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November 30

Displacement receiver v. transducer v. sensor

I'm working on the Displacement receiver page, which formerly had no citations, and the going is difficult because few things actually talk about displacement "receivers" rather than sensors/transducers/etc.. Does anyone know if these three terms refer to the same thing? The initial article talked about a carbon microphone as a displacement receiver because it responds to displacement internally, although what it measures is sound waves, whereas this book says displacement transducers measure the distance between a sensor and a target, and this one says they measure movement and the "occurence of a reference position", whatever that means. It doesn't seem like carbon microphones fit those definitions. But I've also seen e.g. this conference paper use "displacement receiver" to refer to a contact sensor measuring its change in distance from a concrete block to measure stress waves, which is an application actually measuring distance. The article defines it as "a device that responds to or is sensitive to directed distance", which also matches the concrete definition.

Does anyone know if a carbon microphone is really a displacement receiver? And is a displacement transducer the same as a displacement sensor? Mrfoogles (talk) 19:56, 30 November 2024 (UTC)

The intended useful function of a Microphone is to sense incoming sound and deliver a proportional electric signal. As Sound is a varying pressure wave, some varying displacement occurs inside the microphone. However, a microphone is not normally intended or calibrated to measure its internal displacements. They are microscopic movements in the case* of carbon granules under pressure in a carbon microphone. I think it is as unreal (overparticularity) to call a Microphone, whether carbon or any other type, a displacement receiver as it is to call my Eardrum a Barometer. In general a Transducer converts energy from one form to another and receiving input is the first part and not the whole of its action. A Sensor must provide actual useful information about a specific physical phenomenon. * pun on "case" Philvoids (talk) 12:41, 4 December 2024 (UTC)

Smelly plasterboard

This BBC News article about a smelly landfill site quotes a chemist as saying "One of the materials that is particularly bad for producing odours and awful emissions is plasterboard". I thought that plasterboard was a fairly inert substance. Why would it cause bad odours in landfill? (I assume that this is not faulty plasterboard suffering from the in-use 'emission of sulfurous gases' mentioned in the WP article.) -- Verbarson  ^talk_edits 21:07, 30 November 2024 (UTC)

When mixed with biodegradable wastes like manure and sewage, gypsum can produce hydrogen sulphide gas, which is odorous and toxic, and a threat to public health.

Plasterboard Disposal: What You Need to Know

Perhaps somebody who understands the chemistry could add something to our article? Alansplodge (talk) 22:35, 30 November 2024 (UTC)

Well, gypsum is CaSO4·2H2O, which has a significant amount of sulfur and hydrogen in it, and hydrogen sulphide is just HS -- I imagine it's not too hard for a chemical reaction to release hydrogen sulphide gas and therefore as they occur they do. Probably there's a paper somewhere that goes over the various reactions that happen. Mrfoogles (talk) 01:07, 1 December 2024 (UTC)

Hydrogen sulfide (however you like to spell it:) is H₂S. According to our article about that chemical, it arises from gypsum by the action of sulfate-reducing microorganisms that are active "moist, warm, anaerobic conditions of buried waste that contains a high source of carbon". 11:48, 1 December 2024 (UTC) DMacks (talk)

1990s Cathode-ray TV questions.

In the late '90s / early 2000s I remember as a kid looking closeup to the TV screen. For The Simpsons, their yellow skin was red green red green lights next to each other to make yellow. You can't do this with the modern TVs now anymore, but what did cathode-ray TVs use for pink? Would it be dim red by itself, or all 3 colors? How do they make brown? And if Cathode rays can do red green red green, can they do for example, red red green, red red green? Thanks. 2603:8001:5103:AF08:2477:8D7F:1D4B:D0 (talk) 22:41, 30 November 2024 (UTC).

