Wikipedia:Reference desk/Archives/Science/2017 November 21

= November 21 =

Does physics have axioms?
Does physics have some kind of axioms? Should we treat as a given at least the perception of basic units? For example: movement, change, or distance? Or how should we call the basic conceptual units? --B8-tome (talk) 11:24, 21 November 2017 (UTC)
 * Yes, most science has axioms. Much of the more theoretical end of their researches is about either removing such axioms, or at least clearly defining them in their minimal form. An experimental scientist might see axioms as a failure of science thus far, as some form of deus ex machina. A mathematical theoretician though sees them as the basis for a formalised and axiomatic system, an approach that has been powerful in mathematics. For physics, see Hilbert's sixth problem and Hilbert space. Andy Dingley (talk) 11:38, 21 November 2017 (UTC)
 * Causality (physics) is often considered axiomatic in physics. See Axiom of Causality, which is a bit weak for a Wikipedia article, sometimes this is called the "Causality principle".  -- Jayron 32 12:14, 21 November 2017 (UTC)


 * Most of physics is grounded in mathematical theories, which entail the acceptance of the axioms of the underlying mathematical theory.
 * About experimental sciences (rather than strictly physics), Asimov has a good quote: I believe in evidence. I believe in observation, measurement, and reasoning, confirmed by independent observers. I'll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be. (The end of it is merely a wordy explanation of the concept of a Bayesian probability, but the beginning is an assumption that knowledge can be gained by observation, which is not trivial philosophically speaking - see problem of induction.) The foundation of the experimental method is the belief that any well-designed experiment ought to be repeatable at least in some statistical sense. For instance, if I throw a coin many times, each result might be random, but the average result over a large number should depend only on my throwing technique, whether the coin is rigged, etc., and not on extra variables (such as the time of the day or the location on Earth where I perform the experiment) which ought not to matter. If it does turn out that there is some variation, we will ascribe it to extraneous variables that we failed to take in consideration rather than to a variation of the law of physics with space/time.
 * Accepting the above, a lot of experimental stuff can be tricky to classify. For instance consider the Huygens-Fresnel principle. It is "simple" (in an Occam's razor sense), and matches experimental data, but we do not really have a clue about why it is so. Does it count as an axiom, or is it a logical conclusion of the experimental evidence plus the "axioms of experimental method"? Tigraan Click here to contact me 13:15, 21 November 2017 (UTC)
 * Well, if you really want to get metaphysical, anything beyond simple solipsism requires us to accept the evidence of our own senses and intellect as axiom. There's no way to derive, from sufficient logic, that we can trust our own senses and intellect based on outside evidence, we have to work under the assumption we can trust them to some point.  -- Jayron 32 13:20, 21 November 2017 (UTC)
 * Well, yeah, pretty much any knowledge-seeking activity will rely on assumptions that the external world exists, that basic logic rules work, etc. But assuming that the experimental method "works" is different. The modern scientist (à la Bacon, Galileo, Newton etc.), when confronted with a phenomenon they do not understand, assume that there is some reason for why it happens with explanatory power (i.e. the reason does not just fit the current data, it also tells you stuff about other possible experiments or the later repetition of that experiment).
 * Consider radioactive decay which seems to follow some statistics (Poissonian process). We are not sure why it is so, and it may well be that we will never know of any explanation that is further up in the chain of causes and consequences; yet we take the belief that any radioactive decay phenomenon follows those laws, not just the ones we observed. Maybe the immediately-preceding root cause is that some deity is playing dice for each atom at each instant; but we still assume that the dice are thrown the same way when we are and when we are not looking (rather than the deity stacking the results when experiments are made in a lab). Or to take another example: nowadays, we have some reasonable clue as to how thunder strikes; a couple millenia back, "Thor made thunder follow yet-to-be-known laws" was a scientific assertion, when "thunder strikes when and where Thor pleases" was not. Tigraan Click here to contact me 13:59, 21 November 2017 (UTC)


 * Some aspects of the answers are going too far. Math axioms will be valid within physics, but they are not a part of it. I also don't want to enter into the philosophical question whether we are in a simulated universe like The Matrix. I mean axioms within physics. You can break down physical concepts like speed into the rate of distance and time. Rate is a mathematical concept. Distance and time are physical concepts. Can these be broken down in even smaller stuff? Does it stop somewhere? B8-tome (talk) 17:16, 21 November 2017 (UTC)


 * The lead of Special relativity says
 * ''In Albert Einstein's original pedagogical treatment, it is based on two postulates:


 * ''The laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
 * The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.


