Wikipedia:Reference desk/Science

Welcome to the science reference desk.

Main page: Help searching Wikipedia

How can I get my question answered?

• Provide a short header that gives the general topic of the question.
• Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
• Post your question to only one desk.
• Don't post personal contact information – it will be removed. All answers will be provided here.
• Specific questions, that are likely to produce reliable sources, will tend to get clearer answers,
• Note:
• We don't answer (and may remove) questions that require medical diagnosis or legal advice.
• We don't answer requests for opinions, predictions or debate.
• We are not a substitute for actually doing any original research required, or as a free source of ideas.

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

Choose a topic:
 Computing desk Entertainment desk Humanities desk Language desk Mathematics desk Science desk Miscellaneous desk Archives

June 23

Random chance, probability, and predictability

In chemistry, I once learned that the exact locations of the electrons are not known (and probably not important anyway), but the probability of finding the electron at a particular point can explain why something bonds with another. Then in statistics, I learned about probabilities, which then made me think of randomness. In a realistic setting, what is randomness? While in theory "random" means "everything is equally likely", how is this practically possible? If one writes a personal name on an index card and repeats the process for 50 index cards, each having an unique name, and then puts all the cards in a Black Box and shakes the Black Box, then it may seem random that a random card will be drawn, but in reality, the card at the top may be more likely to be drawn than the card at the bottom. Maybe "randomness" is the "unknown"? If one doesn't know or expect something, then it's "random"? Another case is when a teacher lists the students "randomly". Practically, the students may race to the teacher and line up. Whoever's first in line may just happen to be closest to the teacher to begin with or be the fastest person in the room. The last student in line may be the slowest person, or because he/she can't find any open spots, may be forced to be the last in line. At what point is something "random"? At what point is something "biased" or has some sort of tendency/correlation? 50.4.236.254 (talk) 00:37, 23 June 2017 (UTC)

This is trickier than it may appear. Probability and statistics of discrete events (i.e. probability of a 2 dice roll being a seven (1 in 6) verses a 2 (1 in 36)) is different than quantum location where the distribution is a fundamental construct of the particle. Temperature of a gas is the mean of the temperature of individual particles and processes like evaporative cooling and sublimation work on the principle that particles are discrete. An electron in the orbital is a completely different construct that is described in models as a distribution and probability but lacks discrete components that can be separated to define the distribution. It is not a collection of random individuals but the distribution is a fundamental part of the particle. --DHeyward (talk) 01:05, 23 June 2017 (UTC)
Randomness doesn't mean all results are equally likely. It means the next result can't be predicted from the previous result. Iapetus (talk) 11:59, 23 June 2017 (UTC)
There several definition of randomness, yours is one of them. "all results are equally likely" is another definition, perfectly legit provided (big caveat!) you have a correct definition of "result" (ie: equal chance for a dice to produce 1-6, which is correct, vs equal chance for a dice to produce 1 or not, which is not). Unfortunately those 2 definition do not means exactly the same. Gem fr (talk) 14:00, 23 June 2017 (UTC)
Your question is hard, and not solved. There are different definitions of randomness, that do not exactly overlap. A system perfectly deterministic, but chaotic and hence unpredictable ( n-body problem or Double pendulum for instance), can be said random because unpredictable, or NOT random because prefectly determinist.
I would recommend determinism and Bohr–Einstein debates for the famous "god don't play dice" quote and implications. Also maybe Hidden variable theory: obviously in your student example, there are hidden variables (proximity and sprinting abilities), so the order is not random, but pseudorandom. It could however be argued that those variable are themselves randomly distributed, meaning the result is random. But it is no longer, if the teacher is well aware of these abilities, meaning he actually choose the result, hence it is not random. When you think of all the things that make you shift back and forth between "it's random" and "it's not random", you end up at classical Antoine Augustin Cournot's definition of randomness (i just wonder why it is neither in his article nor randomness) « Encounter of two independent causal series ».
for instance : a first series of causes made you arrive somewhere, sometime; an other series made a bus arrive at the same place at the same time. The more common causes there are, the less random (the more biased) your encounter with the bus is. With enough knowledge of the causes, your encounter could be perfectly predicted (or explained afterward and then appear it HAD to happen), but would nonetheless stay random (according to this definition) provided they stay independent.
hope it helped, although it may rise more question than it answered Gem fr (talk) 14:00, 23 June 2017 (UTC)

Can wine and beer be used to clean stuff?

Rubbing alcohol is isopropyl alcohol. Drinking alcohol contains ethanol. What happens if flour (with added water) is left fermented on the kitchen counter for a very time and then the fermented liquid is used to make beer, which is then used to clean stuff? Will ethanol still work as effectively as isopropyl alcohol? Is it safer to use ethanol than isopropyl alcohol simply because you can ingest it safely? If you add a lemon wedge or slice to the ethanol solution, then will it smell like lemons too? 50.4.236.254 (talk) 00:53, 23 June 2017 (UTC)

• Ethanol does make a good cleaner, beer and wine is usually only between 4-15% ethanol. The other stuff in beer and wine will make whatever you want to clean dirtier than before you started, defeating the purpose. Neutral spirits, such as vodka or moonshine can be used to clean things, though: [1]--Jayron32 01:05, 23 June 2017 (UTC)
Even so, rubbing alcohol is usually 70% which is stronger than most vodkas, whiskeys, rums and so on.Sagittarian Milky Way (talk) 02:03, 23 June 2017 (UTC)
Isopropyl alcohol is more toxic to humans' central nervous system than ethanol but it's not that toxic. It'd take about a half shot to be toxic. Methanol is more toxic. Now that's some dangerous stuff. Even so, if the thing being cleaned is 50.4's mouth, using 70% isopropyl alcohol is a bad idea even if you spit it out immediately. The mouth will instantly desiccate and take days to heal. Alcoholic mouthwash is designed to not do that. Sagittarian Milky Way (talk) 02:03, 23 June 2017 (UTC)
"The mouth will instantly desiccate and take days to heal." Some people actually tried that before? 50.4.236.254 (talk) 03:50, 23 June 2017 (UTC)
Well at least the hard palate. Sagittarian Milky Way (talk) 03:55, 23 June 2017 (UTC)
"IPA causes rapid intoxication, so people sometimes drink it to get drunk. Other people use it to attempt suicide". What Is Isopropyl Alcohol Poisoning? Alansplodge (talk) 09:48, 23 June 2017 (UTC)
Some Russians told me it had been popular during Gorbachev's anti-alcohol campaign. Wnt (talk) 11:51, 23 June 2017 (UTC)
Sounds like you're describing this IPA. ←Baseball Bugs What's up, Doc? carrots→ 14:52, 23 June 2017 (UTC)
I'll stick to IPA if you don't mind. Alansplodge (talk) 09:07, 24 June 2017 (UTC)
I'm surprised no-one has mentioned Deglazing (cooking) where wine is used to dissolve the gunge off the pan after frying some meat, especially if the meat has been coated in a seasoning with flour. Dmcq (talk) 10:41, 23 June 2017 (UTC)
The ethanol I'm the wine had nothing to do with that process. You can deglaze with chicken broth or even straight water. It's simply the action of hot water the removes the brown bits. Being wine had nothing to do with the deglazing bit, you use wine because it tastes good.--Jayron32 22:12, 23 June 2017 (UTC)
My experience is that a wine or something like it is much faster at the job than just water. Dmcq (talk) 23:31, 23 June 2017 (UTC)
And my experience is there is no difference. So we both lose.--Jayron32 02:53, 24 June 2017 (UTC)
Well to check I have just now tried Google which knows everything with 'cleaning a pan' and the first entry it comes back with is http://www.apartmenttherapy.com/how-to-clean-burnt-pots-scorched-pans-140547 which advocates first putting water and vinegar in the pan and boiling it as the first step - so they are advocating putting something acid into the pan and wine is acid. They do say to put in baking soda after it has boiled and thats alkali, but the one thing they don't say is to just use water. Dmcq (talk) 10:47, 24 June 2017 (UTC)
Vinegar contains acetic acid, which is an acid. The difference between vinegar and wine is that vinegar is "sour wine". It has vin- in the name, which means "wine". Wine is an alcohol, which can act like an acid (donating a proton) or a base (receiving a proton). I suppose wine can be considered an "acid" if you compare it to pure water. 50.4.236.254 (talk) 14:28, 24 June 2017 (UTC)

Pinhole Camera

1. (Just to double check) Can a pinhole camera contain a lens element?

Assuming the answer is no, then:

2. Are these[2] really pinhole cameras? I can see a little glass-like reflection from little hole. And this site[3] sells what appears to be the cone-like tip of the camera, and it clearly contains a very large lens element.

