Wikipedia:Reference desk/Archives/Science/2011 November 23

= November 23 =

Introducing life
If humans in the future had the capability to travel to a planet near the Goldilocks zone but not exactly in the Goldilocks zone (possibly Gliese 581 g or Gliese 581 d [if it didn't have greenhouse gasses], with pretty frigid climates) and planted microscopic plant and animal life and possibly small insects on the planet, is it possible that the life will evolve to suit the climate and environment? Could this also work for Jovian moons such as Titan and Io as well? If so, does that prove that life doesn't necessarily mean that life can still exist even if the planet isn't in the Goldilocks zone? 64.229.180.189 (talk) 03:17, 23 November 2011 (UTC)


 * The closer the planet or moon was to Earth, the more likely Earth flora and fauna are to survive, and then adapt, to the new environment. Single celled organisms might be the most flexible in that regard, and animals would have to be introduced later, as they would need plants to eat.  Lichen might do well in cold climates, provided they have access to sunlight and a place to anchor themselves. StuRat (talk) 04:01, 23 November 2011 (UTC)


 * See the article Extremophile for organisms that might survive in an extraterrestrial environment. Cuddlyable3 (talk) 04:14, 23 November 2011 (UTC)

Done thoughtfully, introducing life to the planet would almost certainly be an important part of a successful teraforming program. That's not to imply that life will automatically make a planet more Earth-like. Just randomly growing things on the planet could just as easily make it worse. Teraforming must be carefully planned, and would likely involve genetically engineering new lifeforms when we couldn't find one that can perform a particular job in a particular environment. APL (talk) 22:44, 25 November 2011 (UTC)


 * Done thoughtlessly, we could unwittingly create an ecological catastrophe. Annihilating native lifeforms we may not be aware of through direct competition for resources or (less plausibly) infection. It'd be War of the Worlds, except we'd be the invaders.--  Obsidi ♠ n   Soul   15:11, 26 November 2011 (UTC)

Why when we put cold water on ours skin, it bring about the blood to come there?
I know that the cold narrow the artries? 95.35.79.205 (talk) 03:21, 23 November 2011 (UTC)
 * It's an astringent. Here is its mode of operation being discussed in a rather old book (published 1867). I'm not sure why you say it bring about the blood to come there; actually it's the other way around - it bring about the blood to go away from there. Maybe I misunderstood you.  Card Zero  (talk) 05:01, 23 November 2011 (UTC)
 * I have heard a prof' who said that cold water on the scalp flow the blood there. 109.253.190.169 (talk) 19:45, 23 November 2011 (UTC)
 * Seems to be unique to the scalp. I found this which cites this, saying "scalp blood vessels do not vasoconstrict in response to cold as do surface vessels in other body areas" and "Froese and Burton found little or no head skin vasoconstriction in response to cold". Nothing there about cold increasing blood flow to the scalp, but it seems it doesn't decrease it, because the scalp reacts in an unusual way to cold water. (But much more unusual things happen if you pour it in your ear.)  Card Zero  (talk) 23:15, 23 November 2011 (UTC)

Watch Gears
Hello, I'm not sure if a question like this is permitted here, but where would be the best place to purchase watch gears (all sizes and nothing specific, just "watch gears")? Thank you. Marcus Lupus (talk) 03:31, 23 November 2011 (UTC)


 * Sounds like a good place to start would be e-bay, which probably has a variety of used watch gears at low prices, possibly still inside the watches. What do you intend to do with them ? StuRat (talk) 03:57, 23 November 2011 (UTC)


 * Wristwatches, except for antiques and status brands such as Rolex, depreciate rapidly and are often not worth repairing. Any watch repairer (a disappearing breed) will likely have non-working watches to give away. Be quick because people are changing to digital watches that have only flimsy plastic gears or no gears at all. Do I guess rightly that you want unspecific gears for art purposes? Cuddlyable3 (talk) 04:10, 23 November 2011 (UTC)
 * I have purchased unspecific clock gears on ebay for art purposes. They aren't as cheap as "junk" because of how popular steam punk has become, but still reasonable. Vespine (talk) 05:00, 23 November 2011 (UTC)