Current screens also describe colors mostly in RGB (red,green,blue) format, although I don't know the details of how they display it (see LCD for one method) -- this webpage lists some color codes for various shades of pink. It looks like they use full red, plus moderate levels of green and blue. Sort of like red + white. Mrfoogles (talk) 01:03, 1 December 2024 (UTC)

OLED displays use a variety of methods; see OLED § Color patterning technologies.  --Lambiam 03:08, 1 December 2024 (UTC)

Brown is basically a darker shade of orange. Whether this is perceived as brown depends strongly on the context. There is no such thing as a brown light; only surfaces of objects can appear brown.  --Lambiam 03:18, 1 December 2024 (UTC)

In photochemistry/photophysics, we can use dyes to make chemicals fluoresce non-spectral colors. Whether or not there is a brown dye is another question. But I believe pink dyes are known. 2603:8001:5103:AF08:2477:8D7F:1D4B:D0 (talk) 05:45, 1 December 2024 (UTC).

In straightforward terms, most human eyes have three color receptors — red, green and blue. The eye can be tricked into seeing any color of light by the right proportions of those three pure colors. The devil is in the details. Doug butler (talk) 06:41, 1 December 2024 (UTC)

It works out mathematically, but one of those details with a devil is that for some colour mixes you may need a negative amount of one of the primary colours – which is physically impossible. That's why some screens use a fourth colour in the mix. PiusImpavidus (talk) 10:35, 1 December 2024 (UTC)

Please see Gamut before declaring devilry. Philvoids (talk) 14:37, 1 December 2024 (UTC)

The colours are still red, green and blue, mixed in varying proportions. The exact hue may vary a bit and some screens add a fourth colour. The dots are pretty small though (maybe smaller than before; resolution has increased, but so have screen sizes) and you may no longer be able to watch them from as close as when you were a kid. Try a magnifying glass. PiusImpavidus (talk) 10:23, 1 December 2024 (UTC)

You're maybe thinking of printing, where the fourth color is black. Way off topic. The really cool thing about color tubes is how the manufacturer deposits the bunches of three phosphors on the inside of the glass screen. The (iron) shadow mask, with its millions of holes, is spaced a few mm back. Spray guns for each color, located where the electron guns will be located in the final manufacturing stage, blast their phosphors so a trio of dots get through each hole in the mask. Electrons from each gun that get through the mask will hit its respective phosphor. Costly, wasteful and inefficient but it worked. Doug butler (talk) 17:07, 1 December 2024 (UTC)

I remember a TV manufacturer telling they added yellow to the standard blue-green-red to be able to make more intense yellows. It makes sense, as the alternative would be driving the blue component to negative.

Professional printers, like those printing food packaging, often use around 6 colours, chosen specifically for the task. PiusImpavidus (talk) 09:32, 2 December 2024 (UTC)

You might be interested in Additive color and the RGB color model. -- Avocado (talk) 18:58, 4 December 2024 (UTC)

December 1

Fusion power critics

I've stumbled upon a few freak Russian critics in the internet who still allege that fusion power is principally impossible. Perhaps the most notorious seems to be Soviet-era physicist Igor Ostretsov, who published an article in a Russian scientific journal, "On the Lawson Criterion in Thermonuclear Research". Since Ostretsov's criticism is too technical for me, I started to wonder how much weight does it carry, if any. Ostretsov writes in particular:

"It is perfectly clear to every competent physicist that thermonuclear plasma, i.e. plasma at temperatures at which a thermonuclear reaction occurs, cannot be transparent. At thermonuclear temperatures, most of the energy is concentrated in radiation. In the article, I cited Zeldovich on this subject: “In complete thermal equilibrium, a significant portion of the energy is converted into radiation; this circumstance limits the equilibrium average energy of charged particles to a threshold of 5–15 keV, which is completely insufficient for a fast nuclear reaction. A slow nuclear reaction of light elements at an average energy of about 10 keV is practically impossible because the removal of energy by radiation during a slow reaction will lead to a rapid drop in temperature and a complete cessation of the reaction.” If the engineers of thermonuclear fusion in magnetic traps "secretly" assume not a thermonuclear reaction, but the synthesis of hydrogen isotopes in high-energy beams, then this is how the problem should be formulated and consider its "efficiency" as extremely ineffective. The Lawson criterion has nothing to do with that problem, since it was obtained for the Maxwellian distribution of particles by velocity, which is shown in my article".

In a letter to physicist Valery Rubakov Ostretsov further asserts that

1. The Lawson criterion was obtained for the Maxwellian distribution of particles by velocity, which is established as a result of dissipative processes (collisions). 2. As shown in my article, the particle velocity distribution function in magnetic "thermonuclear" traps is determined only by external constant and variable fields, and therefore is not Maxwellian. Due to points 1 and 2, the Lawson criterion has no relation to modern "thermonuclear" research.

