Wikipedia:Reference desk/Archives/Science/2018 November 4

= November 4 =

Land plants
Are land plants a subset of green algae?
 * Take a look at Green algae. Although not strictly a "subset", it appears that land plants are included in the Phragmoplastophyta classification and are descended from organisms that would be considered as green algae. Graeme Bartlett (talk) 03:52, 4 November 2018 (UTC)

Neutrinos, massless particles, speed of light
The article on neutrinos says: "if neutrinos are massless, they must travel at the speed of light". I've occasionally wondered about this "must" qualification. Is this really true? Must massless particles travel at the speed of light? Thanks, Attic Salt (talk) 14:02, 4 November 2018 (UTC)


 * If something has no rest mass and no relativistic mass, then m=0 and E=mc^2=0, i.e. it has no energy to contribute either. Something with no mass or energy isn't going to affect a collision or a decay in most of the ways measured, such as the ways that neutrinos have been measured.
 * But how can something have relativistic mass when that is merely the gamma factor times the rest mass, which is zero? Well, gamma factor can be infinity ... if the velocity is the speed of light.  The relativistic mass is then undefined (and it has to be, otherwise all light would be the same color).  So if you see a neutrino with a non-zero energy and a zero rest mass, the conclusion you'd draw is that it is travelling at the speed of light.
 * Another way to look at it is that if the neutrino were going any slower, an observer could catch up to it and see it as a particle at rest with no mass and no energy. If you could interact with such a particle in any way, it would take zero energy to give it any speed less than light speed (it would still have zero kinetic energy).  The particle's momentum would be precisely zero, so its position would be infinitely uncertain under the Heisenberg principle.  I bet this isn't all the ways this would break physics. Wnt (talk) 14:14, 4 November 2018 (UTC)
 * Thank you, very helpful. Attic Salt (talk) 14:16, 4 November 2018 (UTC)

Okay, following on from Wnt's explanation, if a particle has rest mass, can it then travel at any velocity less than the speed of light? I ask this because neutrinos apparently travel at pretty much close to the speed of light. If they had mass wouldn't we see at least some that are traveling slowly? Attic Salt (talk) 14:23, 4 November 2018 (UTC)
 * The speed of a particle with mass $$m$$ and energy $$E$$ is
 * $$v=c\sqrt{1-m^2/E^2}\approx c\left(1-\frac{m^2}{2E^2}\right)$$,
 * if $$m\ll E$$. For instance, for Solar neutrinos with energy of about 0.7 MeV and neutrino mass of about 1 eV the speed will be $$c\times0.999999999999$$. Such a difference with speed of light $$c$$ is impossible to measure with any fanciable techniques. Ruslik_ Zero 20:37, 4 November 2018 (UTC)
 * Just to add here's the wiki on Measurements of neutrino speed. Dja1979 (talk) 20:56, 4 November 2018 (UTC)
 * Hypothetically, if a particle has imaginary mass, it can only travel faster than the speed of light. However, there is no evidence that such a thing exists, or can exist. Iapetus (talk) 10:38, 5 November 2018 (UTC)

Loudspeakers humming when connected to turned off PC
When my loudspeakers are on and connected to a turned off PC they hum non-stop (pretty annoying). They have their independent power source, no earthing (I don't know if this is pertinent). Why does this happen, and how can I deal with it (besides buying a higher end loudspeakers).--Doroletho (talk) 18:12, 4 November 2018 (UTC)


 * Probable cause: the output of your PC becomes a high impedance when it is turned off, and the turned-off PC becomes an antenna picking up hum from the power lines in your house.