Thank you all for your responses. I am interested in them simply from a mechanical point of view, that is, to create actual working devices (unrelated to steampunk) rather than art. I have tried eBay, but the demand for watch gears and the odd watch part (because of the steampunk fad/craze) has driven prices up quite a bit. Where a typical gear used to be just pennies, they are now 10 to 20 cents a piece, which is quite prohibitive if one is interested in creating large and complex devices through trial and error. I may have to contact watch repairers to see if they have non-working watches to give away, it may be the best source. Thanks again. Marcus Lupus (talk) 06:00, 23 November 2011 (UTC)


 * Well it depends where you live, but car boot sales or jumble sales are often a good source of unwanted items. --TammyMoet (talk) 08:51, 23 November 2011 (UTC)

fluid flow
why do we have stream line flow of a fluid when the rate of flow is low, and turbulent flow when rate of flow is high? 196.0.4.86 (talk) 13:55, 23 November 2011 (UTC)
 * Our turbulence article says: "Richard Feynman described turbulence as "the most important unsolved problem of classical physics."" We also have a short article on Laminar-turbulent_transition. We are pretty sure that the reynolds number has something to do with it, but understanding turbulence is still a very active field of research. SemanticMantis (talk) 14:23, 23 November 2011 (UTC)

Magnetism
Are compounds of magnetic elements still magnetic (e.g. Fe2O3)? --70.250.212.95 (talk) 13:56, 23 November 2011 (UTC)
 * No, not in general. Looie496 (talk) 15:18, 23 November 2011 (UTC)
 * Why not? --70.250.212.95 (talk) 18:57, 23 November 2011 (UTC)
 * Ferromagnetism arises due to a specific type of electron configuration. Chemical bonding with different types of atoms will often cause a different electron configuration than in the pure element.  Dragons flight (talk) 19:10, 23 November 2011 (UTC)
 * Not questioning that it is true, but it is a little counterintuitive, since ferromagnetism would seem like a property of the element, and not something appearing and diszsappearing as it forms compounds with other elements or is liberated from a compound. It suggest some interesting phenomena, if reactants are placed in a variable magnetic field. Could the reaction rate or the ability to form a compound be modulated by varying the magnetic field, and has such a property been used for any useful purpose? Edison (talk) 20:41, 23 November 2011 (UTC)
 * Well, according to the article, the example given by the OP, Fe2O3 IS ferromagnetic, albeit weakly Hematite. Ferromagnetism has more details. I think the answer "not in general" is still right; some specific compounds 'are' ferromagnetic, but they seem to be the minority, I think if you just took some magnetic elements and made a whole bunch of random compounds, you'd be very lucky to actually make any magnetic ones. Vespine (talk) 22:26, 23 November 2011 (UTC)
 * My comment was about modulating a chemical reaction with a magnetic field. It seems intuitively that there could be an interaction between the presence or strength of a magnetic field and the ability of a chemical reaction to take place, or the reaction rate, if the reactants would go from nonmagnetic to magnetic, since that might result in a change of the potential energy of the system. Edison (talk) 19:50, 24 November 2011 (UTC)
 * The magnetic interaction is too weak, compared with the electrostatic interactions, to make a difference to the chemical reactions - except possibly near certain critical points. RockMagnetist (talk) 02:07, 26 November 2011 (UTC)

By "magnetic", I assume you mean "strongly magnetic" (all materials can respond to a magnetic field). The magnetic properties of materials like Fe2O3 are mainly determined by an interaction between electron spins called the exchange interaction. Naturally, the chemical composition determines how the atoms are arranged, and therefore how strong the exchange interactions are and in what direction. Lots of compounds involving iron are paramagnetic (see Table 2 in magnetic mineralogy). Temperature makes a difference too - for example, hematite loses its ferromagnetism (strictly speaking, canted antiferromagnetism) and becomes antiferromagnetic below a temperature called the Morin temperature. RockMagnetist (talk) 02:07, 26 November 2011 (UTC)

Small bowel adenocarcinoma doubling time
It is said that colorectal cancer is one of the most slow-growing tumours in its early stages. Does this also hold true for small bowel adenocarcinomas? If not, how quickly do they spread? Thanks.--109.14.64.253 (talk) 14:46, 23 November 2011 (UTC)

Bridge Records
Either my google powers are failing, or there's a surprising lack of of this bit of trivia:

I'm looking for the answers to these questions:

What is the world's widest bridge?

What is the world's shortest (FUNCTIONING) suspension bridge?

It seems people are mostly concerned with length of bridges!

