Wikipedia:Reference desk/Archives/Science/2012 July 7

= July 7 =

Genetic variation between humans
A little while back, I heard someone (I believe a biologist) make the claim that genetic variation among humans was small. First, what precisely does this mean? And second, is it true? Thanks.

PS I tried reading the article human genetic variation but I couldn't find the answer I was looking for. 65.92.7.168 (talk) 02:18, 7 July 2012 (UTC)
 * Small compared to what? There are different ways to measure genetic variation.  Read the Human genetic variation article again - it lists some of the ways.  Perhaps the easiest to understand is Nucleotide diversity.  The article cites a nucleotide diversity of about 0.001 for humans.  I found an article that suggests that domestic dogs have about the same number.  By this measure, humans contain about the same amount of genetic diversity as the domestic dog.  Chimps and Bonobos also have similar nucleotide diversity, or even somewhat higher numbers than humans.  The nucleotide diversity of tuna, on the other hand, is much higher, over 4%. Buddy431 (talk) 03:37, 7 July 2012 (UTC)


 * As you correctly guessed, by small I meant compared to comparable mammals. But I find the article's claim about domestic dogs to be very surprising. Different breeds of dogs seem so ...different, surely they would have a very high nucleotide diversity, no? 65.92.7.168 (talk) 04:05, 7 July 2012 (UTC)


 * Genetic diversity often does not manifest itself as obvious differences in phenotype. Among humans, for example, there is more genetic diversity within Kalihari Bushmen than across all non-African populations combined.  That's a result of the ancestors of the Bushmen having been in that area for hundreds of thousands of years, whereas all non-African populations passed through a common bottleneck about 50,000 years ago.  The visible differences among dogs are a result of selective breeding and not a sign of large genetic diversity -- the breeders emphasized the genes that make a visible difference and ignored the others. Looie496 (talk) 05:27, 7 July 2012 (UTC)


 * You can think of a human's DNA as a sequence of letters A, G, C, T, lined up in arbitrary order (like AGGCTATTACTTTACCTA...), several hundred million letters long. If you pick two people at random, their letter-sequences will match in over 99% of locations.  It's in that sense that genetic variation is small. Looie496 (talk) 03:44, 7 July 2012 (UTC)


 * "Small" is always going to be in comparison to something else. It's certainly smaller than lay people or even new students to biology intuitively guess. It's pretty average as far as animals go. Someguy1221 (talk) 03:46, 7 July 2012 (UTC)


 * As for why the genetic diversity of humans would be small, it's because we have a relatively long time between generations, and are a relatively young species, which combine to mean there have been a lower number of generations than most other species. (Dogs are an exception, since we bred them from a small number of wolves, fairly recently, they don't have much more variation than those original wolves had.)  Since each generation has the potential to introduce new genetic diversity, more generations mean more diversity. StuRat (talk) 07:24, 7 July 2012 (UTC)


 * Rather surprisingly that is not true. See molecular clock. That depends a bit on whether an animal is cold blooded or hot blooded but does not seem to depend very much on how long the generations are. Dmcq (talk) 16:20, 7 July 2012 (UTC)


 * I don't see how that article contradicts what I said. In fact, it agrees, listing one of the factors as "Changing generation times (If the rate of new mutations depends at least partly on the number of generations rather than the number of years)".  It also provided examples of where longer generations slow the molecular clock: "Tube-nosed seabirds have molecular clocks that on average run at half speed of many other birds, possibly due to long generation times, and many turtles have a molecular clock running at one-eighth the speed it does in small mammals or even slower." StuRat (talk) 16:34, 7 July 2012 (UTC)


