over briefly to see if any adjustments need to be done before the due date. A good teacher will give constructive feedback to a student that does this. --Jayron32 22:35, 12 November 2012 (UTC)

Well, length should be a secondary issue, but, depending on the teacher, it may very well be their primary concern. I've had teachers grade almost solely based on length, and I shoveled lots of "filler" into everything I wrote for them, accordingly. StuRat (talk) 22:55, 12 November 2012 (UTC)

So, the one thing we can be certain of is that Jayron never taught StuRat? μηδείς (talk) 23:11, 12 November 2012 (UTC)

No, that does not follow. Stu had at least two other teachers other than Jayron, that much is certain. --Stephan Schulz (talk) 23:22, 12 November 2012 (UTC)

Nobody can predict how grade school teachers will act. But we have a piece of an article on "oobleck" under Non-Newtonian fluid and Wikiversity has some resources also. (Those are in an unusually advanced condition for Wikiversity, which is to say, it looks like a rough draft of something even if you're not quite sure what yet) Wnt (talk) 23:58, 12 November 2012 (UTC)

Jayron must be an exceptionally helpfull teacher if he gives students a pre-mark off the record assessment, just because individuals ask for it. Whether it is fair to all students is debatable. Remember, there are 3 basic reasons for marking assignments: 1) So the teacher and the teacher's superiors can asses his success at teaching; 2) to ensure students actually do the assignments; 3) Gating - i.e., only let thru students who have a demonstrated level of capability - knowledge, ability to think, and the ability to get work done. Would you be comfortable with a student asking another student look over a draft paper? That is sometimes done, but is generally considered a bad thing unless it is only to get help where you are stuck. It it always a bad thing if the student pays the other student. How is getting the teacher to give a pre-mark off-the-record assesment different? The teacher would need to be VERY carefull how he does this. Having said that, it is quite normal for university staff to provide such help with post-grad thesis papers - but this is done in very controlled ways, is acknowledged to be the norm, and when the thesis is marked, it's usually done by at least 2 people. I agree that it is quite acceptable for a student to ask in general terms what length is expected, just as it is acceptable to get any ambiguity in the assignment question resolved.

Particularly in 1st year university and college courses, there is another reason why teachers/lecturers set a required length for assignments. 1st year students sometimes come with an intense interest in certain topics and may be poor time managers. A student might, for example, happilly spend all week writing a most splendid longish paper on a subject he/she loves, and leave minimal time to do that difficult math assignment - but in fact the big picture is that he/she should have spent the time pondering the math and mastering it. It's not very good if at the end of semesterm you got an A in subject you love and a FAIL in subjects you didn't like - not good for you, and not good for the college or university. It's better to get a whole lot of B's and C's if that's what it takes to avoid any FAILS. Setting a max length on assigments helps prevent students not do more than is required and maitain a balance of effort.

Another reason for setting a maximum length is to force the students to be concise - that keeps the teacher's marking work load down, and sets the student up for his future - being concise is essential in employment. So, look at what you have been asked to write on. If you feel you can cover the subject quite well in 2 pages when 5 have been asked for, perhaps there's an aspect to the topic you haven't thought of?

Wickwack 121.221.32.50 (talk) 00:49, 13 November 2012 (UTC)

Your experiences must be very different from mine. I've never had a teacher who wasn't willing to look over drafts. Most of them refuse to give preliminary grades, but they do give detailed and helpful feedback. I've also never had a teacher who objected to students proofreading each others' essays. In fact, my school has a student-staffed writing center specifically for the purpose of proofreading other students' essays. --140.180.252.244 (talk) 02:12, 13 November 2012 (UTC)

Could be. You (140.180.252.244) geolocate to Princeton University (a very good university incidentally). Where you refering to Princeton or to a highschool? Certainly in my experience, getting such assistance from teaching staff in undergrad courses is just not possible due to their workload. I remember doing first year engineering math - about 2000 students from all engineering & science courses together in a large hall - and way down the front was the lecturer with a microphone. After presenting his weekly 2-hour excited no-stopping-for-anything recital, he would hang around another 10 minutes or so for questions, then nick off - we wouldn't see him until the next week. Each week during the 2 hour lectures in each subject they would set an assignment. We were required to submit them to the department clerk on or before the required date, along with a cover sheet and signed statement that it was all our own work and references credited. The clerk date-stamped them and gave us a receipt. A day or so later, the lecturer got all the assignments from the clerk, took them somewhere (home? his office?) and marked them. At the next lecture, he brought the marked assignments with him, and put them on the table for us to collect during the break. That was pretty much all the contact we had. If only (say) 1% of students somehow made contact and asked for advice, that would obviously take up more of the lecturer's time than his official commitment would. If you wanted him to advise you on a draft, you'd have to give the draft to to the clerk, and there was no reason why the lecturer would turn up until he needed to collect the assignments for marking. However, once you got past first year, the teacher-to-student ratio progressively improved considerably and much more contact/interaction happened. Wickwack 121.221.90.241 (talk) 02:42, 13 November 2012 (UTC)

• Regarding the debate being held above as to whether or not a teacher will give helpful advice to students who ask for it: It depends on whether the job of a teacher is to educated students or to assess students. I tend to think in terms of the former rather than the latter. I'm not particularly interested in assessment for its own sake. Assessment is important to understand what a student is learning, but education is not a game or a contest, and there aren't winners and losers, and I don't look at assessment as a means of deciding between winners and losers. If students wish to learn, my job is to teach them by any means necessary, and I'm not particularly interested in witholding instruction from students for any purpose. When Wickwack says "Whether it is fair to all students is debatable", it isn't debatable. The core of my soul does not allow me to not teach the student in front of me. A student who comes to me with the desire to learn something will not be held back in the interest of "fairness", as though education were some contest that I was unfairly helping this student "win". That's an odd conception of "fairness", in the sense that it presumes the only means of being "fair" is to treat education as a race to a finish line. It isn't, and I don't operate that way. I teach because I would die if I stopped. It is my air. I also recognize that not all teachers work this way. Not all plumbers do a good job of fixing my pipes either. I still hold the expectation that they will, and if they don't, I don't find that acceptable. --Jayron32 04:09, 13 November 2012 (UTC)

Jayron makes some good points and I certainly see where he is comming from. However, it is clear - expecially from his last sentences that he is an exceptional teacher, not a typical teacher. Yes, ideally, teaching is about teaching/educating, and not about gating. But, in practice, the gating part is important - very important. That's why I listed 3 reasons for marking in my earlier post. At the end of the day, employers use academic records (as well as other things) to work out who to hire. This reminds me of a conversation I had with a Head Of Department when I was a student representative: HofD: "This university is not about competition between students. It is about excellence" Me: "It is widely believed by students that it is competitive. Can you give me a tight reassurance I can take back to them?" HofD: "Yes, absolutely - see here - it is writen in core policy." Me: "Very good, now please tell me how the university guards against an individual exam paper being unexpectedly hard or easy affecting students in one particular year." HofD: "We know from experience that students fit a bell curve of ability, and any given assessable assignment or exam should return a certain average result. So, if the as-marked results seem out, we scale the marks up or down for the records." Me: "Ah, yes - actully I heard a rumour that the university uses a computer programme to do the scaling automatically on each and every subject." HoD: "Err... yes, that's correct - it is a well regardled software package used by many universities." Me: "So, in any one semester, if more than the expected number of students pass as marked, the computer will adjust so the right number fail." HofD: "Well, you can put it that way." Me: "So, in fact the assesment, as recorded, IS competitive - only the top approriate fraction of students will pass, not all students that passed the bar?" HofD: "Well, I always enjoy chats with student representatives. But its time for my next appointment. Goodbye!" Wickwack 124.182.149.73 (talk) 05:43, 13 November 2012 (UTC)

Sounds like your school needs some new plumbers. Ones that actually know how to fix broken pipes. --Jayron32 05:46, 13 November 2012 (UTC)

Nah. What we need is more funding to buy more pipe parts. Wickwack 120.145.40.78 (talk) 06:33, 13 November 2012 (UTC)

Or plumbers who don't take some fraction of broken pipes in your house is "acceptable". "It's OK that your downstairs toilet sprays shit all over the walls every time you flush. Both of your other toilets work acceptably well, and your shower is like, the best shower we've ever seen. So, it's alright that the other toilet covers your bathroom in feces. After all, we can't fix EVERY fixture, can we?" --Jayron32 06:51, 13 November 2012 (UTC)

We should be carefull not to stretch the metaphor too far. As I said, ideally we would educate everybody. In parctice, when you graduate from school, college, or university, it is about TWO things: 1) You've learnt something, which ought to be usefull in your life an career, but often in fact has little direct relavence (but if you learnt how to think, that is relevant); 2) You have proved you meet a certain hard to define standard and are thus certified as fitting within a subset of the population. Note that I did NOT say that folk without the certificate are not in that subset. I do say that just what standard you've met is hard to define, because you might have passed because of a) superior intellect; b) you were very determined (that one applied to me), c) you managed your time well, etc etc. When you go for employment, that's what employers look for - not so much what you learnt, but what sort of ability you have. So, if the poorest capability group of students don't get to pass, even if they DO know the subject, that sounds unfortunate, and it is, but from society's point of view that may be for the best. Such students are failures in term of marks, but there is no suggestion that they are as such failures in life or failures to society. We can't all be professors and presidents - some have got to be plumbers. A plumber is not a broken professor. And if you & they try hard, you can educate them to Ph.D standard, but that may degrade the standing of Ph.D's. Wickwack 120.145.40.78 (talk) 07:27, 13 November 2012 (UTC)

Quite frankly, I could give a flying fart what my grades mean to employers. My grades are a measure of what a student knows, and student assessments are just another pedagogical tool I use to adjust my instruction to meet the students needs. That doesn't mean that students don't fail, students that don't know the material for whatever reason get a failing grade. But, if every student in my classroom knows the material at the highest standards possible, their going to get the highest grade. I'm not going to fail some random bunch of students just because an employer expects to have some resumes to reject. I'm also not going to withold knowledge from a student who wants it, just because some employer wants to have some failing grades on a resume to reject. If a student shows the initiative to seek outside help, and they get it, and that's what helps them pass, great. If a student has the innate skill not to need that outside help, and they get the same passing grade as the student with less skill but more initiative, that's fine, because my grades don't differentiate between why a student knows the material I teach. My job as a teacher is to train students: if all of my students are trained to the standard they need to be, then they are all trained to the standard they need to be. I'm not going to fail some at random merely because their employers are bothered by the lack of ability to differentiate them by the grades in my class. That doesn't mean that I don't have students that fail (I do), but my grades are not primarily a tool for others to know anything more about a student than that they know the material on the curriculum. Grades can't tell an employer why the students know that material on the curriculum, whether via innate skill or extra effort. But they never could: grades never tell you that. --Jayron32 14:15, 13 November 2012 (UTC)

