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August 29

Rounding error in large numbers

At the last point of the tabel or matrix about the far future, there is something like:
"Because the total number of ways in which all the subatomic particles in the observable universe can be combined is 10 10 115 {\displaystyle 10^{10^{115}}},[152][153] a number which, when multiplied by 10 10 10 56 {\displaystyle 10^{10^{10^{56}}}}, disappears into the rounding error,"
My question is: How is the point about the rounding error valid? 2A02:8071:60A0:92E0:30AB:357:41DB:5492 (talk) 16:54, 29 August 2024 (UTC)

Multiplying the numbers is the same as adding the exponents. So $10^{10^{115}}\times 10^{10^{10^{56}}}=10^{\left(10^{115}+10^{10^{56}}\right)}$. And $10^{115}$ is negligible when added to $10^{10^{56}}$. --Amble (talk) 17:45, 29 August 2024 (UTC)

Or does the text try to say that $10^{10^{115}}$ is negligible compared to the rounding error in $10^{10^{115}}\times 10^{10^{10^{56}}}$? The intention is not clear to me. Why should these numbers be multiplied and what do rounding errors have to do with it? The argument should be, I think, that since there are "only" $10^{10^{115}}$ possible combinations, the specific combination that results in a repeat of the Big Bang is bound to occur sometime in the next $10^{10^{10^{56}}}$ years. However, this seems to assume that all combinations are about equally likely and ignores the effect of the expansion of the universe. I suspect SYNTH.  --Lambiam 19:57, 29 August 2024 (UTC)

how to calculate 10^{10^{115}}? I thought it's 10^(10×115)? 2A0D:6FC0:8EF:6000:9455:1667:D5E1:3858 (talk) 03:13, 2 September 2024 (UTC)

$(2^{3})^{2}=(2^{2})^{3}=2^{6}$, but $2^{3^{2}}=2^{(3^{2})}=2^{9}$. Exponentiation goes right to left; follow the braces. I trust you'll forgive my not writing out $10^{115}$. —Tamfang (talk) 18:53, 3 September 2024 (UTC)

Photons question.

When an electron collides with positron, and when a proton collides with the anti-proton, both situations they are transformed into 2 photons. Can those 2 photons be distinguished from the 2 situations? That is, can the 2 photons be traced to being formerly an electron or proton? They have different energy of initial states, different total spin? Thanks. 66.99.15.162 (talk) 17:46, 29 August 2024 (UTC).

See electron–positron annihilation and annihilation. At low energy, electron-positron annihilation will produce two photons, and you can be sure they didn't come from proton-antiproton annihilation because the total energy is less than the rest mass of two protons. The annihilation of a proton and antiproton, or an electron and positron with higher energy, can produce a variety of end states including baryons and weak bosons. A proton-antiproton annihilation to two photons would be a fairly rare process, see [1]. So we're talking about fairly uncommon end states. The high energy electron-positron annihilation and the proton-antiproton annihilation could produce the same types of final states, but with different probabilities, so you can make some statistical inferences, especially if you can observe multiple events. --Amble (talk) 19:15, 29 August 2024 (UTC)

Proton-antiproton annihilation in a pair of photons is very rare. The far more common outcome is a pair or triple of pions. Ruslik_Zero 20:05, 29 August 2024 (UTC)

And then, photons that were created from an electron moving up/down an orbital or shell, are obviously different than the above mentioned photons? Have different measurable properties? These properties (or just energy) are measured when a photon hits a solid, the energy measured by a photo-multiplier tube? 66.99.15.162 (talk) 20:22, 29 August 2024 (UTC).

Well these photons have lower energy than those created in annihilation. Also they are usually produced one at a time, rather than a pair or triple. Graeme Bartlett (talk) 00:28, 30 August 2024 (UTC)

But the concise answer to your question is: no. Once a photon is created, its only unique property is its energy. There is no difference between a 511 keV photon created from positron-electron annihilation or one created from any other source of 511 keV photons. See: indistinguishable particles. PianoDan (talk) 16:04, 30 August 2024 (UTC)

August 30

Dead batteries come back to life?

A friend of mine has a battery-powered clock. It stopped running. It was over a week before he got replacement batteries. When he went back to the clock, it was running again. Is there an explanation for this? Bubba73 ^{You talkin' to me?} 16:04, 30 August 2024 (UTC)

This writeup has one theory:[2] ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:08, 30 August 2024 (UTC)

This effect is readily seen with a flashlight when the cells (battery) are approaching the end of their useful lives. After a short period of operation the light from the flashlight becomes dim. If the operator turns the light off and waits a few seconds, then turns it back on, the light is usefully bright again but only for a few seconds before it becomes dim again.

The active materials in the cell migrate to get to their electrode and that takes time. Similarly the exhausted product migrates away from its electrode. Allowing the cells to rest for a short while allows these materials to migrate to/from the electrodes and improve the density of the active materials surrounding each electrode. Dolphin (t) 22:13, 30 August 2024 (UTC)

Temperature also plays a role. Batteries near their end of life are more likely to fail if the room turns cold. If it warms up again the clock may restart. Shantavira|^{feed me} 09:04, 31 August 2024 (UTC)

An additional factor may be a slight irregularity in a clock's mechanism, such as a slightly mis-shaped or corroded cog tooth, that is overriden by the power of a good battery, but provides enough resistance to the weaker power of a near-dead one to stop the clock. If the clock is then moved or disturbed by vibrations, this may jar the tensioned mechanism past the obtruction, restarting the clock for a time. {The poster formerly known as 87.81.230.195} 94.1.209.45 (talk) 09:18, 31 August 2024 (UTC)

it might be that the relationship voltage-current is non-linear and/or hysteretic. when the clock doesn't get enough juice, it stops drawing any current, the battery "recovers" via the chemical mechanism above, the clock starts drawing current again. the system "clock -- battery" thus oscillates which manifests itself in just such intermittent operation Aecho6Ee (talk) 18:07, 3 September 2024 (UTC)

some devices do this (poweroff when voltage drops below a threshold and back on when it rises) intentionally as part of "brownout detection" or "undervoltage lockout" Aecho6Ee (talk) 18:14, 3 September 2024 (UTC)

