Wikipedia:Reference desk/Archives/Science/2013 October 21

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October 21

Metric

The Fisher information metric is understood to describe infinitesimal changes in the Kullback-Leibler divergence. However its quantum analog, the Bures Metric, is understood to describe infinitesimal changes in the Bures distance, which is defined with reference to the Fidelity, and not the Quantum Relative Entropy (the quantum analog of the KL divergence). What is the reason for this breakdown in analogy?

Has a metric describing infinitesimal changes in the QRE been studied? Does its behaviour differ from the Bures Metric? — Preceding unsigned comment added by 144.82.191.250 (talk) 10:51, 21 October 2013 (UTC)

Just linking some of the terms above for reference: Fisher information metric, Kullback-Leibler divergence, Bures metric, Bures distance, Quantum relative entropy. -- The Anome (talk) 11:21, 21 October 2013 (UTC)

Also Fidelity of quantum states. Red Act (talk) 17:18, 21 October 2013 (UTC)

Maybe the Mathematics reference desk is a better place for this ... Ssscienccce (talk) 14:27, 21 October 2013 (UTC)

This is unarguably within the remit of physics. — Preceding unsigned comment added by 86.134.240.24 (talk) 19:45, 21 October 2013 (UTC)

Regarding the absence of a very well known info on SEXUAL DESIRE on wikipedia

This question is related to sexual desire of males. In the article SEXUAL DESIRE i could not find a very well known fact. It is generally seen that in sexually active active males, the time gap between the ejaculations affects their sexual desire. For example, if a man masturbates or ejaculates regularly(say once a day or once every two days or twice a day) and suddenly stops it, his sexual desire(or what we call libido) increases..It is also observed that the orgasm after a period of abstinence are also intense and getting and erection in a short term abstinence period is quicker..Why does this happen..? And why does wikipedia not mention i under the factors affecting sexual desire ..? I know that males are made to release their sperms once in a while. But why could i not find a reason or mention of it on wikipedia. .. Please give a scientifically proven reason for your answer....many thanks — Preceding unsigned comment added by Ed beerman (talk • contribs) 11:46, 21 October 2013 (UTC)

If it's that well-known, you should have no trouble finding valid sources on it, and could improve the article yourself. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:42, 21 October 2013 (UTC)

You are generalizing with no links to any studies. Your statements amount to speculation and can be discarded.217.158.236.14 (talk) 13:24, 21 October 2013 (UTC)

It is noted (with cite) in Libido#Physical_factors. Libido is one of the concepts linked and discussed in Sexual desire, but I don't understand the distinction between those two concepts...seems like there is a lot of overlap between their articles. DMacks (talk) 15:07, 21 October 2013 (UTC)

• The most relevant article is probably refractory period (sex). Looie496 (talk) 16:05, 21 October 2013 (UTC)

yes,i have read about Refractory period(sexual)..But this article only deala with the time period after ejaculaion during which it is impossible for a person to get an erection..i am talking about a different thing(although we can relate it to refractory period)...i am asking about the reason that greater the gap between two ejaculations(at least in short run) , higher is the desire to ejaculate ...why is sit so..? — Preceding unsigned comment added by 117.207.94.35 (talk) 17:27, 21 October 2013 (UTC)

At a guess, I'd imagine that it has at least something to do with the changes in pressure in the ejaculatory system caused by the build-up and release of fluid, but that would be pure unfounded speculation on my part; there are so many mechanisms that could be related to it -- hormonal, nervous (voluntary and involuntary), muscular, vascular, psychological, learned behaviours, etc. etc. that it wouldn't be suprising if it was still an open question. Have you tried to take a look at the medical literature on the topic, e.g. via Medline? -- The Anome (talk) 00:42, 22 October 2013 (UTC)

Are you sure this is actually true? I usually masturbate (or have sex) around 4 times a day, if I'm busy and not able to, and am aware of this, then what you're discussing holds. On the other hand, sometimes I don't notice, in which case it doesn't make a difference at all. In other words, at least for myself, it seems that if I want sex/masturbation and can't have it, then I want it more- and if I don't really want it and don't get it, then I still don't want it. What you seem to be describing is wanting + not getting = really wanting; but that holds for hunger, thirst, the need for a cigarette, or any other desire. I think the key is where you say "and suddenly stops it", rather than doesn't feel like it- if you avoid satiating anything, again, you're going to want it more.Phoenixia1177 (talk) 04:11, 22 October 2013 (UTC)

