Wikipedia:Reference desk/Archives/Science/2011 August 20

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 20

Flying in a whiteout

Is it possible, while flying a helicopter over mountainous terrain in gusty winds and a flat-light whiteout, to transition to a hover over a precise location at exactly 50 feet AGL (as for rescue operations) and maintain that position? If so, what special techniques/risks are involved in this maneuver? (Assume that the helicopter has a full blind-flying panel and a bitching Betty installed. Also note that the weather situation is a flat-light whiteout, where there is no contrast between the ground and the sky but visibility is unimpeded -- not a blizzard/snowfall whiteout where visibility itself becomes minimal.) 67.169.177.176 (talk) 03:45, 20 August 2011 (UTC)

Well, with that equipment, not seeing the ground wouldn't be as much of an issue as the gusty winds. Depending on how high the gusts are, they might be more than the chopper could handle, resulting in it jerking about wildly. That close to the ground, the risk is obvious. StuRat (talk) 06:43, 20 August 2011 (UTC)

Firefly range

I would like to see a range map for Firefly: in particular, are they found in Virginia? Any parts of California? 76.254.20.205 (talk) 06:34, 20 August 2011 (UTC)

These sites may be of interest to you.[1][2]. Fireflies are widely found in the eastern part of the USA because the habitat is damper and cooler for the snail prey that the fireflies need. There are local 'hotspots' where there are larger concentrations but east of Kansas is firefly territory. In California the climate is too warm and dry for widespread snails thus no fireflies. Richard Avery (talk) 06:49, 20 August 2011 (UTC)

body of Jovian outer planets

most of jovian planets are made generally from water Ice and ammonia and methane , what is the reason of this combination? , Plz give me best references .--Akbarmohammadzade (talk) 07:30, 20 August 2011 (UTC)

Actually, Jupiter itself is mostly composed of Hydrogen and Helium, and all the rest is more or less just a minor contamination. Oxygen is the third most common element in the universe (and the solar system), carbon is the 4th most common, and nitrogen is 7th in the universe, 5th in the solar system. See Abundance of the chemical elements. Water, methane, and ammonia are all very simple, very stable chemical compounds composed of those frequent elements. --Stephan Schulz (talk) 08:48, 20 August 2011 (UTC)

basic difference between ac and dc

what is the basic diff between ac and dc supply or current? — Preceding unsigned comment added by 117.195.110.232 (talk) 07:37, 20 August 2011 (UTC)

Is this a homework question? 67.169.177.176 (talk) 08:21, 20 August 2011 (UTC)

AC is a scam - the electricity company is selling you the same electrons over and over again. DC is the real deal, where they have to pump fresh electrons into the line at the power station, and have to withdraw the used ones on the other end. AC/DC, on the other hand, is where old electrons go to get recycled into noise. --Stephan Schulz (talk) 08:40, 20 August 2011 (UTC)

In other words: AC is when the current goes back and forth in rapid cycles (like sound waves or something like that), while DC is when the current flows constantly in one direction, like a river. Each of these has advantages and drawbacks: AC allows the voltage to be changed more easily to suit different applications, and is also required for induction motors; DC is a must for charging batteries, and also allows better speed control for non-induction electric motors. Note that you can change AC to DC with a rectifier, and DC to AC with an inverter. 67.169.177.176 (talk) 09:44, 20 August 2011 (UTC)

Ignore the allegation by Stephan Schulz above. Electricity companies sell energy and not the electrons whose movement transfers the energy to the consumer. Cuddlyable3 (talk) 15:39, 20 August 2011 (UTC)

...or that's what "they" want you to believe! --Stephan Schulz (talk) 16:08, 20 August 2011 (UTC)

This is fair humor, but someone who doesn't know the difference between AC and DC might not get the joke. To be clear, electrons push one another apart - they can't bunch up much at all because the negative charge would accumulate and force them apart with great force. But they flow through a conductor, often a metal, which allows them to move easily back and forth. They stay spread apart pretty uniformly, and the positive charge of the atomic nuclei making up the metal cancels out their negative charge allowing them to move back and forth.