You and I agree on a lot - we just put some things in different words and other things in different emphasis. But you have missed some points. Yep, grades can't tell an employer much. I essentially said that above when I said that sucessful graduation comes from lots of things in a range of combinations. I said it when I said that having a qualification means you have met a standard but what that standard is is not clearly defined. But particularly when an employer is hiring a recent graduate with no relavent job experience, the qualification (degree, diploma, whatever) is his best guide to who is suitable. And if all candidates have the right/appropriate qualification and look pretty equal in interview, which one gets the job? The one with the highest grades, naturally. You might not care a fig what employers want, but your students should and most probably do - employment is what motivates students to do courses. It is not a case of failing some students at random. It is the weakest achieving students that get failed. Again, as I said before, that's not ideal, as it can be that some or all of the failed students actually have what the curriculum says they should have. But it's not a disaster either. A non-competitive education system is something that idealistic teachers (and students) often want. But industry, commerce, and government is always competitive, and there's nothing we can do about it. Competitive strength comes from (amongst other things) hiring the best graduates and this ultimately leads to a desirability to build some performance status into academic qualifications. That's one of the reasons why universities tend to demand students come with a high minimum score in their high school marks or some sort of entry exam - they don't just require a pass. You are idealistic, Jayron. I'm pragmatic. Wickwack 121.215.135.144 (talk) 15:21, 13 November 2012 (UTC)

Well, to be pragmatic, my impression is that cheating has increased to well-nigh universal levels,[4][5][6] so what is actually being assessed is moving away toward rote memorization or creative problem solving into the more practical assessment of whether students are willing to work around ethical obstacles and do what it takes to succeed. In an age where there seems to be little if any distinction between business and corruption, I suppose this reflects employers' priorities. Wnt (talk) 05:18, 14 November 2012 (UTC)

Very good point. The issue of cheating is an interesting one. When I was a student at university (decades ago now), I was fortunate to be at a uni that had very high ethical standards. And when I started my professional career, it was the norm that my employer (a very large company) put us through a 2-week ethics course to prepare us for interaction with customers and suppliers. It was a very good course. However, the personal skills/attributes that enable students to cheat easily (bad) are the same skills that facillitate team work in employment (good). The objectives are different - at school/college/uni, you are there to learn, but in employment you are there to get things done. And nothing in real life is wholely bad or wholely good. Wickwack 120.145.170.99 (talk) 15:14, 14 November 2012 (UTC)

Ye Olde Practical Student's Guide to Writing an Academic Paper on a Topic About Which You Don't Care:

Start with the Introduction. For the typical 5 pager you mentioned, after Title and a Heading which says "Introduction" and margins, this oughta get you to the middle of the first page. Then, basically copy your introduction in different words with "as we have seen", etc. to provide the Conclusion. Now put together your Bibliography, either what was given to you or what you dig up yourself. Maybe 6-8 entries is a good size, if you haven't been told how many. Now open up each of them and dig up something that references your topic/introduction. It pays to reference sources with an index, otherwise you'll have to make do with the Table of Contents to find something. When you find something relevant write it in your own words and/or quote part of it with attribution in half a page or so, between the Intro and Conclusion; needless to say, don't just copy it without attribution, or plagiarism will attack you. If it agrees with your intro/conclusion, great; if not, list it as if it's you considering all sides of the question: "However, blahblah states in his 'Blablahology', 'Blah blah blah'." If two of your sources are having an argument, then you've got a nice 'he said, she said' situation. Anyway, that oughta be enough to at least get you middle of the road grades on your paper/essay. Gzuckier (talk) 05:00, 13 November 2012 (UTC)

Off label use of drugs

How can it happen that some drugs are used off-label? If someone has reliable information about one use, it should be included among the normal uses of a medicine. If no one knows it it's reliable, why is someone using it? Comploose (talk) 22:50, 12 November 2012 (UTC)

Getting "reliable info" about a new use of a drug requires years of research and millions of dollars. It would also be difficult to prevent off-label use. StuRat (talk) 22:56, 12 November 2012 (UTC)

What Stu said. We have an article on off-label use, by the way. Someguy1221 (talk) 22:59, 12 November 2012 (UTC)

Doctors are allowed to prescribe any drug that is approved by the FDA, but they are generally not restricted in what they can prescribe it for. It sometimes happens that a doctor makes a judgement that the possible benefits of using a drug in a certain way outweigh the possible risks -- for example with a patient who will die unless an effective treatment is found. Looie496 (talk) 04:06, 13 November 2012 (UTC)

Basically, drug companies are much more legally restricted than doctors are. A drug company rep or salesperson can't even casually mention to a potential doctor or patient over drinks that they've got preliminary evidence that their new blood pressure drug also cures impotence, without the company being penalized by the FDA; but a doctor is free to prescribe anything for any random reason, with only the risk of malpractice suit to set limits. Because, who are you going to trust? A billion dollar company with a large scientific and statistical staff conducting tests and clinical trails all the time, or good old Dr. Welby who helped Maw Maw when her schizophreens were acting up back in aught five? Gzuckier (talk) 05:15, 13 November 2012 (UTC)

What drug-makers can say (in these United States) about their drugs doesn't depend on solid scientific evidence: it depends on what the Food & Drug Administration explicitly permits. If the FDA is impartial, apolitical, and competent, these two criteria are effectively equivalent; but one may reasonably have some doubts about whether that is the case. —Tamfang (talk) 06:28, 13 November 2012 (UTC)

As the off-label use article says pharmaceutical companies actively persuade doctors to prescribe off label. These companies spend more money marketing than they do developing and testing new drugs. This is well explained in the recently published Bad Pharma by Ben Goldacre an essential and frightening read for anyone interested in how the worldwide pharmaceutical companies operate to patients' detriment. Richard Avery (talk) 08:04, 13 November 2012 (UTC)

Bad Pharma by Ben Goldacre is definitely not an essential read. Ben Goldacre, like many physicists physicians doesn't deal well with statistics and get things wrong at several places. Painting a frightening picture might be quite useful for selling books, a field where Goldacre is pretty good, but it's simply skewing the reality to fit your view. It's not serious. Bottom line: for the several thousand medicines in the market, most are reliable and work as they are intended to work. OsmanRF34 (talk) 14:23, 13 November 2012 (UTC)

NB. Ben Goldacre isn't a physicist, he's a physician (a research fellow in epidemiology according to our article). AlmostReadytoFly (talk) 17:22, 13 November 2012 (UTC)

Sorry, my bad. I certainly didn't mean that physicists doesn't know enough maths. OsmanRF34 (talk) 19:05, 13 November 2012 (UTC)

The off-label use article says "it is illegal for the company to promote off-label uses to prescribers." Companies try to, if not blatantly break the law without being caught, at least skirt as close to the edge of the law as permitted, as in any industry; but the off-label thing is hard to bend directly, even by dropping hints. "Friendly" physicians (i.e. receive large retainers to have their names attaced to things) who have "interesting findings regarding new off label uses" can, however, be counted on to spread the news without urging. The FDA can be somewhat quirky in how they approve or reject drugs, but they're pretty reliable about not taking it well when they catch somebody violating their regs. Since a fine is often just a drop in the bucket compared to potential revenue, they often penalize the company by delaying consideration of a New Drug Application for 6 months or a year, which shortens the profitability life of a drug before the patent expires, which is the real crimp in a drug company's income. "Marketing research" is indeed at this point probably more important to a drug company than biochemical research; but probably tied with it is "patent law research". This is at least partially due to there just not being that much return on actual biochemical pharmaceutical research these days. We're going to have to see a major breakthrough in biological or physiological science before we are going to see the kind of boom in new therapeutics we once thought was going to continue forever. Gzuckier (talk) 19:47, 15 November 2012 (UTC)

November 13

my camera for my experiment recorded video with a 0.80x resampling factor. When calculating centripetal acceleration and curvature, how do I scale?

I run the video through a program and it plots the trajectories of tracked objects over time (based on timestamps). The only issue, my video is 25% faster than reality. Video is taken for 5 min and 35 seconds, but only 4 min 30 seconds of video is recorded. The video is being time-scaled to match, i.e. it is not truncation. I have since corrected this problem, but I have two months of data that were taken in this "time-compressed" fashion.

Am I right to assume that curvature will remain unchanged, that is, the scale corrections applied to velocity and x' * y - y' * x will cancel out?

However for centripetal acceleration, since it is v^2 / r or v^2 * curvature, and curvature will not change with time scale, my values that I am calculating for centripetal acceleration will be (1.25)^2 faster, that is I have to apply a correction factor of 1/1.56 to my values. 137.54.63.225 (talk) 00:12, 13 November 2012 (UTC)

(EC) Why would you assume that that the time scaling would cancel out? Have you tried the calculus yourself? I think to say anything definitively, you'd have to know exactly how the camera samples the frames in time. Can you document that it's uniform? If not, then we cannot say for sure without knowing how frames are sampled over time. Also, is this the fruit fly project that IP questioners have discussed here before? SemanticMantis (talk) 02:13, 13 November 2012 (UTC)

I know it's uniform because VirtualDub explicitly says the resampling rate is 0.80x. When I disabled a time compensation feature (it was because the camera takes 15 FPS in the dark, rather than 30 FPS), it goes back to 1x. Yeah it's the project, but I just want to confirm my intuition. Using dimensional analysis, s^-1 implies one-fold framerate dependence, and s^-2 implies two. Curvature is specified as a inverse length scale, and has no time component, so it won't be affected by a sped-up framerate. However centripetal acceleration has a two-fold framerate dependence, as seen by c = v^2 * k and also looking at the dimensions of centripetal acceleration. Can somebody confirm my intuitions? 71.207.151.227 (talk) 08:35, 13 November 2012 (UTC)

Chemistry problem

I was wondering how to do part B of the following problem.

1) A saturated solution of sulfur dioxide was prepared at 27 °C and 760 torr (millimeters of mercury). A 5.00-milliliter sample of this solution was diluted and an excess of potassium iodate solution was then added. The iodine liberated was equivalent to 32.80 milliliters of 0.100-molar sodium thiosulfate solution. The reactions that occur are given in the equations below:
5 SO2 + 2IO3¯ + 4 H2O --> 5 HSO4¯ + 3H+ + I2
2S2O32¯ + I2 --> 2I¯ + S4O62¯
(a) Calculate the moles of I2 liberated.
(b) Calculate the volume of sulfur dioxide needed to prepare 1.00 liter of the saturated sulfur dioxide solution.

I got .00164 moles of I2 for part a (I am almost positive this is correct, if someone would like to double check feel free to do so and let me know).

NOTE:

• The answer for part B is 40.3 L, but I am not sure how to do this.
• Part B is not a limiting reaction, it is a gas law problem, and may or may not involve stoichiometry.
• You don't need to write a detailed explanation for your steps, if you just write the equations as you progressed and the answers for them, I should be able to figure it out on my own. If I need more help I'll contact you. Or you can just write the steps you'd take.

Alternatively, if someone can't solve it but can just explain what Part B is asking, that'd be appreciated. SO2 and sulfur dioxide are the same, so isn't 1.00 Liter of satured sulfur dioxide the same as the volueme of SO2? I don't understand what they're looking for.

