Wikipedia:Reference desk/Archives/Science/2009 January 17

= January 17 =

Will Uranus and Jupiter eventually lose it's rings?
Will Uranus and Jupiter eventually lose it's rings, since Jupiter and Urnaus rigns is made of charcoal, and it's even too dark to be seen by spacecraft (almost black). Saturn's rings has also been slowly eroding and getting browner, and eventually Saturn's rings will disappear.--69.111.37.66 (talk) 01:35, 17 January 2009 (UTC)


 * If the rings are unable to be seen by spacecraft, how do we know they are there? And I don't know of any significant observed colour change or erosion of Saturn's rings. I think you need to check the reliability of wherever you got this information. As for the stability of ring systems - according to Planetary ring: "Most rings are thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it appears that Saturn's rings might be quite old, dating to the early days of the Solar system." --Tango (talk) 02:03, 17 January 2009 (UTC)


 * Wouldn't the rings eventually get pulled into the planet? 96.242.34.226 (talk) 02:05, 17 January 2009 (UTC)
 * They're just as likely to be perturbed away from the planet, or get pulled into one of the shepherd moons. It's a little unclear why Saturn's rings are so stable, but it presumably has something to do with the shepherd moons. --Tango (talk) 02:39, 17 January 2009 (UTC)


 * The rings are not easily seen - but they were detected by the fact that they occluded the planet's own disk in some kind of characteristic manner. I believe Voyager observed Uranus's rings by observing the occlusion of a distant star by the ring system. SteveBaker (talk) 03:54, 17 January 2009 (UTC)
 * When spacecrafts take pictures they use black-and-white images with purple, orange, and brown lights on the back. To you, when you orbit around Jupiter and Uranus' rings, they look nearly black, but that have nothing to do with this OP. My question is would Uranus and Jupiter eventually lose it's rings? THe article Tango linked me said most planet's rings will deprecate over time, and Earth could have rings a loong time ago. It is not only because planet eats it, meteorites comes any time, and wipe fragments out.--69.111.37.66 (talk) 04:00, 17 January 2009 (UTC)


 * Well Rings of Jupiter says that the material from the Jovian ring system is continually falling into the planet - and is replenished by meteor impacts onto various moons and moonlets. I suppose that means that they are going away - eventually the moons will be eroded to nothing - then no more source of dust and sometime later, no more ring system.  Rings of Uranus says that Uranus's rings are relatively recent - and it also remarks that their formation mechanism is similar to the Jovian rings - so one presumes they'll suffer the same fate.  The article says that without replenishment, the rings might only last 100 to 1000 years!  Also, it mentions that if there we have not detected any 'shepherd moons' adjacent to some of the rings - and that they would be expected to dissipate for that reason if no such moons are there.  I don't think the color of the rings matters in this regard...although in the case of Saturns rings (which are made of ice - not dust) - I suppose a darkening of them might cause them to absorb more sunlight and get a little warmer - which I suppose might in turn cause some evaporation from the ice...but that's a long shot.   I strongly recommend that you read the two articles I've linked to because there is a lot of information in there - too much to convey conveniently in a Ref Desk post. SteveBaker (talk) 05:15, 17 January 2009 (UTC)


 * Steve, the part where you said the moons may erode to nothing may very well take far longer than the projected life of the solar system. Since we are maybe halfway through that life now, waiting for the Sun to go nova die, I'd expect the moons to last until then. StuRat (talk) 06:24, 17 January 2009 (UTC)


 * Our sun will not "go nova". For a start, you probably mean supernova, not nova, and secondly, the sun is nowhere near big enough. It will turn into a red giant and then collapse into a white dwarf, nothing particularly dramatic - it certainly won't do anything to the moons of outer planets. --Tango (talk) 16:07, 17 January 2009 (UTC)


 * Why would I mean "supernova" ? That requires even more mass. StuRat (talk) 17:12, 17 January 2009 (UTC)


 * Because a nova can only happen in a binary system, and we don't live in a binary system. A supernova is not just a big nova, it's a completely different thing (well, 3 completely different things, actually) - read the articles. --Tango (talk) 17:20, 17 January 2009 (UTC)


 * OK, but does this mean that the outer planets (and their moons) will last forever ? Or will they end up sucked into the giant black hole in the center of the galaxy or be destroyed in some other way ? StuRat (talk) 05:32, 20 January 2009 (UTC)


 * If you wait long enough, something will happen to them, although it's not clear what. They could very slowly spiral into and crash into the white dwarf that is left of the Sun due to the radiation of gravity waves, they could be perturbed out of their orbits by a passing star, their protons could decay, they could be ripped apart by the ever accelerating expansion of the universe, they could end up falling into a passing black hole, etc., etc. See Ultimate fate of the universe and the pages linked to from there for details of the possibilities. --Tango (talk) 16:27, 20 January 2009 (UTC)


 * Do we lack an article for the Ultimate fate of our solar system ? StuRat (talk) 20:51, 20 January 2009 (UTC)


