Wikipedia:Reference desk/Archives/Science/2018 June 25

= June 25 =

Tokamaks and actual fusion
Having looked over the tokamak article, I find myself still in doubt about a pretty central point: Have tokamaks, thus far, achieved actual fusion, at a measurable level?

I guess there are two different sorts of "measurable" that could be of interest here. More academically, one could look for the neutrons and other particles that would be emitted from a fusion reaction, and say whether they have been detected and whether other explanations have been ruled out.

Somewhat more practically, there's a metric called Q that indicates the amount of power produced by the reaction, divided by the power needed to keep the reaction mixture at the indicated temperature and pressure. Is it possible to measure this Q and give a confidence interval for it that does not include zero?

And for both questions, if so, then how long has this been so? --Trovatore (talk) 03:14, 25 June 2018 (UTC)
 * It's not technically a Tokamak, but the Wendelstein 7X has achieved fusion. It's always been a net power consuming process though, and the experiment was never expected to generate net energy. 202.155.85.18 (talk) 04:07, 25 June 2018 (UTC)
 * Our Fusion energy gain factor article (the Q factor) says: "As of 2017, the record for scientific breakeven is held by the JET tokamak in the UK, at Q = (16 MW)/(24 MW) ≈ 0.67, first attained in 1997." DMacks (talk) 04:10, 25 June 2018 (UTC)
 * Thanks (both)!
 * I think this should be covered in the main tokamak article. As it stands, you can read the whole thing without learning whether they've achieved fusion with the things or not. --Trovatore (talk) 05:48, 25 June 2018 (UTC)
 * Technically, fusion occurs at all temperatures all the time, just at a very slow rate. Any amount of heating and compression of a gas will increase the rate of fusion. So all fusion experiments achieve fusion in a sense, even Martin Fleischmann's. The question is when you would consider the border to be crossed between a trivial increase in the rate of fusion and one with significant implications for a useful reaction. 202.155.85.18 (talk) 06:28, 25 June 2018 (UTC)
 * Well, sure, in a "what is not forbidden is required" sense (I'm sure we have an article on that but I can't find it). That's why I specified "measurable". --Trovatore (talk) 19:11, 25 June 2018 (UTC)
 * The early uses I can find of it are in religious-law practices, such as Thomas Smyth (note none of these DAB entries is the right person?) in 1858 (, page 482). DMacks (talk) 19:28, 25 June 2018 (UTC)
 * I'm talking about the QM concept, not the phrase in general. --Trovatore (talk) 19:29, 25 June 2018 (UTC)
 * I know that:) But it's a general logical concern too. In physics, it seems to be called "Gell-Mann's Law" (or ...Dictum), for Murray Gell-Mann. DMacks (talk) 19:33, 25 June 2018 (UTC)
 * Ah, that helps. Our article is at totalitarian principle. --Trovatore (talk) 20:11, 25 June 2018 (UTC)
 * I could be wrong, but it was my understanding that the rate at which fusion happens "all the time" is not particularly meaningful. Say, could it turn out to be one fusion reaction in a trillion years per ton of deuterium-tritium mixture at standard temperature and pressure?  In any case it would have to be something you would detect, if at all, only by detecting the high-energy reaction products escaping, and not by measuring the power output of the reaction. --Trovatore (talk) 19:11, 25 June 2018 (UTC)