In case you can't tell, I'm from Boston which is home to the Lenny Zakim, the world's widest cable-stayed bridge, and the pulic gardens bridge which WAS the world's shortest suspension bridge, but has been converted to a girder bridge.

So to recap:

The world's widest bridge of ANY type (and it can't be the Sydney Harbor, because that comes in at a mere 161 ft. whereas the Zakim comes in at 183 ft.)

The world's shortest suspension bridge still in use, and still suspended.

Any help would be greatly appreciated!71.232.14.6 (talk) 18:06, 23 November 2011 (UTC)
 * For question 1, this plaque claims to mark the widest bridge in Europe, but I believe the claim is no longer current. However, if a smallish river is culverted in and you count the length of the bridge as being the short distance across the river, the width could be arbitrarily large. Thincat (talk) 20:27, 23 November 2011 (UTC)
 * So it boils down to: "When is a bridge not a bridge?" (Perhaps when it's a tunnel?) {The poster formerly known as 87.81.230.195} 90.197.66.23 (talk) 20:42, 23 November 2011 (UTC)
 * A quick google came up with Blue Bridge (Saint Petersburg) as the widest bridge at 97.3m or 319ft. The opening paragraph of our article confirms the claim but it does say the claim isn't officially recognized, however it doesn't offer any alternatives either. Vespine (talk) 21:29, 23 November 2011 (UTC)
 * The Lowry Hill Tunnel, while technically a tunnel, is effectively a bridge, and quite a bit wider one than the Blue Bridge. ←Baseball Bugs What's up, Doc? carrots→ 11:02, 24 November 2011 (UTC)


 * It isn't recognized because of the main issue in this question... How long does a bridge have to be to be a bridge? The Blue Bridge is too short to be recognized by Guinness. -- k a i n a w &trade; 21:59, 23 November 2011 (UTC)
 * Oh, well, it looked a genuine sort of bridge to me. Here is a satellite photo of it. My 11-year old is keen on world records and I have looked in his Guinness Records books for 2006, 2008 and 2011 but I can't find any mention of wide bridges. Thincat (talk) 22:21, 23 November 2011 (UTC)
 * Yeah, Blue Bridge does exemplify the 'culvert' problem&mdash;while it's 97 meters wide, it spans a river that is only 41 meters from bank to bank. TenOfAllTrades(talk) 23:39, 23 November 2011 (UTC)
 * For question 2, this one's 86 ft long, at Mill Creek MetroPark - Youngstown, Ohio. Mikenorton (talk) 22:33, 23 November 2011 (UTC)
 * Does the suspension bridge need to be a road bridge? I know of several very short pedestrian suspension bridges. HiLo48 (talk) 23:30, 23 November 2011 (UTC)
 * The OP refers to this bridge in his post as originally the shortest, and it's a footbridge. Mikenorton (talk) 23:35, 23 November 2011 (UTC)
 * That looks like a conventional concrete bridge with the suspension stuff added as mere decorations. ←Baseball Bugs What's up, Doc? carrots→ 10:58, 24 November 2011 (UTC)
 * This one in Australia is pretty short. HiLo48 (talk) 16:46, 24 November 2011 (UTC)


 * Correct. That is not actually a suspension bridge. Locals call it one, but mostly they know it is not one. -- k a i n a w &trade; 17:09, 24 November 2011 (UTC)

How long does it take for water to get from your mouth to your bladder?
How long does it take for water to get from your mouth to your bladder? For a 20 years-old male, not doing sport. The question is not "How long between drinking water and having to pee?", if you don't have much urine in the bladder you don't feel you need to pee.

http://ask.metafilter.com/149519/How-long-does-it-take-for-water-to-get-from-your-mouth-to-your-bladder http://answers.yahoo.com/question/index?qid=20080626235040AAIky8y http://uk.ask.com/beauty/How-Long-Does-It-Take-Liquid-to-Reach-Bladder http://www.intelligentanswers.co.uk/index.php?topic=3216.0 http://au.answers.yahoo.com/question/index?qid=20080422111420AABl71g

Some say 10min, others 45-60min. Some say hours. Who's right? — Preceding unsigned comment added by Trbdvx (talk • contribs) 23:44, 23 November 2011 (UTC)


 * I don't see how the question has an answer, because it presupposes that water gets from your mouth to your bladder. It doesn't. Much of the water you drink enters your bloodstream. Some water from your blood is extracted by the kidneys and stored in the bladder. There is no guarantee that any particular water molecule will ever reach the bladder, and those that do may take widely varying times. --ColinFine (talk) 00:16, 24 November 2011 (UTC)