Note that I'm not suggesting the seller is misrepresenting his product. Linguistically "pinhole camera" certainly covers all miniature cameras, regardless of operating principle. He has every right to sell it as a "pinhole camera". I'm just interested in learning about the operating principle behind these cameras. If they're not true (scientifically speaking) pinhole cameras then how come the front lens elements can be made so small? Is there a limit to how small the front lens elements can be before performance is unacceptably degraded? Scala Cats (talk) 01:16, 23 June 2017 (UTC)

Yes, diffraction. This is the same reason why a telescope lens has to be at least 115.8 millimeters wide to look as sharp as 20/20 vision at 60x magnification. Almost certainly not coincidentally, this is almost 60 times the width of the human pupil in the daytime. Sagittarian Milky Way (talk) 02:09, 23 June 2017 (UTC)

────────────────────────────────────────────────────────────────────────────────────────────────────

There are two completely different things that are called "pinhole cameras".

The first uses a pinhole instead of a lens. It is characterized by having everything in focus no matter how close or far it is, and by requiring a huge amount of light (typically full sunlight, fast film, and long exposures). Example: https://www.amazon.com/Ilford-Obscura-Pinhole-Camera-Only/dp/B00I9Z2PD8/

The second uses lenses, but configured in such a way that you can drill a pin-sized hole in a wall with a cone-shaped space behind it (there is a tool for that) and then take pictures of the inside of the room, hopefully without anyone noticing the pinhole your camera is looking through. Example: https://www.amazon.com/Vanxse-Pinhole-Security-Microphone-Surveillance/dp/B012K4VP5W/

--Guy Macon (talk) 03:10, 23 June 2017 (UTC)

"The second uses lenses" That's what my first question is getting at. Do those tiny cameras that use lenses actually qualify, scientifically speaking, as a pinhole camera? Because the article starts off by saying "A pinhole camera is a simple camera without a lens". I not sure whether that means "A pinhole camera is commonly constrction without a lens" or "A pinhole camera is defined as a camera that uses a pinhole instead of lens" Scala Cats (talk) 17:44, 23 June 2017 (UTC)
• Yes, we have dual meanings here. I have the same problem with herbal tea, which both means tea leaves plus herbs, and herbs alone (tisane). StuRat (talk) 17:55, 23 June 2017 (UTC)
Ah, gotacha. Thanks, that clarifies it. Scala Cats (talk) 18:36, 23 June 2017 (UTC)
Also note that the type of pinhole camera lacking a lens might still have a glass cover over the pinhole, to prevent dust from getting in. StuRat (talk) 17:55, 23 June 2017 (UTC)
Really? In the very unlikely circumstance of dust being a problem, wouldn't the shutter adequately serve that purpose?--Mrs Wibble-Wobble (talk) 18:37, 23 June 2017 (UTC)
No, because pinhole cameras need to keep the shutter open a long time, since very little light is let in through the pinhole, to gather enough photons for full exposure. Also, there could be a subtle breeze into the pinhole, to equalize pressure, due to temperature changes, and this could draw dust in. StuRat (talk) 18:48, 23 June 2017 (UTC)

Blocking gravity waves?

(My apologies if I missed this in the article gravitational wave.) Suppose a planet is near enough to its star that the view of a distant source of gravitational waves behind the star is completely blocked everywhere on the planet.

1. Is the planet affected by gravity from the distant objects?
2. Could detectors on the planet detect the gravitational waves from the distant objects?
3. If either of the answers is yes, then how? Can gravitational waves go through solid light-blocking objects like the nearby star?

Loraof (talk) 16:43, 23 June 2017 (UTC)

No the waves are not blocked so the waves are still detected, and I can think of a number of reasons why gravity and its waves don't interact and disperse like other interactions. But you mustn't take my word for it today though... for I looked and found this to be asserted on the APS website here: [4]. As to how, I'd would just assume that the changes in local curvatures are accrued at the speed of light regardless of the source. --Modocc (talk) 18:35, 23 June 2017 (UTC)
Note that, even assuming gravitational waves pass through stars unmolested, that doesn't automatically mean they would be detectable on the far side. This is because the signal-to-noise ratio may be too low, with all the gravitational perturbations of the nearby star (created by non-symmetric mass movements, like coronal mass ejections, solar flares, etc.), swamping the subtle gravitational waves created perhaps billions of years ago and billions of light years away. StuRat (talk) 18:44, 23 June 2017 (UTC)
The detectors would have to be really really close to the star to create any significant noise however of any kind (not by waves per se). Presently, our Sun can briefly eclipse black hole events (putting them out of telescopic view), but that would not effect our gravitational wave detection. To put this another way, the magnitude of change in curvature produced by the blackhole events are clearly on a different scale than those caused by local perturbations. -Modocc (talk) 19:22, 23 June 2017 (UTC)
Another issue might be that current gravitational wave detectors measure longitudinal:transversal length ratios and could drown in noise even if the detector is on the near side. One would have to use "grav interferometry", rather than a single receiver, to tell signal from noise if they arrive from two opposite points. I think that there might be some issues even if the detector is off to one side - but for a different reason this time: the transversal reference would get disturbed at that point, contributing to noise just as much as when crossing the axis.
There could be some second-order effect if the eclipsing object is really massive, resulting in a "gravitational Einstein ring". But I'm not 100% sure here. While emission of grav waves uses energy (>3 solar masses in the LIGO event GW150914!) it might act on too fundamental a level to get deflected by other massive objects. After all, that grav wave originated from a pair of black holes. - ¡Ouch! (hurt me / more pain) 09:45, 24 June 2017 (UTC)
On a side note, some articles unrelated to GW150914 show up on its "what links here" list, example HIP_41378. Not sure what's going on there. - ¡Ouch! (hurt me / more pain) 09:45, 24 June 2017 (UTC)
However much energy you think is involved in black hole mergers, the correct answer is much, much larger. Even after traveling billions of light years, the gravitational radiation from black hole mergers is still many orders of magnitude larger than the gravitational radiation associated with the sun or any other local source. For LIGO, noise sources are mostly normal sources of terrestrial vibrations (equipment noise, earthquakes, heavy trucks, weather, etc.). Dragons flight (talk) 14:17, 24 June 2017 (UTC)
Nice reference: https://what-if.xkcd.com/73/
I wonder how destructive the grav wave alone would be. Could it even disrupt a planet, or would it just make everything wobble? - ¡Ouch! (hurt me / more pain) 13:50, 25 June 2017 (UTC)
I looked for refraction of gravity waves and found some apparently relevant papers. [5][6] Thanks to Sci-Hub, Springer no longer means checkmate, but I haven't gone there because I doubt I'm going to try to make serious sense out of this anytime soon. I would *guess* that very heavy masses might be able to distort the waves away from a given viewer... Wnt (talk) 18:53, 23 June 2017 (UTC)
The effect is under normal conditions negligible, but the fact that it does exist is a simple consequence of conservation of energy. The fact that matter couples to the gravitational field (which is why you can have gravitational waves in the first place), also means that gravitational waves can interact with matter and be detected. But that means that energy can be extracted from gravitational waves (Thought experiment originally proposed by Feynman to argue that gravitational waves carry energy involves test masses that move due to a passing gravitational wave, Feynman argued that you could use masses with a hole and put a stick through the holes such that the masses would now slide with some friction against the sticks, thereby dissipating energy. Where does the energy come from if gravitational waved do not carry energy?). Then if energy is extracted from passing gravitational waves, the waves that move on will carry less energy than they would otherwise have carried. The difference is caused by the emission of gravitational waves due to extracting energy from the gravitational waves, these are opposite in phase such that the transmitted gravitational waves will have a lower amplitude. Count Iblis (talk) 19:48, 23 June 2017 (UTC)
True. But that is a big if ("if energy is extracted from passing gravitational waves") and neglected by that APS webpage. Bummer. Maybe there are sources predicting the significance/insignificance of such extractions that would be useful for the OP. --Modocc (talk) 20:28, 23 June 2017 (UTC)
Gravitational waves will be stopped by an event horizon. But that is not something that you could fabricate to any shape you like, such as a sheet. Spacetime is really stiff, and all known matter will be soft and deformable compared to space. So it is hard to couple something made from matter to moving spacetime to extract the energy and lose it. Perhaps if your gravitational wave has a tiny wavelength, such as that of an X-ray, you would stand a better chance of making it lose energy in solid matter. Graeme Bartlett (talk) 02:34, 24 June 2017 (UTC)
A common missconception (as is the choice of the term "gravitational wave" imho), these are not ripples in gravity but ripples in spacetime! If there where such ripples in gravity, as frequent and far reaching as these ripples in spacetime seem to be, the whole universe would probably be grinded down to dust and rubble by now. --Kharon (talk) 09:21, 24 June 2017 (UTC)
I'm not following you, but if this misconception is so common, do you have any citation(s) about it? Of course, the term gravitational wave always refers to any propagating ripples of the spacetime metric of General relativity which is the geometric theory of gravitation. Seems like an appropriate term actually. -Modocc (talk) 10:48, 24 June 2017 (UTC)

Can a leaning tower be built in a stable way ?

