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November 30

polywell

Can someone familiar with nuclear physics comment on whether polywell fusion reactors are actually feasible or not? To a laymen it sounds awfully lot like a free-energy device scam. There's been very little peer review on this since the research team is under a publishing embargo.Dncsky (talk) 00:56, 30 November 2012 (UTC)

It's not a free-energy device scam — fusion is real physics — but with all fusion programs there are three major hurdles: 1. Can it reach ignition? That is, can it generate more energy through the reactions than it takes to start the reactions? So far none of the methods pursued have done this, yet, with the exception of thermonuclear weapons. 2. Can it produce useful net energy? That is, can you get electricity out of it? This requires both considerable efficiency and also in some cases elaborate means of extracting the energy from the fusion reaction. 3. Can it produce economically viable net energy? That is, can a fusion plant be economically competitive with other fuels out there? Another big unknown. The rub is that as of yet, we're still trying to find a method that satisfies #1. NIF was supposed to do it; it hasn't so far and it's not clear that it ever will. ITER is supposed to do it, we'll see. Once someone has found a way to do #1, it should be relatively easy to figure out a way to scale it up for #2. As for #3, it really depends on technical constraints we don't know yet, as well as issues unrelated to fusion technology (e.g. the price of natural gas).

I'm not a fusion scientist but my read of the Polywell article suggests that the people working on it really think it will work, and that they have been able to convince other experts at funding agencies that it might work. I don't think it's a scam, but that doesn't mean it will actually work. The fusion quest is nothing if not a field of failed dreams. So far. --Mr.98 (talk) 03:25, 30 November 2012 (UTC)

I am familiar with the current state of fusion research and I support every cent behind the 10 billion spent on ITER. Regardless of whether it will eventually work that money still needs to be spent. I am asking because the only publication I can find on the polywell reactor [1] seems to have been discredited more than a decade ago[2][3]. Yet they are still receiving funding from the Navy. Dncsky (talk) 04:27, 30 November 2012 (UTC)

The "References" list in the article is quite long and includes lots of scientific publications. I guess I'm not understanding why you think there is not much information published on it. The article shows it going through many funding reviews, many publications, and so on. The Navy has given them a few million — not chump change, but not a huge amount by research standards. Again, I can't evaluate the technical merits, but I don't see anything here that's against the laws of physics. The question is just whether it'll work or not, and that's not an easy thing to answer usually without spending some money on it. --Mr.98 (talk) 14:08, 30 November 2012 (UTC)

The References list is quite long, but it doesn't include a whole lot of peer-reviewed publications; it's mostly an assortment of press clippings, contractor and company 'reports' of various types, and patents. Conceptually, 'polywell' devices are fairly well understood. Like their cousins, the Farnsworth fusors, they're able to confine and fuse small amounts of hot ions. In that respect, polywell devices are 'real' fusion devices that actually 'work'—unlike cold fusion setups, there's no question that fusor and polywell devices really do fuse hydrogen atoms. Also like the fusors, however, polywell devices aren't terribly efficient at it. The 'true believers' think it's possible to eventually engineer around the inefficiencies of the technique and create a workable energy source; most physicists remain (highly) skeptical.

The Navy has a history of being a sucker for pie-in-the-sky fusion research of all flavors; they're one of the few stable sources of funding for cold fusion research as well. TenOfAllTrades(talk) 20:24, 1 December 2012 (UTC)

What are the global\general\basic ingredients of sand "regular" earth?

(sorry for possible misterminology, i don't have basic knowledge in geology). thanks 79.182.153.70 (talk) 04:34, 30 November 2012 (UTC)

Sand regular earth? Are you talking about plain old Sand? As our article discusses, the chemical makeup of "sand" is highly variable, as it is defined based on its gross properties rather than its chemical composition. Our article gives some of the more common types of sand and what they are made of. Since you mentioned "earth", you might also be interested in soil. Someguy1221 (talk) 04:42, 30 November 2012 (UTC)

okey, it's becoming interesting. what do you mean by "it is defined based on its gross properties rather than its chemical composition". this sentence is very abstract to a layman like me. i ask what are the most basic and general and typical ingredients of sand (and soil) just like a little boy, a curious boy would ask about them, i rally am a total ignorant in this matter. you are doing an holy work by lifting me out of this mud of ignorance (nice example eh?). — Preceding unsigned comment added by 79.182.153.70 (talk) 06:59, 30 November 2012 (UTC)

In laymans terms, sand is pretty much any finely ground rock or mineral. If it looks like sand and it feels like sand, it's sand, no matter what it's really made of. In the same way wood is wood no matter what tree you cut it from. Someguy1221 (talk) 11:17, 30 November 2012 (UTC)

The most common material is quartz which would make a white sand. Also common are rock forming minerals such as feldspar and pyroxene. Other sand may be made from fragments of rock like basalt, or pieces of shell or coral. Black sand may contain ilmenite or particles or wood or charcoal or discoloured by iron sulfide. Graeme Bartlett (talk) 10:27, 30 November 2012 (UTC)

There is a nice table of the chemical composition of the Earth crust at Composition_of_the_Earth#Chemical_composition. The most abundant substances are silica, alumina, and lime, as far as I know all rock-forming materials (and hence possible sources for sand). --Stephan Schulz (talk) 12:27, 30 November 2012 (UTC)

Our article mentions quartz is common for sand in 'inland continental settings and non-tropical coastal settings'. It also mentions 'The bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material'. There is a bit more useful info, I strongly suggest the OP read it if they haven't already. Nil Einne (talk) 13:47, 30 November 2012 (UTC)

As noted, 'sand' can be from just about any mineral that is ground by erosion and accumulates in one spot. I strongly recommend a visit to Hawaii to examine their...sand. In addition to 'white' sand (predominantly quartz, most common) you can find black sand (Punalu'u Beach, principally basalt), green sand (Papakolea Beach, colored by olivine), and red sand (Kaihalulu Bay, rich in iron compounds). TenOfAllTrades(talk) 13:50, 30 November 2012 (UTC)

Are you sure of the first statement? Haiwaii would generally be consider a tropical region and most sand there is likely to be coastal, and as I mentioned above (perhaps with an EC) our article suggests silica or quartz is actually often not the predominate material in white sand in such settings. [4] [5] [6] seem to agree that silica or quartz sand is not common in Haiwaii. Nil Einne (talk) 14:15, 30 November 2012 (UTC)

How about extremely small pebbles between X and Y mm diameter? Might it also help to identify what sand is not? EG: clay & soil, both with significantly more organic and fine powder-like material?165.212.189.187 (talk) 14:54, 30 November 2012 (UTC)

If the questioner goes to his neighborhood building center or hardware store and asks for "sand" for his child's sandbox, he is likely to get "play sand." This product has a Material Safety data Sheet which says it is made of "crystalline silica," which seems to be another name for silicon dioxide. It has been washed to reduce the dust and dirt. The other type of sand he would find in the store is "all purpose sand," which is darker and contains more dust. It is used for making concrete. Edison (talk) 15:50, 30 November 2012 (UTC)

Without NASA we wouldn't have computers

Someone told me this, is it correct? ScienceApe (talk) 18:08, 30 November 2012 (UTC)

No, computers were already around before NASA was even founded. See e.g. the Atanasoff–Berry Computer and ENIAC. - Lindert (talk) 18:18, 30 November 2012 (UTC)

On the general topic, I have read that entire industries have come out of the space program, because sending rocket ships into orbit requires engineering designs that can vary by at most one part in 10 000, or something like that. I don't know what exactly we owe to the space program, although I know it isn't computers, and it isn't teflon. IBE (talk) 18:27, 30 November 2012 (UTC)

Orange Tang. SteveBaker (talk) 18:43, 30 November 2012 (UTC)

Certainly NASA didn't produce the first computer...who precisely did depends crucially on your definition of the word "computer"...Atanasoff–Berry Computer (circa 1942) is the most likely candidate...but there are reasons to say that it doesn't count. Arguably, NASA owned the first small "microcomputer". NASA paid IBM to build a 19" long computer (weighing in at 60lbs!) in May 1963. It was used by NASA on the Gemini program and had a fairly respectable 16k bytes of memory. However, there is always a problem with "Without A, we wouldn't have B" arguments. Clearly, IBM had the technology to build this thing - so if NASA hadn't paid them to do it, what is to say that six months or a year later, someone else wouldn't have? Since computers were already in fairly widespread use in 1963 - it was only a matter of time before someone else built a tiny one. There is no evidence whatever that NASA's machine was widely copied - or that it is somehow the progenitor of all computers that followed it - to the contrary, it was not much more than a dumb calculator with far fewer features than it's contemporaries. SteveBaker (talk) 18:43, 30 November 2012 (UTC)

It seems according to the Computer page the Z3 was first computer (obviously depending on your definition of computer) in 1941. Dja1979 (talk) 20:20, 30 November 2012 (UTC)

Here in the UK, everybody knows that Colossus was the first computer. I think we should have a List of computers claimed to be the first computer. Alansplodge (talk) 20:38, 30 November 2012 (UTC)

To which we could add Charles Babbage's Analytical Engine of 1837, "the first design for a general-purpose computer that could be described in modern terms as Turing-complete." Alansplodge (talk) 20:45, 30 November 2012 (UTC)

At the Computer History Museum in Mountain View, California, the exhibition hall is set up so that the first items you see are historical computation contraptions dating to pre-history; such artifacts as abacuses and cuneiform tabulations. The exhibition hall progresses forward through more advanced mathematical machines; Babbage engines; punch-cards and time-clocks (other intricate mechanical devices that could perform domain-specific computation); Curta peppermills; and finally, after you've gone through a whole row of historic inventions, you finally see the first of the electronic and electronic-digital-machines that start to resemble what we call a computer. Like any question of history, there is much room for debate and different perspective. You can navigate an online version of the "Revolution" - the first 2000 years of computing, and the Computer History Timeline. The museum used to be free and open to the public; but now charges an admission fee. Nimur (talk) 22:02, 30 November 2012 (UTC)

Colossus was the first programmable computer. We have History of computing hardware, but it could use some work. --Tango (talk) 01:11, 1 December 2012 (UTC)

A more plausible argument is that without nuclear weapons, we wouldn't have modern computers. See e.g. ENIAC, Project Whirlwind, SAGE... It's still an historical fallacy (there's no reason to think that the computer wouldn't have been developed and funded for other reasons, and the history of computing is nothing but an endless string backwards of priority arguments over what counts as the "first computer" anyway), but it's more on target than the NASA reference — nukes, their deployment, and attempts to defend against them were much more influential in the short and long terms. --Mr.98 (talk) 00:37, 1 December 2012 (UTC)

Actually it was Nazis, not NASA or nukes that set America on the road to computing. You had not only the big push to break Axis power codes, but also the IBM sales to help manage the final solution. So it was a win-win, of sorts. :-( Hcobb (talk) 00:42, 1 December 2012 (UTC)

America? Surely you jest. The British were the ones who did the hard work on Nazi cryptography, and the only ones (I believe) who built anything that looked like computers for it. The Americans worked on Japan's and later the Soviet's codes but they were not terrible consequential with regards to the Nazis (and I'm not too sure of the role of computing, per se, in American cryptological efforts). And while I think the topic of IBM during World War II is of great interest, I think saying that the Holocaust really advanced computing is a bit much. The Hollerith sorting machines, while useful, weren't really computers in the modern sense at all. (Neither, really, was Colossus, but it was a step along the way.) For American computing, nukes played a much bigger role than cryptography, initially. --Mr.98 (talk) 03:27, 1 December 2012 (UTC)

As has been pointed out before, Americans who read American books etc think that Americans invented the computer, as is understood in modern times by the term - i.e., a machine that can be programmed, at any time after commissioning, to do a multitude of unrelated things. And the British, who read British books, like to think that the British invented the computer, even though the code breaking apparatus and even the later university computing machines do not conform to this meaning. However, Conrad Zuse, a German, beat all of that, having a fully programmable computer in commercial use before any of that. Yep - the Germans were first. See http://en.wikipedia.org/wiki/Zuse. At the time British and American airplane manufactures were using "loft computers" (rooms full of dozens of junior engineers doing wing design and stress calculations manually), German airplane manufacturers were using a Zuse computer to do it. Be carefull about the claims about the British cracking the German enigma messages. British media have made much noise about it practically ever since, and good on them. But it was very specialised with little or no commercial value, and the Americans did important work too - they just kept good and quiet about it. Keit 121.221.37.153 (talk) 06:08, 1 December 2012 (UTC)

" Americans... just kept good and quiet about it" See U-571 (film). Alansplodge (talk) 12:28, 1 December 2012 (UTC)

I'd argue, without any primary sources, that advances in electronics related to the Vietnam War led to the curve of improving technology and lower prices that made digital electronics so available to all, including personal computers, cell phones, etc. Gzuckier (talk) 03:56, 3 December 2012 (UTC)

Sorry for jumping in late to the party, but here is an interesting retraction to an article [7] -- I don't know what the title of the original article was but I've heard the anecdote before that the Apollo guidance computer led to a large demand for early integrated circuits. 192.220.135.34 (talk) 00:51, 5 December 2012 (UTC)

blocking in experimental design

Resolved

I understand (more or less) what a block is in experimental design, but I don't get what the authors are talking about here when they say "In addition, although blocking subjects on initial interest in the target activity of course eliminated any between-groups differences in this variable,..." What is this "blocking subjects" referring to? In a block design, I thought the blocks were supposed to be arranged in advance, and furthermore, our article states "A nuisance factor is used as a blocking factor if every level of the primary factor occurs the same number of times with each level of the nuisance factor". This doesn't look like something you could arrange after the experiment. What's going on? IBE (talk) 18:23, 30 November 2012 (UTC)

I'm not sure what is confusing you. The article describes the blocking and the assignment of subjects to groups as arranged before the experiment, as far as I can see (p. 131, upper left). Looie496 (talk) 19:30, 30 November 2012 (UTC)

Perhaps it's the use of the word "subjects", meaning "people on whom we experiment" versus the more common meaning of "topics" ? StuRat (talk) 03:35, 1 December 2012 (UTC)

No, I had just missed the bit that Looie pointed out. I did read most of the article, but I would have skimmed over that bit, and my inexperience confused me. I should have at least done a ctrl-f, because the article is searchable. I was just thrown by something that seemed to come out of left field. Thanks for pointing it out. IBE (talk) 08:22, 2 December 2012 (UTC)

Electrical Properties of Tubes

What if instead of wires we were to use tubes (of copper, say for indoor wiring)? Would there be any advantages/disadvantages and are there any special electrical properties of such a configuration? Seems as if the current would travel on the outer surface mostly, but I'm just guessing, honestly. 66.87.126.32 (talk) 20:51, 30 November 2012 (UTC)

I believe this was done, in some places, specifically with the tube carrying either positive or negative, with a regular insulated wire inside carrying the opposite charge. There are several disadvantages, though:

1) Not flexible, so much harder to install and maintain, especially where bends are needed.

