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October 9

Neutrinos interacting with each other?

Suppose we see a supernova that went off in a galaxy billions of light-years away. The neutrinos from that star have been travelling parallel to one another for all that time, and the supernova might have given them a limited range of energies, and the ones travelling near each other now should all be sorted for one particular velocity. So as far as I can figure in my mind, there ought to be a soup of neutrinos that are all pretty near rest relative to one another, chilling out in space for billions of years.

What do they do with each other in that time? I asked a question about "neutrino nuggets" a long time back [1] but that was a fairly unusual idea about accelerons. Is there a way to measure or predict if they really interacted? Wnt (talk) 02:48, 9 October 2018 (UTC)

True proportions of Earth-moon-distance. 240,000 miles or 1.3 light seconds!
There is way to much empty space in our universe for the human mind to "figure". We are build to figure 100 miles at best, when we stand on a high mountain in ideal weather conditions for far sight. You can memorize that sight and recall it pretty exactly. Go try that with the picture at the right sight and then calculate how many times more your billion(s) lightyears is (tip ONE year is 31,557,600 seconds and the distance Earth moon is 1.3 seconds). Next try to calculate how many neutrinos (0.3 attometer) you could line up in a straight line in between. --Kharon (talk) 03:45, 9 October 2018 (UTC)
talk-page fodder
The following discussion has been closed. Please do not modify it.
  • That's a pretty useless answer. Doroletho (talk) 21:04, 9 October 2018 (UTC)
Its better than your comment about it atleast, User:Doroletho. But since you really seem the fresh new wikipedian your fresh account insinuates, we will patiently remain, to see if your aim improves in the future. --Kharon (talk) 21:59, 9 October 2018 (UTC)
Neutrino#No self interaction: "Observations of the cosmic microwave background suggest that neutrinos do not interact with themselves". Rojomoke (talk) 06:12, 9 October 2018 (UTC)
The citation for that quote is a SciAm article which doesn't say much more about it than is in the Wikipedia article, so it's hard to be sure exactly what it's talking about, but my guess is that when it says they don't interact with themselves, it means exactly that — each individual neutrino does not interact with itself. That doesn't mean they don't interact with each other.
That's different from, say, electrons; each individual electron does interact with itself. What does that mean exactly? To be honest, I don't understand that in detail. But if you Google "electron self-interaction" you'll get a lot of hits from which you can start to investigate, and it's pretty clear that they're talking about an individual electron interacting with itself.
So in any case I don't think this rules out Wnt's hypothesis of neutrinos interacting with one another on their long journey. Intuitively I would expect that interaction to be very very tiny, but I'm not an expert. --Trovatore (talk) 07:15, 9 October 2018 (UTC)
No, self-interaction in this case almost certainly means interaction among themselves, one neutrino interacting with another neutrino (as in self-interacting dark matter); the abstract of this paper may serve as an example where "self-interaction" means neutrino-neutrino scattering. The terminology is admittedly rather mushy. What you're thinking of in connection with electrons is presumably self-energy. --Wrongfilter (talk) 07:47, 9 October 2018 (UTC)
Wrongfilter: I did look at the self-energy article, but it wasn't very explicit about electron self-interaction. Here's an example of the kind of thing I'm talking about, I think: [Quantum Electron Self-Interaction in a Strong Laser Field]. From the abstract: The quantum state of an electron in a strong laser field is altered if the interaction of the electron with its own electromagnetic field is taken into account. --Trovatore (talk) 17:49, 9 October 2018 (UTC)
Actually, in the context of electrons, the concept of "self-interaction" usually refers to a common interpretation "one-electron-at-a-time" version of the double slit experiment, and similar phenomenon. When electrons are used in the double slit experiment, they form an interference pattern at the detector even if the electrons are allowed to go through the experiment one-at-a-time so that each electron is detected before the next electron is fired. Since the resulting interference pattern is indicative of interaction between two different waves, a common interpretation of that result means the single electron must be passing through both slits and interacting with itself before being detected. I have no idea how this relates to the neutrino problem above (and reading this apparently not at all), but in the context of self-interaction and electrons, that's what it usually means. --Jayron32 14:11, 9 October 2018 (UTC)
I have my doubts regarding "usually" (and feel pressured to voice my doubts given the rather dismissive "no" from your edit summary). In Wikipedia, Self-interaction redirects to Renormalization because of Self-interactions in classical physics. In the double slit, the wave function interferes with itself. That is not quite the same as saying "the electron interacts with itself". Google gives results where "self-interaction" and "double slit" both occur, but not very many. Be that as it may, relevant references for Wnt's question are [2] and [3]. --Wrongfilter (talk) 15:37, 9 October 2018 (UTC)
Good point. --Jayron32 16:12, 9 October 2018 (UTC)
Is it possible to set up Young's Double-slit experiment (1801) using Neutrinos ? DroneB (talk) 15:39, 9 October 2018 (UTC)
As a thought experiment, certainly. In practice, it'll be hard to find a sufficiently absorbant screen in which to cut the slits... --Wrongfilter (talk) 15:52, 9 October 2018 (UTC)
Sorry my earlier answer went into a direction that has little or no use for you. I am so fascinated by the gap between human imagination capability and reality that this carries me away every time someone comes near that with a question. Since the Double-slit experiment is about interference, maybe Neutrino oscillation#Propagation and interference is helpful. --Kharon (talk) 22:39, 9 October 2018 (UTC)
Neutrinos interact with each other directly via the exchange of Z-bosons. The cross section for neutrino-neutrino scattering becomes large at the resonance energy for creating Z-bosons. This effect has been invoked in the past to evade explain ultra-high energy cosmic rays above the GZK limit, the so-called Z-burst model, also mentioned here.
At low energies, interactions due to higher order effects yielding a finite neutrino magnetic moment will be more important. The neutrino magnetic moment is given by where is the Fermi constant and is the Bohr magneton. This is due to only known Standard model physics, extensions of the Standard model could give rise to far larger magnetic moment, experimental constraints limit to be smaller than about , but it's likely going to be less than as for larger magnetic moments the parameters from the extension of the Standard model needed to get you there, must be fine tuned to conspire with each other so as to not yield neutrino masses that become way too large.
Interactions due to the magnetic moment can be detected due to e.g. effective neutrino decay. Such interactions can induce spin flips, causing left-handed neutrinos to become sterile right-handed neutrinos that don't interact with matter at all. Count Iblis (talk) 15:19, 10 October 2018 (UTC)