Please, if anyone can solve Part B and explain their steps here, it would be much appreciated! Or offer even some steps or help. I've tried this multiple times, but have never been able to get 40.3 (and I know this is the answer). --Jethro B 02:45, 13 November 2012 (UTC)

Part b is ambiguous. Does it assume that all the gas you have is dissolved?--Jasper Deng (talk) 03:46, 13 November 2012 (UTC)

Part b isn't ambiguous. I'm a little sleepy right now, and just got done a bunch of chemistry tutoring, so I'm a little fatigued, but you can calculate the moles of SO2 in solution using simple stoichiometry, and you can use Henry's law to calculate the partial pressure of SO2 needed to form a solution of SO2 at that concentration. I don't feel much like doing the math, but Henry's law is the key to solving this. You would need to know the Henry's law constant for SO2 dissolving in water, but there are tables of values for that, availible on the internet via google; type "Henry's law constant" into google to find a bunch. --Jayron32 03:58, 13 November 2012 (UTC)

Unfortunately, I want to do this without Henry's Law, as it was done in 1969 (when the problem was published) without Henry's Law. Henry's Law could work, but it's possible without. Some equations that may be of use here that I can think of are PV=nRT (or v=nRT/PV), and MM=gRT/PV where MM=Molar Mass. --Jethro B 04:22, 13 November 2012 (UTC)

I'm really not sure how to do it without Henry's law, it is the only way I know to calculate the relationship between the concentration of a gas in solution based on the pressure of the gas over the solution. Also, I'm not sure what 1969 has to do with it, Henry's law had been established for 166 years at that point. It was hardly unknown. Most general first-year chemistry classes teach it today, and though I was not yet alive in 1969, I can't imagine it wouldn't have been covered if the above problem had been given in class. --Jayron32 04:29, 13 November 2012 (UTC)

Addendum: I thought perhaps that Raoult's law would be applicable here, but that only works at ranges where the solute is below its boiling point. In this case, it isn't, as SO2 is a gas at room temperature. --Jayron32 04:34, 13 November 2012 (UTC)

Addendum #2: The key here is saturated is the word in the problem that leads me to Henry's law. But OK. Let me play around with the numbers to see what I can get. --Jayron32 04:37, 13 November 2012 (UTC)

• .00164 x 5 = 0.0082 moles SO2. At the above conditions, PV=nRT gives (1 atm)(V) = (0.0082)(0.08205)(300) so V = 0.201843 liters for a 5 ml sample. To prepare a 1-liter sample of the same concentration, you'd need .201843 * 1/.005 = .201843 * 200 = 40.36 L. QED. So you were correct. Henry's law wasn't needed. Does this all make sense? --Jayron32 04:55, 13 November 2012 (UTC)
  • Thank you, it looks good. I'm just going to write out what you wrote here on paper and make sure I see each step, and then I'll let you know. But thank you so much for the help. --Jethro B 04:59, 13 November 2012 (UTC)
  • Yes, I got it. Thank you so much for your help! --Jethro B 05:02, 13 November 2012 (UTC)

Momentum

Why is p the symbol for momentum? --168.7.228.22 (talk) 06:18, 13 November 2012 (UTC)

Nobody knows. Seriously. —Tamfang (talk) 06:29, 13 November 2012 (UTC)

What he said. The other letters were already taken up, and p was still left over. It was basically like being the last guy picked for the kickball team. If it were "m" or "M" it would have been confused with mass, which would have been bad in equations. Since we couldn't have 2 Ms, it got hung on p. Otherwise, there is no other reason. --Jayron32 06:48, 13 November 2012 (UTC)

If you have as much trouble remembering silly things like that as me, think of it as "pomentum". StuRat (talk) 07:14, 13 November 2012 (UTC)

If you have less time, think at it as p for 'pull'. OsmanRF34 (talk) 17:38, 13 November 2012 (UTC)

We are a reference desk, not a plead our own ignorance desk. So here are several references [7][8][9] each claiming that "p" was inspired by the Latin "petere", which is the root of the English word "impulse" (Latin: "impetere"). It is claimed that the concept we refer to as "momentum" was called "impulse" at the time of Newton, and that this led to the choice of "p" to represent this concept (noting that "i" would have been a poor choice since it was already in use for other purposes). Now, I don't know for sure that the "petere" story is accurate, but it seems plausible enough and clearly has some support on the interwebs. Dragons flight (talk) 12:01, 13 November 2012 (UTC)

"Do You Feel Like An Insider Or Outsider?"

When asked the above question, is this a psychologically sound question? Is it a diagnostic question, or criterion for a DSM illness?Curb Chain (talk) 06:27, 13 November 2012 (UTC)

Can you define "Insider" and "Outsider"? You may be interested in our articles on solitude and emotional isolation. Someguy1221 (talk) 07:24, 13 November 2012 (UTC)

Innie and outie? 124.150.45.187 (talk) 09:00, 13 November 2012 (UTC)

The Outsider (Colin Wilson). μηδείς (talk) 17:37, 13 November 2012 (UTC)

electrons and time-varying electric fields

There is a free electron placed in an electric field that varies as E = r-> * cos(w*t) where r-> is some constant vector. How do I come up with a complex function for its velocity? If the time-varying electric field is external, won't that set up a time-varying magnetic field that will radiate EM waves away from the source? But now the electron will be moving too, and will be radiating away its own energy. 71.207.151.227 (talk) 09:40, 13 November 2012 (UTC)

You seem to be overthinking this. Work out the force f=Ee (vectors here). Then work out acceleration of the electron based on force and its mass. Velocity is time integral of acceleration. Assuming this is homework, you can calculate the resultant formula. You may have to consider initial velocity as well. Graeme Bartlett (talk) 11:31, 13 November 2012 (UTC)

But if the electron accelerates, won't it produce its own changing electric field, that it will respond to? 71.207.151.227 (talk) 12:07, 13 November 2012 (UTC)

In an introductory physics class, which I assume this is, second-order concerns such as how the electron's field will influence the experiment are almost always assumed to be negligible. Dragons flight (talk) 12:42, 13 November 2012 (UTC)

I'm studying graduate-level optics, where EM waves are very important. I last took E&M two years ago. I need to know these secondary effects. Eventually, I have to estimate very small effects like light pressure. However, I missed two lectures and now the homework is ver confusing for me. 137.54.43.148 (talk) 19:27, 13 November 2012 (UTC)

For a lot of electrons as in a plasma you can treat it as a differing dielectric constant k=1-(Ne²)/(ω²mε₀) m-mass of electron ε₀ is permittivity of free space, ω angular frequency N number of electrons per unit volume, e is electron charge. The dielectric constant is real unless you start to worry about collisions between the particles. This is from my textbook on radiowave propagation, where electromagnetic waves meet ionised gas. It says the positive ions can be ignored because they are so heavy. The book does not mention magnetic effects. These will become comparable if the electron is moving at close to the speed of light, which it might do if the wave has extreme intensity. But will not be big normally. To workout magnetic effect you can consider the magnetic field from that electromagnetic field and velocity of electron. However since the electric field is location independent, it is not travelling anywhere, so is not actually an electromagnetic wave. Graeme Bartlett (talk) 11:23, 14 November 2012 (UTC)

My professor asked me to "employ complex form" to compute the velocity of the electron. But the velocity of the electron doesn't have two components. How is this possible? 76.123.35.31 (talk) 23:04, 14 November 2012 (UTC)

You can do a Fourier transform of the electric field and the electron location/velocity. The complex result will reflect the amplitude and phase of the electron response. And you can express the electric field with e^jωt and if you want the real part is the real part of that magnitude or cos (ωt). The Fourier transform of this would be δ(ω). And one more clue I will give you is that the velocity of electron is 90° or π/2 out of phase with the field. So that would give you a factor of i, making the velocity purely imaginary. This situation happens in the night time ionosphere with radiowaves being reflected back to earth. When you transform the derivatives or integrals with time factors of ω will pop out, but since I can't think of what they are without looking them up, I won't write them here. Graeme Bartlett (talk) 11:06, 15 November 2012 (UTC)

0.45 vs 0.2 micron filters (for life sciences)

I'm looking for information regarding the use of 0.45 vs 0.2 micron filters? I guess the 0.45s are cheaper? Can bacteriological sterility be assured with a 0.45 micron filter? — Preceding unsigned comment added by 129.215.47.59 (talk) 10:04, 13 November 2012 (UTC)

0.2 micron filters are usually considered adequate to ensure bacterial sterility, while 0.45 micron filters would generally not be adequate. There are occasional reports of ultramicrobacteria that may have slipped through 0.2 micron filters as well, but this would be pretty unusual. A 0.45 micron filter will still catch many bacteria, and are sometimes used for that purpose, but they are unlikely to stop all of the bacteria present under typical conditions, and hence the filtered fluid is unlikely to be sterile. Dragons flight (talk) 11:01, 13 November 2012 (UTC)

Thanks. Why then do we have 0.45 um filters? 129.215.47.59 (talk) 12:49, 13 November 2012 (UTC)

Bigger holes generally filter faster (everything else being equal), which is an advantage. For some experiments the filter residue (i.e. the material remaining on the paper) is of greater interest than the fluid that passes through, and in those cases the difference between catching all cells and catching most cells may not matter much. For example, if you care about larger cells, such as eukaryotes, then you wouldn't necessarily care what happens to the bacteria. Dragons flight (talk) 13:02, 13 November 2012 (UTC)

It's also worth pointing out that light scattering samples need to be filtered to remove dust (> 1um) and other static scatterers (really big things) from solution. If what you're looking at is bigger than 200nm than you wouldn't really want to use a 0.2 um filter because you'll be filtering out the thing you're trying to analyze. I guess the take-home message is that such filters aren't used solely for sterilization purposes. (+)H₃N-Protein\Chemist-CO₂(-) 15:06, 13 November 2012 (UTC)

One manufacturer says that their 0.45 um filter reduces Serratia marcescens titer by a million-fold. [10] One good reason to use them is for "pre-filtration applications", to reduce "bioburden" as they say (or, I suppose, to get rid of precipitates/insoluble matter etc.) In other words, as a first rough filter to keep all these unwanted goodies from clogging up the one you use for your actual sterilization. Wnt (talk) 05:05, 14 November 2012 (UTC)

Dyson sphere - "Gravity"

Hi,

I have a question regarding the "gravity" caused by a dyson sphere (or ring if you prefer) at the equator due to the centrpetal force of the spinning sphere/ring. I realise that such a structure in unfeasable, but view this just as a thought experiment. Lets assume that the spere is 1 AU in diameter and is spinning at such a rate that the centripetal force is equal to that of gravity, and there are Earth like features (cliffs/mountains etc) on the inside of the sphere. My question is such: What happens if you were to run off the edge of one of these cliffs, would you "fall"? Is this indestinguisable (physically) from the notion of "falling" on Earth (assuming effects of curvature inward/outward are negilgable on both at such a small scale)? Eg. distance travelled, speed attained etc. Also, what happens if you adjust the spin speed of the sphere/ the radius of the sphere with this problem (aside from the obvious heat/cold problems). I am just having trouble imagining this, thanks for any help! Matt 80.254.147.164 (talk) 13:28, 13 November 2012 (UTC)

Gravitational attraction of any object appears to act from a point source located at the centre of gravity. In the case of a thin narrow dyson ring this means that the centre of gravity is at the centre of the sphere. So if it is rotating just fast enough for centrepetal force (acting outwards) to be equal to the gravitational attraction (acting inwards) at the ring, folk located at the ring will appear to be weightless. If they climb an inward facing mountain, the gravitation distance is less but the rotational speed is also less. So gravity is stronger so they will fall towards the centre. Clearly, if the mountains face outwards, centrepetal force will be the stronger of the two. Gravity is weakening in proportion to the inverse square of the distance to centre and centrepetal force is increasing in proportion to distance to centre, that is, faster than gravitational attraction. So folk on the outside of the ring fall away.