2) Takes up more space.

3) Requires more electrical insulation.

4) Since it's uncommon, people might not realize it's carrying current, and be electrocuted.

5) Access to the interior wires is more difficult.

Using the tube as ground/earth with both positive and negative insulated wires inside makes more sense, especially out-of-doors, where the tube provides additional protection from the environment, for the wires. The tube might also function as a structural support, say when using a flagpole as the ground/earth for a light placed on top, with wires running inside. StuRat (talk) 21:51, 30 November 2012 (UTC)

The one-conductor-inside-another layout is termed coaxial cable. As the article indicates, it's used mostly for RF signal transmission (where it has beneficial properties), although the configuration has been used for power in certain situations, though attempts to search for a good reference are swamped by mentions of coaxial power connectors. - By the way, the article Skin effect shows a three-wire-bundle high-voltage power line, mentioning that because of the skin effect, they're effectively one conductor, which is taking the tube-as-conductor idea one step further. -- 205.175.124.30 (talk) 22:20, 30 November 2012 (UTC)

Except that coax cable is flexible, and I think the OP means rigid pipes. StuRat (talk) 03:38, 1 December 2012 (UTC)

The OP asked about using tubes as conductors, saying "thinking most of the currrent flows on the outer surface", indicating he's heard about skin effect - where the magnetic field created by alternating current opposes the flow of current where the field is strongest, which is inside the conductor. However, for skin effect to the significant, the conductor diameter must be large enough, and/or the frequency must be large enough. In the design of high power radio transmitters, both factors apply and the use of tubing instead of solid wire is common. In the design of low and medium power electronics, another solution is used - litz wire. In the transmission of electrical power at 100's of megawatt levels, the diamter is large enough for skin effect and proximity effect (the magentic field from one conductor can aid or oppose the current in an adjacent conductor) to be significant and hollow conductors are used, as well as the grouped conductors mentioned by 205.175.124.30. Often, the conductors consist of a central steel tension member surround by copper strands. The steel supplies the mechanical strength without affecting the electrical properties too much. In domestic house wiring, the wire diameter is way too small for skin and proximity effects at the frequency used to be significant, and it's cheapest to just use solid wire or normal stranded wire. Keit 124.182.170.42 (talk) 01:18, 1 December 2012 (UTC)

Does the "magentic field" cause nearby objects to turn magenta ? :-) StuRat (talk) 03:42, 1 December 2012 (UTC) ~

One for you and one for me, Stu. You can get flexible pipe, and you can get rigid coax, which is often used in professional radio equipment. I agree though, that the OP didn't mean coax. Keit 121.221.37.153 (talk) 05:58, 1 December 2012 (UTC)

Very elucidating, thanks so much! 66.87.126.32 (talk) 01:38, 1 December 2012 (UTC)

Just adding that, in the UK, older 2-wire mains supply wiring to properties is being replaced with a "co-axial" cable where the "live" is on the inside, and the neutral is a stranded "tube" around the outside. This is for safety, of course. I don't know whether it is used in other countries. Dbfirs 08:30, 1 December 2012 (UTC)

In Australia, as in most other countries, you get two applications: 1) Fireproof MINS cabling - this has an active solid conductor in the centre, surrounded by mineral insulation, in turn surrounded by a metal tube. But the tube is not a neutral or return conductor - it is earthed, and only carries current under fault conditions. It's there because the mineral insulation must be surrounded by metal for mechanical reasons. It is only used, in buildings, where the supply of electricity must survive a fire. 2) Underground high voltage street cabling also is coaxial, with one coax for each of the three phases, with the outer "tube", termed the "screen" comprising of strands and functioning as the neutral/earth per Multiple Earth Nuetral practice. Here the coaxial construction is used for three reasons; a) there is no external magnetic or electric field, which is important with underground cables as a significant field could cause soil heating and be a hazard for humans and animals (or power loss/dissipation in the outer steel wire armouring if fitted, which it usually is), b) if the cable is cut by say a backhoe, the screen must be penetrated first, and will always be there to carry away the fault current and avoid high voltage on the backhoe. c) it shields the outer plastic sheath from electrical stress, and avoids a touch shock/tingle hazard on the outer plastic sheath via capacitance. Keit 60.230.222.186 (talk) 10:47, 1 December 2012 (UTC)

Okay, so consider a bare, tubular conductor in air connected in series with a power circuit. Does the majority of the current travel on the outer or inner surface? What if the conductor was dipped in an insulator? What would be the differences between using a DC and AC power supply? 66.87.127.92 (talk) 20:49, 1 December 2012 (UTC)

With DC, the current flows smoothly and continuously. With a tube conductor, the current does not flow on the outside surface, nor does it flow on the inside surface. DC flows evenly distributed throughout the thickness of the conductor, just as it flows with the even density throughout a conductor of any shape. To see why this is so, recall that any electrical conductor offers resistance to the flow of current. Take your tube conductor - you can consider it as consisting of any number of tightly fitting concentric thin tubes - all having the same electrical resistance per unit cross-sectional area, and each thin tube electrically in parallel with the other thin tubes.

With AC, two effects, known as skin effect and proximity effect come into effect. These effects depend on frequency. Frequency is the rate at which the current alternates from one direction to the other. The alternation sets up an alternating magnetic field that causes skin and proximity effect. The higher the rate of alternation, ie frequency, the faster the rate of change in magnetic field and thus the stronger the skin and proximity effects. (DC has zero rate of change, so zero skin and proximity effect). In a solid circular cross-sectional wire, the magnetic field opposes the flow of current, and the opposition is strongest where the magnetic field is strongest, which is in the centre of the conductor. So, at a frequency high enough, the current is minimal in the centre - this is skin effect. The higher the frequency, and the bigger the diameter of the wire, the greater the fraction of current forced to flow near the outer surface. If the frequency is very high, a tube works just as well, as any metal in the centre isn't doing anything - at extreme frequency, the current flows close to the outer surface of a tube.

At power main frequencies, skin effect is negligible at the wire sizes used in houses, and the current is evenly distributed throughout the conductor cross section just as it is with DC, regardless of whether the conductor is a solid wire, a tube, or some arbitrary shape.

At frequencies high enough for skin effect, if the are two parallel routed conductors, carrying current in opposite directions (as in one conductor carrying the return current from a load, the conductors insulated from each other), the magnetic field of one conductor will aid the current in the other conductor. Thus the current in each conductor tends to be concentrated in the part of the conductor cross section nearest the other conductor. This is called proximity effect. If two parallel routed conductors are both carrying current in the same direction, proximity effect will tend to force the current to flow in the part of the conductor cross section furtherest away from the other conductor.

As skin and proximity effects are driven by magnetic fields, the type of insulation around the conductors has no effect on them. However the presence of any ferromagnetic material and other parallel routed conductors (if earthed at both ends) such as steel conduit does affect skin and proximity effects.

Keit 124.182.167.109 (talk) 07:35, 2 December 2012 (UTC)

Okay, I think I understand now. Thanks again Keit for the excellent explanation. Cheers! 66.87.126.240 (talk) 14:48, 2 December 2012 (UTC)

When did the last visible (with naked eye) star appear in the sky?

Did Jesus looked at the same sky as us? Comploose (talk) 21:08, 30 November 2012 (UTC)

Pretty much, with a few exceptions. Some stars only become visible as a result of a nova or supernova. A new star being ignited might only very slowly become visible to us, as the dust clouds around it clear. The Earth's precession also makes different stars into the pole stars every so often (Polaris isn't always the North Star). There are also periodic comets which are only visible certain years, like Halley's Comet. See List of periodic comets. StuRat (talk) 21:18, 30 November 2012 (UTC)

Timeline_of_white_dwarfs,_neutron_stars,_and_supernovae is your friend :) Dr Dima (talk) 21:26, 30 November 2012 (UTC)

Proper motion also has an effect; a star might be nearer or further away, or in a slightly different place relative to others. But 2000 years is a pretty short timescale for such things; while there were differences, most of them were subtle. I don't think any significant naked-eye stars have appeared or disappeared in that time. AlexTiefling (talk) 21:28, 30 November 2012 (UTC)

They haven't appeared of disappeared, but some of them have moved noticeably. According to our article on Alpha Centauri:

"Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions.[63] For example, the bright star Arcturus (α Boo) in the constellation of Boötes showed an almost 0.5° difference in 1800 years,[64] as did the brightest star, Sirius, in Canis Major (α CMa).[65] Halley's positional comparison was Ptolemy's catalogue of stars contained in the Almagest[66] whose original data included portions from an earlier catalog by Hipparchos during the 1st century BCE"

Alpha Centauri itself has a much larger proper motion than Arcturus or Sirius, and moves by 1 degree per millenium. For reference, that's about the width of your thumb at arm's length, or twice the angular diameter of the Sun or Moon.

So in conclusion, Jesus' sky would have looked almost identical to ours, with the exception that the north pole would have been in between Polaris and Kochab instead of very close to Polaris (see http://en.wikipedia.org/wiki/File:Precession_N.gif). --140.180.249.151 (talk) 23:22, 30 November 2012 (UTC)

We're forgetting about something here:

The sky as seen by Jesus:

"Class 1: Excellent dark-sky site. The zodiacal light, gegenschein, and zodiacal band (S&T: October 2000, page 116) are all visible — the zodiacal light to a striking degree, and the zodiacal band spanning the entire sky. Even with direct vision, the galaxy M33 is an obvious naked-eye object. The Scorpius and Sagittarius region of the Milky Way casts obvious diffuse shadows on the ground. To the unaided eye the limiting magnitude is 7.6 to 8.0 (with effort); the presence of Jupiter or Venus in the sky seems to degrade dark adaptation. Airglow (a very faint, naturally occurring glow most evident within about 15° of the horizon) is readily apparent. With a 32-centimeter (12½-inch) scope, stars to magnitude 17.5 can be detected with effort, while a 50-cm (20-inch) instrument used with moderate magnification will reach 19th magnitude. If you are observing on a grass-covered field bordered by trees, your telescope, companions, and vehicle are almost totally invisible. This is an observer's Nirvana!"

The sky we see today:

"Class 9: Inner-city sky. The entire sky is brightly lit, even at the zenith. Many stars making up familiar constellation figures are invisible, and dim constellations such as Cancer and Pisces are not seen at all. Aside from perhaps the Pleiades, no Messier objects are visible to the unaided eye. The only celestial objects that really provide pleasing telescopic views are the Moon, the planets, and a few of the brightest star clusters (if you can find them). The naked-eye limiting magnitude is 4.0 or less."

Count Iblis (talk) 23:30, 30 November 2012 (UTC)

The sky as seen by Jesus. Merry Christmas! Thincat (talk) 12:21, 1 December 2012 (UTC)

Small angle formula

D = X · d / 206,265

What is the difference between D and d? They are both distances so can someone explain the difference to me in an easy way to understand?Pendragon5 (talk) 21:52, 30 November 2012 (UTC)

It appears to me that your formula is a special case of arc length of a circle, commonly denoted as $s=r\times \theta$; but you've got it in a form where the radius angle is presented in normalized units (your constant 206,265). I didn't recognize that constant off the top of my head, but it wouldn't surprise me if it's related to, e.g., a special case of angular resolution. And, lo and behold, it's a conversion constant to arc-seconds, when d is the radial distance to the target, and D is the linear size of the object, and X is measured in radians. 206265, a unit conversion factor I don't have any common use for.

So, in plain english: d is the distance to the object, and D is the size of the object. This case-sensitive notation is a little ugly. Perhaps it derives from an era when ink was much more expensive. Nimur (talk) 22:27, 30 November 2012 (UTC)

What do you mean by the size of the object? Its diameter? Its radius? Its mass? Or what?Pendragon5 (talk) 23:32, 30 November 2012 (UTC)

To be perfectly pedantic, it's none of those things. It's the cross-section, determined by the appropriate projection geometry of your optical system. For simple geometrical objects, like stars and planets that are spherical, this value is well-approximated by the diameter. Nimur (talk) 00:07, 1 December 2012 (UTC)

December 1

Magic bread or...?