The net effect of all this is that you can push on the whole collection of electrons in a wire, and the push rapidly carries all the way to the far end of the wire. It's just as if the electrons are water in a pipe - you could suck it back and push it forward with a bellows, or continuously force more in from a higher pressure supply. The electrons can't escape the "pipe" (provided there's not a short circuit or the circuit is grounded, and within the limits of the insulation, certain types of radio emission etc.) So you can move them back and forth (AC) or in one direction (DC) with only a few minor practical engineering differences. Wnt (talk) 23:44, 20 August 2011 (UTC)

Bird of prey, Lake Washington, Seattle

Two nights ago, we watched a bird hunting along the western shore of Lake Washington, in Seattle. It 'hovered', or flew in place for a few seconds, about 40 feet above the water, then plunged straight in. It seemed to dive head first into the water, rather than feet first. We would like to know what bird of prey this might have been. Osprey's are said to go feet first into the water, which this bird did not do. Also I thought this bird was smaller than an Osprey, and didn't have the fanned feathers at the extreme ends of his/her wings. It wasn't a bald eagle, which also grab up fish feet first. It seemed smaller, and more hawk or falcon like. I read that peregrin falcons mainly take smaller birds, not fish. I'm not sure what other birds of prey would hunt fish in Lake Washington. We thought the underside of this bird was white or light colored. We didn't see if s/he got the fish. Thanks if you can give us an idea what the bird might have been. — Preceding unsigned comment added by 83.98.238.113 (talk) 09:14, 20 August 2011 (UTC)

sooty shearwaters are fairly common along the washington shore,but are normally sea birds. They do dive head-first to fish. A long shot but perhaps you saw one that took the wrong off-ramp and followed the waterways to the lake. animals do occasionally get lost.Phalcor (talk) 20:06, 20 August 2011 (UTC)

The link-magic doesn't seem to like that plural or maybe lack of capitals, Phalcor, so here's the one you intended - Sooty Shearwater. {The poster formerly known as 87.81.230.195} 90.197.66.81 (talk) 23:16, 20 August 2011 (UTC)

Stripes on a battlehsip

• http://www.modelshipgallery.com/gallery/bb/it/veneto-700-sa/sa-index.html

Why did the Italians put red-and-white stripes on their battleships? Did the stripes make it much easier for an enemy's airplane to spot the ship? -- Toytoy (talk) 09:56, 20 August 2011 (UTC)

It's called Dazzle camouflage and does not camouflage a ship so much as makes it difficult to know what the exact class, heading, etc. of the ship is. It's hard to hide a ship in the open ocean, but it was thought that by more or less confusing the contour of the ship it might be possible to mislead a viewer (particularly submarines). Some animals do this with their body coloration to avoid predators. -- Obsidi♠n Soul 10:47, 20 August 2011 (UTC)

It's particularly important in the age of optical rangefinders. The aim is less to confuse the enemy about the presence or type of the ship, but to make it difficult to determine the correct range for aiming gunfire (or torpedos). For both types of weapon, you need both direction and distance (and, in particular for torpedos, speed) of the target to compute a correct firing solution. --Stephan Schulz (talk) 11:03, 20 August 2011 (UTC)

Now that we have radar and homing torpedoes, these "tiger stripes" no longer work. Note, however, that submarines still sometimes use optical rangefinders for greater stealth, but even so, with homing torpedoes, getting the range wrong is less of an issue. (And as for the target's speed, it can be obtained from passive sonar alone using a function that isolates and demodulates the harmonics from the screws.) 67.169.177.176 (talk) 19:22, 20 August 2011 (UTC)