This Q comes up specifically regarding the leaning minaret in Mosul which ISIL recently destroyed, along with the rest of the mosque: [7]. I assume that at some point they would want to rebuild both, but this might also come up if the Leaning Tower of Pisa collapses, etc. So, is there a way to make a tower which appears to lean, but which is stable, as opposed to leaning further with each passing year and finally collapsing ? One thought is to give it a narrower width at the top, as this would allow one edge to slope, while the other remains vertical. Not quite "leaning" but might give that effect from certain angles:

  /    |
/     |
/      |


Also, there could be a cantilevered portion, made of a lightweight material, extending past the vertical wall:

  /    |///
/     |//
/      |/


Do we have an article on such a construction technique ? StuRat (talk) 19:11, 23 June 2017 (UTC)

Are you old enough to remember the 1976 Olympics? Tower was finished in the late 80s. Sagittarian Milky Way (talk) 19:32, 23 June 2017 (UTC)
Interesting. Do we have a link for the pictured structure ? Is it still standing ? Could it stand for 100 years ? 1000 ? StuRat (talk) 19:52, 23 June 2017 (UTC)
Olympic Stadium (Montreal). Sagittarian Milky Way (talk) 19:55, 23 June 2017 (UTC)
Thanks. Looks like inclined tower is the general article, but it doesn't address the stability issue. StuRat (talk) 19:58, 23 June 2017 (UTC)
Apparently the tower wasn't finished in time for the 1976 Olympics, or even 1986. Maybe it was too short to impress in summer '76 so it's not as famous as it could've been. Sagittarian Milky Way (talk) 20:12, 23 June 2017 (UTC)
A structure is stable if its center of mass lies within its "footprint" (our support polygon article refers to this). So a tower can lean arbitrarily as long as the base of the tower reaches over to cover the vertical projection of its center of mass, as can be seen in the stadium picture above. Of course in extreme cases, the material of the tower may not be strong enough to handle the stress required if the base juts out at an extreme angle. CodeTalker (talk) 20:48, 23 June 2017 (UTC)
Figuring it out in terms of center of mass gets a bit hairy when you have something leaning with a cable anchoring it from the back. Does one include the chunk of earth between where the cable is anchored to the base of the leaning object? DMacks (talk) 21:07, 23 June 2017 (UTC)
I would say yes. Talking about whether a building tips over based on its center of gravity is assuming that it's been left lying flat on the ground like a toy block. Once you start digging cables into the ground, then it depends on how much of the ground has to come out with the cables in order for it to tip. Wnt (talk) 23:09, 23 June 2017 (UTC)
Rigid cables? Sagittarian Milky Way (talk) 21:29, 23 June 2017 (UTC)
Is the stadium part of the mass even though the tower is supporting the white thing with the cables instead of the other way around? Sagittarian Milky Way (talk) 21:29, 23 June 2017 (UTC)
Burj Al Arab-Modocc (talk) 22:02, 23 June 2017 (UTC)

Burj Al Arab is a luxury hotel located in Dubai, United Arab Emirates that takes the form of a sail, so it is essentially the world's tallest lean-to (and consistent with your first example of "leaning"). It's very stable for it doesn't have much in the way of any mass that actually leans. --Modocc (talk) 22:02, 23 June 2017 (UTC)

See Block-stacking problem, the tower can lean over as much as one likes with each layer being just as wide and heavy as any other. Dmcq (talk) 23:36, 23 June 2017 (UTC)
Obviously, there are going to be some very real lower limits to any tower's slope (total rises over runs) and length since overhangs of blocks are limited by the blocks' compressive and tensile strengths. For example, for walking on they must be stiff and not thin like cardboard.. Each uppermost block/floor can only be so long for any given thickness of material and the loads placed on them. Even really thin materials have to rise above the horizontal as they are stacked. -Modocc (talk) 02:15, 24 June 2017 (UTC)

I should clarify my Q a bit. As I'm talking about replicating ancient structures, they shouldn't rely on steel cables to keep them from toppling over. Steel beams, inside the structure, might be acceptable, so long as they could be hidden. However, I'm skeptical of how good of a building material steel is in the long term. That is, it's seems to require constant maintenance to keep it from rusting, like repainting it every few years. So, if there's a way to build an inclined tower out of stone, in a stable way, I'd have more faith in it lasting for centuries. StuRat (talk) 00:51, 24 June 2017 (UTC)

It may be harder to make a forever pure stone and mortar tower that imitates an ancient one that was never intended to lean but did anyway. The blocks of the block thing don't lean but the stones of the leaning tower of Pisa do. Also, if it leaned unintentionally the soil on that site might be crap. Sagittarian Milky Way (talk) 01:13, 24 June 2017 (UTC)
I must be clueless, but if the stones are not tilted like the ancient structure you wish to replicate, its not really a replica is it? I mean people do notice their alignments and it is near impossible to hide anything that is too substantial. I venture that carbon fiber cables might eventually be a superior alternative but that's just an educated guess. --Modocc (talk) 02:15, 24 June 2017 (UTC)
Well, you could create angled lines on the structure to make it look like each block is leaning, but you would need to do something at the top or it being level would give away the whole deception. StuRat (talk) 02:33, 24 June 2017 (UTC)
Well, fakes of just about anything anywhere can substitute for the real thing certainly, whether or not the original is preserved. Many structures can be stabilized one way or another, and the one's that can't are usually condemned and demolished. For example, our government recently relocated a tall brick lighthouse further inland away from the shoreline. --Modocc (talk) 03:12, 24 June 2017 (UTC)
Every existing building that tilted over time can and will easily be build brandnew with the help of the regular skilled work practice called Falsework. --Kharon (talk) 09:05, 24 June 2017 (UTC)
Existing buildings can be uprighted, yes, but lots of owners also like to replace or permanently rid themselves of older structures that they don't want or are failing due to rotted timbers or are otherwise unsafe due to older practices. I can understand the desire to rebuild historical structures as StuRat suggested with some or all of the tilt that they had acquired over time (upon a secure and solid foundation) in a manner that is safe and lasting. -Modocc (talk) 11:20, 24 June 2017 (UTC)
I don't see any inherent difficulty in building a new masonry at a tilt. This was actually done in the leaning tower of Pisa (though more sensibly in the opposite direction). So long as a decent foundation can be created - which is the one unique challenge - the rest is millennia-old technology.
Of course, the caveat is that there has to be a point. It won't be the real artifact for study; it is only a tourist attraction. Could not the money be spent in some better way to promote tourism? And it still stands in Iraq, subject to be destroyed again in battle in a year or five. If people want to preserve ancient artifacts, it seems like their best play is to stop spending money, especially in the Muslim world. Every site dug out is going to be destroyed by the Wahhabis in an eyeblink of history, even if it takes thirty years, even if it takes sixty. Every site surveyed will be attended by looters if it is not dug out. Apart from idle study of already open sources (Google Maps) or targeted looting expeditions (Elgin Marbles) it seems like a strict moratorium is in order, perhaps to be revisited in a century or two. Wnt (talk) 12:00, 24 June 2017 (UTC)
While I would argue that mobile artifacts should be removed to a safer part of the world (perhaps leaving cheap copies around for ISIS parts 2, 3, etc., to destroy), buildings are a bit different. Their location is itself historic, so rebuilding them in-situ is critical, for local morale, tourism, etc. If they get destroyed again, rebuild them again. StuRat (talk) 19:58, 24 June 2017 (UTC)
I concur with StuRat. Consider, for example, the rebuilding of the centre of Ypres after its almost complete levelling during the First World War. {The poster formerly known as 87.81.230.195} 94.9.80.133 (talk) 20:23, 24 June 2017 (UTC)

Something like this, perhaps? — Preceding unsigned comment added by 79.237.83.44 (talk) 03:27, 26 June 2017 (UTC)

June 24

What Herbivores and Omnivores could realistically live and even thrive in an arctic land where snow covered the ground most of the year ?

What Herbivores and Omnivores could realisticly live and even thrive in an arctic land where snow covered the ground most of the year, some 3/4 of the year ??

I know Reindeer/Caribou and Bison, just to give two examples, can manage just fine in harsh winter conditions. Bison can reach grass underneath by digging through the snow. But in a land where the summer season barely exists and the grass barely gets time to grow in between winters, I imagine it would be very much different.

So what Herbivores and Omnivores (of any size) could live in such a place and thrive, and how would they eat? Many examples would be appreciated, the more the merrier

Krikkert7 (talk) 09:12, 24 June 2017 (UTC)

We have an article on psychrophiles, not all of which are prokaryotes. --Cookatoo.ergo.ZooM (talk) 11:00, 24 June 2017 (UTC)
One option is to get food elsewhere (ocean fish, for example), then go to the polar region for safety from predators. This is what the documentary March of the Penguins described. Seals also try this approach in the Arctic, but are less successful at avoiding predators, as polar bears often sniff them out in their hiding spots under the snow. StuRat (talk) 20:06, 24 June 2017 (UTC)
This isn't a full answer, but it's worth reading insolation. The light reaching the poles is actually theoretically higher than elsewhere during a portion of the year. Plants have to convert that light into some energy resource at the moment it falls. So in theory, if a plant can find a way to grow that exposes it to the light, and your herbivore can properly harvest and digest the plant, and if it can hibernate or go away for half the year with low energy cost, it should be pretty much happy. And so for example, whales are seen in the deep Arctic doing very well on the phytoplankton that grows abundantly in the summer. Wnt (talk) 17:00, 25 June 2017 (UTC)

User: Wnt: Hm, that's actually quite interesting.