 * Actually this is an incomplete picture and the influence of generational length, in this context is not as great as you might think, though not primarily for the seasons that Dmcq cites above. The length of generations is a contributing factor, but (likely to be) a small one here, since this is not the strongest constraint on species differentiation and we know of more potent influences in the case of humans.   For example, natural selection is not the only factor which could leverage such a split - Sexual selection is often a major contributor to the generation of a subspecies split -- and indeed, human beings do show some propensity for selecting mates who look similar to those they grew up around; for example, many studies have observed that couples, barring outside social pressure on the choice of their mate, tend to have amazing similarities in small details of their facial features (the width of the bridge of the nose, the relative size of the earlobes, ect.) that are incongruent with chance.  This is because human beings, like many complex sexual species, imprint upon those they grow up around (parents and siblings) as their ideals for attractiveness, and, to an extent, select mates accordingly -- this is mostly what accounts for the generation of our phenotypical "races" and differences like skin color (earlier theories that these types of changes were the result of natural selection -- such as protection from UV radiation driving skin color adaptations, have been more or less debunked).  However, these factors, interesting as they are, are not as firm in humans as they are in less social species -- humans apply much more complex psychological equations in the selection of their mates and (baring some historical divides) don't show much qualms in crossing those phenotypical lines (again, as compared to sexual species of similar complexity).   Without the bottle-necking effect of sexual selection, homogeneity is going to increase, and indeed research suggests that at present time this is the trend, as even non-experts can see in the fact that "pure white" races are becoming a smaller and smaller proportion of the population in nations where they used to predominate.  Also consider that, even with natural selection, the pressure for change is not all that great; natural selection is driven, for given species, by the adaption of beneficial traits and we've found one that has, since we adopted it, not ceased being useful in our environmental context: intelligence.  Well, a specific type of intelligence -- abstract thought, and all the attendant advantages that process confers -- language, tools, and mental/practical adaptations that allow us to tackle new environments and new problems without having to develop a more specific physiological trait to cope.  You're quite right to point out that humans are a relatively young species (our modern form being somewhere between 100,000 and 200,000 years old) and that we shouldn't necessarily expect differentiation in any event, but some species have cleanly split into several subspecies in a lesser number of generations than have taken place for humans in that span and the greatest constraints on our doing so moving forward are our willingness to mate freely with one-another (which seems likely at present time to only increase) and our conceptual/technological ability to adapt to new circumstances (which also will likely increase).  Add to this our advancing capabilities with genetic engineering and it seems likely we will be tailoring our genes in a planned manner long before we begin to differentiate via the traditional means. Snow (talk) 19:39, 7 July 2012 (UTC)


 * There certainly are other factors, like isolation of populations, which affect how quickly a species splits into two or more, but, overall, I doubt if the majority of species diverge into separate species with as small a number of generations as there have been in humans. StuRat (talk) 20:31, 7 July 2012 (UTC)


 * Well, in most cases we don't have a completely clear picture, but 6,000 to 10,000 generations has seemingly proven more than sufficient to allow for significant divergence in other mammalian species, possibly even other primates. Consider for example the fact that we co-existed for a short period with other species (Neanderthals, and seemingly others), with whom we were genetically similar enough to interbreed and that the entire span of their existence was not much longer than ours.  But ultiamtely we just don't have the data of a full census of the genetic history of most species to know what is typical in this regard, though we can make decent guesses based on extrapolated information. Snow (talk) 20:59, 7 July 2012 (UTC)


 * I read, I think in Nicolas Wade "Before the Dawn" a quote by some geneticist that the amount of genetic variation in humans, in any other animal would warrant a division into subspecies. Уга-уга12 (talk) 17:50, 7 July 2012 (UTC)


 * This image should give you a very good direct visual grasp of the difference in genetic diversity of humans compared to the other great apes: http://www.familyoriginstree.com/images/tree/tree_of_life.jpg Unfortunately I cannot find the original source.  It would be appreciated if anyone familiar with the image can give the source. μηδείς (talk) 18:04, 7 July 2012 (UTC)


 * Too bad they didn't include any members of the genus Homo other than modern humans and Neanderthals. StuRat (talk) 20:38, 7 July 2012 (UTC)
 * That image is at least about five years old (it might be in Before the Dawn, actually) but I can't remember where I have seen it. In any case, Denisovan DNA info has only been available very recently and something just in the news said it is closer to Neanderthal than sapiens sapiens. μηδείς (talk) 21:51, 7 July 2012 (UTC)


 * Here's a more informative link to the human/great ape genetic diversity image which provides the source from 1999: http://www.icb.ufmg.br/lbem/aulas/grad/evol/humevol/extra/apediversity.html μηδείς (talk) 02:02, 8 July 2012 (UTC)


 * Still no other Homos (there are at least 7 more). StuRat (talk) 06:29, 8 July 2012 (UTC)


