Wikipedia:Reference desk/Archives/Science/2016 February 22

= February 22 =

Lights off the Falklands


What are all the lights in the ocean north and west of the Falklands? Patagonian Shelf doesn't mention oil drilling, and Economy of the Falkland Islands seems to indicate that all oil drilling was in the future in 2009 (when the image was produced) and implies that even in 2016, they've not yet started doing a significant amount. I can't think what else, aside from an error in NASA's imaging equipment, could produce such a large amount of light in the region. Nyttend (talk) 00:01, 22 February 2016 (UTC)


 * .&mdash;eric 00:16, 22 February 2016 (UTC)


 * You can read the NASA article, which the Daily Fail so extensively cribbed, here. DuncanHill (talk) 00:23, 22 February 2016 (UTC)

I think its a error in NASAs imaging equipment 65.175.189.34 (talk) 12:15, 22 February 2016 (UTC)
 * This being a reference desk, could you provide references helping me to understand why you say that? Nyttend (talk) 13:29, 22 February 2016 (UTC)
 * To actually answer the question, they're fishing boats, catching Illex argentinus. Tevildo (talk) 20:39, 22 February 2016 (UTC)
 * I read the links from Eric and Dave; my response was merely to the IP's unreferenced advancement of a different idea. Nyttend (talk) 23:22, 22 February 2016 (UTC)


 * Who's Dave? DuncanHill (talk) 23:24, 22 February 2016 (UTC)
 * You. Obviously I wasn't paying attention!  Nyttend (talk) 00:42, 23 February 2016 (UTC)
 * "Dave's not here, man!" -- Dismas |(talk) 18:12, 23 February 2016 (UTC)

Did someone just 'prove' Bohmian mechanics?
Reading this news report about "quantum surrealism", which per the original paper is based on Bohmian mechanics ... it sounds very exciting. But...


 * Is there any meaningful proof of the Bohmian "interpretation" over others, or is it all still just a math game that can be explained in any of the other "interpretations"?


 * Per our article it sounds like hidden variables are a thing, really; per the phys.org summary, the photons in the double-slit experiment have honest to God classical trajectories through space. Is this so?  And if so... where does the wave function and the ubiquitous Planck constant "come" from?


 * Per our article the Bohm idea requires a foliation of spacetime. Is this foliation scheme unique - is there actually a preferred frame implied by the theory, in which simultaneous events (for the theory) appear simultaneous to an observer?


 * There's some kind of measurement without measurement involved in this paper. How does that work - why does this give useful results?

If you can point to a much more idiot-level intro than the wikipedia article, I should try to understand it... Wnt (talk) 05:10, 22 February 2016 (UTC)


 * Here's a sentence from the paper that seems relevant to your first question: "Indeed, our observation of the change in polarization of a free space photon, as a function of the time of measurement of a distant photon (along one reconstructed trajectory), is an exceptionally compelling visualization of the nonlocality inherent in any realistic interpretation of quantum mechanics." I don't think they've proved anything about which interpretation of QM is correct, only that it must be nonlocal (which I think most physicists are coming to accept anyway, now that almost all the "loopholes" of Bell's theorem have been closed) and that the "pilot wave" theory works mathematically. The "measurement without measurement" is something called weak measurement. Measuring something directly disturbs it a lot and wrecks the quantum wavefunction, but if you measure it very indirectly, you can barely disturb the system at all. One weak measurement tells you almost nothing because you've barely taken any information out of the system at alll, but lots of weak measurements can then be averaged together to give you a lot of useful data. I don't fully understand how they did the weak measurement, but it involved creating an entangled photon pair, putting one photon through a double slit (actually a beam splitter here), and then measuring the phase shift between the two photons via their polarization, which is weakly correlated to the momentum of the photon (from which you can reconstruct its path). This is a very roundabout way of doing things, but because it's so roundabout, it's a weak measurement that produces usable data. Smurrayinchester 10:24, 22 February 2016 (UTC)
 * Aha, just found New Scientist's take on it which is much easier to read. They don't say that it proves Bohmian mechanics, but rather that "Bohm is back in the game" because the experiment resolves a paradox that was thought to totally disprove it. The biggest benefit of the experiment will probably be that because this paradox has been resolved, physicists who want to study Bohmian mechanics will once more be able to get grants etc. Smurrayinchester 10:46, 22 February 2016 (UTC)
 * Simple! You take the good shit and pack it into the bong. then you play a gentle flame over the top of the weed and then you start to...Oh...wait a sec...that's Bohemian mechanics. Just forget I ever said anything about it, ok? Myles325a (talk) 01:51, 23 February 2016 (UTC)