Beginning at about 4:40 in this video, an illusionist does something that I'm pretty sure is impossible... At least, in all the years I've spent studying sleight of hand and the like, I've never seen anything that should enable one to do something like that. Any thoughts on how that's done? Evanh2008 ^{(talk|contribs)} 06:35, 1 December 2012 (UTC)

Looks like a camera trick to me. That is, they stop filming, put a small piece of bread in his hand and let him pull it out, then stop filming and put a larger piece of bread in his hand, etc. I think I can see the cuts. Also, that bread is just too large to have been up a sleeve, etc. This does, of course, mean that the "audience" were just shills, pretending to be impressed. (The trick camera work could also have involved hiding the full piece of bread behind a green screen, then digitally replacing the green screen with the background, but I think it's just jump cuts.) StuRat (talk) 08:48, 1 December 2012 (UTC)

BTW, does the person who posted it know his name is listed as "Yi Fart of Magic" ? :-) StuRat (talk) 08:51, 1 December 2012 (UTC)

I don't see any cuts. It looks like green screen to me, with actors for the "audience reaction". Red Act (talk) 15:48, 1 December 2012 (UTC)

Actually, there is a cut before the audience reaction. So the video could start with a green screen effect, followed by a real audience reaction to a simpler trick that produces a baguette. Red Act (talk) 15:55, 1 December 2012 (UTC)

• It's behind his back and he's pulling it around. Look how flexible it is. Looie496 (talk) 17:03, 1 December 2012 (UTC)

I watched a program once a while back showing how magicians use powerful retractable strings to exchange items from either up their sleeves or from behind their clothes. For the long bread loaf trick, he appears to be using one coming from under his very large shirt collar which becomes pulled upward by the bread, before it drops back down to his chest... perhaps due to the string pulling the large dough ball into his shirt after the bread was pulled out from underneath (the large loaf must be compressed within a smaller diameter tube to reduce its bulk and the tube might be sealed to maintain an increased air pressure within the bread, causing the bread to expand rapidly when removed, and in addition, the air pressure may help expedite its removal from the tube too). He would simply need to extract a difficult to see hook mechanism (behind the collar and partially visible at the 3:24 mark) and then uses his free hand's thumb to plant the hook firmly between his fingers behind the dough. Note also that when the collar is raised it is not folded in place like a typical collar, but it separates from the shirt when this happens, allowing the bread to remain hidden behind cloth from viewers on either side of him. It also appears that the croissant may have been undercooked and turned inside out through a small cut centered on its uppermost edge. He would either had to given it to the deli prior to the taping or switched their dough for the prop, perhaps during the cuts. -Modocc (talk) 23:40, 1 December 2012 (UTC)

I've studied magic for many years and while i certainly wouldn't claim to be an expert, I'm pretty sure there is a camera trick involved in the large baguette production, some magicians are definitely not against using such tricks. Vespine (talk) 05:55, 2 December 2012 (UTC)

Not to my eye or reasons.. The loaf of bread can easily be concealed once compressed and he just so happens to be wearing something that is poking out slightly from beneath his collar (and its not attached to the collar because its poking out from the shirt in later frames), so what is it? Had he used a camera trick, he would have no need to be wearing that unusually tailored shirt or any hardware beneath it. Why would the collar be lifted? He also may also being using string to swap the colored rubber bands and the use of retractable strings (or elastic bands) to remove or disappear objects (the dough in this case) is a common method of concealment. Furthermore, his various audience's reactions appear genuine to me and I don't see any hint of a green screen (I've seen these screens used countless of times and they tend to be noticeable for various reasons). That he is able to pull his tricks off as well as he does simply speaks highly of his skill. For these reasons, I don't believe he resorted to doing a cheap camera trick that just about anyone could do. But of course, I could also be wrong... :-) --Modocc (talk) 06:23, 2 December 2012 (UTC)

I don't see a large collar and a hook. I see a scarf (presumably used in another one of his tricks) and a microphone. And the bread would have to make a 90 degree bend to come out from under the scarf. I believe he just bumps the scarf with the large piece of bread. And, if green screening is done properly, it's not visible on the final video. StuRat (talk) 07:10, 2 December 2012 (UTC)

It is a highly flexible piece of bread, and a narrow tube that conceals it could help with bending it if its been pulled out to his hand (the hook itself, should one be used, could be smaller and less visible than the mic), but his hands may not be close enough to his shirt for that not to be noticeable. The bread seems to expand some, up until he breaks it in two (conformation bias on my part I'm sure). Plus, it would be to his advantage to be able to perform the trick on demand (for more gigs) than only with a green screen production. But that's what his videos are, and of course I was completely wrong about the shirt. Fair enough. My vision isn't what it used to be. -Modocc (talk) 08:23, 2 December 2012 (UTC)

The bread does seem to grow in size between cuts, which is one reason I suspect trick photography. StuRat (talk) 08:45, 2 December 2012 (UTC)

re: green screens, that's called the toupee fallacy, you've seen green screens only when they've been executed poorly, if they were done well you didn't notice them, so you think they are all poor. Also, reproducibility is not a factor for a magician doing a YouTube or television performance, I don't think ALL the tricks were rigged with camera tricks, but I do thing the baguette one in particular was. As for reactions, they are actors, they are paid to give genuine looking reactions. Have you seen this clip? Criss Angel is a far more known magician, in the west at least, and this clip was exposed as completely staged, he used actors, one of them an actress with a Hemicorporectomy and creative editing, that's it, no "magic" involved at all. I agree, it's cheap, but some magicians are not beneath using them. Vespine (talk) 08:46, 2 December 2012 (UTC)

Most seemingly impossible shots are green screen, thus it is likely that it is a well-executed green screen. This too is an inductive leap and I didn't say all green screen are poor. I was just pointing out that there was no validating evidence yet given showing that it is one (and lots of so-called "evidence" and testimony that it is a green screen), but I do agree that its probably a green screen. -Modocc (talk) 09:26, 2 December 2012 (UTC)

@StuRat Yes, trick photography is definitely possible. On the other hand, after its initial rapid inflation (think cosmology here, :-)) the air in the bread still had some residual pressure left within its pores to expand further, albeit more slowly. Modocc (talk) 10:07, 2 December 2012 (UTC)

I'm rather skeptical that instantly rising, edible bread in a tube actually exists. If it does, it should be sold to consumers, where it would be a party hit. StuRat (talk) 18:50, 2 December 2012 (UTC)

Seeing a clever illusionist live can be very impressive. Seeing a film or video of anyone seeming to be an illusionist just makes me sceptical. HiLo48 (talk) 00:04, 3 December 2012 (UTC)

I call "camera trick" (although it's more properly CGI rather than old school in-camera trickery). I think I can see the fuzzy reflection of the entire loaf in the chrome table-top in front of the magician...which would definitely indicate camera trickery. The fact that the camera 'jumps' forward and stops floating around like it was before just as he starts pulling on the bread seems significant. Minimizing camera motion makes graphics trickery much easier...to the point where it's actually easier to put the camera on a tripod and add in 'fake' camera sway after you've done the actual trickery parts of the editing. The very uniformly red background of the lower half of the shot would greatly help if one were doing "blue screen" kinds of computer graphics effects here. SteveBaker (talk) 14:14, 3 December 2012 (UTC)

The red background, the general setup (such as there being no audience is in the shot as he performs it) and the camera work is conducive to graphics trickery, thus I do agree that's almost certainly how it was done, even though I can only see what appears to be primarily his arm and hand in the reflection. In any case, the OP did ask if making the bread appear in one's hand as a magic trick, is possible, and I think it can be done if the bread is spongy and elastic, along with strategically placed cavities within it if necessary to allow it to be adequately compressed. -Modocc (talk) 16:54, 4 December 2012 (UTC)

How should "望闻问切" in Traditional Chinese medicine be translated into English?

Seemed this is not covered in the English article, and the Chinese article zh:中医诊断学 which this term redirects does not have an English correspondent--Inspector (talk) 13:41, 1 December 2012 (UTC)

You might want to delete this and repost it at the language desk instead. μηδείς (talk) 17:39, 1 December 2012 (UTC)

Google Translate renders it as "look and smell". Looie496 (talk) 17:49, 1 December 2012 (UTC)

This is an instance where I would not trust a machine-translation at face value. Reading many of our articles, I found a citation in our Medical Diagnosis article. These characters translate literally as "look, smell, ask, cut." I also found this phrase, commonly coincident with the other characters: 中医四法, which translates as "the four methods of medical diagnosis." Elsewhere, I have seen these enumerated: "looking, listening, questioning the patient, and checking the pulse." The exact methods vary from source to source. It is my opinion that "check the pulse" is a rather euphemistic translation, now that bloodletting has fallen out of favor, but as I read 切, I interpret "make a giant carving." I'm not fluent in any of these languages, though. It appears that these are general descriptions of "methodology," and not a description of an exact procedure. Sometimes that last one is called "cutting" or "incision." Now, if I could only find out which simplified Chinese characters represent germ theory of disease, I could work on bridging the cultural- and linguistic- barrier for the betterment of medical practice everywhere! In all seriousness, is there even such a concept in traditional Chinese medicine? I have not been able to find any reference to the idea - the fact - that for at least some ailments, we have actually ascertained the root-cause to a specific infectious microbe. Our article on traditional Chinese medicine discusses folklore conceptions of disease, but makes no mention of any attempt to consolidating that world-view with new factual evidence. Nimur (talk) 20:21, 1 December 2012 (UTC)

(*facepalm*) Guys/gals, the Chinese article tells you exactly what 望闻问切 means. That's probably why the OP linked to it--he understands the article, but doesn't know how to properly translate the name into English.

Quoting from the Chinese article:

In Traditional Chinese Medicine, the "four methods" is the collective name for basic methods of medical checkup, including "sight checkup", "listen/smell checkup", "asking checkup", "feeling checkup", hence 望闻问切. 望means observing the patient's bodily condition, including facial color, tongue, etc; 闻means listening to the patient talk, cough and breathe, and smelling his breath or body for abnormal odors; 问means asking the patient about his condition, disease history, etc; 切means using the hand to check the pulse or press the abdomen to see if anything is unusual. Through the "four methods", the checkup reveals all types of symptoms and characteristics, for the purpose of understanding the disease's origin, character, and relationship with the internal organs, to provide a sufficient basis for diagnosis.

切cannot possibly mean bloodletting, because that's not a diagnostic method, there's no evidence that anybody used bloodletting in TCM, and 切 has numerous meanings depending on context, and the one for "cut" has a different pronunciation than the one for "ascertain/correspond". That article on 切土 is Japanese, not Chinese, and as the article says, 切土 means cutting through a hill to build a flat path through it. I don't know what the corresponding word is in English, but again, 切 has a different pronunciation in this context than in 望闻问切. The Chinese term for the germ theory of disease is 疾病细菌学说 or 疾病细菌说 (literally "disease germ theory"). As for why TCM doesn't mention it, it's because the germ theory of disease is an 19th century European invention while TCM is an ancient pseudoscience. If TCM made any sense at all, it wouldn't be "alternative medicine" (read: utter bullshit); it would be Western medicine (西医). --140.180.249.151 (talk) 20:25, 2 December 2012 (UTC)

For the record, I'm well aware that Kanji characters can mean totally different things in Chinese or Japanese context. As I mentioned, my interpretation is colored by my prior experience; I recognized a kanji and knew it from somewhere else. Any fluent reader would make the same connection, consciously or otherwise. The words may be pronounced differently, but the language is structured such that these meanings have an implicit connection. How can we ignore that detail when translating? We have to make a tradeoff between clarity and correctness. As you are undoubtedly aware, it is very difficult to "correctly" translate Chinese to English without losing a lot of nuance and context. If you translate the phrase literally, you produce a disjointed sequence of English nouns. If you translate semantically, you lose the context specificity. In both cases, you lose the double-entendre impact of each ideogram. English has a concept of homonyms, I do not believe that concept captures the subtlety of a shared ideogram or a tonal shift. As proof, consider this: Lion-Eating Poet in the Stone Den. In its original form, it's a clever poem. In English, it's formless nonsense that doesn't really satisfy any poetic criteria. Chinese, as a language, is so structurally different from English that it is not possible to directly translate; much effort is needed to tease out the meaning; which is why (as I prefaced my earlier comment) a machine translation is often inappropriate. Thank you, 140.180.249,151, for your insights into the subject. Sorry that you feel the need to face-palm yourself. I do not agree that traditional medicine necessarily refers exclusively to that portion of folklore tradition that eschews factual evidence. Of course, much traditional medicine is clearly pseudoscience. But it seems to be an unfortunate hijacking of the terminology to define "traditional" medicine as "anything and everything that necessarily excludes real, factual knowledge." Nimur (talk) 05:18, 3 December 2012 (UTC)

Why in some countries, HIV\Hepatitis C tests are after 2 months and in others 3 months (of suspicious contact) ?