 * See also Postulates of special relativity. Loraof (talk) 18:17, 21 November 2017 (UTC)


 * Also Mathematical formulation of quantum mechanics. Loraof (talk) 18:32, 21 November 2017 (UTC)
 * The Laws of thermodynamics are axioms or conceptional laws of physics. But since logic is officially part of math you could as well argue that all axioms are mathematical concepts, in the end pushing even philosophy from its holy throne and instead crowning math as highest and most basic of all sciences. However real science does not care about crowns and domination. What it really wants is harmony so it does not matter if an axiom is physics or math, it matters that an axiom works for as many parts of science as possible. --Kharon (talk) 06:37, 22 November 2017 (UTC)

Using a transistor rated for 45 V in a 60 V circuit
According to my thinking, I should be able to use a BC337 transistor rated for a maximum of 45 V across the collector and emitter in a circuit which uses 60 V to power LED filaments because, when the transistor is off/closed, the LEDs (there are about 18 in series, I suppose) should drop some of the voltage themselves.

I don't yet have my LED filaments so I did an experiment with four regular LEDs in series with the transistor and applied 8 V. With the transistor off/closed, I measured the voltage across the transistor to be 2.2 V and the voltage across the LEDs to be 2.4 V even though the voltage across the transistor AND the LEDs was 8 V and 2.2 + 2.4 != 8. I was advised (on an electronics forum) that this discrepancy is because the DMM itself is passing current and affecting the measurement. I was also told that I should just use a transistor rated for 80 V but I think this was a lazy response that doesn't conisder the voltage being dropped by the LEDs. --185.216.48.85 (talk) 16:18, 21 November 2017 (UTC)


 * A DMM or "valve voltmeter" shouldn't pass significant current. It's the function of their high impedance input to reduce this to a low level that won't load a circuit like this. However, when your transistor is switched off the circuit's impedance will itself rise to a level that it again becomes comparable to that of the meter, and your meter will (probably, depending on the meter) become a significant load again, as the forum advised.
 * As to the breakdown voltage, then the VCEO limit for a BC337 depends on the current, but it also has a sharp cutoff at around 45V. The transistor is assumed to fail above this, whether any current is flowing or not. The idea that the LEDs will drop voltage is only true if they're flowing current. In the zero-current situation like this, the transistor voltage will still float high, toward the supply rail voltage (and the transistor fails). Of course, it might work. It might well do, but fail early. It's impossible to know without serious testing, but you are working past the limits of the datasheet, so any failure would be an "I told you so".  I'd probably look at using an MPSA42 rather than a BC337. Andy Dingley (talk) 16:47, 21 November 2017 (UTC)


 * Okay, thanks for the recommendation; they're pretty cheap on AliExpress. --Seans Potato Business 17:49, 21 November 2017 (UTC)
 * See data sheet for BC337. This transistor type, manufactured in quantity with a spread in collector-emitter breakdown voltage, is tested and sorted thus: If VCEO is less than 45V it may be marked BC338, if VCEO is over 45V it can be marked BC337. If you test individually a batch of BC338 you may be able to extract a minority with a higher VCEO than 45V, but why bother? There are lots of good fish in the sea, consider a 2N2484. See Breakdown voltage in Wikipedia. Blooteuth (talk) 13:28, 22 November 2017 (UTC)

Pomegranate pith
From an evolutionary view, do the inedibility of pomegranate flesh (pith) and its relatively thick envelope mean to protect it from harmful birds in the same way as poisonous berries do? Does it also mean that because of that pomegranate relies more on pollination rather than seed dispersal? 212.180.235.46 (talk) 16:30, 21 November 2017 (UTC)
 * This seems to be a good start at answering some of your questions. -- Jayron 32 16:32, 21 November 2017 (UTC)


 * Pollination can't substitute for seed dispersal, it seems to me. — I recently saw some pomegranate trees with many fruits that had split open, exposing seeds; evidently the pith protects the seeds only until they are ready to go. —Tamfang (talk) 05:08, 25 November 2017 (UTC)

Computing with magnetic knots
This is the typical sort of thing that takes but a moment to daydream but so much more to figure out...