For a dyson sphere, the rotation velocity falls off toward zero asr the rotational poles are approached. So folk on the sphere at the poles can only fall inwards, regardles. Everywhere else, they must be between being balanced as on the equator, or at the poles where they always fall inwards. So, everwhere else, they must fall inwards readless of whether the moutains poke inwards or outwards, unless the mountains approach a height just touching an imaginary cylinder tangent to the equator and of radius equal to the sphere radius. On any moutain exceeding that cylinder, they must fall outwards. Floda 120.145.145.21 (talk) 14:45, 13 November 2012 (UTC)

For the full sphere, the shell theorem applies, so the gravity of the sphere cancels out. Only the gravity of the sun (and potential planets) is relevant. That does not change the core statement - rotating a Dyson sphere would be a bad way of generating (pseudo-)gravity, since would change from the equator to the axis of rotation. On a Ringworld, the effect of the centrifugal force is very nearly the same as that of gravity on Earth. The major difference is that the Coriolis force affects things slightly differently. But this effect is rarely noticed directly in daily life. It might cause weather to do interesting things in a ringworld. --Stephan Schulz (talk) 15:33, 13 November 2012 (UTC)

OK, that makes sense, but what if we assume that the mass of the ring is zero and the ring is spinning fast enough so standing on the inside of the ring the centripetal force is "pulling" you towards the ring (ie the star would appear to be directly above you as if you were on earth) with the same force as gravity on earth. Also the height of the "mountains"/"valleys" is negligable compared to the distance to the central star. I am mostly interested with what happens when you jump off a "cliff" on the ring, really. I would think that since you are on in contact with the ring, the only forces acting is the force of the star, so you would orbit it or fall towards it. 80.254.147.164 (talk) 15:27, 13 November 2012 (UTC)

No, you would still "fall" towards the ring, because even though you're no longer touching it, you are still moving at the same speed as it, due to inertia, so centrifugal force will still act on you. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 15:29, 13 November 2012 (UTC)

Okay, call me Mr Pedantic, but someone has to say it. There is no centrifugal force. When you jump off the cliff you move in a straight line at constant velocity (ignoring air resistance and the effect of the star's gravity, which will be much smaller than 1g) until that straight line intersects the ring. You appear to be falling towards the ring but in reality the ring is rising to meet you. Gandalf61 (talk) 17:26, 13 November 2012 (UTC)

No, Mister Bond, I expect you to die! (as linked above ;-) --Stephan Schulz (talk) 17:42, 13 November 2012 (UTC)

Given the equivalence principle, the centrifugal acceleration is just as real as the gravitational acceleration that it's intended to replace. -- BenRG (talk) 20:42, 13 November 2012 (UTC)

That makes much more sense, the idea of the ring exerting force on a body it wasn't in contact with was confusing me. If someone wants to be extra nice , they could calculate the RPM/ velocity of the disc to require an earth like "gravity" - although when my brain is working i'll get round to it 46.64.43.100 (talk) 19:29, 13 November 2012 (UTC)

Or you could just read Ringworld, which is worth doing anyway. IIRC Niven got 770 miles/second. Of course his world wasn't exactly 1 AU in radius and the surface gravity wasn't exactly 1 g, but there's an order of magnitude for you. --Trovatore (talk) 19:57, 13 November 2012 (UTC)

To specifically answer what would happen when you fall off a cliff, you would fall as normal. Everything would seem equivalent. Until you fell far enough to notice the curvature of the ring, nothing would seem strange. Someguy1221 (talk) 22:59, 13 November 2012 (UTC)

This question is suspiciously coincidental with the release of Halo 4, which features both a set of concentric dyson spheres, and the usual ringworld the series is known for. That game has some crazy stuff in it, like solids that aren't; fake stars, objects that float in place. Apparently, it can all be explained by ideas like spacetime manipulation, quantum phenomena, exotic particles, etc. Plasmic Physics (talk) 09:08, 14 November 2012 (UTC)

No doubt, a lot of it is pseudoscience and quantum fruitloopery, but it is interesting to think about. Plasmic Physics (talk) 09:10, 14 November 2012 (UTC)

Certain electric weapons, use micro wormholes to teleport electrical current instead of using wiring, resulting in strange concepts such as the weapon fireing a few atoseconds before you actually pull what counts as a trigger. Plasmic Physics (talk) 09:53, 14 November 2012 (UTC)

Atmosphere and oceans on Dyson ring

In the scenario above, with the ring rotating quickly enough to produce 1 g for anyone standing on the interior edge, I wonder about oceans and atmosphere on such a ring. Clearly, you would need edges on the ring to prevent them from flowing over the edge. But, would you need an inner ring or would centrifugal/centripetal force be sufficient ? StuRat (talk) 18:31, 13 November 2012 (UTC)

It depends on the time scale. The classical Ringworld had no cover, being open to the sun (with a secondary ring of shades providing day and night). If you have not read the book, do it now! Niven's ring had rims 1000 miles high - more than two times higher than the limits of the exosphere, and about four times the altitude of the ISS. Of course some atmosphere spills over, but little enough to be only noticeable over geologic time periods. And then you probably have atmosphere plants to fix it... --Stephan Schulz (talk) 18:58, 13 November 2012 (UTC)

Sufficiently high walls will do the trick (I think Niven's Ringworld called for 1000 km walls). You'll still get some atmospheric loss over the edges (disproportionately weighted towards non-critical stuff like hydrogen and helium), but if you can postulate "create a Dyson ring", "replace lost air" is comparatively trivial. — Lomn 19:00, 13 November 2012 (UTC)

Earth's atmosphere is gradually losing air too - as long as your ringworld doesn't lose air faster than Earth, I don't think anyone can really complain. --Tango (talk) 23:14, 13 November 2012 (UTC)

Well, at least Niven's Ringworld does not have tectonic processes that replenish the atmosphere with new volatiles. But on the other hand, if it's only good for 10 million years or so, that should be good for most civilisations ;-). --Stephan Schulz (talk) 23:21, 13 November 2012 (UTC)

Do they even let people graduate high school without having read Ringworld? μηδείς (talk) 03:12, 14 November 2012 (UTC)

Yes. Smaller rings are all the rage nowadays. Someguy1221 (talk) 03:24, 14 November 2012 (UTC)

OP here, I ironically graduated with a maths degree as Niven did, and haven't read ringworld Oops! 80.254.147.164 (talk) 09:27, 14 November 2012 (UTC)

Fatigue=drunkenness?

Is it true that fatigue produces most of the same effects as drunkenness? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 15:22, 13 November 2012 (UTC)

No. OsmanRF34 (talk) 16:15, 13 November 2012 (UTC)

Fatigue, however, can be one of the sequelae of drunkenness. μηδείς (talk) 17:35, 13 November 2012 (UTC)

... and some of the symptoms of ordinary fatigue could be mistaken for those of drunkenness (e.g. impaired driving ability). Dbfirs 18:00, 13 November 2012 (UTC)

No, not fatigue. But Oscar Wilde made an important discovery: that alcohol taken in sufficient quantities produces all the effects of intoxication. :) -- Jack of Oz ^[Talk] 21:51, 13 November 2012 (UTC)

Also, lack of sleep due to to staying awake for a long time (say 40 hours). Count Iblis (talk) 03:39, 14 November 2012 (UTC)

Ah, yes, that reminds me of summer camp when I was twelve. Several of us stayed up all night twice in a row playing cards and were giddy on the third day so that it would indeed have looked like drunkenness. μηδείς (talk) 17:10, 14 November 2012 (UTC)

Why are flat screens not flexible?

Are they too thick? Conductive materials can be made flexible, so why aren't they flexible? OsmanRF34 (talk) 18:49, 13 November 2012 (UTC)

"Can be" and "must be" are not synonyms. Most flat screens are constructed with one or more layers of glass which needs to be thick enough to maintain structural integrity and thus inflexible. Roger (talk) 18:55, 13 November 2012 (UTC)

Well, I see a couple of advantages for a product with a flexible screen, or a foldable screen. If some screens are an array of LEDs, why didn't we get yet the iPhone with a huge foldable screen, but in pocket size? OsmanRF34 (talk) 19:03, 13 November 2012 (UTC)

LCD screens are made on a silicon substrate, which is not flexible. In the not-too-distant future there will probably be OLED displays, which can be built on a flexible substrate -- see also our rollable display article. Looie496 (talk) 18:57, 13 November 2012 (UTC)

Nice link. OsmanRF34 (talk) 19:03, 13 November 2012 (UTC)

The problem with such displays will be durability. After you fold a map a few times, it tends to want to tear at the folds. The same would be true of a screen. So, folding is out of the question. On the other hand, rolling up a screen inside a tube could protect it, much as it does with engineering drawings. However, if you want a screen 2 meters by 3 meters in size, you still end up with a roll over 2 meters long, so it's not all that portable. Another problem is that after having been rolled up for a while, it won't want to lie flat, but a fold-out frame, possible stored inside the tube, could hold it flat. Another option is sufficient weight so that gravity will hold it flat, but, in the above example, that would mean a 3 meter long roll, if you want a "landscape" display. Also, lugging around that weight might be unpleasant. StuRat (talk) 21:06, 13 November 2012 (UTC)

The United States Army funds the Flexible Display Center, where the goal is to advance the technology for reliable, full-color video displays. Like most Army research, this technology would be incredibly useful if it worked. There are plenty of neat videos on their website, and you can even visit in person. If you're flush with cash, you can even buy in to the technology. Nimur (talk) 22:11, 13 November 2012 (UTC)

And note that the calculus is a bit different for the military, where they are willing to pay quite a bit more for portability than the average consumer. For example, a front-line infantry unit could use one to view the video feed from a surveillance drone. StuRat (talk) 22:21, 13 November 2012 (UTC)

November 14

Observation in quantum mechanics

Since wave function collapse requires someone to observe the system, does this mean that before humans (or other life), the Universe didn't exist? --168.7.231.3 (talk) 02:05, 14 November 2012 (UTC)

"Observation" is just the word we use for the event that occurs when a wavefunction collapses. It simply means that a certain kind of physical interaction has occurred. It does not require that any living thing be present. The confusion is understandable, but wavefunction collapse can happen whether or not humans are looking at the results. In fact, even physicists have a hard time defining what interactions will or will not constitute measurement. See measurement in quantum mechanics, wavefunction collapse and quantum decoherence. Dragons flight (talk) 03:26, 14 November 2012 (UTC)

Perfect number

I think I've found an error that make the information in the article contradicting. In the "even perfect numbers" section, it says: "As of June 2010, 47 Mersenne primes and therefore 47 even perfect numbers are known." Then later in the section it says: "It has not yet been proved that there are (or are not) others after the 41st." So I think it should have said "...other after the 47th."?174.20.101.190 (talk) 06:17, 14 November 2012 (UTC)

This is science reference desk, you should have asked the same on mathematics reference desk. Sunny Singh (DAV) (talk) 07:48, 14 November 2012 (UTC)