"Oh, no, not again. The creature stirs..." (Momentarily contrasting the desperate intellectual deprivation of the 80s with our fortunate forum, though I think Harry would have missed the sound of a human voice.) Wnt (talk) 05:20, 22 October 2013 (UTC)

Four times a day sounds like above average frequency for male orgasm. I have heard "Age 20: tri-weekly. Age 40: try weekly. Age 60: Try weakly." So no scientist has published reliable sources looking into the temporal link between getting off and horniness? Amazing. Think of all the studies that could be done with experimentally varying the hydrostatic pressure in the seminal vesicles and seeing how it affects horniness, or just measuring the horniness as a function of the interval from last orgasm, or administering hormones and seeing their effects, whether on rats, monkees or human volunteers. Edison (talk) 04:02, 23 October 2013 (UTC)

It is above average frequency, I suppose, I've always had a strange fascination with sex. I think, though, as far as the question goes, the focus is too narrow- the same relations hold for most wants. If you miss a meal, you might eat more at your next, or at least think more about it. The main element seems to be having an unsatisfied desire, not anything specifically sexual.Phoenixia1177 (talk) 05:06, 23 October 2013 (UTC)

'You regularly have it(like you 4times a day) and you suddenly stop it, then you want it more(if you are aware and not having it), this is what i want to say..does the urge to masturbate not really increase if you dont have sex for a,say a week(when you regularly have it)..? are the orgasms after a peiod of short term abstinence not more intense than the usual..? Also it is so common to say that we get a wet dream to release the fluids when the tank is full..Sexual desire is also like hunder or any other biological need.. Now i arrive at two simple questions: When will the sexual desire(or stimuli) be more strong--When you are masturbating fouth time in a day..? Or when you are masturbating after four days Should "time gap between ejaculations" be a factor affecting sexual desire on wikipedia..? If no, Please tell why..many thanks 12:16, 22 October 2013 (UTC)~~ — Preceding unsigned comment added by Ed beerman (talk • contribs)

I would think that the plumbing, hydrostatics and hydrodynamics contribute very little. The main stimuli are psychological and pheromonal. Dbfirs 12:56, 22 October 2013 (UTC)

I can only speak from personal experience to answer, so bear that in mind. I notice that the more I want sex, the more intense the experience is, and the more I think about it- and the worse it is if I can't have it (I work long hours some days). Some weeks, I get wrapped up in other things of interest and sex is far from my mind (as are things like regularly eating and sleeping), in those cases, I don't really miss it, and, when it returns, it doesn't come back in a "flood", but slowly increases back to normal (hunger and the interest in sleep do the same). On the other hand, sometimes I'm very fixated on sex, in those cases, if I can't have it, it's all I want to think about- in that case, I masturbate more, think about it more, etc. As for when the sexual experience is the most intense: during those times when I am masturbating more frequently, the experience is stronger- since I'm not doing this back to back, but a few hours apart, each experience is equally intense (there is variance, but it is a factor of time of day and schedule more than where the event is in sequence). As I said, I think the major factors in intensity and want is desire and availability- if I really really want sex and get stuck at work for a 20 hour shift, it's very amazing when I finally get home- on the other hand, if I'm at a low period of interest and work a 20 hour shift, when I come home, sex is less of a "yippee!" and more of a "I'm sleepy, but my wife likes sex, so okay".Phoenixia1177 (talk) 05:06, 23 October 2013 (UTC)

Opportunities for Exploring Comets and Minor Planets

• List of missions to minor planets
• List of missions to comets

Where can I get information about proposed projects and/or theoretical opportunities of unmanned space explorations?

Is there a list of future "windows" for the exploration of solar system objects?

Launch windows are determined partly by the available propulsion power and the location of your launch facility. If you have an ion rocket and a very light-weight spacecraft and your rocket base is on the equator, you probably have larger windows for visiting every solar system body. Otherwise, you may have smaller or no windows at all.

