Wikipedia:Reference desk/Archives/Science/2007 December 12

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December 12

Amount of ohms and stereo speakers

Hello, I'm getting a new stereo and the speakers are 6 ohms (it's 400w, 200 per channel, 6 1/2 inch sub, 3-way bass reflex). A pretty smart guy told me that less ohms means less resistance and better response. I have some pretty small speakers here that are 3.2 ohm, how can they have better response than these high quality 6 ohm ones I'm getting? It seems like bigger speakers have more ohms, then how could my crappy little Audiovox speakers out-respond some top of the line ones with 6 ohms? Is more ohms bad? NIRVANA2764 (talk) 00:40, 12 December 2007 (UTC)

Ohms has no effect on sound or response. The only thing the ohms rating is for is to match the impedance. If you connect your 3.2-ohm speakers to an amplifier that wants 6-ohm speakers, you will probably blow out the amplifier. You can connect speakers with an ohms rating higher than 6 to an amplifier that wants 6 ohms, but the higher the ohms rating of the speakers, the less sound you'll get. This is for amps that don't run on vacuum tubes; that's a horse of a different color. By the way, a good rule of thumb is to get speakers with a power rating double that of the amp. Another good rule is to put most of your money into the speakers, and better speaker simply sound better—compare by ear. --Milkbreath (talk) 01:21, 12 December 2007 (UTC)

According to the impedance matching article, stereo amplifiers actually use impedance bridging (which is news to me). Also, I've used speakers with lower than recommended impedance before (on old equipment, so I wasn't that worried about damaging the equipment), and it worked OK. I don't remember if I've ever used speakers with half of the recommended impedance before, but I may have plugged two speakers into one speaker output at one time, and wouldn't this cut the total impedance in half? It seemed to work, but I wouldn't try it if you're not willing to fry your amplifier.

Also, if I recall correctly, most home audio equipment tends to be 8 ohms. I only remember seeing 6 ohms on smaller "shelf systems". Maybe larger speakers have higher impedance, in general. Philbert^2.71828 02:26, 12 December 2007 (UTC)

Speaker impedance used to matter back in the days of vacuum tube amplifiers, but it's practically meaningless nowadays. Nowadays, impedance only matters in the following two ways:

1. A given amp can drive more power into speakers with a lower impedance than speakers with a higher impedance. But a 200W amp can probably go plenty loud with speakers of any reasonable impedance.
2. There's usually some minimum impedance that a given amp will be happy driving, but it's usually quite, quite low. Go below that impedance and the amp will think that the output is shorted and will current-limit (which you'll hear as clipping). This usually matters the most if you try to drive multiple sets of speakers simultaneously; placed in parallel, the multiple speakers may end up presenting too-low an aggregate impedance to the amplifier.

I haven't worried about speaker impedences for decades.

Atlant (talk) 13:24, 12 December 2007 (UTC)

Benzoylecgonine levels

Is a level of 0.06 mg/l pleural fluid of Benzoylecgonine considered high? 99.251.194.41 (talk) 00:58, 12 December 2007 (UTC)

Do you mean high for a cocaine user or high for a non-user? Rockpocket 01:28, 12 December 2007 (UTC)

I mean high for an occasional user.My son was in an accident (fatal) and his remains showed this level. He told me he never did cocaine. Obviously that was false. I am wondering if he did a lot or this level shows "occasional" use only.Houseman (talk) 22:29, 13 December 2007 (UTC) —Preceding unsigned comment added by 99.251.194.41 (talk) 02:03, December 12, 2007 (UTC)

Based on [1] and related Google results, 0.06 mg/L is a low but significant number. For typical cocaine consumption amounts, it suggests use more than 12 hours prior to death but less than 60 hours. Dragons flight (talk) 02:35, 12 December 2007 (UTC)

Please accept our sympathies for your loss. As Dragons flight notes, assuming typical consumption amounts, the data you provide can some idea as to the last time your son took cocaine prior to the accident. However, with this data alone its not really possible to infer anything meaningful about frequency of use. The best source of this information would probably be your son's friends. Rockpocket 18:02, 12 December 2007 (UTC)

Thank you Rockport, and thank you both for your replies. My sons friends aren't saying anything. They are 5000k from us in north western Canada. (I am on the east coast) Until we get out there and actually talk to them I guess we are in the dark as to what really happened. Houseman (talk) 22:28, 13 December 2007 (UTC) —Preceding unsigned comment added by 99.251.194.41 (talk) 01:52, 13 December 2007 (UTC)

Medicine

Do doctors use a lot of physics? Is it necessary to learn physics in order to get in medical school especially in new zealand? —Preceding unsigned comment added by 118.90.0.137 (talk) 02:35, 12 December 2007 (UTC)

No, and probably. I can't speak for NZ, but the equivalent of two semesters of college physics is required in the US. Dragons flight (talk) 02:38, 12 December 2007 (UTC)

I just remembered, medical school in the US is a post-collegiate program. Outside the US, many medical programs are post-secondary programs. In other words in those countries you enter a 6-8 year medical program instead of going to college, while in the US it is 4 year program beginning after college. Given that fundemental difference, the entry requirements are likely to depend on the structure of NZ medical school programs. Dragons flight (talk) 02:45, 12 December 2007 (UTC)

The amount of physics used, if any, depends heavily on the area of treatment that a doctor specializes in and the specific area of physics you are referring to. Both are extremely wide fields of study with a little overlap. -- kainaw™ 03:26, 12 December 2007 (UTC)

In practice, most doctors would not use much that we would recognize as 'pure physics' in their day-to-day work. Exceptions would include physicians in fields like radiation oncology and radiology, as well as certain branches of sports medicine, kinesiology, and the like. (Often doctors in these fields are supported by medical physicists: individuals with PhD rather than MD degrees who have intensive training in physics.) Some knowledge of physics is important (arguably, essential) to all doctors in order for them to be able to properly understand and interpret a wide range of diagnostic tests and physical maladies. I can't comment on the level of formal physics training required for doctors in Oz. TenOfAllTrades(talk) 04:06, 12 December 2007 (UTC)

Doctors practicing medicine do require an understanding of basic physics up to a certain degree, ie, interpreting CAT scan results or even perhaps reading the heart in some sort of way. —Preceding unsigned comment added by Hey mrs tee (talk • contribs) 04:37, 15 December 2007 (UTC)

I was curious about the system in NZ and had a look. It seems (but definitely don't take my word for it) that both Otago and Auckland have systems with a few different entry level one year courses common for all health sciences, from where you then move on to the actual Medical course. Auckland Uni has a web page[2] with a matrix of entry requirements. It seems physics is not explicitly required except as part of a requirement of having a certain number of credits in sciences. I might have misunderstood things, so I suggest you read through the pages linked from that one. There are contact phone numbers for questions about entry requirements, which might be a very good idea to call if you're about to make career decisions /85.194.44.18 (talk) 18:18, 15 December 2007 (UTC)