 * Not a single article, but Red giant discusses the death of the Sun, and the various fate of the universe articles give an idea of what might happen after that. --Tango (talk) 22:25, 20 January 2009 (UTC)


 * There's a huge amount of time between the two, though, especially if we are talking about proton decay as the end of the universe. Won't something else destroy the solar system on that time scale ?  What about our Milky Way galaxy, won't it eventually be destroyed prior to the death of the universe, due to collisions with other galaxies and/or the black hole at the center, and won't that take our solar system with it ?  StuRat (talk) 14:58, 21 January 2009 (UTC)


 * Collisions between galaxies rarely have any effect on individual stars - galaxies are mostly empty space, they pass straight through each other (gravity and tidal forces distort them and slow them down so they eventually merge, but the individual stars just carry on as they were). Apart from radiating gravity waves (which is an extremely slow process) there is nothing to cause stars orbiting a black hole at a safe distance to fall into it. Once the Sun dies, the solar system will just sit here slowly cooling down for billions of years undisturbed (passing stars may perturb a few comets out of the Oort cloud towards the inner solar system every now and again and we may get irradiated by nearby supernovae, but that's about it). Wait long enough and random interactions and perturbations and whatever else will eventually result in everything ending up in black holes, but you're talking trillions of years for that, I think... we have a timeline somewhere... Here it is: Future of an expanding universe --Tango (talk) 23:19, 21 January 2009 (UTC)


 * Thanks for the answer, but I disagree that collisions of galaxies rarely have an effect on individual stars. The stars are pulled out of their orbits by gravity and strewn all about, as shown on this page: .  I'm not sure what would happen to the planets associated with such stars, they might be thrown out of their stellar orbits, too. StuRat (talk) 04:08, 22 January 2009 (UTC)
 * Sure, the arrangement of stars is affected, but nothing happens to the star itself. Planets are very tightly bound to their stars, another star would have to pass very close to cause any significant change in planetary orbits (which will happen if you wait long enough, but you're talking on the scale of quadrillions (1000s of trillions) of years according to the timeline I linked to). --Tango (talk) 15:54, 22 January 2009 (UTC)


 * The Earth is believed to have had a ring formed of debris from a massive collision which coalesced into the moon over the course of a few hundred or thousand years. See giant impact hypothesis. --Tango (talk) 16:07, 17 January 2009 (UTC)

Darvocet (Propoxyphene) Overdose Dosage
This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis or prognosis, or treatment recommendations. For such advice, please see a qualified professional. --Jayron32. talk . contribs 01:52, 17 January 2009 (UTC) --Jayron32. talk . contribs 01:52, 17 January 2009 (UTC)


 * Jayron, I have not asked for a prognosis. I have asked for a fact. Propoxyphene has a toxic dosage, and I have asked what that dosage is based on weight and gender. I am not asking for advice or counsel. I need not a qualified professional to answer a simple question.


 * The fact you asked for can only be given by a qualified medical professional. If you have a question about the correct dosage of a drug to take, consult a doctor, not random strangers on teh intertubez... --Jayron32. talk . contribs  02:09, 17 January 2009 (UTC)


 * Considering the nature of my question, consulting a doctor would be... undesirable. Once again, Wikipedia proves itself to be full of cancer. If I thought it was important enough to find a doctor to talk about this, I would have done just that. I'm asking for a simple answer to a simple question. I could be mistaken, but I believe general toxicity dosages are available for other substances. Whatever, though. I'm not going to piss around with this with you. Hopefully it won't matter tomorrow. 71.115.157.126 (talk) 02:30, 17 January 2009 (UTC)

Suicide response


 * Well, if asking a doctor becomes undesirable, then that implies that the drug has not been duly prescribed by a doctor. This is quickly becoming a legal issue.  We don't give that sort of advice either. --Jayron32. talk . contribs  02:36, 17 January 2009 (UTC)
 * If you believe someone may have taken an overdose, they need to see a doctor. They can do so completely confidentially, they don't even have to give a name, but they do need to seek medical advice immeadiately even if there's only a small chance they have overdosed. The toxicity dosages you can find online won't help you since they won't take into account the patient's medical history and any other medication they may be on, only a doctor can give you accurate numbers. --Tango (talk) 02:44, 17 January 2009 (UTC)


 * Since it was deleted I'm not sure how exactly the question was phrased, but it sounds like people are getting paranoid again. Not every question is a medical or legal advice. If someone wanted to take an overdose, all they'd have to do is take too much. This person asked for the exact documented dose which simply means digging in the literature and finding an LD50 value. - Mgm|(talk) 11:53, 17 January 2009 (UTC)
 * Although I think suicide prevention would be an important thing. While we can't give medical advice, we have gone above and beyond on things like dating advice,a and I think it's important to mention that there are many positive thigns that life has to offer. the person may need to make a total change of his or her life, but it can be done. As a lot resort, I will add this, as odd and out of place as it may seem. But, Jesus Christ loves you, and has made grat changes in my life. he can in yours too, 71.115. Please choose to live.209.244.30.221 (talk) 14:00, 17 January 2009 (UTC)
 * My first thought was that it sounded like a request for advice on suicide, but the "Hopefully it won't matter tomorrow." comment suggested the dose had already been taken and the OP was wondering if whoever took it needed medical help or not and was too afraid to go to a doctor unless it was absolutely necessary. --Tango (talk) 16:16, 17 January 2009 (UTC)