Metric units in the UK

In the United Kingdom, is room space and building height measured in metric units? And has rail transport seen any metrication in its history? --40bus (talk) 20:38, 30 August 2024 (UTC)

Rightmove are the UK's biggest online property agents. As you'll see from their website, they quote room space in sq ft but with conversions to sq m. I don't know about rail transport. Mike Turnbull (talk) 20:49, 30 August 2024 (UTC)

40bus: "On 5 May 1975, rail traffic switched to metric measurements for loads, capacities, tare weights and brake force. On the traffic side of the railway we are chiefly concerned with distance and weight. As to distance, no metrication is planned for the time being... (i.e. distance is still measured in miles, as are road distances). [3] Alansplodge (talk) 16:22, 31 August 2024 (UTC)

And are there any everyday things that are measured in metric units in the United States, as oppsoed to scientific things? --40bus (talk) 21:11, 30 August 2024 (UTC)

Some commodities are. You can get liter bottles of soft drinks, for example. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:23, 30 August 2024 (UTC)

2 quart soda bottles haven't been used in decades and decades. Did they keep the same price for awhile to say at least you're getting more? Sagittarian Milky Way (talk) 04:22, 1 September 2024 (UTC)

Highway distances: metric signs in Tennessee. (Not a representative sample!)  Card Zero  (talk) 21:39, 30 August 2024 (UTC)

Other states also post both miles and kilometers. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:13, 30 August 2024 (UTC)

Indeed, that site has more.  Card Zero  (talk) 22:36, 30 August 2024 (UTC)

Engine capacities in cc, esp. motorcycles. Doug butler (talk) 21:55, 30 August 2024 (UTC)

I thought Harley-Davidson measured their V twins in cubic inches. --TrogWoolley (talk) 10:38, 1 September 2024 (UTC)

Ammunition is measured in mm. So are camera lenses. And mechanical pencil lead widths. And a lot of other things that are too small to be comfortably measured in inches -- such as jewelry components. Wrench sets are made for both metric and US customary units. Backpacks and other luggage are sized in liters. We run 5K and 10K races.

A lot of components of food and supplements (carbohydrates, cholesterol, sodium, caffeine, etc.) are measured in grams or mg, especially on standardized nutrition labels -- and they've crept into the vernacular from there. Nobody talks about how many ounces per day of protein or grains of caffeine one should consume. -- Avocado (talk) 02:03, 31 August 2024 (UTC)

Guns and ammo are in inches. .357 Magnum, for example. Though much less common than they once were, Mile run events are still held in America. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:25, 31 August 2024 (UTC)

The Dream Mile race is held in Oslo (Norway metricated in 1875). Alansplodge (talk) 16:16, 31 August 2024 (UTC)

Miles are still used in Norway, but those are metric miles of 10 kilometres. The traditional Norwegian mile was 11298 metres. English miles have obviously never been in common use in Norway, but the abovementioned race is an English mile long. PiusImpavidus (talk) 17:48, 31 August 2024 (UTC)

Oh, yeah, not saying we don't use customary units for those things at all. Just that we do commonly use metric ones for them, too (such as 9mm for ammo). -- Avocado (talk) 13:53, 31 August 2024 (UTC)

Ammo calibres are more like names than actual measurements. .223 Remington and 5.56mm Nato are the same size, despite 5.56mm not being the exact conversion of 0.223", and neither being the actual measurement of the bullet. Iapetus (talk) 10:47, 2 September 2024 (UTC)

Most things related to electricity, such as a 120-volt receptacle or a 9-volt battery. Jc3s5h (talk) 14:03, 31 August 2024 (UTC)

Small lengths of time in the US are commonly specified in the corresponding SI base unit, the second. --Amble (talk) 22:47, 30 August 2024 (UTC)

I hope that the US will metricate at least some everyday things in next 20 years so that US-related articles will use metric units first in 2044. --40bus (talk) 17:25, 31 August 2024 (UTC)

I hope we don't we're the last bastion of old units, just big and isolated enough to prevent zero Earth unit diversity Sagittarian Milky Way (talk) 04:34, 1 September 2024 (UTC)

I second that emotion. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:56, 1 September 2024 (UTC)

I don't know everything about British railways, but if I'm to believe openrailwaymap, it appears that some British lines have a speed limit in kilometres per hour and some have a limit in miles per hour that is an obvious conversion from a round number in kilometres per hour. Look at the high speed lines like High Speed 1 (not the older, improved lines with 125 mph limit), DLR and some of the more recent lines of the London Underground. So it looks like a conversion has started. PiusImpavidus (talk) 16:39, 31 August 2024 (UTC)

The Grenfell Tower fire led to the United Kingdom cladding crisis. Most of the building height measurements are only or primarily given in metres. -- Verbarson  ^talk_edits 20:49, 31 August 2024 (UTC)

But article Tower Bridge, for example, uses imperial units first. And it is a building. --40bus (talk) 21:44, 31 August 2024 (UTC)

Tower Bridge is 130 years old. The sources (and the sources' sources) are therefore far more likely to use Imperial units. The response to the Grenfell fire is contemporary, so better reflects current practices. -- Verbarson  ^talk_edits 05:06, 1 September 2024 (UTC)

Our Miles per hour article says: Miles per hour is also used on British rail systems, excluding trams, some light metro systems, the Channel Tunnel and High Speed 1 (the Channel Tunnel and its High Speed link obviously extend into France and beyond). Alansplodge (talk) 11:30, 1 September 2024 (UTC)

It's a surprisingly modern bridge really. The Time Machine was written the next year.  Card Zero  (talk) 13:06, 1 September 2024 (UTC)

Real steampunk, in fact. (Edit) My mistake - it was hydraulic. -- Verbarson  ^talk_edits 13:35, 1 September 2024 (UTC)

Not mistaken at all, Verbarson! The operations were performed by hydraulic pressure, supplied by hydraulic accumulators, but "[w]ater at a pressure of 750 psi (5.2 MPa) was pumped into the accumulators by a pair of stationary steam engines" – see Tower Bridge#Hydraulic system. {The poster formerly known as 87.81.230.195} 94.1.209.45 (talk) 21:45, 1 September 2024 (UTC)

Ah, thank you. When I saw it was 'hydraulic', I somehow assumed it used the London high-pressure network (which was itself steam-powered). What makes it feel most like steampunk is the combination of ancient (Gothic turrets), more recent (suspension spans for the approaches) and up-to-date (steam-hydraulic powered bascules) styles and technologies. -- Verbarson  ^talk_edits 22:17, 1 September 2024 (UTC)

September 2

Is the nose of a grizzly or a brown bear cold?