Physics is not an explicit entry requirement in Auckland as far as I know. However during your first year (the common health sciences first year) you will do a Physics course, Physics 160 (Physics for the Life Sciences). In my opinion, this paper is very easy, too easy if you have done physics before at secondary level such as at A-level. If you have not, you may find it a bit harder but, and I don't mean to be rude by this, if you find any of the first year papers too hard I'm not sure whether medicine is your calling. Bear in mind that medicine is extremely competitive. There are two entry streams I believe. Either you can make it in in your first year or after your first year. Your GPA or GPE is important in either case so if you don't make it in the first time, you would need to do well in the Physics paper Nil Einne (talk) 20:45, 17 December 2007 (UTC)

Chemistry (gases)

I'm supposed to find out the volume of NH3 (g) produced from 200 L of H2 (g), and the gases are measured at 350C and 400 atm.

I tried the PV/T, and a couple of other methods, but none of my answers still don't match up. Someone please help. --Jeevies (talk) 02:44, 12 December 2007 (UTC)

Chemistry always happens in ratios of moles: find out how many moles of H₂ you have, and then how many moles of NH₃ that makes (write a balanced net ionic equation for the reaction). Then figure out the volume of ammonia that that number of moles is. Are you given the amount of N₂ also (and therefore have to figure out the moles of that, and then the limiting reagent)? DMacks (talk) 02:55, 12 December 2007 (UTC)

How do I get the moles? Apart from that all the info I got was N₂ + 3H₂ -> 2NH₃ --Jeevies (talk) 05:56, 12 December 2007 (UTC)

For a rough answer you do not need to know as each mole of gas takes up the same volume at a given pressure and tempreature. You can see that there is 1 and a half times as many moles of hydrogen as ammonia, so the volume also follows the same ratio. However still you are assuming that there is enough of the nitrogen, and that the reaction goes to completion, this will need to be checked out. Graeme Bartlett (talk) 05:57, 12 December 2007 (UTC)

Oh, thanks a lot! I thought I was going insane for a while there. --Jeevies (talk) 06:36, 12 December 2007 (UTC)

Orange Ice Cream

I was discussing orange ice cream with some friends. This delicious treat sounds like a great idea. But, two questions come to mind.

1. Would the orange's citric acid curdle the milk?
2. Follow-up - what techniques do commercial orange-ice-cream makers use to prevent milk from curdling when adding acidic flavorings?

Thanks! Nimur (talk) 05:08, 12 December 2007 (UTC)

It doesn't seem to be an issue. (Perhaps the other ingredients, like sugar, dilute the acidic effect?). There are lots and lots and lots of recipes for orange ice cream on the web, and they all seem to just mix the ingredients together. - Nunh-huh 05:14, 12 December 2007 (UTC)

I had this idea that the citric acid doesn't affect the milk so much. I've seen it added to yoghurt for example Rfwoolf (talk) 10:05, 12 December 2007 (UTC)

What about having an orange sorbet instead? Lanfear's Bane | t 13:03, 12 December 2007 (UTC)

I've eaten lemon ice cream so it's obviously possible to make citrus-flavoured ice creams.

Atlant (talk) 13:31, 12 December 2007 (UTC)

It would be an easy enough experiment to do. Now, I have some citric acid but I have no milk here to try it with... --BenBurch (talk) 16:42, 12 December 2007 (UTC)

Citrus ice creams may be flavored mostly with zest, oils, or extracts, instead of juice. -- Coneslayer (talk) 16:46, 12 December 2007 (UTC)

Frozen Desserts by Liddell & Weir contains a recipe for orange ice cream, using the zest and juice of 3 oranges. While I will not reproduce the recipe here, the liquids are about 250 ml orange juice, 125 ml milk, and 500 ml heavy cream. -- Coneslayer (talk) 03:00, 13 December 2007 (UTC)

Today's xkcd

In today's xkcd, what's the solution to the problem? My intuition says 0, since $R_{t}^{-1}=R_{1}^{-1}+R_{2}^{-1}...+R_{\infty }^{-1}$, which the right side will be infinite. Since ${\frac {1}{\infty }}=0$, $R_{t}$ must also equal to 0 is well. Am I correct, and is there any formal proof on this problem? --antilived^{T | C | G} 06:07, 12 December 2007 (UTC)

At first I thought it might be zero, but now I think otherwise. Every possible path is in parallel. However, most of these paths are "very long" - so they act as a large resistor in parallel. At a certain path-length, these large resistors in parallel are negligible. The total resistance is dominated by the shortest path(s). Nimur (talk) 06:18, 12 December 2007 (UTC)

I'm going to boldly assert that the answer isn't zero or infinity. Here is a thought experiment: You have a close analogy of this problem if you stick two probes into the ocean about 1cm apart and try to measure the resistance of the sea water between those two points. The oceans of the earth are pretty darned close to being infinite compared to 1cm - and you don't expect to get either zero or infinity as the answer in that case. In fact, I think you'd be surprised if you could measure a different resistance in a bucket of sea water compared to the entire oceans of the world. So we know that the answer doesn't depend significantly on those longer paths. So it's easy to get an approximate answer.

So the real question here is: Why are physicists are only worth two points and mathematicians three? It ought to be the other way around because the physicist can say - "oh about three ohms" and carry on walking. Mathematicians are going to end up worrying about the contribution of an infinite number of paths - each with infinite resistance and wind up with infinity-divided-by-infinity - then FOOOOOMMMM! Maybe they mean theoretical physicists? (You see how you'd find it so hard to score points from a Ref Desk person? We're worth a LOT more than three points!) SteveBaker (talk) 06:42, 12 December 2007 (UTC)

It's definitely less than 3 ohms, since disregarding all the resistor outside the little rectangle drawn up by the two dots, there're 3 paths of 3 ohms resistance so it would be somewhere below 1 ohm IHMO. --antilived^{T | C | G} 07:02, 12 December 2007 (UTC)

F-ing xkcd! 4/pi - 1/2 ≈ 0.77 Dragons flight (talk) 08:16, 12 December 2007 (UTC)

How did you get that? --antilived^{T | C | G} 08:53, 12 December 2007 (UTC)

Q10 on the Google Test answers carries the answer; the paper Application of the lattice Green's function for calculating the resistance of an infinite networks of resistors explains in more detail. Laïka 12:32, 12 December 2007 (UTC)