Why in some countries, HIV\Hepatitis C tests are after 2 months and in others 3 months (of suspicious contact) ? Thanks. 79.182.153.70 (talk) 22:32, 1 December 2012 (UTC)

Two or three months after what? Can you clarify and give some context for your question? RudolfRed (talk) 22:38, 1 December 2012 (UTC)

i did that (read the header again). — Preceding unsigned comment added by 79.182.153.70 (talk) 23:06, 1 December 2012 (UTC)

I think the short answer is that such decisions are driven by logistical considerations, after a scientifically-reasonable range of options has been established. For both HIV/AIDS and Hepatitis C, seroconversion occurs 1-2 months after onset of infection (i.e. an exposure resulting in infection). Examples of evidence for this are here and here, more specifically for HCV PMID. So, testing at 2-3 months will detect most seroconversions, though some have suggested testing at 6 months as well to avoid missing late seroconversions (this is exceedingly uncommon with current tests). -- Scray (talk) 05:38, 2 December 2012 (UTC)

More data for HIV PMID 20846033 and HCV PMID 22715213 and PMID 11264728. -- Scray (talk) 05:55, 2 December 2012 (UTC)

December 2

Garlic supplements

I take garlic supplements and usually buy whatever's on sale at the grocery store. I happened to notice that brand I bought last time, Sundown Naturals has 75 mgs of garlic. The brand I just bought today, Nature's Bounty has 1000 mgs of garlic. I realize that there's no established RDA for garlic, but I was surprised that the amounts are so wildly different. The lower dosage pill is bigger, too. I'm guessing that one of these companies is playing 'accounting games' with the amount of garlic and that one pill isn't really 13 times more potent than the other, but I don't know that. Can anyone shed some light on what's going on here? Thanks. — Preceding unsigned comment added by A Quest For Knowledge (talk • contribs) 01:26, 2 December 2012 (UTC)

The label for the Sundown product says it contains 400 mg of garlic extract, equivalent to 2000 mg of fresh garlic bulb. Looie496 (talk) 01:39, 2 December 2012 (UTC)

(edit conflict) Herbal supplement#Government regulations should provide some light; basically the entire world of herbal supplements is an impenetrable dark morass of unproven, untested, and unregulated products foisted on the public as "health". It's not harmful (maybe), so there's no reason you shouldn't be taking them, but otherwise, don't actually expect any real consistency or meaningful efficacy from over-the-counter herbal supplements, most of which are about as useful as snake oil. --Jayron32 01:41, 2 December 2012 (UTC)

@Looie496: Oh, that's interesting. The label has apparently changed since I bought it. A Quest For Knowledge (talk) 01:51, 2 December 2012 (UTC)

@Jayron32: I did a bit of research on garlic a couple months ago, and I don't think this was the article I read, but there appears to be some support for garlic and hypertension, Garlic 'remedy for hypertension'. A Quest For Knowledge (talk) 01:51, 2 December 2012 (UTC)

I am in no way doubting that there could be medically useful compounds in garlic. There very well may be. The issue is that the herbal supplement industry is essentially an unregulated industry with no set standards the way that actual medicines have. So yeah, the fact that there are compounds within garlic that reduce hypertension is to me unimportant to the discussion at hand. The question is whether pills in stores by random manufacturers which report some content of "garlic" are useful or consistent. They are different questions. --Jayron32 01:55, 2 December 2012 (UTC)

Plus, with all these vampire shows, like Twilight: Breaking Wind, I assume there's a plague of vampires right now, so having some garlic in your system to ward them off is a good idea. :-) StuRat (talk) 07:17, 2 December 2012 (UTC)

From a culinary point of view, eating garlic is much better than eating garlic pills. 2000mg is two grams, or about one clove - a homeopathic dosis, as far as my cooking is concerned. The natural unit of measurement is the bulb, not the clove ;-). --Stephan Schulz (talk) 08:52, 2 December 2012 (UTC)

There are some of us who hate garlic. And, even for those who love it, do you really want to smell like garlic ? StuRat (talk) 09:12, 2 December 2012 (UTC)

Well, the unenlightened need to become enlightened. And I'm not sure that "smell" is the proper term to use for the heavenly fragrance that garlic and its disciples contribute to the universe. More seriously, I've cooked some meals for people who claim to not like garlic, and they where quite happy with it. It always depends not only on "how much", but also on "how", with garlic added early and sautéed a bit being much easier on the stomach and more discrete on the taste buds than raw garlic. --Stephan Schulz (talk) 09:20, 2 December 2012 (UTC)

Those who love it have no problem with it, by definition. Garlic on the breath of someone who's eaten it in the past few hours is not unpleasant. In fact, it might be a damn sight better than it would otherwise be. It's only a problem on people who haven't showered for a day or more, and the weather is warm. Then, it comes out in their pores and mingles with their sweat and becomes pretty yucky. But a garlic eater who attends to their daily personal hygiene doesn't normally find their smell is a problem for others. -- Jack of Oz ^[Talk] 10:23, 3 December 2012 (UTC)

I want to know some intersest things about how metal found

Any kind of metal — Preceding unsigned comment added by Summeru (talk • contribs) 11:04, 2 December 2012 (UTC)

Iron can fall from the sky in the form of an iron meteorite. It can then be found lying on the ground. You may also be interested in the metal detector. Graeme Bartlett (talk) 12:04, 2 December 2012 (UTC)

In the spirit of providing interesting things about how metal is found, you might enjoy reading some excerpts from Roughing It, a narrative account by Mark Twain about his experiences as a silver miner in Nevada (and other excitement in the old West). Perhaps the author embellishes a little bit, but as far as I can tell, he presents an incredibly interesting (and quite potentially factual) account of ore extraction and processing. Chapter XXIX, Out Prospecting; Chapter XXXVI, A Quartz Mill and Ore Processing; Chapter LX, Pocket Mining, Placer Mining, and Mining Technicalities; and essentially everywhere else in the book, scattered bits of wisdom about silver and gold mining in the mid-nineteenth century. For an encyclopedic overview, we have articles on silver mining. Nimur (talk) 14:02, 2 December 2012 (UTC)

Maybe this is about discovery. (From our article on nickel): "In medieval Germany, a red mineral was found in the Erzgebirge (Ore Mountains) that resembled copper ore. However, when miners were unable to extract any copper from it, they blamed a mischievous sprite of German mythology, Nickel (similar to Old Nick), for besetting the copper. They called this ore Kupfernickel from the German Kupfer for copper. ... In 1751, Baron Axel Fredrik Cronstedt was trying to extract copper from kupfernickel—and instead produced a white metal that he named after the spirit that had given its name to the mineral, nickel."--Fuhghettaboutit (talk) 14:43, 2 December 2012 (UTC)

metal from Lithium and magnesium to cooper and silver and gold had been exist in earth crust and found by man for duration of some thousand years,we dont know clearly about first mining and how could man find minerals,but the usage of minerals and metal goes to age 8-9 thousand years ago. now we have instruments and high technology for mining ,the way of separating of mineral matters on earth crust refers to its molten core and diffraction of layers for duration of its life(4.6 billion years)--Akbarmohammadzade (talk) 15:03, 2 December 2012 (UTC)-

Akbarmohammadzade, I know you are trying to help, but if you want to answer a question in English, please follow some basic rules of the English language and of this page. It is really hard to read otherwise, and therefore not so useful to the original poster of the question.

1. The first word of a sentence should a capital letter. ("Now we have... ", not "now we have...".)

2. Proper nouns should also be capitalise (Earth crust, not earth crust).

3. Don't use automatically translated language, it is so wrong that we can't make sense of it.

4. Questions should end with a question mark.

5. Here we provide references (this is the reference desk), so the idea is that we point the user to specific pages within or outside wikipedia. So if you could link your answer to the relevant wikipedia page (for example did you mean molten core ?). --Lgriot (talk) 14:32, 3 December 2012 (UTC)

Photobiolysis

Do we have an article? Is there a missing redirect? Did I make a typo? See [here] for an explanation of what the term means. As of 13:32, 2 December 2012 (UTC), the title of the question is a redlink. Thanks, NorwegianBlue ^talk 13:32, 2 December 2012 (UTC)

Biohydrogen, Photohydrogen, and Biohydrogen reactor all seem to be related. To which, if any, "Photobiolysis" should redirect I'll leave to others more versed in the material. Deor (talk) 17:08, 2 December 2012 (UTC)

Thanks. I came across the word when trying to assist my daughter with homework; it appeared in a chapter about using hydrogen as fuel. It now occurred to me to try search for Biophotolysis instead of Photobiolysis. The term Photolysis is well established, and biophotolysis is a special case, driven by biological processes, so the latter term makes more sense than the former. And indeed, on google scholar, biophotolysis seemed to be the preferred term, and also occurs in our article Hydrogen production, so maybe that would be the best place to redirect biophotolysis. A bit confusing though, if a user gets redirected there, and searches for photobiolysis to locate what he's looking for. NorwegianBlue ^talk 20:56, 2 December 2012 (UTC)

what does burn in nature

we had discussion about matters does burn by fire . they said :only matters made of carbon does burning.what about others? --Akbarmohammadzade (talk) 14:41, 2 December 2012 (UTC)— Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 14:40, 2 December 2012 (UTC)

Many elements burn, including metals and hydrogen gas. Most pyrotechnics do not seem to have carbon. DMacks (talk) 17:02, 2 December 2012 (UTC)

If you can oxidate it, you can burn it. When you "burn" something you simply combine it with oxygen, you oxidize it. Magnesium burns, and it's not organic, for example. OsmanRF34 (talk) 20:11, 2 December 2012 (UTC)

And the oxidizer doesn't even have to be oxygen -- it can be hydrogen peroxide, any of the halogens, any nitrate or perchlorate compounds, etc., etc. In fact, alkali metals spontaneously combust in water, while the aforementioned magnesium burns very well in carbon dioxide... 24.23.196.85 (talk) 00:38, 3 December 2012 (UTC)

Could somebody explain what is meant by "the oxidiser can be a halogen" please? thankyou.124.191.177.160 (talk) 07:23, 3 December 2012 (UTC)

Oxygen's role in burning is to capture electrons from other atoms, to raise their oxidation state (see redox). But oxygen is not the only molecule that can fill this role. Many materials will also burn in a halogen atmosphere, such as chlorine, even when no oxygen is present. Someguy1221 (talk) 07:29, 3 December 2012 (UTC)

And of course I think most people would consider the sun to be "burning", but there's no oxygen anywhere near it, but that's a very different kind of burning then what we would see naturally on earth. Vespine (talk) 21:38, 2 December 2012 (UTC)

what do you thinking about this questions?

پرسش از چگونگی اندر کنش گرانش و فضا؟

گراویتون ها(ریزگانهای نامزد ترابری نیروی گرانش ،موجب خمیدگی فضا نمی شوند . حضور میدان در فضا با مبادله ذره تفاوت ماهوی دارد و این را اینشتین به خوبی درک کرده بود. ببینید شما روی صندلی نشسته اید و صندلی برروی سقف قرار دارد و سقف روی ستونها و ستونها روی قشری از پوسته زمین . مثال ساده فوق را اینطور تصور کنید : ذرات گراویتون از زمین (معلوم نیست از کجای زمین)به شما واجسام یادشده گسیل میشود . 1- چه عاملی باعث می شود زمین تشخیص دهد شما اینجائید تا ذره گسیل دارد؟ 2-چرا مواد و اشیا زیر شما بین شما و زمین سایه نمی اندازد؟(مانع برای رسیدن گراویتون) 3- چه چیزی به زمین می گوید شما چقدر جرم دارید تا همان اندازه گراویتون گسیل نماید. 4-گراویتون به هیچ و جه توجیهی برای افزایش شتاب به نسبت عکس مجذور ثانیه و شتاب یافتن ذره در میدان نمی آورد. 5-هیچ مبادله کوانتومی ذره استثنا از قانون پلانک و اصل طرد پائولی نیست. 6-ترازهای انرژی میدان جاذبه از اصل عدم قطعیت هایزنبرگ تبعیت نمی کند. 7-مبادله ذره قوانین کپلر و میدان جاذبه و حرکت در میدان جاذبه را نمی تواند توصیف کند. — Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 14:43, 2 December 2012 (UTC)

This is the English Wikipedia and so this question written in Persian is not well suited here. Moreover, from what I can make out from machine translation, this appears to be a homework question, which we will not answer for you because it robs you of the ability to learn yourself. I will post machine translation of this answer into Persian. I hope it's less garbled than the translation I was given of your text.ویکیپدیای انگلیسی است و بنابراین این سوال است از این بخش است که به زبان فارسی نوشته شده است و مناسب اینجا نیست.علاوه بر این، از آنچه که من می توانم از ترجمه ماشینی، این به نظر می رسد یک سوال برای مشق شب، که ما آن را نمی خواهد جواب را برای شما به دلیل آن را به شما محروم می سازد از توانایی خود را یاد بگیرند. من به ترجمه ماشینی از این پاسخ را به زبان فارسی ارسال کنید. من امیدوارم که آن را در کمتر از ترجمه من از متن خود را به او داده شد درهم است.--Fuhghettaboutit (talk) 14:52, 2 December 2012 (UTC)

Gravitons (gravity transport candidate, not the curvature of space. Field presence in space has an inherent difference between the particle and the Einstein exchanged well understood. If you see sitting on the chair and sits on the roof and on the roof of the cortical columns and pillars of the earth's crust. Imagine a super simple way: Graviton particles from the ground (not sure which part of the land) will be sent to you Ballistics above. 1 - You are here to determine what causes the particles are emitted? 2 - Why the materials and objects between you and the ground you would not have a shadow? (Barrier for gravitons) 3 - What is the land mass tells you how you can send the same size gravitons. 4- Graviton anything quite excuse for the acceleration inversely square s and acceleration of particles by the field 5 - No swap Pauli exclusion principle is a quantum particle exception of Planck's law. 6 - Energy balances the gravitational field does not obey the Heisenberg uncertainty principle. 7 - sharing particles of the gravitational field and Kepler's laws of motion in the gravitational field can not describe. is this translation clear enough ? — Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 15:12, 2 December 2012 (UTC)

your name is as difficult as my name,i cannot spell it. — Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 15:18, 2 December 2012 (UTC) If you want to justify graviton field must first be applied to the total impedance playing field with those particles like photons in space to justify. Each crime should be displaced between Dvtraz energy part of the gravitational energy is emitted. According to Maxwell's equations for the gravitational field Nakarast and radiation or certain wavelengths of emitted gravitational field graviton is not so controversial as the transferor remains Abtr field. The concept of gravity as the curvature of space electrodynamic equations of motion of a particle with space-time with virtual Chharbdy tensor is solved. The extent of the equations Bapkhsh just purely particle radius dependent impedance or density or mass or unresolved issue is resolved--Akbarmohammadzade (talk) 15:46, 2 December 2012 (UTC)

I don't understand what you are trying to say, but you don't seem to have asked a question. You have just made a series of statements. --Tango (talk) 02:06, 3 December 2012 (UTC)

difference between theory and law

we have Faraday or Kepler or thermodynamic laws . so we have several theories about differences between theory and low: A common misconception is that scientific theories are rudimentary ideas that will eventually graduate into scientific laws when enough data and evidence has been accumulated. A theory does not change into a scientific law with the accumulation of new or better evidence. A theory will always remain a theory; a law will always remain a law.

http://en.wikipedia.org/wiki/Scientific_theory

A law differs from a scientific theory in that it does not posit a mechanism or explanation of phenomena: it is merely a distillation of the results of repeated observation. As such, a law is limited in applicability to circumstances resembling those already observed, and is often found to be false when extrapolated.

http://en.wikipedia.org/wiki/Scientific_law--Akbarmohammadzade (talk) 14:49, 2 December 2012 (UTC)

What's your question? --Mr.98 (talk) 15:32, 2 December 2012 (UTC)

something about how we say newton formulas law? — Preceding unsigned comment added by Akbarmohammadzade (talk • contribs) 15:43, 2 December 2012 (UTC)