The explanation is quite simple. There are 47 known Mersenne primes, but only the first 41 are known to be consecutive - not all possibilities after the 41st Mersenne prime have been checked, so there may be as yet undiscovered Mersenne primes in the gaps between the 41st and the 47th on the known list. There is a 1-1 correspondence between Mersenne primes and even perfect numbers, so there are 47 known even perfect numbers but only the first 41 are known to be consecutive. This is what the perfect number article is trying to say - but I agree it is not very clear, and the wording could be improved. Gandalf61 (talk) 10:41, 14 November 2012 (UTC)

Thanks, Gandalf61. I've clarified it there. Duoduoduo (talk) 14:53, 14 November 2012 (UTC)

Heat energy vs thermal energy

Is correct to say that heat energy and thermal energy are different ? Sunny Singh (DAV) (talk) 07:36, 14 November 2012 (UTC)

There is a difference in a subtle way. Note that the term heat energy is bad english - heat is energy, so the term is like saying energy energy. The terms heat and thermal energy are often used interchangeably. However, more correctly in a scientific or engineering paper, heat is energy being interchanged from one media to another (e.g, the heat rejected by a gas to its surroundings when it is compressed), whereas thermal energy is the thermodynamic energy in a system. You may like to carefully read the WP articles Heat and Thermal Energy. Wickwack 58.169.249.183 (talk) 09:00, 14 November 2012 (UTC)

I was taught that heat was another term for waste energy. Plasmic Physics (talk) 11:46, 14 November 2012 (UTC)

In most engineering applications, heat is waste energy, i.e., unwanted or unusable, but in some cases it is the heat that is wanted. For example, the heat rejected in an internal combustion engine is waste, unless it is utilised in co-generation. The heat rejected by the burning fuel in a power station is not waste heat, but what's left over after boiling the feedwater and has to be lost in the cooling towers is waste. You may have mis-remembered or misunderstood your teacher. Wickwack 120.145.170.99 (talk) 15:00, 14 November 2012 (UTC)

No, I didn't mis-remember or misunderstand my lecturer, he used the second law of thermodynamics to explain it. Plasmic Physics (talk) 22:30, 14 November 2012 (UTC)

The 2nd law is essentially that in the conversion of heat into mechanical work, not all the heat can be so converted. How does that mean that that heat is waste energy? Not only is not all the heat "wasted", heat may be the desirable output as I said. Wickwack 120.145.143.165 (talk) 00:48, 15 November 2012 (UTC)

He didn't use "heat" in that sense. Plasmic Physics (talk) 05:04, 15 November 2012 (UTC)

It seems odd to read that heat is waste. Although most of the heat produced throughout the world become waste, yet it is a useful form of energy. It is true that we don't notice most of the heat produced, but it is not like that heat is waste. I don't think so because heat is used in steam engines to evaporate water, in ovens, in heating effect of many devices and many more. Talking about the second law of thermodynamics, it says all the heat cannot be converted into mechanical energy but some can. Sunny Singh (DAV) (talk) 09:16, 15 November 2012 (UTC)

It isn't that "heat is waste", which is a rather trite saying and like all trite saying has a tiny bit of truth packed inside too much approximation to have any real meaning. We should avoid triteness when describing things scientifically. Heat is waste in the sense that when two substances are in contact they exchange heat: the warm one heats up the cold one, until such time as the temperature between the two is equal. Here's the thing: The energy transferred from the hot body to the cold body cannot be utilized again to do work. That's because commensurate with the energy transfer is an increase in entropy, and you cannot recover that energy without "stealing" entropy from somewhere else in the universe, which would require you to heat up that part of the universe by more than you recover. That's the inescapable part of the second law of thermodynamics. Now, you can use heat to do work, as it is in the process of moving from the hot place to the cold place, but once the two places are in thermal equilibrium, you can't use that energy to do work again in an absolute sense: that energy is lost to the universe, i.e. there is a loss of free energy. So, in one sense, the trite statement "heat is waste" is correct, however it is not wholly correct because you can do work with heat; you just can't recover the energy after the work is done. --Jayron32 14:02, 15 November 2012 (UTC)

First, that is off the mark. Take a red hot body and a cold body, say a large fired up rod and a cup of cold water. Let's put these together. Energy flows from one to the other, and yes, that energy can still do work, since it will create steam that can be used in a heat engine. As for other misconceptions regarding the second law, I intend to knock it out of its ivory tower too. --Modocc (talk) 14:22, 15 November 2012 (UTC)

Um, why do you start a response with, "that is off the mark" and then say something that agrees with everything I say 100%. I am confused as to what parts of my explanation are incorrect. --Jayron32 14:33, 15 November 2012 (UTC)

To clarify, you said this, "The energy transferred from the hot body to the cold body cannot be utilized again to do work." Since you said "cannot be utilized again" I assumed you understood that the hot body already did work on the cold body, by heating it up, with the kinetic energy of the molecules being transferred. Its incorrect to conclude that this energy that was transferred cannot do any work again as you said though. In any case, entropy itself is elusive in the sense that it is inherently system dependent. In the 70's, entropy and Gibbs free energy were used to explain why diamonds, at the time, could be created only under extreme pressure and temperature, but in the 80's new catalysts circumvented the presumed entropy. -Modocc (talk) 15:01, 15 November 2012 (UTC)

Really? Once the rod and the water are the same temperature, how do you presume to use the thermal energy that moved from the rod into the water to do work again? Energy which is transfered from one form to another, or from one location to another, can be used to do work indefinitely except energy which is transferred as heat. You can only use that energy once; as it makes its trip from the hot stuff to the cold stuff you use that transfer to do work; but only that one time. Other energy transfers can be used indefinitely to do work along the way. That's the difference between the first law and second law of thermodynamics. --Jayron32 18:12, 15 November 2012 (UTC)

Let the temperature of the rod be 200celius. Let the rod cool to 199degrees and the water heat up to 199degrees. Assuming my cup has a lid on it, the water has turned to steam and is in equilibrium with the rod. Now, some of the heat of that rod has done work and you have said its not able to do work again, but it can. Open the valve on the lid, so the steam enters an engine. The heat energy of this steam gets transferred to the engine such that it is again does work. -Modocc (talk) 18:45, 15 November 2012 (UTC)

Seems to me this discussion has lost the plot with all this talk of triteness, reusability, and diamonds. Seems to me Jayron has merely established that heat can be waste (and in any thermodynamic process, some will be waste in a component process of a larger process), but heat isn't necessarily waste. Maybe that is a trite (= not novel) thing to say, but it is clearly different to saying heat IS waste (always), which is what PlasmicPhysics said. Plasmic said something wrong. Wickwack 120.145.0.141 (talk) 16:06, 15 November 2012 (UTC)

Incidentally, in Engineering Thermodynamics, D B Splading and E H Cole, Edward Arnold publ, a standard undergraduate text for many years, in Chaper 5 Heat, it gives a formal definition of heat: "the interaction between systems which occurs by virtue of their temperature difference when they communicate" (page 86 in 3rd Edition). It then goes on with 7 more pages about what heat is and what it is not. There is not a word about waste or any similar word like it. I have several other textbooks on thermodynamics and they all give the same definition, albiet in their own words, without any statement about "waste" until they come to actual examples, where for example, in a Carnot process some of the heat must be unusable (wasted). Wickwack 120.145.0.141 (talk) 16:06, 15 November 2012 (UTC)

All too true, and the above usage of "waste" is simply being used synonymously for the concept of entropy. So to define entropy is to define waste. In my neighborhood, we do a great deal of recycling though, so as to reduce the amount of waste that gets wasted. We've been binging on fossil fuels for some time now, but its time to sober up and do a far better job of recycling energy than we have. -Modocc (talk) 16:31, 15 November 2012 (UTC)

Vitamin D

Vitamin D is already present in cow's milk or it is added to milk by humans. Sunny Singh (DAV) (talk) 07:52, 14 November 2012 (UTC)

Most of the vitamin D in milk is artificially added. I can't find a source online that says exactly how much vitamin D is naturally present. Someguy1221 (talk) 08:20, 14 November 2012 (UTC)

Yes, I read the same thing in the last line of second paragraph of the article vitamin D. I am also confused how much vitamin D is naturally present in cow's milk. What about mother's milk (referring human), it contains vitamin D or not. Sunny Singh (DAV) (talk) 08:40, 14 November 2012 (UTC)

I found the same thing as with cows milk. It contains some vitamin D, but not enough to meet even a baby's daily requirements, but I didn't find a source that gave the exact amount. Someguy1221 (talk) 09:42, 14 November 2012 (UTC)

This site says that milk in the UK is not routinely fortified with Vitamin D. I also found this: "Whole cow’s milk was found to contain 38 i.u. vitamin D/I. Whole human milk contained 15 i.u. " from "The total content of vitamin D in cow's milk and human milk", Leerbeck (1980) - Journal of Human Nutrition. (There is a link on Google, but I've had a lot of trouble trying to get it to download and I'm not sure about putting the reference in here.) --TammyMoet (talk) 10:37, 14 November 2012 (UTC)

According to recent research, mother's milk contains enough vitamin D for the baby (more than 400 IU per liter) provided the mother's calcidiol level is above 120 nmol/l, which requires a daily intake for the mother of at least 4000 IU/day [11]. This is one of the arguments for higher calcidiol levels (120 nmol/l or higher instead of 50 nmol/l) despite there not being strong evidence for better health outcomes for adults, or that evidence being disputed. Obviously if babies would routinely need vitamin D pills because they would otherwise not even get the 400 IU/day for which we know their health is adversely affected, then that also implies that adults need more than 4000 IU/day. If we are right about this not being necessary for cancer or heart disease, then for other reasons we don't know anything about yet.Count Iblis (talk) 16:56, 14 November 2012 (UTC)

The use of ultraviolet light to enrich milk with vitamin D was discovered by Harry Steenbock. It's not so much that the vitamin D is added (there's is not some separate container of vitamin D that is mixed into the milk), rather the milk is vitamin D enriched/fortified, as there are pre-vitamin D compounds in milk that are transformed by the ultraviolet light into active vitamin D, in the same fashion sunshine (which contains ultraviolet light) can convert pre-vitamin D compounds to vitamin D in the skin. -- 205.175.124.30 (talk) 22:46, 14 November 2012 (UTC)

USDA's National Agricultural Library has all sorts of interesting information. This and this compare the nutrient content of 3.25% whole milk with and without added Vitamin D. Zoonoses (talk) 07:07, 15 November 2012 (UTC)

Forest fire vs candle flame

Candle flame goes off by the effect of wind but forest fire increases with increasing velocity of wind. Why ? Sunny Singh (DAV) (talk) 09:03, 14 November 2012 (UTC)

If you had a wind in proportion to the size of the flame, you could blow it out. For a forest fire, you'd need a hurricane, at least. StuRat (talk) 09:33, 14 November 2012 (UTC)

Also a forest fire is much, much hotter than a candle flame. Even if you could "blow out" the actual flames of a forest fire, the timber would still be burning. You can't really blow out any fire other than a tiny one such as a candle flame.--Shantavira|^{feed me} 09:45, 14 November 2012 (UTC)

I disagree. You'd just need a long and powerful enough wind to allow for the heat to dissipate. This might be several hours at several thousands of miles per hour, though. StuRat (talk) 09:49, 14 November 2012 (UTC)

I think Shantavira was talking about the real world. Caesar's Daddy (talk) 15:52, 14 November 2012 (UTC)

Is it correct to say that- wind in case of forest fire bring flame from burning tree to neighboring tree and in this way fire increases rapidly, but this doesn't happen in the case of candle flame. Will same thing happen if we stick some candles in a row and blow wind ? Sunny Singh (DAV) (talk) 11:09, 14 November 2012 (UTC)

Yes, wind helps a forest fire to spread, but as Shantavira says, pre-existing heat is a major factor. It's worth considering a blacksmith's forge, where moving air from a bellows or similar is used to increase the intensity and heat of a fire.

