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February 21

Black hole

what is meant by black hole?does it exists in the universe? — Preceding unsigned comment added by Sadhana savkare (talk • contribs) 03:32, 21 February 2014 (UTC)[reply]

You might try reading the Wikipedia article titled Black hole. --Jayron32 03:54, 21 February 2014 (UTC)[reply]

Here is a simple explanation:

Stars are huge - and they have immense amounts of gravity. The only thing that stops them from collapsing inwards is the pressure of all the light that they produce pushing back outwards and balancing the gravity forces.

When a star begins to run out of energy, the amount of light it produces reduces and sooner or later, it isn't enough light pressure to stop it from collapsing under it's own weight. A smaller star may collapse into a dense, heavy ball called a "neutron star" with all of the atoms crushed together very tightly. But a larger star may have so much gravity that nothing whatever can stop it from collapsing to a dot. Smaller than a period on this page - smaller than an atom...smaller than anything you could imagine - a point with literally zero size.

We know that gravity gets stronger the closer you get to the center of a star or a planet. So that tiny dot has all of the mass of the original star - but because it's so small, you can get much closer to the center of it...so close in fact that the gravity becomes strong enough to prevent anything from ever escaping from it. Even light can't escape once it gets closer than a certain distance from the center. We call this distance "the event horizon".

Because nothing falling into it can ever escape, it's called a "hole" - and because light can't escape, it's "black"...hence the name "black hole".

Black holes are amazingly dangerous - but thankfully, they aren't exactly common. But we know for sure that they definitely do exist - and that there is one at the center of our "Milky Way" galaxy. We suspect that every galaxy has a black hole at the center - and that many collapsed stars have formed black holes. They are impossible to see directly (because they are "black") - but you can tell that they are there by the way their gravity bends light from more distant objects - and from the fact that they suck in other stars, planets, dust and debris - and that all of that 'stuff' starts to glow as it falls faster and faster into the black hole.

Discussion of black holes gets very complicated - both space and time are bent by the black hole, extremely strange things happen when you get close to one of them, they are thought to slowly 'evaporate' because of quantum theory effects, there is a huge amount of very complicated science relating to them. They're also enthusiastically written about by science fiction authors, who often claim things about them that science doesn't always know to be true.

SteveBaker (talk) 16:26, 21 February 2014 (UTC)[reply]

It has been suggested^[who?] that a true black hole can never really exist, because the star's mass takes infinite time to cross its own event horizon. —Tamfang (talk) 21:46, 21 February 2014 (UTC)[reply]

That requires a very insular view that amounts to saying "us outside here will never see it, so it never happens". As a counterpoint, Roger Penrose has seriously proposed considering how the universe's evolution can be projected (conformally) to beyond an infinite amount of time, into later epochs. —Quondum 18:09, 23 February 2014 (UTC)[reply]

The view from everywhere except the island is insular? ;) —Tamfang (talk) 09:13, 24 February 2014 (UTC)[reply]

To understand how light is captured, there are two intersecting definitions of energy: E = ħν and E = mc². The speed of light is constant, so some notations define it as 1 and m = ħν giving photons of a given frequency the property of mass propertional to the frequency. From that, you can treat light as any other mass. Once light or any mass crosses the Schwarzschild radius, it cannot escape. The Schwarzschild radius can be calculated for any object but for non black holes, the mass of the object is outside the radius. For black holes, all the mass is contained within that radius. As an example, the sun would have to collapse all it's mass to a diameter of about 3 kilometers to be a black hole (conservation of angular momentum would still apply so the rotation and toroidal magnetic fields would be extremely intense). Blackholes are complex in how they absorb and radiate mass and energy (rotation, magnetic fields and quantum mechanics at the event horizon all are thought to play parts). Read the article as the off the top of my head memory may be dated. --DHeyward (talk) 06:26, 22 February 2014 (UTC)[reply]

Can a human have fleas or other biting parasites?

Like an animal? — Preceding unsigned comment added by 174.65.117.118 (talk) 11:33, 21 February 2014 (UTC)[reply]

Absolutely. See our long list of parasites of humans.--Shantavira|^{feed me} 11:45, 21 February 2014 (UTC)[reply]

Agreed. Both fleas and lice require hair, but humans still have enough for them. Interestingly, the lack of thick hair on some parts of our body has led a single species of lice splitting into two, head lice and pubic lice. The presence of those two species in archaeological sites even allows us to time when our human ancestors lost their thick body hair.

