# Wikipedia:Reference desk/Archives/Science/2013 November 30

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# November 30

## Density Matrix

How does one calculate the density matrix of a system? 150.203.188.53 (talk) 03:16, 30 November 2013 (UTC)

I think that question is too broad to have an answer other than what you'll find in the article. The density matrix is $\sum_i p_i | \psi_i \rangle \langle \psi_i |$. If it's the notation that's confusing you, $| \psi_i \rangle \langle \psi_i |$ is the outer product of the vector $\psi_i$ with itself. In more conventional column-vector notation it would be $\sum_i p_i \psi_i \psi_i^\dagger$, where † indicates the adjoint. -- BenRG (talk) 07:49, 30 November 2013 (UTC)
But how do you calculate the density matrix of a system from the data you have about the system? 150.203.188.53 (talk) 10:00, 30 November 2013 (UTC)
So, for example, how would you calculate the density matrix if you just knew some of the macroscopic thermodynamic quantities, like temperature? 150.203.188.53 (talk) 15:00, 30 November 2013 (UTC)
In that case it is the sum over n of exp(-E_n/(k T)) |n><n|

where the summation is over all the energy eigenstates of the system.

But how do you find the probabilities, in general, for the data you have? I.e. how would you calculate $p(|n\rangle|data)$? For example, $p(|n\rangle|T) \propto e^{-\frac{E_n}{kT}}$. And how would you find the energy eigenstates? Wouldn't you need to know the Hamiltonian? But the Hamiltonian is dependent on the number of particles, isn't it? And that's not always known, or constant (e.g. see Grand canonical ensemble) 150.203.188.53 (talk) 01:37, 1 December 2013 (UTC)

## Bird ID

What is the bird depicted here? It's tagged "bird of paradise" but doesn't look like any of those species. 69.111.73.99 (talk) 04:17, 30 November 2013 (UTC)

It is hard to believe that is a real image. This site says japanese paradise flycatcher and we have Japanese Paradise Flycatcher. μηδείς (talk) 14:33, 30 November 2013 (UTC)
The picture does look fakey (more like a taxidermy display than a snapshot of live birds), but that seems like the right bird. Thank you! 69.111.73.99 (talk) 17:46, 30 November 2013 (UTC)
I think you are right. The tail is too long, the log too clean, an altricial chick like that would not be outside the nest or running about flapping its wings. The background is uniformly out of focus. It's either stuffed, or photoshopped, or both. μηδείς (talk) 03:01, 2 December 2013 (UTC)

## Waste products

(a) Is there any living entity that has no waste products? If, as I suspect, the answer is No:

(b) Could the universe, taken as a single entity, be considered a counter-example? -- Jack of Oz [pleasantries] 09:18, 30 November 2013 (UTC)

This is more a question of the definition of "living entity". See Life#Definitions, where metabolism is listed as a (possible) essential characteristic of life, although there isn't a universally-agreed list of such characteristics. However, I don't think the universe as a whole could be considered "alive" - it doesn't reproduce (and therefore isn't visible to natural selection), and it doesn't maintain a local negative entropy gradient. Tevildo (talk) 10:53, 30 November 2013 (UTC)
I see. Thank you. -- Jack of Oz [pleasantries] 00:55, 1 December 2013 (UTC)
While living thing create waste products, from their own POV, those waste products can still be quite useful to other organisms. The honeydew produced by aphids and consumed by ants is a prime example. StuRat (talk) 01:36, 1 December 2013 (UTC)
Viruses might qualify as not creating any waste, in that they are just genetic code that inserts itself into the host cells' DNA. However, the host cell produces waste, and you could allocate some of that waste to the virus. Also, some may not consider viruses to be a life form. Similar considerations apply to prions. StuRat (talk) 01:43, 1 December 2013 (UTC)
• Plants don't really have waste products per se, since they can reuse the oxygen (and carbon dioxide) they produce. They give off waste heat and leave husks when they die, but this is not the same as producing feces. μηδείς (talk) 19:20, 1 December 2013 (UTC)
Leaf litter is much like a waste product - the plants can't reuse it directly and rely on other organisms to recycle it; it acts as a sort of fertilizer to improve the environment, which I suppose is marginally analogous to animal dung. To find something without a waste product you really have to have a sealed container of it growing/metabolizing/converting energy in the absence of any other species (including fungi), I think. Wnt (talk) 23:45, 1 December 2013 (UTC)
A semi-closed system with an influx of light and an outflow of heat can be fairly balanced. My grandfather had a large sealed bottle with insects and salamanders and a lot of ferns, moss and algae that was never opened as long as I remember. See Ecosphere. But entropy increases, and even Biosphere 2, with humans, needs influxes of nutrients rather quickly in its basically failed two-year mission. μηδείς (talk) 02:58, 2 December 2013 (UTC)

## FOOD DIGESTION

We eat three times a day but we pass stools once. Where the digested food accumulates? In the large intestine? Thank you.175.157.126.249 (talk) 09:37, 30 November 2013 (UTC)

Indeed it does. See Alimentary canal and Large intestine. Tevildo (talk) 10:48, 30 November 2013 (UTC)
The digested food doesn't accumulate - it slowly makes it way through the alimentary canal and the useful components are extracted along the way to be used by the body for many different processes. The waste products - the parts that can't be digested - accumulate in the large intestine until they are eliminated. See Digestion. Richerman (talk) 11:13, 30 November 2013 (UTC)
Regarding your "pass stools once", it is a quite interesting fact that human defecation rates vary enormously; anything from three times a day to three times a week is considered normal.[1]--Shantavira|feed me 14:14, 30 November 2013 (UTC)

## Phase-Change Material

Hello,

I would like to ask the author for this article about the Thermophysical properties of selected PCMs (the table in the article). How do I communicate with him?