Ostretsov also claims that the "during thermonuclear fusion reactions, high-energy neutrons constantly fly into the inner walls of tokamak" and "it's difficult to withstand such bombardment, while a thermonuclear reactor must operate for many years". Is anything of it true? Brandmeister^talk 16:57, 1 December 2024 (UTC)

Check who cites the article and see what they say. Abductive (reasoning) 19:23, 1 December 2024 (UTC)

There is an article about him in Russian Wikipedia. Based on it, he looks like some kind of freak. So, I think that his opinions can be safely ignored. Ruslik_Zero 20:40, 1 December 2024 (UTC)

Plasma confinement is a primary issue in the design of fusion reactors. If the plasma is insufficiently confined, which could happen in a badly designed reactor, but also due to a malfunction, the inner walls will briefly be bombarded by high-energy neutrons. But insufficient confinement also means that the fusion process stops. Of course there will always be some stray neutrons, however excellent the confinement may be. Whether the damage they inflict significantly limits the lifetime of a reactor cannot be predicted without a detailed study of the specific design of a given reactor, but this is not an issue that the designers are somehow unaware of.  --Lambiam 15:27, 3 December 2024 (UTC)

Neutrons, being electrically neutral particles, are not confined by magnetic field. They will just freely leave the reactor's volume. So, 17.6 MeV neutrons will constantly bombard the walls of the reactor. This is a serious problem but it is thought to be solvable. Ruslik_Zero 20:28, 4 December 2024 (UTC)

And something else that has to put up with neutrons for years-to-decades: fission reactors. And particle accelerators. Neutron embrittlement is a known problem, but it's an "engineering problem" kind of thing: we have ways to build things that have acceptable tolerances to certain amounts of it. It's just a question of how feasible it is. At least with the neutron stuff he's actually answering a different question: "how feasible is X", not, "is X physically possible in this universe or is it impossible". Very hard: designing and building a rocketship to Mars and getting it there intact. Impossible: eating the Sun. --Slowking Man (talk) 04:34, 12 December 2024 (UTC)

December 2

Velocity and acceleration in special relativity

I was thinking that acceleration can always cause time dilation (clocks tick slower) in special relativity but when I tried to imagine the following, I got confused.

Imagine 3 frames A, B, C such that frame A is our ancestors stationary frame, B is an intermediate frame with velocity v1 relative to A, and C is our stationary frame after our ancestors traveled to it with a precise clock. Frame C has a relative velocity v2>v1 (all are in the x direction, in empty space without gravitational effects for simplicity).

We were born in Frame C without knowing anything about our ancestors journey and we decided to visit Frame A. (Accelerating first to frame B then decelerating to frame A). In this case how come we will have another time dilation (additional slow ticking in clock) while we were just travelling back to the original (supposedly stationary frame)?

We are supposed to assume that we were stationary in frame C without knowing the truth, and so we will assume that we will have time dilation during our journey from C to A not the reverse (and if I am right then even our ancestors should not had been confident that they had time dilation unless they witnessed it). I hope you can explain where I got wrong.Almuhammedi (talk) 20:05, 2 December 2024 (UTC)

The essence of the theory of relativity is that notions such as velocity are only meaningful relative to the frame of reference of an observer. Observers using different frames will measure different values. This is not a matter of being right or wrong. It is meaningless to say that an observer is stationary in their frame of reference "without knowing the truth". They are stationary by definition. Time dilation of a moving clock can only be observed from a frame of reference relative to which the clock is moving. For an observer holding the clock, the clock is not moving, so they will not themselves observe time dilation during their journey. Only outside observers can observe this.  --Lambiam 01:40, 3 December 2024 (UTC)

I introduced the 3 frames to simulate what happens to an atomic clock on a traveling plane.

Of course there is a reference relatively (stationary clock) that is supposed to show the difference.

In this case assume that our ancestors traveled with 2 atomic clocks x, y to frame C but we used only one of their clocks, x to travel to frame A and then returned back with it to frame C.

From our perspective, we considered the travelling clock (x) as the accelerated clock (as well as us) which should suffer time dilation after returning to our frame C.