Backing off a bit, let's consider the inner lattice of 7 resistors between the two points. Is the resistance in that 7-resistor lattice going to be ${\frac {1}{{\frac {1}{2}}+{\frac {1}{3}}}}$ ohms? It's been ages since I've studied physics or electronics so other than simple series/parallel stuff, I'm not necessarily up to snuff. Do I need to consider every possible non-repeating path between the two points in the calculation? Donald Hosek (talk) 17:19, 12 December 2007 (UTC)

For any reasonable answer, you need to consider every path that influences the answer. For example, if I had a 1 ohm resister in parallel with a 1 trillion ohm resistor and asked how many ohms the combined two are to the nearest ohm, it would be 1 ohm. The larger of the two is so large that it doesn't come into play enough to affect the answer. So, you have to consider every path between the two points that changes the answer within the precision you are looking for - which is not an infinite number of paths. -- kainaw™ 17:32, 12 December 2007 (UTC)

So for the seven-resistor lattice, I need to do something along the lines of ${\frac {1}{{\frac {1}{3}}+{\frac {1}{5}}+{\frac {1}{3}}+{\frac {1}{3}}}}$ then (assuming I found all the paths)? Donald Hosek (talk) 17:58, 12 December 2007 (UTC)

Just counting the paths isn't going to work, if those paths overlap... consider, two 2Ω resistors in parallel, vs two 1Ω resistors in parallel with another 1Ω resistor in series. Both have two paths, and all the paths are 2Ω... but the total resistances are 1Ω and 1½Ω, respectively... since, in the latter case, the two paths share a resistor. To solve the small 7-resistor case, you can collapse two pairs of series resistors (the corners that aren't the nodes you're trying to measure), and get 5 resistors arranged like a Wheatstone bridge (except with another resistor in the middle instead of a voltmeter... I only link to that article for the picture). You then need to use the Y-Δ transform on one of the two triangles, and you'll get two resistors in parallel with another two resistors, and a fifth resistor in series with the whole thing... which is solvable with just the series/parallel rules, to get exactly 1.4Ω. Phlip (talk) 22:49, 12 December 2007 (UTC)

We're also discussion that question in the german wikipedia. Thanks for your answer, Dragons flight - I'll post it on the german board. I didn't solve the problem, but was sure, that $0\Omega$ can't be right, since you need at least $0.5\Omega$ because each one of the two points is surrounded by 4 resisters with $1\Omega$ each.--Slartidan (talk) 12:37, 14 December 2007 (UTC)

Geomagnetism

It is taught that the earth has a molten iron core.How does the molten iron core of the earth support permanent magnetism.Molten iron is not usualy permanently magnetic.82.15.53.173 (talk) 10:00, 12 December 2007 (UTC)

It's not a permanent magnet. See geodynamo. It is more analogous to an electromagnet, where the magnetic field is generated by moving charged fluids. Dragons flight (talk) 10:10, 12 December 2007 (UTC)

Also, your initial statement was not completely correct. While the Earth's outer core is molten, the inner core is solid, due to the higher pressure. StuRat (talk) 12:04, 12 December 2007 (UTC)

Dragons flight and StuRat are both correct. The earth's core is theorized to be a solid, mostly iron, crystal that is extremely hot, due in part to the enormous pressure that it is under from the gravitational energy of all the mass of the planet pushing in to the center. However, there are also several other possible reasons that the core of the planet is so hot: 1) leftover heat from the initial accretion phase during which time most of the planet was molten, and 2) heat that is generated from radioactive minerals that are within the earth, 3) the theory that within the earth's inner core there is a fission reactor process known as a Georeactor that produces heat, and 4) heat created through the solidification of the inner core. The reason that the inner core of the earth is not molten, though, is due to the fact that there is just so much pressure that it has been squeezed into a solid mass, instead of a liquid ooze. As one journeys away from the center of the earth, the pressures begin to let up, since there is less mass lying above to push down, and as the pressure is eased, the extremely hot iron no longer is being forced into a solid state, and therefore it becomes liquid, or molten. This is why the outer core and the mantle layers of the earth are molten. Then, of course, the crust of the planet's surface is hard, due to the lack of pressure and also to the cooling effect of being exposed to the thin blanket of ocean and atmosphere before encountering cold space.

Now, as the earth spins on its axis, parts of the interior of the planet spin at slightly different rates from one another. Think of this in terms of when you try to spin a fresh unboiled egg. The egg will very quickly lose its spin. However, if you hard boil the egg, it spins much better because it is solid all the way through. The liquid parts inside a fresh egg want to spin at different velocities from the outer shell, and this is why it doesn't spin so well. With the case of the earth, the inner layers are spinning at different rates from one another - specifically, the inner solid core is spinning at a slightly different rate than the liquid outer core and mantle, which also spin at a slightly different rate from the crust. What this creates is the condition in which iron molecules are passed against one another as these different layers rub against each other due to their different spin rates. Whenever you have such a situation of iron molecules rubbing against other iron molecules, you have created a dynamo (or generator), as Dragons flight described above). What happens as the iron molecules pass one another is that ions are stripped from the molecules and then caught up in the pre-existing magnetic current that makes up the magnetosphere through a process known as Faraday's law of induction. Think of this in terms of a pump pushing water along a closed system. So this is what creates the magnetosphere of the earth, which in turn helps to shield the earth from bombardment of harmful levels of radiation that come from the sun. Most planets that we know of do not have a magnetosphere. We're lucky to have one.

References:

Schneider, David (Oct 1996) A Spinning Crystal Ball, Scientific American

Lehmann, I. (1936) Inner Earth, Bur. Cent. Seismol. Int. 14, 3-31

Herndon, J. Marvin (1996) Substructure of the inner core of the Earth Vol. 93, Issue 2, 646-648, January 23, 1996, PNAS

http://www.nytimes.com/2005/08/25/science/25cnd-core.html

-- Saukkomies 09:27, 12 December, 2007. (UTC)

Sigh. For the second time in two days on this board, the mantle is a solid (specifically a rheid). Dragons flight (talk) 15:16, 12 December 2007 (UTC)

Another small correction to Saukkonies' explanation - pressure by itself does not generate heat, because a stationary force does not transfer energy. Gravitational energy could only be transformed into heat energy if the Earth were contracting, which it is not. Our article on geothermal geology gives a list of the sources of geothermal energy. Gandalf61 (talk) 16:52, 12 December 2007 (UTC)

I stand corrected. Thanks for pointing this out to me, since geology is one of my favorite subjects, and lo! I find I've been under misconceptions in these regards for so many years. ::-- Saukkomies 16:39, 12 December, 2007. (UTC)

sources

wiki pls give two sources of information about water quality in kerala202.88.234.8 (talk) 10:02, 12 December 2007 (UTC)