@Count Iblis: Outstanding but perplexing answer. To begin with, our article on the Z-burst says that "This process proceeds via a (virtual) Z-boson..." and I'm already off track. How can a reaction of real particles proceed via a virtual particle intermediate? How do you get one virtual particle at all? I hope they're wrong, or before long I'm going to have to wonder whether the transition state of a Krebs cycle intermediate bound to an enzyme is "virtual". Then there is the neutrino magnetic moment you're calculating, which I agree reflects this ... I suppose I knew a neutron has a magnetic moment, but it still seems surprising to think a fundamental "point particle" with no charge would have one. (though our article makes it sound uncertain) I don't feel confident to calculate whether there can be a meaningful interaction based on this; and if there is, can energy be released as they approach one another to leave them in a bound system? Wnt (talk) 10:11, 12 October 2018 (UTC)

October 10

Could elephants be smarter than humans?

If they were smarter, I don’t see a clear way for them to act that would indicate ite fact, since they have mainly just their trunks and tusks to manipulate objects. Are there observations that imply they are less intelligent than humans? — Preceding unsigned comment added by Lassen Listening (talkcontribs) 02:24, 10 October 2018 (UTC)

Define "smarter" and "intelligent". ←Baseball Bugs What's up, Doc? carrots→ 03:07, 10 October 2018 (UTC)
Our article Elephant cognition is pretty good and has over 80 references. Cullen328 Let's discuss it 03:15, 10 October 2018 (UTC)
(e/c) I'm reminded of a quote from The Hitchhiker's Guide to the Galaxy, "For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons." We humans essentially define intelligence as being "like us", so it's hardly surprising that no other animals ever meet that standard. It's fairly clear that humans have a far greater capacity for abstract thinking, communication, and environmental manipulation than any other species. We kind of define intelligence as being able to do those things. If dolphins - or elephants - are capable of conscious thought, they probably define intelligence as being dolphin-like (or elephant-like, as the case may be). Less philosophically, elephants seem to have issues with even passing the mirror-test, though it's clear some can. See elephant cognition and the many many links therein, for a great deal more information. Matt Deres (talk) 03:23, 10 October 2018 (UTC)
You could also change the question a bit: Could a elephant "outsmart" a human? For example notice and avoid a trap you set or calculate like a hunter where to best confront you and then "get you" if it thinks you are a problem that can and should be "solved very easily". In that way some animals may turn out to actually be much smarter than humans in some situations! --Kharon (talk) 14:20, 10 October 2018 (UTC)

Potassium nitrate and nitrosamin formation

Can potassium nitrate lead to nitrosamines formation when in the stomach or at high temperatures? — Preceding unsigned comment added by AnonymousJohn25 (talkcontribs) 04:50, 10 October 2018 (UTC)

Dietary nitrate can spontaneously convert to nitrite, which can spontaneously form nitrosamines, so I don't see why not. But I don't see any references calling out that specific sequence. Someguy1221 (talk) 00:50, 12 October 2018 (UTC)

where is Fram Crater

Hi, I have been trying to find the coordinates of Fram Crater and it seems I cannot find it at all on the IAU Gazetteer of Planetary Nomenclature. Is it not an official crater? I found many references on the NASA website but not coordinates. Thanks --Golan's mom (talk) 06:14, 10 October 2018 (UTC)