1) Can you model a finite state machine or some useful computing task in the unravelling of a trivial knot, i.e. one that can be pulled out to a simple loop, perhaps with the output of the task stored in the final shape of an interlocked simple loop that would be forced into a trivially knotted state in the process?

2) Can a frictionless trivial knot definitely be unravelled to a loop by simply applying a force?

3) I know a magnetic field can be knotted.  Some of what I'm thinking might somehow be related to magnetic skyrmions... I'm not sure.  But is a knot in a magnetic field frictionless?

4) Is there a way to control how readily magnetic reconnection occurs in a sample region, so that a complex trivial magnetic knot (and "answer" knot) be set up in an arbitrary state, then the system is changed so that if pulled on it will unravel rather than reconnecting/decaying?

Extra: If magnetic reconnection occurs near Earth (as the article says) does that mean that there are magnetic knots wandering through our bodies as we sit here, as subtle small scale variations in the Earth's magnetic field or something? Is there a way to "harvest" these, trap them as they pass through some kind of collection screen? Could you use them for energy, clandestine communication, or some other high weirdness? Wnt (talk) 21:03, 21 November 2017 (UTC)
 * For the Extra, these magnetic effects that you talk about are in plasma. This is not the state of matter in our bodies or the Earth's atmosphere, so we only have more distant effects of electric currents in the ionosphere or magnetosphere. These could have some dramatic effects in a geomagnetic storm. But the fields will be large, hundreds of kilometers in size. Graeme Bartlett (talk) 22:16, 21 November 2017 (UTC)
 * Magnetic field lines from the Earth's magnetic field pass through us right now. Why can't they be knotted in or near our bodies? Wnt (talk) 23:44, 21 November 2017 (UTC)
 * You can use Maxwell's equations to get a relationship between the magnetic field and the electric field and any currents. There are electric currents in the body, but they are minuscule.  Read Bioelectromagnetics. The forces due to magnetic fields in the body will not be large enough to move material around, and mechanical or chemical forces will predominate. This is different to a plasma, where the material is free to move, and conducts, and is highly affected by electromagnetic fields. The overall magnetic field around us is to the Earth's magnetic field modified by ionospheric currents. (see Ionospheric dynamo region and Magnetospheric electric convection field.)  You may be interested in Geomagnetic secular variation and Geomagnetic excursion. You will find that the sizes of these magnetic structures are huge - planetary sized. Graeme Bartlett (talk) 00:35, 22 November 2017 (UTC)
 * You may also be interested in https://books.google.com.au/books?id=6JgFyk-zku8C and Knot theory. I think the answer to (2) is no, as you can get a tangle formed. Graeme Bartlett (talk) 00:43, 22 November 2017 (UTC)


 * (ec) Hmmm... Maxwell's equations famously describe the motion of magnetic and electric fields through hard vacuum! Are such oddities completely impossible at less than lightspeed, or could some strange coiled path retain them at a lower speed?  And I'm also not sure why the tenuous nature of the atmosphere and the relative uninterestingness of our bodies wouldn't simply make the knots weaker when examined at a human scale.  Still, I have to admit what you say seems to match common sense. Wnt (talk) 00:47, 22 November 2017 (UTC)
 * Those magnetic knots etc will only be possible if there are also electric currents. So that means they are happening in some material, which would likely have friction. Perhaps in a superconductor there is no friction, but when magnetic field lines move through that there would be some kind of friction. Graeme Bartlett (talk) 01:00, 22 November 2017 (UTC)
 * A paper about knots being used for computation http://www.mdpi.com/2073-8994/7/3/1289/htm I don't know if it is any good though, or if you can understand it. Maxwell's equations describe the fields though materials too. Dielectric constant and electric currents may vary in a substance. Graeme Bartlett (talk) 07:19, 22 November 2017 (UTC)
 * MDPI is kinda-sorta-not-really predatory. Tigraan Click here to contact me 09:57, 22 November 2017 (UTC)
 * You might like, though there the knots are to protect from noise. Dmcq (talk) 12:23, 22 November 2017 (UTC)
 * Like Magnetic-core memory? --Kharon (talk) 23:42, 22 November 2017 (UTC)