User: StuRat: I believe I've seen the March of the Penguins. Interesting documentary as I can recall it. But Seals are Carnivores, and unless I am mistaken so are Emperor Penguins - you said yourself; to hunt fish. If they can, the females eat so much fish and crustaceans they nearly burst, then they return to their baby penguins to share that fish with them in the form of delicious puke. So they survive by eating meat. They're neiher herbivores nor omnivores, as far as I can tell.

But let me ask a different question; it wouldn't necessarily take much summer for things to grow, would it ??? Grass, at least, grows very fast, and can be very hardy. If things can just grow for a short period of time, during a short summer, then animals such as the bison and reindeer and probably many more than I am aware of can reach it through the snow. In the hardiest conditions, I also know that Snow hares can feed mainly on twigs, rather than greenery. Krikkert7 (talk) 18:20, 25 June 2017 (UTC)

Bison are more a non-Arctic animal. And there is plenty of plant life on the tundra during our short summer season, see Category:Arctic flora. Even though we don't have the bison the caribou can get at the plants under the snow in winter and lemmings live under the snow. Of the herbivores muskox are probably the largest and lemmings amongst the smallest. For more take a look at List of mammals of Nunavut, it includes carnivores and animals like the moose that are only found in the deep south, and see Category:Arctic land animals. StuRat, you have it partially right. The bears do hunt seals but seals don't hide under the snow. Seals keep breathing holes holes open in the sea ice, as much as 3 m (9.8 ft) thick, all winter long. The hole may have a thin cover of ice or snow over it. The bear finds the hole and lies in wait for the seal. Alternatively, the bear may observe a seal on the ice. It is able to creep up and get the seal. Some people will say the bear will cover its nose with its paws to hide the black. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 01:54, 26 June 2017 (UTC)
The cold is more of a problem for plants than the lack of light. That is, the plants would need to be able to resist freezing to death in winter. In more temperate climates they can retreat to underground roots, bulbs, and tubers in winter, but once you get to an area with permafrost, those would freeze, too. Specifically, water forms ice crystals which bursts their cell walls. Some type of anti-freeze would be needed for them to survive. Then there's a problem with a plant colony living on a glacier, which would likely mean it would eventually die when it calves off into the ocean. So, plants would need to find an outcrop of rock, both to remain anchored and to get the minerals they need. Then, as you mentioned, plenty of herbivore mammals could exist in such a climate, if they could find plants to eat, so there would be lots of animals eating any plants that do manage to survive. So, it's tough going for a polar plant. StuRat (talk) 01:47, 26 June 2017 (UTC)
I know that viable plants have been found under glaciers but I don't think there are many growing on them. Here's a short article that explains how they survive the winter. Mosses and lichens do grow on rocks but there are plenty of plants as well. This says there are about 1,700 species in the Arctic but this says 140. Note that lack of water is a problem. I also found Arctic vegetation which I hadn't seen before. As this points out some plants can be used by humans and other animals such as bears and Jstor has something on that as well. Here's an interesting bit on edible plants just don't eat the mushrooms. By the way there is one group of omnivores that lives and thrives in the Arctic, Category:Indigenous peoples in the Arctic. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 05:11, 26 June 2017 (UTC)
For comparison, there are something like 400,000 total species of plants worldwide: [8]. So, whether it's 1700 or only 140 that manage to survive in polar regions, either way it's a tiny portion. There are also problems related to reproduction, since pollinators like bees, ants, and hummingbirds don't survive there. So, probably best to use asexual reproduction there. Then there's the issue of how to distribute seeds. Water-borne plants would have an advantage here, at least when the water isn't frozen as ice. There may not be any birds to eat berries and poop seeds out far away, so they would need to rely on mammals for that (although there are some polar birds). Hitching a ride on passing animals could work, as in burrs, but there may not be enough animals to make that method viable. StuRat (talk) 05:28, 26 June 2017 (UTC)
Would not the brightness at higher latitudes and elevations be affected by snow and ice? In the Arctic the glare can be painful. 81.27.92.74 (talk) 06:34, 26 June 2017 (UTC)
The discussion is at Wikipedia:Reference desk/Archives/Miscellaneous/2017 February 3#Daylight intensity by month?. 81.27.92.74 (talk) 06:34, 26 June 2017 (UTC)
StuRat Please don't make stuff up. While I haven't seen any there may be some ants of the Formica genus. There are certainty bees in the Arctic, Bombus polaris and Bombus hyperboreus which play a small role in pollination. Flies are the major plant pollinators. As for birds only the snowy owl and ravens overwinter but Category:Arctic birds shows that plenty send several months here and some, Arctic tern, common raven and Canada goose, eat berries. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 06:57, 27 June 2017 (UTC)

Is it a goat or a cow?

When I first saw this short clip, I thought the climbing animal was a baby goat. Then someone suggested it was a baby calf and looking around I'm not sure now. So is it a goat or a cow? [9] Nil Einne (talk) 13:59, 24 June 2017 (UTC)

Goat - no doubt at all. That short tail sticking up in the air is the clue - cows all have long dangling tails. Wymspen (talk) 14:26, 24 June 2017 (UTC)
Off topic: Here's a fun one. Wnt (talk) 17:06, 25 June 2017 (UTC)

Ethidium bromide

Can anybody here explain where the name Ethidium and the former name Homidium come from? I didn't manage to find a proper etymology for neither of the two.

PS: Since I'm not a native speaker and interested in linguistics (as my question probably already implies): Is the double negative in my last sentence incorrect? If so, how could I avoid it [changing as little as possible]? Hoping for your support, best--Curc (talk) 17:44, 24 June 2017 (UTC)

It would have been more correct to say "didn't ... find ... either", as in "one or the other", as opposed to "neither" (not one or the other). No information as to your first question. General Ization Talk 17:50, 24 June 2017 (UTC)
You could also say "I found proper etymology for neither of the two" but it sounds more formal, and is maybe less likely to be understood by non-native speakers. Dbfirs 19:37, 24 June 2017 (UTC)
Thank you very much for at least helping me along with the English! Now, to round off this issue: Would there maybe have been a more natural / idiomatic formulation (do you use "formulation"?) you would have used? Best, --Curc (talk) 23:39, 24 June 2017 (UTC) ;-)
I probably would have said: "I couldn't find a proper etymology for either one." General Ization Talk 23:57, 24 June 2017 (UTC)
Thanks once more! Now we only need somebody who can tell us about the etymology itself... ;-)--Curc (talk) 00:21, 25 June 2017 (UTC)
I could guess it comes from a simplification of the systematic name 3,8-Diamino-5-ethyl-6-phenylphenanthridinium bromide. Graeme Bartlett (talk) 00:56, 25 June 2017 (UTC)
@Curc: I looked homidium up in PubMed, sorted by oldest results (they go back to 1954) and took the first paper (1957) that I could access text on (none had abstracts). Inadvertently, I'd pulled up ethidium because I didn't qualify the search. Anyway, the paper provides a useful clue:

Certain phenanthridinium compounds, originally synthesized by Morgan, Walls, Browning, Gulbranson & Robb (1938), possess trypanocidal properties (Browning, Morgan, Robb & Walls, 1938) and have been used with success against Trypanosoma congolense and T. vivax infections in cattle. Walls (1945) demonstrated that high trypanocidal activity in the phenanthridine series is a property of quaternary salts containing a primary amino group in the 7- and a phenyl group in the 9-position; the activity is much increased by the presence of a second amino group, thus 2:7-diamino-9-phenyl-10-methyl phenanthridinium bromide (dimidium bromide) and the 10-ethyl analogue (ethidium bromide) are particularly effective (Watkins & Woolfe, 1952; Woolfe 1952).