In this specific case though, the red and white strpes on the deck are likely to be an aircraft recognition feature. In the early part of the war, there was very little chance of British air attack (or so they thought until Taranto) but an attack by friendly aircraft could be averted by prominent and mutually understood markings. German ships carried a swastika on a broad red stripe across the forecastle until 1942.[3] The grey stripes on the side of the model ship are indeed dazzle painting. Alansplodge (talk) 19:03, 21 August 2011 (UTC)

I should have known that Wikipedia Has An Article About Everything! See Ship camouflage#Other navies in World War 2: "Italian foredecks had a high-visibility pattern of red and white diagonal stripes so that their own aircraft would not attack them." Alansplodge (talk) 19:10, 21 August 2011 (UTC)

But these diagonal stripes are quite easy to forge. If an enemy ship found itself surrounded by the not-too-useful Italian bombers without friendly support around, they could paint their own stripes within hours. -- Toytoy (talk) 06:48, 24 August 2011 (UTC)

A cunning plan... but you'd have to be even quicker with the paint if one of your own aircraft turned-up ;-) Seriously, recognition stripes have been widely used, most recently in the Gulf War (white chevrons on vehicles) and the Invasion of Iraq (orange panels). Alansplodge (talk) 18:13, 24 August 2011 (UTC)

Magnetic Field

Do magnetic fields heat metal? If so, why? Thanks. 82.3.242.27 (talk) 12:12, 20 August 2011 (UTC)

See Induction heating. Plasmic Physics (talk) 12:29, 20 August 2011 (UTC)

More reliable helicopter?

Why do most helicopters, including military ones, have two rotors? Wouldn't they be more reliable, including against enemy fire, if they had another architecture? For example, having 2 pairs of 5 smaller rotors, or compress the air to go up. Quest09 (talk) 13:19, 20 August 2011 (UTC)

Short answer is that the wetted area would increase. Wetted aspect ratio (wing). Extra rotors would also not stop the rotor assembly going out of balance. Finally, helicopters are complicated enough already.--Aspro (talk) 15:16, 20 August 2011 (UTC)

A Quadrotor helicopter has four rotors. I expect that failure of one of its rotors is not catastrophic.

The Fairchild XH-26 used internal compressed air to start jet engines at the tips of each rotor blade and the Sud-Ouest Djinn (in production) uses compressed air alone this way. Cuddlyable3 (talk) 15:35, 20 August 2011 (UTC)

I'm not an expert by any means, but it seems to me that it's quite hard to hit the rotor of a flying helicopter with small arms fire. Put more rotors in to me would seem to INCREASE the chance you'll hit something, not decrease it. Vespine (talk) 05:02, 22 August 2011 (UTC)

Also, military blades consist of a tough skin over a filling of foam. If a round hits it is just makes a nick or a hole. In practice, even a burst, at rate 600 rounds a minute, its not easy to get a tight group of hits on one section of blade. Single holes can sometimes be repaired with speed tape. A steel wire held aloft by a kit is far more hazardous.A safer solution is to leave said aircraft in hanger until hostilities cease.--Aspro (talk) 12:09, 22 August 2011 (UTC)

In Vietnam, the Vietcong sometimes fired steel-shafted arrows through the US helicopters' rotor disk to damage the blades, with great success (this was especially effective against Bell choppers like the Huey or the JetRanger). 67.169.177.176 (talk) 23:07, 22 August 2011 (UTC)

Can you cite a reference for that? After a fairly vigorous Google, I've drawn a blank and there's a LOT of stuff out there. Alansplodge (talk) 18:26, 24 August 2011 (UTC)

To be honest: No, I cannot -- I got this info firsthand from a Vietnam veteran. So it may or may not be accurate, and is definitely not verifiable. 67.169.177.176 (talk) 00:09, 25 August 2011 (UTC)

Stable

What is the definition of stability? really, it confuses me... it's like everything is stable somehow considering the environment it is in (like saying :"it couldn't be any other way"), and everything is unstable in some sense because everything keeps changing....--Irrational number (talk) 18:26, 20 August 2011 (UTC)