For these general sorts of questions, google is often a great place to quickly find what you want. See here. Someguy1221 (talk) 10:56, 12 December 2007 (UTC)

Pigeons always pecking the tarmac for invisible food

Does anybody know what the city (and suburban) pigeons are always pecking? They seem to always walk around pecking even when there is clearly no food. I considered that might be pecking small seeds or breadcrumbs but from my observation this doesn't seem to fit - such things aren't scattered so abundantly in the suburbs and city. Maybe they are actually eating small stones, or grains of salt. Rfwoolf (talk) 10:04, 12 December 2007 (UTC)

Some birds' digestive systems rely on having small stones in their digestive tract to help grind food. They're sometimes referred to as "gizzard stones", but are apparently properly known as Gastroliths. EvilCouch (talk) 11:38, 12 December 2007 (UTC)

Thanks! That seems to answer the question. "...Particles as small as sand and stones the size of cobbles or greater have been found" " Domestic fowl, for instance, require access to 'grit', for the purpose of food-grinding." Thanks. Rfwoolf (talk) 11:47, 12 December 2007 (UTC)

why frogs disappear

give me atleast two explanations for why some frogs are disappearing world wide202.88.234.8 (talk) 10:05, 12 December 2007 (UTC)

As in the above, google is a great first place to look. Someguy1221 (talk) 10:57, 12 December 2007 (UTC)

Decline in amphibian populations and Frog#Distribution_and_conservation_status are other good places to look. SteveBaker (talk) 12:48, 12 December 2007 (UTC)

But some are on the increase - see cane toad, particularly about what's happening Down Under. -- JackofOz (talk) 19:44, 12 December 2007 (UTC)

carrying capacity

define human carrying capacity202.88.234.8 (talk) 10:09, 12 December 2007 (UTC)

Carrying capacity is very well defined. Applying the concept to humanity is notoriously debatable, and attempts are discussed in that article. Someguy1221 (talk) 10:54, 12 December 2007 (UTC)

I think we can all agree only on the concept that there is a carrying capacity. Personally, I think we are ultimately limited by arable land unless we can make some real breakthroughs in hydroponics, sea-farming, or completely synthetic foodstuffs. --BenBurch (talk) 16:48, 12 December 2007 (UTC)

It is very well proven that there is a carrying capacity for most species in the wild. This is a dubious concept when applied to the global human population as humanity as a whole has yet to reach a point where further growth is completely unsustainable, as shown by the fact that the human population of the world has yet to cease to grow. While this may be accounted for by the fact that many populations overshoot their carrying capacities, it seems unlikely as humanity seems to be able to increase its available food supply due to advances in agricultural technology and placing more land under cultivation. That being said, there is much histrical evidence to suggest that there were carrying capacities for localized, pre-industrial human populations such as that of France immediately prior to the Black Death. This population reached a plateau immediately before the population crisis of the 14th century, and this plateau is theorized to have been the maximum possible population supportable by the area. Thus, while some human populations have reached a "carrying capacity" in the past, it is a matter of opinion whether this point can be reached in the human population at large, as technological advancements and population isasters are unforseeable in the future, and would cast into doubt any unequivocal statement that there is, in fact, a human carrying capacity.137.186.246.104 (talk) 21:33, 13 December 2007 (UTC)ÊĴÁŶ

No matter how far technology advances, there is a finite carrying capacity for humans.

• Assuming that the entire surface area of the Earth (land and water) is used for photosynthesizing food as efficiently as possible, the carrying capacity is on the order of 10¹⁴ people.
• Assuming that the entire solar output intercepted by the Earth is converted directly to food, the carrying capacity is on the order of 10¹⁵ people.
• Assuming that the entire solar energy output is converted directly to food, the carrying capacity is on the order of 10²³ people.

Of course, at these population levels, the standard of living wouldn't be very good: all available resources are being used for the sole purpose of keeping people alive. --Carnildo (talk) 22:50, 13 December 2007 (UTC)

Gravity

When I worked in a coal mine, I felt that coal was lighter to shovel, than when on the surface.Are things lighter when below the surface considering the mass of the earth that is now above and acting contrary to the normal pull of gravity?Could this effect be percieved by a young miner?Are things perceptively lighter in a submarine as it goes deep under the sea?82.15.53.173 (talk) 10:18, 12 December 2007 (UTC)

If you assume the Earth is uniformly dense, then by Newton's law of universal gravitation the strength gravity inside the earth is directly proportional to your distance from the center of the earth. So even if you're three kilometers beneath the surface of the earth, you're only 0.05% closer than you were at the surface, and so gravity is only 0.05% weaker. While you could certainly measure the difference with a good balance, this would not be perceptible at all. Someguy1221 (talk) 10:51, 12 December 2007 (UTC)

See shell theorem. StuRat (talk) 11:48, 12 December 2007 (UTC)

This is a wild guess, but if the tunnels are on a slight gradient - and I think that it usually the case - you wouldn't normally notice it as you have no frame of reference underground such as a horizon. However, shovelling coal would be easier if you were moving it downhill, even though your brain assumes you are moving it on the level.--Shantavira|^{feed me} 11:15, 12 December 2007 (UTC)

If gravity is 0.5% weaker, a 30-pound shovelful will be 2.4 ounces lighter. As one who has shoveled all day several times in his life, I think I can say that that weight reduction would become noticeable by about 3 in the afternoon. --Milkbreath (talk) 21:28, 12 December 2007 (UTC)

But Someguy1221's calculation is off by a factor of 10 – i. e. 3 km is only 0.05%. Icek (talk) 22:35, 12 December 2007 (UTC)

I have no idea what you're talking about. Someguy1221 (talk) 04:04, 14 December 2007 (UTC)

*LOL*. Correcting a mistake and saying it never happened... Icek (talk) 11:37, 14 December 2007 (UTC)

Maximum/minumum temperature forecasts.

In South Africa the temperature forecast is first the minimum (morning) and then the max (afternoon)[3]; but I've seen British[4] and American[F] forecasts put the max first. Are these minimums the next morning's, or are they just the wrong way round? -- Jeandré, 2007-12-12t13:47z

In US forecasts, the high usually comes first, followed by the "overnight low" that occurs after it. So they give the "Saturday High" that probably occurs Saturday afternoon, then the "Saturday Low" which occurs "Saturday night", but is probably actually early Sunday morning. (It is common here to say "Saturday night" to describe the time between going-to-bed Saturday and waking up on Sunday. This imprecision results in people saying things like, "So Saturday night, the phone rings at 3 in the morning...") -- Coneslayer (talk) 14:06, 12 December 2007 (UTC)

Respiration protocols

Some mammals were treated to three different respiration schemes: group 1: 6.2±0.4 ml/kg VT (PIP 40/0), 30bpm, 95% O2; group 2: 6.2±0.4 ml/kg VT (PIP 40/0), 30 bpm, 95% N2; group 3: PEEP 5cmH2O, 95% O2. I'm trying to figure out what this means. Where it says VT, by the way, T should be subscript.