• Oil well fires are typically put out using the "Wind" from a large high-explosive blast: the shockwave from a high-explosive charge "blows out" the flame in exactly the same manner as you blow out the candle: Bigger flames require bigger winds to blow them out. --Jayron32 15:40, 14 November 2012 (UTC)

See also the fire whirl article. Count Iblis (talk) 17:27, 14 November 2012 (UTC)

Parrot Drone or Quadrotor

Is there any project that try to develop Parrot Drone or Quadrotor that can lift a person or two persons? What's the obstacles to built it? Is it motor problem or battery problem? Is Parrot drone can fly very stable? What if another two motor added horizontally, can it fly faster? Will these two motors cause disruption in flight stability? Thanks... roscoe_x (talk) 09:10, 14 November 2012 (UTC)

What's a parrot drone ? I don't think a quadrotor is particularly safe. If any one of the four rotors fail, I assume it then crashes. StuRat (talk) 09:36, 14 November 2012 (UTC)

Parrot AR.Drone apparently. No flying machine weighing more than a few ounces can get off the ground it runs from a battery.--Shantavira|^{feed me} 09:52, 14 November 2012 (UTC)

Not so. The Sikorsky Firefly is a manned electric helicopter. For that matter, a bit more searching reveals the E-volo VC1 (see picture in the article), a manned electric quadcopter-style rotorcraft. That said, there are major design tradeoffs that make battery-powered helicopters completely impractical given current or near-term future tech, just as the quadrotor concept isn't really solving a problem in the field. — Lomn 15:45, 14 November 2012 (UTC)

I suspect that mostly it's a function of "no need" once you scale up to manned aircraft. Helicopters and gyrocopters are well-developed technologies that don't need the additional complexity that a four-rotor design introduces. For your final question, there's the compound helicopter, which adds horizontal thrust for additional speed -- the Sikorsky X2 is a recent example that reached 250 kts in level flight. There's also the V-22 Osprey, a tiltrotor. Both types of aircraft have considerably more engineering difficulties than traditional helicopters. — Lomn 14:43, 14 November 2012 (UTC)

I found an idea for a two-person quadcopter named The Skyflyer and a suicidal Chinese man. I think less is more when it comes to personal helicopters. Trio The Punch (talk) 19:36, 14 November 2012 (UTC)

It seems odd that those two quadrotor designs put the pilot above the rotors. This design is less stable and obscures the view of the ground, which is critical for landing. StuRat (talk) 19:55, 14 November 2012 (UTC)

Its probably much safer to sit on top of them in case of a crash landing and if the rotors are above you you'll need extra big and strong (=heavy) landinggear. Trio The Punch (talk) 22:20, 14 November 2012 (UTC)

Interesting answers, thank you. Especially the news about the farmer who built his own flying machine. roscoe_x (talk) 00:58, 15 November 2012 (UTC)

Some of those farmers are pretty innovative - see Richard Pearse. Zoonoses (talk) 07:10, 15 November 2012 (UTC)

This thing is pretty cool, but unmanned. Trio The Punch (talk) 15:08, 15 November 2012 (UTC)

chemical / molecular cross-section

What is the formula for the chemical reactivity cross-section of a molecule? The article cross-section only gives info about light absorption cross-sections and neutron cross sections. 128.143.1.142 (talk) 12:12, 14 November 2012 (UTC)

Reaction rates depend on the details of the chemicals involved as well as their physical states and abundance. In general, there isn't a single cross-section that would be useful for all molecules or all possible reactions, though you can use tables of things like standard electrode potential to determine what chemical reactions are likely to proceed. If you want more detail, you probably need to ask about specific molecules and reactions. Dragons flight (talk) 18:24, 14 November 2012 (UTC)

this is what is the compuound of Splenda, sugar and cloro form this, can we understand if it is a natural usefull compound for the human body?

1,6-dicloro-1,6-dideossi-β-D-frutto-furanosil 4-cloro-4-deossi-α-D-galattopiranoside o C12H19Cl3O8. — Preceding unsigned comment added by 195.110.143.127 (talk) 16:26, 14 November 2012 (UTC)

It looks like you've given the IUPAC name for sucralose. Splenda is the brand name of a sucralose-based sweetener that also contains dextrose and maltodextrin. Do the sucralose and Splenda articles answer your question? They both have referenced sections on health and safety. 209.131.76.183 (talk) 18:49, 14 November 2012 (UTC)

Artificial 2-toned lobsters

Is it currently possible to manipulate a lobster so that it looks like this one? Are we able to control the pigmentation selection process at the first cell division of the embryo (of certain species)? Is this technically possible in all multicellular beings (if you invest lots of time and money for research)? Are 2-toned humans theoretically possible? Trio The Punch (talk) 16:27, 14 November 2012 (UTC)

It's called heterochromia which in this case is caused by mosaicism, an early mutation in one cell after fertilization; but can also be caused by Chimerism, the merger of two embryos. There are people with such a condition, but due to differences in the splitting of cells in mammals and arthropods a symmetrical half-and-half mixture would be unlikely. See these images. Yes, a mad scientist could make one, but almost certainly not as striking as the lobster. μηδείς (talk) 17:04, 14 November 2012 (UTC)

As a rule, the kind of straight-down-the-middle color boundary so beautifully illustrated here is more common in arthropods. See gynandromorph for other such examples. (A Google Image search will be rewarding also) The split doesn't have to honor this midline boundary, however, even in arthropods. In mammals, the contribution of different cells in chimeras is pretty close to completely unpredictable. Wnt (talk) 18:29, 14 November 2012 (UTC)

Thanks a lot guys, very interesting links! Trio The Punch (talk) 19:14, 14 November 2012 (UTC)

Cthulu, before he escaped the lab

Note again that the lobster in this case is a mosaic, not a chimera. Humans can be both mosaics due to a mutation in an early embryonic cell or chimeras of fused non-identical twins. Interspecies chimeras have been created, such as this horrific house mouse/deer mouse chimera. Note the monstrous lack of symmetry in the eyes due to the developmental gene expression. Do that with a Human and a Chimp and you'll get something out of H. P. Lovecraft. μηδείς (talk) 20:33, 14 November 2012 (UTC)

I hope our mad scientist creates a Cthulhu-like creature. Trio The Punch (talk) 22:12, 14 November 2012 (UTC)

Yah, I've been daydreaming the human-chimp thing since the eighties... we could use an interpreter or two for interspecies communications. :) Wnt (talk) 22:44, 14 November 2012 (UTC)

Nyeah, bad idea. Reminds me of Asimov's The Ugly Little Boy. -- OBSIDIAN†SOUL 10:13, 15 November 2012 (UTC)

I don't know why would someone ever want to have 2 skin colored. It looked pretty abnormal and doesn't look good at all to me. Plus it is not ethical to do that anyway. You can't just test that on people.174.20.41.202 (talk) 21:58, 15 November 2012 (UTC)

Agreed, see wrongful birth and wrongful life. The picture of the chimerical mouse I linked to should scare the pants off any ethical experimenter. A human-chimp hybrid would be symmetrical and even-colored--but it's still not worth the risk. μηδείς (talk) 22:23, 15 November 2012 (UTC)

Named particles?

The Oh-My-God particle was a specific particle which was named for its surprisingly high speed. This is the first example I'd ever heard of a specific particle having a name. (Not a class of particles, but this particular proton.) Are there other examples? Staecker (talk) 16:35, 14 November 2012 (UTC)

Not a particle, but of the same idea: See Wow! signal. --Jayron32 17:09, 14 November 2012 (UTC)

This is strictly a case of notability. Anyone can arbitrarily name anything they want. For example, I name all of my protons, and some of my better mesons, but I'm not famous enough for any of my pets to have their own encyclopedia articles. Nimur (talk) 17:11, 14 November 2012 (UTC)

Yes I know. I'm asking if any other individual particles have achieved this level of notability. Staecker (talk) 00:15, 15 November 2012 (UTC)

gravity = side effect of vacuum?

Could it be that the displacement of vacuum by massive objects causes the density of the vacuum closest to the object to be denser than the vacuum any distace farther away from the object, thus causing the denser vacuum to "suck" smaller objects to that area, which just happens to be the surface of the object?165.212.189.187 (talk) 20:45, 14 November 2012 (UTC)

Gravity is the effect of mass causing curved space-time. (relax.... its non intuitive and complicated) Spacetime — Preceding unsigned comment added by Ap-uk (talk • contribs) 21:25, 14 November 2012 (UTC)

Thanks, (and no offense) but that sound like regurgitated double-talk165.212.189.187 (talk) 21:31, 14 November 2012 (UTC)

It's much easier to regurgitate sound-bytes describing our most non-intuitive physical explanations than to understand them! In any case, whether we understand it or not, gravity is a phenomenon that we observe in the universe. It is one of the most fundamental interactions we know, and it appears to contribute to the dynamics of almost everything we know about (with only a very few exceptions). We can describe the effects of gravity in many different ways. For many purposes, it is useful to describe gravity as a force, whose strength depends on a quantity we call mass. This is the "classical" gravity that physicists attribute to Isaac Newton. Einstein and more recent physicists used a different mathematical formulation to avoid the trouble with inexplicable action-at-a-distance, and to resolve some of the more subtle effects of gravity, like the way it interacts with electromagnetic waves. But, whichever formulation of gravity you use to model your observations, very few physicists ever talk about the "density" of a vacuum. And I have never heard any reputable physicist suggest that vacuums "cause" gravity as a side-effect; I can't really even figure out what that would mean. "Vacuum" is just a term we use to describe a region of empty space. It sounds as if you're trying to stretch an analogy of gravity to some type of buoyant force, and you're hypothesizing some type of gravitational phlogiston fluid - but if you carefully study all the implications of that hypothesis, I think you'll find that theory does not correspond to what we observe in nature. You are not alone; many great scientists throughout history have tried to use a hydraulic analogy to describe as-yet unknown effects of heat, electricity, and chemistry; but in each case, these hypotheses have been superseded by more parsimonious explanations. Nimur (talk) 22:26, 14 November 2012 (UTC)

Dammit, Jim, I'm a lawyer not a physicist, but 165.212.189.187 it seems to me that you've got the cause and effect wrong. Vacuum density is a measure of how much matter — gas and other particles — are within a space. Those things are denser near an object because of gravity, or to say it backwards, if it were not for gravity the density would not be higher near the object. Someone who's a real physicist will probably tell me I'm wrong, but that's my first impression. Regards, TransporterMan (TALK) 21:30, 14 November 2012 (UTC)

No offence taken , it's to do with perspectives. Mass, space and time are interlinked but our general default brain setting is one of 17th century mechanics. The proof of Einsteins curvature of space time explained 1) Mercury's weird orbit 2) Displacement of starlight around the sun. Both are shown here : Tests_of_general_relativity ... To show that time is not absolute a 1971 experiment is good fun :check out the "Hafele–Keating experiment" Ap-uk (talk) 23:00, 14 November 2012 (UTC)