Our article is at Fram_(crater). As you say, it's definitely not on the IAU list, so presumably unofficial. HenryFlower 14:22, 10 October 2018 (UTC)
1.9°S 5.5°W - according to our article List of craters on Mars. It may not be listed as it is very small (8 metres in diameter) so is only known from images taken by the Opportunity rover - it may not be visible on images taken from orbiting satellites. Wymspen (talk) 14:27, 10 October 2018 (UTC)
Fram Crater is not presently in the database at NASA JPL's MarsTrek. The only reputable source cited by our article is this JPL PhotoJournal website, where "Fram Crater" is described in "scare quotes." However, I went looking for other reliable sources and found the Science Magazine Special Issue (Vol. 306, Issue 5702, December 2004): Opportunity at Meridiani Planum. This issue mentions Fram Crater in multiple papers:
...and the penultimate paper for you, our reader-with-interest-in-interplanetary-localization:
There is a lot of very difficult theory and practice that pertain to describing a location on another planet - especially when we start talking about putting landers and rovers on said planet with the intention of driving to features of interest - so that last paper is the one you should read first.
Additionally, the 2006 issue of IEEE Robotics included a paper from the JPL navigation experts: Jeff Biesedecki et al.: Mars exploration rover surface operations: driving opportunity at Meridiani Planum (IEEE Robotics & Automation, 2006), in which the authors take credit for informally naming the Fram Crater. They also present lots of fun technical details about why nobody knows where this crater really is:
"With considerable slip and time constraints preventing use of visodom, we knew the rover's internal position estimate would not be very accurate. Not making use of the internal position estimate precluded the use of Go_To_Waypoint and Turn_To commands, conditional sequencing based on estimated distance to a Cartesian location, and even remote sensing commands designed to image specific X, Y, Z coordinates. Instead, our mobility sequences were almost all geared to using combinations of Turn_Absolute and Arc commands based on predictions of what our slip would likely be."
..."Drives along the rim were all done with Arc and Turn_Absolute commands. We were driving on generally rocky berm, on a slope that was away from the crater interior (so slip would take us away from the rim itself, which we liked). The drives were kept short enough that we had a clear view of our drive path and could verify it was clear of obstacles and ejecta. We avoided doing sharp “dog legs” because we were driving far enough that stereo range data was not precise, and we did not trust the precision of our localization in the orbital maps. So most days were straight drive segments, approximately 40 m/sol."
I have added links to these references to our article.
So - the location of the landing site is known to better than 10 meters accuracy; the location of certain other crater features visible from orbit can be known with excellent precision - but the positions for many of the features that Mars Exploration Rover Opportunity actually visited and photographed are not accurately known. We can safely say that Fram Crater is a few days drive - perhaps a couple hundred meters - from the nearest big rock, informally called "Bounce Rock" - but otherwise at unknown distance and bearing, on the featureless terrain of the plains of the Meridiani.
Nimur (talk) 17:31, 10 October 2018 (UTC)
Other sources which discuss the crater, although probably don't help with the localisation, that I came across earlier are Crater gradation in Gusev crater and Meridiani Planum, Mars and Opportunity rover localization and topographic mapping at the landing site of Meridiani Planum, Mars. BTW this source [4] which I added to the article discusses how these unofficial names for the crater came about. The unofficial names don't comply with the IAU naming rules, so I don't think they were intended to be possible official names. Nil Einne (talk) 18:25, 10 October 2018 (UTC)

Does the hypothetical compound XeH2 have a dipole moment?

This was one of the last questions on a recent general chemistry I exam at Columbia University. I'm a tutor and my student was thrown off by this question.

Firstly based on MO theory this bond arrangement would be very weird. The most stable bonding arrangement would involve Xenon exciting a single electron from each of two nonbonding orbitals to two new higher energy orbitals to form a bond with H•. However this bond is much weaker than the H-H bond and would decompose almost immediately. I don't think this is a fair question, but I also anticipate seeing bent geometry. Should my student appeal this question? Yanping Nora Soong (talk) 18:38, 10 October 2018 (UTC)

I think my student's professor may have merely wanted her to have memorized XeF2. in XeF2, xenon is able to use its "expanded octet"(having ten valence electrons) because F lowers the energy of the resulting molecular orbital involving dsp3-sp2 atomic orbitals from Xe and F respectively. H cannot do this. It simply isn't electronegative enough.

to me, concepts should supersede using the mere memorization of facts to extend a motif to a molecule that doesnt exist. Yanping Nora Soong (talk) 19:03, 10 October 2018 (UTC)

Calculated properties of XeH2 (abstract). DroneB (talk) 20:09, 10 October 2018 (UTC)
"Hypothetical compound", therefore there is no grounds to appeal based on whether it would be even slightly stable. Based on DroneB's nice ref, you now have deeper information about the concepts, and see if your understanding of what "cannot" happen is on target, and how widely applicable the XeF2 pattern is. The only ref to this compound on en.wp is in polyhydride, which cites doi:10.1063/1.4931931 for several XeHn. I cannot access the fulltext at the moment:( DMacks (talk) 21:30, 10 October 2018 (UTC)
  • Hypothetically this is stable from 1 to 200 GPa. XeH2 in this state is solid in orthorhombic crystal form, space group Cmcm and so is symmetric with no dipole. Graeme Bartlett (talk) 09:16, 13 October 2018 (UTC)
  • If you're just supposed to be learning entry level VSEPR theory, without getting into the higher-level bonding theories like MO, then the Lewis diagram would have 5 electron domains around the central Xenon, which would give a trigonal bipyramid electron arrangement; and in that arrangement, maximal electron pair repulsion would put the three lone pairs in the equatorial position, and the two H-Xe single bonds in the axial position. The H-Xe-H bonding angle would be 180 degrees, which would have no dipole moment. Which is one way to approach the problem. I would ask the professor which particular principle, theory, or idea the problem is supposed to be allowing you to explore; the answer you give to a problem like this depends entirely on which ideas you are supposed to be applying. --Jayron32 22:32, 10 October 2018 (UTC)
  • I'm teaching an "atoms first" paradigm -- where my student uses approximations like pedagogical effective nuclear charge from the very beginning to get an intuition for the energy of orbitals and why hypervalent bonding occurs. The common VSEPR bonding structures naturally arise from these considerations. I'm frustrated by questions which have students have to discard common sense chemistry concepts, because they have to use these concepts for organic chemistry. Yanping Nora Soong (talk) 00:06, 15 October 2018 (UTC)
  • I justify my student the existence of XeF2 by using a very simplified Lewis model discussion of the three-center four-electron bond. I dislike having my students memorizing stuff they have to unlearn later that really messes them up in organic chemistry, and my teaching paradigm is that undergraduate introductory organic chemistry is "90% concepts 10% memorization". This Lewis model helps them also learn or reinforce Lewis acidity/basicity, resonance and the HOMO/LUMO interactions which will be used extensively later. Having the student extend XeF2 to XeH2 meaninglessly does none of that. However, my student did not wish to "cause a ruckus" and decided not to appeal. Yanping Nora Soong (talk) 00:15, 15 October 2018 (UTC)
  • All of that may be true, but you're missing the point here. Pedagogy is not merely the listing of true things that students are supposed to magically understand simply because you told it to them. Pedagogy requires knowing how to build up knowledge in students through a series of approximations and simplifications so that students can be intellectually prepared to get more and more accurate views of the world. I do agree that in general one should avoid blatantly incorrect examples when teaching simplified models, but it isn't really the end of the world here. The theoretical basis for this sort of "building up" of knowledge in the learner probably dates to psychologist Lev Vygotsky (see Zone of proximal development) and more fully developed as the practice of Instructional scaffolding. See also Wittgenstein's ladder, which is sometimes derisively referred to as the lie-to-children, but really is a valid instructional practice. In simple terms, you don't start with general relativity when teaching new learners about simple motion. You start with classical mechanics and build them up to it, even though you as a physicist know that the Newtonian equations of gravitation are incorrect, you don't drop the Einstein field equations on them on day one of the lesson. Similarly, introducing advanced models of molecular bonding theories on a student without them having a solid basis in the simpler models isn't necessarily beneficial here. Again, in order to know how to help your student solve their problem with determining the polarity of the putative XeH2 molecule, it is important to know why did the professor give the XeH2 problem to begin with that is what principle is the professor trying to teach through the use of that example. It really isn't important if the example is even correct (again, it is a Wittgenstein's ladder), merely that it can be used to demonstrate a specific concept. Perhaps the XeH2 problem was intended to allow the student to explore the ideas you are bringing up, but unless we know that, we can't know how the student is to approach the problem. If you lead them down that route, you could be leading them into topics they aren't going to be able to incorporate into their own knowledge base because they have not been properly primed for them by learning simpler concepts. --Jayron32 12:44, 16 October 2018 (UTC)