So clearly the eth comes from the 10-ethyl component in that reference -- note that is the N+ in our current article diagram and 5-ethyl in the IUPAC name. The one sure thing about numbering these things is that there's always more than one way it has been done in the literature. ;) Wnt (talk) 01:35, 25 June 2017 (UTC)
The Walls 1945 reference is putatively available here; I at least see it cites B. PP. 511,353 and 520,273 (I take those to be British patents). I didn't find these on Espacenet but I did find a US patent that mentions them ([10]) but without etymology, or indeed the word "ethidium" that I noticed. Wnt (talk) 01:52, 25 June 2017 (UTC)
@Wnt: Thanks alot for your research! While you were at it, have you found anything about the Homidium by any chance?--Curc (talk) 17:45, 26 June 2017 (UTC)
@Curc: I'd forgotten about it, but it only takes a moment to go to PubMed, which is a free resource for public use, and type "homidium", and then (this time) click Advanced Search and specify title/abstract for the word, to exclude all the MeSH synonym hits. That gives a list of 61 papers instead of 10,000; choosing "last" I get [11] which uses the term but doesn't explain why, from 1960. I also noticed [12] which potentially might be useful. Well, acting on a hunch, I went to plain Google with homidium "trade name", expecting it was the trade name -- surprising thing is, according to this [13] ethidium is the trade name. This is plausible - I've seen cases where drug companies make a well known name proprietary and then the capitalist acolytes dutifully scurry to use some little known generic substitute name. In this instance that appears to have been singularly unsuccessful! But I'm kind of stumped to think of where to trace the trail further back - there's a source you can chase from that 1960 paper, but I bet it will turn out to just use "ethidium", because it should be a paper indexed by NCBI. My guess would be that "homo" is used to indicate that an extra -CH2- is added somewhere in a compound, like homocysteine and homoserine. So "ethidium" might have been coined to indicate the CH3-CH2- was substituted for CH3, while "homidium" was coined to suggest -CH2- was added to CH3-. Wnt (talk) 18:55, 26 June 2017 (UTC)
@Wnt: Sorry, but I'm a little confused now: In your hypothesis, the CH3-CH2 compound was there in the first place, then substituted to only CH3 (ethidium) and finally added again to CH3-CH2 (homidium), or did I misunderstand you here?--Curc (talk) 19:15, 26 June 2017 (UTC)
@Curc: Somewhere you got confused. Methyl is CH3-. Ethyl is CH3CH2- And "homo" means adding -CH2-. So "ethyl" anything could be "homomethyl", but I do sincerely hope that abomination has never made it into print. ;) (Well, I see on a search it has, but mostly as a mis-scan of homomenthyl salicylate) Wnt (talk) 19:19, 26 June 2017 (UTC)
Well, now you've really lost me! You write: "So "ethidium" might have been coined to indicate the CH3-CH2- was substituted for CH3, while "homidium" was coined to suggest -CH2- was added to CH3." — Now, this sounds to me as if you wanted to say the formula had been originally with the CH3CH2, which was then "substituted for CH3" and finally re-substituted to the original CH3CH2 ("to suggest -CH2- was added to CH3") by a different company using the new (generic) name homidium. Where is my fallacy?--Curc (talk) 19:31, 26 June 2017 (UTC)
@Curc: I'm not sure what you're not understanding. (Just in case there is a language issue, I should point out that A was substituted with B matches Romance languages where you lose A and get B, but A was substituted for B is more common in English and means you lose B and get A) The names are not systematic - they added a second amino group, so they called it dimidium (di- meaning two). That compound happened to have a 10-methyl. Then they changed 10-methyl (-CH3) to 10-ethyl (-CH2-CH3) and they called that both ethidium and homidium. Because replacing methyl by ethyl or adding -CH2- to methyl is the same thing. Wnt (talk) 15:49, 27 June 2017 (UTC)

Wetness meter

I've purchased a hydrometer they actually call wetness meter, for my garden at Amazon[14]. It has a scale and a hand. It works well. When I put it in the ground the hand promptly moves to the right if the ground is wet and ends up in a green zone. Where's the battery? Is it a Perpetuum mobile? Simple examination shows there is no compartment for the battery and it is not mentioned in the instruction. "Made in China," of course. How does it work? Thanks, - --AboutFace 22 (talk) 22:35, 24 June 2017 (UTC)

See galvanic cell. Basically the water in the soil and the two electrodes form a battery, and the meter measures the output of the battery, which is high when sufficient water is present and low or absent when it is not. The key (and non-obvious) aspect is that the electrodes are made of two different kinds of metal. General Ization Talk 23:13, 24 June 2017 (UTC)
Also remember not to leave in the ground permanently as the anode will dissolve away and it will stop working. Aspro (talk) 00:18, 25 June 2017 (UTC)

Clear. Thank you --AboutFace 22 (talk) 12:53, 25 June 2017 (UTC)

Resolved

StuRat (talk) 03:20, 25 June 2017 (UTC)

June 25

Species in which females are aggressive and males passive

Are there any species in which females are aggressive while males are passive?Uncle dan is home (talk) 04:49, 25 June 2017 (UTC)

The spotted hyena is a species of hyena that is very unique in the class Mammalia in that the females of the species have fewer nipples , lack an external vaginal opening, and possess a pseudo-penis. (Essentially they possess a very large clit; keep in mind the anatomical and development similarities between the clitoris and the penis.) The species operates in a matriarchal social system; the females are larger than the males and dominate over them. Even the lowest ranking females of the group have dominion over the highest ranking males. The female spotted hyenas tend to stick together over a large period of time while the male does not often form strong social ties to the den and is more migratory. A mother hyena is extremely protective of her cubs especially towards male hyenas. Penile erection is a sign of submission the male displays to a female hyena. Additionally, the spotted hyena is considered to be one of the most intelligent species of mammal in the world, ranking as smart or smarter than many species of primate. UltravioletAlien (talk) 06:18, 25 June 2017 (UTC)
Homo sapiens? Greglocock (talk) 06:45, 25 June 2017 (UTC)
Many species of hymenoptera. In bees, for example, the males (drones) only exist to mate, then they die. The female queen and workers run the place. StuRat (talk) 14:36, 25 June 2017 (UTC)
Popular culture is well acquainted with the black widow; among insects I think the preying mantis also. But the anglerfish is stranger than them all. Wnt (talk) 16:36, 25 June 2017 (UTC)
Aren't there some grasshoppers where the female decapitates the male after mating ? "Would you care to come in for a decap... uh, nightcap, I mean." StuRat (talk) 05:40, 26 June 2017 (UTC)
Actually Preying mantids, although subsequent research showed that this happens far more frequently when they're in captivity than in the wild, as the article discusses. {The poster formerly known as 87.81.230.195} 2.221.82.167 (talk) 11:44, 26 June 2017 (UTC)
Thanks, that's what I was thinking of. StuRat (talk) 05:43, 27 June 2017 (UTC)

Physics: Can a changing spacetime be used to generate unlimited energy?

I discovered that energy is not conserved in general relativity after reading this by Sean Carroll. He seems to argue that energy is not conserved if spacetime is changing.

The work done on a mass by a force F over a distance L is

${\displaystyle \int _{0}^{L}F\cdot dx}$

If energy is not conserved in general relativity due to a changing spacetime, I wondered if it could be exploited to generate unlimited energy. The first example I considered was Hubble's law. Hubble's law says

${\displaystyle {\dot {x}}=Hx}$.

Taking the derivative of both sides gives

${\displaystyle {\ddot {x}}=H{\dot {x}}=H^{2}x}$.

Now, by Newton's second law and special relativity

${\displaystyle F={\frac {m}{\sqrt {1-{\frac {{\dot {x}}^{2}}{c^{2}}}}}}{\ddot {x}}}$.

But work can only be recovered up to the Hubble radius, so

${\displaystyle L={\frac {c}{H}}}$

Substituting all of that and evaluating the integral gives

${\displaystyle \int _{0}^{\frac {c}{H}}{\frac {m}{\sqrt {1-{\frac {(Hx)^{2}}{c^{2}}}}}}H^{2}x\cdot dx=mc^{2}}$

The energy generated is exactly the energy content of the mass used to generate that energy!

In fact, if you use different rules for the expansion of space (e.g. ${\displaystyle {\dot {x}}=Hx^{2}}$), you get the same answer (${\displaystyle mc^{2}}$). Here's the WolframAlpha for the example ${\displaystyle {\dot {x}}=Hx^{2}}$.

This means you can't really use the expansion of space to generate energy, as the energy content of the mass used to generate that energy is the same as the amount of energy generated (and that mass disappears over the horizon once it reaches c/H).

That made me wonder if mass really IS conserved in a sense in general relativity, or if there is some other law preventing the generation of unlimited energy.

I know only the basics of physics. If you are an expert on physics (or if you know someone who is an expert on physics), can you let me know if a changing spacetime can or can't be used to generate unlimited energy? Perhaps a generalization of my reasoning above?

PeterPresent (talk) 09:41, 25 June 2017 (UTC)

The blog piece you quote seems very misleading. I mean, directly contrary to what he says, there is a difference between saying that energy is not conserved, versus saying that energy is conserved when the negative energy of the gravity field is taken into account. The difference is that you understand right off with the second option that if you make any circular set of changes to the gravity field, if in the end you have the same field you started with, you have the same energy you started with.
But the key issue here is what he says near the end:

First, unlike with ordinary matter fields, there is no such thing as the density of gravitational energy. The thing you would like to define as the energy associated with the curvature of spacetime is not uniquely defined at every point in space. So the best you can rigorously do is define the energy of the whole universe all at once, rather than talking about the energy of each separate piece. (You can sometimes talk approximately about the energy of different pieces, by imagining that they are isolated from the rest of the universe.) Even if you can define such a quantity, it’s much less useful than the notion of energy we have for matter fields.