Tidal lock with the sun
Let's imagine that the Earth had a tidal lock with the Sun. Obviously the half that was facing the Sun all the time would be roasty-toasty to a degree intolerable for life, and the half always facing away would be too frozen over for life. But what about the border region, right on the line? How big of a line would that be, what temperature would it be? Obviously I'm not asking you to work out the specifics but I'm just curious what would happen generally speaking. Would it be a strict sun/not sun divide, or would there be any sort of overlap? Assuming the planet still had an atmosphere, would that lead to all sorts of crazy weather phenomena at the interface line? Just a strange idea I had today, thought I would ask on here... --98.217.8.46 (talk) 02:45, 17 January 2009 (UTC)


 * The ring around the middle would get sunlight in the same way we get it during sunset or sunrise. What affect that would have on the planet, I don't know, although extrasolar planets like that have been discovered, I think (although they are gas giants, not Earth-like). I do seem to remember such a planet appearing in a scifi book, but I don't recall the details. --Tango (talk) 02:48, 17 January 2009 (UTC)
 * There are, I believe, many such sf stories, with the earliest being set on Mercury when it was still believed to be locked. I can't remember any titles though (might have a look tomorrow). Algebraist 02:51, 17 January 2009 (UTC)


 * Weather wise your biggest issue is likely to be wind due to the atmosphere on one side of the planet being rather warmer than the other side.Geni 05:40, 17 January 2009 (UTC)


 * The condition of the earth would depend on the temperatures of the lighted and dark sides. If the dark side is cold enough to freeze solid the oxygen and nitrogen of the atmosphere, and the temperature on the warm side keeps them a gas, the atmosphere on the warm side would have flowed into the vacuum produced by the freezing, and all the atmosphere would be a frozen mass on the dark side.
 * At first, libration would alternately freeze and unfreeze some of it in the libration zone, but the gas released would have flowed into the dark side, be frozen there, and will stay there permanently. Some of the released gas will stay on the warm side, but its pressure will gradually force all of it to the cold side. Eventually, no frozen atmosphere will be left in the libration zone. There would be a liquid phase between the solid and gas states in the libration zone.
 * Various other conditions could exist if the temperature on the cold side is a bit higher. Oxygen liquifies at about 54 K and solidifies at about 90 K. Nitrogen liquifies at about 63 K and solidifies at about 77 K. I started to write up the various conditions for the two substances on the two sides of the planet, but it got too lengthy to include here. Briefly, one gas could be liquid and the other solid, or both gases could be liquid, or both solid. Also, the various minor gases in the atmosphere are a complicating factor.
 * If the temperature on the cold side is above 54 K, the atmosphere will be a gas all around the earth. But atmospheric pressure will be less on the warm side, causing a complex wind pattern, probably with wind in opposite directions at different altitudes. Libration will introduce further wind complication. I can't predict what the winds would be, but neither can professional meteorologists reliably predict winds on present-day earth, so I don't feel too dumb about that. – GlowWorm. —Preceding unsigned comment added by 174.130.253.174 (talk) 06:04, 17 January 2009 (UTC)


 * Surely they condense at a higher temperature than they freeze? And the condensing point will presumably depend on pressure, so it's all rather complicated. --Tango (talk) 16:22, 17 January 2009 (UTC)


 * The oceans on the dark side would eventually freeze solid, and water from the light side would evaporate and the water vapor would cross to the dark side and fall as snow. So, eventually, all the water would find it's way to the dark side as ice. The dark side temps might drop to those which would allow for dry ice. If so, the carbon dioxide in the air would all freeze out when it circulated to the dark side. I don't think we'd get temps anywhere near as low as is needed to liquify, much less solidify, oxygen and nitrogen.  The cold side would still be heated by tidal forces from the Moon, reflected sunlight from the Moon, radioactivity, volcanism, and heat transfer from the lighted side, after all. StuRat (talk) 06:15, 17 January 2009 (UTC)


 * Because of the large tidal influence of the Moon, I believe it would be impossible for the Moon to still exist at the same time that the Earth was tide locked to the Sun. The tendancy to tide lock on the Moon would dominate.  Dragons flight (talk) 16:12, 17 January 2009 (UTC)
 * I agree, you might be able to get some system with the moon at L4 or L5, but with it in orbit around the Earth, you're not going to get a tidal lock to the sun. --Tango (talk) 16:22, 17 January 2009 (UTC)


