Wikipedia:Reference desk/Archives/Science/2021 January 22

= January 22 =

Mythbusting James Bond
If you fill an incandescent light bulb with concentrated nitric acid (as was done in the movie Wrong is Right), (1) would it explode when turned on, and (2) if so, would it set nearby objects on fire (as was the case, again, in the same movie)? 2601:646:8A01:B180:A0FA:C24:4E90:ED94 (talk) 06:03, 22 January 2021 (UTC)
 * Nitric acid is not by itself flammable; it needs something combustible to react with, so on its own, even (or particularly!) as pure 100% concentrate, it won't do much. --Lambiam 11:55, 22 January 2021 (UTC)
 * Nitric acid is a pretty good oxidizing agent, and many of its reactions are exothermic, so concentrated nitric acid can do a LOT of damage, and I suppose under ideal conditions could cause a flame. But I would never expect it to create open conflagrations like being described above.  If it gets on your skin, it is corrosive and will cause you some bit of damage, I certainly wouldn't want a bottle of it poured over my head; a few drops in your eye could also be particularly nasty, but the worst that usually happens if you get a few drops on you and wash them off right away is some yellow stains on your skin that peels off in a few days.  -- Jayron 32 14:52, 22 January 2021 (UTC)
 * And confirms that you have protein in your skin. Mikenorton (talk) 15:54, 22 January 2021 (UTC)
 * These acids are very, very dangerous.  See acid attack. 194.53.186.133 (talk) 16:29, 22 January 2021 (UTC)
 * Yes they can. -- Jayron 32 16:50, 22 January 2021 (UTC)
 * The light bulb contains some metals: two leads, a tungsten filament and maybe some supports. I assume they will dissolve in the acid, producing some gas in the process (some nitrogen oxide, I guess, but it's some time ago that I did these chemistry problems). This will build up pressure inside the bulb. When the light is switched on, an electric current will run through the acid (assuming there's enough left of the electrodes), starting some more reactions. I'm not sure of the resulting gases, but they may build enough pressure to make the bulb explode. I'm not sure about further effects. (Using sulphuric acid instead of nitric acid makes the chemistry simpler: it will produce hydrogen and oxygen.) PiusImpavidus (talk) 19:13, 22 January 2021 (UTC)
 * The filament is very thin, so it's likely to be dissolved first, creating an open circuit; in that case, nothing will happen when the switch is turned on. 51.9.103.0 (talk) 11:48, 25 January 2021 (UTC)
 * Another factor to bear in mind is that nitric acid is 1,000 times denser than argon (the gas that incandescent bulbs are normally filled with), so it would dissipate much more heat from the filament than argon would. Assuming the filament is not dissolved by the acid, it would take much longer to heat up than usual, and would be very unlikely to reach an incandescent temperature.  It's also unlikely it would get hot enough to vaporise the acid or induce a chemical reaction within it.  All in all, it'd be a pretty pointless exercise.  Rhythdybiau (talk) 17:56, 25 January 2021 (UTC)

What mistake did Newton in Perihelion precession of Mercury?
What mistake did Newton in Perihelion precession of Mercury? I know Einstein proved Newton was wrong in gravity thing. Rizosome (talk) 07:22, 22 January 2021 (UTC)
 * Newton didn't make a mistake per se. He computed Mercury's orbit with Newtonian mechanics, and found a slight unexplained mismatch with observations.  Einstein explained it centuries later by developing general relativity, but that really relied on hundreds of years of theoretical and experimental advances. 2601:648:8202:96B0:0:0:0:313A (talk) 07:54, 22 January 2021 (UTC)

Can you show "unexplained mismatch" Newton did? Rizosome (talk) 07:59, 22 January 2021 (UTC)


 * Before Newton, it was thought that the planets followed Kepler's laws of planetary motion, forever traversing planetary orbits that were unchanging ellipses. It was not understood why they should do that; these laws resulted from careful observations and a lot of work to come up with laws that agreed with the observations. Then Isaac Newton came up with a set of laws of motion and a law of universal gravitation. Together, they explained many known physical phenomena, including Kepler's laws. These turned out to be a mathematical consequence of Newton's laws. In fact, Newton's laws also predicted that the planets exerted some influence on each other, leading to slight perturbances of the Keplerian elliptical orbits. These predictions confirmed what some astronomers had already observed: that Kepler's laws were a very good description of the behaviour of the planets, but not perfect. Newton's laws explained these differences, in agreement with the observations. So far, so good. Nobody make any mistakes; they all just did the best they could with the available observational data. Because of the proximity of Mercury to the Sun, it was the hardest to observe among the planets then known. It was only in 1639 that Giovanni Battista Zupi definitely showed that Mercury orbited the Sun. For a long time, no one had any reason to suspect that Mercury might not obey Newton's laws perfectly. It was only in 1839, long after Newton had died, that French mathematician and astronomer Urbain Le Verrier, using a much improved telescope, showed that the actually observed so-called perihelion precession of Mercury did not precisely conform to the predictions of Newton's laws – the differences were larger than could be ascribed to measurement errors. It took to 1915 before Einstein came up with an explanation based on his general theory of relativity. --Lambiam 11:40, 22 January 2021 (UTC)