Other parasitic insects, like ticks, mosquitoes, and blackflies, don't require hair.StuRat (talk) 16:03, 21 February 2014 (UTC)[reply]

Why can't warm-blooded animals tolerate a wide range of body temperatures ?

At least some can't, like humans (there are some hummingbirds that seem to do much better). Warm-blooded animals evolved from cold-blooded animals, which did have the ability to tolerate a wide range of body-temperatures (although slowing down noticeably when they get cold). Humans, on the other hand, tend to die or slip into a coma at least, at such temperatures. So, how and why did we become so sensitive to changes in our internal temperature ? Does our ability to control our internal temp somehow cause this ? (The only theory I can come up with is that there is a large cost to being able to tolerate such wide ranging temperatures, and since that's less important in many warm-blooded animals, it's no longer worth the cost.) StuRat (talk) 16:11, 21 February 2014 (UTC)[reply]

It's a matter of tradeoffs. Many biochemical reactions, particularly those involving proteins, are extremely temperature-sensitive. A change of a few degrees can cause a tenfold change in reaction rates. Keeping the body temperature within bounds allows the system to be engineered without having to do a lot of difficult things to compensate for huge changes in metabolic reaction rates. Looie496 (talk) 16:42, 21 February 2014 (UTC)[reply]

Most chemical reactions are influenced by temperature (e.g. reaction rate constant), and some proteins are also temperature sensitive (e.g. denaturation (biochemistry)). Animals are ultimately just big bags of chemistry, so these things matter if the animal is going to exposed to a variety of temperatures. As different reactions can have different temperature dependencies, ensuring different chemical systems work together efficiently across a range of temperatures can be challenging. Ectothermic (i.e. cold-blooded) animals have to ensure that their internal chemistry stays in balance over the whole range of temperatures that the animal is expected to tolerate. As a result, their proteins and regulatory systems may emphasize temperature tolerance over efficiency of reaction. Endothermic (i.e. warm-blooded) animals, by contrast, have the ability to optimize their chemistry for a much narrower target temperature range. This allows them to adopt more efficient chemistry and regulation targeted to their body temperature, but with the trade-off that they will have trouble tolerating changes in their internal temperature. See also: thermoregulation, hypothermia, and hyperthermia. Dragons flight (talk) 17:12, 21 February 2014 (UTC)[reply]

The above comments are correct. An endothermic animal's normal body temperature is a compromise between chemical reactions going faster the higher the temperature, versus the tendency of higher temperature to cook (as mentioned, denature) sensitive proteins. Animals can survive lower body temperatures as long as tissues don't freeze or congeal and rupture, as long as the demand for oxygen doesn't outstrip what can be provided by a slowing heart. The diving reflex response to cold apparently shuts down the brain's demand for oxygen, which is why people can survive submerged in cold water for more than 10 minutes, while submersion in warm water for that time s inevitably fatal. μηδείς (talk) 22:35, 21 February 2014 (UTC)[reply]

Thanks all. StuRat (talk) 17:22, 24 February 2014 (UTC)[reply]

What's the Move the Earth article?

I can't seem to find the correct page for plans to move the Earth.

As Sol will kill us if we stay, we've got to move anyway, whatever the short term climate changes.

Some external links (but no Wiki-page I could find):

So move the Earth, I'd rather not get off. Hcobb (talk) 16:14, 21 February 2014 (UTC)[reply]

If we could literally move the earth, we could probably control the weather too, and then it would be no problem. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:07, 21 February 2014 (UTC)[reply]

Earth's orbit is stable, hence controllable. The weather is subject to Chaos theory and so is innately uncontrollable. Hcobb (talk) 19:33, 21 February 2014 (UTC)[reply]

If the current weather is uncontrollable, how would adding another variable improve that situation? ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:14, 22 February 2014 (UTC)[reply]

What an excellant idea my good friend. This is not a new problem as history tells us that Archimedes was quoted on the very same sublect...something to the effect: "Give me a place to stand, a lever long enough and a fulcrum. and I can move the Earth." Now, I can't help you at the moment with all of the details, but you will need to go about getting a very large Lever and a solid Fulcrum. Once you have these nessesities, I say that you will find yourself in a favorable position to take on the task at hand. Good show and have a safe adventure. Phineas J. Whoopee (talk) 19:50, 21 February 2014 (UTC)[reply]

The energy needed to move the Earth to a safe distance from the Sun's red giant phase would, I imagine, suffice to terraform more than one extrasolar planet. —Tamfang (talk) 21:48, 21 February 2014 (UTC)[reply]

