Wikipedia:Reference desk/Archives/Science/2018 October 5

= October 5 =

When do fetuses/babies start feeling pain?
And how can we scientifically test this? --Qlearn (talk) 01:15, 5 October 2018 (UTC)


 * Start reading the references in our article on nociception, and read our article on clinical research ethics. Reputable scientists will not harm humans - especially not young ones - to satisfy curiosity.  There would have to be a very good reason to study the problem, and the methods would necessarily be non-invasive and subject to review by an ethics committee.
 * Here's a peer-reviewed article authored by a couple of medical doctors from Stanford: Towards a Physiology-Based Measure of Pain..., (2011).
 * Just up the road from their lab is the Pediatric Pain Management facility at the Lucile Packard Children's Hospital. They are a real working clinic and they also publish active scientific research.  They publish a guide for parents to help understand pain and related symptoms in children, including those that are too young to verbalize their reports.  If scientific publications aren't your forte, they even have a video explaining how things work in simple words that even a child can understand.
 * So the short summary is, real scientists and clinicians do care about this kind of question, but they approach the topic with appropriate sensitivity and decorum.
 * Nimur (talk) 03:30, 5 October 2018 (UTC)


 * I have the impression the OP wants to know an age-threshold, when fetus are developed enough to feel and perceive pain. This is a recurrent issue in abortion debates and the basis for fetal-pain laws in Utah (who would trust Stanford scientists more than Utah Republicans in such matters?) requiring doctor to use anesthesia during some abortions (that is, given to the mother for reaching the fetus).
 * For a scientific review of the evidence around fetal pain see:
 * The nociception article could be complemented with a section about fetal pain--Doroletho (talk) 10:50, 5 October 2018 (UTC)


 * One can set a lower limit by making the quite reasonable assumption that there is no pain before the nerve cells (including the pain receptors) start connecting to each other and sending signals. If I remember correctly, this happens after the first trimester and before the third trimester. --Guy Macon (talk) 17:48, 5 October 2018 (UTC)

Ideal pattern for a camera trying to focus automatically
What kind of pattern is a camera looking for in order to focus automatically? What pattern could I print on paper to best maintain the focus of an auto-focusing camera? — Preceding unsigned comment added by 185.230.100.66 (talk) 04:11, 5 October 2018 (UTC)
 * We have an article Autofocus. The whole business is complex but basically lots of sharp edges help. Dmcq (talk) 10:36, 5 October 2018 (UTC)


 * The article autofocus focuses on focus technology. --Doroletho (talk) 10:35, 5 October 2018 (UTC)


 * If you want a practical answer, use a Datacolor SpyderLensCal Autofocus Calibration Aid, available here: I would be interested in seeing an engineering analyses comparing various autofocus test patterns --Guy Macon (talk) 17:57, 5 October 2018 (UTC)


 * If you want a pattern to test autofocus instead of a pattern that will "best maintain the focus of an auto-focusing camera", look here: --Guy Macon (talk) 18:02, 5 October 2018 (UTC)

Stink but serious
I have noted that some flatulence is close to odorless at 1st (will not go into detail as to how I know that) but after couple of seconds start to stink. I guess there is some reaction behind that, but what it can be? — Preceding unsigned comment added by 37.26.148.231 (talk) 07:54, 5 October 2018 (UTC)


 * Fascinating observation. Looking into this a moment, I see that the odor of flatus is very largely attributed to hydrogen sulfide .  And in fact the detection of sulfur odor requires binding of the sulfur containing compound to copper before it can interact with the odorant receptor.   As the GPCR cascades are rapid and work rapidly for other odors, I assume that once the OR is bound there ought not be any noticeable delay (caveats about biology being immune to theory applying as always).  But copper is a trace element and its binding to H2S seems like the sort of thing that no specific catalyst may exist to speed up, so you plausibly could be observing the rate of that reaction directly.  But I don't know that, and there may be data on how quickly H2S is smelled elsewhere.  But I do seem to recollect experiencing a brief delay in noticing sulfur odors with other compounds... but I never thought about it! Wnt (talk) 10:06, 5 October 2018 (UTC)