However, to an external observer relatively stationary to frame A, who witnessed our ancestors travel he will understand that Clock x only reduced its speed when traveled to its original frame A and then returned to frame C which means it suffered temporary less time dilation than clock y.Almuhammedi (talk) 06:50, 3 December 2024 (UTC)

So there are two clocks at C that show the same time. One clock, y, remains at rest at C. The other clock, x, is moved from C to A and back to C. Then, on return, x will be running behind y. What happened before x's journey from C to A and back is not relevant.  --Lambiam 15:14, 3 December 2024 (UTC)

What makes you so sure?

Just return both clocks to their original frame A and compare the results with a third stationary clock in frame A. I think you will see the opposite of what you you've said. Almuhammedi (talk) 16:50, 3 December 2024 (UTC)

I may have some confusion between acceleration and deceleration here which caused my wrong conclusion.Almuhammedi (talk) 17:52, 3 December 2024 (UTC)

I suggest that you read our article on the twin paradox. BTW, I think that the (sourced) statement that "[t]here is still debate as to the resolution of the twin paradox" is misleading. The twin paradox is only paradoxical in the sense that it is a counterintuitive effect predicted by the laws of both special and general relativity. The issue is that the explanations commonly provided – other than "this is what the laws tell us; do the maths yourselves" – are ad hoc explanations for special cases and do not cover all conceivable scenarios exhibiting the counterintuitive effect.  --Lambiam 08:54, 4 December 2024 (UTC)

Snow questions

Two questions related to snow that I have wondered in recent times, not homework.

1. Why do most European countries lack snowfall data in their weather observations? Without data, snowfall cannot be specified since snowfall is not same as change of snow depth from one day to next.
2. Can Lake Geneva, Lake Constance and Balaton ever produce lake-effect snow? --40bus (talk) 21:58, 2 December 2024 (UTC)

@40bus 1. Presumably because in a temperate climate it's almost impossible to measure. What falls as snow on higher ground (which may or may not settle as snow) may fall as sleet or rain on lower ground, or it will turn to water or ice in the rain-gauge. Shantavira|^{feed me} 10:01, 3 December 2024 (UTC)

But US, Canada and Japan have continental climate (at least in some areas), so why then they measure? And is snowfall deducible from precipitation value so that 5 mm of precipitation equals 5 cm of snowfall? --40bus (talk) 10:54, 3 December 2024 (UTC)

No, not accurately. Snow comes in many different consistencies and levels of moisture, from tiny dry flakes to huge wet masses that fall as almost pre-made snowballs. Our (Canada) weather forecasts include estimates for amounts of snow to land, but they're hilariously inaccurate for the simple reason that snow, unlike liquid water, can pile up and drift. We had a dumping of snow this past weekend and the thickness of snow on one varied quite a bit just across the width of my driveway. So, should the record show the 15 cm in my front yard, the 10 cm in my driveway or the 8 cm in my neighbour's driveway? Depending on the type of snow falling, that ratio would change as well. Matt Deres (talk) 18:15, 3 December 2024 (UTC)

"Hilariously inaccurate" seems a gross exaggeration to me. The measurement should indicate the average depth of new snow over an area large enough that the variations between your front yard, your driveway, and the next driveway are irrelevant. --142.112.149.206 (talk) 09:17, 4 December 2024 (UTC)

Spoken like someone unfamiliar with snow. It's not really a knock on the forecasters; it's just the nature of the material. To measure rainfall, it's not so complicated: rain may get blown about, but it typically only lands once. Not so with snow. It lands, gets picked up, lands, gets picked, and so on. If you picked a spot in your yard to measure, you'd find the level going up and down as the day transpired. So, from 6pm to midnight you'd get 10 cm of accumulation, then from midnight to 6am you'd get -3 cm of accumulation. Rain also doesn't "pile up" in areas. It lands unevenly, of course, but that hardly matters because it drains and gets absorbed. Snow piles up in chaotic ways, depending on the wind, the nature of the snow, and the terrain. Some of the worst whiteout conditions occur when there's no precipitation at all. Matt Deres (talk) 20:21, 4 December 2024 (UTC)