If that is true, then it certainly seems like a worthy challenge for theoreticians. Surely there must be some way to divide up this energy neatly, without making any simplifying assumptions ... mustn't there? And if you have that math worked out, it seems like there ought to be other uses for it. Wnt (talk) 16:49, 25 June 2017 (UTC)
1. law of conservation of energy only applies to isolated system. A system in a changing gravity field (i.e., a changing spacetime, in relativity framework) is NOT isolated, and can gain (or lose) energy (for instance : pendulum energy is mgh, if g increases, so does energy). No big deal, even in a newtonian framework.
2. This law of conservation itself derives from the principle of least action, that general relativity also respects (ie: Einstein had not to get rid of this principle, he only had to adopt Einstein–Hilbert action instead of previous action (physics)).
So you wont generate energy through changing spacetime: you need as much energy to change space-time as it would get through the change. No free lunch (according to current theory, that seems to work pretty well, so we have no reason to change).
Gem fr (talk) 14:21, 26 June 2017 (UTC)

Feynman Lectures. Exercises PDF. Exercise 7-4 JPG

copyvio, see talk
The following discussion has been closed. Please do not modify it.

...

DELETED We now turn to the case of elliptic orbits. This is essentially about three ellipses: both bodies move along ellipses (light-heavy, heavy-on-small) and, in addition, the relative motion of bodies also occurs along an ellipse (see Lectures, Chapter 7 ). All three ellipses are similar to each other, that is, they share the same eccentricity. If we also take into account that the center of mass of the system remains stationary (it lies in the common focus of the orbits of both bodies), and the distances from the center of mass to the bodies are inversely proportional to their masses, we come to the conclusion that the arrangement of bodies and their orbits is as in figure.

Denoting by v_1 and v_2 the velocities of the bodies M_1 and M_2 at the time when they are at apogee. As can be seen from the figure,

${\displaystyle {\tfrac {v_{1}}{v_{2}}}={\tfrac {a_{1}+c_{1}}{a_{2}+c_{2}}}={\tfrac {a_{1}(1+e)}{a_{2}(1+e)}}={\tfrac {a_{1}}{a_{2}}}}$
(Indices 1 and 2 denote quantities related to ellipses along which the masses M_1 and M_2 move).

To get the same expressions for elliptical orbits as for circular ones, remember that an ellipse can be imagined as a circle visible at some angle to its plane or (which is the same thing) as the projection of a circle onto an inclined plane. In other words, an ellipse can be obtained from a circle if you change the scale along one of the coordinate axes. Acceleration of the body while moving it along the circumference was calculated in "Lectures" (fig. 7.4). To obtain the acceleration of the body (for example, M_1) in the case of interest to us, imagine that its orbit is obtained from a circular increase in the scale in the "vertical direction" a_1 / b_1 times. The value of x will not change, and s will increase and become equal to s1 = (a1 / b1) s.
Substituting x2 = 2Rs (valid for the circle) their values ​​after increasing the scale x1 = x, s1 = (a1 / b1) s and R = b1 (The "horizontal" dimensions have not changed, so the small semiaxis of the ellipse is equal to the radius of the original circle), we obtain

${\displaystyle x^{2}=2{\tfrac {b_{1}^{2}}{a_{1}}}s_{1}}$

Thus, the radius of curvature of the ellipse at the point of intersection with the semimajor axis is ${\displaystyle {\tfrac {b_{1}^{2}}{a_{1}}}}$. Assuming that for a very small period of time the first body moves along a circular orbit of this radius, we can write

${\displaystyle {\tfrac {v_{1}^{2}a_{1}}{b_{1}^{2}}}={\tfrac {GM_{2}}{(a+c)^{2}}}={\tfrac {GM_{2}}{a^{2}(1+e)^{2}}}}$

(Here a and c are the orbital parameters of the relative motion of the bodies: a = a_1 + a_2, c = c_1 + c_2). Similarly for the second body

${\displaystyle {\tfrac {v_{2}^{2}a_{2}}{b_{2}^{2}}}={\tfrac {GM_{1}}{a^{2}(1+e)^{2}}}}$

Adding the last two equalities and expressing v_2 in terms of v_1, we obtain

${\displaystyle {\tfrac {v_{1}^{2}(1+e)}{a_{1}^{2}(1-e)}}={\tfrac {G(M_{1}+M_{2})}{a^{3}}}}$
It remains only to find out what relation the quantity on the left-hand side of this equation has to the period of revolution. First of all, note that the area that the radius vector of the body M_1 (drawn from point 0) "sweeps out" per unit time is 1/2 v_1 (a_1 + c_1) = 1/2 v_1a_1 (1 + e). Although in fact we calculated the rate of change of the "swept out" area for the moment when the body M1 is at apogee, according to Kepler's second law, this speed does not change when the body moves in orbit. Therefore, the value 1/2 v_1a_1 (1 + e) ​​T (here T is the period of revolution) is equal to the area of ​​the orbit of the body M_1. The area of ​​the ellipse is easy to calculate if you realize that when you zoom in on one of the axes, the area of ​​the figure increases by the same factor as the scale. Therefore, the area of ​​the ellipse is
${\displaystyle \pi b_{1}^{2}{\tfrac {a_{1}}{b_{1}}}=\pi a_{1}b_{1}=\pi a_{1}^{2}{\sqrt {1-e^{2}}}}$

It is now easy to see that

${\displaystyle T={\tfrac {2\pi a_{1}}{v_{1}}}{\sqrt {\tfrac {1-e}{1+e}}}}$ , and ${\displaystyle T^{2}={\tfrac {4\pi ^{2}a^{3}}{G(M_{1}+M_{2})}}}$

—  MEPhI , Solutions (Google Translate)

Apparent WP:COPYVIO, see talk μηδείς (talk) 23:27, 25 June 2017 (UTC)

Here is my solution png. The question is can we use 3rd Kepler's law for relative circular orbit and relative elliptical orbit? Or the 3rd Kepler's law is just approximation.

Username160611000000 (talk) 12:46, 25 June 2017 (UTC)

See Kepler's third law -- it is an approximation. Wnt (talk) 16:53, 25 June 2017 (UTC)
Hm, they uses 2nd Kepler's law during the proof. So we cannot derive the exact formula for circular orbit and then extrapolate the approximation formula on case of elliptical orbits, can we? Username160611000000 (talk) 09:46, 26 June 2017 (UTC)
The second law should not be an approximation so long as the right center is used - the actual barycenter in the case of a two-body problem, rather than the center of the larger body. This is because of conservation of angular momentum. Naturally, any torque from additional masses would tend to throw it off. Wnt (talk) 17:12, 26 June 2017 (UTC)
Looking at the problem myself, you have a force of GmM/D^2 pulling inward. Note I use D for the distance between them which is only approximately r (the radius of the circle) for the smaller mass. The centripetal acceleration needed is -w^2*r, where w is angular velocity (I'll be lazy and avoid the omega). The second is a vector; the first is a magnitude, so I'll wave my hands and make the "-" go away, then convert acceleration to force by multiplying by the mass, leaving me with GmM/D^2 = mw^2*r for the little mass = and GmM/D^2 = Mw^2*r for the big one. But what is r? Well, it's a (literally) weighted average of the full distance D for one mass and 0 for the other, i.e. (0*m + D*M)/(m+M) or (D*m + 0*M)/(m+M). The first (DM/m+M) defines r=0 at the small mass and the second (Dm/m+M) puts r=0 at the large. Either way we flip that and get GmM/D^2 = mw^2*DM/(m+M) = Mw^2*Dm/(m+M), depending on which mass you consider. So G(m+M)/D^3 = w^2. The period is 2 pi w, so this is a period inversely proportional to D^(3/2) and directly proportional to the square root of m+M.
Note that we use the inaccurate Kepler's third law version in the first section of orbital period, without warning people it is wrong, even saying that mu depends on the 'more massive body' rather than 'total mass'... then get out the right version in the third section down. I think I actually tried to fix that once and got rebuffed, but there is an article that looks way more mysterious than it ought to if anyone wants to hack at it. Wnt (talk) 17:44, 26 June 2017 (UTC)

Bringing home the process of making industrial nutritional yeast - how to do it?

I know very little about the industrial process of making nutritional yeast other than the reading on Wikipedia. Some kind of unnamed strain is used to culture the yeast cells on top of the agar plate, I presume. The agar plate probably contains glucose from molasses or sugarcane, which is food for the nutritional yeast. Then, the yeast cells are heat-deactivated, harvested, washed, dried, packaged, and sold in grocery stores. In a home kitchen, an agar plate can probably be replaced by a tomato sauce jar lid. Maybe the agar can be replaced by gelatin (may be easier to buy than agar). The person may want to heat-sterilize whatever metal (has to be metal, as plastic will just melt) instrument he's using and layer the Baker's yeast (same species, but may be a different strain) on the culture plate with the sterilized rod. The culture plate (lid) may be screwed back on the tomato sauce jar container, but the container is placed upside-down, because of the yeast cells. Incubate (in the conventional oven?) for 24 hours or until the culture plate seems to be covered by yeast. Then, the person should select for "desirable" phenotypic characteristics. But, what is considered desirable? Maybe it's best to harvest everything? For the washing process, a cheesecloth, perhaps, may be used to filter the yeast, but that assumes that the yeast will not fall through the holes of the cheesecloth. For drying, the yeast cells may be gathered into a jar. The jar is mostly sealed except for a space to allow a blow-dryer in and dry the yeast cells. Afterwards, the yeast may be used as topping on foods.