 * I just want to clarify some things as well; Lambiam's explanation is excellent, but one very important thing he said in passing needs to be highlight. Einstein did not prove Newton wrong.  Newton did not make any mistakes.  Newton's laws were, and are, still very correct and very useful for most measurements.  Newton's laws worked because the predictions they made extremely accurately match observed behavior, and continue to do so until this day.  You can happily use Newton's laws to predict how balls roll down hills, how colliding hockey pucks on a frozen pond will behave after the collision, how pendulums will swing, and all sorts of physical phenomena.  The problem is that around the edges of our ability to measure things, some aspects of Newton's laws start making predictions that don't match observed behavior.  That doesn't make Newton's laws wrong, it makes them incomplete.  Einstein didn't prove Newton's Laws wrong, he improved them by adding additional calculations that make them more accurate in the 1% of edge cases where Newton's equations make predictions that don't match observed behavior.  Improving the work of others does not make them wrong, it just makes our current understanding better.  But it bears repeating that Newton's laws still work for just about anything you'll need physics to predict, except in extremely large cases (where Einsteins General relativity is needed, like at extremely high velocities or very close to very large mass objects) and in the extremely small cases (where quantum mechanics comes in, like on the scale of atoms and molecules).  For just about anything in between, Newton is sufficient.  I mean, you can do the Einstein field equations for, say, calculating the rate of velocity of a ball dropped on the moon.  You'll get, to within the tolerance of any measuring device capable of being made, the same answer as Newton's law of universal gravitation.  The math is stupidly hard in the case of the Einstein field equations; whereas any middle-school aged student can work out the answer to solving Newton's version.  That's the point; any new science doesn't prove well-tested science wrong usually, it improves upon it; as long as Newton is useful (and it continues to be useful) it's good science.  Where it isn't useful, we need better science, and Einstein provided that.  -- Jayron 32 13:11, 22 January 2021 (UTC)

What does "predictions of general relativity " mean?
Sentence: Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light.

SOURCE

What does "predictions of general relativity" mean? Rizosome (talk) 18:49, 22 January 2021 (UTC)
 * It means this
 * An experiment to test general relativity is designed
 * The results of the experiment are calculated from theory *(The prediction)*
 * The experiment is carried out, and the results are measured
 * The difference between the predicted results and the actual results are measured
 * The differences, as the article notes, are very small. LongHairedFop (talk) 19:08, 22 January 2021 (UTC)


 * See also my answer to the previous question, in which I talk about the predictions of Newton's laws. In the physical sciences, a scientific theory generally contains laws or rules that describe quantitative relationships between measurable physical quantities, often expressed in the form of mathematical equations. An example of a quantitative law is the law known as Archimedes' principle. Given a fluid of a known density, and a body of a homogeneous material of a known, lesser density, we can calculate what fraction of the body will be immersed when it floats on the fluid. That fraction is the ratio between the densities. So we can calculate this: it is a prediction of what we will witness if we actually place the body in the fluid. Now the body and the fluid have not studied physics and have never heard of Archimedes and his principle, so why should we expect them to obey it? Nevertheless, application of the law has resulted in a prediction: it has predicted what we will observe if we put the law to the test and do the experiment. It is just the same for Newton's laws, or the laws of general relativity. They can be used to calculate the magnitude of quantities that can be measured before they have been measured. These are then predictions. In the case of general relativity, one such prediction is that time slows down on board of a fast moving vehicle. This can be tested, as follows. Take two very precise atomic clocks that are synchronized. Place one in a supersonic jet, send it off, and leave the other on the ground. The general theory of relativity predicts that when the one in flight returns, it will be a bit behind the one that remained stationary. According to Newton's theory, they should still be running in perfect synchrony. --Lambiam 22:09, 22 January 2021 (UTC)


 * As Lambiam stated, a theory is a set of explanations and predictions that describe what should happen in some real situation. They may be developed in response to observations; a good theory will not just be able to explain existing observations, it will also be able to tell what future observations will show as well.  General relativity is an explanation of (among other things) how gravity works in high-mass/energy situations.  Its predictions are counterintuitive to people who have lived their lives at low-mass/energy situations, but among the things it predicts are things like curvature of spacetime, time dilation, length contraction, black holes, etc.  These predictions are fantastically accurate; many of the things that GR predicted to be true about the universe were later confirmed by observations.  -- Jayron 32 12:00, 25 January 2021 (UTC)


