Wikipedia:Reference desk/Archives/Science/2010 August 31

= August 31 =

Absolute vs. invariant
What is the difference between both terms: absolute, invariant?--Email4mobile (talk) 01:55, 31 August 2010 (UTC)


 * Have you seen these two terms used in some scientific application? If so, it would help us if you describe the scientific application so that we can explain the difference in a way that is meaningful for your application.  If not, your question should be re-posted at the Language Reference Desk WP:RD/L.  Dolphin  ( t ) 02:08, 31 August 2010 (UTC)


 * When talking about light speed, we almost say it is invariant and it is rarely described in some sites as absolute (for instance: Principle of Invariant Light Speed).--Email4mobile (talk) 03:01, 31 August 2010 (UTC)


 * Not sure if this is what you want, but absolute usually refers to a limiting invariant. Thus, 'absolute zero' is the invariable lowest limit of temperature, while other temperatures (such as the boiling point of water at standard pressure) are invariant without being absolute.  but without knowing the context of your question, it's hard to be sure if that helps.  -- Ludwigs 2  03:40, 31 August 2010 (UTC)


 * One could say that the speed of light is an "absolute maximum" speed, but this is usually taken for granted and is seldom spelled out in those words. "Invariant" just means that it doesn't change, but note that only the speed of light in free space is invariant.  Light "speed" actually depends on the medium through which it is travelling.    D b f i r s   08:19, 31 August 2010 (UTC)
 * Light only travels in a vacuum, and at only one speed. If it runs into say an atom it gets absorbed and (perhaps) re-emitted -- at light speed, back into the inter-atom vaccum.   Wikiscient  (talk) 08:38, 31 August 2010 (UTC)
 * Well, yes and no. Yes, for the particle description of light.  For the wave description, not so much.  How can they be true at the same time?  Not so easily.  You have to swallow one or more of the various weirdnesses of quantum mechanics.
 * By the Copenhagen interpretation, I think you would say that the light wave, in matter, is a superposition of various possible world lines where the photons are absorbed and re-emitted, as you say &mdash; but there isn't necessarily a fact of the matter as to which of these (mutually incompatible) world lines is the one that "really happened". --Trovatore (talk) 09:25, 31 August 2010 (UTC)
 * Yes, thanks for the correction on interpretation. I've put "quotes" round "speed".  The word is used informally to mean "speed of propagation", as in the experiment to "stop light".    D b f i r s   18:28, 31 August 2010 (UTC)

Solar system
When was the last time Jupiter, Saturn and Neptune were in line from Sun, estimated years to next time planets will form straight line from Sun, how does this effect earth's orbit--Golfforgeorge (talk) 04:37, 31 August 2010 (UTC)
 * It's called an astronomical syzygy and it's really not that hard to look up past and future planetary alignments, if you're interested, GIYF. And no, the planets are so far apart that there is no appreciable effect on earth's orbit.. Vespine (talk) 06:49, 31 August 2010 (UTC)


 * This link, http://www.space.com/scienceastronomy/solarsystem/planets_align_020402-1.html, talks about the alignment in April, 2002 of the Sun, Mercury, Venus, Mars, Earth, and Saturn -- and says that in the next 100 years there will be only three other times when at least five planets are so closely aligned: September 2040, July 2060, and November 2100. (You can also find astronomy software online that will calculate the dates etc. of various planetary configurations for you, if you want.)   Wikiscient  (talk) 08:02, 31 August 2010 (UTC)


 * It should also be noted that such predictions only work on relatively short (centuries or millenia) timeframes. When extrapolated over longer time periods, the system degenerates into an n-body problem, and becomes chaotic.  That's because over long time periods, the total of the small gravitational effects of the planets on each other begin to add up, and aren't negligible anymore.  -- Jayron  32  05:43, 1 September 2010 (UTC)


 * Vespine - could you please not use that offensive acronym, especially when you fail to do it yourself and find out just how difficult it is to find a list of historical alignments? Most of the results are blogs about 2012, no matter what I search for. The previous suggestion, of downloading a simulator, seems about right - see the post on physicsforums.
 * As for the effects of such alignments, I crunched the numbers for you quickly. At closest approach to Earth, each planet exerts their largest gravitational force on us, and during an alignment, all those forces add up. Jupiter's max force on the Earth is 2.2*10^18 N, Saturn's is 0.16*10^18 N, and Uranus and Neptune both exert about 4*10^15 N, giving a total force during alignment of 2.368*10^18 N, which is barely more than what Jupiter exerts normally (about 7% more force). All of this is a counteracting force against that exerted by the Sun on the Earth - 3.5*10^22 N - which is 10,000 times greater than the admittedly-significant force exerted by Jupiter. So how much of a difference do we, as Earthlings, feel in gravity during this largest-planet alignment? The answer is that we feel 0.0005% lighter, which is very insignificant. If you want to change your body weight by this amount, walk a couple steps up a flight of stairs - your distance from the Earth will increase enough to reduce your mass by the amount that this alignment would (source for analogy). [[User:SamuelRiv|SamuelRiv] (talk) 17:16, 2 September 2010 (UTC)
 * I never thought the acronym was offensive, in fact i thought it was quite purposefully inoffensive, like a backlash to RTFM. As for whether the information the OP seeks could be found by google, well that's just a matter of how much effort you put in, which by the naivete of the question appears to have been NONE up to the point of deciding to post here. I advocate learning for yourself, i gave a lead and i encouraged self discovery, I meant no offense. Your answer is very good, but takes a different approach. I would argue that to anyone who even slightly believes in astrology it will be easily dismissible; magic doesn't require gravity as a medium. Vespine (talk) 23:31, 2 September 2010 (UTC)
 * It would be nice if people would answer the questions rather than bicker about irrelevant BS. 1) This conjunction occurs every 179 years with a slight but increasing "drift" in its closeness; as a result, in a few centuries the 179-year cycle will shift to a different "starting point" in the A.D. year-number (i.e., you won't be able to just add 179-year increments to the previous dates).  It occurred, for example, in 1345, 1524, 1702, and 1881, but by 1881 the closest the three got was a relatively wide 7 degrees of geocentric longitude.  The closest recently was January 20, 1524, when the three got to within 1.3 degrees in longitude and 0.9 degrees in latitude.  2) The next time will be June 2060, when the closest triple conjuntion will be 8 degrees in longitude.  Jupiter and Saturn conjoin every 20 years, minus a few months.  That increment, added to June 2060 repeatedly, will reveal the next "beginning year" of the 179-year cycle once the "drift" causes the closeness of the conjunction to disappear in the current "...1345/1524/1702..." cycle (which centers on 1524).63.17.54.204 (talk) 03:26, 4 September 2010 (UTC)