Kind Regards Dudi [redacted] 79.176.222.17 (talk) 14:26, 30 November 2013 (UTC)

Wikipedia articles are built collaboratively. Phase-change material was started in 2005 and has had numerous contributors. All past versions of a Wikipedia page, along with the contributors, can be seen by clicking on the "view history" tab at the top of the page: here is the edit history of the page in question. You can communicate with any other user by editing their user talk page and leaving a message there, but if that user has not edited recently, you may not necessarily get a speedy reply. - Karenjc (talk) 23:13, 30 November 2013 (UTC)

## Cold effect on stomach

It is often said that people should eat less fatty foods when they have a cold as the stomach can be affected by a cold. But is the stomach directly affected by the cold virus or is it more an indirect effect, such as having less energy to digest food as well or irritation from medication or the stress of being ill? 82.40.46.182 (talk) 15:37, 30 November 2013 (UTC)

I think they are being a bit sloppy when they use the word "cold", and also including things with similar symptoms, like the flu, which do affect the digestive system. StuRat (talk) 01:31, 1 December 2013 (UTC)

## Swimming in water in space

How would swimming in a "pool" of water (or rather a room filled with water) in space differ from swimming in a regular pool of water on Earth? --CesarFelipe (talk) 17:03, 30 November 2013 (UTC)

If it was in freefall, you would stand a good chance of drowning since the water and air would become thoroughly mixed and the air "bubbles" would be too small to breathe.-- 18:08, 30 November 2013 (UTC)
If you were suitably equipped with breathing equipment and the room were complete filled, it would still be more like floating in space, but with more resistance. You wouldn't float or anything as there still is no up or down to cause the water to have increasing pressure at greater "depth". It would be of uniform pressure, whatever it might be. Mingmingla (talk) 19:14, 30 November 2013 (UTC)
And, if you rotate the space station, or just the pool, then it would be more like swimming on Earth, allowing for normal breathing, with the unique feature that you could have a cylinder of water, and swim continuously in one direction (a completely new type of endless pool). The rotation speed and diameter set the g-force, and whatever it was at the surface, it would be more at depth. The result is that pressure would increase more quickly as you dive, as compared to a static g-force environment. StuRat (talk) 01:27, 1 December 2013 (UTC)
Of slight relevance might be the Neutral Buoyancy Laboratory in Houston where astronauts train underwater for Extra-vehicular activities. HiLo48 (talk) 02:40, 1 December 2013 (UTC)
In weightless conditions, swimming underwater in scuba gear wouldn't be much different than in a swimming pool here on earth. But for swimming unaided on the surface, life would be chaotic and dangerous! For one thing, you'd no longer float preferentially on the surface of the water - you'd be equally likely to drift up into the air "above" the pool or to sink down into it...and of course the water wouldn't remain neatly separated from the air for very long - pretty soon the air and water would be all mixed up and breathing would become difficult at any depth. Getting your head sufficiently clear of the water to take a breath would be a scary, dangerous business. SteveBaker (talk) 05:48, 1 December 2013 (UTC)
(I'm just making this up, I wonder if it's true:) Hmmm, if you had sufficiently hydrophobic clothing - or an oily beard :) - maybe you could get away with it like a diving bell spider. Assuming the water was kept reasonably well churned up with the air by some means, you could have a layer near your body that attracts air, and exchanges it with the bubbles you encounter. Wnt (talk) 21:19, 1 December 2013 (UTC)

## Why do newborn chickens have yellow plumage?

It seems yellow is noticeable in the wild: among the green grass, the brown ground etc., so for predator it is easier to catch yellow chickens. It's like against evolution.--Lüboslóv Yęzýkin (talk) 21:08, 30 November 2013 (UTC)