True, but irrelevant to reporting or predicting the amount of snow that falls. Which I was shoveling today, by the way. You accuse the forecast of inaccuracy because it does not report what you want it to, that's all. --142.112.149.206 (talk) 06:23, 5 December 2024 (UTC)

I'm not accusing them of anything; just reporting the plain fact that there's no accurate way of measuring it. If we could easily see accumulations of rain, we'd recognize that they too are broad estimates. Snow is worse, as I've detailed above. We just don't have a methodology for measuring snowfall that accounts for the fact that the amount that came out of the clouds bears little resemblance to what builds up on the ground. Matt Deres (talk) 16:11, 6 December 2024 (UTC)

The Dutch weather office collects hourly snowfall data at some (not all) staffed weather stations, most of them at airfields, but apparently not at the more common unstaffed weather stations or the even more common precipitation stations. Maybe it's hard to measure automatically.

Snow can fall in temperatures slightly above freezing, rain can fall slightly below freezing, so the combination of precipitation and frost doesn't tell you about snow. Usually the snow melts within hours. On most days with frost, it only freezes part of the day; we used get about 50 freeze-thaw cycles per year in the east of the country, fewer along the sea, but I think that has halved in recent years. PiusImpavidus (talk) 14:54, 3 December 2024 (UTC)

Re your question 2 - According to our article that you linked above "a fetch of at least 100 km (60 mi) is required to produce lake-effect precipitation". Lake Geneva, the largest lake in Europe, is only 95 km (59 mi) along its longest side (it's crescent-shaped, so the longest straight line would be somewhat shorter), so it seems unlikely (FYI: "fetch" is the distance that an air mass travels over a body of water). Alansplodge (talk) 21:15, 4 December 2024 (UTC)

What's more, any lake effect would be overwhelmed by the effect of the surrounding mountains. This would also be the case for Lake Constance. Lake Balaton has no surrounding mountains, but is only 75 km long and so shallow that it can cool quickly, reducing the lake effect. There are several larger lakes in the north-east of Europe (Vänern, Vättern, Ladoga, Onega).

BTW, interesting etymology. Lake Geneva, a name appearing only in the 16th century, is named after the English exonym for the city of Genève, derived from Latin Genava and originally Celtic Genawa (compare the Italian city of Genova). The older local name of the lake is Léman, from a (Celtic?) word for lake, or pleonastically Lac Léman (already Lacus Lemanus in Roman times). Lake Constance, a name in use since the 15th century, is named after the German city of Konstanz, in English known by its French exonym Constance, derived from Latin Constantia, probably after emperor Constantius. Locally, the lake is since the 6th century known as something like Bodensee. Names from Roman times are known, but no longer in use. PiusImpavidus (talk) 11:22, 5 December 2024 (UTC)

December 3

How long is this problem in molecular biology?

In 2016, DeepMind turned its artificial intelligence to protein folding, a long-standing problem in molecular biology.

How long is this problem in molecular biology? Source HarryOrange (talk) 10:20, 3 December 2024 (UTC)

Even before the process of protein biosynthesis was discovered, it was known that small changes in the amino acid sequence could lead to major changes in protein structure. How the amino acid sequence determined the protein structure was an open question, but at the time one with no practical relevance, initially drawing little theoretical interest. That changed in 1969 when Cyrus Levinthal published the paper that gave rise to the term Levinthal's paradox. With the possibility to edit genes and synthesize proteins in the lab, it has now also become a problem of high practical relevance, but 1969 is a good starting date for the standing of the problem.  --Lambiam 15:05, 3 December 2024 (UTC)

I just came across this YouTube video: "How AI Cracked the Protein Folding Code and Won a Nobel Prize". It also gives the history of the problem.  --Lambiam 09:20, 6 December 2024 (UTC)

December 5

Birds with white cheeks

• 
  Silver-eared Mesia
• 
  White-cheeked Starling
• 
  Great Tit
• 
  White-cheeked Bushtit
• 
  White-cheeked Bullfinch
• 
  White-cheeked Bulbul

What is the evolutionary advantage - or purpose - of white "cheeks" on these disparate birds? Andy Mabbett (Pigsonthewing); Talk to Andy; Andy's edits 14:54, 5 December 2024 (UTC)

In great tits, the immaculateness of the black border of white cheek patches predicted social status and reproductive success, but there was no clear evidence that it played a role in mate choice (Ferns and Hinsley 2004).