The lack of progress in motor vehicles
It seems to me the US can't depend on car manufacturers to help progress cars into efficient vehicles. So we should be hopeful of independent means, right? The porn industry changed the way we look at movies, and NASA has used space to aid the progress of medical and other scientific technology. Likewise, Richard Branson is pushing civilian space traveling like no one has (or would, if it weren't for him). So why on earth is the combustion engine still even being used? Why hasn't NASCAR or F1 or whatever independent racing corporations invented the car that has 80–90% (or better) efficiency, awesome MPG ratings, etc etc? What on earth does NASCAR do with all the advertising money they get from those six thousand decals on every car? How come cars are so ... embarrassing? –  Ker αun oςc op ia◁ galaxies  04:37, 31 August 2010 (UTC)


 * It seems to me that this request is asking for opinions. The Science Reference Desk does not provide opinions so it is likely that the OP will not receive satisfying responses.  Dolphin  ( t ) 04:45, 31 August 2010 (UTC)


 * Hmm... what if I rephrase the question with less whining involved (my fault): Students and science contests don't count. –  Ker αun oςc op ia◁ galaxies  05:11, 31 August 2010 (UTC)


 * Far superior? I doubt it.  See Carnot cycle to get an idea on the thermodynamic limit on efficiency.  Dolphin  ( t ) 05:34, 31 August 2010 (UTC)

Sure, battery electric vehicles are far superior to internal combustion vehicles in efficiency, if not energy density. Until battery technology improves enough to address that latter issue, you can have the best of both worlds with plug in hybrids. 208.54.5.78 (talk) 06:31, 31 August 2010 (UTC)


 * Thanks for both responses. From what I've heard from owners of hybrids, they're somewhat of a sham. I wouldn't consider them "far superior" by any means. I was hoping someone might know if there are plans in the works to eliminate the internal combustion engine entirely. I just feel like it's 2010 and we're still stuck in the 40s or something. –  Ker αun oςc op ia◁ galaxies  06:53, 31 August 2010 (UTC)


 * Battery electric vehicles can be extremely efficient if the batteries are charged by some sustainable source such as solar or wind power. But if the batteries are charged from the mains supply that same efficiency will never be available because at the source is a heat engine of some sort.  It depends to a large degree on what you have in mind when looking for improvements.  For example, are you looking for improved thermal efficiency and are willing to sacrifice in the area of capital cost?  Or are you looking for reduced greenhouse emissions and are willing to contemplate increased nuclear waste?  Or are you looking for reduced weight and are willing to sacrifice in the areas of capital cost, thermal efficiency and greenhouse emissions?


 * When designers go looking for improvements they need to know the criteria for determining merit. Similarly it would be good to define some criteria for determining merit here.   Dolphin  ( t ) 07:29, 31 August 2010 (UTC)
 * Battery electric vehicles are just as efficient when the batteries are charged for ANY source. Although coal and petroleum-fired generation plants pollute, they are still much better than having lots of IC-engines since the former are concentrated sources and can thus their pollutants be more easily controlled and contained. -- Sjschen (talk) 14:41, 31 August 2010 (UTC)
 * If you compare car engines from 40 years ago, they have improved a lot, especially in the power they can make, and reduction in noxious emissions. Graeme Bartlett (talk) 11:38, 31 August 2010 (UTC)


 * Are you asking where you flying car is? -- ToET 14:18, 31 August 2010 (UTC)

It's a little unfair to be too critical. The manufacturers of cars (and their engines) are reacting to what the public will buy and at what price. If everyone in the world refuse point blank to buy a car that does less than (say 40mpg) then within a very short period of time, all cars would be able to do 40mpg. But because people will still buy cars that only do 20 to 25mpg - and there are no laws to prevent their manufacture, there is no incentive for things to improve.

Secondly, as others have pointed out, there are hard theoretical limits to the efficiency of heat engines. Also, there aren't many other readily available energy storage systems with the 'energy density' of gasoline. Over the years, people have made all-electric cars, hydrogen cars, pneumatic cars, clockwork cars, propane powered cars, steam-powered (wood/coal/oil fuelled) cars, cars that ran on chemically treated sawdust, etc...but none of them have enough energy density to compete with gasoline. So unless the general public are prepared to live with less range/acceleration/interior-space or less weight (which probably means less safety equipment) - there isn't a whole lot that can displace gasoline until we have laws to force a change - or until it becomes so seriously un-cool to drive a gasoline powered car that nobody will do it anymore.

Thirdly, a lot of the problem is with infrastructure. The technology for hydrogen powered cars already exists. There are many working prototypes of hydrogen powered cars and they work pretty well. But if there is nowhere within 20 miles of my home and/or office that sells hydrogen in the bulk and with the filling convenience that I need, I can't buy a hydrogen car. This produces a chicken and egg situation - there are almost no hydrogen filling stations in the world - so there is no market for hydrogen cars - so there is no incentive to build the filling stations.

Breaking the infrastructure problem is something that can be solved with electric cars - electrical outlets are everywhere! But electric cars don't solve the energy density problems - so you're likely to be stuck with 100 mile(ish) range or (vastly overrated) hybrids until some truly magical battery technology comes along.