Between Turing's conception of a computer in 1936 and the introduction of Transistor computers, Elliot sold computers that used magnetism to calculate. They exploited the magnetic hysteresis property of hard ferrites to create elements of a computer such as logic gates and memory. Blooteuth (talk) 11:36, 23 November 2017 (UTC)


 * Hey : not sure what precisely you mean for 2), but it's clear that there are many snarled unknots on which some applications of force will not unravel it. Consider holding a closed loop of twine, then putting several trefoil knots into the double strand. In that configuration, no amount of force applied to the free bights will untie this higher order tangle, but there is of course some direction(s) in which the proper force will free the whole assembly to look like a simple loop. See Reidemeister move for more on that.
 * As for 1), sure, why not. I can think of many ways to model/encode the computation, but none of them will "work". Consider that you can visualize/model/encode a turing machine as a little box that crawls along a chain and flips toggles in each link. It's a fine model, but it's also no way to build a computer. SemanticMantis (talk) 17:44, 27 November 2017 (UTC)


 * Thanks for the links -- it has been apparent from these answers that the theory of knots gets very complicated. I have to admit that I honestly believed that grabbing any two free bights in the example you gave would unravel the knot if it weren't for friction, but you're the second person telling me not so, and I'm still in no position to argue.  I could make some kind of wacky argument that thermal forces could do random reidemeister moves on a knot at a small scale so if you pull it tight enough it ought to explore all possible moves and unravel itself anyway, but I have no idea if that's true either nor could you really do that without some kind of reconnection or something, I think, even given the plasma I'm told I would need to work with to do any of this.  Maybe someday I'll figure out enough about the details to come back and get smacked down again. ;) Wnt (talk) 23:33, 27 November 2017 (UTC)
 * User:Wnt, maybe you misunderstand my example. I think that grabbing almost any two bights would unravel it, and if you picked two at random and repeated a few times, I think it would almost surely unravel in finitely many iterations. What I was trying to point out was that no amount of force applied to two specific bights would leave you with a simple loop, even with frictionless lines. Knot theory is indeed surprisingly deep stuff. I haven't studied it seriously since around '01, and at that time there were still no known universal knot invariants, i.e. algorithms that take finite steps and can tell you whether any two knot presentations are the same or different. A skim of knot invariant would seem to indicate that's still the case. So imagine trying to do math where you can reliably distinguish prime numbers from each other, but you can't be quite sure if 12 isn't just a funny-looking version of 11! Along those lines prime knots may also be of interest. SemanticMantis (talk) 14:53, 28 November 2017 (UTC)

Medieval chemistry
How did medieval alchemists make sulfuric acid? 2601:646:8E01:7E0B:404:F3D3:C557:159A (talk) 23:47, 21 November 2017 (UTC)


 * Oil of vitriol was made from vitriol. For more see  (you can find a PDF download) which I think should cover it. Wnt (talk) 00:06, 22 November 2017 (UTC)
 * The sulfuric acid article gives particular credit to roasting green vitriol (Iron(II) sulfate) in an iron retort. The iron (II) sulfate occurs as a hydrate, so there is plenty of water to crack apart, yielding Fe(OH)2 + H2SO4, I presume.  <--- actually the source below says it is FeO + H2SO4.  Any neutral H2SO4 formed would instantly evaporate, thus being removed from the reaction and driving equilibrium toward the product, and could be condensed elsewhere.  Still, I would think the presence of some other acidic component in the reaction would greatly assist this process by increasing the fraction of SO42- that is protonated... so I don't think I've told everything. Wnt (talk) 00:38, 22 November 2017 (UTC)


 * This source seems to be adequately researched, but otherwise unverified. It includes Biringuccio's description of the process. 2606:A000:4C0C:E200:E958:86E3:541F:E7F1 (talk) 00:42, 22 November 2017 (UTC)


 * Interesting! What kind of equipment did they use -- I presume that the dry distillation had to be carried out at red heat? 2601:646:8E01:7E0B:1984:C5C9:D8FC:B368 (talk) 06:13, 23 November 2017 (UTC)