30 bpm? Beats per minute? I tried searching with google but can't figure it out. I've found out that PIP = positive inspiratory pressure and PEEP = positive end expiratory pressure - does PEEP make it harder to breathe out and therefore generally harder to breathe than PIP? Also what's the 6.2±0.4 ml/kg VT in groups one and two about and the reference to H2O in group three? Thanks for any insight. --Seans Potato Business 16:04, 12 December 2007 (UTC)

I assume that bpm in this context is breaths per minute. 5 cmH₂O is a pressure of five centimeters of water. (A column of water five centimeters tall will exert this much pressure at its base.) One centimeter of water is equivalent to 0.76 mmHg or 98 Pascals: [5]. I wouldn't want to guess at the rest. TenOfAllTrades(talk) 16:19, 12 December 2007 (UTC)

VT = VD + VA. That is... The tidal volume (VT or TV in some textbooks) = the alveolar ventilation (VA) + the dead space (VD). For someone who is not a critical care nurse.... The tidal volume is the amount of air which is normally inhaled and exhaled during breathing.

PIP is the Peak Inspiratory Pressure (the pressure difference between the ventilator and the lungs results in inflation until the peak pressure is attained, and passive exhalation follows)

PEEP is correct = Positive end-expiratory pressure. You will only see the reference to H2O when talking about PEEP. It is the positive resistance that is applied to exhalation. This pressure keeps the small airways open though the entire ventilation cycle.

So. Group #1 had 5.8 to 6.6 mL per kg of 95% oxygen in their tidal volume with a PIP (Peak Inspiratory Pressure) of 40/0 and a rate of 30 BREATHS per minute.

Group #2 was the same ventilator settings but they used Nitrogen as opposed to Oxygen.

Group #3 I suspect was the same as group #1 with the addition of the PEEP to the ventilator settings.

Look here for a crash course: http://www.emedicine.com/emerg/topic788.htm

Hope that helps... ICU RN

Gold that's heavier than pure gold?

I've finally decided to finish the damn Baroque Cycle, having read the first two when they came out and then just put it aside. So I read the second one again (having totally forgotten everything that had happened) and have now just begun with the third book, The System of the World. A major plot point in books two and three is concerning something called "Solomonic Gold", that is, Gold that is heavier than ordinary gold. Newton and others use a lot of alchemical mumbo-jumbo to try and explain this ("It is infused with the Philosophick Mercury!", etc.) , however what is clear is that in this book this is a real metal. Is there such a thing? Can you perhaps make "heavy" gold much like heavy water, by only selecting larger isotopes? Or is it an alloy with some other heavier metal (depleted uranium-gold?) Or did Neal Stephenson just make this up? 83.250.203.75 (talk) 18:43, 12 December 2007 (UTC)

Gold only has one stable isotope (197). The only other nearly stable isotope is 195 with a half-life of 186 days. So, you'd find it very difficult to make heavy gold using heavy isotopes. -- kainaw™ 18:52, 12 December 2007 (UTC)

Gold-Osmium alloy? Osmium is the densest known element. Although, platinum is also more dense than gold and is used in jewelery, too. — Scientizzle 18:56, 12 December 2007 (UTC)

I haven't read the book, but I've spoken to people who have, and I'm reasonably sure the damn stuff is infused with the Philosophick Mercury. Thus Stephenson has decided to make some of alchemy true rather than make up an isotope. Algebraist 21:33, 12 December 2007 (UTC)

He really doesn't explain why the gold is heavier than it should be. A modern reader should probably assume he's talking about an isotope - but (as already explained) that doesn't really work. The point is that the book is written from the point of view of a 17th century alchemical society - and the impact of screwy gold on their currency would indeed be problematic. I suspect (from Stephenson's writing style) that we'll find out more in subsequent books. The business of the same family names carrying on between the Barque Cycle and the Cryptonomicon - and one of the characters in it seeming to be immortal means that the science within those stories cannot be taken as literally true. The mythic compactness of the Qwelgmian language - and a whole lot of other things he talks about - are also impossible. However, the Cryptonomicon is certainly in my top 10 list of best ever works of fiction - the Baroque cycle...is too long to re-read - so I'm not sure! SteveBaker (talk) 00:18, 13 December 2007 (UTC)

• Is it possible he's referring to Growth of High-Density Gold Nanoparticles on an Indium Tin Oxide Surface Prepared Using a "Touch" Seed-Mediated Growth Technique? I've only read Cryptonomicon, so I don't have a complete grasp as to whether Stephenson was attempting to make a real-world reference or creating a fictional substance. --M@rēino 14:34, 13 December 2007 (UTC)

Aliens

What is the probability for the existence of aliens? 64.236.121.129 (talk) 19:19, 12 December 2007 (UTC)

See Drake equation, and consider refining your question. Do you mean aliens we will make contact with in your lifetime? Aliens in our galaxy? Aliens anywhere in the universe? If it's aliens anywhere in the universe (with no requirement that we ever find out about them), then the only realistic answers are 0 (if you believe that we are the unique creations of God, for example), or 1 (if you don't). It would require incredible fine-tuning for life to exist on one and only one planet in the universe. -- Coneslayer (talk) 19:26, 12 December 2007 (UTC)

The probability for aliens to exist somewhere in the universe at some time is so high as to be almost certain. The probability for aliens to exist within the brief (geologically speaking) lifetime of the human race with the ability of physically visiting us on earth is so low as to be almost impossible. There's a lot of wiggle room in between. Personally I take a very pessimistic view of the Drake equation—it makes the idea of any sort of communication (defined as two-way contact, not simply "an alien picks up one of our radio signals someday, long after we're gone") at all seem practically impossible. --24.147.86.187 (talk) 19:48, 12 December 2007 (UTC)

The Drake equation and all speculation on the matter depend on an unknowable variable, what the probability is that life will spontaneously arise given the right conditions, leaving aside mystical ideas about the genesis, no pun intended, of life. Some think the probability is high, or even one. With a tip of the hat to Descartes, it's obviously not zero. The area in between is the very best sort of ground to grow unending pointless debate in, so I'm not going to tell you what I myself think. --Milkbreath (talk) 21:21, 12 December 2007 (UTC)