 * Here's a working link to: "To People the World, Start With 500" By Nicholas Wade NYT November 11, 1997 AS few as 500 or so people, trekking out of Africa 140,000 years ago, may have populated the rest of the globe.... μηδείς (talk) 02:07, 8 July 2012 (UTC)


 * I happen to think Wade's 140kya date is too old, and find the Toba catastrophe theory fits the linguistic and archeological evidence better. μηδείς (talk) 02:10, 8 July 2012 (UTC)

Sweet Potatoe vs kumara
I remember reading sometime ago, that the sweet potatoe is not native to New Zealand. I've read that the Maori brought a tuber vegetable with them which was called a kumara. Apparently, it was a sweet, relatively small vegetable that resembled a knobbly ginger root. Later, europeans introduced the sweet potatoe in the nineteenth century. The sweet potatoe is easier to cultivate, or something like that, so the Maori adopted the sweet potatoe as the kumara instead. Where can I find a source for this? Plasmic Physics (talk) 03:50, 7 July 2012 (UTC)
 * Sweet potato Snow (talk) 07:42, 7 July 2012 (UTC)
 * Reading around that section, it appears that kumara is simply the name for sweet potato in various languages and that the Maori only had a small variety which was replaced later by a bigger one after wider trade contact. Rmhermen (talk) 15:04, 7 July 2012 (UTC)
 * That was my take-away as well, though the wording could be clearer in the article. Snow (talk) 14:21, 8 July 2012 (UTC)

large hadron collider
what could make the particles to collide together during big bang as now it is collided by the LHC? — Preceding unsigned comment added by Haresiba (talk • contribs) 05:01, 7 July 2012 (UTC)


 * Just after the Big Bang the universe was very hot and very dense. So collisions or interactions between particles were both very energetic and very frequent. According to the LHC Brochure, when it is running at full power the collisions created by the LHC will have the same energy as typical collisions just 10-12 seconds after the Big Bang, when the temperature of the universe was about 1016 degrees C. Gandalf61 (talk) 10:26, 7 July 2012 (UTC)


 * The LHC generates a bunch of nearly identical collisions in a single place, so that we can build a huge detector there and observe what happens. Conditions like that have never existed naturally. If you're just talking about random energetic collisions between particles, you don't have to go back to the big bang for that—cosmic ray collisions of higher energy than the LHC energy happen all the time in Earth's atmosphere. There's no good reason to say that the LHC "recreates conditions of the big bang". It's just hype. -- BenRG (talk) 13:01, 7 July 2012 (UTC)

Large Hadron Collider (Part 2)


What is large - the hadrons or the collider? Roger (talk) 10:59, 7 July 2012 (UTC) The collider.-- Gilderien Chat&#124;List of good deeds 11:14, 7 July 2012 (UTC)
 * The collider itself, which has a length exceeding 27 kilometers (see map on right) and which weighs in at a healthy 38,000 tonnes. The particles are your run of the mill sort (protons composed of quarks).  Snow (talk) 12:42, 7 July 2012 (UTC)
 * The collider is big but they are searching for "large hadrons" like the new Higgs-like boson at 133 times heavier than a proton. Rmhermen (talk) 14:59, 7 July 2012 (UTC)
 * The Higgs boson is believed to be an elementary particle; it is not a hadron. (A hadron is any one of a family of composite particles formed from two or more bound quarks.) The LHC facility has plans and facilities to accelerate protons and sometimes a bit of non-exotic hadronic matter (heavy ions like lead nuclei).  TenOfAllTrades(talk) 15:19, 7 July 2012 (UTC)
 * An easy mistake to make, because some non-experts in popular treatment of the matter have implied as much, but in fact: Scientific American interview with Frank Wilczek. Also, as Ten points out, the Higgs boson is an an entirely different class of particle, being a much smaller excitation of the Higgs field than other elementary particles. Snow (talk) 18:09, 7 July 2012 (UTC)
 * My understanding was, the "large" qualifies the "hadrons." Any kid can build a hydrogen ion accelerator in his back yard and truthfully claim to be accelerating small hadrons, but only CERN has the resources to accelerate heavy nuclei like lead (Pb) to supermassive TeV states.  And of course, it's also a particle collider, not just a heavy ion beam; every silicon shop from here to Hong Kong has an ion-beam epitaxy station; but only CERN has the energy to smash these heavy nuclei into other heavy nuclei.  In fact, in senior year physics lab, I seem to recall building a proton cyclotron out of a tuna fish can and a neodymium magnet.  That technology was Nobel-prize-worthy in 1939, but today it's a toy.  It's one thing to excite a hadron and measure its presence - you do that every time you cook grapes in your microwave - but it's a totally different and much more difficult thing to collide a hadron with another one.  Nimur (talk) 18:03, 7 July 2012 (UTC)
 * All true perhaps, but these hadrons are no larger than any other hadrons would be under the conditions of CERN project experiments. Also there's the fact that every physicist connected with CERN who I've ever heard refer to the installation and whose native language is English places the grammatical stress in the phrase in such a way as to suggest that "large" is an adjective modifying the combined compound noun phrase "hadron collider" as opposed to an adjective modifying the word hadron.  Also, the acceleration of heavy ions take place in only a very small subset of the CERN projects and are not the main focus of research at the installation, which would make it an odd choice if the large were to refer to this work. Snow (talk) 18:28, 7 July 2012 (UTC)