Right now, there are ridiculous legal restrictions that prevent some of the technology we do have from being used! I drive a MINI Cooper - and in Europe there is a version that automatically cuts the engine when you stop the car - and then when you pull away again, starts rolling the car forwards using the starter motor rather than immediately restarting the engine. Only when you reach a certain speed (10mph or so) or if the battery starts to run low, does the engine actually start running. This is a ridiculously simple thing to do - it's hardly more than a software change - but it means that in stop/go traffic, you have an electric car. It could be retro-fitted onto most modern cars - and would cost almost nothing to do. However, it's illegal in the USA! So my car is perfectly capable of doing this - but prevented from actually doing it by a software change they had to put in to meet US regulations. Aaaarrrggghhhhh!!!!

SteveBaker (talk) 18:58, 31 August 2010 (UTC)
 * (Only partly a reply to Steve.) An article I have read puts an emissions rating of about 100g/km on electical vehicles, based on the way the United Kingdom generates energy. On the development front, Volkswagon's "L1", an internal-conbustion machine, gets 39g/km, or 180 miles per gallon. It's not quite your average car, however, but you can't say they're not trying. On the more achievable side, the article (quoting Richard Parry-Jones) suggests that cars running at about 70g/km should be mainstream by 2025. By comparison, the average UK car (and that includes what Americans would call "SUVs", and cars more common here) gets about 150g/km, it says. I think that's about 30 MPG, but I'm not sure (it would depend on other factors, I would imagine). - Jarry1250 [Humorous? Discuss.] 20:14, 31 August 2010 (UTC)


 * (ec - also only a part-way reply to the above): as long as the United States produces most of its electricity using coal and natural gas ((see this Department of Energy table for details), switching to electric cars will actually increase the carbon footprint per each mile traveled. (Coal is a much dirtier fuel than petroleum in many ways). So if the objective is to reduce greenhouse gas consumption, electric cars won't do this - what they will do is break down the dependence on petroleum. There are many reasons to make this transition; but it is not accurate to say that carbon emissions would be lower. Nimur (talk) 20:20, 31 August 2010 (UTC)


 * I think you're missing the point though. Cars are fairly long-lived things.  Cars that are 10 years old are still on the road.  While it's obviously important to get fossil fuel power generation switched over to wind/solar/wave/hydro/nuclear - we shouldn't wait for that to happen before we start to switch the world's fleet of cars & trucks over to electrical power.  In fact, it can be argued that the additional profits made available to power generation companies would hasten their ability to invest in new technology.  Another argument (which I'm a bit dubious about) is that by centralizing the pollution to one large facility (as opposed to a bazillion separate tiny ones) would make it easier to capture the pollution and fix it.  However, that has not historically been the case - cars have cleaned up their emissions considerably over the last few decades - power plants, much less so.


 * As for your claim that "Coal is a much dirtier fuel than petroleum in many ways" - well, yes, perhaps in "many" ways - but not in the one very specific way that most concerns us...CO2 production. A 4-stroke steel car engine has a maximum possible theoretical efficiency of 37% - and most cars manage around 18% if 'normally aspirated' and 20% if supercharged or turbocharged.  Contrast that with coal fired power stations which actually achieve 33% and natural gas stations which get about 50%.  The trouble is that it's not that simple.  It costs energy refine oil into gasoline - it takes more energy to haul it to gas stations which are widely and inconveniently scattered.  It takes less energy to get raw coal onto a 2 mile long train and haul it to just one destination - which is probably situated conveniently close to the mine.


 * But coal is more of a nuisance than just the CO2 - all of the ash piles (which are more radioactive than the vast majority of the waste produced by nuclear power!) - the damage they do to the water table, the sulphur compounds they generate into the air...lots and LOTS of nasty side-effects.


 * But for sure, we need to start switching our cars over to electricity in the hope that we'll be ready when clean electricity DOES start appearing.


 * SteveBaker (talk) 22:08, 31 August 2010 (UTC)


 * Well, the way I see it, even if electric cars initially are fueled by coal, there is the probable eventuality of switching to nuclear energy or geothermal or wind some other source of energy. In other words, electric vehicles have an "exit strategy" from the polluting fossil-fuel base.  So, they are advantageous for that reason.  But in the short term at least, petroleum is a more efficient fuel - economically, thermodynamically, and so on.  I think these issues all elucidate the trouble with the OP's original question: "progress" is very hard to define, because it depends on the timescales involved and the ultimate objective we are aiming at.   Nimur (talk) 01:48, 1 September 2010 (UTC)

Vision at the speed of light
This is more fiction than real world science but I'm curious none the less...

If it were possible for a person to move at the speed of light (without any other physiological changes than just whatever gave them the ability to do this), how would their vision be affected? After all, we see because light hits our retinas but what if we're moving at that speed. Then would we only see what was directly in front of us since those photons would be going directly into our eyes? Dismas |(talk) 07:56, 31 August 2010 (UTC)

At light speed, length contraction makes everything have zero length. Any photon you encounter (that would let you see in the first place) would be blue-shifted to infinite energy, or red-shifted to zero energy. Human eyes can detect neither, so you wouldn't see anything. 157.193.175.207 (talk) 08:01, 31 August 2010 (UTC)
 * Two photons colliding head-on do so with infinite(x2?) energy...?
 * Definitely not, because each has zero rest mass. The total energy is hν1 + hν2.  See our article on photon.    D b f i r s   18:19, 31 August 2010 (UTC)
 * You should start by reading through the Special relativity article, probably, Dismas.  Wikiscient  (talk) 08:11, 31 August 2010 (UTC)

You wouldn't have time to see anything. The proper time experienced along any path at the speed of light is zero. This might sound theoretical, but it has a serious consequence &mdash; it's how it was originally recognized that neutrinos have nonzero (rest) mass. If they had zero mass, they would have to travel at the speed of light; in that case, they could not change flavor, because there would be no time in which the change could happen. But there aren't enough electron neutrinos coming from the Sun; only about a third as many as there should be. The explanation offered was that they are changing flavor on the way to Earth. But then they must have mass. --Trovatore (talk) 10:00, 31 August 2010 (UTC)