 * StuRat suggests that if the oceans on the dark side freeze solid, water vapor in air moving to the dark side will freeze and be deposited there. The end result would be that all water on earth will accumulate as ice on the dark side. I wonder why that doesn't happen now in the Arctic and Antarctic regions. I suppose melting around the edges of those areas, including iceberg melt and ice-pack melt, keeps liquid water on the rest of the earth's surface. I wonder what the relative humidity of air in the polar regions is in present conditions. It will be low but it won't be zero - so why isn't it zero (even in nonpolar regions) when the air temperature is below freezing?. – GlowWorm. —Preceding unsigned comment added by 174.130.253.174 (talk) 18:44, 17 January 2009 (UTC)


 * I asked why the relative humidity of the atmosphere is not necessarily zero when the air temperature is below freezing. Perhaps the reason is that water molecules are kept aloft by air turbulence. I think that if the air is in a container and left long enough for turbulence to vanish, all grouped water molecules would become ice and fall out. I said "grouped water molecules" because a single water atom cannot freeze - for freezing to occur, a number of water molecules must be involved. I don't think Brownian motion would keep grouped water-ice molecules aloft because of the relatively heaviness of the grouped water ice molecules. Individual water molecules might remain in the air, keeping relative humidity above zero, unless their weight took them below the gaseous components of the air. – GlowWorm. —Preceding unsigned comment added by 174.130.253.174 (talk) 19:38, 17 January 2009 (UTC)


 * For the first question, "why doesn't all water end up frozen at the poles", you essentially got it right, the moisture in the air that falls as snow forms glaciers that eventually flow to the sea, break off, and melt. The summer season, when sunlight occurs 24 hours a day in polar regions, is critical to this process, however, and I wouldn't expect this to happen in the absence of sunlight.  As for why the humidity isn't zero in the air above the polar regions, there is a continuous supply of moisture from warmer areas, and some water vapor can sublimate (evaporate from ice).  The colder the temps, though, the less water will sublimate. StuRat (talk) 02:15, 18 January 2009 (UTC)

What's a good resource for algae?
I'd like to see a list of algae species (most common ones, I guess, or atypically interesting ones) that includes things like preferred temperature, nutrient requirements, light requirements, oil content, colour, etc. Anyone know of anything? My Google searches have been fruitless. Thanks! -- Kickstart70 T C 03:52, 17 January 2009 (UTC)
 * Um you can try reading our article Algae. It may give you an idea of why your question is way too open ended to be answerable. For example "algae are national foods of many nations: China consumes more than 70 species". It's rather likely most of these species are resonably common otherwise they wouldn't be consumed. Look at the number of groups there. Care to take a guess of how many species there are? After you have take a look at the section on numbers and see "The Algal Collection of the U.S. National Herbarium (located in the National Museum of Natural History) consists of approximately 320500 dried specimens, which, although not exhaustive (no exhaustive collection exists), gives an idea of the order of magnitude of the number of algal species" (although we have no idea how many of these are unique species, I would expect the ratio is higher then 1/1000). If you just want a list of some algae, just pick some at random from the article and try researching them. However it appears to me you're in Algae fuel. If so rather then randomly looking at algae, it may be better to nail down what charestics you want and look for those that fulfill them. You can also take a loon at the ones currently of interest in that field (e.g. Category:High lipid content microalgae and SERI microalgae culture collection) and perhaps others that have been cultivated to some degree. In the end, your list will be extremely far from complete, which isn't surprising. Many scientists are likely looking into this at the very moment because there is no simple list they can consult to give them the answer. Nil Einne (talk) 16:01, 17 January 2009 (UTC)


 * For use as a source for making fuel (or for any other large-scale industrial application), it might be better off to breed your own algal strain that would meet the criteria more exactly. I guess they have a pretty short generation time - you could presumably devise some means to apply evolutionary pressure to produce the characteristics you need. SteveBaker (talk) 17:29, 17 January 2009 (UTC)


 * Wikipedia articles are unlikely to contain the specific information you are looking for over such a wide range of algae species, but you may be able to find the information you're looking for by following the links found at List of algal culture collections. 152.16.16.75 (talk) 02:21, 18 January 2009 (UTC)

mass of photon
what is the rest mass of photon? is it zero? if the rest mass is zero, then according to the formula of SR formula that states that the mass of an object that moves will have a relative mass that can be found by dividing the rest mass by the square root of 1-v2/c2(sorry! i don't know to type the formula here). so the relative moving mass must also be 0. then its momentum is 0. so how will NASA use solar sails to construct a space ship? --Harnithish (talk) 14:20, 17 January 2009 (UTC)


 * In general, real (as in, non-virtual) particles compy to the energy-momentum relation $$E^2=c^2p^2+m^2c^4$$, where $$m$$ is the rest mass of the particle. For photons (and in general, massless particles), the momentum is thus equal to the energy, which, for photons, is $$h\nu$$. baszoetekouw (talk) 14:28, 17 January 2009 (UTC)


 * For completeness, the concept of relativistic mass that you seem to be referring to is obsolete and no longer commonly used in modern physics. For the case of massless particles, it is also ill-defined as it implies a division of zero rest mass by a zero factor $$\sqrt{1-v^2/c^2}$$ with $$v=c$$ baszoetekouw (talk) 14:34, 17 January 2009 (UTC)