 * Yes, Bohmian mechanics at the fundamental level violates Lorentz invariance. To make the universe seem Lorentz invariant you have to choose a special form for the quantum potential that hides the underlying nonlocality. I think this is the main reason that most physicists aren't interested in Bohmian mechanics: there's no reason in a Bohmian world for relativity or quantum mechanics to seem to be true. You'd expect them to be obviously false with probability near 1.
 * Even without understanding this paper you can make the following argument: they didn't find anything that contradicts quantum mechanics (since otherwise everyone would be talking about it—it would be vastly more important than the first detection of gravitational waves), and if the outcome of an experiment is correctly predicted by orthodox quantum mechanics then it probably won't convince anyone to support your favorite interpretation who didn't already support it.
 * I don't agree with Smurrayinchester that this experiment resolves a paradox in Bohmian mechanics. The paradox was based on a thought experiment, and was resolved already by Hiley et al according to the paper (or not resolved, depending on how convinced you are by their argument). Everyone involved believed that the outcome of the thought experiment would be the outcome predicted by quantum mechanics—thought experiments are useful precisely because everyone agrees on the outcome. If there were any reason to actually do the experiment, it wouldn't be a good thought experiment. It reminds me of the bit in Dirk Gently's Holistic Detective Agency where someone decided to actually do the Schrödinger's cat experiment. They hired a psychic to remote-view inside the box. Richard sensibly pointed out that if remote viewing works then it's just another way of looking inside the box. Here, they use weak measurement, and it's the same deal. All measurements are weak because they tell you nothing new about the vast majority of the matter in the universe. So-called weak measurements are just somewhat weaker than usual, and there's no standard for how weak they have to be before you can call them weak. Weak measurement is measurement. There seems to be no reason to use it here except to make the experiment more complicated and so less obviously unnecessary. -- BenRG (talk) 02:15, 23 February 2016 (UTC)
 * You provided a keyword (Lorentz invariance) that makes a search easier, and I found an impressive-looking paper on the subject here. A bit too impressive for me, really - I understood little before the frame dependence of "quantum equilibrium (ρ=|ψ|2)" was broached, and nothing thereafter, though I certainly did not put in much work in reading, as I think even an expert in the field doubtless would need to.
 * In general, ideas that bring foliation of spacetime into play seem interesting to me because this is not a relativistic universe. If you accelerate by .1c in any given direction, everything behind you will be red, and anything nearby you are approaching will be blue.  So I keep thinking relativity needs to be taken down a notch somehow ... I just don't know how. Wnt (talk) 18:17, 23 February 2016 (UTC)

Transcription enzyme
Hey, My grade recently had a science test in which during research an argument developed whether or not the enzyme RNA helicase, DNA helicase, or RNA polymerase unzips the DNA strands to make the template strands. The general consensus was RNA helicase, until some students found textbooks stating it was RNA polymerase. I just thought of asking the desk what the answer was after the test. Thanks JoshMuirWikipedia (talk) 05:16, 22 February 2016 (UTC)
 * You might like to look at our helicase article. Basically the story is that DNA helicase is what unzips the strands.  RNA helicase acts on RNA rather than DNA; RNA polymerase constructs an RNA molecule by trascribing the sequence encoded in a DNA molecule. Looie496 (talk) 05:57, 22 February 2016 (UTC)

Water purification
Is there any way (short of distillation) to remove all traces of iron from tap water? (By all traces I mean that even 1 ppm of iron may be an unacceptably high level.) 2601:646:8E01:9089:516D:646:9764:EDC1 (talk) 08:10, 22 February 2016 (UTC)