Gallium and miscibility with other metals

Is gallium miscible with mercury, sodium, lithium, potassium and NaK? Are these alloys liquid around room temperature? Thanks!--82.137.12.135 (talk) 23:46, 10 October 2018 (UTC) posted by request from my talk page --Jayron32 23:55, 10 October 2018 (UTC)

The large electronegativity difference between gallium (1.8) and the alkali metals sodium (0.9), potassium (0.8) and lithium (1.0) makes it unlikely that they will form metallic bonds. Looking at a Ketelaar triangle, such bonds would fall in the Zintl range. I would expect gallium to dissolve in liquid mercury, but I can't find any references relating to it. Handschuh-talk to me 01:15, 11 October 2018 (UTC)
This article contains some examples of gallium-alkali metal zintl phases. Handschuh-talk to me 01:38, 11 October 2018 (UTC)
This NBS report covers attempted alloying of gallium with various metals with the specific aim of finding new dental-filling materials. DMacks (talk) 02:40, 11 October 2018 (UTC)
Mercury + gallium non-alloying (if you trust YouTube). DMacks (talk) 02:48, 11 October 2018 (UTC)
Thought i have seen it mentioned in newer battery/anode technology, which is a huge, extremely active research field since fossil fuels became troublesome. The percentages for the Gallium part where actually extremely low however. --Kharon (talk) 13:43, 11 October 2018 (UTC)
Thanks for that video - that's a fun one! I've long wondered how many simultaneous immiscible liquid phases you could have in a container if you really tried, and this adds one more. More to the point... can you make a separatory funnel with mercury and gallium and use it to purify and separate metals? Can you categorize metals as "mercurophilic" or "gallophilic"?? (I looked ... but "mercurophilic" seems to mean some other obscure thing that affects bond distances in a molecule) Wnt (talk) 10:23, 12 October 2018 (UTC)
The elements' tendency to prefer dissolution in one fluid medium over another is usually categorized using the Goldschmidt classification. According to this system, both mercury and gallium are chalcophiles, which again, would point towards them being miscible, though obviously isn't definitive. Handschuh-talk to me 01:55, 13 October 2018 (UTC)
That reminds me of The Beach (film) where the two boys utterly failed to consider resolving their sexual rivalry amongst themselves. Not all that love oxygen love one another. Wnt (talk) 16:33, 13 October 2018 (UTC)

October 12

Drinking human blood

As a matter of curiosity, in general what are the effects and especially the health risks of drinking human blood? FreeKnowledgeCreator (talk) 09:43, 12 October 2018 (UTC)

The consumption of blood as food is not uncommon; there are tribes that have long taken blood from cattle as one might take milk, due to lactose intolerance not having been bred out of their heritage from millennia of milk-drinking. However, as that article notes, there is a risk of excessive blood consumption (I assume from iron toxicity). Now human blood is another ball of wax, due to blood-borne disease. I haven't checked quantitatively how the risk of infection from blood compares to the risk of infection from semen, but conceptually you can compare risks of oral sex, bearing in mind that the dosage potentially could be different. Wnt (talk) 10:30, 12 October 2018 (UTC)
Some human societies drink blood (Hematophagy) or use it to manufacture foodstuffs and delicacies, see Blood as food. However note religious prohibitions and vampirism. DroneB (talk) 12:07, 12 October 2018 (UTC)
A medical or possibly chemistry textbook once told me those vampire wannabes could only have a pint (or was it quart?) in their stomach cause more causes vomiting (a US pint is 473 milliliters, a lot less than the 2 liters of food needed to cause vomiting). Sagittarian Milky Way (talk) 14:34, 14 October 2018 (UTC)