I don't understand your question. You might like to read Newton's laws of motion and Physical law.--Shantavira|^{feed me} 18:03, 2 December 2012 (UTC)

His question is: Why do we say that Newton's Laws are laws, rather than theories? Good question. My stab at it would be this: We observe through a very large number of observations that they are true. Therefore we take them as basic principles (laws) which can form the starting point of theories. For example, Newton's laws of motion says Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion. So one thing explaining another thing is a theory, while the one thing itself is an assumption (and the assumption is a law if there's a lot of solid evidence for it). My question: If a law is subsequently shown to be not perfectly true, do we say that it remains a law? For example, are Newton's laws considered to be untrue under relativistic conditions? Duoduoduo (talk) 17:15, 3 December 2012 (UTC)

Thanks,the relativistic condition solved newton's laws shortages but all scientific references tell law to newton formulas for their role in classic mechanics.--78.38.28.3 (talk) 05:51, 4 December 2012 (UTC)

Looking for research on Tetrahydrocannabinoids (THC)

use as appetite simulant in cats diagnosed with CHF vs: azotemia (cardiac vs: kidney) problems - common in some cats.--78.38.28.3 (talk) 05:54, 4 December 2012 (UTC) Tried the orexigenic route via Wiki without much success. We are currently using Mirtazapine per vet with mixed results.This guy is a lovable little furry person! — Preceding unsigned comment added by 99.146.148.92 (talk) 15:53, 2 December 2012 (UTC)

I think we probably ought to avoid giving advice here, even if it is for a cat rather than a human. Plus this is such a technical question that nobody except a vet could give useful advice anyway. Looie496 (talk) 16:27, 2 December 2012 (UTC)

Humans, being primates, have large livers meant to metabolize chemicals in plants. Carnivores do not. The popular press has a lot on the toxicity of marijuana to pets. I'd speak to your veterinarian, if I were you. μηδείς (talk) 16:45, 2 December 2012 (UTC)

Cats in particular are reputed to have livers which are not as good as detoxing things; phenols (pinesol, etc.) being a commonly cited example. Gzuckier (talk) 04:06, 3 December 2012 (UTC)

We are proscribed from providing medical advice (human and otherwise) here. This needs to be closed.Dncsky (talk) 17:42, 2 December 2012 (UTC)

Although it's obvious he wants to treat his cat, he has asked for research, which we can provide. μηδείς (talk) 17:56, 2 December 2012 (UTC)

Have you just tried more tempting foods ? I know my cat could never resist the oil from a can of tuna. (You can get the tuna packed in water, too, but that won't have as many calories for your cat.) Also, have you had the cat's mouth and throat checked ? A sore tooth or throat might make it avoid solid foods. StuRat (talk) 18:42, 2 December 2012 (UTC)

Synchronous belt as serpentine belts

I noticed that for most engines the serpentine belt is a multi-V belt. Is there any engine out there that uses synchronous belts as their serpentine belt? I can't see any reason they are not used other than the higher cost. But if cost is the only concern then some of the luxury and racing companies must've done it already. Google hasn't been helpful due to the synchronous belt/timing belt conflation.

Just to make it clear I'm not asking about timing belts; I'm asking about the belt that's used to transfer power from the crankshaft or camshaft to the various accessories. I'm wondering whether synchronous belts (commonly called "timing belts") have been used for this purpose.Dncsky (talk) 16:21, 2 December 2012 (UTC)

I'm sure some accessories have been driven off the camshaft drive. As an extreme example, Lotus built an Esprit for Mike Kimberley that had its PAS pump driven off the camshaft. I'd also take a punt that somebody has used an ancillary as a timing belt tensioner. Greglocock (talk) 22:18, 2 December 2012 (UTC)

Thanks, I'll look into that Esprit.Dncsky (talk) 22:28, 2 December 2012 (UTC)

Toothed belts offer lower friction power loss than V-belts - so toothed belts tend to be used where the power to be transmitted is higher. Ratbone 124.182.8.228 (talk) 00:06, 3 December 2012 (UTC)

Precisely. So I'm left wondering why doesn't all the high-end automakers use toothed belts instead of the basic V-belts. A thinner toothed belts can do the job of a wider V-belt, so it can help to shave a few grams off.Dncsky (talk) 02:04, 3 December 2012 (UTC)

Not only could you save weight, you could use a sync belt longer and transfer more power. If the belt is uniformly V-shaped (or multi-V for that matter), it seems to be compressed more on its way from the crankshaft and expanded by a greater amount on its way to the crankshaft than a similar sync belt. This does not only imply more physical stress due to the greater amount of expansion, but also due to abrasion if it slips and thermal stress because part of the deformation is converted into heat, which will age the belt.

And now to the really, really bad OR part (I still think it's correct...) All of the above is over-simplified, in that it assumes that a sync belt has uniform stress at "teeth" and "gaps". It's not the pull that counts, but the pull over cross-section, and this is not uniform in a sync belt. The gaps are the weak spots of the sync belt, and more precisely, the transition from teeth to gaps.

ASCII art of a sync belt (the numbers are for reference purposes only)

   ____      ____      ____      ____      ____   
__/    \____/    \____/    \____/    \____/    \__
01234567890123456789012345678901234567890123456789
--------------------------------------------------

The "12" and "78" parts of the belt are more fragile than the rest. If there is only a slight imperfection, it will grow under stress and snap.

On top of that, the teeth per revolution of either shaft must be an integer. Sync belts could still be of use for very large engines, where thermal losses could be an issue, but I think you could go the full nine yards there and use a chain, which is even more efficient.

This is OR too, but I'd guess that the efficiency of a chain would outweigh its weight penalty in these cases. - ¡Ouch! (^{hurt me} / _{more pain}) 07:19, 3 December 2012 (UTC)

Chains are not used for things such as driving the fan, as they would cost a lot more. A chain requires lubrication, which means it need to be enclosed. V-belts give an acceptable life without any lubrication. Ratbone 121.215.26.126 (talk) 08:41, 3 December 2012 (UTC)

Thanks for your input. I'm a little lost on your "must be an integer" point though. It was my impression that vehicle accessories did not need precise timing or speed; they must be able to function over a wide range of RPMs. Dncsky (talk) 14:52, 3 December 2012 (UTC)

Out of curiosity, might it be possible that a little bit of 'slip' in the serpentine belt is seen as a feature, and not a bug? When you turn the air conditioner on and off, for example, you're connecting a pretty substantial load. If the a/c clutch is a bit clunky then you'd be applying a nasty jerk to the rest of the peripherals – and directly to the crankshaft – if there wasn't room for just a little bit of slip on the pulleys. Worse, what if a bearing seizes suddenly on the power steering pump? With a sync belt or chain, that sudden stop is fed back instantly to everything that's connected, with potentially expensive consequences. Could the small efficiency penalty (2-5%) operating the parasitic loads be seen as a reasonable price to save wear and tear, and to protect all the rest of the components from a catastrophic failure of one? TenOfAllTrades(talk) 15:54, 3 December 2012 (UTC)

You cannot design cogs with, say, 23 degrees spacing. Either 24 (that is, 15 teeth per revolution) or 22.5 (16 per revolution), but nothing in-between. Thus, sync belts would introduce some constraints on the transmission ratios, and there would have to be many different belts for different shafts.

Ratbone: you're right, and these issues are less (in a percentage POV) with huge and/or expensive engines. If the engine is 10 times the weight, the added complexity (e.g. for lubrication) is not 10 times the cost. For small-scale components like the fan (which receives only a tiny percentage of the total mechanical power), it pays off to use a light-weight belt which doesn't rely on lubrication. For the primary power transfer (not "accessories"), the added efficiency of a chain, with the added benefit of lower heat buildup) could be significant. - ¡Ouch! (^{hurt me} / _{more pain}) 15:58, 3 December 2012 (UTC)

Using your example, if 23 degrees is the ideal number, but we're forced to work with 22.5 due to the teeth constrain, then the accessory will be driven 2.2% slower (0.5/22.5). But since the crankshaft goes anywhere from 500 to 5000 rpm, all the accessories must be able to handle a 10x input range as well. In that respect 2.2% is negligible. Dncsky (talk) 03:39, 4 December 2012 (UTC)

Fungus ID

I found this fungus after a couple of rainy days, in Southern California. Can anyone identify it? 69.111.189.155 (talk) 17:20, 2 December 2012 (UTC)

Possibly Sparassis crispa, known as the 'Caulifower fungus'? Mikenorton (talk) 17:33, 2 December 2012 (UTC)

Looks like its probably a species within that genus, but I dont think it's that particular species. The species in the picture looks more "curled-up" that S. crispa. I'll ask if someone at wikiproject fungi can have a look for us. douts (talk) 00:08, 3 December 2012 (UTC)

I don't think it's Sparassis crispa, but it might be a different species of Sparassis. I highly recommend posting at Mushroom Observer, there are many California residents who frequent the site and would probably be able to id it for you. Sasata (talk) 00:27, 3 December 2012 (UTC)

Actually, it doesn't look like a fungus at all, but rather like a Slime mold. Morphology of slime molds ranges widely and fantastically, as can be seen here: [[8]]. The specimen in question resembles the sixth picture from the bottom, which is, alas, not identified. Dominus Vobisdu (talk) 00:36, 3 December 2012 (UTC)

Here's a atlas with 18 pages of pictures of identified slime molds: [[9]]. If you click on a picture, and then click on the species name after the word "Title", you end up with a page full of pictures. Dominus Vobisdu (talk) 01:37, 3 December 2012 (UTC)

+1 slime mold. Especially the way it engulfs the blades of grass, rather than pushing them aside. de Bivort 03:14, 3 December 2012 (UTC)

Battery and voltage

Can you construct a battery with any combination of voltage and material? I know that certain materials have different electromagnetic forces when loaded, but you could always 'pile' them to get a higher voltage. And in the way down, you could just put a resistance to deliver a lower voltage than the output. Strangely, I am not seeing a lithium-ion battery in the form of AAA or AA battery on that market, although I would prefer it instead of alkaline AAA/AA batteries. OsmanRF34 (talk) 18:31, 2 December 2012 (UTC)

Googling "lithium-ion AAA" and "lithium-ion AA" gets me plenty of hits. They are certainly out there in the market. You don't see them more often because their voltage is too high. Lithium-ion chemistry provides 3.6V, much higher than the 1.5V expected in standard AAA and AA cells. The higher voltage might even damage the electronics. Your idea of adding a series resistance unfortunately won't work. Consider two loads, one 1MΩ load and a 1Ω load. Whatever series resistance you add to the battery will either make the current too high or too low.Dncsky (talk) 18:42, 2 December 2012 (UTC)

Yes, you are right that google gets you lots of hits, but they are by no means a replacement to the 'normal' AAA. OsmanRF34 (talk) 18:51, 2 December 2012 (UTC)

Like I explained, there's no easy way to step the voltage down. Putting a 3.6V battery in my TV remote will likely ruin it. These 3.6V batteries are for people who design their own electronics for that voltage, RC cars for example. Dncsky (talk) 19:00, 2 December 2012 (UTC)

And what's the point of making it round like an AAA battery? Couldn't they have used a flat cell-phone battery? OsmanRF34 (talk) 19:04, 2 December 2012 (UTC)

Allowing the use of AAA battery holders, I guess? I don't see any advantage in it myself. I would never buy these, since the risk of accidentally putting them in a $200 camera is too high. The vast majority of LiPo batteries are in the flat brick type. Google imaging "lipo pack" gets me the flat rectangular shapes for the first few pages.Dncsky (talk) 19:09, 2 December 2012 (UTC)

It seems as too much trouble just for keeping a minor piece. OsmanRF34 (talk) 19:28, 2 December 2012 (UTC)

They are usable in certain devices like some LED flashlights which can accept both types (although that isn't the primary reason they are made, the market is too small for that). Anyway lithium ion cylidrical cells of various sizes are commonly used in battery packs of various products like laptops, cordless drills, eletric cars, etc. The 18650 is I think the most common size but by no means the only one used. 10440 ('AAA' size) is often used in electronic cigarette. Note that many lithium ion chemistries including the most common lithium cobalt oxide are less flexible in manufacturing then the Lithium polymer battery chemistry so producing flat packs with them isn't so easy. (Anything which looks like a flat pack but has non polymer chemistry most likely just has a bunch of cylindrical cells.) AFAIK cylindrical cells are by far the most common production method for such non polymer chemistries. And when you aren't aiming for excellent packing like in phones, tablets and ultrabooks, the minor amount of wasted space isn't a big enough issue to be of concern. (There are other advantages of lithium polymer cells.) And lithium cobalt oxide still I think predominates among lithium ion chemistries (including polymer). So while lithium polymer batteries are rarely cylidrical, most lithium ion cells produced are likely cylidrical. While you can purchase individual cells included protected ones (with a small protection circuit on top), this is a market which developed for certain specialist hobbyist uses. The cylindrical cells aren't really intended for the end consumer, instead to be used in battery packs for devices or sometimes built in to the device. (And if you are using such lithium ion batteries, putting them in the wrong device and destroying the device should really only be a minor concern compared to the other precautions you have to take.) P.S. Generally speaking, it's best to avoid anyone who sells 'AA' or 'AAA' lithium ion cells. These would generally be called 14500 or 10440 by anyone who you should trust enough to buy such cells from. Nil Einne (talk) 20:18, 2 December 2012 (UTC)

If you want a voltage not available from a standard commercial battery, use a switch mode power converter. This is often done, in order to use an old battery powered device that was designed to use a battery no longer available - e.g., portable scientific instruments, multimeters, and gieger counters that were designed to use 22.5V and 45V batteries designed for 1940's and 1950's hearing aids and portable radios based on tubes instead of transistors. Such instruments were made up until the 1970's when the batteries were still available, and many are still perfectly serviceable. Because modern batteries have a much higher energy density, by using a physically smaller modern battery it is possible to fit both the battery and the switchmode converter in the space provided for the original battery. Keit 58.167.254.96 (talk) 00:02, 3 December 2012 (UTC)

Since nobody else has mentioned it, I'll point out Energizer Lithium-Iron batteries which retain the familiar carbon zinc (are they still made?) sizes, from AAA to 9 volt, I believe. [10]Gzuckier (talk) 07:01, 3 December 2012 (UTC)

As our Zinc–carbon battery article mentions, zinc-carbon batteries are still made and in fact are resonably common in some countries. Lithium-iron primary batteries are mentioned in our Lithium battery article, however the sizing issue isn't the key point. As the above discussion attests, lithium iron (rechargable) batteries are made in the same sizes. However they aren't suitable as replacement in most uses because the devices because the voltage range is quite different from what is provided by most primary cells in those sizes. (They aren't generally sold to consumers for these and other reasons.) Lithium iron batteries have a similar enough voltage range to those in common primary cells that they are generally compatible. Nil Einne (talk) 07:38, 3 December 2012 (UTC)

what are the specific enthalpies of fusion of the chocolate phase transition temperatures?