This kind of thing reminds me of something Will Rogers said during World War I, that the way to get rid of German U-Boats is to boil the ocean. When asked how to do that, he said, hey, I'm just the idea man. It's up to engineers to come up with the details. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:16, 22 February 2014 (UTC)[reply]

To answer the original question: the answer to any question of the form "Why hasn't Wikipedia got an article on X" is some combination of "because nobody has written it" and "because the topic is not notable, i.e. there do not exist sufficient reliable sources to write such an article. If you spot a gap in Wikipedia's articles, and think that suitable sources exist, you are welcome to try to fill it, either by writing it yourself (see your first article) or by requesting an article. --ColinFine (talk) 11:55, 23 February 2014 (UTC)[reply]

Planetary engineering#Geoengineering has one sentence: "Future geoengineering projects may preserve the habitability of Earth through the sun's life cycle by moving the Earth to keep it constantly within the habitable zone.^[1]" PrimeHunter (talk) 00:16, 25 February 2014 (UTC)[reply]

^ "[1]",Moving the Earth out of harm's way.

Calvin cycle

In most textbooks discussing the Calvin cycle, the focus of discussion is explaining how the carbon from carbon dioxide eventually comes out as G3P. This is pretty much the same on the relevant Wikipedia articles. However, the amounts of oxygen and hydrogen entering and exiting the cycle are not very well discussed. I am wondering specifically where one of the oxygens in carbon dioxide goes (every CO₂ in produces a CH₂O, so one oxygen is unaccounted for), as well as where the hydrogens in the product come from (water from the hydrolysis the produces 3-phosphoglycerate? or hydrogens from NADPH?) Bramble claw x 16:33, 21 February 2014 (UTC)[reply]

Let's add a link to the relevant article for all readers to acquaint themselves with the topic: Calvin cycle. StuRat (talk) 00:53, 22 February 2014 (UTC)[reply]

As far as the oxygen, that's probably released into the air as O₂. Plants do give off oxygen, after all. StuRat (talk) 00:56, 22 February 2014 (UTC)[reply]

Track the structures of each intermediate carefully to at least figure out at which stage of the cycle the second oxygen is lost. Then look at that stage carefully (check links or discussion of the relevant enzyme, for example) to see what other substrates are involved or what other changes are occurring (see if something else is being oxidized, or a "total of a water molecule" is being lost, etc.). Our RuBisCO#Products article suggests that common textbook descriptions might omit the 3-Keto-2-carboxyarabinitol-1,5-bisphosphate intermediate (coming from the reaction of ribulose-1,5-bisphosphate altogether, which makes tracking atoms through the process difficult in those older sources. This [2] appears to include that intermediate (note that googling for the image of this chemical by name gives two contradictory diagrams of it, and that many of our basic biochem enzyme/chemical articles omit actual structures in favor of cartoon images!). DMacks (talk) 19:34, 22 February 2014 (UTC)[reply]

Incubation periods

Why is that the more deadly illnesses tend to have much longer incubation periods? For example, common colds, flus and mild infections normally only have incubation periods of a few days to a week or 2 at most. However, illnesses six as HIV have incubation periods of months or even years and also prion diseases can have incubation periods of decades. Why is this? — Preceding unsigned comment added by Clover345 (talk • contribs) 23:20, 21 February 2014 (UTC)[reply]

Speaking as a non-expert, if a deadly disease killed off its host quickly, it wouldn't have a good chance to spread. Clarityfiend (talk) 00:03, 22 February 2014 (UTC)[reply]

Also, diseases like HIV must first undermine your immune system before they can take hold, and that takes time. Cold viruses, on the other hand, seem to have a strategy of reproducing and spreading as much as possible before the immune response wipes them out. StuRat (talk) 00:50, 22 February 2014 (UTC)[reply]

ClarityFiend has the basic idea. See virulence, optimal virulence, and endemicism. Diseases that only have human vectors tend over time to become less virulent, but highly prevalent. Many people carry such diseases like the herpes viruses without ever developing, say, cold sores, genital herpes, or the shingles. Diseases that have animal vectors, like anthrax and rabies, tend to be endemic in their hosts, but very virulent when they, on occasion, cross over into humans. Since they are not transmitted person-to-person, killing the human host doesn't effectively lessen their long-term chance of survival in the wild. μηδείς (talk) 02:49, 22 February 2014 (UTC)[reply]

[1] "[1]",Moving the Earth out of harm's way.

[1]