Certain breeds might have mostly yellow plumage but most of the 8-10 breeds that I have raised have varied in color with quite a few combinations of black, brown, and grey. Dismas|(talk) 21:22, 30 November 2013 (UTC)
See, for instance, this image to the right. Dismas|(talk) 21:28, 30 November 2013 (UTC)
I don't know from chickens, except the packaged ones in the stores. Is the yellow color of (some) chicks a fictional stereotype, or is it only in certain breeds of domestic chickens? (Which would suggest it was maybe unintentionally a consequence of the artificial selection process used to develop a given breed.) ←Baseball Bugs What's up, Doc? carrots→ 22:32, 30 November 2013 (UTC)
Not sure about these ones.  Card Zero  (talk) 22:41, 30 November 2013 (UTC)
(ec)The traits of domesticated animals are influenced by selective breeding rather than the selective pressures of evolution. Many of the characteristics we select for, or those that come about as a by product of the traits we select for, would be a disadvantage in the wild. And yes, the chicks from some of the common breeds are bright yellow. However, as Dismas said, they also come in many other colours see: [2] Richerman (talk) 22:42, 30 November 2013 (UTC)
I am not an expert in chicken breeds but while living in the countryside during my childhood I saw predominately clear lemon yellow or pale yellow chickens. Only a few had dark plumage. When they raised to several weeks or so they became more camouflaged and reminded of adult birds.--Lüboslóv Yęzýkin (talk) 02:21, 1 December 2013 (UTC)
• The ancestors of chickens are thought to be the red junglefowl and grey junglefowl, which mainly inhabit environments where there are plenty of large plants to provide concealment for chicks. Looie496 (talk) 01:04, 1 December 2013 (UTC)
• As far as possible reasons, one might be so the mother hen can find them, as lost chicks are almost certain to die. And, if this trait was indeed selected for during breeding, that would both be so the mother and farmer can find them. It's also possible that this trait evolved all on it's own, in domestic chickens, since there the advantage of being visible to the mother hen outweighs the disadvantage of being visible to predators, since presumably domestic chicks are kept relatively safe from predators. Therefore, this keeps them from getting lost outside the chicken coop and freezing to death at night, etc. StuRat (talk) 01:20, 1 December 2013 (UTC)
• Seems legit. But as I remember local crows, magpies and similar birds were real disaster they could wipe out an entire chicken brood.--Lüboslóv Yęzýkin (talk) 02:21, 1 December 2013 (UTC)
• You also lead me to another thought. Many animals while domesticated lost their camouflage and became clear white, black, or of other simple colour. Though I don't know the exact mechanism of this.--Lüboslóv Yęzýkin (talk) 02:26, 1 December 2013 (UTC)
• The yellow of those chicks is a rather drab shade, probably even moreso to colorblind mammalian predators. Note also they are countershaded, dark on top, and lighter on the bottom. μηδείς (talk) 02:00, 1 December 2013 (UTC)
Many decades ago at the Royal Melbourne Show one of the gimmicky exhibits was green and pink and blue plumaged chickens, apparently created by injecting dye into the eggs before hatching. HiLo48 (talk) 02:36, 1 December 2013 (UTC)
This and the sale of died bunnies is common in the US where legal at Easter. Fortunately, that is an American holiday not celebrated overseas. μηδείς (talk) 01:08, 2 December 2013 (UTC)
Typically those critters were dyed before the holiday, and too often died after it. ←Baseball Bugs What's up, Doc? carrots→ 02:28, 2 December 2013 (UTC)
Sheeit. μηδείς (talk) 02:49, 2 December 2013 (UTC)
Easter is certainly a holiday in the UK. Good Friday and Easter Monday are public holidays and Easter Sunday is one of only two days of the year - the other is Christmas Day - that big shops have to stay closed. However, I have never, ever heard of dyed animals being sold before and I hope I never will again. Richerman (talk) 23:08, 2 December 2013 (UTC)
I assume that must be due to all the US GI's "over there" during WWII, popularizing the holiday in Britain. In the US Easter was started by FDR as a way to stimulate the chicken and rabbit-meat industries. Oddly enough, Dingus day is not a public holiday in the States, possibly due to its sexist nature. μηδείς (talk) 23:24, 2 December 2013 (UTC)
Er, no it's because it was always considered the most important time in the Christian calendar and some of the Easter traditions such as egg rolling and egg decorating date back to pagan times. I don't think there was much time for holidays in Britain during WWII. Richerman (talk) 23:47, 2 December 2013 (UTC)
Do be aware we Americans don't call a week off from work a holiday--that's vacation. Like Christmas Vacation. (During the War, American vacations were postponed by law, with interest. That's why the entire Baby Boomer generation of the 50's and 60's was conceived between Dec 1, 1945 and March 1, 1946, during the three month vacation after demobilization was complete.) Rather, holidays are Christian holy days, like Thanksgiving, established by New Testament tradition. μηδείς (talk) 01:38, 3 December 2013 (UTC)
When Thanksgiving was invented, the New Testament was already 1,524 years old. Also, there is a strong Biblical tradition of the Valentine's Day and July 4th dating back to the Apostles. 12.196.0.56 (talk) 17:04, 5 December 2013 (UTC)
"Injecting dye into the eggs before hatching" to get green and pink chickens yet doesn't date that much back. Thus in any case of no relevance to the argument of "colourblind predators". --84.137.115.34 (talk) 10:38, 8 December 2013 (UTC)
Mimikry comprises looking like something else too, like looking like all the yellow flowers or fruits around, or like a dead yellowish autumn leaf, (or like neonyellow plastic-bottle or tin-can, nowadays) not necessarily look like and vanish in front of ground or grass. --217.84.69.206 (talk) 19:15, 6 December 2013 (UTC)