• One theory of gravity is that the vacuum exerts a positive pressure, and that the proximity of two bodies blocking that pressure from each other causes them to move together. I am not sure what that theory is called, so I can't get you references, but perhaps someone else here does. μηδείς (talk) 23:59, 14 November 2012 (UTC)
  • You may be referring to Le Sage's theory of gravitation. Richard Feynman says it doesn't work, and that's good enough for me. Someguy1221 (talk) 04:37, 15 November 2012 (UTC)
• Three things. First, gravity exists and can be measured in the absence of vacuum or pressure forces, or even in the presence of opposing forces - a heavy weight dropped in the ocean will sink, even though the water pressure surrounding it in every direction is roughly equal, and that the water pressure increases with depth rather than decreasing. If low pressure (or vacuum) caused gravity's attraction, the weight would more likely float up into the sky. Second, attraction due to vacuum cannot explain how a small, very dense object like a black hole or neutron star can exert a much stronger gravitational effect on larger but less dense objects. This observation also affects the positive pressure theory Medeis mentioned. Third, because pressure in a given area tends to like staying equal, vacuum 'sucks' on an object in space roughly equally in all directions. With equal force pulling the object in every direction, the net force effect is zero, not 'sucked toward the nearest other object' as the question suggested. So in short, no, vacuum cannot explain the observable properties of gravity. – NULL ‹talk›
  ‹edits› 04:32, 15 November 2012 (UTC)

Arent atoms' volume 99% empty space?165.212.189.187 (talk) 16:35, 15 November 2012 (UTC)

@ Someguy and Null. Le Sage's theory of little particles is not the same one I was thinking of, but rather a Casimir effect caused by vacuum energy. The geometry is the same, but in the second theory it is a force, not particles. This is mention in this section of the Le Sage article, and no criticism or refutation is given. Second, the vacuum energy is someowhat of a misnomer. It's supposed to exist every, including in relative vacuums, not just in or because of vacuums. Perhaps the OP means vacuum in a different way from vacuum energy, he'll have to speak for himself. μηδείς (talk) 17:54, 15 November 2012 (UTC)

A question on economics.

Here in the UK a lot of economics statistics and also general work statistics (ONS) are used by the media who report it as fact.

Many years ago I got a degree in science and struggle to see how accurate these statistics actually are especially with economics. The general trend is to make it all look too difficult to the average person so they don't question things and also there is a lack of a glossary and explanation as to what has been left out or included.

1) Is economics a pseudoscience?

2) Is there some kind of bias in the statistics to make things look slightly less intimidating to the public? I notice for example that with regard to wealth stats the 69% of people who die in debt are left off the table, the wealth stats are then tabulated to take the super rich out (in fact you are in the top percentile with a net worth of only £423,000).

If economics is a pseudoscience why are no top scientists complaining or is it safer (as regards future financial science funding) to just pick easy targets like religious people ;)

--Ap-uk (talk) 21:19, 14 November 2012 (UTC)

1) Economics is a social science. 2) It would really depend on what you are looking at, who compiled the wealth stats you are looking at? They might have reasons to present data in a particular way other then "making it less intimidating".. And of course, nothing stops an economists biasing data if they have an agenda to push. Vespine (talk) 21:38, 14 November 2012 (UTC)

Your answer 1) is open for discussion. I wouldn't call it a science at all, social or not. Unless you are testing and improving your theories, do not call it a science. Science operate on the principle of falsifiability. The most amazing thing about economists is that they don't even care if their theory is wrong or not. It's an ideology pretending to be a science.~On the other hand, they indeed do use, although poorly, scientific tools like statistics, but that's not enough. OsmanRF34 (talk) 23:42, 14 November 2012 (UTC)

I got the wealth stats from the ONS (Office of National Statistics) however it was not straight forward to find out the answers that I wanted and I had to glean it from several documents even then it was not 100% clear. The ONS did become separate from the government recently however I still see traces of "non-controversial" bias. — Preceding unsigned comment added by Ap-uk (talk • contribs) 21:55, 14 November 2012 (UTC)

This reminds me of the control systems lecturer we had when I was an electrical engineering student. He loved to poke fun at fields that were not like engineering, which is 100% based on fundamentals. He used to say that when an Engineering lecturer writes this year's version of the exam, he changes the questions. When an economics lecturer rewrites the exam for the year, he keeps the questions and changes the answers!

More seriously, economics when compared to things like engineering is a bit like psychology. Both economics and psychology DO have fundamentals, but the subjects are not fully understood and day-to-day issues have to be solved by postulation. Examples of fundamentals are: In psychology - all learning is either clasical conditioning (http://en.wikipedia.org/wiki/Classical_conditioning) or operant conditioning (http://en.wikipedia.org/wiki/Operant_conditioning); in economics, in a closed economy, the total value of money is equal to the total value of goods, services, and property. But can a psychologist predict what sort of responses will be triggered by my post, or can an economist predict how the price of widgets will react to Hurricane Sandy? Well, yes they can, but not by a process solely involving calculation and logical deduction starting with fundamentals. Engineering is quite different. In engineering, all solutions are derived from calculation and logical deduction starting with fundamentals like ohm's law. Then you get climate change science, which is not even as solid as psychology or economics. Wickwack 120.145.143.165 (talk) 01:13, 15 November 2012 (UTC)

• The answer is sort of complicated. When economics is quantitative, it is a science. But because economics is so closely tied to politics, there are aspects of it that are not scientific, as for example the work of Karl Marx and the Austrian School. Looie496 (talk) 03:31, 15 November 2012 (UTC)

The category of pseudoscience may not be as useful as you have been led to think. Classifying something as pseudoscience begs the question whether there is a generally applicable criterion by which to do so, the famous demarcation problem. Certainly no-one would argue that the economy is something beyond scientific study, and certainly most economists are quite earnestly trying to understand, quantify, and describe it. On the other hand, there are many scientists, including economists, who have made scathing critiques of the state of mainstream economics, for example that in economics, the theoretical side and the empirical side exist in fatal separation from one another (in a very interesting paper by an economist, but naturally I forget her name – I think she was from the University of Chicago). This, if true, would make economics bad science but not pseudoscience.

As to official government figures relating to the economy, you do well to take them with a good pinch of salt, since massaging the figures is usually the easiest way to make a problem go away. Inflation can be conveniently suppressed by adjusting the consumer price index, unemployment be kept down by defining people out of the ranks of the unemployed, etc. (I have no idea if and how much this is done in the UK, but would be surprised if it isn’t.) The “official” net worth of an individual person may not reflect their actual wealth with any degree of accuracy, since much of what the very rich own will often be nominally owned by some corporation or otherwise be distanced from them. But if you know how to read the figures correctly, they do provide useful information about real life. If the media report them uncritically, it’s the media’s fault, not the statisticians’.--Rallette (talk) 07:23, 15 November 2012 (UTC)

There are websites who specialise in checking out statistics, especially those quoted by politicians to back up their claims. check out Full Fact and Fact Check. (UK urls given as OP is UK) --TammyMoet (talk) 09:54, 15 November 2012 (UTC)

November 15

the material rising in the thermoscope \ thermometer

I would like to know if when the material rises in the the thermoscope meter (or in the thermometer), it indicates that the air in the thermoscope is shrunk or spreading. According to what I understand, the material (in example water ) spreading becaouse the warm and that is the couse of the material rising of the material inthe meter. What doyou think about? 46.210.166.245 (talk) 01:40, 15 November 2012 (UTC)

If the thermoscope is sealed, the liquid expanding will simply compress the air. If the temperature increases the air would also be warmed up at the same time as the liquid and if it was free to do so it too would expand, but since the liquid is far more dense then the air, the resistance the air applies to the fluid would be negiligable and the pressure inside the container would increase. I don't know for sure but I wouldn't be surprised if thermometers were created in such a way as to minimize the amount of air in the space before they are sealed. Vespine (talk) 02:36, 15 November 2012 (UTC)

I think it's more about the compressibility of a liquid (not very compressible) vs gas (easily compressible) rather than density itself as the underlying reason that the liquid expands to compress the gas even when ∆T says they should both be trying to expand. Our Alcohol thermometer article makes mention of the headspace above the liquid level. DMacks (talk) 07:10, 15 November 2012 (UTC)

Suppose, we have a thermometer of range 0 to 100 degree celsius. The thermometer is taken to a region of 100 degree celsius, I don't think that it will show 100 degree celsius if the glass cap of thermometer is attached at the same height as of 100 degree celsius mark. Air in the thermometer will compress to a certain amount, but will not get dissipate. If the cap is attached above the highest temperature scale (about 1 inch), the material inside is able to manage air compression and the thermometer will show correct temperature.

According to Vespine, pressure exerted by air on mercury will be negligible but what will happen if the mercury has expanded to the highest temperature. Will the air exert the same pressure at that temperature ? Air will also expand as the mercury (due to the heat of surrounding medium) and hence the kinetic energy of air molecules also increase and this time air will exert more pressure on mercury. Think about mercury barometer, it has vacuum in its mercury column but in case of mercury thermometer it has air. I do not know why ?Sunny Singh (DAV) (talk) 10:39, 15 November 2012 (UTC)

The air is confined to the volume left by the rising mercury. So the air pressure will increase due to 2 reasons 1) the pressure increases due to the reduced volume available, and 2) it increases due to the increased temperature. However, the increase in air pressure has negligible effect on calibration, as the mercury, being a liquid, is vitually incompressible. Floda 121.221.27.52 (talk) 11:14, 15 November 2012 (UTC)

I'm sorry, but I don't understand what does the meter shows when the material rises in the meter. the question is: Is the air in the meter is shrunk or spreading? (the thermoscope is open to get influences of atmosphere) 176.13.83.244 (talk) 18:17, 15 November 2012 (UTC)

The volume of the space at the top of the thermometer is smaller. That is the definition of "shrunk". --Jayron32 18:31, 15 November 2012 (UTC)

(edit conflict) As explained above, it doesn't make much difference whether the meter is sealed or open because it is the expansion of the material (usually coloured alcohol or mercury) that is being observed as the temperature rises. The only occasion when the compression of air is important is when a thermometer is heated well above its maximum range, and the air gets compressed almost to zero volume. The high pressure usually causes the bulb to explode. There may well be a partial vacuum above the material in a sealed thermometer, but this is not essential in a thermometer (though it is important in a barometer). Dbfirs 18:34, 15 November 2012 (UTC)

Electromagnetic radiation

At the beginning of the statement, it says: "electromagnetic radiation (EM radiation or EMR) is a form of energy emitted and absorbed by charged particles". So are charged particles mean it is either positive or negative? How about neutral particle with no charge? Can neutral particle emit and absorb electromagnetic radiation?174.20.41.202 (talk) 03:17, 15 November 2012 (UTC)

Consider blackbody radiation. It can be emitted by matter that has, in bulk, no net charge. But, if you zoom in all the way to the fundamental interactions that are responsible for the thermal emission of radiant energy, you will discover that in the very microscopic sense, the radiation is emitted by a process that could be classically described as a movement of charged particles. We might say that warm objects have electrons in excited atomic orbitals, and that the photons are produced when the electrons decay. That explanation treats the photon as an emitted particle, and doesn't concern itself with the electrodynamics of the photon-as-a-wave. The exact details are actually somewhat subtle; for most purposes, we gloss over this detail; and if we want to study it closely, we have to use some very heavy-duty mathematical physics. Often, when we consider blackbody radiation, we are not interested in electrodynamic interactions; we only care about the energy that is being conveyed. But, any emitted electromagnetic radiation - whatever its source - is attributable to microscopic movement of charge.