I think it will be weak compared to water, but I wonder if the phase transition energy can make up for the lack of an precise thermometer when melting chocolate (while preserving the beta crystals) for use in dessert simply by watching where the temperature rise slows down. 128.143.1.238 (talk) 20:15, 2 December 2012 (UTC)

Can you back up and tell us exactly what you're trying to do with chocolate ? If trying to melt it without burning, I recommend a double boiler. If you want a mixture of solid and liquid chocolate, toss some chocolate chips in the dessert, along with the melted chocolate. StuRat (talk) 20:17, 2 December 2012 (UTC)

I'm trying to melt chocolate without melting the beta crystals so I don't have to temper it. 128.143.1.238 (talk) 21:37, 2 December 2012 (UTC)

OK, here's an article that explains the issue: [11]. I'd simply use one of the products sold specifically for this purpose. That is, chocolate with additives so it will properly harden and form a shell, without the precise temperate control needed for 100% chocolate. (Do be sure to avoid trans-fats, though, as those are unhealthy.) One such product is Magic Shell. StuRat (talk) 22:02, 2 December 2012 (UTC)

Goggling on this question, I happened upon an answer: 93 kj/kg given on an engineering forum here. -Modocc (talk) 23:35, 2 December 2012 (UTC)

The birds and the bees

spam
The following discussion has been closed. Please do not modify it.
How is babby formed? Jessqueen99 (talk) 22:01, 2 December 2012 (UTC) See sexual reproduction, gestation, blastocyst, embryo, and fetus. StuRat (talk) 22:04, 2 December 2012 (UTC) There's also fertilization, zygote, implantation and gastrula, as well as several hundred other relevant concepts. μηδείς (talk) 02:10, 3 December 2012 (UTC) Not to mention babby.--Shantavira|feed me 09:13, 3 December 2012 (UTC) This is a popular internet meme, and should be deleted as spam. This is not an honest question, the OP is having a joke by posting this here. OP, don't do this. "How is a Babby Formed?"[[12]] — Preceding unsigned comment added by 217.158.236.14 (talk) 10:11, 3 December 2012 (UTC) Closed per IP 217's explanation. μηδείς (talk) 17:32, 3 December 2012 (UTC)

Placebo overdose

What happens when you overdose a placebo? OsmanRF34 (talk) 23:28, 2 December 2012 (UTC)

That could mean 2 different things:

1) An actual overdose, say of sugar pills. That wouldn't be terrible, unless they have diabetes, but would cause their blood sugar to spike.

2) Tell them they took too many. In this case, they could possibly will themselves into getting sick. StuRat (talk) 23:49, 2 December 2012 (UTC)

I mean 1). Under placebo I mean something without active components (that wouldn't be the case of sugar in people who have diabetes). Why wouldn't it be terrible? Placebo is something without active components, but it, for some mysterious reason, has a real effect in some people. Wouldn't a big dose of that have a bigger effect. OsmanRF34 (talk) 00:20, 3 December 2012 (UTC)

Not really, since a placebo, by definition, has no pharmalogical effect. All that occurs is a psychological effect, in other words, they think themselves into getting better so I wouldn't have thought that a bigger dose would result in a bigger effect. douts (talk) 00:25, 3 December 2012 (UTC)

I don't agree that placebos are just psychological. They have a real effect on your body, although the relationship between believing and physical effect is not well-known. OsmanRF34 (talk) 00:38, 3 December 2012 (UTC)

There have been studies showing that a larger dose of placebo has a greater effect (or, at least, the physical size of the pill does - that's all that is mentioned in our article, and I think my source is QI so that may be all they meant). I think they would need to know it was "an overdose" - obviously there is no such thing if it's just an inert substance, so it can't be a real overdose. If they did know, then a psychosomatic illness (or, more accurately, a somatoform disorder type illness) could easily result. --Tango (talk) 01:10, 3 December 2012 (UTC)

• Are you suggesting the subject has willfully "overdosed" on something he has be prescribed as a placebo? If so, are we to assume he hasn't looked on line to find out how much sucrose (or sawdust) he needs to take to kill himself? Can you be more specific? Thanks. μηδείς (talk) 02:05, 3 December 2012 (UTC)

It is quite possible to be prescribed a drug that has an active ingredient that works as a placebo, as the placebo effect is psychological. See also the nocebo effect. I don't feel it is possible for the lay person to know whether the drug is working because the brain thinks it should work, or whether there is an actual physical effect on the body. So be very careful. --TammyMoet (talk) 10:26, 3 December 2012 (UTC) See this report in the New Scientist which claims that some drugs work by amplifying the placebo effect. --TammyMoet (talk) 10:27, 3 December 2012 (UTC)

December 3

Chromosomes Question

I apologize if this is a stupid question, but are the chromosomes of every human being different? Also, in regards to the chromosomes in cancer cells, these chromosomes are always duplications or alterations of one or more of the (original) chromosomes of the human being where this cancer now resides, correct? Futurist110 (talk) 03:40, 3 December 2012 (UTC)

Since the genes of every human being are different, the chromosomes (the material that contains the DNA within it) must also be different. Wikipedia has a (fairly poor) article titled Genetics of cancer which has a little bit of information for your second question. --Jayron32 04:13, 3 December 2012 (UTC)

The one exception is identical twins, who, theoretically, could have identical DNA. However, even though they may start out identical (or nearly so), there are subtle changes in the DNA which occur throughout our lives, such as telomere shortening (based partly on the environment), causing twins to slowly diverge. In fact, your DNA isn't even completely identical from one cell to the next, due to oxidation damage, etc. StuRat (talk) 06:22, 3 December 2012 (UTC)

BTW, in case there is any confusion on the part of the OP, Genetic recombination in the form of Chromosomal crossover during Prophase I of meiosis means that the each chromosome is different, people do not inherit chromosomes wholesale from their parents. (Chromosome abnormalities as discussed below in some cases arise from problems occuring during crossing over.) Edit: So yes, those genetic differences reflect differences in chromosomes (genes which are close together in a chromosome will not be inherited independently hence genetic linkage). Probably should also mention independent assortment. See also Mendelian inheritance. (End edit.) Even sex chromosomes generally undergo limited crossing over (mentioned in our article on meiosis and discussed in more detail at Pseudoautosomal region). Nil Einne (talk) 18:44, 3 December 2012 (UTC)

Most humans have the same number of chromosomes in most of their somatic cells - 23 pairs, so 46 altogether. What differs from one human to another is the contents of those chromosomes - the genes. A small proportion of humans have a non-standard number of chromosomes, usually due to a chromosome abnormality such as Down Syndrome or Turner syndrome. Chromosome abnormalities such as aneuploidy (having an abnormal number of chromosomes) are also often found in cancer cells, although I am not sure whether they are considered a cause of the cancer or a side-effect. Gandalf61 (talk) 10:39, 3 December 2012 (UTC)

Are there any blood vessels in the skin? (Skin = Epi, Dermis, Hypo)

thanks. — Preceding unsigned comment added by 109.64.163.33 (talk) 04:06, 3 December 2012 (UTC)

See the Wikipedia article titled skin. --Jayron32 04:11, 3 December 2012 (UTC)

Capillaries, certainly, or you wouldn't bleed when you prick your skin. Unless you have spider veins or varicose veins, you probably don't have much larger blood vessels in your skin. StuRat (talk) 06:18, 3 December 2012 (UTC)

Bicameralism (psychology)

Is this idea accepted as plausible by psychologists, or almost scholars of ancient literature? It seems extraordinary to claim that people were not conscious 3000 years ago, yet I can't find much about how mainstream this idea is. --140.180.249.151 (talk) 06:44, 3 December 2012 (UTC)

"Is this idea accepted"? No. Is it mainstream? No. If it was there'd be more recent discussion of the question. The scholarly consensus is probably that human cognitive abilities have changed little over the last few millennia - but if it has, it may well have deteriorated, for environmental rather than genetic reasons. AndyTheGrump (talk) 07:05, 3 December 2012 (UTC)

Thanks for your answer. I didn't know what the most recent discussion of the question was, being completely unfamiliar with psychology. But I do think human cognitive abilities have improved drastically due to increased education, better living standards, and a more advanced world--this is reflected in the Flynn effect. --140.180.249.151 (talk) 18:22, 3 December 2012 (UTC)

Looks like total BS to me. Very little evolution occurs in 3000 years, certainly not enough to change the core nature of the human mind. StuRat (talk) 07:46, 3 December 2012 (UTC)

To my knowledge, Jaynes did not hypothesize that genetic evolution played any part in the breakdown of bicameralism. Rather, his thesis is more reminiscent of the evolution of memes, which was curiously proposed in the same year as bicameralism. Someguy1221 (talk) 09:52, 3 December 2012 (UTC)

This bicameralism theory is from 1976, the meme theory was popularized by Dawkins in a publication of the same year, but it's much older. OsmanRF34 (talk) 17:53, 3 December 2012 (UTC)

Binary Black Holes

This is a long one.

Let us assume that we have an observer O, a distant star S, and a Black Hole H between S and O. Thus the light going straight towards O will enter H; however, due to H's gravity, light can go around H in an arc. Within a plane, there are two solutions: one arc goes through X, and one goes through Y.

ASCII art:

. S'
.
.
.              X
.
.S             H             O
.
.              Y
.
.
. S"

So there will be two apparent stars S: at locations S' and S". In the 3D case, O will see a ring with H and the true S at its center (although they won't see H or S). S' and S" will be red-shifted equally in that case, and the amount of redshift will depend on the escape velocity of S and the celestial body O is on. Which can even be negative if, say, S is an asymptotic Red Giant and O is on Mercury (planet) deep within the Sun's gravity well. But whatever, S' and S" will have the same spectrum.

So far, so good. Now let us assume that H is not a single, but a binary Black Hole, rotating clockwise within the plane shown above. Both equal mass, centered at H. I'm interested in the spectra in that case.

My friend argued that because the Black Holes emit gravity waves, S" will have a lower redshift; "its" light passes through Y and hits the gravity wave head-on, which will compress the wave. OTOH, the light from S' will pass through X and move in the same direction as the gravity wave; there will be no collision at all, thus the full redshift will be observed.

I suspect that that's wrong, and that the true way of interaction is completely different; that light does not get compressed like some kind of spring. I have the feeling that S" would exhibit the highest redshift, because the gravity wave takes some momentum from the light when it passes through Y. S' would be the other way around; it would gain a comparable amount of momentum, and thus the wavelength will be the shortest.

Bonus question: Is X or Y closer to H? (I'd guess X, because light passing through X gains momentum, which helps escape the gravity well - thus the light which passes X can make the closer pass and still escape at all).

Is the effect significant? From what it looks like, it takes not only a binary Black Hole, but a quite massive pair, lest the orbit size be negligible to the distances H-X and H-Y. So I guess the pairs which do exist are unlikely to exhibit a significant asymmetry. - ¡Ouch! (^{hurt me} / _{more pain}) 10:13, 3 December 2012 (UTC)

You may get an effect from a massive spinning black hole using the Kerr metric. An orbiting pair would have s similar effect. However the light rays will have to pass fairly close. If a binary black hole merges into one a significant fraction of mass is converted to gravitational waves, and as they pass your observer O space can be shifted permanently. I expect that this will cause a temporary red or blue shift. Using gravitational waves observers also hope to see variations in pulsar timings. Graeme Bartlett (talk) 10:42, 3 December 2012 (UTC)

Thanks. Kerr metrics seem to support my point, at least as far as the light paths and the X, Y points are concerned. Particles gain or lose momentum as I predicted, but will that effect red/blueshift light? - ¡Ouch! (^{hurt me} / _{more pain}) 16:06, 3 December 2012 (UTC)

shifting of the frequency could happen if the observer or source are near the black hole(s) so that they are in the gravitational potential well, and then experience gravitational red shift. But it do not see that one side of the gravitational arc would be shifted differently to the other. The gravitational potential for emitter and observer should be roughly constant for the light traveling via different parts of the arc. Graeme Bartlett (talk) 20:22, 3 December 2012 (UTC)

Origin of Intelligence and Mental Illness Linked to Ancient Genetic Accident

Or so claims this article. The claim is so sweeping I find difficulty in even taking it seriously. What do you think? --Halcatalyst (talk) 12:32, 3 December 2012 (UTC)

It looks like they just reprinetd the University press release [13], which can often be pretty far off the mark. I don't have access to anything other than the abstract of the original paper [14], but that is the place to look for the real (although harder to read) version of the story. 209.131.76.183 (talk) 12:40, 3 December 2012 (UTC)

relationship between conductivity and mean free path

This is a very basic formula found in most textbooks, but I don't have my textbook and the absurdity of this simple relationship not being on Wikipedia is aggravating me. Where can I find it? 128.143.1.238 (talk) 13:44, 3 December 2012 (UTC)

Are you referring to thermal conductivity or electrical conductivity? douts (talk) 13:52, 3 December 2012 (UTC) Not sure if it's of any use, but a quick Google search found this paper [[15]] entitled "Calculation of the thermal conductiwity and phonon mean-free path" douts (talk) 13:57, 3 December 2012 (UTC)

Thermodynamics of a microwave

Why is there no heat or work transfer in a microwave heating something or a microwave being used for radiotherapy or sterilising? Thanks. — Preceding unsigned comment added by 138.253.210.27 (talk) 16:32, 3 December 2012 (UTC)

I'm not sure I understand what you're asking here. Heat transfer does occur in a microwave - the water in the food is heated by the microwaves, cooking the food. Microwaves generally aren't used for sterilising objects - normally either gamma rays or temperatures above 200 degrees centigrade are used. If I've mis-understood the question please let me know. douts (talk) 17:03, 3 December 2012 (UTC)

Less than 200 °C is enough, as it's the case of an autoclave. OsmanRF34 (talk) 17:30, 3 December 2012 (UTC)

About 125 °C in a saturated steam environment, a bit more (I've used 160-180ish before) in a dry environment. Fgf10 (talk) 18:37, 3 December 2012 (UTC)

Watertight compartments

• Ship floodability

Watertight bulkhead compartments were written of by the Song Dynasty Chinese author Zhu Yu, in his book Pingzhou Table Talks of 1119 AD (written from 1111 to 1117 AD). Watertight compartments were frequently implemented in Asian ships, and had been implemented in the warships of Kubla Khan.[2][3][4] Chinese sea-going junks often had 14 crosswalls, some of which could be flooded to increase stability or for the carriage of liquids.