The Superconducting Super Collider would have been a lot larger. Count Iblis (talk) 19:57, 7 July 2012 (UTC)
 * Shhhh, American physicists are still in mourning. Snow (talk) 21:56, 7 July 2012 (UTC)


 * From the LHC website: "The LHC is exactly what its name suggests - a large collider of hadrons." LukeSurlt c 22:57, 7 July 2012 (UTC)
 * Well, I'd say that's case-closed. Snow (talk) 23:05, 7 July 2012 (UTC)
 * Just to point out, the hadrons that are being collided are protons.Dja1979 (talk) 01:40, 10 July 2012 (UTC)

Gas phase 3rd order reaction rates for various M
For gas phase 3rd order reactions of the form A + B + M → AB + M, A + A + M → A2 + M, and the like, there is a large body of published reaction rate data (constants to insert in arrhenius or modified arrhenious equation) for all sorts of atoms A and B, but for each sorts of A and B, only for a limitted range of M.  The reaction rate is strongly dependent on what atom or molecule is M.  How does one handle a situation where measured data is not available for a particular M?. Is there a way given reaction rate data for a few given types of M, you can roughly estimate rate data for a different M (say based on molecular weights or radius or something)? For example, for the reaction Br + Br + M > Br2 + M, data has been published for M = N2, Ne, Br, Br2, O2, etc, but not CO2 or H2O. Ratbone124.182.0.102 (talk) 14:40, 7 July 2012 (UTC)
 * I'm pretty sure that there's no way to estimate the effects of a catalyst without actually measuring it. The intermediate species are vital, so you'd have to a) know what the intermediate steps are, and b) understand their kinetic properties. The catalysts that you list in your example have wildly different properties in terms of lone pairs, etc. so they will form a wide array of different activated complexes through a wide array of different mechanisms. 203.27.72.5 (talk) 07:58, 9 July 2012 (UTC)

Aren't these reactions elementary (single hump) reactions? Or, if the reaction IS elementary (perhaps not the example I gave), is there a method? Is "catalyst" the correct term for M? The only role for M is as somewhere for excess kinetic energy to go to. If it is not possible to at least roughly estimate rate coefficients, then what does one do? The chances of someone having measured the exact same combination of A, B, and M is not very high. Ratbone120.145.62.207 (talk) 09:38, 9 July 2012 (UTC)