 * Muons catalyse fusion reactions and they are constantly incident on the earth from cosmic sources. About 10,000 impact per square meter of earth every second. Under laboratory conditions each muon can catalyse up to 150 fusion events before they decay or bind thus ending the catalytic cycle. Deuterium molecules confined to a lattice at room temperature can undergo fusion at a rate of around 3000 events per second per mole . I don't know the exact measurements for background fusion rates, but they're reasonably frequent and definitely within the range of measurability. 202.155.85.18 (talk) 00:06, 26 June 2018 (UTC)
 * Ah, interesting. I wasn't considering muon-catalyzed fusion.  But since the tokamak concept is not relying on muon catalysis, it seems like a bit of a distraction.
 * My vague thoughts on the subject are that a tokamak has to create an environment where D-D and D-T collision events greater than the energy barrier for fusion are common. If you're below the temperature where a typical deuteron/triton has kinetic energy of that order, then you can still get a fusion event if (a) just by luck you get a couple of nuclei colliding that happen to be on the far-right tail of the energy distribution, or (b) the energies aren't that high, but they tunnel over the barrier.
 * And I think both of those possibilities go down at least exponentially with the energy deficit, correct?
 * So would it be fair to say there is no measurable non-muon-catalyzed fusion at room temperature? --Trovatore (talk) 00:17, 26 June 2018 (UTC)
 * The linked article claims ~3000 events per second per mole for deuterium. While any given experiment might not being specifically trying to use muon catalysis, if muons are present, it will benefit from them anyway (they may not be present if the magnetic confinement that keeps the plasma in also keeps the negatively charged muons out...not sure). Without a catalyst of some kind the energy required to overcome the coulomb barrier between two nuclei is prohibitively high. Without it, the sun wouldn't achieve it's large fusion rates (the nuclear fusion inside stars is catalysed via the CNO cycle, positron catalysis and probably also a fair bit of muon catalysis, and possibly also quarks and other poorly understood, speculative processes). You could crunch the numbers and determine the fusion rate at room temperature excluding all catalysis if you really wanted to, but I'm not sure it would be very relevant for a comparison to real systems that have all kinds of catalysis happening all the time. 202.155.85.18 (talk) 00:38, 26 June 2018 (UTC)
 * "[I]f muons are present it will benefit them anyway" -- sure, but not proportionally. You have muons creating those catalyzed events.  You increase the temperature and pressure to the levels of a tokamak that's actually generating power, you don't get any more muons, so who knows, maybe they catalyze a few more events, but presumably not enough to be noticed compared to the the thermally generated ones.
 * That's why I think it's a distraction to talk about muons. OK, I hadn't thought of them, and it's an interesting point, so thank you for raising it.  But I don't think it's really relevant to my question.  I was talking about events created by the mechanism the tokamak relies on, and I think muon catalysis is completely irrelevant to that. --Trovatore (talk) 00:47, 26 June 2018 (UTC)
 * Well, ok then. Here's another irrelevant fact that you might nonetheless find interesting, and it also highlights the banality and commonness of fusion reactions. Commercially available neutron generators for industrial applications like moisture detection use D-T fusion reactions to generate their neutrons. 202.155.85.18 (talk) 04:09, 26 June 2018 (UTC)
 * That is indeed interesting, thanks! Following on that link, I also found the fascinating fusor article, which seems more "thermal". --Trovatore (talk) 05:06, 26 June 2018 (UTC)