I just want to see a list of the "ON SALE" (least expensive) days for visiting comets and minor planets. -- Toytoy (talk) 15:14, 21 October 2013 (UTC)

I think you will need to narrow you criteria; there are over 1 million asteroids and comets known. Rmhermen (talk) 15:43, 21 October 2013 (UTC)

There must be some useful rules for solar system tours.

If you want to travel beyond Mars, you may need to use Mars as a slingshot. Then you have to look-up the nearest window for Mars at first.

I think there must be many rules and restrictions to narrow down available possible destinations. -- Toytoy (talk) 07:41, 22 October 2013 (UTC)

See Exploration of Mars#Launch windows. Duoduoduo (talk) 17:43, 22 October 2013 (UTC)

Also Exploration of Venus#Under study, BepiColombo, Solar Probe Plus#Trajectory and mission, Venus In-Situ Explorer, Venus Entry Probe, Venera-D, Europa Jupiter System Mission, and Titan Saturn System Mission. Duoduoduo (talk) 17:54, 22 October 2013 (UTC)

I heared a presentation on which asteroids you can visit and bring a sample back. Although there are a lot of asteroids out there the number was small like 5 or 10. First you have to get ride of all with high inclination. The ones which are far out are off limits because of the too high reentry speed. Third is to eliminate all fast spinners. At the end you elimated all but 10 and you end up at the point that OSIRIS-REx and Hayabusa 2 had too look for the same conidates.--Stone (talk) 18:21, 22 October 2013 (UTC)

The problem is always you can go for venus mars and earth flybys to get to speed and with that you have a lot of oportunities. --Stone (talk) 18:21, 22 October 2013 (UTC)

Soda pop can exploding when frozen

Why do soda pop cans explode when put in the freezer too long? I've put 20 fl oz plastic bottles of cola in the freezer, but they don't explode. --209.203.125.162 (talk) 17:51, 21 October 2013 (UTC)

Aren't cans usually full to the top, with no room for the ice to expand, while bottles usually have air in the neck? μηδείς (talk) 17:56, 21 October 2013 (UTC)

Plastic bottles seem like they can stretch a bit from internal pressure, whereas metal can certainly deform but not as readily to increase volume? DMacks (talk) 19:00, 21 October 2013 (UTC)

Yes that's it. Soft plastics can undergo a great deal of plastic deformation (no surprise there), while metal isn't as flexible. StuRat (talk) 20:55, 21 October 2013 (UTC)

However, as I understand, the recent incidents of "dry ice bombs" in LAX were plastic bottles stuffed with dry ice and then capped tightly. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:59, 21 October 2013 (UTC)

That's because it's easy to put dry ice in a plastic bottle and seal it. If you fill it completely with dry ice, then it will have to pop. For it not to pop would require it to be able to contain a gas at a pressure of about 1000 bar. Count Iblis (talk) 00:26, 22 October 2013 (UTC)

Which is essentially what makes them work. If you tried the same with a can (somehow, not sure how you'd seal it), you'd probably get a smaller pop, since it would rupture at a much lower pressure. MChesterMC (talk) 08:50, 22 October 2013 (UTC)

So if the cola inside is frozen, why does it explode all over the freezer? Why doesn't the can just burst open, leaving a frozen cola cylinder ? --209.203.125.162 (talk) 01:17, 22 October 2013 (UTC)

Not all of the cola is frozen. Enough freezes for it to break the seal, through which the remaining liquid escapes. 81.147.166.89 (talk) 01:31, 22 October 2013 (UTC)

The freezing process involves the unmixing of the water and the cola ingredients, you get pure ice from which the rest of the cola is expelled. The freezing temperature depends on the concentration of the ingredients, so, when a little ice forms the cola ingredients have been expelled from the ice, the concentration will have increases in the remainder. So the remainder will only freeze at lower temperatures. This means that at some given temperature, the freezing process will stop before everything is frozen. But of course, if the can cannot hold a large pressure it will break open. Most of the contents will be a liquid which then explodes out of the can as it was held under pressure. Count Iblis (talk) 01:35, 22 October 2013 (UTC)

There are a number of phenomena going on here. The big ones are the liquid freezing from the outside in, forming a rigid container of ice around an ever-decreasing volume of liquid, and freeze distillation forcing dissolved carbon dioxide out of the liquid as it freezes, forming a bubble of high-pressure gas. In a relatively inelastic can, at some point the pressure reaches the point where the ice shatters, the can ruptures, and the remaining liquid and gas spray all over the place. In an elastic bottle, the bottle will stretch as the ice breaks, letting the formerly-dissolved gas join the bubble at the top of the bottle. --Carnildo (talk) 01:45, 23 October 2013 (UTC)