 * All photons move at the speed of light, so it's meaningless to talk about the rest mass of a photon. A moving photon (which is the only type that exists) has momentum due to its energy, you can calculate a mass from that, but it's a pretty meaningless number, it's momentum that matters. --Tango (talk) 16:25, 17 January 2009 (UTC)


 * Certainly you can't stop a photon - so it's meaningless to talk about a "rest mass". Indeed, if it had a non-zero rest mass then its mass at the speed of light would be infinite.  This doesn't affect solar sails because they are mirrors that take the momentum of the photon coming towards the spacecraft and turn it around into a photon moving away from the craft.   That reverses the sign of the photon's momentum - and conservation of momentum requires that the spacecraft accelerate.  A black solar sail that would simply absorb the photon would also work - but it would heat up and have to radiate that heat away as infrared which would reduce its efficiency. SteveBaker (talk) 17:20, 17 January 2009 (UTC)


 * it doesn't HAVE to radiate any of the new energy away. you could glue on one-sided black holes for example (so that the black hole doesn't suck the spacecraft in the other side)  —Preceding unsigned comment added by 82.124.85.178 (talk) 18:30, 17 January 2009 (UTC)
 * Yeah - that's a REALLY practical solution! "Your answer is wrong because you could use 'one-sided-black-holes'."...very helpful. Thanks for the correction.  SteveBaker (talk) 18:44, 17 January 2009 (UTC)
 * It wouldn't help anyway - it would still (probably) radiate Hawking radiation, which isn't significantly different to thermal radiation for these purposes. --Tango (talk) 18:50, 17 January 2009 (UTC)
 * More to the point - if such an impossible thing as a 'one-sided' black hole existed - you could simply aim your one-sided holes at the nearest massive object and let them pull you towards it! But 'one-sided' gravity is a craziness - if gravity is a distortion in space-time, how could you possibly have a distortion only in one direction?!  82's answer is equivalent to saying "Steve you're wrong because Harry Potter can just wave his magic wand and make the spaceship fly". SteveBaker (talk) 20:50, 17 January 2009 (UTC)
 * Of course, but I decided to ignore the obvious criticisms of the idea and pick a more obscure one - it's more fun that way! --Tango (talk) 00:01, 18 January 2009 (UTC)

hey, you said that photons can't be stopped but in this article it is given that some scientists actually stopped light. so they must have stopped photons. Anyway i understood the solar sail thing. thank you.Harnithish (talk) 15:22, 18 January 2009 (UTC)
 * They stopped the light as a whole, they didn't stop any individual photons. It's complicated and not something I've read up on, so I'll leave someone else to explain the details. --Tango (talk) 16:49, 18 January 2009 (UTC)


 * The explanation is complicated - and it also relates to the speed of light in air, glass, etc being slower than light in a vacuum...when it's not possible to slow down photons AT ALL. The answer is to do with 'group frequency'.  But suffice to say that slowing down (and even 'stopping') light isn't the same thing as slowing down or stopping photons - which is flat out impossible.  Because an object's mass at the speed of light is infinitely larger than its mass at (say) 99.99999% of the speed of light - a photon's mass has to be zero if it were slowed down even by the smallest amount.  Because E=mc2 - the photon would have to give up ALL of it's energy in order to drop in speed AT ALL - it can't slow down without ceasing to exist.  Which must essentially be what happens when a photon is absorbed by (for example) an atom. SteveBaker (talk) 17:06, 18 January 2009 (UTC)

Vitamin D and Tanning
I was wondering how tanning (specifically artificial tanning) is related to production of vitamin D? After browsing through Vitamin D, Ultraviolet, Tanning Bed, and many other related articles, I have determined that the output of the ‘good’ kind of UV, UVB, in tanning lamps is typically very little (5-6.5%) while the ‘make you darker then give you cancer’ type of UV, UVA, is very high. However, wouldn’t it be healthier for the ratio to be more like 50-50, possibly even more towards the UVB side? This would increase vitamin D and decrease cancer potential, wouldn’t it? Since we can control the % of different UV light output this is probably easily done, and it would probably get a lot of people to go tanning once they knew it was safer. Since this hasn’t been done yet there must be a flaw in my theory. Maybe UVB isn’t as great as I think it is? -Pete5x5 (talk) 17:37, 17 January 2009 (UTC)
 * Q: Who uses tanning beds? A: Those people who appear to think orange coloured skin is a good look. Therefore reducing the tanning efficiency of a bed is entirely defeating the point of using one for most users. The whole vitamin D theory of tanning is a bit misleading. For a vast majority of the world's population, incidental exposure to sunlight provides more than enough vitamin D. Moreover, the more tanned you are, the more UVB you need to synthesize the same amount of vitamin D. Therefore tanning is completely counterproductive for this goal. Its also important to realize that there is no "safe" level of tanning. The very process of of one's skin darkening is a physiological response to UV damage. Rockpock  e  t  18:42, 17 January 2009 (UTC)