Crystal methodist

Reports say that the Securities and Exchange Commission have filed court papers stating that the offer of Elon Musk, the chief executive of Tesla, to purchase the shares at 4 dollars 20 cents each is related to the date April 20, which is significant in marijuana culture. What happened on April 20? 92.8.223.143 (talk) 16:07, 12 October 2018 (UTC)

I don't think this really belongs at the science desk however given the protection I guess some misplaced posts are to be expected. Anyway, if you mean why is 420 and 20 April associated with cannabis culture, our article 420 (cannabis culture) explains. According to the article, nothing in particular happened on 20 April. The association began because of the time 4:20 pm. The association with 20 April arose later due to the association with 4:20 pm and then 420 and so April 20th. BTW, AFAIK Elon Musk did not offer to purchase any shares at $4.20. Musk suggested privatising Tesla with an offer of $420 per share [5]. Note the absence of a decimal separator in that number is correct and intentional. Nil Einne (talk) 17:34, 12 October 2018 (UTC)
Shucks. And we all though Musk was a closet head. But talking of "high finance".... here's the UK's celebrated Crytsal Methodist. Martinevans123 (talk) 17:42, 12 October 2018 (UTC)
The Daily Mail gives a plausible-sounding explanation here ([6]) - supposedly a 20% premium is standard for taking a company private, which worked out to $419, and he rounded it off apparently because the "420" reference made it funny to his girlfriend. Note the tweet was made in the morning of August 7, so there were no other joke references aside from the extra dollar a share. Wnt (talk) 11:12, 13 October 2018 (UTC)
Ah-ha! So we can all blame Grimes for such an inspired choice. Too bad we have WP:DAILYMAIL (?) Martinevans123 (talk) 11:27, 13 October 2018 (UTC)

October 13

Reproduction question

I previously asked this question here, but did so anonymously and thus I am wondering if I will get more responses here if I openly asked this question. Anyway, here goes:

If a man's epididymis is removed (on both sides, of course) and his vas deferens grows and attaches itself to his rete testis, is this man going to once again have sperm in his ejaculate, but still be incapable of ever impregnating a woman through sexual intercourse as a result of his sperm not being motile enough? Futurist110 (talk) 02:04, 13 October 2018 (UTC)

Have you considered handjobs? Sagittarian Milky Way (talk) 14:38, 14 October 2018 (UTC)
I see no particular reason why there would not be sperm in the semen, but there might well be less of it, given the loss of its principal storage vessel. So while a modicum of fertility may remain, I have to imagine that it would be reduced. Motility is another question, the answer to which would depend on a number of factors. I am definitely not going to say "incapable of ever" causing pregnancy: far too many "longshots" have resulted in conception in the past. After all, it only takes one sperm. --Piledhigheranddeeper (talk) 01:51, 15 October 2018 (UTC)

Negative differential resistance in fluorescent light bulbs?

Graph.
Graph showing the relationship between current and voltage across a neon lamp.The portion of the curve between points A and B is often referred to as the negative resistance region. DroneB (talk) 17:34, 13 October 2018 (UTC)

Our article on negative resistance gives a sourced sentence (and I checked the sources) that "an increase in voltage across the device's terminals results in a decrease in electric current through it." It depicts as an example a fluorescent light bulb, where the current that flows starts an arc that reduces resistance. That too is sourced, but there the sources say "negative resistance characteristics". An online discussion seems skeptical. [7] (The article also lists these in a table as a passive negative resistor)

The problem I have is that I can't picture that a circuit that can actually increase the voltage on a fluorescent light bulb in real time at any point during its cycle would actually observe decreased current flow. Either the flow would increase minimally, or a lot -- I think. The phrasing for the definition of negative resistance would seem to impose a very stringent criterion this way - one met by a tunnel diode, where increasing the voltage (as the independent variable, you might say) really does cause it to choke off the flow of electrons.

This was discussed on the article talk page before I looked at it, but can we get an answer on the overall fact: is a fluorescent light bulb a negative resistor in any meaningful sense at all? Wnt (talk) 16:28, 13 October 2018 (UTC)