Yes, it's not hard to get nuclei to fuse: just accelerate them to high enough speeds to overcome the Coulomb barrier. This can be done with a fusor you can build yourself that fits on a table, as noted. The thing is, this takes more energy than is released by the fusion reactions. No big deal if what you want is, for instance, a neutron source. Projects like the tokamak are trying to accomplish fusion that gives a net energy release, for generating power. Stars accomplish this because of their enormous mass that compresses the nuclei in the core. The problem is we can't fit a star inside the building next door, so we instead have to use extremely strong magnetic fields that take a lot of energy to produce. --47.146.63.87 (talk) 07:16, 26 June 2018 (UTC)

Moment of Inertia, Torque and Angular Momentum
This excerpt from my textbook appears to contradict itself. It seems to give two different definitions for torque: one being the first derivative of the moment of inertia with respect to time, the other being the first derivative of angular momentum with respect to time (both circled in red). I can't see how it's possible for both to be true. In a normal circumstance we wouldn't expect the moment of inertia to change with respect to time anyway during the course of applying a torque (maybe orbits are a different story where accelerating a body will cause the radius of its orbit to increase, but simple, classical systems like a spinning disc or whatever wouldn't see a change). By the analogy implied by the textbook itself, force is not the first derivative of mass in linear kinematics either. Is this a mistake in the text, or am I missing something? 202.155.85.18 (talk) 04:06, 25 June 2018 (UTC)
 * Lousy textbook. T=I*d(omega)/dt, or as they also say T=dJ/dt. Typo maybe. Greglocock (talk) 05:02, 25 June 2018 (UTC)
 * Yes, probably the printers mixing up Is and Js, and not knowing any mathematics or physics. This is the sort of error that a good specialist proof-reader should have corrected.   Dbfirs  06:29, 25 June 2018 (UTC)
 * It's possible it's a typo... or, it's an advanced physics book, and they're analytically decomposing the formula for torque by applying the product rule to calculate it for a nonrigid object.  Conceptually: if the object is deforming inelastically, there can be a torque that relates to the change in shape, hence change in the moment of inertia.  Which textbook are you using?
 * Torque is the derivative of angular momentum; so it is $$\frac{d (I \omega)}{d t} = I \frac{\partial \omega}{\partial t} + \omega \frac{\partial I}{\partial t}$$. It is very rare - in textbooks, at least - for the partial of the moment of inertia to be non-zero.  A non-zero rate of change in the moment of inertia means that mass is moving in the object; a classical case would be a complex pendulum, which is invariably the pedagogical instrument that is specifically used to knock physics students out of Newtonian mechanics and into advanced dynamical analytic tools, mostly by shaking a lot of time-varying force equations at them.
 * By analogy to linear dynamics - it is rare but not impossible to impart momentum by changing an object's mass with respect to time: $$\frac{d p}{d t} = \frac{d (m v)}{d t} = m \frac{\partial v}{\partial t} + v \frac{\partial m}{\partial t}$$. Although it is rare in elementary physics to change mass with respect to time, it is a critical element in non-rigid body analysis.  This full expansion, it turns out, is the foundation of the rocket equation.  The other textbook example cases are a heavy chain falling off a table, or a massive train derailing, changing the mass of its cars.  If we zero out that second term, we get our familiar "F=m a" formulation, which is the kind of ancient, centuries-old Newtonian physics they teach to teenagers in schools.
 * My copy of Marion & Thornton - which is now apparently available online at Archive.org - introduces torque in a few important places - notably, in the chapter on, ahem, "rocket dynamics," where a younger version of myself penciled in the margins: "very horrible algebra." Every physicist should read this book, and real physicists should own a copy so they can pencil in their own commentaries.
 * Sadly, in the case brought by our OP, it looks like it's just an unfortunate typographical error, and not a foray into exciting nuances of physics. But why waste a learning opportunity?
 * Nimur (talk) 14:39, 25 June 2018 (UTC)
 * The textbook is ATKINS, P. W., & DE PAULA, J. (2006). Atkins' Physical chemistry. Oxford, Oxford University Press. The probable typo is from page 11 in the "Foundations" section; essentially a review of basic chemistry and physics that is assumed knowledge for the rest of text. 202.155.85.18 (talk) 00:17, 26 June 2018 (UTC)
 * I agree that formula (B.6) contains a typo but still the partial derivative of I by time is quite often non-zero when, for instance, a non-spherical body rotates. This of course depends on choice of the reference frame. Ruslik_ Zero 18:25, 25 June 2018 (UTC)
 * In such cases, it's more common to expand the scalar moment of inertia into an inertia tensor so that it is non-time-variant, irrespective of the axis of rotation. (...grumble, grumble, something about why this is always guaranteed to be possible).  But you could, of course, use a wacky coordinate system and encapsulate the object's apparent change in moment-of-inertia by writing it as a time-varying scalar property, functionally coupled to the rate and axis of rotation.  I'm not sure that would be easier... but hey, it presents an awesome opportunity to conduct some fun with equations!  Nimur (talk) 20:49, 25 June 2018 (UTC)

Swallowtails follow-up (last question on this subject, I hope)
Hello again, since it was shown to me a few days ago that swallowtail butterflies are found pretty much everywhere in North America and I can never be sure to avoid them completely (unless I move to the western half of Alaska, or to the Yuma Desert), I've decided to see if I can overcome my fear of them instead -- and I'm trying to think of a strategy to do that. But for this, I'll absolutely need the following info:

As before, NO PICTURES PLEASE (but maps are welcome) -- I don't want to get an anxiety-induced heart attack from seeing close-ups of those critters, seeing them every summer IRL is bad enough! 2601:646:8A00:A0B3:4960:40AC:D40E:12AC (talk) 08:14, 25 June 2018 (UTC)
 * 1) What is the native range of Papilio cresphontes (the biggest of all, and therefore the one which would scare me the most)?
 * 2) How large (in terms of wingspan) are each of the most common North American swallowtail species, P. appalachiensis, P. canadensis, P. eurimedon (sp.?), P. glaucus, P. multicaudata and P. rutulus? (I need either the range of wingspans, or the maximum wingspan, in either inches or centimeters.)
 * 3) Excluding the above-named species, I need the following info on any and all swallowtail species which are found in the USA and which can reach a size (in terms of wingspan) of more than 3 inches (excluding Battus philenor, because I have already looked up the info for that one and discovered that photos of it do not scare me): species name, native range, wingspan (either maximum wingspan or range of wingspans), and whether or not the species is tiger-striped (i.e. has any pattern of contrasting dark stripes or veins on a bright background, like the above-named species, or vice versa -- this is a major factor in my phobia of them).
 * A tentative suggestion: in the UK we have recreational Butterfly farms (as I am accustomed to calling them, but our article is at Butterfly house – NB: one photo of NOT-Swallowtail butterflies lower down in the article), where one can visit hothouses containing (in both senses of the word) native and exotic butterflies that one can walk amongst – if one so wishes. (Understandably, this might not be something you've previously sought out in your locality.) At such a venue one could, perhaps, arrange to observe the problematic butterflies from the other side of glass or mesh barriers without immediate "danger" of physical contact. I will not presume to discuss the details of what amounts to medical treatment (as I suppose you have already given this topic much thought), but our articles Specific phobia and Exposure therapy might be of broad interest.
 * That first article has a link to List of butterfly houses (NB: photos of large butterfly-shaped billboards, though none clearly showing actual butterflies), which contains a lengthy list of such venues in the USA. {The poster formerly known as 87.81.230.195} 2.125.75.224 (talk) 15:40, 25 June 2018 (UTC)
 * Yeah, in fact the strategy I have in mind is a sort of impromptu DIY exposure therapy (in fact, I've been doing this since early childhood and this has helped me to overcome my fear of many other butterfly species, from cabbage whites to monarchs) -- I just want to take this to the next level, now that I've narrowed down the problem species to only tiger-striped swallowtails, I think that if I deliberately occasionally expose myself to the smaller-sized species in this category (while avoiding the big ones, at least until my fear has faded significantly), my fear will fade over time. The only problem is, this takes time (several years for each "kind", as defined by wing shape and coloration), and my place of residence is within the native range of some VERY large swallowtails (including, possibly, P. cresphontes, the worst offender of them all) -- so every spring and summer, I see the really big ones and it traumatizes me all over again and undoes any kind of exposure therapy I might have done, and in fact leaves me worse off than before.  So tell me, where can I go to see the smaller ones while avoiding the bigger ones?  And, once again, what is the size of the common North American species listed above (P. canadensis, P. glaucus, P. multicaudata, etc.) -- would they be too big for me, or not? 2601:646:8A00:A0B3:4960:40AC:D40E:12AC (talk) 09:19, 26 June 2018 (UTC)
 * I am on the wrong side of the Atlantic to be giving detailed advice about either sizes and distributions of North American lepidoptera, or the species bred at particular establishments. For the latter, you could contact conveniently located venues directly and discuss your problem (about which I'm sure they'd be sympathetic); for the former, you might search for and read the Wikipedia articles on the individual species with the pictures turned off – I don't know how to do this but I'm fairly sure it can be done, so an editor with more Wiki-fu will hopefully be along shortly to advise about this. {The poster formerly known as 87.81.230.195} 2.125.75.224 (talk) 16:03, 26 June 2018 (UTC)
 * There are a number of options here: Help:Options to hide an image -- To do so specifically for Wikipedia requires logging on as a registered user and editing a file; otherwise, there are browser-specific options. —2606:A000:1126:20CE:0:98F2:CFF6:1782 (talk) 19:05, 26 June 2018 (UTC)
 * Thanks, 2606 IP -- I'll do this shortly. 2601:646:8A00:A0B3:157E:9722:AD27:3562 (talk) 10:03, 27 June 2018 (UTC)
 * Have checked out several articles (with the pictures turned off as you advised), and Alaska is looking more and more appealing as a place of residence for me with each one I read. :-( 2601:646:8A00:A0B3:157E:9722:AD27:3562 (talk) 11:01, 29 June 2018 (UTC)

You should be able to configure your browser to turn off the display of images altogether (exact procedure depends on your browser). I think you are best off trying to treat the phobia instead of rearranging your whole life around it. We can't give medical advice but I like to think "see a doctor" is not medical advice, and therefore it's still ok to suggest it. Or in this case, see a professional who works with this type of problem, rather than trying to self-treat. You can see from the phobia article that many of the treatment approaches aren't really amenable to DIY. 173.228.123.166 (talk) 04:31, 1 July 2018 (UTC)