 * So then in order for it to be used as a 'vitamin D supplement' of sorts the UVA/UVB ratings would basically have to be switched. Then you wouldn't get darker, but you also wouldn't get Rickets! -Pete5x5 (talk) 00:09, 18 January 2009 (UTC)
 * Sure, but there are far easier ways to avoid rickets. Just eating a good diet can be enough, you don't need any sunlight (consider people imprisoned in basements for years on end, eg. the recent incest case in Austria). --Tango (talk) 00:19, 18 January 2009 (UTC)


 * In what world is UVB the "good" UV? UVB causes direct DNA damage by photo-dissociating base pairs in DNA.  UVA, which has somewhat greater penetrating power, causes indirect DNA damage through the creation of free radicals.  Yes, melanoma seems to be caused 9 times out of 10 by the action of indirect damage, but it is rather big leap to conclude therefore that direct damage is "good".  Dragons flight (talk) 06:26, 18 January 2009 (UTC)
 * It helps the production of Vitamin D, which is good. But I agree, it does more harm than it's worth in any large quantities - just go for a nice walk outside every now and again and you'll be fine. --Tango (talk) 16:50, 18 January 2009 (UTC)
 * And that's exactly why we don't offer medical advice, because that may very well be true for you but it is far from being the case for everyone. Scotland and Canada, for example, are places where it is quite possible to not get enough vitamin D simply by going 'for a nice walk outside every now and again'. There are a lot of factors, and blanket statements that apply in California and Australia do not apply everywhere. 79.66.92.148 (talk) 21:42, 18 January 2009 (UTC)
 * Perhaps, but the treatment for them isn't more exposure to UV, it's a better diet or vitamin supplements. The article on Scotland seems to be saying (I haven't read it fully) that the problem is people not going out in the sun enough or wearing too much sunscreen when they do - in that case, my advice is entirely appropriate. I've only read the abstract of that Canadian study, and it doesn't give any explanation for the deficiency, just that it exists, it could be poor diet rather than lack of sun (although, obviously, going for a walk in the sun doesn't work during an Arctic winter - as with any advice, mine needs to be interpreted with a little common sense). --Tango (talk) 21:53, 18 January 2009 (UTC)
 * PS On second thoughts, me being right isn't really an excuse for giving medical advice... the rule exists because we can never be 100% sure we are right. The issue I should have responded to is whether or not my comments constitute medical advice, and I think it's borderline. It was a general comment and was not aimed at any specific person with any specific medical problems, which would suggest it's not medical advice, just a comment about medicine. So, I think I'm just the right side of the border. --Tango (talk) 22:21, 18 January 2009 (UTC)
 * Yeah, it was the 'you' in your last statement that really made me comment... 79.66.92.148 (talk) 23:13, 18 January 2009 (UTC)
 * That's just sloppy English, I meant "one". --Tango (talk) 23:43, 18 January 2009 (UTC)
 * (e/c) Note, though, that the Scottish study samples those "... who were overweight; or Asian; or housebound in some way resulting in little expose to sunlight." Those people very likely lead a sedentary lifestyle which results in minimal amounts of incidental UV exposure, or their skin tone means the require more exposure in such northern latitudes. For most people, who have even a moderately active lifestyle and a moderately varied diet, incidental exposure is sufficient even countries with little sunshine. Thats said, when deficiencies do occur in high risk groups, health professionals typically recommend they should be addressed by diet or supplement, not by tanning. Rockpock  e  t  22:16, 18 January 2009 (UTC)
 * Just to clarify I'm definitely not looking for medical advice. I have no desire to artificially tan/take vitamin D supplements, I was just wondering if something generally viewed as unhealthy could theoretically be used to increase someone's health instead.  Also, even though this clearly isn't the place to bring it up, I've noticed that like 10% of all the questions on the science RefDesk have this rule brought up in the answer.  Although I see why it exists, too many questions are being viewed as medical instead of curiosity in my opinion.  Turns out that a lot of people want to know about things that relate to the human body, but hardly any of them want medical advice.  It's a good rule, it just bothers me slightly.  -Pete5x5 (talk) 23:22, 18 January 2009 (UTC)

sun collapsing
i heared that at some point the moon will be pulled by the sun ... and abig explosion will be the product of that impact...is that true....? —Preceding unsigned comment added by Mjaafreh2008 (talk • contribs) 20:01, 17 January 2009 (UTC)


 * No. In order for the Moon to collide with the Sun it would need to lose almost all its orbital velocity and seeing as it is moving at around 30km/s (that's Earth's orbital speed, the Moon's will vary a bit from that depending on where it is in its orbit around the Earth) and has a mass of 7x1022kg, that would take an enormous amount of energy. It is highly possible, however, that when the Sun turns into a red giant (in about 5 billion years) it will engulf the Earth and Moon, but that's about it. --Tango (talk) 20:27, 17 January 2009 (UTC)