 * If the presence of M lowers the activation energy, then yes, it is a catalyst. I don't really understand why you need it just to act as a sink for excess energy; why can' the A2 or whatever just take the energy (or would that cause it to decompose again)? If that's all it's doing, then isn't the rate just going to be proportional to the heat capacity? 203.27.72.5 (talk) 22:18, 9 July 2012 (UTC)
 * Good point - lowering the activation energy means it's a catalyst by definition. However, in reactions of the type A + A + M > A2 + M, the activation energy is essentially zero regardless of what M actually is.  Only the pre-factor and temperature exponent vary.  Without any M, there is no reaction at all.  (Note:  in published data, you sometimes see the activation energy value, Ea, given a quite small value.  I think this just a dodge to make the measured data fit the modified arrhenius equation, remembering that this equation only approximates reality, and especially remebering that the measurement accuracy is seldom very good by engineering standards.)  As to why M is needed, I've never seen that in any of the text books I've looked at, but previous questions in this forum produced a consensus that unless M is present, the energy made available by creating the bond means that the bond must be immediately broken apart again, as in these types of reactions, the bonded form is always lower in energy than the monatomic form.  The reaction rate is significant but varies only slightly with temperature (usually decreasing slightly with temperature), which supports the idea that it only requires the 3 atoms to collide in suitable directions without an A & the M combining.  Ratbone121.221.26.137 (talk) 01:34, 10 July 2012 (UTC)

Law of cause and effect
I was reading the iron chariots website which is a counter-apologetics website for atheists. Specifically here, where it addresses Kalām cosmological argument, and counters the premise that everything that begins to exist has a cause.

The counter argument is this, "Within quantum mechanics there seems to be real counter examples to the first premise of the argument. "Everything that begins to exist has a cause." For example, when Carbon-14 decays to Carbon-12 the radioactive decay is a perfectly random causeless event and thus though the Carbon-12 began to exist it wasn't caused to exist."

My understanding is that Carbon-14 decays due to the weak nuclear force, so couldn't it be said that the weak nuclear force is the cause?

Correct me if I'm wrong, but wouldn't a better example be the decay of pions? Even though they decay because of the weak nuclear force, it has two decay modes for both charged and neutral pions. But how it determines which to decay into is completely random, and thus is uncaused. Correct? ScienceApe (talk) 15:04, 7 July 2012 (UTC)


 * Carbon-14 decays to Nitrogen-14, not Carbon-12. Roger (talk) 15:16, 7 July 2012 (UTC)
 * I take this position: the deeper you dive into the semantics of a teleology debate - whether in the context of religion and philosophy, or in the more mundane examples of the application of the first principles of physics - the more ill-defined "cause" and "effect" become. I think this is basically what  Feynman hoped to illustrate when he drew those ridiculous line-diagrams.  Everyone else uses those diagrams as if they actually reveal some useful physical intuition - but Feynman never did that!  The goofy diagrams are just examples to illustrate a philosophical point.  If you draw time on the "x-axis," and you read from left to right, it looks like everything on the right half was caused by everything on the right side.  But if you draw the energy on the x-axis, and time on the y-axis, ... suddenly everything at higher energy looks like it was caused by everything at lower energy.  Your brain is (if you read English) conditioned to treat change in the +x direction as a "causal relationship," when in fact, you're the one making the assumption of cause/effect.  The decaying particle makes no such assumption!  The universe is doing its thing, exactly as it should, without worrying about what causes what.  Unfortunately, most people don't get it - they're spending all this time trying to make nature fit into their tiny and ultimately limiting ideas about causality, drawing ridiculous squiggles to represent particle evolutions as if they mean anything... and that's probably why our fabulous article on Feynman diagrams doesn't actually cite Feynman, and why he's even got a whole chapter in his book about whether reading from right to left or from left to right is "correct"! Nimur (talk) 16:49, 7 July 2012 (UTC)
 * Not to mention, he has another entire chapter about how ridiculous it is to apply scientific methodology to settle disputes about medieval mysticism. Apologies to the now-deceased author for the link to a probable a copyright-violation - but at least hopefully it may spread some enlightenment.  ... And spur some enthusiastic physics student to actually go track down where exactly in Feynman's text he actually drew that squiggle.  Nimur (talk) 16:57, 7 July 2012 (UTC)