Stability, in a vague sense, is a resistance to change, or tendency to recover from perturbations.[4] But there are a number of different ways that stability is defined precisely, depending on what field you're interested in. See stability for a list of those. Red Act (talk) 18:43, 20 August 2011 (UTC)

Stability is closing the door before the horse bolts. Mitch Ames (talk) 01:37, 21 August 2011 (UTC)

um... I saw them, but none had general information about what I want... actually, the "metastable" caught my attention, I can't really understand what it means, neither the stability itself...--Irrational number (talk) 20:22, 20 August 2011 (UTC)

Actually, I was disappointed by the lack of a good article on Stability Theory, a subset of control theory. "Metastable" has a precise meaning in the formalized mathematical study of systems: it means that along one axis of a generalized coordinate space, the system is at a minimum of its potential; and along another axis, it is at a maximum; in other words, a saddle point in the generalized coordinate space of the system; so it is susceptible to perturbations along only one coordinate. Some day, when I have more time, I may sit down and expand the article stability theory. Nimur (talk) 22:23, 20 August 2011 (UTC)

That's not my understanding of the word metastable. Metastable could be defined as stable under very small disturbances but unstable under larger disturbances. Another interpretation of a metastable state is an unusually long lived unstable state. Supercooled water would be an example of a metastable state. Dauto (talk) 03:52, 21 August 2011 (UTC)

In chemistry, at least, Dauto has the correct interpretation, or very close to it. A metastable state is at a local potential energy minimum that is very far from the absolute potential energy minimum for a system. That is, a small disturbance will destabilize it and send it towards the absolute minumum. Classic metastable systems are supersaturated solutions and superheated liquids which are metastable insofar as, if left entirely unperturbed, will not do anything interesting, but with a small perturbance will suddenly destablize very quickly to reach the new equilibrium. --Jayron32 03:58, 21 August 2011 (UTC)

And, since no one bothered to link it yet, we do have a halfway decent article at Metastability. --Jayron32 04:01, 21 August 2011 (UTC)

I'm willing to concede that I'm not a chemist, so I don't know the usage of such terminology in a chemistry context. I guess this is an example of domain-specific terminology. In the lingo used by a physicist or a controls engineer, that scenario would be a "local minimum," not a "meta-stable state." And a local maximum is not a stable state at all, meta- or otherwise. Even if there is no net force acting on an object at that local-maximum spot (e.g., an L4/L5 Lagrange point), the system is unstable - because it is sensitive to perturbations of infinitesimal magnitude. See sensitivity analysis. Clearly, in the domain of chemical thermodynamics and molecular kinetics, the terminology is used differently. Nimur (talk) 16:53, 23 August 2011 (UTC)

It might help if we knew what field(s) you are interested in. For example, a "mentally stable" individual is one not subject to emotional breakdowns, while a "stable nuclear isotope" is one which is not subject to spontaneous nuclear decay. StuRat (talk) 04:42, 21 August 2011 (UTC)

I don't know... for example diamond is said to be metastable... but it looks more stable than many other things that are considered to be stable.... or I don't understand why they say that electron is unstable in excited state, since you can say that electron is somehow unstable in ground state when it meats a proper photon... I mean in depends on the environment, and considering the environment something is in, one can say that everything is stable with regard to its environment (even if it's wrong, I don't know how to answer that...)--Irrational number (talk) 08:33, 21 August 2011 (UTC)

Diamond is metastable because it spontaneously converts to graphite (albeit very slowly). Excited electrons spontaneously revert to their ground state, whereas going the other way requires outside interference (a photon). Because it's this spontaneity that defines stability, the environment isn't relevant. (One could probably come up with situations where the environment "assists" stability in some sense, like a very small marble rolling in a very shallow bowl which would escape in vacuum but which suffers Brownian motion and remains within the bowl (longer, anyway) in the presence of air. But then maybe the air is just part of the system, then…) --Tardis (talk) 14:23, 21 August 2011 (UTC)