The can bursts because water expands when it transforms from liquid to solid, unlike many other fluids. I wanted to create a smooth hemispherical dome on the bottom of a Diet Coke can so it would work well as part of a Van de Graaf generator using "found" materials .I had observed that a cola can has a recess at the bottom and that when it freezes, the recessed bottom as well as the top expand out and become convex. I used Diet rather than Regular Coke because the sugar lowers the freezing point. Anyway, after a while the can at about zero F had bulged out at the bottom into a dome with a few rings and the top was deformed outward as well. It was placed bottom up for better observation. I decided to give it another 5 minutes to try and smooth out the dome a bit more with increased interior pressure. When I came back, the poptop had failed and the Coke slurry inside had come out like a rocket, and the can had clearly bounced around in the freezer. The actual instant of failure and resulting rocket action would make a cool Youtube or Mythbusters episode (perhaps motivated by checking the "myth" that Diet Coke freezes and bursts before regular Coke in a deepfreeze or very cold outdoor environment). Edison (talk) 03:46, 23 October 2013 (UTC)

Meat from unstressed animals

The claim that meat from unstressed animals tastes better is regularly made, but is there any scientific evidence that it is actually true? All the sources that I can find merely assert it without presenting any evidence, or use words such as "people say that...". 86.176.215.115 (talk) 19:42, 21 October 2013 (UTC)

Science has certainly backed this up. Lack of references may come from generations of our forefathers experience that take this as a given – so our forefathers didn’t require any authority to tell them what they already knew - so that's the origin of "people say that...".

• This 'patent' give some of the science bit as background (with refs) to explain the patent:[1]
• This link explains why it is important in to the meat industry:[2]

--Aspro (talk) 21:13, 21 October 2013 (UTC)

The type of food an animal eats supposedly can make a significant difference in taste (I've been told that deer that feed on corn taste a lot better than deer that have to resort to leaves and twigs), so it's not unreasonable to expect that the animal's general well-being could also affect the taste. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:01, 21 October 2013 (UTC)

That's an interesting point which is worthy of a separate discussion. In short: You know how Coca-Cola did blind consumer market research and tried to introduce New Coke only to find it was then rejected – as not the real thing. I think is a bit like corn feed deer. Some people used to eating domesticated animals I can well imagine, find that the taste of 'farmed deer' more familiar. I myself, prefer what I am more familiar with - i.e., Scottish wild deer. My tasted bud are now getting a bit jaded (alas) by the passage of time but younger members of the family refer to farmed deer, as a bit like funny tasting lamb! In other words, lamb has been slaughtered before it has developed the richer taste of mutton. Farmed deer are slaughtered before they have developed a rich flavor and so like lamb, non have the taste that my hungry mouths around the dinning table associate with meat from a deer hunted on a scottish estate where they feast on a natural Highland diet. But that has nothing to do with the stress that the animal is put through before slaughter. PS. The food the animal eats does has a very big effect on taste - there is no supposing about it.--Aspro (talk) 18:18, 22 October 2013 (UTC)

You may check out this article.

• http://www.journalofanimalscience.org/content/83/2/440.full

If a pig has been scared for too long before slaughter, you may see its meat whitened unnaturally and lose its meat juice. -- Toytoy (talk) 08:09, 22 October 2013 (UTC)

Cline variation

What is meant when it is said that human variation is clinal not racial? — Preceding unsigned comment added by 70.49.46.102 (talk) 19:54, 21 October 2013 (UTC)

It basically means that the prevalence of specific human genes and phenotypes generally varies gradually instead of suddenly across geographic regions, making it difficult and arbitrary to try to meaningfully divide humans into discrete races biologically, if you consider humanity as a whole. See Cline (biology). Red Act (talk) 20:10, 21 October 2013 (UTC)

So essentially it is hard to draw the line where for example black race or white race or asian race starts? But it is easy to do that, we all know what an Asian, or African or white person looks like? — Preceding unsigned comment added by 70.49.46.102 (talk) 20:15, 21 October 2013 (UTC)

No we don't. There is so much regional variation those terms are almost meaningless sometimes. That of course doesn't mean there aren't' genetic differences between different populations, but they don't necessarily map to the traditional 'human races'. Fgf10 (talk) 20:49, 21 October 2013 (UTC)

A couple points:

1) It isn't always easy, with people of mixed ancestry.