Do bears have a cold nose like a dog? Wasn't able to find an answer with Google and finding out myself would be against the policies of my health insurance. --Question123Ka (talk) 04:46, 2 September 2024 (UTC)

Dog noses are cold because they are cooled by the evaporation of moisture on their noses. Dog noses are moist because dogs lick their noses when they get dry. So the question amounts to, do bears lick their noses, regularly? I suppose your health insurance covers the risks of an investigation if carried out in a zoo or using binoculars or a zoom lens.  --Lambiam 06:29, 3 September 2024 (UTC)

I don't go outside. That's where bears live.

But thanks, you helped me: If one specifically searches for bear and wet nose or rhinarium, one might find some results. At least one in Google Books: International Wildlife Encyclopedia, where they say brown bears have a wet nose.

Google as such is so dead, however. Just stock photo spam and ads for sweatshop products. --Question123Ka (talk) 12:32, 3 September 2024 (UTC)

This polar bear's nose is at about 15°C, but I can't find a thermogram of a brown bear.  Card Zero  (talk) 08:40, 3 September 2024 (UTC)

Thanks, that's exactly what I found out too, that polar bears have a cold nose (means: colder than the rest of their body). I'm mostly interested how it would feel to touch a bear's nose. --Question123Ka (talk) 12:32, 3 September 2024 (UTC)

According to the book "Grizzly Bear Science and the Art of a Wilderness Life," all bears lose heat primarily through their paws, but also through area with limited hair such as their nose and ears. That makes the nose appear warmer than the surroounding face because it is radiating more heat. Checking "Ecology and Behaviour of North American Black Bears," it agrees that all bears lose heat through their paws, ears, and nose. It also states that the grizzy and black bears have a stronger sense of smell compared to other bears and they further accentuate it by using their tongue to force air into their nose in a motion that looks like licking. A side effect of this is that the water evaporates on their wet nose, cooling it down, and allowing more heat to escape. As a result, grizzly and black bears tend to have more and larger blood vessels just under the surface of their nose. Being so close to the Appalacians, we have plenty of books on bears, but most are aimed towards children. I didn't check those because I assume they won't have anything of interest and, usually, they don't have an index where I can quickly look for pages about the bear's nose. 75.136.148.8 (talk) 10:44, 5 September 2024 (UTC)

Hydrogen storage

Quote from the article: "A potential efficiency loss of only 12.79% can be achieved, or 4.26 kW⋅h/kg out of 33.3 kW⋅h/kg." What is the reason for using 4.26 out of 33.3? Is there some significance to those numbers? I get 12.79 out of 100 but what is special about 4.26 out of 33.3? 196.50.199.218 (talk) 13:49, 2 September 2024 (UTC)

The section was introduced in this edit, you could try to contact the author (but they haven't edited since 2018). I cannot find those numbers in the reference given. Not sure what to make of that. --Wrongfilter (talk) 14:36, 2 September 2024 (UTC)

Hydrogen energy density which is 120mj/kg is 33.3kwh. 156.155.127.55 (talk) 15:05, 2 September 2024 (UTC)

September 4

Cat

Can anyone explain how it is technically possible for a cat to be simultaneously alive and dead at the same time? I've seen this referenced in pop culture a few times, but I don't understand. 146.200.107.107 (talk) 00:59, 4 September 2024 (UTC)

See Schrödinger's cat and Schrödinger's cat in popular culture. It's one of those things that if you understand it you probably don't really know what it's really about. DuncanHill (talk) 01:14, 4 September 2024 (UTC)

The cat knows if it's alive. It's just that someone outside the cat's box might not know. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:51, 4 September 2024 (UTC)

If the cat in the box has a philosophical bend and knows Latin as well as the Schrödinger equation and wave function collapse, it might think, Miror si vivo vel non. Ergo vivo. But, actually, we as outside observers do not know whether the cat knows anything. If it does not know whether it is alive or dead, it is more likely dead. The cat may be in a superposition, though, of a state of knowing and a state of not knowing it is alive.  --Lambiam 08:43, 4 September 2024 (UTC)

Cats that are alive know they're alive. They don't have to overthink it. :) ←Baseball Bugs ^{What's up, Doc?} carrots→ 09:32, 4 September 2024 (UTC)

The question is, does a cat count as an observer for quantum physical purposes? And can its quantum state be collapsed inside the box but not yet outside? {The poster formerly known as 87.81.3230.195} 94.6.83.137 (talk) 12:24, 4 September 2024 (UTC)

This all sounds like animal cruelty. Better they should seal Schrodinger in a box and see how he likes it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:18, 4 September 2024 (UTC)

To make it clear to others, this was a thought experiment that Schrödinger described to illustrate his opinion that the Copenhagen interpretation of quantum mechanics led to (in his opinion) absurd conclusions: neither he nor any other physicist has proposed carrying it out with a real cat. For one thing, the cat would likely have to be listed as one of the authors of the resulting paper, which would be silly :-). {The poster formerly known as 87.81.230.195} 94.6.83.137 (talk) 04:30, 6 September 2024 (UTC)

Silly indeed. That could never happen. PianoDan (talk) 03:35, 10 September 2024 (UTC)

Whenever quantum whatever is mentioned in pop culture it's probably better to just enjoy it as it is, because it's probably completely off the rails.