Bird Coloration, Volume 2 (p. 186)

Alansplodge (talk) 15:47, 5 December 2024 (UTC)

Here's recent a review article about what's known about the genetics of bird color patterning. We know a lot less about this topic than about the genetics of patterning in insect wings. It strikes me that all birds follow that same general pattern scheme, with only the colors varying. So in a bird that is all one color, the scheme is there, but not apparent. As for the face, there are many selection pressures that could be occurring–or that might have occurred in the past–to be tested. First, if the pattern is found only in males, there's a good chance it is sexually selected (some trait is getting sexually selected for, but the face color might just be genetically or developmentally tied to it and just along for the ride). In some species, fights between males drive selection, and drawing one's opponents to peck somewhere other than the eyes would be strongly selected for. If female choice is strong, then costly-to-maintain signals are selected for. But there is also selection for confusing predators (such as about the size and position of the eyes), and for confusing prey. Finally, the feathers near the beak get a lot more wear and tear, so need to get replaced more often. Skipping adding color might make this process faster and/or cheaper. All this is guesswork on my part so make of it what you will. Abductive (reasoning) 19:09, 5 December 2024 (UTC)

Time dilation

I can't seem to get a straight answer: How many parts per trillion between Earth's most time travelly places+where are they? (1 answer for all points a "stationary" non-"antigraviting" (i.e. helicopter/airship) human could be that exist now (i.e. Mammoth Cave/the Chunnel/2 WTC's temporary roof but not the much higher place the permanent roof's planned to be or 10ft below the deepest ice dig a human could put their body. Humans could theoretically go 10ft lower but not as is), 1 answer for if under liquids also doesn't count Mariana Trench=sea level)

Some ppl say everywhere on an equipotential surface has the same speed of time from the 2 dilations canceling out. So Everest+Mariana should be extremest? Or the Kidd Creek Mine if under liquids doesn't count. I haven't been able to reproduce cancellation with the formulae or calculators though. Some gravitational dilation calculators want distance to center which is NOT geopotential (Chimborazo's furthest, Arctic seabed closest, or North Pole if has to touch air), some want g-force???. It's not g-force unless that calculator only works for the surfaces of spheres. Earth's gravitational dilation's strongest at the base of the gravity well where you'd be weightless. Google AI dumbass can be made to say both ellipsoid+geoid for the equal dilation surfaces. Some human who might know says it's the geoid. Some probably different human I don't remember says it's only equipotential on one of rotating vs inertial reference frame. How the hell can it depend on reference frame? Clocks can't both be later than each other when they reunite (very slowly to infintesimalize kinematic dilation from the trip). Some clock pair has to be most disparate when they reunite. Maybe it can still depend in some way without violating this logic? Presumably Cayambe's the place with the most kinematic time dilation? Furthest point of Earth's surface from the axis. Presumably axis points avoid more kinematic time dilation than any other points of the planet? Sagittarian Milky Way (talk) 00:20, 6 December 2024 (UTC)

Although the Earth can be considered a rotating sphere, I think the effect of its rotation on gravitational time dilation is small. Using the formula at Gravitational time dilation § Outside a non-rotating sphere, I compute that the fractional difference is about 1.1 × 10⁻¹⁶ per metre height difference (above sea level). The fractional difference of time dilation by the velocity difference between the poles and the equator is about 1.2 × 10⁻¹², so this will beat gravitational time dilation.  --Lambiam 02:41, 6 December 2024 (UTC)

How is Rainbow considered as application ?