 * Uh... the point of those "ridiculous diagrams" was to simplify computations in quantum electrodynamics, not to make a philosophical point. And if Feynman didn't believe that the diagrams were physically meaningful, he certainly did a good job of faking it in QED. And if there's one thing about cause and effect that everyone can agree on, it's that causes precede effects. That's not the hard part. -- BenRG (talk) 00:05, 8 July 2012 (UTC)
 * It's funny that you should say that, because the exact quote I recall from Feynman was, "if (event A) is later than (event B), you're inclined to say the photon went from (here to there); and if (event B) is later than (event A), the photon went from (there to here); ...but it's a funny thing in relativity, it's very difficult to say for things that are nearly at the same time, which is ahead.... and this whole business of trying to decide which one is ahead, which one is emitting the photon, which one is absorbing the photon, is an irrelevancy." This is, incidentally, from QED Lecture # 3.  Nimur (talk) 14:21, 8 July 2012 (UTC)
 * Roger's very reasonable objection aside, the point of the argument is that the time of the decay doesn't depend on anything that came before, and I don't think it's strengthened by considering a case where the products of the decay are also random. -- BenRG (talk) 00:29, 8 July 2012 (UTC)


 * I think we've run into a limitation of the word "cause" here. If you're dealing with quantum mechanics, you're dealing with probabilities. Let's consider the two-slit experiment, with a detector at each slit. What "causes" the photon to be detected at one slit instead of the other? Nothing. There was a finite probability that it would go through either slit, and it was detected where it was detected by random chance. In much the same way, nothing "causes" a radioactive atom or other unstable particle to decay; there's just a finite probability that it will occur at any instant, so when it does it is consistent with the laws of physics.
 * If you want to force a "cause" in there, you could say that the creation of the unstable particle was the cause of its decay. The laws of physics dictate that this particular particle is unstable, therefore it will decay at some point: that's a cause, albeit a fuzzy, hand-wavy one. - Running On Brains (talk) 03:13, 8 July 2012 (UTC)
 * The Kalām cosmological argument is philosophical nonsense. You can't DEFINE a being into existance. Anything that begins to exist has a cause, but God didn't begin to exist, therefore he didn't need a cause. I can JUST as easily make a premise "that which didn't begin to exist, doesn't exist", 2 "god didn't begin to exist" 3 god doesn't exist". It's JUST as logically valid as the kalam argument, it's either special pleading or circular reasoning to say god didn't "begin" to exist. To invalidate my argument, you would have to demonstrate something which exists but never begun to exist, which is the same thing you are trying to prove with the 1st argument. You're basically using your own argument to prove something into existance. Secondly, WHERE did God exist before the universe begun to exist? and WHEN did God exist? The answers are NOWHERE and at NO TIME did God exists. This is the law of non contradiction, God can't EXIST and NOT EXIST at the same time. Apologists say there was no time or space but God existed 'somewhere else' in some "supernatural dimension" well this is special pleading at its best, if they want to argue that there was truly NOTHING before the big bang (which we truly do not know and possibly CAN NOT know", there was no 'other special realm'. If they want to posit a "special realm" then maybe the universe simply emerged from this "special realm" it self? Why not? It's just as valid. They also say that "real" infinites are not possible, but God was eternal, but since there was no "time" before the big bang, God's existance doesn't count as "infinite", it's complete nonsense. Vespine (talk) 00:09, 9 July 2012 (UTC)
 * My feeling is that efforts to make the Big Bang stand in for the "Let there be light" of Genesis are confounding two entirely different and unrelated things, and wrong religion as much as they wrong science. The Big Bang is a mathematical catastrophe.  I bet that during that little interval from 3 to 20 minutes after the Big Bang, when fusion was occurring, that there were strange and intelligent beings made up of magnetic flux-lines or catalytic quark-gluon plasmas, who built whole civilizations that lived and died in fractions of an attosecond, during which they wrote depressing science articles about the coming "cold death with a whimper" when the universe, so slightly short of flat and ever so slowly failing to contain itself, would cease to produce new energy by fusion.  The Big Bang isn't a moment of creation, but an endlessly regressing wonderland of different laws of physics carried out within a universe always vast in space and time to its inhabitants.  The means of the origin, the theoretical First Thing, is not something waiting one last equation from a theorist, but lies concealed behind wave after endless wave of stranger and stranger phenomena that grow ever more impossible to model using our tiny energies, just as its future extends endlessly into incomprehensibly long times and large distances during which impossibly cold particles find new meaning in the most minute of interactions, things far too subtle to observe in a mere billion years. Wnt (talk) 19:37, 11 July 2012 (UTC)

stupid and speculative optical media question
the fastest DVD-RW blanks today are 4x. The equivalent of 4x in CD speeds is 36x (both mean roughly 5.3 MByte/s). So, why are there no 36x CDRW blanks (12x is max), can't they just use the coating they use for DVD-RW blanks for CDRW? What am I missing? Уга-уга12 (talk) 17:29, 7 July 2012 (UTC)