Anyone can build wood ships with watertight compartments. However, without watertight doors, the compartments are not very useful. You probably have to install many trapdoors on the deck for each compartment so you can climb up to the deck to go to another trapdoor.

I really don't think it's easy to build watertight doors on a ship made by wood. How did Chinese inventors solve this problem? -- Toytoy (talk) 16:36, 3 December 2012 (UTC)

I don't know about the Chinese but pitch can be produced from plant material and has been used for many watertight application in the past. The introduction of the linked articles talks a little about it. Dauto (talk) 17:07, 3 December 2012 (UTC)

Don't know if this is the case, but could it not be that the space between the keel (if they had one) and the lowest deck was compartmentalised? So as to compartmentalise at least part to the buoyant volume. In that case the habitable decks wouldn't need watertight doors. Lower compartments could easily be sealed with something like pitch, as suggested above. Fgf10 (talk) 18:36, 3 December 2012 (UTC)

Also note that the watertight compartments don't need to be 100% waterproof, from the top. Say the boat is swamped by a rogue wave. You only need to keep it afloat long enough to bail out most of the water. If some water leaks into the watertight compartment in that time, it's OK, so long as the boat doesn't sink before you can bail it out. StuRat (talk) 18:42, 3 December 2012 (UTC)

Yes, with very primitive watertight compartments, you can still buy time to plug the hole and let the sailors bucket brigade the compartment dry. Without compartments, you're dead with the fish.

However, doors made of cheap wood can be very imprecise. They they can absorb moisture. Unless you pay big money for really good woods and treat the parts with tung oil, I don't think it was a good idea to build doors between compartments. -- Toytoy (talk) 04:18, 4 December 2012 (UTC)

Neutron decay and atomic weapons

So I was just watching this video, http://www.youtube.com/watch?v=o_EBqZPCZdw

And the guy in the video (thunderf00t) said that a neutronium bomb would be very destructive but wouldn't release its energy all at once, but rather over the course of 10-14 minutes because that's the time it takes neutrons to decay (10 minute half-life, 14-15 life-time).

Well in a fusion reaction, about 80% of the energy released is in the form of high energy neutrons, and that made me think of the tsar bomb. According to the article, the bomb was tested without a uranium 3rd stage so almost 97% of the energy released in the test came from fusion alone. But yet the explosion came all at once, instead of being spread out, continuously exploding for 10-14 minutes. I thought about this, and I reasoned that it must have been due to the lead tamper they used to absorb the neutrons. So I thought about the neutron bomb. If the tamper were made out of nickel or chromium, allowing the neutrons to escape, would the explosion be spread out over the course of 10-14 minutes like the video suggests would occur? ScienceApe (talk) 17:47, 3 December 2012 (UTC)

All neutrons will quickly lose their energy due to scattering off atomic nuclei in the air/water/soil. Then a significant part of the neutrons will be absorbed by some nuclei. The energy that will be released as by radioactive decay of the remaining neutrons is quite small. Ruslik_Zero 19:28, 3 December 2012 (UTC)

Note that even if neutrons did behave that way — that is, they went until the end of their "life" and then decayed to release their energy — their energy would be incredible diffuse. D-T fusion neutrons travel at a speed of 52,000 km/s; if they survived for 10 minutes they'd be well, well outside the radius of the planet by the time they decayed! It doesn't work that way, obviously. As for the tampers, they do matter with regards to what happens to the neutrons; neutron bombs are just hydrogen bombs where the fusion neutrons are given a direct route outside of the bomb without being use either for fissioning or for just thermal energy. They still all come out in one burst, and still matter for only a fraction of a microsecond. --Mr.98 (talk) 00:25, 4 December 2012 (UTC)

Why issue placebos to control groups for objective treatments?

Reading placebo effect, I saw "The placebo effect is highly variable in its magnitude and reliability and is typically strongest in measures of subjective symptoms (e.g., pain) and typically weak-to-nonexistent in objective measures of health points (e.g., blood pressure, infection clearance)." (Yes, I saw "citation needed", but is the statement really indefensible?). I wonder, if it's an all-but sure bet that sugar pills are *not* really going to be effective against a given cancer, why do testers still do controls with them? It seems about as likely for a control group taking sugar pills to see effective cancer treatment in any of the control group subjects as would be if the control group held water balloons. Is the only purpose in objective (not asking patients how they feel) placebo-using trials to have double-blind studies to keep the doctors honest and from tampering with administration protocols, such as giving more to patients they know are on the real thing so there's a better chance to have success and continued funding? 20.137.2.50 (talk) 18:01, 3 December 2012 (UTC)

The idea is to make the differences between control and test group as small as possible. The placebo effect is a complex thing, for instance, speaking to a doctor will often also have a beneficial effect. By making the control group as close as possible to the test group, you can rule out as much as possible any effects not due to the drug being tested. An other aspect is that without placebos it would be easy to determine for the trial doctors which group is control, and it's then easy to subconsciously treat both groups differently in for instance bed-side manner. It has been shown (bear with me while I hunt for a ref) that a drug given with a positive talk by the doctor will have a better effect than the same drug given with a disparaging talk. Just to rule out effects like that, we do double-blind placebo-controlled trails. Fgf10 (talk) 18:33, 3 December 2012 (UTC)

Blood pressure is something I'd expect to respond to placebos, since just relaxing can change it, and somebody thinking they just took some good meds might be more relaxed. Also note that many meds are approved even though they only have a slight benefit, like in 10% of the patients. So, even a tiny placebo effect could have a major effect on such results. StuRat (talk) 18:37, 3 December 2012 (UTC)

A sugar pill with a smile can rival an alpha blocker with a frown in terms of effectiveness? 20.137.2.50 (talk) 18:53, 3 December 2012 (UTC)

As others have stated, placebos can actually have an effect in patients even in cases of cancer. In fact the American Cancer Society has a long discussion of placebos [16] and while it doesn't mention cancer that much it's a good read if your understanding of the placebo effect is poor. [17] and [18] may discuss the placebo effect and cancer somewhat although I haven't actually checked the articles. As Fgf10 said, there are also other advantages to having a placebo, namely in preventing differences in the interaction between the patient and other people. In other words a placebo or something similar makes it possible to do a double blind which is generally considered the gold standard in medical research. However I would note that not all double blind trials are placebo controlled, as our Blind experiment and Clinical trial mention, it's also common to use existing medicines for the condition being treated. Nil Einne (talk) 20:16, 3 December 2012 (UTC)

I'll look at those sources promptly. But I'm impatiently curious; do those papers even hypothesize what could possibly be going on (in terms of chemistry) in any single case where someone taking a placebo sees a significant improvement in their cancer treatment? 67.163.109.173 (talk) 22:30, 3 December 2012 (UTC)

BTW the Declaration of Helsinki article which is linked from the clinical trial article goes in to a fair amount of detail about the controversy surrounding the usage of placebos when existing treatment exist. Nil Einne (talk) 20:44, 3 December 2012 (UTC)

In a case like that, I'd think using an existing med instead of a placebo would make sense. You would then compare the results to find out if the new med is better than the old med. StuRat (talk) 20:51, 3 December 2012 (UTC)

Yes definitely, when an effective treatment exists for a specific disease, the control group will most likely receive the current treatment, not just a placebo. You aren't necceserily trying to find out if a new treatment is just better then a placebo, you're trying to find out if it's better then the current treatment. Of course "better" might not just be efficacy, it could also be cost, side-effects, how invasive the treatment is, etc... But in most cases where a treatment already exists, it would be considered unetihical to withold it for the purposes of research. Vespine (talk) 21:58, 3 December 2012 (UTC)

As I mentioned in my post I was replying to with references to our articles, that is often done. But as the Helsinki article attests and Vespine mentioned even if it's generally regarded as unethical to withold an alternative existing treatment (although there is some dispute in cases when the consequences and risks aren't high), that doesn't stop it happening particularly in the developing world. (In case there's still some confusion that's the primary reason I brought the Helsinki article in to the mix after already mentioning that it's common to use an existing medication when one exists instead of a placebo.) Nil Einne (talk) 00:43, 4 December 2012 (UTC)

Global warming

Is there a way of identifying exactly whether a certain CO2 molecule was created by man or naturally made? Could this identify whether global warming is caused by man or occurring naturally? I've heard that Carbon isotopes can identify CO2 which has been produced by burning fossil fuels but is this an accurate test? Thanks. 138.253.210.27 (talk) 18:01, 3 December 2012 (UTC)

No. A particular CO₂ molecule will contain the carbon-12, carbon-13, or carbon-14 isotope, with the first 2 being stable, and carbon-14 being unstable with a half-life of around 5,730 years. So, if it contains carbon-14, the chances are that it's not a fossil fuel from the age of dinosaurs. However, the molecules containing carbon-12 or carbon-13 aren't necessarily "old", as only 1 in a trillion carbon atoms is carbon-14, even in new CO₂. But, if you look at a large sample, then the ratio of carbon-14 will tell you something about the average age of the carbon molecules.

But now for the complication: carbon-14 is being continually generated in the upper atmosphere: [19]. So, the carbon-14 we find up there might be generated in that way, from carbon from burnt fossil fuels, or it might be from wood somebody burned, or it might be from what a human or other animal exhaled. And, of course, the carbon-14 generated up there doesn't stay aloft, it moves throughout the biosphere. StuRat (talk) 18:19, 3 December 2012 (UTC)

(edit conflict) No, not by individual CO2 molecules. However, that doesn't mean that reliable correlations cannot be drawn to decide if the bulk of CO2 molecules is not from man-made sources. To take an analogy: Imagine you're watching a New York City street and every single person is an exact clone, with the exact clothes. Now, imagine you can only take snapshots of the street, so you don't see people moving, but you can count the number of people at any given time. You don't know where each individual person comes from, but you can correlate the number of people on the street with certain times of day, and you can also correlate other events that occur at those times, so you can say that the increase of people on a specific street at a specific time may be due to a train arriving at a nearby subway stop. You don't need to see the people get off the train to consider that a reasonable conclusion. You don't even need to know which specific people got off the train, the fact that there are more of them at the same time every day, immediately successive to the train arriving, is enough to establish the reasonable conclusion that some of those people got off the train. It's the same thing with CO2 increases. You don't need to know which CO2 molecules came from, say, a volcanic eruptions and which come from burning coal. We know that some increase in CO2 is going to come from burning fossil fuels, because it is patently obvious to anyone with a high school chemistry level knowledge of combustion that burning hydrocarbons makes CO2. We don't need to watch each CO2 molecule individually from the moment it formed and track each one to know that, in the bulk, burning more stuff makes CO2, any more than you need to track every pedestrian to know that the number of people on the street will increase after the train arrives. --Jayron32 18:25, 3 December 2012 (UTC)

• There's another aspect to this. As our article on isotopes of carbon points out, plants take up ¹²C in preference to ¹³C, and that preference carries over to fossil fuels. The main sources of atmospheric carbon are fossil fuel burning, respiration, and volcanoes. Volcano-derived carbon has a different isotope ratio than biologically generated carbon -- this ratio is known as δ¹³C. There is also a smaller difference between carbon derived from fossil fuels and carbon derived from recent respiration, because of differences in the atmosphere when the fossil fuels were created. Looie496 (talk) 18:49, 3 December 2012 (UTC)
• The effect of fossil fuel burning changing the isotopic ratio of carbon in the atmosphere is known as the Suess effect and was first described in 1955, when its influence on radiocarbon dating was noted. That said, we have many additional lines of evidence that the increase of CO₂ is anthropogenic. The simplest is basic chemistry. We (roughly) know how much fossil fuels we burn, and hence how much CO₂ we emit. We also can measure the amount of CO₂ in the atmosphere. The increase in atmospheric carbon dioxide corresponds to roughly half of what we release (much of the rest is absorbed by the oceans). We can also see a corresponding decrease in atmospheric oxygen. Note that the question of where the CO₂ comes from is different from the question of what its effect is. That said, the spectroscopic properties that give rise to the greenhouse effect have also been known for more than 100 years and the effect of adding carbon dioxide was first quantified by Svante Arrhenius around 1900. --Stephan Schulz (talk) 20:41, 3 December 2012 (UTC)

Travelling to star

Scenario: I pick a star 100 million ly away to travel to. And i can get there because i can live forever. the star is on the very ege of a galaxy 200,000 ly across. When i plug the coordinates of the star into my space craft, HAL, will i travel in a straight line and wind up possible 200,000 ly off my mark? or will i have to adjust my course as i get closer? or something else?165.212.189.187 (talk) 18:40, 3 December 2012 (UTC)