When we discuss subatomic particles, we have to generalize the way we think about emitted radiation. Energy can be emitted in other forms besides electromagnetic waves, especially if it's radiated in a way that isn't related to wiggling electric charges. Sometimes, certain "exotic" energetic interactions between particles result in an emitted particle - which carries energy - that is not an electromagnetic wave. A neutrino can be emitted and can convey energy, without any interaction with electric charge. A phonon is energy that is emitted that takes the form of propagating wave in bulk condensed matter. There are many such examples where the radiated energy is not electromagnetic in nature, because the source of energy did not participate in any electromagnetic interaction. Nimur (talk) 06:19, 15 November 2012 (UTC)

I think Nimur is wrong. Consider a quantity of non-reactive gas at some finite temperature. The gas molecules, which are not charged particles are, according to the kinetic theory, continually flying about in the x, y, z tanslational axes and also rotating about these axes. The gas will be emitting, in all directions, heat in the form of infrared radiation, which is electromagnetic radiation. If there is incoming infrared radiation of total wattage greater than that being emitted, the temperature will increase - that is the gas will absorb energy by increasing the translational velocity of the molecules. If the molecules are not symmetrical in all x,y,z dimensions some of the heat energy will be converted into increased rotational movement. Some will be absorbed by lengthening the atomic bonds, thus increasing the "flywheel effect" Charged particles have no part in this beyond forming the atomic bonds, which are not changed by the increse in temperature. It is possible to have a "gas" in this thermodynamic sense consiting not of complete atoms or molecules, but instead consisting entirely of one sort of particle (electron, nuetron, etc), which may or may not be a charged particle. Floda 121.221.27.52 (talk) 11:08, 15 November 2012 (UTC)

Um, "Charged particles have no part..." Is completely wrong. If, as you say, there is "lengthening of the atomic bonds" then, by definition, that is a change in shape of molecular orbitals which involves a change in energy of electrons, which requires the electrons to change energy levels in some way, which requires the emission or absorption of photons, QED. It is inescapable: if photons are produced, something about the motion/location/energy of electrons changed (which "property" of electrons changed depends on your perspective, but the initial statement that all EM radiation originates in changes to charged particles is fundamentally true.) --Jayron32 13:50, 15 November 2012 (UTC)

Jayron, you did not mention clouds of sub-atomic particles. So, if you (& Nimur) are correct in saying charged particles are necessarily involved in the emission and absorption of EM radiation, then a cloud of nuetrons either cannot exist, or cannot have a temperature? I always thought that the Kinetic Theory of Gasses applies to sub-atomic particles as well as atoms and molecules when they are not forming a solid or liquid. (A cloud of particles should obey the KTofG if the following are true: Sum of particle volume << cloud volume; number of particles is very large; except in collisions, interactions are negligible; Number of collisions between particles is large compared with collisions of particles with container). Floda 60.230.192.18 (talk) 15:34, 15 November 2012 (UTC)

I don't follow the connection that is being made here between kinetic theory and thermal radiation. A "gas" of sub-atomic particles will always have a temperature and other thermodynamic properties, but it will only emit or absorb thermal radiation if the particles carry an electric charge or have components that carry an electric charge. Atoms and molecules have no overall charge but they still emit and absorb thermal radiation because they have charged components and hence they have an electric dipole moment. A "gas" of neutral fundamental particles would have a thermodynamic temperature but would not emit or absorb thermal radiation. Gandalf61 (talk) 15:57, 15 November 2012 (UTC)

Such neutral fundamental particles will have to be completely decoupled from electromagnetism, otherwise there will be higher order effects leading to the emission of photons. This means that the particles must be hidden sector particles that are completely decoupled from the Standard Model. And even then there will be extremely small effects mediated by gravity. Count Iblis (talk) 17:37, 15 November 2012 (UTC)

To expand on Iblis's point: neutrinos don't have any electric charge, and are not mediated by photons, so interactions involving neutrinos only involve exchange of particles like the W and Z bosons. If something neutrino does causes the release of a photon, it is only of the "higher order effects" that Iblis mentions: the weak bosons that neutrinos release interacting in some way with a charged particle, and THAT charged particle releasing a photon. It should be noted (not mentioned yet as a point of confusion, but it could be given the way the discussion is going) that even neutrons are not fundamentally neutral; they are composed of charged quarks, and thus are subject to EM interactions just as other charged particles are. --Jayron32 20:18, 15 November 2012 (UTC)

God, I'm so lost... Didn't expect the answers to be this lengthy. What you guys saying are beyond what I can understand, I'm just a curious ordinary person who doesn't know physic very well. Can you someone simply answer my question in simply way? Can neutral particle emit and absorb electromagnetic radiation? If it can then how the article only say "emit and absorb by charged particles"?174.20.41.202 (talk) 21:43, 15 November 2012 (UTC)

The answer depends entirely on what you mean by "neutral particle". If you mean "any particle with no net charge" then the answer is "It can still emit and absorb EM radiation if the particle is made up of smaller particles that themselves have electric charges". IF you mean "a fundamental particle which is not composed of smaller particles, and which is fundamentally neutral at all levels of organization" then the answer is "No, it cannot emit and absorb EM radiation" Thus, both the hydrogen atom and the neutron, which are each neutral but which are also each composed of smaller bits that themselves are charged (a proton and electron in the former, up and down quarks in the latter) CAN interact via electromagnetic radiation. However, a particle which is both indivisible and neutral, like a neutrino cannot. Does that make any more sense? --Jayron32 22:15, 15 November 2012 (UTC)

[Edit conflict]Electrically neutral particles that are composed of equal amounts of positive and negative charge emit and absorb radiation, so yes, neutral particles do, but only because of the equal presence of both kinds of charge which can emit or absorb the radiation. Does that help? -Modocc (talk) 22:23, 15 November 2012 (UTC)

A related question: why don't neutral objects create EM fields all the time as they move about? (Like say, a human running and sprinting and then slowing down to catch his breath and then running again?) Individually, the objects are made of charged particles, so shouldn't they generate their own fields, that then would mostly cancel out? Photons would be emitted though. 71.207.151.227 (talk) 23:42, 15 November 2012 (UTC)

Recharging smartphone

Why does it take up to an hour to go from 99% charged to 100% on my HTC One S? 67.243.3.6 (talk) 15:48, 15 November 2012 (UTC)

See Lithium-ion battery#Battery charging procedure. The charging is not linear, but consists of three stages. Dbfirs 17:45, 15 November 2012 (UTC)

Doppler effect: two moving objects at an angle

What's the formula for the Doppler effect if two moving objects are moving at an angle subtending the velocities between the two? For example, an airplane overtakes a motor vehicle on the ground-- how would you calculate the frequency shift of the engine roar emitted or received at any specific moment? 128.143.100.179 (talk) 18:41, 15 November 2012 (UTC)

First, you would subract the velocity vectors (assuming you have an external frame of reference) to get the relative velocity between the two points. Then you use the Dot product to project the velocity vector onto the vector between the two points. This will tell you how quickly the points are moving towards or aways from each other, which can then be fed into the Doppler formula. 209.131.76.183 (talk) 18:55, 15 November 2012 (UTC)

You could also use a full-wave-equation modeling technique, such as the Finite-difference time-domain method, to estimate the wavefield at every point. Then, you could sample the simulator to approximate the signal that would be observed for any trajectory at any velocity. This method is a lot more difficult to implement in practice, but if performed correctly, can account for many non-ideal effects, like the interaction between the sounds of the two vehicles; or imperfections in the propagation material. (Our article on FDTD has a strong bias toward the application for solving electromagnetic wave equations, but you can equally-well apply the numerical technique to solve the acoustic wave equation, and many other multidimensional PDEs). Nimur (talk) 20:05, 15 November 2012 (UTC)

Okay, I would have to do this in an exam. (It's a problem that I anticipate, as my professor likes to give us "climax of all that we know" questiosns. Also, according to my lecture notes, converting the two velocities into a relative velocity doesn't really work as v becomes on the same scale as c (e.g. 0.1c) . 71.207.151.227 (talk) 23:38, 15 November 2012 (UTC)

Origin of the Nebraska sand hills

Hi. Currently, I'm researching the Sand Hills (Nebraska) for a world geologic location and hazard research assignment, and I'm exploring the geologic origins and proneness to mass wasting of these ancient sand dunes. What I've found so far is that the sand hills region is composed of Quaternary-age "Eutric" regosols, bounded by much sediment and/or uplift of Eocene to Mesozoic (think Niobrara Sea) age, located at the edge of the Denver Basin, with loess deposits extending to its east and southeast (yes, I have citations for all this info), reaching all the way to the Mississippi River and beyond, while the loess sediment also reaches down to the mouth of the Mississippi River - perhaps this makes sense, since the Platte River runs through the southern sandhills. I'm asking anyone for more relevant information about this area that they can find, including the depth of the sand (soil and rock core profiles would really help), the presence of any halite or gypsum salt deposits (I'm noticing that Nebraska is downwind of Lake Bonneville of similar age), and the effects of past explosive volcanic eruptions including at Yellowstone caldera on the region, and any evidence of past vegetation or absence thereof - for instance, the area's dunes were active during the Medieval Warm Period. Might the dunes creep east of Omaha and Lincoln, NE in the not-too-distant future? Also, what is the source of the sand dunes scattered throughout the Lake Michigan shoreline - are they direct deposits from the Laurentide ice sheet, as suggested by the USGS map on the "loess" article, or might the Nebraskan sandhills contributed to their origin sometime during the Medieval Warm Period? Any information on the influence on the stability of the dunes from the underlying artesian aquifer - the Ogallala Aquifer, which has its thickest portion underneath the sandhills and which is used for irrigation in mile-wide circular drip irrigation systems, which has dropped several metres in the past decades. Also, what about tornado alley - can you get sand tornadoes over devegetated portions of the dunes? What type of dunes are they - Barchan, Transverse, Longitudinal, Concave, Parabolic or Star? Is there any evidence of erosion by water on the dunes, and what are their permeability, porosity and density like? What are the differences and similarities between Nebraska's sand hills and other sand hills regions across North America and worldwide?

I'm also willing to contact certain universities and research/environmental organizations who might have more relevant information or current research on the topic, either by email or by phone - can you recommend any groups I might contact?

Thanks. ~AH1 ^(discuss!) 21:57, 15 November 2012 (UTC)

I found my answer to the Michigan sand dunes at Lake Chicago - by the way, remember that any extra research done here could also go into improving the article(s) discussed. ~AH1 ^(discuss!) 22:10, 15 November 2012 (UTC)

Bok choy is the same species as the common turnip, really? It resembles nothing like a turnip

I know that artificial selection can really bring out the variety in a species, but this amazes me. How is it possible? It tastes nothing like turnip leaves. 71.207.151.227 (talk) 23:32, 15 November 2012 (UTC)