 * The time issue is nasty. The instant you hit lightspeed - the entire universe would be over for you.  Whatever ultimate fate befalls the universe is where you are.  The incoming energy issue is also fatal - you get hit by an infinite number of photons and each has infinite energy...that'll ruin your entire day right there!  The universe would have contracted to zero dimensions along your line of flight - so you'd be everywhere in the universe at once - which would guarantee you'd smack into a star or something...at the speed of light! (Ouch!)  Everything you can possibly imagine about this is utterly and instantly fatal.  Fortunately, it's impossible, so this is one less thing to worry about! SteveBaker (talk) 18:36, 31 August 2010 (UTC)


 * That explanation of neutrino oscillation is incorrect. A particle of zero mass won't oscillate into some other flavor, but neither will a particle of any definite mass. There are three neutrinos with well defined masses that don't oscillate into each other (ν1, ν2, ν3). The three traditional named neutrinos (νe, νμ, ντ) are mixtures of those and do oscillate into each other, which means that the mixtures have no definite mass, which means that ν1, ν2, and ν3 don't all have the same mass. It follows that they don't all have mass zero, but the value zero plays no special role in any of this. -- BenRG (talk) 22:19, 31 August 2010 (UTC)
 * Hmm, OK, that's interesting, but just the same, doesn't it independently follow from zero proper time that there can be no oscillation? --Trovatore (talk) 22:29, 31 August 2010 (UTC)
 * I don't think the universe is "over for you" when you hit lightspeed, in terms of time dilation. Remember, from your perspective it's the universe moving at lightspeed, and it is not moving forward in time.  Of course, that infinitely blueshifted light may mean the universe is over for you in a more mundane way!
 * There's a nice simulation video of the changes in angle as you accelerate toward lightspeed... somewhere... eventually all the light compresses toward a point. I didn't find that today.  (But while looking I did find the most glorious crank theory of gravity I've ever seen, complete with a comparison of spin-2 graviton theory to Adolf Hitler... ) —Preceding unsigned comment added by Wnt (talk • contribs) 18:58, 1 September 2010
 * No, you're missing the point. Take any two events along a lightlike path; what's the interval between them?  It's zero.  And that's the same as the proper time experienced by an observer along that path.  So such an observer does not experience time passing at all.
 * The bottom line is that there is no such thing as an inertial frame of reference traveling at lightspeed. You can't set up such a coordinate system at all; you'd have to divide by zero. --Trovatore (talk) 19:11, 1 September 2010 (UTC)
 * Hmmm... I did miss some points. An observer "at lightspeed", i.e. in the limit as he approaches lightspeed, should perceive a vast swath of the universe flatten down to a two-dimensional disk moving past him at the speed of light (Lorentz contraction).  But time for that seems frozen.  The key thing that we're neglecting is that reaching lightspeed involves acceleration, and lots of it.  From the perspective of outside observers the acceleration to speed up that last little bit is very small, but the mass is enormous, and so an enormous amount of energy is involved.  From the perspective of someone traveling "near" lightspeed, adding another 0.9c involves a tremendous amount of acceleration - and of course, even an object moving relative to him at 0.9c is still slower than light in every frame.  So we're talking about just incredible amounts of acceleration, over and over.  This is discussed a bit in [], but the formula they give there "t' = t(1 + Φ / c2)", with Φ = gh, confuses me - there's a unitless "1", and it seems like the clock-ticks could go negative for objects behind the accelerating observer.
 * I am not well-versed in general relativity, but I think that the gist is that you can't really just reach the speed of light and your life is over. Rather, you can burn your Star Drive at full blast and watch time gradually slip away for as long as you like, never reaching lightspeed. Wnt (talk) 00:56, 2 September 2010 (UTC)

Direction of Earth's Revolution
Suppose one were "above" the plane of the solar system, such that, when looking at the Earth, one saw primarily the northern hemisphere. From this perspective, would the Earth appear to revolve clockwise or counterclockwise about the sun? —Preceding unsigned comment added by 203.97.79.114 (talk) 11:17, 31 August 2010 (UTC)


 * Earth's rotation says counter-clockwise. Dolphin  ( t ) 11:23, 31 August 2010 (UTC)
 * (ec)Just imagine if you are looking south from the north, the earth is rotating towards the east, which means it will go anticlockwise. Graeme Bartlett (talk) 11:25, 31 August 2010 (UTC)
 * I'm afraid you're not answering the question I asked. You've both told me about the Earth's rotation about its own axis; I'm asking about its revolution about the sun.  —Preceding unsigned comment added by 203.97.79.114 (talk) 11:44, 31 August 2010 (UTC)


 * Sorry about that. Earth's orbit says counter-clockwise.  Dolphin  ( t ) 11:48, 31 August 2010 (UTC)