 * Reverse osmosis and Deionised water are two possibilities.--Phil Holmes (talk) 08:44, 22 February 2016 (UTC)


 * Semiconductor fabrication plants need extremely pure water, so probably whatever they do. Some chemical plants probably require similar standards as well. Obviously the required equipment is quite expensive. I kind of wonder what the motivation behind your question is, and whether you're trying to do something that either isn't a good idea or could be better done another way ("XY problem"). It's just not that common for someone to know they need water at such a purity level but not know how to go about obtaining it. Or you could just be curious. --71.119.131.184 (talk) 09:24, 22 February 2016 (UTC)
 * We have an article about purified water that covers the various methods. DMacks (talk) 10:07, 22 February 2016 (UTC)
 * When I worked in one, semiconductor fabs used reverse osmosis (RO) to treat the incoming water, then deionisation (DI) to finish it. DI led to water so pure that, if the lid was lifted from the bath, the carbon dioxide from the air dissolving in the water changed its conductivity to a considerable degree.--Phil Holmes (talk) 13:57, 22 February 2016 (UTC)
 * And would tanks made of substances normally considered inert (like pure SiO2, common glass, low solubility plastics) have solubility saturation limits high enough to ruin the water? What substances are suitable for a tank liner? How unreactive a metal would work? Platinum? Gold? Less noble than that? Sagittarian Milky Way (talk) 17:13, 22 February 2016 (UTC)


 * The question, as asked, needs clarification. The original question emphasizes removal of all traces, but proceeds to use units of parts per million.  When we discuss water purity, one contaminant part per million is huge.  There is a reason why we use a logarithmic scale to measure molar concentration!
 * The United States Environmental Protection Agency publishes this data sheet: National Recommended Water Quality Criteria. As a baseline, 300 micrograms of iron per liter of water is recommended for avoiding taste and discoloration effects.  Wolfram Alpha can automatically convert this to ppm, if you're too lazy to do the stoichiometry yourself... it comes out to just about 5 micromoles per liter of water.  In plain english: if you have 0.1 parts of iron per million parts of water, you can see it with your naked eye - let alone if you use scientific equipment or chemical assays!
 * If we purify water using ordinary equipment suitable for household use, we should expect to get less than one part iron per million parts water. If that's accurate enough, we surely don't need to bring in the advanced methods used to chemically or physically purify water as one might need for certain industrial applications.  By the time we are talking about water for semiconductor fabrication, we had better be measuring contaminants in logarithmic scales, because even 1 parts per billion of contaminant may be unacceptably high.  Semiconductor scientists can no longer use units of "parts per large number" to discuss contaminants; we just don't have accurate-enough equipment to measure that - so instead, they switch over to units of, e.g., sheet electrical resistance.  My reference text, Microchip Manufacturing, lists 18 megaohm-cm megaohms per square centimeter for fab-grade deionized water.  Golly, my ohm-meter doesn't even go up that high - that's water so pure it's off the chart!
 * We can't produce a purification scheme that eliminates trace elements beyond what we can actually measure.
 * Perhaps the purest water is the stuff they use in, e.g., Super-K or its equivalent experiments, for equipment related to the detection neutrino collisions. It has been claimed, in the very famous Homestake experiment, that physicists were able to detect something like fifty atoms of contaminant in a hundred thousand gallons of (mostly) perchloroethane.  (They didn't use water because it couldn't be made pure enough to count such small numbers of contaminant atoms!)  This might be the purest collection of molecular stuff ever created by humans, and it ought to have contained exactly zero atoms or ions of Iron - (but it's also possible that the experimenters simply had the technology to accurately ignore contaminants-other-than-argon).  Another competitor for "very pure molecular compound" (other than water) surely comes from some scientific glass used in physics experiments; Gravity Probe B used fused quartz crystals that ought to have contained exactly nothing except crystals of silicon dioxide.  It is not entirely clear to me whether we could measure the presence of a small but countable number of contaminant atoms in such glass.
 * Nimur (talk) 17:58, 22 February 2016 (UTC)