The problem with the Drake equation is that we have a good idea of the first factor, are working on the second and third, and know nothing whatsoever about the remaining 4. Incidentally, the fact something has happened does not imply its probability was non-zero, at least in the usual mathematical theory of probability. That reminds me: someone ought to point out that questions of the form 'what is the probability of [event in real world]' are fraught with philosophical difficulties. For an extreme example, one could say that the existence of aliens is nonrandom (they either exist currently or they don't) so the answer is 1 or 0 depending on whether they exist or not. Algebraist 21:30, 12 December 2007 (UTC)

If you're interested, there's a recent Damn Interesting article on the odds of aliens and radio communications with them titled "Space Radio: More Static, Less Talk" that includes an interactive Drake equation calculator. You can plug in your own figures and it will give an estimate for the number of communicating civilizations in the Milky Way galaxy. -- HiEv 23:42, 12 December 2007 (UTC)

Technically - there is nothing wrong with the Drake equation - the problem is only with the numbers you plug into it. However, because we know that humans exist, none of the parameters of the equation can possibly be zero. Hence, if the universe is large enough - there will be other lifeforms out there. However, knowing that there are definitely aliens out there is a very different thing from being able to detect them - let alone communicate, meet, cooperate with. The section of the universe that we can ever reach is finite - and that section could be utterly devoid of alien species if the numbers in the equation are small enough. SteveBaker (talk) 23:53, 12 December 2007 (UTC)

Oh - yeah - the other thing about the Drake equation is that it doesn't predict the number of populated alien worlds (which I guess is what the OP is asking) - but the number that have intelligent aliens, that know how to make big radio transmitters and who decide to transmit and who are in roughly the same phase of technology that we are (so they aren't trying to communicate by wormholes or something) and who havn't died out from global warming or nuclear holocaust or something. If you just want to know whether there are primitive alien bacteria out there - then the odds are not only much better - but the answer has a lot fewer unknowns in it. SteveBaker (talk) 00:02, 13 December 2007 (UTC)

SETI doesnt seem to have found any evidence of extraterrestrial intelligence. So draw your own conclusions--TreeSmiler (talk) 02:16, 13 December 2007 (UTC)

I lost what faith I had in SETI when I read somewhere (source forgotten, I fear) that if our present radio emissions had (by the intervention of mischievous time-travelling aliens perhaps) by humanity 50 years ago, then they would have considered it random noise due to the complexity of modern coding. (Hopefully someone here will know if this is a load of marsh gas) Algebraist 02:49, 13 December 2007 (UTC)

That seems quite unlikely. Digital signals are most certainly not natural analog noise. Would the encoding be decipherable? Perhaps not; MP3 isn't really intended as a first-contact codec. It would absolutely be artificial though. — Lomn 04:13, 13 December 2007 (UTC)

Power and duration are also obvious give-aways. Artificial radio signals will always make the detected signal louder than noise, since they add to the noise, and a sudden burst of radio waves that lasts three minutes before suddenly disappearing is definitely conspicuous. The Wow signal is the strongest candidate for an extraterrestrial transmission because it was the most powerful signal Big Ear ever detected, and because it was only received once. --Bowlhover (talk) 06:05, 13 December 2007 (UTC)

That's not entirely true. To make the most possible use of the available spectrum, you need to use data compression techniques. An advanced civilisation still only has the same amount of available spectrum as we do - but they need to send full holographic smell-o-vision movies instead of boring 2D television and such - so they need to resort to techniques like data compression to make the optimum use of it (this is already happening here on earth with analog TV being replaced with digital TV). Our Satellite TV (for example) is both data compressed AND encrypted. The thing about data compression is that it's goal is to remove any semblance of regularity from the signal since regularity can be exploited to further compress the data. The goal of encryption is also to take away any patterns that might be recognisable and to turn them into patternless noise. So compressed and/or encrypted data comes to resemble white noise - a completely irregular, unpredictable stream. It follows then that any civilisation that's even a teeny bit more advanced than we are (say 20 years ahead) - or who simply made better decisions than we did 50 years ago when designing TV transmission standards - would be producing signals that closely resemble white noise. Worse still, we know that 'spread-spectrum' transmission (such as military battlefield communications) and 'frequency hopping' (as in cellphones) makes more sense than the system of descrete bands that we currently use for most of our radio traffic. If the aliens did that, we'd have a heck of a time figuring it out. We'd see white noise, evenly distributed across the parts of the spectrum that their atmosphere transmits comfortably. So the idea that we can use signal-to-noise ratio to detect their routine internal transmissions is a non-starter because no sufficiently sophisticated societies are sending anything with a noticable signal - it's ALL noise to the uninitiated! If we detect a high power white noise source, we're going to assume it's any one of a large number of natural sources - stars put out tons of noise - we would just assume that there was a strong, natural, radio source in that direction. The only signals we're going to be able to figure out are the ones that are sent deliberately towards us - and using a simple enough scheme that any reasonable civilisation would be able to understand it - a prime number sequence - the binary digits of PI - that kind of thing. SteveBaker (talk) 18:28, 14 December 2007 (UTC)

I've always found it interesting that science fiction portrays aliens as being only a little more advanced than us. If there really were intelligent beings who were capable of visiting us, there is a good chance that they would be not just thousands but millions of years ahead of us technologically, and would regard us as we regard animals. Rather than beaming "hi there" messages into space we should be keeping our heads well down.--Shantavira|^{feed me} 09:39, 13 December 2007 (UTC)

Indeed. Think of how we treat chimps, then wonder whether you want advanced aliens noticing us... Skittle (talk) 15:59, 13 December 2007 (UTC)

Absolutely. People slaughter animals that pose no threat to us what so ever, all the time. Indeed, people hunt some of them for fun even. People test experimental drugs on them to see if it will kill them or not. And if it doesn't, they kill them anyway (see animal testing). Hell, people even enslaved other humans because some people thought they were inferior. Why would an alien civilization millions of years ahead of us, treat us like equals? 64.236.121.129 (talk) 15:26, 14 December 2007 (UTC)

• If you'd like an entire book on the subject, I recommend Lonely Planets by David Grinspoon. Or anything by the late Carl Sagan; his theories have held up pretty well over time. --M@rēino 14:36, 13 December 2007 (UTC)

Backcrossing

MyD88-/- mice were generated as described and backcrossed for 9 generations on an H-2d (BALB/c) background. - what's the backcrossing necessary for? --Seans Potato Business 20:15, 12 December 2007 (UTC)

When generating knockout mice, it's generally necessary to mix two different strains of mice. Backcrossing is used to reduce the genetic components of one of the founding strains. — Scientizzle 21:16, 12 December 2007 (UTC)