 * High speeds mean more violent airflow around the disk, which causes the disk to flex and flutter. For reading, the CD-ROM section (4th para) talks about increasing inaccuracy as speed increases. The need for accuracy is greater still when writing, an as CD-R notes, write speeds above 20x can be achieved only using a Constant linear velocity where the high speed is used only when writing to the inner portion of the disk (where the disk is stiffer). At higher speeds still (beyond the ambit of your question) the forces on the disk are such that compact Disc shattering is a concern. But lastly there's simply the issue of demand - if someone can burn a CD-ROM in 5 minutes, how much more are they willing to pay to do it in 2? - it seems like the CD-R and -RW makers have concluded that not enough people are willing to pay the necessary increment for them to make the investment in a factory to produce these wholesale (and they probably don't want the reputational damage they'd get when people with cheap burners can't reliably burn at those speeds anyway). -- Finlay McWalterჷTalk 19:08, 7 July 2012 (UTC)
 * Especially as the medium is moving (slowly) towards obsoletion (or at least a more limited role). Snow (talk) 19:46, 7 July 2012 (UTC)


 * Not sure that I agree with Finlay McWalter's answer. The CDRW is for CD writers of 'older' laser technology.  Any CD manufacture must produce blanks to record within those limits. DVD writers have superior laser optics and tracking. So a DVD writer can write to both DVD and CD where as an 'old'  CD writer can not write to a DVD.  To mix the two up, would mean that customers would be buying CD's claiming to have a capacity that their writers can not achieve, thus leaving the manufacture liable to law suits for false advertising.  --Aspro (talk) 19:56, 7 July 2012 (UTC)

thanks for all your answers. Уга-уга12 (talk) 10:27, 8 July 2012 (UTC)


 * Note that your basic premise is flawed. The fastest DVD-RWs are 6x . And DVD+RWs which aren't really that different from the - ones (at least not when you're comparing them to CD-RWs) are available in 8x variants  . Meanwhile Ultra Speed+ CD-RWs are available in 32x variants as our article mentions and shown  . However it's a bit pointless to compare raw transfer speeds of CDs and DVDs as the above answers have hinted at but perhaps not explained very well. DVDs have a much higher data density so naturally you can record more while travelling at the same speed (whether you actually can spin and record reliably while spinning at the same speed may be a different matter). They also use different lasers and I think (definitely for the recordable not sure about rewritable) substrates (dyes in the case of recordables, ) from CDs. Even if you could apply some of the stuff from DVDs and produce generally compatible CD-RWs and no one has done it yet, considering that Ultra Speed CD-RWs are rare let alone Ultra Speed+, I think this tells you how much interest the market has in such products. Remember that while the writers may need to be upgraded, most people usually expect their media to work in any reader which works with the older discs (CD-RWs do have problems with some readers particularly audio CD players and of course all recordable optical media can sometimes be a bit hit and miss). If you're only going to use the same burner to read the point of CD-RWs becomes even more questionable considering the price of rewritable DVDs. Remember there's nothing to stop you burning most CD images to DVD except for those odd ones like Mode 2 form 2 or audio CDs, and if you're only going to read them in the original it'll be rare burning the images as files will be a big disadvantage so even with those you can still burn them in a fashion. So burning whatever you plan to burn to CD-RW to DVD+/-RW will likely be a more compatible option then burning to a CD-RW someone makes which can only be read in new burners or drives. Nil Einne (talk) 08:22, 9 July 2012 (UTC)

Whitewashing coal
In the early days of coal mining the Davy lamp was used by miners for lighting. These lamps give out a paltry amount of light, so did anyone think of whitewashing the coal face to increase the illumination for the miner whilst he undercut the seam before blasting it away? --92.25.99.4 (talk) 19:28, 7 July 2012 (UTC)


 * That is ridiculous. Why would they whitewash the seam that they were hacking away? Whoop whoop pull up Bitching Betty 20:02, 7 July 2012 (UTC)