 * Just a correction on units. The quality of water is expressed in units of Resistivity, which are ohms-length; extremely pure water (as we used in my fab) is around 18 Megohms-cm.  Sheet resistance is expressed in terms of ohms per square (dimensionless in length); in semiconductor fabs, it's used to measure the doping levels of an ion implanted or diffused layer.  A very highly doped layer would be something like 5 ohms per square; a lightly doped layer could be one thousand or so ohms per square.--Phil Holmes (talk) 19:03, 22 February 2016 (UTC)


 * Thank you for catching my error! In haste, I had incorrectly typed the units, and I failed to catch that error.
 * After triple-checking the source, it is in fact ρDI-water=18x106Ω-cm, which is a unit of electrical resistivity.
 * Nimur (talk) 14:21, 23 February 2016 (UTC)

Thank y'all very much for the input! Just to be clear: the question has to do with my earlier question about synthetic turquoise, for which application I need water which is free of iron -- other impurities normally present in tap water may be OK, but iron is unacceptable because it would discolor the final product. And once purified, I plan to store the water in non-ferrous containers (copper, glass or plastic, whichever is more convenient), so repeated contamination will not be an issue. 2601:646:8E01:9089:F88D:DE34:7772:8E5B (talk) 05:38, 23 February 2016 (UTC)


 * Is there a reason why you can't just buy distilled water? It cost less than $1 USD per quart at my local supermarket.


 * Getting back to volume resistivity (AKA specific resistance) of water, MΩ cm is measured by taking the resistance of a 1cm × 1cm × 1cm cube of 25°C water between two 1 cm square conductive plates spaced 1 cm apart. For pure water this is roughly 18.2 MΩ cm. MΩ cm (actually Ω meter) is the SI unit but some books use the term megohms per cubic centimeter.


 * You can't just immerse a couple of plates in a bucket of water, though. The water outside the imaginary cube passes current and reduces the reading. And the 25°C is important: the resistivity changes a lot with temperature. Also, "pure water" does not mean "pure water with dissolved gasses." You can take 18.2 MΩ cm pure water, expose it to air, and gasses are immediately absorbed into the solution -- including carbon dioxide, which forms carbonic acid, reducing the resistivity of the water to below 10 MΩ cm. This usually takes less than a minute. --Guy Macon (talk) 09:42, 23 February 2016 (UTC)

Danger of asbestos after constructing the building, and before demolishing it
Is asbestos still dangerous during the life-time of a building? Processing asbestos is obviously dangerous, but once it is in the building, are we still exposed to it? --Scicurious (talk) 22:36, 22 February 2016 (UTC)
 * It's only dangerous if you're exposed to it, so it's not a problem if it's contained in some way. I was in a building that had asbestos-containing tiles and/or other flooring materials, nobody cared unless they were doing construction that would disturb the flooring, they would do regulated asbestos abatement in the area of the work before actual work began. But only in that room. I had a chemical storage cabinet with asbestos boards, nobody cared unless a board were to break or start to get substantially scuffed on the edges, then we'd have a contractor pick it up for disposal. Wiring, insulation, whatever...as long as it's not releasing fibers to a place that people were, it was all okay. DMacks (talk) 22:58, 22 February 2016 (UTC)


 * Remember that asbestos isn't chemically dangerous, as silicon dioxide compounds don't react with the skin. It's clear from the asbestos article that the problem is the physical damage asbestos fibers cause to the lungs and the significantly heightened risk of cancer occurring during the lungs' self-rebuilding process.  Nyttend (talk) 23:28, 22 February 2016 (UTC)