 * Great, thank you. —Preceding unsigned comment added by 203.97.79.114 (talk) 11:49, 31 August 2010 (UTC)
 * Interestingly, Earth's rotation does answer it though, in the very first article section, with two diagrams including one from with the questioner's actual perspective. DMacks (talk) 11:50, 31 August 2010 (UTC)
 * I would argue that the first diagram doesn't, since there's no way to tell if we're viewing the day side or the night side of the Earth. The second does, though.  —Preceding unsigned comment added by 203.97.79.114 (talk) 12:00, 31 August 2010 (UTC)
 * Sure enough! The image's own description page has it ("Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left)") but the caption on the image in the article does did not. Fixed, good catch! DMacks (talk) 12:12, 31 August 2010 (UTC)
 * Your caption change didn't last. -- ToET 14:08, 31 August 2010 (UTC)
 * Le sigh. DMacks (talk) 14:57, 31 August 2010 (UTC)
 * Wait, no, it would be going from left to right regardless of which side you were on, as long as you kept north as up. I disagree with your caption.  —Preceding unsigned comment added by 174.91.10.238 (talk) 16:50, 31 August 2010 (UTC)
 * The concern is with the direction of revolution, not rotation. -- ToET 17:19, 31 August 2010 (UTC)
 * ...same confusion (wrong technical term for poster's actual question) that got several of us off in the wrong direction (so to speak) with the answer. DMacks (talk) 18:16, 31 August 2010 (UTC)
 * Note also that the moon rotates and revolves the same way as well (counter clockwise when viewed from well above the earth's north pole). See Orbit of the Moon. -- ToET 14:08, 31 August 2010 (UTC)
 * There is of course nothing except our own interpretation that makes north "up" and south "down", it would be just as valid to call the Antarctic "up", it's purely because theres people in the northern hemisphere who sit more on top of the globe then below it. However it's so ingrained on every single map and every globe that even I in Australia struggle to make the switch just in my head. Vespine (talk) 23:56, 31 August 2010 (UTC)
 * Yeah, I imagine a lot of mental exercises are more difficult, given that you're distracted by having to hang from the ground by your feet all the time. --Trovatore (talk) 20:55, 2 September 2010 (UTC)

"Reverse osmosis"
Should "reverse osmosis" really be called "reverse osmosis," as if it's the reverse of osmosis? Osmosis is the movement of water across a semipermeable membrane from an area of high water potential to an area of low water potential. If I've understood reverse osmosis, all they are doing is increasing the pressure on one side of the membrane with the "dirty" water, so that the water moves across to the clean side. But since water potential is already defined in terms of pressure, among other things, the water is still just moving from an area of high water potential to an area of low watert potential. So it's still just regular ol' osmosis, right? 76.24.222.22 (talk) 11:37, 31 August 2010 (UTC)
 * Yep, thermodynamically it's just regular ol' osmosis: as you say, the chemical potential of the water is increased on the "dirty" side of the membrane so that it's higher than the chemical potential on the "clean" side. It's called "reverse" osmosis because most people think of osmosis as water moving the other way, so that solute concentrations on the two sides of the membrane are equal, but that is just a limited conception of osmosis. Physchim62 (talk) 12:48, 31 August 2010 (UTC)
 * Alternatively, osmosis is defined as the movement of solvent from the low concentration side to the high concentration, then the opposite is correctly called reverse osmosis.   D b f i r s   17:51, 31 August 2010 (UTC)

I have a theory on what the crystal skulls actually are
i would like someone to contact me about the crystal skulls it may be that its a far feched theory but i believe they hold some sort of message could you please contact me asap as i believe i know the key to opening this infomation.. thankyou contact me on (e-mail address removed) —Preceding unsigned comment added by 86.139.245.30 (talk) 12:23, 31 August 2010 (UTC)


 * Wikipedia has nothing to do with the crystal skulls. You should read the article to find someone who may be interested. --  k a i n a w &trade; 12:28, 31 August 2010 (UTC)


 * We don't contact people by e-mail, and you should not post your e-mail address here. I have removed it, to combat spam. --Mr.98 (talk) 13:10, 31 August 2010 (UTC)


 * Wikipedia is specifically not interested in ideas you have just dreamed up (see WP:NOR) - this reference desk is for asking questions and getting answers - not for recruiting people to listen to whatever ideas you've had. If you had a specific question about the crystal skulls that our article was unable to answer - then it would be appropriate to ask it here. SteveBaker (talk) 18:28, 31 August 2010 (UTC)


 * His email address is still available in the history. Aren't these supposed to be permanently removed? I'm just curious. –  Ker αun oςc op ia◁ galaxies  18:57, 31 August 2010 (UTC)


 * Please tell me how to find it !  Jon Ascton    (talk)  16:17, 1 September 2010 (UTC)


 * Permanently removing ANYTHING from Wikipedia is amazingly difficult. Even an admin can't completely eradicate data from the system - it takes yet higher levels of system privilege to do that.  Here on the ref.desks we seem to have evolved a system whereby we simply remove the email address from the current version and leave it in the history.  IMHO, that's a waste of time - if our OP's wish to leave their email addresses and risk getting spammed - that's their own call.  I leave my email address exposed on my User page and I've not noticed any change in the volume of spam I get as a result.  Anyway - what really matters is that we're not going to email answers to people - much less get involved in some kind of private debate. SteveBaker (talk) 21:46, 31 August 2010 (UTC)


 * Anyway it's how we generally handle things. The issue is less a bot going through the history looking for addresses (what a waste of time!) than it is the many pages that mirror "active" content and put it all over the place. --Mr.98 (talk) 00:48, 1 September 2010 (UTC)

Eye Surgery
Strictly speaking how safe is the surgery that gets you rid of spectacles ? Jon Ascton   (talk)  14:09, 31 August 2010 (UTC)


 * See Refractive surgery -- k a i n a w &trade; 14:11, 31 August 2010 (UTC)


 * This section doesn't mention that certain measure may be OK only for spherical glass - problem and not for cylindrical


 * Also if you want a lot of angry, vocal testimonials from unfortunate people who have had permanent eye damage from the surgery, google lasik disaster or the like. Comet Tuttle (talk) 16:28, 31 August 2010 (UTC)


 * Someone deleted this question as a request for "medical advice". However, general discussion of a medical procedure, including its risks, is well within the scientific purview of Wikipedia - it's no different than the question above about whether you can be electrocuted by standing on a 220V plate.  As always, be disclaimed that we can't tell you what to do. Wnt (talk) 04:15, 1 September 2010 (UTC)


 * Is it a myth or a fact that it works only on cases with spherical lens and not cylindrical lenses ?

