Wikipedia:Reference desk/Archives/Science/2018 August 19

= August 19 =

Why cant you hear the jet engine anymore before the jet takes off?
Everytime i watch a jet take of before the take of the engine spools up but then you cant hear it anymore the jet takes off and only what you hear is the noise of air but the actual sound of the engine is gone. Saludacymbals (talk) 15:37, 19 August 2018 (UTC)
 * Do you mean while observing from the ground, or while riding in the airplane? ←Baseball Bugs What's up, Doc? carrots→ 17:41, 19 August 2018 (UTC)
 * See Aircraft_noise. Ruslik_ Zero 17:45, 19 August 2018 (UTC)

Speed of electrons
How is the speed of an electron calculated? —Eli355 (talk &#124; contribs) 16:11, 19 August 2018 (UTC)


 * Do our articles on Speed of electricity, Drift velocity, and Electron mobility answer your question? --Guy Macon (talk) 16:49, 19 August 2018 (UTC)


 * Of course the universe is more confusing than that...
 * "What if you point to some particular electron and ask how fast it is moving? Unfortunately, that’s a question we can’t answer, because in quantum mechanics, individual electrons in the metal really don’t have well-defined velocities. Rather, there is a probability distribution of many different possible velocities that they can possess. Since the uncertainty principle tells us it’s impossible to measure a particle’s position and velocity at the same time, successfully pinning down an electron’s velocity also means that you lose information about its position. Thus, you can’t tell which electron you are looking at." Source:
 * --Guy Macon (talk) 16:55, 19 August 2018 (UTC)


 * With laboratory electric experiment setups combined with an Interferometer. --Kharon (talk) 04:04, 20 August 2018 (UTC)
 * Answer to the original query: Very carefully! Sorry, I couldn't resist. Anyway, that's discussing electrons in a metal, but the questioner didn't specify any particular medium. In experiments in particle accelerators where electrons or other particles are accelerated, discussing particle velocities is common. For instance our own Large Hadron Collider article states: …the protons have a Lorentz factor of about 6,930 and move at about 0.999999990 c… Now I'm not sure how that figure is arrived at. Is it data from the particle collisions, or from just calculating the amount of energy injected and subtracting losses? I took a gander at a few of the article references but they didn't seem to answer this. Also, this is measuring groups of particles and, I presume, giving their average velocity. From my layperson's understanding, I know about the issues with the uncertainty principle. But, and the source seems to agree, I think you can give an average velocity if you have a decent number of particles that you measure. --47.146.63.87 (talk) 05:10, 20 August 2018 (UTC)
 * The speed of an electron can be extrapolated from its momentum. By the Heisenberg uncertainty principle, an electron's momentum can be well defined so long as its position isn't; and if we know the mass of that electron (which we do), velocity is just momentum divided by mass.  Of course, that's treating an electron as a particle in orbit around an atom; which isn't really all that right.  But you can still make the calculation.  -- Jayron 32 14:35, 20 August 2018 (UTC)
 * A bit more here is the formula for calculating electron orbital velocity for a specific energy level in a hydrogen atom per the Bohr model of the atom; how much this calculation applies to an actual electron in actual motion is debatable, but for a simple one-electron atom according to the Bohr model, it's a calculable thing. -- Jayron 32 17:57, 20 August 2018 (UTC)

Helpful Parasites?
Are there any parasites or worms that can be beneficial to a human host? Makuta Makaveli (talk) 22:37, 19 August 2018 (UTC)
 * Gastrointestinal tract has some information about bacteria used as part of digestion. — Paleo  Neonate  – 23:45, 19 August 2018 (UTC)
 * Adding: Commensalism may also be of interest, but I don't remember of particular ectosymbiosis cases in humans. — Paleo  Neonate  – 23:50, 19 August 2018 (UTC)
 * More: There is Maggot therapy (potentially repulsive image warning), but myiasis is generally not considered beneficial. — Paleo  Neonate  – 23:55, 19 August 2018 (UTC)
 * The distinction is maggots used for maggot therapy are species that preferentially eat dead tissue, whereas harmful myiasis usually involves species that eat dead and live tissue indiscriminately. --47.146.63.87 (talk) 04:16, 20 August 2018 (UTC)
 * You have to be careful with terminology. Parasitism is defined as being a situation where one organism survives at the expense of another. Of course, nature is rarely clean-cut. Some related topics: hygiene hypothesis, effects of parasitic worms on the immune system, and helminthic therapy. Matt Deres (talk) 01:34, 20 August 2018 (UTC)
 * Thank you for the link to effects of parasitic worms on the immune system, I was looking for something similar when I wrote the above but couldn't find it (I looked in the nematode article as I remembered that some were voluntarily swallowed). — Paleo  Neonate  – 01:56, 20 August 2018 (UTC)
 * Yes, to be pedantic, parasitism means a situation where one organism (the parasite) benefits in a way that reduces another organism's (the host's) fitness, but without killing the host, at least immediately. This is only one type of symbiosis; see that article for others. Our gut and skin flora are commensalistic or mutualistic, as long as they stay where they "belong". An interesting case that isn't exactly what was asked about, but kind of similar: malarial therapy. Before antibiotics, intentionally infecting people with malaria was sometimes used successfully to cure syphilis. The fever produced by the malaria infection killed the syphilis bacteria, which are temperature-sensitive. But the malaria pathogen did not directly affect the syphilis; it just happened to induce an immune response that harmed the syphilis bacteria. Another interesting example along this same line: innate resistance to HIV. A mutation in the CCR5 receptor that appears to have been selected for because it confers resistance to smallpox also happens to "accidentally" confer resistance to HIV. --47.146.63.87 (talk) 04:16, 20 August 2018 (UTC)
 * We're also seeing cancer treatment testing, which involves infecting a tumor with the polio virus, which then tricks the immune system into attacking the polio virus and also the tumor. ←Baseball Bugs What's up, Doc? carrots→ 17:50, 20 August 2018 (UTC)
 * Well it's not really a "trick" since the immune system should be attacking both the virus and the tumor. The immune system normally attacks cancerous cells; cancers can only develop when they are able to evade the immune system. --47.146.63.87 (talk) 03:58, 21 August 2018 (UTC)