@DroneB: Thanks for the figure - I see it is at neon lamp. Then again, I also see it has had a "dubious-discuss" tag on it there since 2015, so I could use a bit more convincing. More to the point, I still don't know if this chart shows a correlation rather than a causation -- I mean, can this bulb really stop halfway between A and B, and increase current if you decrease the voltage, or decrease it if you increase it? Wnt (talk) 20:58, 13 October 2018 (UTC)
Yes it can. Supply the bulb from a very high voltage in series with a very high resistance, which creates a virtually constant current supply. Of course there won't be any current until after the arc is struck at "A" by some kind of over-current trigger. DroneB (talk) 21:13, 13 October 2018 (UTC)
Wnt, it seems that your confusion about "correlation" or "causation" is because the relation between voltage and current is not simple. More precisely, this graph shows a non-Ohmic relation between current and voltage. The relation is a multivalued function; the physical system has state. We could describe that state as a result of hysteresis or we could describe it as a complete set of other physical parameters; but in such cases, the relation between voltage and current is neither simple nor linear. Nonetheless, it is still true that the applied voltage is the causative agent for the flow of electric current. The magnitude of that current, however, depends on additional factors.
This is one case where we can't use simple math to describe the physics: if we tried to describe the current-voltage relationship as a single-parameter function, we would just get the wrong answer. The amount of current in a fluorescent light-bulb depends on voltage, but also depends on the time-history of that voltage, and on the ambient temperature, and the pressure of the gas in the bulb, and so on. Even a very simplified model would be a function of many variables, and when you try to address such a problem without using the right mathematical tools, you just get wrong numbers in your answers - and even worse, you may even draw incorrect physical conclusions (such as the confusion about causation that Wnt appears to have described).
In the case of a light bulb, the volts represent energy applied to the system; the amps represent the system's response to that applied energy, converting it from electric potential energy into the kinetic energy of moving charged particles. The equation that governs how that energy conversion occurs is complicated because there's a non-negligible amount of real-world plasma physics happening in that tube - so the simple "V = I R" linearization just isn't good enough.
Nimur (talk) 00:55, 14 October 2018 (UTC)
@Nimur: I really do understand all of that. I understand that a light bulb at the point shown on the graph at right that has an x-value halfway between A and B could have either value for the current depending on circumstance. Other things like the tunnel diode have a similar situation. The key issue for me is still that whereas the tunnel diode actually can be shown to have a true negative differential resistance, where turning up the voltage reduces the current, I still strongly doubt that the fluorescent or neon light bulb can be made dimmer as a consequence of turning up the voltage at a given moment. I mean, turning up the voltage can't reduce the temperature, it can't reduce the amount of current flowing through an existing plasma arc, what can it possibly do (aside from interacting with the ballast...) that would dim the bulb? But that's the definition the article gives for negative resistance. Wnt (talk) 02:23, 14 October 2018 (UTC)
Does this paper help?[8]
One thing that helps people to think about negative resistance is to consider an ordinary fuse. Up to a point, an increase in voltage causes an increase in current (fuses do have a small resistance, and would not function is the resistance was exactly zero). Past that point, an increase in voltage results in a dramatic decrease in current. And every wire is a fuse. --Guy Macon (talk) 06:49, 14 October 2018 (UTC)
@Guy Macon: I don't think a fuse is a good example, because the decrease in current with a fuse is a function of time more than anything else. Put a very rapid spike of voltage across it (enough not to induce heating...) and it would be just a conductor.
However, the paper is great. I may not have really understood the step between equations 7 and 8, let alone the heavy math at the end, but I think I can fairly say that "a plasma discharge has a negative differential resistance when the slope of the filament characteristic curve exceeds the slope of the plasma production curve at their intersection." That said, the discharge curve they show is discharge current as a function of electron probe current; then they show both discharge current and plasma production as functions of plasma density. In figure 7 it is clear that in the negative resistance region (C to D), the filament characteristic is increasing faster than the plasma production. The problem is, I still don't really understand why that means the current drops from increased voltage. I have a sense that, perhaps, the flow is being "choked off at the filament" because the emission of electrons is somehow saturated, but as best as I can tell that seems backwards from the increased "filament characteristic"? I don't feel like I understand well enough to explain it in an edit, beyond possibly the quote at the beginning of this paragraph. But it's progress. ;) Wnt (talk) 13:41, 14 October 2018 (UTC)
Re: "the decrease in current with a fuse is a function of time more than anything else", that's a really useful way of looking at it. The plasma discharge also involves time -- it doesn't create new ions and electrons instantaneously, nor do they revert to ordinary atoms instantaneously. But then again, a resistor does not act as a resistor unless you give it some small amount of time after a sudden change in voltage -- because every resistor is also an inductor. I would think it fair to say that both the fuse and the plasma show a negative differential resistance when subjected to a slowly-changing voltage, and that pretty much everything acts differently at very short time scales. One could say that everything is a perfect insulator if the time scale is fast enough that the speed of light is the limiting factor. --Guy Macon (talk) 15:08, 14 October 2018 (UTC)
With the neon lamp biased at a point midway between A and B by a current through a high value resistor as I described[9], apply a small voltage through a capacitor. That should demonstrate that the current and voltage swings are in the opposite direction consistent with the negative I/V slope. Also you may see the lamp brightness modulated with decreasing brightness with increasing voltage swings and vice versa. However a constant voltage supply is not good for testing the lamp because of the danger of the current leaping into the possibly destructive segment B->C. It's better to make a constant current supply be the controlling variable, which enables straightforward plotting of the whole V = f(i) characteristic. Note that this is not an efficient way to create a fluorescent lamp Dimmer. This is better achieved by a solid-state circuit that delivers a variable pulse-width voltage. DroneB (talk) 20:25, 14 October 2018 (UTC)
  • The question has been answered by now, but I will still leave a link to electrical breakdown. (Though not exactly the same thing, it helps understand how resistance can decrease by the application of strong voltage.) TigraanClick here to contact me 11:46, 16 October 2018 (UTC)

October 14

Nuclear shell model, alpha particles=nuclei within nuclei?