If you accept that reality is real, then a macroscopic event really happens in real life and the cat really lives or dies, one or the other. So the original trigger to the experiment (a beta decay, say) is probabilistic based on a superposition of states. Note we need to remain ignorant for as long as possible as to whether the particle has decayed if we keep the superposition across the system -- see Zeilinger's quote in the superposition article -- you can see how that would be a problem if you expect the cat itself to be entangled with the particle. The question is, when and by what nature does the system decohere? In some sense, we know how to describe the experiment from when it begins to some midpoint and how to describe it from some midpoint to when it ends, but somewhere in the middle the magician critically slips the other cards into his pocket -- so when did that happen (or maybe it wasn't a card trick to begin with)? SamuelRiv (talk) 18:21, 4 September 2024 (UTC)

I don't get the other people's responses, but I thought it had to do with multiple universes. That is, after an incident happens to a cat in a box, the scenario is split into 2 additional universes: 1 in which you open the box and the cat is dead, and the other where you open the box and the cat is alive. 66.99.15.162 (talk) 20:55, 4 September 2024 (UTC).

The many-worlds interpretation is just one of many attempted interpretations of quantum mechanics. Although Hugh Everett III is usually credited as the originator of the interpretation, he merely aimed at presenting a mathematical model including "measurements". The popularization framing this as splitting into multiple universes is due to Bryce DeWitt; Everett had no high opinion of this, considering it "bullshit".^[4]  --Lambiam 22:41, 4 September 2024 (UTC)

Self-moved to New Question Sep 10 (talk) 04:20, 10 September 2024 (UTC)

Colour of PmCl₃

At the infobox for promethium(III) chloride we find two different sources for this compound's colour, one calling it purple and the other calling it yellow. Probably few have seen this compound (considering the short half-life of Pm), but what's the story here? Is it perhaps similar to how the colour of NdCl₃ varies under lighting conditions (between purple and yellow as well)? Double sharp (talk) 04:40, 4 September 2024 (UTC)

I would expect most trivalent compounds of promethium have similar colour. But there may also be radiation damage to the material. Also perhaps the compound is not pure and contains neodymium. Graeme Bartlett (talk) 21:49, 6 September 2024 (UTC)

Checking my textbook, (Advanced Inorganic Chemistry by Cotton and Wilkinson) it says trivalent salts of Pm are pink. So none of the above. Graeme Bartlett (talk) 10:44, 8 September 2024 (UTC)

Native germanium

Resolved

Is native germanium actually a thing? Mindat lists it, but the source is rather lacking in details, and does not mention Ge proportions beyond 37.6%. Maybe I'm misunderstanding the paper. Double sharp (talk) 13:08, 4 September 2024 (UTC)

According to the definition in Native element mineral, it would be counted as "native", even though it is an intermetallic compound with palladium, rhodium (or possibly platinum and nickel in your ref). But not known as pure or close to pure as a mineral. Graeme Bartlett (talk) 00:44, 5 September 2024 (UTC)

Thanks. Double sharp (talk) 15:44, 6 September 2024 (UTC)

September 6

Stress-energy tensor

Why is it called "stress energy tensor", although its components intended to refer to the energy - actually refer to the energy divided by the speed of light squared - i.e. actually refer to the (relativistic) mass? HOTmag (talk) 13:14, 6 September 2024 (UTC)

Check out the Stress-energy tensor#Components section where the components of the tensor are normal or shear stress, momentum or energy. The energy is the sum of energy and mass. Graeme Bartlett (talk) 21:55, 6 September 2024 (UTC)

It seems that your last sentence contains some logical mistake. That's why I couldn't figure out your answer.

Anyway, don't you agree, that whenever any component of this tensor refers to the energy of a given body - this component actually refers to the energy divided by the speed of light squared - i.e. actually refers to the (relativistic) mass? HOTmag (talk) 18:49, 7 September 2024 (UTC)

Relativistic mass just is energy. The only difference is the units, and not even that if you use units in which the speed of light is 1. So it's a little hard to figure out what your question means. --Trovatore (talk) 18:43, 8 September 2024 (UTC)

September 7

Dilithium in real life

The article about real-life dilithium has the potential to be quite interesting, but because of search result overlap with the Star Trek substance, it's difficult to nail down references on this topic. We could use some to establish - for example - if it's a gas at standard temperature and pressure, and if it's "stable", what its typical lifetime is. I would be interested if anyone could put their fingers on sources with this sort of info. -- Beland (talk) 08:22, 7 September 2024 (UTC)

IIRC all the alkali metal dimers are known in the gas phase (not sure how much of the vapour is monatomic vs diatomic, though). But they will condense back into metallically bonded structures when cooled below the boiling point. Double sharp (talk) 08:25, 7 September 2024 (UTC)

@Beland The general way to find decent references is to use Google Scholar. This search is a start and removes some hits from articles about a piece of software called dilithium by searching for co-occurence of that word with "lithium". Mike Turnbull (talk) 11:05, 7 September 2024 (UTC)

You can also search in a standard search engine using its InChIKey, SMBQBQBNOXIFSF-UHFFFAOYSA-N Mike Turnbull (talk) 11:11, 7 September 2024 (UTC)

The search term "lithium dimer" also seems to work well. Double sharp (talk) 12:19, 7 September 2024 (UTC)

Excellent; thanks for the tips! -- Beland (talk) 16:28, 7 September 2024 (UTC)

Colour of radiation glow

The description of this picture of actinium states that the blue comes from Cherenkov radiation, but I'm not sure can Cherenkov radiation produces other colours. --Nucleus hydro elemon (talk) 11:27, 7 September 2024 (UTC)

Actinium glows blue, curium glows purple, while radon glows yellow. What decides the glow colour of a radioactive element? Nucleus hydro elemon (talk) 11:27, 7 September 2024 (UTC)

Radioactive elements and their isotopes release, for example, alpha particles of various energies. These ionise surrounding material and the colours come from the ions relaxing back to their ground states. There is a chart which gives the energies (zoom in to the isotope of interest). Mike Turnbull (talk) 12:16, 7 September 2024 (UTC)