Just to expand a bit on what Tardis is getting at; one very common mistake people make is confusing chemical thermodynamics with chemical kinetics. Thermodynamics is the study of energy movement; stability (and spontanaeity) are thermodynamically defined only. If one state has more potential energy than another, it is by definition less stable and thus the less stable (higher potential energy) state will always spontaneously revert to the more stable (lower potential energy) state. Always. Thermodynamically, diamond has more internal potential energy than graphite, so it is a metastable state. The other half of the problem, that of chemical kinetics, deals with the related factors of mechanism and rate. That is, by what method does the change in states occur, and how fast does it occur. Kinetics is wholly independent of thermodynamics, so (as is obvious with diamond) the rate at which the less stable state relaxes to the more stable state can be very slow indeed (on the scale of thousands or even millions of years), but that has nothing to do with the thermodyamics; diamond is still at a higher potential energy, regardless of how long it takes to release that energy. It is metastable because it releases that energy very slowly, but we cannot say it is truly stable because it is not in the absolutly lowest energy state it can exist in (that would be graphite). --Jayron32 19:21, 21 August 2011 (UTC)

Identify the bug, if possible?

The bug. Click to enlarge, as usual.

What species of bug is this? It was seen near Ferndale, Arkansas (in southwest Pulaski County, Arkansas), and I photographed it as best I could (sorry that that wasn't very good...the bug was moving very fast and I didn't want to get too close because of the bright colors and what that can mean in nature about toxicity) but I have absolutely no idea what it is. Thanks in advance, Ks0stm ^(T•C•G) 18:36, 20 August 2011 (UTC)

A velvet ant perhaps, in which case you were wise to be cautious. Mikenorton (talk) 20:09, 20 August 2011 (UTC)

The one shown here has similar markings to the insect you saw. Mikenorton (talk) 20:12, 20 August 2011 (UTC)

At the Ugly Bug Home Page they have some close-ups of what they are labeling Velvet Ants. Bus stop (talk) 02:07, 21 August 2011 (UTC)

Uncertainty principle

I've been exposed to two different interpretations of the uncertainty principle:

1) If an electron is in a certain state, a measurement of its position will yield a definite result. However, if after the measurement the electron could magically be returned to the same state, then a repeated measurement of its position will yield a different answer. Same holds for measurements of momentum. However, the standard deviation of the distribution for positions * the standard deviation for the distribution for momentum will always be greater than a certain constant. As Griffiths puts it: "Please understand what the uncertainty principle means: Like position measurements, momentum measurements yield precise results - the "spread" here refers to the fact that measurements on identically prepared systems do not yield identical results".

2) Measurements are fuzzy. Measurements of, say, position will never yield a definite result, unless the momentum becomes completely unknown. In Townsend, it is said "we cannot obtain a single value for the position of a particle when we measure it". This also seems to be in the spirit of the Bohr-Einstein debates.

Which one is right? And why? Thanks. 65.92.4.13 (talk) 20:24, 20 August 2011 (UTC)

They are both right. Dauto (talk) 21:44, 20 August 2011 (UTC)

In other words: both descriptions predict the same measurements in an experimental setup. You can form any opinion you like about the universe in its un-measured state: maybe the position has no value until you measure it; or maybe it has a value that can't be precisely known until measured. Doesn't matter: after you measure it, the result is the same. Nimur (talk) 22:18, 20 August 2011 (UTC)

I'm not asking a realist/anti-realist philosophical question, nor am I asking whether or not it makes sense to discuss what an electron's position is prior to a measurement. Rather, if I were to take a reading of an electron's position, would I get a clean value, like x = 5cm (with the understanding that if I were to repeat the experiment, I could very well get x = 4cm or x = 6cm) or would I get a splotch, like x = 4-6cm? 65.92.4.13 (talk) 00:47, 21 August 2011 (UTC)