No, if you travel in a straight line at a star on the other side of the galaxy, when you get there the star will not be there, because the galaxy is in motion: it is moving relative to where it is now in several ways, it is rotating about an axis, and it is moving in a line away from other galaxies. You would need to aim for where the star will be when you plan to arrive, based on where the star is now and where it will move over the course of your trip. This is not trivial, because a) the star is not now where you see it (because light takes time to travel) and b) the star's location needs to be calculated taking into consideration a) your current motion b) the stars motion and c) the complete motion, including acceleration, deceleration, and top travel speed, of your spaceship. This is all hypothetically possible to calculate, so other than the tricky math, there's nothing wrong with calculating a straight line path that should make you arrive at the same point in spacetime as that star some place in the future. --Jayron32 18:52, 3 December 2012 (UTC)

And this is similar to planning trips within our own solar system, although the other planets are within a few minutes or hours of where they appear to be. If it's a round trip, that makes it even more complex. StuRat (talk) 19:26, 3 December 2012 (UTC)

Or you could just program your ship's computer to adjust your course every (insert period of time here), so that as you get closer, the error factor will be reduced with each course change. douts (talk) 19:03, 3 December 2012 (UTC)

You'll have to compensate continually for the near infinite number of disruptions from gravitational influences along the way. Fgf10 (talk) 19:21, 3 December 2012 (UTC)

Practically yes, but hypothetically, if you could keep track of the entire galaxy in a single computer, and reliably predict the entire gravitational field of the galaxy, as well as changes to that field over time, you could hypothetically have all those corrections made before the trip. That is, any corrections made to your trip would come from incomplete knowledge rather than anything which is physically impossible to know. Any corrections made along the course due to incomplete knowledge could be corrected before the trip if the information had been known ahead of time. --Jayron32 19:24, 3 December 2012 (UTC)

I don't think that's even theoretically possible. That is, storing the location and vector of every atom in the universe would require a computer made of more atoms than the universe. There's a thought experiment along these lines, but I forget the name. Of course, if your margin for error is large enough, like a solar system, no corrections may be needed. If, on the other hand, your goal is to dock with a spaceship in that solar system, then corrections will be required at some point. StuRat (talk) 19:31, 3 December 2012 (UTC)

Well, I suppose yes, you are correct. The question is when the corrections need to be made: constantly along the way, or only at the end. I think the analogy of the airliner is appropriate: Airplane pilots basically aim their plane at the airport and don't adjust much during the length of the trip: autopilot is capable of keeping the plane on the correct course, usually a fairly easy-to-calculate great circle course (the Earth equivalent of a straight line), and the pilots only need to be involved in the take off and landing portions. For most of the trip, the pilots don't do much except monitor the plane to make sure nothing goes wrong. For a properly working plane, 99% of the trip doesn't need any adjustments at all. Hypothetically, traveling to a distant star should involve a similar level of involvement: aim and fire, and then adjust the final destination when you get close. You aren't likely to end up ridiculously off course if the proper calculations are made ahead of time. --Jayron32 19:39, 3 December 2012 (UTC)

Are you sure that autopilot isn't making tiny corrections all along ? StuRat (talk) 19:51, 3 December 2012 (UTC)

At a more practical level, unless the target star has a circular orbit with precisely known velocity, making predictions will depend on the distribution of dark matter in the target galaxy, which we know only at poor resolution. Looie496 (talk) 19:34, 3 December 2012 (UTC)

Add to all this above that your target star maybe doesn't exist anymore or disappear during your journey. OsmanRF34 (talk) 22:02, 3 December 2012 (UTC)

So couldn't the galaxy also be moving not just directly away from us but also some other arbitrary direction? What do you mean "in Aline away from other galaxies" GeeBIGS (talk) 00:44, 4 December 2012 (UTC)

The expansion of the universe causing movement away from distant galaxies should be the dominant factor at that distance, but there would also be a smaller factor in another arbitrary direction. At shorter distances this factor is dominant. See for example Andromeda–Milky Way collision. PrimeHunter (talk) 01:22, 4 December 2012 (UTC)

No, gravitationally bound objects like galaxies do not expand due to metric expansion. -- Finlay McWalterჷTalk 01:32, 4 December 2012 (UTC)

The thought experiment could be expanded. If the person can live forever, they could travel to a star that is 50 billion light years away. Under that scenario metric expansion might come into play. Would that be correct? Would it still be possible to make almost all calculations before setting out on the journey? Bus stop (talk) 01:58, 4 December 2012 (UTC)

I don't think any stars (we know about) are that far away. Plus the star would be unlikely to exist when we got there. StuRat (talk) 02:30, 4 December 2012 (UTC)

[20] Bus stop (talk) 03:04, 4 December 2012 (UTC)

What I'm trying to get at is that in some cases the star is actually closer than the calculations predict because the arbitrary direction could be directly at us. Right?GeeBIGS (talk) 05:17, 4 December 2012 (UTC)

At shorted distances the arbitrary direction becomes dominant = the closer you get the more the arbitrary direction matters? So it seems like any path looks like a really long straight line with a sharp turn towards the end of the trip?GeeBIGS (talk) 05:30, 4 December 2012 (UTC)

Mars Curiosity findings

There are reports of interesting chemicals being found on Mars. However, they caution that it might be contamination from Earth. Why didn't they take care of that possibility before they sent it? They had it in a clean room, why didn't they clean the damn thing to preclude the possibility of what they find being from Earth? Bubba73 ^{You talkin' to me?} 23:48, 3 December 2012 (UTC)

I'm sure they tried, but it's quite difficult to keep it sterile. What if an adjustment needs to be made after it's cleaned ? Do you clean the whole thing again ? And do you store it in a vacuum until launch, to prevent contamination by the air ? StuRat (talk) 23:57, 3 December 2012 (UTC)

This episode of NPR's Science Friday talks about the degree to which a planetary probe is cleaned and sterilised. -- Finlay McWalterჷTalk 00:18, 4 December 2012 (UTC)

Thanks for the info and link. But if they are going to send it off not clean, and they have those doubts about their initial results, will they ever be able to have confidence in them? Bubba73 ^{You talkin' to me?} 00:53, 4 December 2012 (UTC)

I recall reading that initial scoops of Martian soil are discarded because of the possibility that they will be contaminated. If I recall correctly, the operation of the mechanism is designed to clean itself with the first few scoops of soil. After that it is not expected to be able to still contain contamination from Earth. But I am just recalling this from memory (faulty). I think I read it in Scientific American. Bus stop (talk) 02:05, 4 December 2012 (UTC)

Here it is: "But before Curiosity fired up its CheMin (Chemistry and Mineralogy) instrument to analyze the soil, it first had to purify its sample-collection instruments using Martian sand as a cleansing abrasive."[21] Bus stop (talk) 02:21, 4 December 2012 (UTC)

They go for 50ng/g of sample contamination. If anybody here has a clue how clean that is? To get lower than that you have to built a polymere free instrument only using ceramics and metals. No cables with isolation no electronic boards .... This is impossible. The cleanliness they have is amazing.--Stone (talk) 03:21, 4 December 2012 (UTC)

To put 50ng/g in perspective, the standard for the purest form of water calls for less than 100ng/g of solid contaminants.Dncsky (talk) 03:58, 4 December 2012 (UTC)

What i like best is the fact that curiosity find the same stuff Viking found in the 1970s. Klaus Biemann reported that they fond dichloromethne and chloromethane, but they argued that it was contamination from solvents used for cleaning. But now it looks more like the chloromethanes were produced by the reaction of organics wit the chlorine from the perchlorates.--Stone (talk) 03:25, 4 December 2012 (UTC)

December 4

How long do water molecules last (live?)

If a snowflake falls on a glacier, (3.5 x 10^19 molecules to a snowflake I read)...and we could follow one molecule when the snow melts, the water runs into a river, then to the ocean, and then at some point evaporates becoming clouds, and pushes onshore to fall as snow again...does that one water molecule exist throughout, changing states from solid to liquid to gas, around and around? Forever? Does something break up an H20 molecule at some point? Does a water molecule have a birth, life, death, or will some of them go around and around for years, ages, eons? How does it work? Thanks if you can help me understand and picture it. — Preceding unsigned comment added by 94.208.75.76 (talk) 03:10, 4 December 2012 (UTC)

Water molecules in liquid and solid change their specific atomic associations over time (exactly which two H are attached to exactly which O). The origin of "neutral water pH is 7" is that some aren't even associated as a 2:1 form at all. See self-ionization of water. DMacks (talk) 03:17, 4 December 2012 (UTC)

I am wondering though, at any point do H2O molecules cease to be H2O molecules? For instance are there any naturally occurring events or processes causing H2O molecules to cease to exist? Do any H2O molecules drift off as hydrogen atoms and oxygen atoms? Or do any hydrogen atoms and oxygen atoms combine with other atoms or molecules to form new substances? Conversely, are there any naturally occurring processes on Earth which create H2O? Bus stop (talk) 04:40, 4 December 2012 (UTC)

There are certainly biological processes which do both. Also, an acid plus a base produces a salt plus water, and that reaction happens in nature both inside and outside of organisms. StuRat (talk) 04:46, 4 December 2012 (UTC)

Also, I'd expect water ice to be more stable than either liquid water or water vapor. On Earth, water ice tends to only last for a few hundred thousand years, in Antarctica, say. However, liquid ice deep inside Pluto or beyond might have formed when the solar system formed, some 4.3 billion years ago, and never melted since (unless tidal forces from Charon (moon) heat it enough to melt the ice). In that case, those molecules should mostly be the same as they were when formed. StuRat (talk) 04:57, 4 December 2012 (UTC)

• In quantum mechanics, particles do not have individual identities that persist over time, and molecules, which are arrays of particles, don't either. When two water molecules come close enough together for their wave functions to overlap, it is impossible even in principle to say which is which afterward. So really the question is meaningless, and some of the responses above are too. Looie496 (talk) 05:50, 4 December 2012 (UTC)

If we consider an oxidanyl group to be the core of a water molecule, how long is the average lifespan above sea level an oxidanyl group with nothing else attached, besides more hydrogen? Plasmic Physics (talk) 06:05, 4 December 2012 (UTC)

Certainly we can calculate the rate of the reverse reaction of H2O --> 1/2 O2 + H2. It's vanishingly small, but finite at 300K ==> long but finite lifetime. John Riemann Soong (talk) 06:12, 4 December 2012 (UTC)

centripetal acceleration and linear acceleration, assuming "constant effort"

Linear plot of the "w1118" genotype

Logarithmic plot of the "fumin" genotype

Following literature concerning velocity and curvature, I decided to explore the power law among my fruit flies by taking their trajectory data and plotting log v against log R for each fly, where R is 1 over the calculated instantaneous curvature at a point. If v = K * R^b, the slope of this plot should be b and the intercept log K. Different genotypes (at least 4 different ones) appear to have per-fly average slopes ranging from 0.465 to 0.499, often with statistically significant differences between the averages: i.e. 95% confidence intervals are on the order of +/- ~0.008 whereas the difference between w1118 and fumin is ~0.03. To the right are two histograms of two genotypes, each a histogram of a collection of 60-75 flies over 4-5 experiments per genotype. One histogram has frequency colored linearly, and the other has frequency colored logarithmically-- I'm doing this to highlight different aspects of my analysis. Despite the differences, the values of the slope imply that b would be almost half, which would be consistent with centripetal acceleration being constant, since if v^2/R = K^2, where K is some constant, then v = K * R^0.5, consistent with the power law. It's the small deviations from this law that are interesting, since I think this is what separates different genotypes from each other.

The Spearman correlation (or even just the plain old Pearson r) is quite decent, on the order ~0.85. However, there is much overlap between the genotypes' distributions, though the means are statistically significantly different probably because of each genotype's sample size being on the order of n = 60-75. I was thinking that part of the spread in the plot could be due to the fact that centripetal acceleration isn't really constant and as linear acceleration will vary too, if we assume that the total "effort" would be nearly constant or fall within a tight range.

I'm actually surprised that my fit is that good, so I'm thinking, how do I get even tighter distributions that can distinguish between genotypes? The thing is, I can't think of what relation would constrain linear acceleration and centripetal acceleration together. I don't think it's really accurate to model linear acceleration + centripetal acceleration = constant, because I think the centripetal acceleration in an arbitrary non-circular path would include portions of linear acceleration. (And centripetal acceleration is a fictitious force and all that, so one might imagine it as a convenient way to talk about linear acceleration in a different reference frame.) Yet I don't think centripetal acceleration = linear acceleration either, even if we model an arbitrary path as the sum of tiny circular paths with ever changing radii (and directions) of curvature. (Ideally, each circular portion is infinitesimal in size, but we sample only at 15 Hz.)

The idea that effort is constant isn't too ridiculous, because the flies' activities are being driven by super-intense blue light (the wavelength they are most sensitive to) and if they rest, they only rest for a brief moment. You can see that to avoid taking the log of 0, and avoid the effects of noise at the same time, I've filtered all the data points where velocity dipped below 0.1 mm/s (that is, all points below log v = -1).

The basic power law relation I'm using predicts that velocity should be infinite for a straight-line path (infinite curvature), and that curvature should be infinite at zero velocity. Some literature offer a correction of R_eff = R / (1 + alpha*R), where infinite R would imply a finite R_eff of 1/alpha, so K*(1/alpha)^b would be the "max velocity". Curiously, I don't appear to have hit the upper limit of this relation, because the straight-line trend without the correction appears to carry to 90 mm/s (log v = ~2, the upper cutoff point, to avoid using data with random tracking errors, which are usually detectable because velocity is some ungodly amount for flies, like 360 mm/s), and using this correction generally makes the overall linear trend worse. In an arbitrary path model, would centripetal acceleration be basically linear acceleration even for seemingly straight line paths? How do I come up with a better "constant effort" model that would also account for the change in intercept (and in the transition zone, slope) at low velocities ? John Riemann Soong (talk) 05:54, 4 December 2012 (UTC)

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