Two things about the image-caption details. First, our Cherenkov radiation article discusses the origin of the color via the Frank–Tamm formula. Second, is the glow from Cherenkov radiation or simple ionization? The linked image's description page says "the Cherenkov blue glow that originates from the ionization of surrounding air by alpha particles", which sounds like it's conflating those two. File:Actinium_sample_(31481701837).png's description, also from ORNL sources, simply says "ionization of surrounding air by alpha particles." DMacks (talk) 05:25, 9 September 2024 (UTC)

Effect of elevation on sunshine

Sunshine duration says that locations on the Arctic Circle get 4,647 hours of sunshine (disregarding the effects of clouds), the most of any location worldwide due to the effects of atmospheric refraction. Imagine that Surtsey-type eruptions produce a new mountain in an ocean location on the Arctic Circle, and it's extremely high — say, 5000m. Disregarding the effects of clouds, will it get much more sunshine than locations at sea level? I assume it will get some more than those locations, since the sun's above the horizon longer for an elevated location, but I don't know how much longer. Nyttend (talk) 21:49, 7 September 2024 (UTC)

It doesn't have to be on the Arctic Circle, you could build a (thought experiment) 100,000 km tall tower on the Equator, and as the Earth rotated, the top of the tower would only briefly be in Earth's shadow, if at all. Abductive (reasoning) 05:14, 8 September 2024 (UTC)

Let $R\approx 6378\,{\text{km}}$ denote the radius of the Earth (taken to be a sphere) and let $\varepsilon \approx 23.44^{\circ }$ stand for the obliquity of the ecliptic. Then, for the top of a tower erected at latitude $\phi$ to avoid the shadow cast by the earth day and year around, its height should be at least $R(|\csc(\varepsilon \,{\dot {-}}\,|\phi |)|-1),$ where $a\,{\dot {-}}\,b=a-b~{\text{if}}~a\geq b,$ and equals zero otherwise. [edited 11:27, 9 September 2024 (UTC); edited again 11:56, 10 September 2024 (UTC)]

• At the Poles, with $|\phi |=90^{\circ },$ this comes out at about $574\,{\text{km}},$ a mere 700 Burj Khalifas stacked on top of each other. This is the height of a Low Earth orbit, so the tower may be hit by satellites in polar orbit.
• On the Arctic Circles, with $|\phi |=90^{\circ }-\varepsilon ,$ the tower needs to be $2953\,{\text{km}}$ high.
• In the tropics, from the Tropic of Cancer to the Tropic of Capricorn, where $|\phi |\leq \varepsilon ,$ the tower needs to be infinitely high, which is impractical from an engineering point of view :).

 --Lambiam 09:40, 8 September 2024 (UTC)

I dunno if that makes sense. Consider Saturn's rings; they are often (always?) partly in shadow. But a tower on the axis will stick out into perpetual sunlight at a height much less than the rings. Abductive (reasoning) 14:55, 8 September 2024 (UTC)

I must have made a mistake; if $\varepsilon$ had been zero (meaning that the equatorial and ecliptic planes coincide), any tower at a pole would always be in sunlight. Possibly, the fix is simply to replace $\sec$ by $\csc .$ I can't examine this further right now.  --Lambiam 17:52, 8 September 2024 (UTC) — Now corrected and hopefully now correct.  --Lambiam 11:27, 9 September 2024 (UTC)

But wait a minute. If at some point in the year the sun is directly overhead at noon (i.e. between the two tropics) then if the tower is normal to the geoid it's going to have to be very long, because at midnight sun-earth-tower form a straight line. This is analogous to a total solar eclipse, where the straight line is sun-moon-earth. But you can have annular eclipses because the sun is so much larger than the moon, and when the moon is beyond a certain distance we can "see over it" and catch a glimpse of the sun. As the sun is much bigger than the earth, the earth's shadow is not infinitely long and at some point the top of the tower must emerge from the gloom. 2A00:23D0:CCD:CE01:1197:733B:D027:4118 (talk) 15:27, 9 September 2024 (UTC)

Just like my calculation assumed a spherical Cow Earth, it assumed an infinitely distant Sun. Since we have total lunar eclipses, we know the Earth's shadow extends to far beyond the Moon's orbit. Ignoring the effect of sunlight being bent by our atmosphere, the shadow cone extends to 1.38 Tm, almost 1/100 of an astronomical unit.  --Lambiam 11:56, 10 September 2024 (UTC)

September 8

What is the systematic name for elements Z=(<100)?

Like, is the systematic name of uranium Ennbium? Is the systematic name of fluorine Ennium? Is the systematic name of caesium Pentpentium? HAt 05:12, 8 September 2024 (UTC)

The rules for the systematic element names, approved by IUPAC in 1978, were designed solely for the purpose of assigning temporary names to unknown or not-yet-named chemical elements. Since all such elements have an atomic number greater than 100, the system only caters for such higher numbers, up to 999 (Ennennennium). The extension to other natural numbers for elements with IUPAC-approved names, while obvious, does not carry the IUPAC stamp of approval. Note that Nilium (a potential name for proton-free muonium or neutronium) and Quadium have already been given away.  --Lambiam 08:31, 8 September 2024 (UTC)

In fact, the IUPAC rules only cover elements 101 onward, so technically even unnilnilium for fermium would not be IUPAC-approved. :) Double sharp (talk) 08:39, 8 September 2024 (UTC)

Given that atomic numbers are integers, "greater than 100" and "101 onward" have the same meaning here.  --Lambiam 09:43, 8 September 2024 (UTC)

Is there a specific name for the opposite of a virus?