I have a number of concerns. (1) Does the strain of yeast really matter? Can Baker's yeast strain replace the Nutritional yeast strain? (2) Is the umami flavor of Nutritional yeast caused by the glutamate release of the cells? (3) Will the protocol above actually work as planned? Are there any holes or false assumptions (such as the cheesecloth which may have holes that are too large for the yeast cells)? (4) What "desirable" characteristics are people selecting for when they make nutritional yeast on an industrial scale? 50.4.236.254 (talk) 19:09, 25 June 2017 (UTC)

As a side question, what is the formal job title of the person who cultures the plates industrially? How can one apply to such a position at a company? 50.4.236.254 (talk) 19:21, 25 June 2017 (UTC)
I'm sure, without even looking, that industrial yeast is raised in a broth and filtered out. I'll take a look a bit later. Abductive (reasoning) 21:27, 25 June 2017 (UTC)
As you know there are different strains of yeast. So the starting culture has its growing medium, tailored to suit the type of yeast that one wants to be in abundance ( by adjusting the mediums pH and things), (which insistently, did not require modern science – just experience gained over many generations). By this process, one can produce starting cultures selected for either bread making, wine, beer, larger, etc. This was achieved long before the invention of the microscope. Aspro (talk) 23:34, 25 June 2017 (UTC)
Just so you know, I don't think that's an answer to my question. My question is not about the different strains of yeast. I already know that. Nor is it about the process of making different strains of yeast in a pre-modern fashion. It is, at the most basic level, about whether deactivated baker's yeast/brewer's yeast/wild yeast can replace "nutritional" yeast and still have that umami-rich flavor. 50.4.236.254 (talk) 11:27, 26 June 2017 (UTC)
[Adding the 'close small' coding that 50.4.236.254 forgot, thus rendering the entirety of the following queries small. {The poster formerly known as 87.81.230.195} 2.221.82.167 (talk) 11:39, 26 June 2017 (UTC)]
A liquid growing medium should be far more productive that a plate. Grow the yeast in a jar full of a sugar solution with added micronutrients. Adapt methods used for making ginger beer to optimize yeast growth. The yeast will settle in the bottom as brown "sludge". Roger (Dodger67) (talk) 08:12, 27 June 2017 (UTC)

June 26

A guy where I work contends that CSPAN is actually harming democracy and functioning of usa legislature.

He says because of cspan, politicians can't make deals, and can't have personal friendships across party lines. Is this true? Also, are there political scientists and journalists who have discussed this claim, either pro or con?65.103.249.243 (talk) 00:00, 26 June 2017 (UTC)

In the wake of the shooting of a Republican congressman, I heard some members of both parties talk about how they all get along personally, they just differ over policies. ←Baseball Bugs What's up, Doc? carrots→ 00:33, 26 June 2017 (UTC)
Question transferred to the Humanities Desk, whose topics include politics—here. --76.71.5.114 (talk) 00:33, 26 June 2017 (UTC)

Engine knocking

According to the engine knocking article, the phenomenon is distinct from, but routinely follows, instances of pre-ignition. Lower in the article, I read that "pre-ignition can destroy an engine in just a few strokes of the piston". Presumably you wouldn't have knocking if the engine were destroyed, so how does this work? Is it simply that pre-ignition sometimes doesn't destroy the engine in such a short period of time, so the knocking has a little time to manifest before the whole thing becomes useless? Nyttend (talk) 00:04, 26 June 2017 (UTC)

I think you are reading too much into it. I know of one engine that runs in preignition and knock for 10s of hours on the dyno, as a durability test. I suspect a liberal sprinkling of [citation needed] tags in that article would reveal that it is mostly factoids from on line forums. Greglocock (talk) 03:39, 26 June 2017 (UTC)
It depends on how far advanced in the power cycle the knocking or pre-ignition is taking place. The effect of a slightly advanced (a few degrees of rotation) spark can usually be heard when the accelerator is pressed hard under load. This causes engine wear, because the spark takes place while the piston is still rising on its compression stroke, and rings, pistons and big end bearings are subjected to loads they were not designed for. Obviously, the more advanced the spark, or the more advanced the pre-ignition due to other causes, the greater will be the damage. It's not unusual for piston rings to be cracked as a result. Sometimes, small broken pieces of the rings manage to fall out of their grooves and if they get sucked up by the oil pump, they may ruin the pump's gears very quickly or travel through the oil bores and score crankshaft or camshaft bearings. A kind of compounding of the problem may develop: the more advanced the spark, the weaker the performance of the engine, leading the driver to put his foot down harder on the accelerator, and causing more wear. Akld guy (talk) 05:42, 26 June 2017 (UTC)
• Knocking is after spark plug ignition, pre-ignition (also called rumble) is before spark. The basic reason for either is if the gas-air mixture stays too hot for too long it auto-ignites (depending on thermodynamical parameters), which is usually not desired as the spark allows combustion control. Knock autoignition is easier, because as the flame front develops around the spark plug gases outside the flame get compressed and heated a bit, plus there is more time for the auto-ignition delay to run out. Obviously, if you can have the hard thing (pre-ignition) you can have the easy thing (knock) unless pre-ignition is triggered by a solid surface that is a lot hotter than other places in the engine (e.g. the exhaust valve), so knock often follows pre-ignition.
The problem in both cases is that the pressure buildup in the cylinder takes place faster than intended, which can damage the engine. Pre-ignition is worse because it makes more gas burn in the uncontrolled mode and hence the pressure buildup is faster. Some modern engines are intended to run in permanent "preignition" mode but most are not (and consequently the dimensions, material strengths etc. cannot withstand rumble or prolonged knock). TigraanClick here to contact me 11:01, 26 June 2017 (UTC)
It's mostly a question of BMEP. The worse the engine is for predilection to knocking, the less damaging it can be. If the engine knocks at low loads, then the forces arre low and nothing gets damaged. If an engine is heavily loaded though, without knocking, and then suddenly begins to knock (such as a fault in the timing suddenly occurring, or failure in something like a water injection system), then it can indeed fail in a few strokes. Many engineers will have started out with a single cylinder variable compression research engine, built like a brick cowshed, in a test cell at university, happily listening to it knocking all day whilst demonstrating high compression and low fuel octane. They'll do it indefinitely, that's what they're for. But tune the 'nads off something highly strung, have some part of the control gear lose the plot momentarily, and it can be expensive.
That's a dreadful article though, hopelessly confused. To understand the fundamentals, there's still no substitute for sticking down quietly with the old editions of Ricardo and reading them cover to cover. Andy Dingley (talk) 11:18, 26 June 2017 (UTC)
Do you mean these books by Harry Ricardo 1885 - 1974 ? Blooteuth (talk) 15:05, 27 June 2017 (UTC)
Yes, particularly the 2nd, 3rd and 4th editions. They're not simply editions, they're pretty much different books (and a different author for the 3rd). The pre-war editions are expensive, late copies (and the modern paperback) are OK. Andy Dingley (talk) 15:28, 27 June 2017 (UTC)
Engine knocking on gas engines causes damages. Diesel engines rely on the self ignition. Diesel fuel burns slower. A cold diesel engine is hard and dirty to start. For this reason, have the glowplugs heat up, before turning the starter. Engine knocking on gas engines occurs form early ignition or wrong engine timing. Incomplete burned fuel or engine oil which came into the cylinder head by any other failure is still glowing from previous ignition. This ignites the new fuel earlier than the controlled ignition over the spark plug. This effect can be seen like stepping the pedal of a bike before it reaches the top position. As the engine has no free run, the pressure of the combustion forces a little reverse to the drive shaft but most against the curb shaft bearings and the cylinder head. Increasing the thermodynamic efficiency of the engine needs to use a more lean gas-air mixture, causing more heat drop on the cylinder and cylinder head. This is the reason for installing a knocking sensor into the engine. When knocking is detected, the mixture ratio will be changed by the engine controller unit. --Hans Haase (有问题吗) 17:54, 27 June 2017 (UTC)
Knocking is not the ignition process in a diesel engine. Nor is ignition in a diesel engine, or even a semi-diesel engine, the same process as knocking in a petrol engine. Andy Dingley (talk) 18:06, 27 June 2017 (UTC)

See THIS comment!

{{xt|Would not the brightness at higher latitudes and elevations be affected by snow and ice? In the Arctic the glare can be painful. 15:05, 26 June 2017 (UTC)

The discussion is at Wikipedia:Reference desk/Archives/Miscellaneous/2017 February 3#Daylight intensity by month?. 92.62.8.1 (talk) 15:05, 26 June 2017 (UTC)

See snow blindness Wymspen (talk) 15:17, 26 June 2017 (UTC)

Feynman Lectures. Exercises PDF. Exercise 7-11 JPG

...