At North Pole
If I stand with a magnetic compass exactly on the North Pole, what direction will it point ? Downwards ? Jon Ascton   (talk)  14:31, 31 August 2010 (UTC)


 * If you hold it sideways then yes, it will point straight down. -- k a i n a w &trade; 14:35, 31 August 2010 (UTC)
 * Wouldn't it just point to the North Magnetic Pole? Deor (talk) 14:38, 31 August 2010 (UTC)
 * Well, slightly to that side of vertically down, if by North Pole you mean the point of intersection of the meridians. Incidentally, in scientific terms, the North Magnetic Pole is really a south pole, which is why the "north" of the magnetized needle is attracted to it.  At some unknown time in the (arguably fairly near) future, it will probably flip and become a true north pole.    D b f i r s   17:45, 31 August 2010 (UTC)


 * Actullay if you take a compass that's constructed for use by the equator and use it in for example Sweden, the needle would point a little downwards scraping the compass floor effectively making it less usable. Instead they come with a small counterweight adjusted for the latitude where it's intended to be used, so the needle will stay horizontal. The needle will be attracted more and more downwards as you get clooser to the north pole. Moberg (talk) 14:41, 31 August 2010 (UTC)


 * There are "three-dimensional" compasses if that's the right term, they should exactly point downward ?


 * Historically, this vertical component of Earth's local magnetic field – with the delightful name magnetic dip – has been measured by using a separate instrument called a dip circle or dipping needle. TenOfAllTrades(talk) 15:29, 31 August 2010 (UTC)
 * Right - but a dip circle is really just a sideways compass. They take some trouble with the bearings - but aside from that, it's nothing special.  If you imagine a "3D compass" (maybe a needle mounted on a ball-and-socket joint or something) then as you'd approach the magnetic north pole, the needle would continue to point towards it - but more and more weakly - and start to dip downwards more and more strongly.  Right at the pole, the 'pointing North' force has dropped to zero and the thing would point straight down at your feet. SteveBaker (talk) 18:24, 31 August 2010 (UTC)
 * I think the above explains it very well, but it may be worth just mentioning that that there isn't really a single "point" where you are exactly at the north pole. Well there is but you'd be hard pressed to figure out you are there if all you had was a compass. If you look at the diagrams at Earth's magnetic field it becomes easier to see that it's more of a north "zone" rather then a point. It's not like your compass is pointing to your front, then you take a couple of steps and all of a sudden it's pointing backwards, or even worse, spins like a top, like I've seen depicted on more then one occasion. Even with a special compass like Moberg suggests, when you are in the "zone" walking in any direction would not make much noticeable difference. To really work out if you are right on the north pole, I think you'd probably have to plot several points around a fairly substantial area and then derive an average. Vespine (talk) 00:31, 1 September 2010 (UTC)
 * Yes, indeed. The amount of force rotating the needle in a horizontal plane drops off dramatically - even at 100 miles from the magnetic pole.  You need an increasingly sensitive compass at those latitudes to get any reading whatever.  The dip angle is more useful though - but (as you say) it's still not exactly sensitive enough for accurate navigation.  There are lots of additional complications - for one, the earth isn't a perfect dipole and the magnetic poles wander around a lot from year to year.  Right now, the magnetic south pole is out in the ocean somewhere - it's nowhere even close to the true pole about which the planet rotates.
 * But the ambiguity in our latitude/longitude/heading system at the poles is very real. There is an old puzzle that goes:
 * Q: A hunter leaves his tent, walks one mile south, one mile east, shoots a bear and then walks one mile north to arrive back at his tent...what color was the bear?
 * A: White.
 * Most people think that the North pole is the only place that this story would work - but actually, there are a bunch of places near the South pole where the story would work just as well...except that there are no bears there. It takes a bit of a mental stretch to visualize where those places are though!  The easiest ones to imagine are points that are one mile North of the line of longitude that wraps around the south pole and is exactly one mile long - so in walking east for one mile, the man makes a complete lap of the pole and ends up following his own footsteps back for another mile, heading north.  However, there is another place that's one mile north of the line of longitude that's a half mile long...then the man makes two laps of the pole as he walks east before returning to his tent.  SteveBaker (talk) 04:38, 1 September 2010 (UTC)


 * In case you asking about the geographic North Pole, then at that location a compass will point towards the magnetic North Pole (and also downward). Of course, if you are standing at the geographic pole, then it will be to your South since all directions from there are South by definition.  Dragons flight (talk) 05:03, 1 September 2010 (UTC)
 * ...which of course would be very odd. Your compass would (weakly) point 'North' - and the magnetic pole would be to your south!  SteveBaker (talk) 12:26, 1 September 2010 (UTC)
 * "Weakly"? See Geomagnetic field. And it would point south towards magnetic North from the geographic pole.  Wikiscient  (talk) 13:36, 1 September 2010 (UTC)

DIET PILL OVERDOSE
Hello,Wikipedians I was wondering what happens when a person overdoses on diet pills? Thank You and Have a great day. —Preceding unsigned comment added by 173.56.147.239 (talk) 15:27, 31 August 2010 (UTC)
 * A warning in advance...we can't give medical advice here. That said, we do have an article on anti-obesity medication, which covers a big range of drugs. Find the one you're interested in on the list, click its article and see if it has overdose information on the page. You'd have to be more specific with exactly what drug you're interested in to get a more detailed answer. Vimescarrot (talk) 15:56, 31 August 2010 (UTC)
 * You should never overdose drugs... that's why they set doses! --Chemicalinterest (talk) 16:21, 31 August 2010 (UTC)
 * The hazards of overdosing on individual drugs will vary depending on the drug. Members of a class of drugs such as "diet pill" will not necessarily have the same, or even similar, hazards.  -- Ed (Edgar181) 19:44, 31 August 2010 (UTC)

In my OR experience, in high school there was a dieretic that would give you energy and thus you would not need to eat, we all ate about 4 boxes each, (50 pills per box, take 2 at a time 3 times a day) It was very much like an adrenalin boost, then we felt really sick, then started taking other cocktails so I cant remember the effect after about 2 hours. But I would never do it again, the effect was overall not enjoyable, and not narcotic as we had hoped. try strepsyls or cough mixture instead.