2) The genes that describe things we think of as race (skin color, hair color and texture, etc.) are only a small number of the total genes. So, you could have Asian genes that control your appearance, and yet have non-Asian genes as well, which don't contribute to your appearance. StuRat (talk) 20:52, 21 October 2013 (UTC)

It can sometimes work reasonably well as a model to divide people into races, if you only consider the people who live in one geographic area, due to historical immigration patterns from separated locations which have obscured the clinal nature of the genetic and phenotypic variation among humans as a whole. For example, a lot of people arrived in North America from Europe, mainly Northern Europe and Western Europe, during the European colonization of the Americas, and a lot of people arrived in North America from Sub-Saharan Africa during the Atlantic slave trade. People in northwestern Europe and people in Sub-Saharan Africa tend to look rather different, so if you're only considering people who now live in North America, it works moderately well to categorize at least some of the population in North America into "white" vs. "black" races. However, if you take into consideration everybody in the world as a whole, what people typically look like varies gradually if you consider a sequence of populations extending from Sub-Saharan Africa up through northwestern Europe. For example, people from Egypt (in Northern Africa) don't typically look like they came from England or France, but they don't look like they came from Senegal, either; instead, they look very roughly somewhere in between (although the variation is more than one-dimensional). But if you're considering only people who are currently in North America, you aren't taking into account too many people with largely Egyptian ancestry, because there wasn't a large wave of immigration to North America from Egypt in North America's history. Similarly, the concept of "Asian" as a race also works somewhat in North America, because historically a lot of people arrived in North America from East Asia and Southeast Asia, but not as many people arrived in North America from Central Asia or Eastern Europe, so the clinal nature of genetic and phenotypic variations along the populations in between East/Southeast Asia and northwestern Europe is obscured. Red Act (talk) 22:18, 21 October 2013 (UTC)

This is even further obscured by our limited understanding of history. If a large wave of immigration to North America from South America (or vice versa) occurred before recorded history, we would have no knowledge of the event. --Auric talk 22:38, 21 October 2013 (UTC)

Archeologists could figure it out, say if the pottery style from one South American tribe suddenly turned up in North America. StuRat (talk) 17:55, 22 October 2013 (UTC)

Yes, exactly. The Timucua of Florida have as their closest relatives speakers of languages in northern South America. The same path from South America up through the Caribbean was taken by the ancesters of the Taino language and other Arawakan language speakers. μηδείς (talk) 02:26, 25 October 2013 (UTC)

Thank you very much, that clears the confusion for me! 70.49.46.102 (talk) 01:11, 22 October 2013 (UTC)

Resolved

StuRat (talk) 02:05, 23 October 2013 (UTC)

Why are screw propeler more efficient than paddle wheels?

Couldn't well-designed paddle wheels (not just a flat board) be equally efficient? OsmanRF34 (talk) 22:00, 21 October 2013 (UTC)

Intuitively paddle-wheels seem less efficient because they waste energy lifting water and submerging air. Also, only a portion of the water they do move goes in the proper direction. StuRat (talk) 22:28, 21 October 2013 (UTC)

Is a screw propeller actually more efficient? They are superior for many purposes because paddle wheels don't work well if the water is not flat, but I would like to see a reference before accepting that there is an advantage in efficiency. Looie496 (talk) 00:09, 22 October 2013 (UTC)

The advantage of screw propellors is that the direction of flow imparted to the water is parallel to the screw rotation axis. That allows propellors to be made with blades of shallow pitch at the tips, gradually increasing to a more agressive pitch as you move toward the centre. In this way the velocity imparted to the water is the same throughout most of the blade length, even though the tangental velocity of the blade is a lot less near the centre than it is at the tips. Also, the blade tips can be rounded, so that in the last 5 to 10% or so of the blade, the water velocity is gradually brought down, avoiding a large shear, which would waste energy in turbulence.