Measurements yield definite results. I'd say (1) is correct and (2) is wrong. "We cannot obtain a single value for the position of a particle" is only correct if you interpret it as a different phrasing of what Griffiths said. -- BenRG (talk) 01:43, 21 August 2011 (UTC)

All measurements have uncertainty associated with them. Dauto (talk) 03:31, 21 August 2011 (UTC)

Is what I said below what you mean? 65.92.4.13 (talk) 03:39, 21 August 2011 (UTC)

(ec) That's what I thought too. But if you're right, seeing as QM applies to measuring apparatuses, is it possible that position measurements become 'fuzzy' because the measuring devices don't have well-defined positions? So that, in effect, (1) and (2) are correct? (This is my interpretation of Dauto's comment) [this was meant for BenRG] 65.92.4.13 (talk) 03:37, 21 August 2011 (UTC)

Yes, that's why both quotes are correct (within their own contexts which are slightly different). Dauto (talk) 15:48, 21 August 2011 (UTC)

If a photon hits a CCD, it will be counted by at most one of the pixels. If it hits photographic film, it will darken at most one of the silver halide crystals. If it hits your retina, at most one of the rods will fire. The measurement yields a definite, discrete result, in that sense. But, you're right, that result doesn't tell you the exact position of the particle.

The quote from Townsend occurs at the beginning of chapter 6 (online here) where he's talking about the fact that you can't actually do a measurement in the position basis (which would tell you the exact position of the particle and leave it in a position eigenstate). But I still can't make any sense of it. For some reason he uses a lens to form an image. All this does is (a) move the position detection problem from the "particle plane" to the image plane and (b) gratuitously introduce a new source of error. He treats that new source of error as the only source of error, and never talks about the actual position measurement at all. Then he ends with "In fact, as our discussion of the Einstein-Podolsky-Rosen paradox emphasized, we should not try to view the particle as having a definite position at all", which is a non sequitur. Entanglement is irrelevant here. You can do EPR with spin instead of position and momentum, yet he seems to think that spin eigenstates are "real". -- BenRG (talk) 04:54, 21 August 2011 (UTC)

Towsend seemed confused to me as well. Thanks a lot. 74.15.137.168 (talk) 03:27, 22 August 2011 (UTC)

Double-slit, single electron experiment

What is the best description of where the electron passed through?

1. The electron passed through both slits.
2. The electron passed through neither slit.
3. The electron passed through either slit.
4. There is no answer.

Plasmic Physics (talk) 23:53, 20 August 2011 (UTC)

I'd say "both", since the electron's observable behavior depends on the properties of both slits. But some people might prefer "no answer". -- BenRG (talk) 01:43, 21 August 2011 (UTC)

An 'electron' is merely a mathematical/scientific construct, used to explain the results of our observations. And so is 'a slit' ;) AndyTheGrump (talk) 02:59, 21 August 2011 (UTC)

5. All of the above. --Jayron32 03:04, 21 August 2011 (UTC)

"Both", but I agree with AndyTheGrump: the electron is not a particle or a wave. It is what it is, and quantum mechanics tells us how to predict what this thing will do. Intuition based on classical particles and waves is just not that useful.--Srleffler (talk) 05:13, 21 August 2011 (UTC)

The two mutually exclusive possibilities where the electron passes throug one slit and not the other, interfere with each other. This is what interference in QM is, it is not the electron splitting itself in two parts, but it's the two possibilities existing side by side and then the final state depends on both. Count Iblis (talk) 14:37, 21 August 2011 (UTC)

The article you want is Wave-particle duality that explains the inability of classical concepts like "particle" and "wave" to fully describe the behavior of quantum-scale objects. Cuddlyable3 (talk) 15:33, 21 August 2011 (UTC)