This is a terminology question about organisms, not diseases. In the grand scheme of things, there are two ways nucleic acid strings propagate themselves. In one, the nucleic acid string encodes enough information to build a cell with enough machinery to copy the nucleic acid string and make enough of the machinery to allow the cell to split, each with a copy of the nucleic acid string. In the second type, the nucleic acid string encodes a simple shell designed to penetrate the first type and hijack its machinery to copy itself and the shell. We have a very specific term, Virus, for the second type. Is there an equally specific term for the first type? Something better than "ordinary cell." --agr (talk) 18:10, 8 September 2024 (UTC)

Why would you expect there to be a word words for non-Virus, non-Archaea, non-Bacteria or non-Eukarya, meaning 'all the others except this one'? They are not really opposites. -- Verbarson  ^talk_edits 20:31, 8 September 2024 (UTC)

User:Verbarson, I thought "eukarya" had an opposite term, "prokaryote". Is that wrong somehow? Nyttend (talk) 07:47, 9 September 2024 (UTC)

I admit to not being an expert in this area. According to Prokaryote, cellular organisms can be divided either

• into two domains (Prokaryote/Eukaryote) - in which case they may be 'opposites', though I don't know whether viruses fit in either half, or
• into three domains (Bacteria/Archaea/Eukarya) - which again may not include viruses

I suppose if Prokaryotes include Bacteria, Archaea and viruses, then they function as the 'opposite' (in the sense of 'exclusive or') to Eukaryotes. I don't know of a term that covers Bacteria, Archaea and Eukarya but excludes viruses, which is what OP is seeking. (Apologies for my sloppy nomenclature to anyone who knows the definitions of these words!) -- Verbarson  ^talk_edits 08:16, 9 September 2024 (UTC)

Viruses are not considered to be cellular.  --Lambiam 12:06, 9 September 2024 (UTC)

Re: "a term that covers Bacteria, Archaea, and Eukarya but excludes viruses": I think said term would be "life" (perhaps with the caveat "as we know it" appended). Viruses are biological entities, but do not meet the qualifications to be considered life. Though I think some, or maybe all, viruses are descended from various types of living organisms..some maybe from life during the RNA world age. 73.2.106.248 (talk) 01:57, 10 September 2024 (UTC)

Organism?  Card Zero  (talk) 00:59, 9 September 2024 (UTC)

There is a name for this grouping of living things with cells: Cellularae proposed by H. P. Traub. There are also other lifelike things that are not viruses and don't have cells: viroids and Obelisk (biology) and prions. — Preceding unsigned comment added by Graeme Bartlett (talk • contribs)

Maybe autocatalytic genes are in there too, the transposons. Sean.hoyland (talk) 09:20, 9 September 2024 (UTC)

References

September 9

The photoelectric effect, but then outside of matter?

When with an incident ray a photon of sufficient energy meets an electron it can eject it from matter. But outside of matter, we still have the probability that other photons meet this electron and orient its course in the direction of the radiation. Has this been observed? If yes or no, what is the explanation? Malypaet (talk) 09:20, 9 September 2024 (UTC)

Well in the ionosphere radiation ejects electrons from atmospheric molecules and then they can interact with radio waves. Graeme Bartlett (talk) 10:10, 9 September 2024 (UTC)

Compton scattering, the effect of the interaction of a photon and a charged particle, usually an electron, also applies to the interaction of photons and solitary electrons. If no energy is needed to release bound electrons, this form of scattering already occurs with low-energy photons and is then known as Thomson scattering.  --Lambiam 13:06, 9 September 2024 (UTC)

I read these articles, but what is missing is the notion of time. A radiation is a flow of energy (a rate), so an electron in a volume crossed by a radiation will experience a force in the direction of the radiation, as long as it is in this volume, ok. As it accelerates, it releases energy in the form of radiation (synchrotron effect?), so it loses acceleration. I suppose that the trajectory of the electron is a curve that brings it in the direction of the incident radiation? Do we know the equation of this curve? Malypaet (talk) 21:14, 9 September 2024 (UTC)

Acceleration is a change in velocity. In the scenario of a photon interacting with a free electron, the only change in velocity is at the moment of interaction. Before and after, in the absence of external forces, their velocities are constant.  --Lambiam 22:45, 9 September 2024 (UTC)

How can you have an electron outside of matter?? PianoDan (talk) 03:31, 10 September 2024 (UTC)

I've interpreted this as "outside of other matter", that is, a solitary electron.  --Lambiam 08:17, 10 September 2024 (UTC)

But in radiation that is a photon flow, there are more than one photon, so the probability that other photons successively (in time) hit the electron is not null. Then, in this term of probability, you can have a trajectory and a curve with a point in space and time for each interaction, isn't it? Malypaet (talk) 08:35, 10 September 2024 (UTC)

The trajectory will be more like a 3D random walk superimposed on a drift than a curve that can be described with an equation.  --Lambiam 08:42, 10 September 2024 (UTC)

Thanks Malypaet (talk) 19:56, 10 September 2024 (UTC)

Rarest colour of a vertebrate?

I've just been trying to think. There are lots that are black/brown/white and I can think of several yellow, green, blue and red vertebrate animals (mostly birds), but very few that are predominantly purple. Is this the rarest colour in nature? Iloveparrots (talk) 21:44, 9 September 2024 (UTC)

Transparent skin seems very rare to me, but with the recent tartrazine discovery it may become common. --Error (talk) 23:10, 9 September 2024 (UTC)

left: azzurro; right: blu

One problem in addressing the question is that colour names cover a fuzzy region in a multi-dimensional space of colours. The 23 examples of purple vertebrates shown here display a wide range of purplish colours. For a reasonable comparison between named colours, the regions need to have similar sizes. The region we call "blue" is split in Italian into two regions considered to have different colours: azzurro and blu.  --Lambiam 23:24, 9 September 2024 (UTC)

Same in Russian. Dark blue is синий (siniy), and light blue is голубой (goluboy). -- Jack of Oz ^{[pleasantries]} 17:28, 10 September 2024 (UTC)

September 10

Recatquista

In looking into my answer to Cat above, I stumbled on the quantum immortality article, and I feel dumber for having read it. I'm getting that it's premised on some mystic consciousness woo, but I still don't understand what the experimenter dying, or the experimenter understanding QM, has to do with anything. Or does the thought experiment just exclude hard materialists at its premise? SamuelRiv (talk) 04:23, 10 September 2024 (UTC)