 * Related: https://www.peoplespharmacy.com/2012/09/03/bismuth-helps-combat-smelly-gas/ --Guy Macon (talk) 17:34, 5 October 2018 (UTC)
 * Much thanks! — Preceding unsigned comment added by 77.126.11.38 (talk) 18:39, 5 October 2018 (UTC)
 * Something that seems obvious to me, but maybe I'm missing something: it takes time for gas to travel to your nose. Flatulence is a mixture of gases, most of which are odorless to humans. As mentioned, hydrogen sulfide is the main gas responsible for the odor of flatulence. To smell something, you need to expel H2S, and then it has to find its way to your nose. --47.146.63.87 (talk) 07:28, 6 October 2018 (UTC)


 * I was assuming the OP had come up with some pleasant way to pass the time which, occasionally, gave him an opportunity for research under conditions where this interval was negligible. ;) Wnt (talk) 12:18, 6 October 2018 (UTC)

Could a cell phone case actually make the device more fragile?
I have dropped unintendedly my phone several times without any damage to the phone proper (just the cover would crack a little bit more after each strike). When hitting the ground, the phone would normally burst open, the cover and battery would fly off. Until its original cover broke up completely, I did not use any case. But after being forced to replace the cover I decided to put it in a rubber case, which held the phone together after a fall. And just one fall was enough to crack the whole screen. I wonder whether this case actually transmitted the shock instead of dissipating it. Are phones designed this way? Does some official instruction mentions the fact that cases can damage your phone? --Doroletho (talk) 11:06, 5 October 2018 (UTC)
 * A supplier of impact resistant smartphone cases claims the material they use is "ultra efficient, absorbing and dissipating impact force and stopping that force from passing into your device, whilst at the same time ensuring the device continues to work as intended. Less efficient materials let the force pass straight through to your phone giving minimal protection, increasing the likelihood of damage, and can even negatively affect phone performance." Cell phone cases are also available to protect the screen aginst scratches and to block harmful Electromagnetic Radiation (EMF) and RFID. DroneB (talk) 11:42, 5 October 2018 (UTC)
 * And if you can't trust a company that makes a product making claims about a competing product, who can you trust? :(  --Guy Macon (talk) 01:43, 8 October 2018 (UTC)

Angular Momentum of Electrons
How is the angular momentum of electrons calculated? — Eli355 (  talk  •  contribs  ) 14:18, 5 October 2018 (UTC)
 * ... The angular momentum of an electron is calculated very carefully, using the excruciating mathematics of quantum mechanics. We have an article on spin, including a very detailed encyclopedic introduction to its mathematical formulation - but beware: these calculations are not easy.  The angular momentum of an electron can be calculated by anybody - but it is usually calculated by people who have spent a few years of full-time formal mathematical study in preparation for the difficult math that follows.
 * As I frequently remark: perhaps our reader is some sort of extreme genius who is able to skip past the several years of preparatory work that most very smart physicists require - but if the reader is a mere mortal like the rest of us, the best place to start is an elementary physics book like Tipler's Physics for Scientists and Engineers. Start at page one of the first book, and by the time you reach the third book, (Volume 3, Chapters 34 through 41), you'll be ready to follow exactly the calculations for electron angular momentum that are covered in there.  Chapters 35 (Applications of the Schrödinger equation) and 41 (Elementary particles...) both work the math in detail.
 * Nimur (talk) 15:20, 5 October 2018 (UTC)


 * From Electron: "The issue of the radius of the electron is a challenging problem of the modern theoretical physics. The admission of the hypothesis of a finite radius of the electron is incompatible to the premises of the theory of relativity. On the other hand, a point-like electron (zero radius) generates serious mathematical difficulties due to the self-energy of the electron tending to infinity." Now think about what "spin" and "angular momentum" mean if the radius is zero. (head explodes). --Guy Macon (talk) 17:41, 5 October 2018 (UTC)
 * "You can think of quantum spin as just like a spinning top, except there is no top." I saw that somewhere but I don't remember where. --47.146.63.87 (talk) 07:25, 6 October 2018 (UTC)

Does electron spin have any connection at all with classical angular momentum? Thanks. (Not the OP). 173.228.123.166 (talk) 00:29, 6 October 2018 (UTC)


 * I don't think so but it might have an axis direction of the clockwise pole which is analogous to planets spinning but not literally spinning like a little electron ball. Sagittarian Milky Way (talk) 04:24, 6 October 2018 (UTC)


 * I should remind everyone that the angular momentum of electron is not same as its spin. It also includes the orbital angular momentum. Ruslik_ Zero 07:22, 6 October 2018 (UTC)