 * Suggested solution: a resistor (start with 100K, them try 10K and then 1K) across the input. If you don't have the skills, any electronics technician can do this for you. You could experiment by cutting up some cables and wrapping the wires around the resistor leads to see if it fixes the problem. --Guy Macon (talk) 22:04, 4 November 2018 (UTC)

women has only 24 hours each month in which they can become pregnant?
I read in past that a ovulation of women takes 24 hours (I didn't find now where). Assuming that it's correct, does it mean that women has only 24 hours each month in which they can become pregnant? 2A02:ED0:4270:7A00:8086:1E:CCBF:E33F (talk) 18:21, 4 November 2018 (UTC)
 * This is incorrect. Please read our article Menstrual cycle which is a featured article. Pay attention to the section on Fertility, which has a very informative graph. A woman with a normal menstrual cycle will have a "fertile window" of about six days, and the most likely time to conceive is as a result of intercourse one day before she ovulates. The odds taper off before and after that peak. Remember that sperm cells can survive in a woman's body for five days or more after intercourse, so there cannot be a window as narrow as 24 hours. Cullen328  Let's discuss it  18:39, 4 November 2018 (UTC)
 * Calendar-based contraceptive methods suggests a set of more empirical measures. Klbrain (talk) 00:41, 5 November 2018 (UTC)
 * The "24-hour period" does seem to be roughly correct, though you have to be careful about what it implies. : After being released from the ovary, the egg is swept into the fallopian tube by the fimbria, which is a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg will disintegrate or dissolve in the fallopian tube. So there is a window of about a day in which an egg is able to be fertilized. However, this does not mean one must have intercourse during that time in order to become pregnant. As others noted, sperm can survive for up to about a week in the female reproductive tract. --47.146.63.87 (talk) 09:37, 11 November 2018 (UTC)

Black box and Flight recorder
This says the two not combined into one box. I thought they were. Flight recorder says they are.

Convenience link: Black box

Anna Frodesiak (talk) 23:26, 4 November 2018 (UTC)


 * The Flight Recorder article says that "The two devices may be combined in a single unit", not that they always are. Mikenorton (talk) 23:31, 4 November 2018 (UTC)


 * Certainly when the TV series Mayday depicts the recovery of the recorders, it always shows them as two separate units, and there have been accidents when one recorder provided usable data and the other did not.
 * Originally they used different technology. The first widely-used FDRs would physically scratch graph lines onto a piece of foil: they only recorded a small number of parameters back then, and I guess the foil sheet was hard to damage beyond readability.  But with CVRs, the fact that for privacy readings they only retain a limited amount of audio time (30 minutes originally, now I think 2 hours) meant that a loop of magnetic tape was the medium of choice.  So it then made sense to have two separate recorders.
 * With digital memory chips things are different, though. --76.69.46.228 (talk) 00:05, 5 November 2018 (UTC)

I see. Okay. Thank you very kindly. :) Anna Frodesiak (talk) 00:30, 5 November 2018 (UTC)
 * This suggests that combination devices are possibly still not very common on large aeroplanes although the 787 does have a combination recorder but it has two redundant recorders [//aviation.stackexchange.com/questions/8310/why-are-fdrs-and-cvrs-still-two-separate-physical-devices]. This suggests that the general recommendation for large aeroplanes is to put the CVR and FDR as far apart as possible to increase the chances one will survive [//www.easa.europa.eu/sites/default/files/dfu/NPA%202018-03.pdf]. That is obviously one advantage with two devices even if they aren't redundant, you may be somewhat more likely to get at least one device more recoverable. Two redundant devices would be even better if you don't increase the chances they both will break or whatever. According to this, the NTSB wanted two such redundant combination devices for aircraft manufactured from 2003 [//www.faa.gov/regulations_policies/rulemaking/recently_published/media/23532.DOC] but I'm pretty sure it never happened. Nil Einne (talk) 09:18, 5 November 2018 (UTC)