 * To see what they were doing my little friend.--92.25.99.4 (talk) 00:33, 8 July 2012 (UTC)


 * There are areas of coal they can't mine, either because it would destabilize the mine or because it's on property to which they don't have mining rights. Whitewashing that might make sense, depending on the cost of the paint and the flammability/toxicity of the fumes. There might also have been a problem with it drying in a cave. StuRat (talk) 20:22, 7 July 2012 (UTC)


 * Mine owners of the time wouldn’t care a dime about toxicity. It is very cheap. Flammability is an attribute that whitewash does not have. It works OK in damp cellars too (don't use whitewash distemper – it contains binders such as gums which can support some types of fungi and bacteria). It doesn’t 'dry.' It cures by absorbing CO2 -of which there is lots of underground and in cellars.--Aspro (talk) 20:42, 7 July 2012 (UTC)


 * That depends on how toxic it was. If it kills all the miners or they come running out of the mine clawing at their bleeding eyes, this will affect productivity. StuRat (talk) 06:21, 8 July 2012 (UTC)


 * Coal is brittle. One just hacks away it it with a pick axe. Back in those days they didn’t blast the coal but the rock blocking the way. Second. Back in those days, mine owners made the miners pay for their own explosives – he ruddy weren't going to pay for any white-wash. Thirdly. People get used to working in dark conditions. The feel from the pick axe handle tells them if they are hacking at coal or not. So, people may have thought about it but I shouldn't think it was ever put into practice until modern times.The last coal mine I was in (Copperfields up near Brum) had a lot of whitener in the access tunnels.--Aspro (talk) 20:18, 7 July 2012 (UTC)


 * Also, while I'm think about the safety aspect and mine-owners practices. The introduction of the Davy-lamp resulted in more deaths rather than less, because it encouraged working in seams that otherwise would have been considered to dangerous to contemplate. Something that the science books always forget to mention. --Aspro (talk) 20:51, 7 July 2012 (UTC)
 * Sorry You are talking thro the wrong orifice. Coal is definitely blasted with dynamite.--92.25.99.4 (talk) 00:36, 8 July 2012 (UTC)
 * Also, so a thin seam is darker than a thicker seam? Where do you get that from?--92.25.99.4 (talk) 00:41, 8 July 2012 (UTC)
 * "In the 1800s, coal was dug with a pick. Crouching or lying on his side, the collier carefully undercut the seam until a wedge or small powder charge brought the coal crashing down". Dynamite was invented 50 years or so after the Davy lamp. Alansplodge (talk) 01:04, 8 July 2012 (UTC)
 * The word "Collier" is actually a contraction of "coal hewer", who was the man with the pick who physically dug the coal in the mine. Coal was dug in this way until the 1980s at some pits in South and West Yorkshire. I used to have business dealings with a man who was the last coal hewer in Wakefield, who used his redundancy money to set up a brewery. --TammyMoet (talk) 08:11, 8 July 2012 (UTC)
 * The OED doesn't agree with you, Tammy: " 			Middle English colier, colyer , etc., <  col, coal n.1, apparently after words from French in -ier suffix, q.v." --ColinFine (talk) 14:27, 8 July 2012 (UTC)
 * Someone better tell my ex-collier friends in Yorkshire that then! And the OED does say "apparently", so even they're not sure. --TammyMoet (talk) 14:59, 8 July 2012 (UTC)
 * "Coal hewer" sounds like a highly motivated folk etymology. μηδείς (talk) 18:10, 8 July 2012 (UTC)
 * Collier Row on the Essex / London border is named after the chaps who made charcoal there a while ago. I don't think they did any hewing. Alansplodge (talk) 01:49, 9 July 2012 (UTC)

So, did anyone think of using whitewash to give more effective illumination or not?--89.243.129.28 (talk) 15:05, 8 July 2012 (UTC)
 * Can't think why they would want to paint something that they were about to smash-up (sorry "hew") with a pick. But it's not easy to prove why somebody didn't do something. Alansplodge (talk) 01:53, 9 July 2012 (UTC)
 * I explained all that: Its to enble the miner to make the most of the paltry light from his Davy lamp.--92.25.103.101 (talk) 14:07, 10 July 2012 (UTC)