 * Yes, I read the part on the article explaining how asbestos harms the lungs. However, would an old building release asbestos through normal wear-and-tear? Or, at least, could the asbestos be released through small home improvement? Crumbled panels and tiles, chipped plaster, worn isolation: these all are going to happen early or later. Won't that release asbestos' fibers? --Scicurious (talk) 00:34, 23 February 2016 (UTC)
 * Indeed, normal wear can cause panels and tiles to become crumbly from normal wear, and obviously and drilling or sawing releases partciels. Friability was the term I couldn't remember earlier as the major concern about existing asbestos-containing installations, affecting whether it is okay to leave in place and maintain in good condition vs need to perform Asbestos abatement. DMacks (talk) 06:18, 23 February 2016 (UTC)
 * Exactly. Asbestos in plaster may be sealed in, but it would be a rare building that doesn't have some cracks (or holes drilled in walls for things like cables and picture hooks) somewhere. See this. But fundamentally asbestos is fricking dangerous stuff - if you need to know about it, hire a professional.Blythwood (talk) 19:44, 26 February 2016 (UTC)

How does the operator get inside the crane ?
This is really an engineering question, so I think it is better to ask it here than at the Miscellaneous Desk. As the section on tower crane in the Crane article notes, the crane is usually, but not always, operated from a cab near where the mast meets the jib arm. The question is: How does the crane operator get to the raised cab? Is there an elevator that is on the ground when the crane is not in use and goes into or next to the cab when the crane is in use, or does the crane operator climb the mast as if it were a staircase? The article doesn't address the question of how the crane operator gets to the top of the crane. (The article does say that sometimes the crane is operated remotely, but I know that it is often operated from a raised cab, sometimes raised one hundred to two hundred feet (thirty to sixty meters), which, among other things, provides the operator with a better view of both the signaller and the equipment. Robert McClenon (talk) 22:55, 22 February 2016 (UTC)


 * Robert McClenon, did you leave this response in the wrong spot by accident? Your comment is the only spot on the current version of this page where "crane" appears.  Nyttend (talk) 23:29, 22 February 2016 (UTC)


 * I split this Q off and added a title. StuRat (talk) 23:34, 22 February 2016 (UTC)


 * It was accidentally put in the section above, the one discussing asbestos. Thanks, StuRat!  Nyttend (talk) 01:08, 23 February 2016 (UTC)


 * I've seen cranes with a ladder on the side, presumably for the operator to use. I suppose they could also use something like a cherry picker to lift them up there, too.  In large cranes, it might even have a built-in elevator. StuRat (talk) 23:37, 22 February 2016 (UTC)


 * US OSHA regulations Access to crane. Access to the cab and/or bridge walkway shall be by a conveniently placed fixed ladder, stairs, or platform requiring no step over any gap exceeding 12 inches. AllBestFaith (talk) 01:06, 23 February 2016 (UTC)


 * For reference a stairway step is normally about 8 inches. I once saw scaffolding that had about at least a 2 foot step but only for the first one off the ground. The steps were really shitty until the second floor of scaffolding but that's probably to discourage people from climbing it (ladder rungs barely wide enough to put one shoe on) Sagittarian Milky Way (talk) 02:52, 23 February 2016 (UTC)


 * Even in cranes tall enough to require an elevator, there would have to be a ladder to use in the event that the elevator were to fail. So just because you see a ladder, doesn't mean that there isn't also an elevator. SteveBaker (talk) 03:47, 23 February 2016 (UTC)

I've seen some cranes with a climber elevator but most of the large cranes build themselves and each section has a platform and a staircase (just like a building staircase except narrower and back tube). https://www.youtube.com/watch?v=y39152VWCPk is a self-erecting crane animation showing each section. --DHeyward (talk) 07:52, 23 February 2016 (UTC)


 * I'll use this topic as a good place to comment on the previous week's question about the crane collapse that killed one in Manhattan. Every other week or so I visit elderly friend of the family was widowed last year.  He is a retired steel worker.  I asked him simply, "Did you see that crane collapse in Manhattan"?  He said "Boy, you could hear that mother singing."  I said, "What do you mean?"  He said, "You know how the crane is built as a framework?  Well, when it fails, the cable rubs against the framework, and makes a violin-like sound called 'singing' as it scrapes against the framework under strain.  I've seen it half a dozen times."  I asked him if that sound could have been a siren or a power tool, and he said there ain't no such siren for a failing crane, and that sound wasn't no power tool." I didn't lead him or coach him on any of his comments, or mention that it was a subject of discussion here. μηδείς (talk) 18:03, 23 February 2016 (UTC)