Since alpha particles come out of radioactive nuclei so often, and alpha particles are notoriously stable, why does the nuclear shell model insist that the shells have separate shells for neutrons and protons, why not even a shell made of alpha particles?...it seems likely to be energetically somewhat favorable, even within a nucleus, for alpha particles to wander around intact.Rich (talk) 00:07, 14 October 2018 (UTC)

I went looking and found a fringey-looking website that didn't link to any existing work, but makes kind of an interesting case. This looks more serious, and describes a weaker variant of the idea where nucleons simply can form alpha particles transiently within a nucleus, altering its properties; it's called alpha-clustering. That brings up scads of papers I know nothing about, describing many decades of work; a downloadable reference is [10] which sounds halfway between the first two. Better to search ArXiv and come up with [11] which looks promising, and many others. At this point I should cede the floor to an actual nuclear physicist... Wnt (talk) 02:33, 14 October 2018 (UTC)
While alpha particles are stable in sense that it is difficult to split them apart, their binding energy is still significantly less than that of heavier nuclei. In addition in heavier nuclei neutrons outnumber protons. So, it is difficult to imagine that heavy nuclei are literally made of alpha particles. Ruslik_Zero 19:19, 14 October 2018 (UTC)
Ruslik, your reasoning seems to me to imply the opposite of what you concluded. You seem to imply that not only would alpha particles be present as subnuclei, but heavier yet particles would be subnuclei. (By the way I didn't mean to suggest that a heavy nucleus would be made up entirely, or even almost entirely, of alpha particles, just some or a few.). Imo the binding energy argument argues against a nucleus consisting entirely of free protons and free neutrons. Thanks to both of you.Rich (talk) 20:11, 15 October 2018 (UTC)
I am not sure why you came to these conclusions? The nuclear forces have a short range and tend to saturate. So, as the coordination number increases, the binding energy increases. The coordination number in an alpha particle is around 3. It is higher in heavy nuclei. However returns diminish quickly due to the saturation. Ruslik_Zero 20:34, 15 October 2018 (UTC)
In a sense, I think alpha particles "fit through the gaps" in the nuclear shell model. After all, they have two protons and two neutrons with opposite spins, which mean they can always fall within a single energy level for either protons or neutrons, and often can be in the same state for all 4 (except spin). Supposing nuclear orbitals are like the somewhat more familiar electron orbitals, then knowing the angular momentum numbers means giving up any idea about the position, which is to say, if l is 1 and ml is +1 for each of the four particles, we have no way to say if they are together in an "alpha particle" or not. I think that means that sometimes they are alpha particles, just by random chance? But to say they stay alpha particles would mean giving up some of the certainty about their quantized momenta. Speaking of which, does the sharpness of the nuclear gamma decay frequencies thereby place any Heisenberg constraint on whether they can be permanently bound as alphas in the nucleus? (This is so not my field, sorry!) Wnt (talk) 02:19, 16 October 2018 (UTC)

October 15

Diesel-electric rail emissions

Someone in the UK House of Commons recently asked if switching freight from road to rail was a priority for the government in its bid to reduce carbon emissions, and the Prime Minister said she thought that would be a good idea. However having looked into it, it seems the majority of freight trains in operation in the UK are Diesel-electric British Rail Class 66's. Would this specific train model offer up any improvements over say road haulage, on a quantitative basis? The British Rail Class 66 article just states:

Numbers 66 752-779 were the last Class 66s ordered for service in Great Britain because of increasingly stringent emission regulations. 66 779 was the last Class 66 to be ever built. Although the Class 66s meet stage 3a of the regulations, they do not meet stage 3b. Stage 3b would have required additional exhaust treatment equipment that could not easily be accommodated within the UK loading gauge. The same restrictions apply to the Class 68 and Class 70. The restriction does not apply to second-hand locomotives, provided that they are imported from within the European Union. The purpose of the regulation was to put a cap on the total number of non-compliant locomotives in the EU.

Many thanks. Uhooep (talk) 01:16, 15 October 2018 (UTC)

This looks like it may be relevant or at least links to useful sources, especially on page 44-45 [12] Nil Einne (talk) 03:11, 15 October 2018 (UTC)
The key source cited there seems to be [13] and if you just care about their estimate, I think page 65 of the report (or 68 of the PDF) may be the key one. Nil Einne (talk) 03:26, 15 October 2018 (UTC)
I didn't do an exhaustive search on this subject, but the "additional exhaust treatment equipment" is probably to reduce soot and/or NOx emissions. Soot is of course mostly carbon, but the carbon emissions the government is talking about are carbon dioxide emissions. So those are unrelated. As far as carbon dioxide emissions are concerned, every somewhat modern diesel locomotive outperforms lorries by a large margin.
Switching from road to rail will require additional locomotives. Some second-hand Class 66s will be available from the continent (I see them there a lot less than a few years back, as they got replaced by electrics), but this switch will require new locomotives too. And I understood that the UK is rapidly electrifying its rail network. PiusImpavidus (talk) 10:21, 15 October 2018 (UTC)
Railways are generally assumed to be up to 5 times more energy effective in a similar logistic task compared to road transportation. This is mainly because of the Rolling resistance, which is very low with steel wheels on a steel railway, and the common very low Grade (slope) of railtracks compared to the much higher grades common in road construction. --Kharon (talk) 18:49, 15 October 2018 (UTC)
Energy_efficiency_in_transport#Trains has some relevant information.--Wikimedes (talk) 04:46, 16 October 2018 (UTC)
No, the UK is not "rapidly electrifying its rail network".  I can't cite specific articles, but this subject comes up regularly in Modern Railways magazine, which I read.  While there have been electrification projects established in recent years, they have tended to overrun their budgets and have been subject to delays and partial cancellations.  Currently the people in charge are embracing bi-mode trains that can run on routes that are only partly electrified. --76.69.47.223 (talk) 08:50, 16 October 2018 (UTC)

Innate responses to tastes

What evidence is there of innate human responses to tastes? For example, is there evidence of an inherent tendency in humans to dislike things that taste bitter? FreeKnowledgeCreator (talk) 22:40, 15 October 2018 (UTC)