 * 1) The Moon and Earth form a system orbiting the sun, if the moon was to be "pulled" (as in brought closer and closer) by the Sun, so would the Earth.
 * 2) The Sun does in fact "pull" the Earth constantly due to gravity, the happy consequence of which is that the Earth doesn't wander off in the darkness and coldness of interstellar space, instead the Sun keeps the Earth (and Moon) at a safe and cozy (for us) distance in a stable orbit. Equendil Talk 20:41, 17 January 2009 (UTC)

Collisions with the sun(four related questions)

 * 1) If an asteroid collided with the sun, would it produce any effects that could be detected from Earth?
 * 2) Would a comet colliding with the sun be detectable?
 * 3) If a larger object(say, the size of a planet) hit the sun, how large would it have to be to make a detectable impact?
 * 4) Could any such collisions possibly endanger life on Earth by, for instance, disrupting the sun's output? 69.224.37.48 (talk) 20:29, 17 January 2009 (UTC)


 * Asteroids (and comets) are, more or less by definition, small (compared to a planet or the Sun that is), a collision with the Sun would be detected from Earth if the asteroid/comet itself was large enough to be detected. The "effects" of such a collision would be short lived and confined to a tiny region of the Sun, it would be a mere astronomic curiosity as was the collision between Comet Shoemaker-Levy 9 and Jupiter a decade ago. The largest asteroids are grains of dust compared to the Sun and couldn't possibly disrupt it in any significant way let alone endanger life on Earth. Equendil Talk 20:49, 17 January 2009 (UTC)


 * I think at worst such a collision could cause some kind of solar flare or similar solar activity, but they happen all the time and we survive just fine. Asteroids, and even small planets, are so small relative to the Sun that I can't see them causing anything beyond a local disturbance. --Tango (talk) 22:45, 17 January 2009 (UTC)


 * There's certainly no chance of a collision "disrupting" the sun's output. If any effect were to be observed, it would be an increase in output rather than a decrease, due to an increase in available fusible material. A super-Jupiter (or Sub-brown dwarf) may contain enough hydrogen to significantly increase the luminosity of the sun, possibly rendering Earth uninhabitable, but it's a long shot.  Anything larger than that and you're talking about smashing two stars together, and, well, that's never good.- Running  On  Brains  23:57, 17 January 2009 (UTC)


 * Like the Earth, the surface of the Sun is relatively cool, so any object which temporarily displaced a noticeable portion of the surface would expose the much hotter interior, which might create a temporary bright spot. StuRat (talk) 02:07, 18 January 2009 (UTC)


 * Indeed, that's the kind of thing I was thinking might cause flares - especially if the asteroid if ferrous-heavy and can mess (locally) with the Sun's magnetic field. --Tango (talk) 02:13, 18 January 2009 (UTC)


 * See Kreutz Sungrazers and there is also video at http://sohowww.nascom.nasa.gov/bestofsoho/Movies/movies2.html#comets "A popular misconception is that sungrazing comets cause solar flares and CMEs (coronal mass ejections). While it is true that we have observed bright comets approach the Sun immediately before CME's/flares, there is absolutely no connection between the two events. The sungrazer comets -- in fact all comets -- are completely insignificant in size compared the Sun."  So, the Sun would just absorb a comet or asteroid.  Also, the temperature of the photosphere is about 10,000 F.  --mikeu talk 03:48, 18 January 2009 (UTC)


 * Which is far cooler than the Sun's interior. StuRat (talk) 11:09, 18 January 2009 (UTC)

Tachycardia
Does anyone know the fastest recorded heart rate for a survivor of tachycardia? Just curious :) Fastest I heard of was my dad's and that was 220, so I suspect it's a lot higher than that. —Cyclonenim (talk · contribs · email) 20:32, 17 January 2009 (UTC)