How To Pronounce "Feynman"
Sorry if this question is a bit trivial, but I couldn't find anyone pronouncing his name. Even his online lectures have edited out the start, where he presumably introduces himself. My questionn is, quite simply, after reading Surely You're joking and starting on the Feynman Lectures on Physics (I need to improve my calculus skills), it struck me that I don't know how to pronounce his name, having seen it only in print. I had a look at the article, but couldn't make head 'nor tail of that IPA stuff. Two options presented themselves to me: "fine-man", or "fain-man" (Fain rhyming with vain). In case it matters, I would pronounce it with an english accent, living as I do in England. Thanks very much.--HarmoniousMembrane (talk) 16:01, 31 August 2010 (UTC)
 * FINE-man (stress on the first syllable). Physchim62 (talk) 16:08, 31 August 2010 (UTC)
 * Actually FINE-munn is more like it, I believe. Looie496 (talk) 16:17, 31 August 2010 (UTC)
 * You're quite correct. Indeed, carefully listening to this video to see how he pronounces that semi-vowel, I would write it FINE-muhnn (as you might expect from someone from New York). But I'm a native speaker of British English (of the Northern English variety), so pronouncing non-stressed "man" as "munn" is second nature to me ;) Physchim62 (talk) 23:18, 31 August 2010 (UTC)

Thanks Very Muchly, guys. :)--HarmoniousMembrane (talk) 16:30, 31 August 2010 (UTC)
 * If you use Firefox, you might consider this Greasemonkey plugin which translates IPA into "B as in Boy": . --Sean 17:34, 31 August 2010 (UTC)
 * Thanks for that link Sean, I do indeed use Firefox, and that script is getting installed right now!--HarmoniousMembrane (talk) 21:27, 31 August 2010 (UTC)


 * Sadly, I find that script fails to read the 'Feynman' IPA: it doesn't give anything for /a/ and fails to read /aɪ/ as a diphthong :( A shame, since it's a good idea. 86.161.108.172 (talk) 21:53, 31 August 2010 (UTC)
 * That is unfortunate. It definitely needs some love as I haven't found it to be useful for non-English IPA at all. Still better than nothing, though, IMO. --Sean 18:26, 1 September 2010 (UTC)
 * FYI, I've fixed this bug and reported it to the developer. See here if you want to update your local copy of the script. --Sean 19:15, 1 September 2010 (UTC)
 * Yay! Thanks, worked well. 86.161.108.172 (talk) 20:47, 1 September 2010 (UTC)

Number of boobies
Did humans evolve from something with more nipples, or did cats evolve from something with fewer nipples, or did they both evolve nipples independently from a mammal that didn't use nipples, like a platypus? —Preceding unsigned comment added by 174.91.10.238 (talk) 16:40, 31 August 2010 (UTC)


 * As humans are thought to have descended from a small shrew like critter mammalian line (Epitheria),  it is very possible. Even today, some  people are born with  Supernumerary nipples.--Aspro (talk) 17:20, 31 August 2010 (UTC)


 * Humans and cats almost certainly didn't develop nipples independently; they are both Eutherian mammals. However their evolutionary histories split around 100 million years ago (humans belong to Euarchontoglires, cats to Laurasiatheria), so there was a lot of time for differences to develop. Looie496 (talk) 17:40, 31 August 2010 (UTC)


 * Re-reading, I think the question has not been clearly answered yet, especially as two different taxonomies have been used which might confuse. So here is some more:
 * Fossil records suggest that the platypus, cats and man derived originally from the Eutherian group. A new group diverged away from this when the modified sweat glands (that produced the proto-milk) evolved into   clearly defined  nipples.  Palaeontologist have named  this group  Epitheria (but perhaps not for this reason alone). Cats and man came from this later group which divided several more times.  Fossil evidence further suggests that, the animal  from which man evolved from was quite small. Evolution appears to disfavour small litters in small animals and so it is thought likely by some that these animals would have at least one pair of nipples in order to suckle larger litters. However, by the time they had split off again and finaly evolved into primates the number had been reduced to one pair and single births. Fossils do not usually record soft tissue so the actual stage of evolution that this happened can only be guesstimated  at.  So, it is quite likely that originally several sets of nipples evolved all at once in the common ancestor of 'human and cat' and then reduced down to the one pair in man today.--Aspro (talk) 20:23, 31 August 2010 (UTC)
 * So in short, going from little furry thing to big primate means smaller litters, which means you don't need as many nipples. Thanks.  --174.91.10.238 (talk) 03:06, 1 September 2010 (UTC)
 * That all makes sense except the bit about the platypus, which as a monotreme doesn't belong with the others. Looie496 (talk) 22:33, 31 August 2010 (UTC)
 * You might be right, as I first did this before man first walked upon the surface of the moon, and jeans where something one wore... and my memory is going! Which taxonomy are we taking about? Fossil evidence is dependant on morphology.  Looking at the  genes and their occurrence in other animals don't over lap exactly with fossil evidence. The two taxonomies  don't yet match perfectly (but they could go on to explain horizontal transmigrations and other esoteric theories). The OP asked a simple question and I think we have answered that now. --Aspro (talk) 23:16, 31 August 2010 (UTC)
 * The sub section:  Fossil-based family tree of placental mammals shows the palaeontological tree.  It might come down to tectonics and the geological dates of continental separation. Which group does the platypus not belong to? Molecular phylogenetics however, cannot state, what they state as all 'fact', as they have not a shred of  hard evidence (i.e. fossils). Modern mammal  (aboriginal man) only arrived  60,000 years ago. --Aspro (talk) 23:57, 31 August 2010 (UTC)
 * The one problem with doing it by tectonics is figuring out how the opossum managed to get all the way to the Americas. Unless it was already there before all of the placental mammals showed up.  --174.91.10.238 (talk) 03:06, 1 September 2010 (UTC)
 * Marsupials are now thought to have developed first in South America, probably after it had split off from Africa (in the breakup of Gondwana) but while it was still connected to Antarctica and Australia. The ones in Antarctica have vanished for obvious reasons.  Opossums, and other South American mammals such as armadillos and sloths, did not reach North America until the land bridge opened around 10 million years ago. Looie496 (talk) 04:12, 1 September 2010 (UTC)
 * The platypus is not a placental mammal, it lays eggs. Monotremes are generally believed to have branched off at the earliest node that has surviving offshoots, earlier even than the branching between placentals and marsupials. Looie496 (talk) 03:57, 1 September 2010 (UTC)