Paddle wheels have the direction of water flow at right angles to the direction of rotation. That means neither variable pitch nor blade tip tapering can be used. As StuRat said, paddle wheels move some water in the wrong direction, they also move some of it at the wrong velocity, and the large shear at the bottom of the paddles causes much useless turbulence.

The problems of paddle wheels could be overcome to a limitted extent by using co-axial nested paddlewheels of huge diameter, with the inner wheels turning faster, and the very outside wheel turning very slowly. Or by using some sort of linkage to keep the paddle blades vertical. But its never going to be as good as a propellor, with anything like a practical robust assembly. Why would you bother, when a simple propellor does the job. 60.230.213.251 (talk) 00:43, 22 October 2013 (UTC)

There are various tracked amphibious vehicles which propel themselves using their tracks ("some sort of linkage to keep the paddle blades vertical"). The DARPA Captive Air Amphibious Transporter looks fun.  Card Zero  (talk) 01:04, 22 October 2013 (UTC)

besides every movement in a propel contribute to the ship displacement, in the wheel you have to waste energy raising up the paddles over the water surface, plus the wheel weight more and require more space, etc.

Planes have to be very very efficient, so they has “propel like” devices (helices turbofans etc)

Iskánder Vigoa Pérez (talk) 02:13, 22 October 2013 (UTC)

Actually, the raising of paddles is not a problem. It takes energy, stored as potential energy in the paddles, but it is balanced out by other paddles moving down into the water, which give back their potential energy. Wheel weight also have no effect on efficiency. A high wheel weight just means that the wheel acts as a flywheel, and a flywheel running at constant speed does not consume energy. In any case, propellors have considerable weight too. 60.230.213.251 (talk) 06:17, 22 October 2013 (UTC)

See also here. Count Iblis (talk) 02:39, 22 October 2013 (UTC)

Hmmm... what if the paddles were made up of some sort of "molecular origami" that was held by some thin, strong, rod, but otherwise had substantial freedom to furl and unfurl in an arbitrary shape by computer command throughout the stroke? If you had the ability to force a succession of, say, cupped parallel planes through the water, might it work better than the screw shape? Wnt (talk) 05:15, 22 October 2013 (UTC)

Well, as I said above, a paddle wheel cannot be more efficient than a propellor, and any real design will be less efficient because with a paddle wheel the water flow is at right angles to the axis of rotation, whereas for a propellor the water flow is parallel to the axis of rotation. That allows profilling the blade width and pitch along the length of the blade, so over most of the blade, the motion imparted to the water is consistent in the desired direction. Yet shear at the tips is minimised by tapering the blades, reducing turbulence. Have a look at the photo of a modern propellor in the propellor wikipedia article. See how the blade pitch increases as you move to the centre. See how the blade width is taperred of to nothing at the blades tips. You can't do that with a paddle wheel - it has no geometical meaning.

Incidentally, while a paddle wheel can be improved by arranging the paddle blades to remain vertical, that still leaves us with excessive turbulence due to shear at the bottom of the blades. And increased shear at the sides. Again, why go for a complex mechanism when a simple propellor does the best possible job? Also, cupped shaped blades do not alter any of this.

60.230.213.251 (talk) 06:30, 22 October 2013 (UTC)

I don't think that one can say that either is more efficient, without fixing some parameters. One could fix the impeller area and total thrust, with the Reynolds number being very large. Under these conditions, the impeller that accelerates water over the largest area will generally be most efficient (least total kinetic energy imparted to the fluid per unit time for a given thrust). Since a paddle wheel only effectively accelerates the fluid over the area of a blade, but a propeller does so over the area that it sweeps out, the latter's advantage is kinda obvious. At a low Reynolds number (imagine treacle), viscous drag dominates and adds a significant energy loss mechanism, when this argument does not apply. — Quondum 03:14, 23 October 2013 (UTC)

Really? The value of computing the Reynolds Number is that it tells you whther or not the flow is going to be turbulent (high Reynolds) or laminar (low Renolds), just from the speed of the fluid past surfaces. You don't want turbulence, it is just a waste of energy. So, paddlewheels or propellors, you want to operate at a Reynolds Number below the transition value. Below that value things are pretty much linear with speed. 124.178.152.227 (talk) 03:31, 23 October 2013 (UTC)