 * This question may require refinement for an accurate answer. For example, I would guess you mean ventricular rate; if not then some extremely high atrial rates have been survived, even tolerated with few symptoms.  If we focus on ventricular tachycardia (VT), it is not always clear in the clinical setting when VT gives over to ventricular fibrillation (VF), for which some very high rates can be observed e.g.: [[File:Lead_II_rhythm_generated_ventricular_fibrilation_VF.JPG]] which is shown on the VF page and appears to display a rate around 375 (I did not pull out my calipers).  I've seen VF rates like this in a number of people whom I treated and survived, and I'm sure we could find reports of same.
 * If we focus on classical ventricular tachycardia (VT), and my guess is that this was your original intent, then 220 is very high, but it might not be clear whether your father was in VT or VF. My guess is that such comparisons are not very meaningful.  --Scray (talk) 22:39, 17 January 2009 (UTC)
 * Not at all, that is quite helpful thank you. To clarify a bit further, I was referring to supraventricular tachycardia as that was what my father had. I was just curious to see what the highest recorded rate was. —Cyclonenim (talk · contribs · email) 00:23, 18 January 2009 (UTC)
 * This article mentions successful treatment of a 300 beats per minute tachycardia, atrial flutter w/ 1:1 conduction due to Wolff-Parkinson-White syndrome.&mdash;eric 03:47, 18 January 2009 (UTC)
 * Here's atrial fibrillation + WPW, i don't see how this guy could have had a pulse.&mdash;eric 04:32, 18 January 2009 (UTC)
 * Am I right in thinking that when atrial flutters are that fast, there's little, inefficient contractions rather than stronger contractions present at more normal heart rates? Or is it that there is still a full contraction happening, but requiring a lot more energy to do so? —Cyclonenim (talk · contribs · email) 09:59, 18 January 2009 (UTC)
 * If the ventricles are contracting at such a high rate, they have little time to fill with blood. The end-diastolic volume would be very low, leading to a very low stroke volume. The EDV also partially determines the strength (intropy) and energy (ATP burned and oxygen consumed) by each contraction. A larger EDV means a greater preload and more forceful contraction and greater oxygen demand.
 * In the above two cases, even though each individual contraction might be weak, i would think there would be a huge oxygen demand because of the high rate, along with almost no cardiac output. Maybe one of the docs around here could explain how these people lived long enough for someone to get a 12-lead on 'em. (it took me so long to write this response that one showed up before i could hit the save button.)
 * Usually with atrial flutter or afib, conduction is blocked in the AV node, the ventricles will be contracting at a much lower rate. The contractions are less efficient because the atria do not contribute to the ventricular preload, but it's not nearly such a dangerous arrhythmia as those above where the AV node was bypassed by a high velocity conduction pathway.&mdash;eric 18:54, 18 January 2009 (UTC)


 * The maximum (ventricular) rate is about 300/min. Rates above this tend to provoke VF. A discussion of VF isn't really meaningful because different parts of the ventricles are all contracting at different times. With a fast atrial rate (as might be seen in atrial fibrillation or atrial flutter), the ventricles are usually limited to about 160/min because the atrio-ventricular node is refractory to conduction. In Wolff-Parkinson-White syndrome, the accessory pathway allows transmission at higher rates, hence the reports noted above. These high ventricular rates lead to impaired diastolic filling of the ventricles. This significantly reduces the stroke volume and causes hypotension and dizziness. Pulse rates cannot be maintained at this level because the reduced cardiac output leads to cardiac arrest. Axl  ¤  [Talk]  18:17, 18 January 2009 (UTC)

Limit of Cold
Is there a limit at which point a human can no longer tell that it's any colder?

Say it's 20 below (fahrenheit) and then the next day it's 30 below, i'd say BRRRR it's COLDER!!!

Is there a point where'd I'd cease to notice the increase in cold?

What about on the other end of the scale?192.136.22.5 (talk) 20:35, 17 January 2009 (UTC)


 * This question brings up an interesting, related question: does human perception of temperature work on a logarithmic scale? Several aspects of human perception seem to be log-based, but I've never heard whether or not temperature perception works on a similar basis.  152.16.16.75 (talk) 02:34, 18 January 2009 (UTC)


 * The elapsed time between exposures would confound the comparison, mightily. hydnjo talk 02:59, 18 January 2009 (UTC)


 * Id say that you could no longer tell its colder when you are unconscious (or dead).--GreenSpigot (talk) 03:51, 18 January 2009 (UTC)


 * They do say that just before you die you start to feel pleasantly warm (although how this was discovered is not clear).--GreenSpigot (talk) 04:16, 18 January 2009 (UTC)


 * It is worth noting that humans don't feel temperature; they feel temperature differences. If your arm (for example) becomes chilled to the same temperature as the ambient environment then your arm will no longer feel cold.  In general, good blood circulation should prevent that from happening, but with very harsh conditions one can start to lose the sensation of cold as your extremities become chilled.  From personal experience at the National Ice Core Lab, I can tell you that -35 C (-31 F) still feels a lot colder than -25 C (-13 F), so if there is a lower limit it must be less than that.  Dragons flight (talk) 05:19, 18 January 2009 (UTC)


 * AFAIK, significant local cooling may result in either pain or loss of sensation; it depends which way and how fast you cool. Either way, there is certainly a limit beyond which you can't tell whether the skin temperature is decreasing further or not. Suggestion that human perception works rather generally on a logarithmic scale is called Weber–Fechner law. I don't know if it works for perception of temperature. There is a paper published 68 years ago: Herget, Granath, Hardy, Am J Physiol 134: 645-655, 1941; which suggests that the law does apply, but in a rather limited range. It certainly does not apply at the extremes. Note also that they tested for "hotter" and not for "colder", presumably because that's easier. All the best, --Dr Dima (talk) 05:43, 18 January 2009 (UTC)


 * Temperature perception is very tricky. For example, if you touch a piece of plastic and a piece of metal that are at the exact same temperature - you'll judge the metal to be cooler because it conducts heat away from your skin faster than the plastic does...unless both are above body temperature - in which case the opposite will happen. SteveBaker (talk) 06:23, 18 January 2009 (UTC)