7-11. In making laboratory measurements of g, how precise does one have to be to detect diurnal variations in g due to the moon's gravitation? For simplicity, assume that you laboratory is so located that the moon passes through zenith and nadir. Also, neglect earth-tide effect.

—  R. B. Leighton , Feynman Lectures on Physics. Exercises

First, I can't understand how the laboratory can be just below and above moon (zenith and nadir). It is only possible if the north and south poles of the earth are equidistant from the moon (e.g. at vernal equinox moon is on the line sun-earth) and simultaneously the laboratory is situated on the earth equator. Does Feynman mean this?

To solve the problem we should say how many decimal digits we need. And it returns us to my unanswered question here.

Now I understood what you meant by the word "lag". There is a lag 6 hours between the bulge and the moon-earth line. But first I must understand the bulge appearance (and height) explanation and only then consider the lag. E.g. I don't understand why is the tides effect explained by the gap of (4.51 - 4.36)×10-5 but much bigger gap of equator point is ignored. The point on equator moves through lunar orbit plane. When the point intersects the plane, acceleration = 4.36×10-5 , but at leftmost and rightmost positions acceleration = 3.38 ×10-5 [15]. So gap =(4.36 - 3.38)×10-5 . Username160611000000 (talk) 20:21, 13 June 2017 (UTC)

Username160611000000 (talk) 16:47, 26 June 2017 (UTC)

The exercise is intended to measure the difference in gravity felt on Earth as the Moon gets closer and further away. The Moon is not always exactly the same distance from the Earth. It get closer and further away, which is the zenith and nadir. So, for simplicity, we assume that your lab is directly below the Moon when it is at the zenith and directly below the Moon when it is at the nadir. In other words, you will measure the absolute maximum difference in gravity change because you will be at the point where you have the maximum difference between close and far distance measurements. 209.149.113.5 (talk) 17:08, 26 June 2017 (UTC)
According to [16] the lunar tidal acceleration at the Earth's surface along the Moon-Earth axis is about 1.1 × 10−7 g. If we calculate gmax/gmin = (384 400 + 6400)2 / (384 400 - 6400)2, we get 1,068 or difference = 6.8× 10-2g what is absolutely distinct from article value 1.1 × 10−7 g. Username160611000000 (talk) 17:26, 26 June 2017 (UTC)
Hehe, which g is that? You just calculated how much the gravity of the moon varies, but now you want to compare it to the gravity of the Earth. Though I'll admit I didn't look up what the ratio of those two is to make sure this works out right. Wnt (talk) 19:28, 26 June 2017 (UTC)
Thank you. It was mistake.
gmax = 6.67•10-11 • 7.35• 1022 / ((384 400 - 6400)• 103)2 = 3.43• 10-5 m/sec2
gmin = 6.67•10-11 • 7.35• 1022 / ((384 400 + 6400)• 103)2 = 3.21 • 10-5 m/sec2.Username160611000000 (talk) 20:17, 26 June 2017 (UTC)
Difference = 3.43• 10-5 - 3.21 • 10-5 = 2.2 • 10-6 m/sec2 = 2.2 • 10-7 • 9.8 m/sec2 = 2.2 • 10-7 • g. It seems, article uses this non-uniform field , generated by the moon. But it's wrong. On the surface of the earth there is a centrifugal acceleration due to revolution about the barycenter and a gravimeter counts it. Username160611000000 (talk) 12:17, 27 June 2017 (UTC)
There's no mistake. The centrifugal acceleration does not show the diurnal ("daily") variation that the question asks about. R. B. Leighton did not mispell "you[sic] laboratory". Blooteuth (talk) 14:52, 27 June 2017 (UTC)
The earth is rotating about its axis, so the centrifugal force applied to a piece of the earth (e.g. 1 kg of rock) will not change its direction and magnitude (period = 27,3 days) , but earth surface will move (period = 1 day). Therefore there are two fields : one due to gravitation, second due to centrifugal force. Both fields have a period of 27 days. There is no difference in the effects shown by these fields. Username160611000000 (talk) 16:22, 27 June 2017 (UTC)
Now you've got me confused. ;) An object orbiting about a barycenter generally does not experience centrifugal force (in its rotating frame of reference), because that force is counteracted by gravity. The whole point of an orbit is to keep the bodies apart by "centrifugal force", you might say. On the other hand, it does occur to me that a phase locked body like the Moon does move its far side further each revolution than its near side. So ... at least by the Kepler third law approximation ... the far side is in the "wrong orbit", whereas the center of the Moon is just right. Well, I think this greater "centrifugal force" is just the flip side of the tidal gradient seen in a usual calculation made in a non-rotating frame, but now I'm not entirely sure. Wnt (talk) 16:01, 27 June 2017 (UTC)
An object orbiting about a barycenter generally does not experience centrifugal force (in its rotating frame of reference), because that force is counteracted by gravity. It is correct for material point, and as Feynman mentions moon’s attraction just balances the “centrifugal force” at the center of the earth [17]. We should return to the previous discussion and consider a model in which the earth is made up of bricks that do not interact with each other. Username160611000000 (talk) 16:22, 27 June 2017 (UTC)

Have there ever been lunar calendars where a solar eclipse makes the month start early?

Presumably they would have to not set before the moment of New Moon else that'd just be the old Moon. Sagittarian Milky Way (talk) 22:52, 26 June 2017 (UTC)

Have there ever been any calendars where eclipses have any function whatsoever? --jpgordon𝄢𝄆 𝄐𝄇 02:33, 27 June 2017 (UTC)
I don't know but lunar calendars start when the Moon is seen or calculated to be seen. The world record for young crescent sighting is about 11.5 to 15.5 hours after New Moon (depending on "found with naked eye", "binoculars to find it", "binoculars to see it", "telescope", "high altitude, computer to aim telescope just to find it, have to hide under a sheet for x minutes first or you're not dark adapted enough", that kind of thing). However you can see a c. 0-1 hour old Moon during a solar eclipse. Sagittarian Milky Way (talk) 03:19, 27 June 2017 (UTC)
Solar eclipses always occur at new moon. In the Hebrew calendar, at least, new moon marks the beginning of the month (they no longer use the crescent moon method). ←Baseball Bugs What's up, Doc? carrots→ 03:03, 27 June 2017 (UTC)
There are limitations on the length of the lunar month. It may not have fewer than 29 days or more than thirty. So I doubt that a solar eclipse would prompt anyone to declare a new month beginning at nightfall if it happened on the 28th day. There might be something in Babylonian records, but in our culture, when this happened the actual date of the eclipse was suppressed Computus#History. If the regulation is impeded by clouds the new month will start at the end of the thirtieth day. There could be a succession of months where the starting date was empirically determined in this way - if a new moon appeared at the close of the 28th day I wonder how the regulators would handle the situation. 94.195.147.35 (talk) 09:15, 27 June 2017 (UTC)
According to this a solar eclipse cannot occur at the beginning of Ramadan, because it takes a day or two for the crescent to become visible. So at least that lunar calendar rules it out, even though it relies on direct observation of a crescent. (Then again, there are often schisms in Islam, and I've noticed that sources about it seem uncommonly willing to omit the differing point of view). I haven't heard of another lunar calendar where people were actually watching to sight a fresh crescent, but I don't know them all! Wnt (talk) 16:06, 27 June 2017 (UTC)

June 27

Can anyone have in their mind a representation of four or more dimensions?

Can people (who presumably study higher dimensionality) hold in their mind a representation of those dimensions like I think I can just about hold in my mind three-dimension arrangements? ----Seans Potato Business 17:07, 27 June 2017 (UTC)

It depends on what the meaning of "like" is. We don't have experience with four spatial dimensions, and I'm not aware of anyone who can visualize four-dimensional objects in the same way that you would visualize three.
But there are workarounds. Take a Klein bottle, for example. It can't be embedded into three-dimensional space without self-intersection. But you can, say, think of most of it as green, and the part that comes around to self-intersect shading into red, and then back to green, whereas the part it pokes through is green. Or you can imagine coming up to the self-intersection and then looking at the area rotated, so that you see a path passing around rather than through. Or you can map one dimension to time and think of an animation.
Five is obviously even harder. --Trovatore (talk) 17:41, 27 June 2017 (UTC)

Some people claim to be able to do so, and that you can get better with practice. Charles Howard Hinton invented a set of colored blocks that he claimed could be used to train yourself to visualize the fourth dimension. See also: [18] CodeTalker (talk) 18:07, 27 June 2017 (UTC)

I either use time as my fourth dimension and have a four dimensional concept be a short movie, use color, or in the case of polytopes where the majority of vertices are in a limited number of planes, just offset them with dashed connectors. (So w+1 is equivalent to x+50).Naraht (talk) 19:18, 27 June 2017 (UTC)