 * Which of the three links on the disambiguation page you just linked to are you referring to? --Guy Macon (talk) 18:31, 6 October 2018 (UTC)
 * I'm sure he's referring to angular momentum operator, specifically, L. He's stating that J=S+L i.e. total angular momentum = spin + orbital angular momentum. 139.194.67.236 (talk) 02:56, 7 October 2018 (UTC)

As an aside, I don't have the impression that the math in this stuff is really that bad? Some basic calculus and linear algebra, is there more? It's hard for me to tell because the articles I look at are full of physics jargon that I don't understand. But that's separate from the math context. 173.228.123.166 (talk) 08:15, 7 October 2018 (UTC)
 * Well, the mathematical tools you require will depend on what you wish to calculate!
 * Many students in upper-level high-school classes are able to use simple quantum-mechanical methods to estimate angular momentum for standardized problems. You might see electron angular momentum in an AP Chemistry class at many American high-schools; students probably memorize a few simple rules and work a few problems with basic arithmetic and logical problem-solving.  However: that is "enrichment" that is out-of-scope for their class, because canonically, officially, the AP Chemistry syllabus clearly states: "Assignment of quantum numbers to electrons is beyond the scope of this course and the AP Exam..." because ..."assignment of quantum numbers to electrons does not increase students’ conceptual understanding of quantum theory."  An advanced high-school-age student probably doesn't yet know the mathematical tools to meaningfully solve those problems - so at best they are memorizing some esoteric cruft that really just dilutes their conceptualization of the physics.  In other words, for a high-school student, it's not just hard, it might actually be "bad" - because memorizing math (instead of learning math) actually harms the student's conceptual understanding.
 * If you wish to describe the evolution of a non-trivial system over time, you must solve the time-varying Schrödinger equation with non-homogeneous boundary conditions. To do so requires solving a multidimensional complex partial differential equation.  In most universities, that would involve methods that are taught to advanced mathematics students some two to three years after they complete "basic" calculus.  Outside of the math and physics departments, many students do not not even realize that there do exist so many additional years worth of calculus classes!
 * A study of university undergraduate programs, endorsed by APS, suggests that 18% of a physics student's curriculum should comprise quantum mechanics and mathematical physics in specific - in addition to all other math and physics classes. At most universities, that would equate to somewhere in the neighborhood of 5 to 8 semester-long university-level classes just studying this type of problem, in addition to all the other math and physics you plan to study!  (They importantly emphasize that all university programs are unique and there is no "one-size-fits-all" curriculum).
 * Far be it from me to call this math "bad" - I firmly believe it's a "good" thing to study mathematics in such detail! I just don't want the novice student to be surprised when they find themselves ten minutes into solving a math-problem that normally takes years to learn to do.  The equations, the notation, and the methods, are all very unfamiliar to people who have not specialized in physics.
 * Nimur (talk) 14:08, 8 October 2018 (UTC)
 * I'm not sure why you would need a time-varying equation or a non-trivial system to calculate "the angular momentum of an electron". In any case, I found a computation here, and to be sure, it involves some annoyingly specialized concepts just to read the equations.  That said ... I always suspect QM has a right to be far simpler than it is, if only the right way to break it down intuitively could be hit on. Wnt (talk) 02:09, 9 October 2018 (UTC)
 * If anybody - anywhere - had a good way to correctly simplify this type of physics, that person would probably write a good textbook and it would become very popular among the community of people who study and teach physics. Here's a wonderful list of good textbooks on quantum mechanics: the bibliography at the Plato Encyclopedia entry for Quantum Mechanics.  Overwhelmingly - a lot of university classes are now using course-notes as a primary text, and teaching without reference to any specific book, all the while publishing syllabi appendices that reference huge lists of "optional" texts in the hope that the students will all read more than one book.  This way, students receive a proverbial "second opinion" on any difficult diagnosis.  If you doubt that a time-varying method is required, why not read six different books to see whether any of them provide you with an alternative to the standard formulation?  ... so you see why it may take you some good length of time to become proficient.  One does not read six physics textbooks in an ordinary afternoon.
 * A favorite new book is Sakurai and Napolitano. A classic is Griffiths' Introduction.  There are many other books with widely varying pedagogical value.  Nobody sincerely thinks this stuff is easy, and if they say they think it's easy, they're lying to themselves and others.
 * Nimur (talk) 21:05, 9 October 2018 (UTC)