Here is an ugly graphical representation. After several generations of backcrossing, one can assume that the only non-BALB/c DNA in these mice is from the regions flanking the knocked-out gene — Scientizzle 21:21, 12 December 2007 (UTC)

That's great; thanks. So you only need to perform backcrossing when you have the knockout in a different genetic background than the one you want? Would it not be simpler to have performed the knockout in the genetic background in which you wanted it in the first place? Are some backgrounds in some way easier to use for producing knockouts? --Seans Potato Business 22:07, 13 December 2007 (UTC)

The [[formation of a genetic knockout mouse is moderately complex. A genetically modified embryonic stem cell of strain 1 is placed in the developing blastocyst of strain 2. In order to determine the efficacy of the implantation (and to plan subsequent breeding), it's useful to produce a chimera of strains with different coat colors. There's more information at Knockout mouse. — Scientizzle 22:22, 13 December 2007 (UTC)

But the diagram you link to uses three different strains. One cell type endures the knockout and is put into a blastocyst of another type. They then try to get the transgene in a C57BL/6 background by "backcrossing". If they (the scientists involved) had used C57BL/6 cells in the first place, they could put those cells in the same blastocyst as before, breed with a C57BL/6 mouse, and just like that, all the black offspring are heterozygous for the knockout and fully C57BL/6; no backcrossing necessary. --Seans Potato Business 13:30, 15 December 2007 (UTC)

Ice Cream

I've heard that you can die from eating re-frozen ice-cream, because of salmonella. Is this true? —Preceding unsigned comment added by 89.242.7.45 (talk) 21:23, 12 December 2007 (UTC)

I assume it would depend on how long it was defrosted for. If it was long enough for a significant salmonella colony to develop then yes - you could certainly have active salmonella in the icecream when you eat (and thereby warm it up). But if it only defrosted a little bit on the way home from the store, I can't believe there would be a problem. SteveBaker (talk) 23:30, 12 December 2007 (UTC)

Not to mention salmonellosis isn't typically fatal, although it isn't the most fun experience in life. --Bennybp (talk) 04:58, 13 December 2007 (UTC)

• Some (nice) ice cream is made with a custard that contains lightly-cooked eggs, so the risks are the same as for raw eggs. --Sean 00:01, 14 December 2007 (UTC)

Light perception and evolution

Little background: Sun and moon have been present around earth since quite some time before the evolution started taking place. Specifically, humans have always seen during the whole evolutionary process either sun or moon at any point of time in the whole day. During the day, sun emits light in the whole electromagnetic spectrum, but due to the intense temperature in the sun, its peak wavelength of radiation lies around 550 nm (middle of the spectrum that we actually see). In the night also, moon shines earth by the reflected light of sun that falls on the moon, then again the peak is around the 550 nm. Since light around nature is abundant in this wavelength range, we have actually adapted ourselves to sense the light in this range only.

The question: However, I must say that all bodies on earth (e.g. solid bodies or living organisms etc.) have been emitting infrared radiation also at all points of time ever since humans started evolving. Yet, we can't sense infrared radiation. My question is what could be the reason for this selective evolution. Is it because of the fact that intensity of infrared radiation always tends to be much less than that of the normal visible light that is found in the nature. But then, why did some other creatures develop this infrared sensing feature? And ultraviolet light is generally not present at all in the nature in normal circumstances, yet some creatures develop this sensing also. Why did humans lose? Does it have to be something related to the human intelligence also? DSachan (talk) 22:21, 12 December 2007 (UTC)

You've got a lot of "human exclusivity" in that question ("Why did humans lose?") but keep in mind that homo sapiens may not have gained/lost anything that primates, mammals, etc. had before that. To my knowledge no mammals can see very far into the infrared or into the ultraviolet (as we humans define the visible spectrum), it's not a human-specific thing, you're probably talking about mammal eyes in general. (Some reptiles can see in the infrared, and with bugs and shellfish and etc. there is a whole variety in types of eyes and what they can see.) It probably has nothing to do with humans in particular. --24.147.86.187 (talk) 22:38, 12 December 2007 (UTC)

Given that infrared radiation has also been present all the time in addition to the visible radiation, why did we not evolve to the see this kind of radiation? You did not answer my question. DSachan (talk) 22:45, 12 December 2007 (UTC)

I wasn't trying to answer the question, I was just pointing out that you're probably asking the wrong question. The "right" question (in my mind) is not centered on humans but mammals in general; if mice can't see in the infrared (and as mostly noctural animals you'd think they'd have a lot more to gain from it than humans, who sleep all night and wouldn't benefit much), then humans probably aren't going to have evolved it. --24.147.86.187 (talk) 00:47, 13 December 2007 (UTC)

It would be very hard for an animal that can see in the infrared to get around during the day. Everything would be really bright to them. It would give some advantage when it is really dark but most nocturnal mammals get by without it. Shniken1 (talk) 23:13, 12 December 2007 (UTC)

I don't understand this answer. Why would everything necessarily be "really bright"? Why couldn't they just have a less sensitive retina, or smaller pupils, or something? What part of the spectrum are we talking about here, anyway? —Keenan Pepper 23:19, 12 December 2007 (UTC)

First, let me point out that infrared refers to a much wider range of wavelengths than visible light. "Thermal infrared", emitted by objects around room temperature and with a wavelength about 10 microns, is very different from "near infrared", which is only emitted by very hot objects and has a wavelength around 1 micron.

I think the main reason why animals didn't evolve eyes capable of seeing thermal infrared is because it's fundamentally harder to see those frequencies. It's the same reason why thermographic cameras are in general so much more expensive than visible light cameras. One of the problems that comes up in making thermal infrared cameras is that the inside of the camera should be dark to make good low-noise images. For visible light, this is easy, but for thermal infrared it's quite difficult, because most things glow in the thermal infrared spectrum. That's why good thermal infrared cameras are cryogenically cooled. If an animal had some kind of thermal infrared eyes, it would have to have a way of keeping them cold, which would probably take a lot of metabolic energy that could be used for other things.

That's just the first problem with thermal infrared eyes that comes to mind. I don't think it has anything to do with humans in particular. —Keenan Pepper 23:16, 12 December 2007 (UTC)

The question is not what we COULD evolve (there are owls that can see in the infrared and bees can see in ultraviolet) - it's what is efficient for us to evolve. We would need more sensors in the back of our eyes to be able to see in IR. So either:

• We'd have fewer sensors for colour - but we need daylight colour perception for finding juicy ripe red fruit in the green leaves of a tree - and to distinguish a brown rotten apple from a fresh red one. That failure would be selected against - so IR vision would vanish from the gene pool.
• Or - we'd need bigger eyes...think "Owl". But bigger eyes would require either:
  • A smaller brain. (definitely contrary to what humans needed to survive).
  • A bigger head. But we already are right on the edge of what size head will fit down a womans' birth canal - the death rate from giving birth (without medical assistance and before ceasareans) is already alarmingly large - making it worse would be a problem. Women could perhaps also evolve larger birth canals - but I'm sure there are problems consequent on that change.