Why it's called "bitter".[14] And the continued popularity of beer undermines your premise. ←Baseball Bugs What's up, Doc? carrots→ 23:35, 15 October 2018 (UTC)
No not really. You'll note my use of the word "tendency" in my question. Tendencies allow exceptions. FreeKnowledgeCreator (talk) 23:38, 15 October 2018 (UTC)
A lot of people drink beer. I don't. Why? Because of its bitter taste. But maybe I'm the exception. The problem with your premise is that it's switched around. It's called "bitter" because it tastes bad. ←Baseball Bugs What's up, Doc? carrots→ 23:50, 15 October 2018 (UTC)
Google "innate responses to tastes" and you'll see a number of options which may address your questions. ←Baseball Bugs What's up, Doc? carrots→ 02:58, 16 October 2018 (UTC)
Humans have like other species special built-in reactions on taste and smell, but in some cases can train to overcome them like with beer or wine (which children do not like at all but adults even prize as Delicacy). Surströmming (fermented fish) is an exceptional example how build-in smells already even block us from trying to taste some food. Its likely because Homo Sapiens is not as able to devour rotten food without getting seriously sick as all Vultures and many Reptiles are naturally, because their "strong" Digestion is very capable of.
Checkout Americans Try Surströmming (Youtube 3:45) for a vivid impression--Kharon (talk) 04:26, 16 October 2018 (UTC)

October 16

corrosion on zinc platted MS parts when coming in contact with corrugated sheets

WE are facing problem of corrosion on zinc platted parts when they comes in contact with corrugated sheets .I would like to understand what is the chemical reaction happen when zinc platted MS parts comes in contact with corrugated sheets — Preceding unsigned comment added by WP-Kalyanbelsare (talkcontribs) 09:57, 16 October 2018 (UTC)

  • The answer to your question is somewhere in the article redox or in the "see also" below it. If you want more from us, you will have to provide more details ("corrugated" is not extremely precise from a chemical point of view; plus, are you sure the contact is the cause of zinc oxidation?). TigraanClick here to contact me 11:43, 16 October 2018 (UTC)
  • See Galvanic anode. Zinc is a fairly active metal (which is to say, it has a lower reduction potential compared to other metals); for that reason it is often used as a protective coating on galvanized metal; when zinc comes into contact with other metals, it prevents the other metals from corroding by instead corroding itself. --Jayron32 12:31, 16 October 2018 (UTC)

Are vector components vectors too?

If we decompose the vector V (1, forming an angle of 45° above the x axis) into its x,y components (0.707,0.707): aren't these components vectors too? You could think of them as two vectors with origin in 0, and perpendicular directions. One aligned with the x-axis and directed in ascending order. The other the same, but for the y-axis If we treated these components as vectors and added them, we would get the original vector V.

This however conflicts with two sources that I checked: "the x component of the ball’s velocity. The x component is a scalar (a number, not a vector), " from [15] and "Caution components are not vectors " from Universal Physics by Young.

What could go wrong if we treated these components as vectors? How can I know they are not vectors? What's the difference between adding two vectors (that match the x and y directions respectively) and decomposing the resulting vectors? Are these not just the inverse operation?--Doroletho (talk) 16:04, 16 October 2018 (UTC)

It's a matter of perspective rather than absolute right or wrong. You can look at the x component as a vector whose origin is (0, 0) and whose direction is along the x axis, OR you can look at the x component as the point on the x-axis where the head of that vector reaches. The final vector is either a) the sum of the two axial vectors or b) the vector whose start point is the (0, 0) and whose end point is the scalar coordinates (x, y). You get the same result either way. this page explicitly makes the case that the components of a vector are also themselves vectors. --Jayron32 16:16, 16 October 2018 (UTC)
  • It depends how you decompose them.
If you decompose a vector as "the sum of two vectors", then they're vectors.
However, if you decompose them as "x,y components", then you've decomposed them into two scalars, each implicitly coupled to an axis. You can then consider each as the product of that scalar and a unit vector in the direction of that axis, then sum them again to return the original vector. But if you talk about them as "X or Y values along an axis" (that named axis implying the direction vector), then the quantity is just a scalar. Andy Dingley (talk) 16:29, 16 October 2018 (UTC)
Mm, I'm afraid this risks getting a little philosophical, but I don't really think it's correct to call them scalars. Scalars are rank-0 tensors, which means they're supposed to transform tensorially under coordinate transformations. In the case of rank 0, that means they don't change when you change coordinate systems. But the "components" are irreducibly tied to the coordinate system; use a different set of coordinates, and you get different components.
There's another word for things like components; that is, real numbers associated with a vector or tensor field, but that depend on the coordinate system. I think they might be called "numerics" or something like that? --Trovatore (talk) 18:27, 16 October 2018 (UTC)
  • Certainly there's a semantic point in here, but I think they're either scalars or vectors, nothing inbetween.
They start out as vectors. When we strip away their implicit connection to an axis, by stating that axis explicitly through some other means, such as saying, "x units along the x axis", then all we have left is that scalar. If you solve a quadratic equation by plotting its function on a graph, then the values you obtain as solutions are purely that, purely real number values (i.e. scalars). Andy Dingley (talk) 18:54, 16 October 2018 (UTC)
Well, my point is not that they're "in between" scalars and vectors, but that they are neither one. Scalars and vectors are supposed to be objective, in the sense that they don't depend on the coordinate system. Components are subjective; they depend on the angle at which you look at the vector. --Trovatore (talk) 19:12, 16 October 2018 (UTC)