Reducing neutronium
Is it possible in theory form a BCS pair from two disimiliar fermions, namely the neutron and electron? If so, can this be considered to be analoguous to reduction in the chemical sense; can one then consider a BCS paired, neutron infused electride to be a neutronide compound, even though it would only be stable at nano-Kelvin temperatures? Plasmic Physics (talk) 13:10, 25 June 2018 (UTC)
 * These usually requires identical particles as their Fermi energy should be the same. Ruslik_ Zero 18:43, 25 June 2018 (UTC)
 * 'Usually' or 'absolutely' ? I thought that it would be determined by the common denominator. Plasmic Physics (talk) 19:43, 25 June 2018 (UTC)

Radiation Symbol T-Shirts
Are there any efforts to ban this (or similar uses) type of use of the radiation icon? Wouldn't that dilute the powerful message the icon is meant to conceive? --Doroletho (talk) 17:24, 25 June 2018 (UTC)


 * To my knowledge, there are many regulations and laws in the United States that require markings and placards for certain hazardous materials - (e.g. for purposes of transportation, 49 C.F.R. §172.300 and onward); but I am not aware of any American laws that prohibit incorrect or unauthorized use of markings or placards in cases where a real hazard does not actually exist. On a technicality, you could probably find some regulation or local law that prohibits these types of unauthorized markings... or if you can technically classify wearing the t-shirt as "transportation" then you could cite §172.303 or §172.401 or §172.502, but those are DOT rules and  probably do not apply to a t-shirt.
 * Generally, especially when I'm around lab newbies, I try very hard to indoctrinate that warning-signs are not "cool-looking decorations." They are actual, functional warning signs, and should be used correctly.
 * Nimur (talk) 20:43, 25 June 2018 (UTC)


 * The first amendment of the constitution of the US protects freedom of speech, which includes display of icons. In theory, no law can prohibit the display of these symbols. However, if such display results in actual harm, then the person who displayed it can be prosecuted for causing that harm: the "don't yell fire in a crowded theater" scenario. -Arch dude (talk)
 * If you could earn a dollar for every time you found an instance in which the conjunction of two laws yielded a logical contradiction, then mathematicians would get paid more than lawyers. Anyway, I'm pretty sure that constitutional protections of free speech are not equivalent to a universal inalienable license to speak or display any thing, at any time, at any place.  Here's a philosophical rumination on the topic from the Plato Encyclopedia; and here's a 2014 summary on the limits of free speech, a report prepared by the attorney for the Library of Congress on behalf of the Congressional Research Service.  Nimur (talk) 12:52, 26 June 2018 (UTC)

Technically, people are radioactive, so the shirt isn't even wrong. I am a radiation safety officer in Australia, and the regulations require us to affix radiation warnings if certain criteria are met, but do not prevent us from using the signage just to be safe in cases that may fall short of the criteria. 61.247.39.121 (talk) 23:27, 25 June 2018 (UTC)
 * If you're in a counting lab, it's not so unusual to see things warning stickered (bananas?) if they merely have a high background count (far from a hazardous one), and so shouldn't be left near or in the shielded counting chambers. I've even seen this on packets of welding rod - specially prepared low-background rods were to be used, not the standard grade stuff. Some scrap armour from the German fleet was spray-stencilled with a green logo when it was recovered in the 1980s to distinguish it. Andy Dingley (talk) 09:15, 26 June 2018 (UTC)
 * The radiation trefoil, like other symbols, is legally considered speech. The recognized exceptions to speech protected by the First Amendment to the United States Constitution all relate to specific and imminent harms (shouting "fire!" in a theater can be such a harm, although libertarians argue back that the cure for bad speech is good speech - "He's lying, there's no fire!").
 * Misuse of hazard placards has to be considered in context. A college campus with an active Nuclear Science laboratory, an oilfield community in which there is naturally occurring radioactive material from used downhole well casing, a town with a nuclear power plant or an Interstate or US highway on which hazardous materials (including radioactives) are apt to be transported are highly susceptible to social harms such as unnecessary deployment of hazmat teams or panic from "false alarms" resulting from misuse of the purple trefoil on yellow background.
 * Even in those areas, the wearing of a yellow T-shirt embossed with the purple radiation trefoil is not a plausible signal of radiological hazard, and cannot be justifiably outlawed. A plausible hazard isn't being signalled  - just, using hazardous material argot, that the wearer is "hot".
 * The same applies to biohazard "flowers", the skull-and-crossbones "poison" symbol, and Department of Transportation hazard placards. The plausibility of a false signal has to be weighed against the necessary prejudice in favor of freedom of speech. loupgarous (talk) 01:46, 1 July 2018 (UTC)