 * Interestingly, Lion Air does not operate in the United States, so it might not be following the rules set forth by American regulators. Most international companies try to follow the American and other international guidelines (for example, an operator in Indonesia almost surely complies with local Indonesian civil aviation rules, and perhaps local military aviation oversight; but they might also voluntarily comply with American rules, ICAO standards, laws of the European Union, and the Australian CASA to some degree).
 * Ultimately, this all means that NTSB and FAA rules are only a small part of the story. Nonetheless, the NTSB is sending investigators, because Americans care when serious aircraft accidents happen almost anywhere on our planet - especially with an American-made airplane.
 * The aircraft was built in the USA and almost certainly met all American standards when it was built, but as soon as it leaves the country, the operator could theoretically do anything they like to it - including modifying it to meet local laws. Since Lion Air has no routes to the USA, they have less incentive to follow American laws, other than the indirect consequences.
 * If you feel like sparking a long argument, ask a regulator whether American or international aircraft safety standards are more stringent. Here's a link to the ICAO Flight Recorder Specific Working Group, a panel whose advisories will probably some day propagate into American, European, and other international laws, and then some years later, manufacturers will comply with those guidelines and rules when building new aircraft.  Airline fleets will gradually assimilate the newer aircraft, and older airplanes will be phased out or upgraded over the typical lifetime of the airframe - perhaps as long as fifty years, especially for international and budget airline operators.  Perhaps by the 22nd century, you will board an aircraft that meets the 1993 guidelines.
 * Nimur (talk) 16:19, 5 November 2018 (UTC)
 * "The aircraft" (unspecified) above presumably refers to Lion Air Flight 610 that crashed on 29 October 2018. DroneB (talk) 17:06, 5 November 2018 (UTC)
 * IMO this is one area for better or worse, the major powers especially the US and EU have massive ability to set the standards. If the FAA had followed through with the NTSB's recommendation, I strongly suspect the majority of Airbus and Boeing aircraft manufactured after 2003 would now have redundant combination recorders. Doubly so for aircraft manufactured in the past few months. No matter whether the airliner operating them had any intention of ever flying to the EU or US. Countries with weaker standards may have allowed these devices to be replaced with non combination devices when they broke (although I wonder how easy it would have been to fit them) and in particular, may have allowed the aircraft to operate even if one of the redundant devices was broken, but I'm not convinced it would be a massive percentage. Things may change as other manufacturers start to gain a significant foothold in the lowcost market, e.g. Comac, Bombardier Aerospace and Embraer but I'm not entirely convinced. More likely these manufacturers will also manufacture planes which comply with the FAA and EASA regulations, as well as any other significant regulations like the Chinese ones could be. The only real problem would be if there starts to be a real conflict between the regulations. As I understand it, at the current time while EASA and FAA regulations may be different, they're often not in significant conflict meaning complying with both is difficult because aircraft regulations are incredibly onerous, but it's not generally the case that the effort you make to comply with one would often actually be the opposite of what you want to do to comply with the other. Nil Einne (talk) 18:06, 5 November 2018 (UTC)

Thank you for the further feedback. This is quite interesting. Oh, and "...Perhaps by the 22nd century, you will board an aircraft that meets the 1993 guidelines..." was very funny. :) The crash was not, though, and I do hope they get to the bottom of it and prevent it from happening again. Anna Frodesiak (talk) 19:38, 5 November 2018 (UTC)

From a Wikipedia comprehension perspective, I think it doesn't help that we don't have separate articles for the Flight Data Recorder and Cockpit Voice Recorder. When they are installed as separate units on an aircraft, and there is a crash, one of them is almost always found before the other. But our articles on the air incident concerned always redirect to one single article. Martinevans123 (talk) 19:53, 5 November 2018 (UTC)

Possible hacking
I've heard a few conspiracy theories, if one could call it that. Is this even possible? Anna Frodesiak (talk) 19:38, 5 November 2018 (UTC)
 * Anna, what exactly are you thinking of, or have you seen? Thanks. Martinevans123 (talk) 19:46, 5 November 2018 (UTC)
 * I don't remember the sources. I will try to find them. Not RS, I guess. I read that someone one the ground might have hacked into the plane's computer system and forced it to crash. Anna Frodesiak (talk) 19:48, 5 November 2018 (UTC)
 * Oh, I see. Nothing to do with the FDR or CVR directly then. Martinevans123 (talk) 19:54, 5 November 2018 (UTC)
 * Nothing at all. Sorry to confuse you. :) Anna Frodesiak (talk) 20:04, 5 November 2018 (UTC)
 * "I've been editing Wikipedia for so long it's not true. I wanted an editor, I never bargained for you." Martinevans123 (talk) 20:22, 5 November 2018 (UTC)