 * AHA! But this question DOES have an answer, it just happens to be related to something I've been reading about recently:) The answer is Biological_half-life. Paraphrasing: It's a continuous "probabilistic" process, it takes 7 to 14 days for HALF the water you drank to be expelled. Vespine (talk) 00:55, 24 November 2011 (UTC)


 * Of course, not all of that water makes it out of the bladder. Depending on a plethora of factors, lots of that water ends up being expelled via sweat or in the breath as well.  -- Jayron  32  02:11, 24 November 2011 (UTC)


 * That answer seems similar to if somebody asked how long it takes electricity to get from one end of the wire to the other, and you responded with how long it takes the actual electrons to move the entire length of the wire. In the case of the wire, the electrons you add at one end push the electrons down the wire until a few other electrons come out the far end.  Similarly, in the case of biology, drinking quantities of water rapidly causes water molecules to accumulate in the bladder, in order to maintain a proper water balance in the body, although they may not be the same molecules you just drank (but I'd expect some would be). StuRat (talk) 05:35, 24 November 2011 (UTC)


 * But that's the answer the questioner wants. The question explicitly says he wants not how soon after drinking do I have to pee but when does the water I drunk come out.  In other words, he wants to know about the electrons not the current. --Colapeninsula (talk) 10:01, 24 November 2011 (UTC)


 * Every time this kind of question comes up I point out that, because of the quantum nature of particles, it's quite meaningless to ask which ones are which once they've all mixed together in the circulatory system. The same goes for a radioactive tracer, except that in that case, because it's not normally present, it will have an elimination half life in the body (once you stop drinking it), and that half life can be measured. -- BenRG (talk) 19:47, 24 November 2011 (UTC)


 * The answer to this may be found in one of the diagnostic tests for kidney problems. For example, I once had to drink some radioactive liquid, which was then tracked through my urinary tract and videoed on its way. From drinking the liquid to being asked to pee took 20 minutes. --TammyMoet (talk) 10:49, 24 November 2011 (UTC)
 * Exactly, Tammy. The same thing applies when you eat asparagus or too much beetroot your pee will smell of asparagus within three-quarters of an hour and the beetroot will colour your pee within the same time. If I allow 15 minutes for my bladder to fill we get about the same time. (the smell of asparagus will not be apparent to all people). Richard Avery (talk) 11:31, 24 November 2011 (UTC)
 * These arguments are not convincing - they tell you how long certain solutes take to make the journey, but these don't necessarily move at the same rate as the water they were originally dissolved in. But Vespine's answer above makes sense, and I stand corrected. --ColinFine (talk) 22:39, 24 November 2011 (UTC)


 * I don't know about water, but my own research with a few more beers than a doctor world suggest, indicates about 2 pints drinking time (~one hour for me) before you have to pee for the first time since entering the pub. Of course, after that you have to pee every half hour or so to make room for the third, fourth, etc.  Astronaut (talk) 13:31, 25 November 2011 (UTC)

why does a dft bin's output noise standard deviation (rms) is proportional to sqrt( N ) ?
why does a dft bin's output noise standard deviation (rms) is proportional to sqrt( N ) ? — Preceding unsigned comment added by Zhaoyin214 (talk • contribs) 23:55, 23 November 2011 (UTC)


 * I have no idea what a dft is (What makes you think I would?), but I'm pretty sure that the answer is closely related to the fact that the standard deviation in a random walk is proportional to sqrt( N ). ~


 * DFT = discrete Fourier transform. The reason can be imagined due to conservation of energy.  A noise energy enters the process, which splits the input into N bins.  One bin will contain 1/N of the noise energy.  The values used to encode the signal are proportional to the voltage, and the power is proportional to the square of the voltage.  So the total noise power is the sum of the squares of noise level in the bins Sum nbin2 = ntotal2.  If you assume similar noise in each bin ( or if not look at the definition of standard deviation with is a square root of sum of squares anyway)  from this you can get Standard deviation nbin=√N ntotal. Graeme Bartlett (talk) 01:24, 24 November 2011 (UTC)
 * Note the standard assumptions that the noise is Gaussian noise and stationary. Graeme Bartlett (talk) 20:48, 26 November 2011 (UTC)