 * The first indication that classical mechanics needed updating was the result of the Michelson-Morley experiment.  The deflection of light during the 1919 solar eclipse was accurately measured to confirm Einstein's theory. 95.148.229.55 (talk) 16:56, 25 January 2021 (UTC)
 * Yes, from an experimental perspective. From the theoretical side, Maxwell's equations (especially the Oliver Heaviside formulations) were also a major source of inspiration for the development of special relativity.  Heaviside's formulations came at around the same time as the Michelson-Morley experiment.  General relativity (and its implications for the theory of gravity) came about later because of the need to expand "special" relativity from inertial frames of reference to accelerating frames of reference. Curved spacetime and gravity kind of "came along for the ride" during the said expansion from the "special" to the "general" theory, and ultimately became its most enduring legacy.  -- Jayron 32 13:48, 27 January 2021 (UTC)

transparency in the Visible spectrum and metallic bonds.
Thinking about Star Trek's transparent aluminum, what is the reason that substances that are joined with Metallic Bonds aren't transparent in the Visible spectrum? The article on bonds doesn't go into depth.Naraht (talk)


 * Relating a material's optical properties to its molecular microstructure is fraught with complication, but we can probably direct you to the Wikipedia article on Absorption (electromagnetic radiation) (I know, I know - how does absorption explain reflection? ... well, read onward); and if you're really really looking for detail, I'm sure we can send you to some fine textbooks that treat the problem more completely.  For what it's worth, my book on classical and modern optics - Meyer-Arendt - doesn't waste any time explaining mechanics.  It just presents some useful and practical equations to model optical absorption while positing no theory.  Some of the greatest optical physicists in history  pointed no fingers  glossed over the details, because they were too busy using the results to stress about any mechanistic underpinnings.  Besides, my books on electrodynamics are downstairs - my upstairs office provides me with limited options for theoretical musings.
 * To some extent, if you aren't intimately familiar with the equations of electrodynamics that govern how light travels, any answer we provide is going to sound like gobbledygook - something about Compton scattering and dielectric constants and statistical descriptions of atomic lattice spacings. I'm nearly certain you'll find extensive treatment of these details in Jackson; but if you try to use those details to make empirical predictions, you're setting up for a lot of disappointment.
 * To hand-wave over a lot of the details, metals can be approximated as things where electrons float freely between atomic nuclei - free, at least, over the distances that we compare to a wavelength of incident electromagnetic radiation. Using only this simplified model, we can deduce from classical physics that the wave should be attenuated quickly - (extinguished, absorbed, thermalized - hey, anybody know of a good thesaurus of physics?)
 * But this simplified model doesn't account for a lot of complications - so it doesn't predict well over a huge range of wavelengths; it doesn't account for weird features of atomic physics that are non-classical; it only satisfies a statistical ensemble kind of description of light. Gosh, it doesn't even acknowledge the existence of the photoelectric effect, which kind of matters when we study light hitting a metal surface.
 * But at least this model points you toward some reading material that helps you see how physicists frame the problem: optics is a neat branch of physics, where practical considerations are usually more important than theoretical considerations - so if I may approach the problem with that powerful sledgehammer of empirical observation: metals are opaque to many wavelengths of visible light, and the fun thing is what we can do with that knowledge. When you try to explain why they're opaque, you will inevitably encounter a lot of corner-cases that defy your explanation - there are lots of physical interactions that we can't easily ignore.
 * Haven't had enough? From the archives of the fine folks at MIT, 6.732 Solid State Physics, several full-length books and lecture slide sets, including Part I, §2.2.3 Polyvalent Metals (...a case study using aluminum as the worked example); and the lovely Part II §6.2 Optical Properties ... which references several more sources, and like every other good physics resource, ends with a citation to Jackson.
 * Nimur (talk) 21:32, 22 January 2021 (UTC)
 * Oooookaaaayyyy.... :) Floatie electrons wiggle over large enough distances that the light is more likely to strike them and not pass through. :) I also looked at the references in the article on Gold as to why Gold isn't colored gray and instead like Copper reflects more at lower wavelenghts. Didn't help much. Sometimes simple questions in science get simple answers and sometimes, they start entire fields of study, this sounds more like the latter. Thanx!Naraht (talk) 04:55, 24 January 2021 (UTC)
 * The nature of metals and the metallic bond arises from electrons in the element's outmost electron shells being only loosely attracted to the nucleus. This means the electrons are easily able to absorb enough energy to jump to other atoms. Light is composed of photons, which experience the electromagnetic interaction, just as do electrons. This allows the electrons to absorb photons, and a bulk metal does this enough to render it opaque.
 * A neat thing is that gold is so malleable that it can be beaten into exceedingly thin gold foil, and very thin gold foil becomes translucent. Also, unraveling the behavior of electrons in metals was a big deal that laid the foundations of quantum mechanics. Albert Einstein received his Nobel Prize for his explanation of the photoelectric effect in metals, a conundrum that baffled physicists and is inexplicable under classical physics. --47.152.93.24 (talk) 02:13, 25 January 2021 (UTC)
 * This is of course assuming it was a metallic allotrope of aluminium, as opposed to some kind of exotic compound or alloy, or even just a figurative name for a glass-like material with comparable strength to aluminium. Perhaps wisely, the writers didn't go into any detail. Rhythdybiau (talk) 18:49, 25 January 2021 (UTC)