The basic argument has nothing to do with consciousness. In the many worlds interpretation, a measurement that causes the wave function to collapse to a definite state actually makes the universe split into two: one for each of the two possible outcomes. Now imagine a qubit being measured again and again, until the outcome is 0. Each time there will be a branch in the tree of universes in which the outcome was 1, so there is a path in which the qubit never "dies". The death of a living organism is the result of many measurements eventually leading to its demise, but, analogously to the immortal qubit, there should be a path in which all outcomes are such that they keep the organism alive and, pace Tegmark, well.  --Lambiam 08:36, 10 September 2024 (UTC)

In the article it says Tegmark's thought experiment "must be virtually certain to kill the experimenter" and "on a time scale shorter than that on which they can become aware of the outcome of the quantum measurement". I don't understand what this has to do with what you're saying about the MWI. It should be sufficient proof enough of MWI to simply have an experiment running measuring the spin of a random qubit that never ever measures 0 (or I guess to see an event in any known process whose expected duration is orders of magnitude smaller than the lifespan of the universe). Or, if personal experience is insisted, I don't see what dying, or instantaneous dying for that matter, has to do with 'traveling' so-to-speak through the many worlds. SamuelRiv (talk) 15:34, 10 September 2024 (UTC)

In your universe the qubit may come up 0 while another version of you, in a sister universe, sees a 1. But that other you cannot communicate this outcome to your you.  --Lambiam 17:31, 10 September 2024 (UTC)

Ok? But if I instaneously kill myself I can?

I feel like there's a great deal of unarticulated premises about consciousness here that everyone in the article seems to know instinctually, but I am completely lost by. (Not that I can't sympathize, but that there are a lot of interpretations of spiritual consciousness and the self around the world, so I can't follow the logic of the argument until I know what premises they're using.) SamuelRiv (talk) 17:40, 10 September 2024 (UTC)

I think bringing in consciousness is a red herring. Everything would go just the same with philosophical zombies – the laws of physics don't care.  --Lambiam 17:48, 10 September 2024 (UTC)

Ok, so then could you explain Tegmark's reasoning in the article? I understand the laws of physics as far as I've studied them, but I'm trying to understand the argument as written (and maybe even salvage the article). SamuelRiv (talk) 17:57, 10 September 2024 (UTC)

There isn't much to it. You have two people (let's make them people to make it easy). One is in the box. One is outside the box. The one in the box knows if he is alive or dead. The one outside the box doesn't know if the one inside the box is alive or dead and, therefore, must continue with the assumption that the person in the box is both alive and dead at the same time. What is being done that requires this? Let's assume that the person outside the box is filing taxes for the person inside the box. Is this the final tax statement for someone who is dead or a normal tax statement for someone who is alive? The person outside the box does not know and has to fill out both, one for someone who is dead and one for someone who is alive. Now, let's assume the person in the box is alive. He knows that the person outside the box is filing both and giggles to himself that he making the extra work. But, what if the person inside the box is dead? The person outside the box is treating him as if he is still alive... which is overhyped as "life after death." It isn't that the person in the box is alive. It is that the person outside tbe box is treating them as they are alive (and dead). The complication isn't in the concept of being alive and dead. The complication is in the quantum formulas that use the two states combined. 75.136.148.8 (talk) 17:18, 11 September 2024 (UTC)

Sorry, but I'm not following what this has to do with Tegmark's 3 conditions in the article I linked at the beginning of this topic header (the question to which you immediately replied), or the subject of the quantum immortality/suicide generally? SamuelRiv (talk) 01:05, 12 September 2024 (UTC)

Voltage and speed of electrons

For a resistance $R$, a voltage $U$ and a current $I$, with the relation $U=RI$, the electrical power is $P=UI=RI^{2}$ ​​. As $I$ is the intensity of the electron current in a section, that is to say the number of electrons that pass through this section per unit of time. Then should we consider that the number of circulating electrons is constant and proportional to $R$, therefore with the intensity proportional to their speed which is then considered as the voltage, or a mixture between the number of circulating electrons and their speed? In the latter case what is the rule giving the relationship between the number and the speed of the electrons?
Malypaet (talk) 08:47, 10 September 2024 (UTC)

The concept that current flow is the same as electron flow is acceptable for learning about circuits, but it is not real. It is similar to using water flow to explain the concept of electricity. Electrons do move, but very slowly in comparison to electrial current flow. There are many websites and videos that explain the actual flow of electromagnetic waves through a circuit. If you ever happen to get into radio or microwave circuitry, understanding the electromagnetic nature of electricity is important. 12.116.29.106 (talk) 12:29, 10 September 2024 (UTC)

Your first sentence is entirely correct. Your second too, assuming that the current is carried by electrons, which is normally the case in solid or liquid metals. Then your question. I don't fully understand the question, in particular the part “the number of circulating electrons is constant and proportional to $R$”. Both the number of free electrons and the resistance are static properties of the circuit, independent of the voltage or current applied, but with both constant, you cannot say that one is proportional to the other. Otherwise, the answer to the question appears mostly yes, although it's worded in an uncommon way. If you increase voltage, the drift speed of the electrons increases, but the number of free electrons is constant (again, in a solid or liquid metal).