So it's impossible to get IR vision without losing something else. Since we sleep at night and gather food in the day - there isn't a VAST benefit to IR vision - but there are significant costs in terms of lost colour vision or smaller brains or larger incidence of birthing problems.

But in any case, evolution doesn't always find the optimum path - it selects the best genes from what shows up from breeding and mutation. If an IR vision mutation never shows up - we'll never evolve it. SteveBaker (talk) 23:21, 12 December 2007 (UTC)

Great answer as usual, Steve! —Keenan Pepper 23:43, 12 December 2007 (UTC)

SteveBaker really should have his own phone-in show! He knows about everything from Mathematics to Computer Programming to Evolution to Red Squirrels !!! Unbelievable! It is sad that someone so smart and helpful is here on the Wikipedia (only as in relation to) while a certain other man, not so smart or helpful is the most powerful person on the planet. That said, hopefully the Wiki will be here forever. I think it is one of the most important projects of our time! So please stay SteveBaker! Saudade7 23:49, 12 December 2007 (UTC)

I have a hard time deciding whether or not I'd like to be Oliver ;-) hydnjo talk 00:52, 13 December 2007 (UTC)

There are animals that can see thermal infrared: the pit vipers. However, they also demonstrate the limitations of being able to see thermal infrared. They can only detect objects that are warmer than the environment (imagine not being able to see plants or any cold-blooded animals) and the resolution is far lower than that of the human eye (imagine not being able to tell the head of an elephant from its rear). --Carnildo (talk) 00:43, 13 December 2007 (UTC)

Hold on. Pit vipers can sense thermal infrared, but they also have functioning eyes - they see plants and cold-blooded animals as well as any other snakes. They also happen to use a niche that's quite different from people and which heavily favours their use of IR sensing. Being a reptile, they don't need to worry so much about 'overloading' their sensor pits since they assume background temperature. They also hunt warm-blooded prey in the cool night-time desert, where IR detection would be very useful and normal sight less so. Matt Deres (talk) 00:31, 15 December 2007 (UTC)

First of all, evolution is not "directed", it's "opportunistic". This means that species usually don't evolve properties that will become useful, but are not currently useful, nor do they have fully-formed complex features spring into place. Instead, minor traits are adapted into complex features if there is an opportunity and a benefit for that to occur, and then evolution continues to refine it if possible. In other words, just because a trait may have advantages, doesn't mean a species will ever get the chance to evolve it.

Second of all, larger animals don't sense infrared with their eyes because IIRC the lens is opaque to infrared. (I couldn't find any good site that says owls see IR, and here is an unscientific test where an owl couldn't see IR.) Precision thermoception is normally done using pits, such as the ones on the heads of snakes or in the noses of some bats, that contain an especially thermo-sensitive membrane. However, as the latter article notes, the accuracy of that sense is rather poor.

Finally, some adaptations have tradeoffs. For example, such a sense organ might be easily damaged by cold or prone to damage and infection. Or, it could be that it also requires "expensive" to produce materials to create such a sensor. Thus, such an adaptation might not be worth the cost. The fact that so few species have thermoception suggests that this might be the case.

So, the answer is that it may not have been worth it for most mammals to evolve thermoception, and even if it is/was, there may have been little or no opportunity to it to evolve. -- HiEv 02:21, 13 December 2007 (UTC)

• "there are owls that can see in the infrared"<-- can you provide some references? --JWSchmidt (talk) 02:52, 13 December 2007 (UTC)

evolution is not "directed", it's "opportunistic". Love this! Excellent quick summary of a lot of ideas. DMacks (talk) 03:03, 13 December 2007 (UTC)

Directed evolution being something else, obviously:) DMacks (talk) 16:10, 13 December 2007 (UTC)

• "We would need more sensors in the back of our eyes to be able to see in IR" <-- why? What kind of sensors are you talking about? --JWSchmidt (talk) 06:06, 14 December 2007 (UTC)

The answers so far are mostly dealing with infrared vision. What about the other end of the spectrum: Ultraviolet? I believe that I have read that common whitetail deer can see in the ultraviolet. Some things that look dull and drab to us, supposedly 'glow' with brilliance to them. And deer, though not primates, are mammals. So, what about ultraviolet vision? Zescanner (talk) 19:45, 13 December 2007 (UTC)zescanner

some animals that see ultrviolet, some reading. --JWSchmidt (talk) 06:06, 14 December 2007 (UTC)

Some people with a damaged or missing lens in their eye(s) can see ultraviolet light too. See the Aphakia article. Of course, without the lens blocking UV light, the retina is more prone to damage by it. Sunburn your skin, you'll recover; sunburn your retina, that's bad. -- HiEv 23:45, 14 December 2007 (UTC)

This was actually a question on the general/introductory chemistry final (hi, Prof. Doll!) I had in college: why we see in the range we do, vs deep (an order of magnitude or so) into IR or UV. DMacks (talk) 06:28, 15 December 2007 (UTC)

While I too am a fan of Steve’s, I DON’T think this is one of his best efforts at all, nor is evolutionary theory a strong suit of his. What he says is that we don’t see in ultraviolet because we would need bigger eyes, ergo bigger heads, ergo the birth canal would not be equipped to deal with it. This is just plain silly. Does that mean marsupials could see in ultra violet? This is treating evolutionary theory as a series of Just So stories. You can always invent ad hoc stories about “how things got that way”. Try this. Humans have purple skin because they don’t have fur, so they need something to camouflage them in the dark. I would guess that most living things can’t see in outside the conventional spectrum because there is very little there that would enhance their survival. In fact, most animals apart from primates, birds, and some sea creatures don’t require much in the way of colour vision at all. Bees can see in the ultraviolet, possibly because some flowers have patterns that can be discerned in that range. In such a scenario, infra and ultra vision can be enhanced by way of symbiotic evolutionary processes. In a way, it is a pity. If humans could see just a little in the infra and ultra ranges, we would see a far more spectacular night sky, more in line with the Hubble Space Telescope images of vast red and purple galaxies. But Nature sure is a no – nonsense boss – we don’t need such tools to survive, so we don’t get them. Myles325a (talk) 00:19, 18 December 2007 (UTC)