 * Actually, a question that has always interested me is when breast tissue migrated to the chest. in every non-primate I can think of (including most monkeys) nipples are situated on the belly, and often (as with cows) on the lower parts of the belly - on humans, chimps, and gorillas, though, they are placed over the rib cage.  I can see the evolutionary advantage of this (allowing an infant to be fed while held in the arms, which makes it possible to walk and feed a child at the same time), I just don't know where the evolutionary spit for it lies. -- Ludwigs 2  22:32, 31 August 2010 (UTC)


 * The article nipple claims that humans develop additional nipples that regress during fetal development. Unfortunately it provides no source.  I found this paper, which examines tissue from uncertainly dated 14-18 week old fetuses, and finds no evidence of milk line segmentation (barely of the milk line at all).  But they are looking too late - mostly, they complain about lack of availability of fetal samples.  I'd hoped that other countries wouldn't have this ethical notion that it's OK to abort a fetus but not to look at the tissue afterward (at least, not if it could benefit scientific knowledge or human health...), so I wouldn't rule out that the Wikipedia article really did have some valid source for events around 4-6 weeks development. Wnt (talk) 19:06, 1 September 2010 (UTC)


 * I was actually curious about phylogony, not ontogeny. -- Ludwigs 2  01:49, 2 September 2010 (UTC)


 * Not really true. Consider a pig (and lots of other animals are similar) - see here for example. The nipples go pretty much right up to just below the front legs, similar to the location in humans. I'd also argue most (all?) primates have nipples up on the chest, not the belly - consider even something relatively distantly related like a lemur, those nipples are right up there. It's also wrong to say they're on the chest to allow "an infant to be fed while held in the arms, which makes it possible to walk and feed a child at the same time" - not many non-human primates could accomplish this, even our close cousins chimps and gorillas are knuckle-walkers, which would make this scenario almost impossible. Most likely the ancestor had multiple nipples, with the extra ones lost and just the upper ones retained. The small number of young raised by primates at any one time - usually one or two at most - would mean no need for the extra nipples and they would be selected against. Retention of the upper nipples would be favoured as it would allow for 'nursing' the young to feed while in dangerous positions like high up in trees. --jjron (talk) 14:28, 2 September 2010 (UTC)

Sodium bicarbonate deodorant properties
I use sodium bicarbonate as a deodorant. What makes it work? --Chemicalinterest (talk) 19:05, 31 August 2010 (UTC)
 * Sodium bicarbonate is a weak base that can react with some odor-causing acids in sweat (such as butyric acid) to form non-volatile salts. -- Ed (Edgar181) 19:41, 31 August 2010 (UTC)

F5
Has an F5 tornado   ever touched down in ireland and is it possible for one too touchdown here. --86.41.136.150 (talk) 20:54, 31 August 2010 (UTC)
 * Our list of F5 and EF5 tornadoes does not include Ireland; however, our list of European tornadoes and tornado outbreaks notes that Ireland was home to the first recorded European tornado. As such, I expect that it's possible, albeit unlikely, for an EF5 tornado to form in Ireland. &mdash; Lomn 21:21, 31 August 2010 (UTC)


 * In the US there is a formal process to survey tornado damage and estimate intensity (managed by NOAA, I believe). Unless there is a similar process in Ireland, there may be know way of accurately knowing the intensity of most tornadoes in Ireland.  Dragons flight (talk) 21:33, 31 August 2010 (UTC)


 * Climatologically speaking there is very, very little chance that this will happen. Violent (F4-F5) tornadoes are rare outside of the United States, and this does not mention that strong tornadoes form that often in the UK or Ireland. Also, if my memory serves me correctly the only locations that have a tendency to suffer strong-violent (F2-F5) (relatively) frequently are the United States, Canada, Australia, Bangladesh, and parts of mainland Europe (don't take this at face value, my memory may be wrong). The list of European tornadoes says that the strongest British tornado is probably the London Tornado of 1091, which should give an idea of what strength Irish tornadoes could have. Ks0stm (T•C•G) 01:14, 1 September 2010 (UTC)

Binoculars - minimum focusing distance
How can the minimum distance for binoculars be calculated? —Preceding unsigned comment added by Kumalo (talk • contribs) 21:40, 31 August 2010 (UTC)
 * I don't think this can be worked out from a formula because it depends on how the focus mechanics are designed. Theoretically, to focus on something closer you'd just turn the focus knob more, however you'll hit a mechanical limit long before you hit a optical limit to how far you can turn the knob. Vespine (talk) 23:28, 31 August 2010 (UTC)

Cats' senses
hi. I asked this question as a follow up to a different question but nobody responded. How much do the acuity of the senses vary between house cats and wild cats (such as cougars, lions, tigers, etc.) and I read the article but I didn't understand how exactly a cat's vision is inferior to a humans in light. Are they nearsighted, farsighted? Or do they see blurrier? Or what? 76.229.235.27 (talk) 23:22, 31 August 2010 (UTC)


 * The Cat senses article says: "Testing indicates that a cat's vision is superior at night in comparison to humans, and inferior in daylight."
 * "Night vision" in mammals mostly involves "rod cells" in the retina; cats have more of these rod cells in their retina than humans do. But that means they have proportionally fewer cone cells than humans, which are the cells in the retina that specialize in responding to relatively bright light.  These cone cells are also more "tuned" to respond to color, detail, and rapidly changing images.  So it is in those ways that cats have "inferior" bright-light vision than humans, because they are relatively specialized for seeing in low-light conditions.   Wikiscient  (talk) 07:19, 1 September 2010 (UTC)