The density of free electrons (electrons per cubic metre) depends on the material used. In semiconductors, there's a strong temperature dependence too. The specific resistance (ohm-metre) also depends on the material and temperature. The current density (ampère per square metre) equals the free electron density (electrons per cubic metre) times the drift velocity (metres per second) times the electron charge ($-1.6\cdot 10^{-19}$ coulomb per electron). The current density also equals the local electric field (volts per metre) divided by the specific resistance, none of which are constant throughout the circuit. All of that assuming that magnetic and electrostatic induction can be ignored (i.e., DC) and that electrons get up to speed in a negligible distance compared to the length scale of the circuit. PiusImpavidus (talk) 12:34, 10 September 2024 (UTC)

One does not apply a voltage or a current, but only a voltage, which will then give a current depending on the circuit's resistance.
So I will clarify my question. I consider the resistance in a restricted circuit volume and having a certain section (elsewhere the resistance is zero), all in a solid. If I understand your answer correctly, the number of (free) electrons moving in the resistance is constant and it is the voltage divided by the resistance which gives the drift velocity of the electrons. I know that this current carries an electromagnetic wave, more precisely a flow of energy and at constant speed. So I am looking for the relationship between the current of the electrons whose number is fixed and the flow of electromagnetic energy whose speed is constant, which gives the equation $P=RI^{2}$. If the number of electrons is fixed, it seems logical to me that the voltage is proportional to their drift velocity (speed). Maxwell is for the energy on one side, and on the other, Ampere is for the electrons that carry this energy.
f the electrons do not move, there is no electromagnetic wave with its transport of energy. Malypaet (talk) 20:48, 10 September 2024 (UTC)

The electron drift velocity is proportional to the electric field which has the units volts/meter. See Drift velocity and Electron mobility. Our article Speed of electricity further calculates the medium-dependent electromagnetic wave velocities of their interactions. Modocc (talk) 13:01, 11 September 2024 (UTC)

Thanks.
It confirms that the voltage and the speed of electrons are proportional. However, I am not sure that "drift velocity" is an appropriate term here because it is zero in an alternative current. It seems to me that the average instantaneous speed of the electrons is more appropriate. Malypaet (talk) 19:35, 11 September 2024 (UTC)

Kinds of Herability

Is there one scientific-mathematical test to distinguish between different forms of genetical herability?
I mean, if a certrain phenotypic property is genetic during Mendel's rules or additiv or something? I wonder whether we are able to find out just by looking at the offsprings and the parent generation. 2A02:8071:60A0:92E0:25A6:B013:4618:1FCD (talk) 10:20, 10 September 2024 (UTC)

When looking at individual organisms, you see phenotypes. The forms of heredity (biological inheritance) apply to genotypes. The relationship between genotype and phenotype is not straightforward. If the phenotypical statistics of the offspring of a couple form a typical Mendelian pattern, it is an indication that Mendelian inheritance is at play, but it is not a proof. And conversely, the absence of a typical Mendelian pattern need not mean the underlying genotypical inheritance is not Mendelian.  --Lambiam 17:45, 10 September 2024 (UTC)

Presumably you're referring to Heritability. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:28, 11 September 2024 (UTC)

The reference to Mendel's rules shows that the OP means heredity (aka inheritance), not heritability.  --Lambiam 17:14, 11 September 2024 (UTC)

See below. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:54, 11 September 2024 (UTC)

What is below? Did you review the article you linked? SamuelRiv (talk) 01:10, 12 September 2024 (UTC)

No, the point is that Lambiam stated the OP was asking about heredity, but it appears the OP was asking about heritability. Though it could be a language issue. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:06, 12 September 2024 (UTC)

There are different kinds of heritability. For instance one way via the rules of Mendelian; some properties are heritabil with additive effects and others with combinated effects.

The question is whether there is a methode to make clear via which way one given property is heredite. 2A02:8071:60A0:92E0:1465:9402:7F53:FBFB (talk) 18:24, 11 September 2024 (UTC)

Take a look at the diagram labeled "A ’Broken Stick’ Model" in the article Additive genetic effects. In general, the observed phenotypic variation is the combined effect of additive and non-additive effects. If it is known that the phenotypic variation is controlled by just a single gene, it is relatively easy to determine which variants the gene has and which variants, if any, are dominant or recessive with regard to their phenotypical expression. But control by just a single gene is exceptional; it implies that the phenotypes can be split into a limited number of discrete categories. The converse implication is not necessarily valid.  --Lambiam 06:19, 12 September 2024 (UTC)

Does "narcissist personality disorder" contradict itself?

I.e. labelling "excessibe grandiosity" using one person's name (fictional or not) achieves grandiosing that person (narcissus) thus communicates a double message? As in while the word "disorder" says "it is severe", the name of one person trivializes grandiosity. Thus it seems not quite medically consistent. Ybllaw (talk) 12:37, 10 September 2024 (UTC)

I fail to see how saying someone has narcissistic personality disorder (assuming that's what you mean) "achieves grandiosing that person". Do you mean to suggest the term is an oxymoron? No it isn't. Shantavira|^{feed me} 12:59, 10 September 2024 (UTC)

You misread what I said. Your comment "grandiosing that person" is not what I said. I said it grandioses "narcissus". I also didn't use the word "oxymoron". I think there is no need to introduxe extra terminology. My question was sufficiently clear. "No it isn't" is an unuseful/not very dilligent reply. Ybllaw (talk) 13:19, 10 September 2024 (UTC)

I said "grandiosing that person" about the (fictional) person whose name is used (narcissus), not a person diagnosed with NPD. Ybllaw (talk) 13:22, 10 September 2024 (UTC)

Thank you for clarifying that you are referring to the mythological Narcissus. The capital letter makes all the difference. I still don't see how the label is grandiose. If anything it demeans him. Shantavira|^{feed me} 14:22, 10 September 2024 (UTC)

Any observed similarity between Hero syndrome and FDIA?

I read on the Hero syndrome page.. "The term is used to describe individuals who constantly seek appraisal for valiant or philanthropic acts, especially by creating a harmful situation which they then can resolve". Isn't that very similar to FDIA? There is no mention on the Hero syndrome page of FDIA.

The FDIA page even literally mentions.. "These proxies then gain personal attention and support by taking on this fictitious 'hero role' and receive positive attention from others, by appearing to care for and save their so-called sick child", but doesn't reference the Hero syndrome page either. Ybllaw (talk) 12:56, 10 September 2024 (UTC)

A statement such as "A is a kind of B" requires a reliable source. One issue why such sources are hard to come by in this case may be that Factitious Disorder is a recognized disorder (300.19 in DSM-5, F68.1 in ICD-10), whereas "hero syndrome" is journalese and has no generally accepted diagnostic criteria.  --Lambiam 17:17, 10 September 2024 (UTC)