How is Rainbow considered as application ? Source

I believe Rainbow is just a Rainbow, not a something to use. HarryOrange (talk) 22:42, 5 December 2024 (UTC)

The Okapi Framework has an app named "Rainbow", which we describe by, "Rainbow — a toolbox to launch a large variety of localization tasks." (Other than this I know nothing about Okapi and its app.)  --Lambiam 01:48, 6 December 2024 (UTC)

The link to the article about rainbows has been in the "applications" section from the start, in this edit, where the applications listed were Rainbow, Cosmic microwave radiation, Laser, and Laser fusion. The first two of those are phenomena, not technologies, so it's certainly unclear how to apply equations to them - with what end in mind? Subsequently Radio wave, Gravitational lens, and Black-body radiation joined the list. Although radio waves are phenomena there are many technological things we might seek to do with them, and in the course of trying to make things work we might need numbers that come from an equation. In other cases the application might simply be to obtain numbers, to study a phenomenon like radiation. But I agree, I can't imagine in what way we could even investigate a rainbow with these equations, and so I don't understand how it's an "application". I think it might be a reference to this Feynman lecture. Near the bottom is a discussion of rainbows:

“While I’m on this subject I want to talk about whether it will ever be possible to imagine beauty that we can’t see. It is an interesting question. When we look at a rainbow, it looks beautiful to us. Everybody says, “Ooh, a rainbow.” (You see how scientific I am. I am afraid to say something is beautiful unless I have an experimental way of defining it.) But how would we describe a rainbow if we were blind? We are blind when we measure the infrared reflection coefficient of sodium chloride, or ...”

Then

“On the other hand, even if we cannot see beauty in particular measured results, we can already claim to see a certain beauty in the equations which describe general physical laws. For example, in the wave equation (20.9), there’s something nice about the regularity of the appearance of the x, the y, the z, and the t. And this nice symmetry in appearance of the x, y, z, and t suggests to the mind still a greater beauty which has to do with the four dimensions, the possibility that space has four-dimensional symmetry, the possibility of analyzing that and the developments of the special theory of relativity. So there is plenty of intellectual beauty associated with the equations.”

So, OK. But it's tenuous, and would be better removed or explained.  Card Zero  (talk) 05:15, 6 December 2024 (UTC)

The disambiguation page for Rainbow treats the various uses of the word equitably without over indulgence in any isolated usage such as the artistic to the unfair extent of shunning the physical reality that the electromagnetic wave understanding of light is the physicist's most applicable tool and that for this its equations are fundamental. Philvoids (talk) 11:47, 6 December 2024 (UTC)

OK? But this question is about Electromagnetic_wave_equation#Applications (which is easily missed, since it's hidden under the word "source"). Should that really list "rainbow" as an "application"?  Card Zero  (talk) 12:37, 6 December 2024 (UTC)

I agree not, and others in the 'Applications' list are also inappropriate ('black hole'?). Perhaps a further list of 'Phenomenon' (or similar) should be created? {The poster formerly known as 87.81.230.195} 94.1.211.243 (talk) 13:20, 6 December 2024 (UTC)

That's Black-body radiation, but yeah.  Card Zero  (talk) 15:03, 6 December 2024 (UTC)

That stuff was added on Feb 9, 2006,[1] by a user who's no longer active. But if their email is available, someone could try sending them a note. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:42, 6 December 2024 (UTC)

December 6

Geodesics for Massive and Massless Particles

In general relativity, do massive and massless particles follow the same geodesic? Why or why not? Malypaet (talk) 23:19, 6 December 2024 (UTC)

According to the Einstein field equations, the worldline traced by a particle not subject to external, non-gravitational forces is a geodesic. Each particle follows its own worldline. Two particles that share their worldline are at all times at the same location and so have identical velocities.  --Lambiam 08:46, 7 December 2024 (UTC)

A massless particle must follow a null geodesic and massive particle must follow a time-like geodesic (in my limited understanding). catslash (talk) 22:20, 7 December 2024 (UTC)

So a massive particle with a velocity infinitely close to that of a photon (under the influence of a massive object) will have a geodesic infinitely close to that of the photon, right? Or is there another explanation and which one? Malypaet (talk) 22:11, 9 December 2024 (UTC)

I believe that is correct (perhaps there is an expert to hand who could confirm this?). catslash (talk) 23:42, 9 December 2024 (UTC)

In some frame of reference, the massive particle is at rest and so its spacetime interval along its geodesic is as spacelike time-like as can be (and thereby as non-null-like as can be for a non-tachyonic particle). So it depends on the point of view of the observer. Simplifying the case to special relativity and considering a particle traveling with speed $v$ in the x-direction, the spacetime interval $\Delta {s}$ between two events separated by a time $\Delta t$ is given by:

$(\Delta s)^{2}=(\Delta ct)^{2}-(\Delta x)^{2}=(\Delta ct)^{2}-(\Delta vt)^{2}=(c^{2}-v^{2})(\Delta t)^{2}.$

In frames of reference in which $v$ approaches $c,$ the interval can become arbitrarily small, making it experimentally indistinguishable from that of a massless particle.  --Lambiam 07:40, 12 December 2024 (UTC)

@User:Lambian, could you re-read the spacetime interval section? I reckon that if there exists a frame of reference in which an interval is purely a time difference, then it is time-like, and if there exists a frame of reference in which the interval is purely a difference in location, then it is space-like. catslash (talk) 10:14, 12 December 2024 (UTC)

Yes, I used the wrong term, now corrected.  --Lambiam 07:30, 13 December 2024 (UTC)

December 7

Source

The articles Radium dial and Radium Girls blithely speak of the element as though infinitesimal quantities of pure metal were employed, whereas the iron law of economics dictate that some partially processed yellowcake with a minuscule (and difficult to extract) percentage of some radium salt would be the raw material. Does someone have this information? Doug butler (talk) 22:02, 7 December 2024 (UTC)

The paint, marketed as Undark, was a powdery mixture of radium sulfate, zinc sulfide and phosphor.^[2] The young women had to mix this powder with water and glue before it could be applied. The radium-226 percentage had to be high enough to produce sufficient luminosity. For its pernicious effect, its chemical form is immaterial.  --Lambiam 23:19, 7 December 2024 (UTC)

the chemical form is mostly immaterial. Radium sulfate is insoluble enough that it's unable to get a hold in the physiology and so has only minimum effects. 176.0.131.138 (talk) 09:45, 8 December 2024 (UTC)

Because radium is not an actinide it can be easily separated from the other elements. So the economic pressure is not to give away something to a customer what you can sell to another customer. 176.0.131.138 (talk) 09:52, 8 December 2024 (UTC)

December 8

Unit questions

1. How widely is the metric system used in the Philippines? Do people there use metric for both short and long distances? Is centimeter a widely used unit in the Philippines? Does Philippines use metric mass and volume units almost exclusively?
2. How widely is the metric system in former British colonies in Africa (Gambia, Sierra Leone, Ghana, Nigeria, Rwanda, Uganda, Kenya, Tanzania, Malawi, Zambia, Zimbabwe, Botswana, Namibia, South Africa, Eswatini, Lesotho)? Are there still some applications for which some people might use imperial units?
3. How widely is the metric system used in Caribbean island countries? Do these countries use imperial system widely?
4. Is there any application that commonly uses fractions with metric units?
5. Can exact one-third of a meter be measured in most devices, as its decimal representation contains just repeating threes? --40bus (talk) 20:56, 8 December 2024 (UTC)

It's worth pointing out that item 5 is one reason the English System is preferable, because feet, yards and miles, as well as acres, are easily divided by 3. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:19, 8 December 2024 (UTC)

This Australian, having now worked with the metric system for two thirds of his longish life, has never screamed "I wish this unit was divisible by three!" HiLo48 (talk) 06:58, 9 December 2024 (UTC)

Is there any metric unit, other than units of time, which is easily divisible by 3? --40bus (talk) 06:14, 9 December 2024 (UTC)

1 metre is easily divided by 3. A third of a metre is 1/3 meter. Do you mean 1/3 meter cannot be precisely written in decimal form? Just use fractions. problem solved. 2001:8003:429D:4100:186E:C147:C792:1055 (talk) 09:25, 9 December 2024 (UTC)

The Metric system article lists the basic units. For several of them, division by 3 doesn't seem like it would be all that useful. Temperature, for example. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:28, 9 December 2024 (UTC)

1. Have you read Metrication? The article says The Philippines first adopted the metric system in 1860 because of the Spanish Colonial government; imperial units were introduced by the American Colonial government; however, the metric system was made the official system of measurement in 1906 through Act No. 1519, s. 1906. US customary units still in use for body measurements and small products while the metric system is used for larger measurements; e.g. floor area, highway length, tonnage. Shantavira|^{feed me} 09:30, 9 December 2024 (UTC)

December 9

I'm collecting examples of a purely "physical property of a physical property" (of a body).

By (purely) physical property, I mean any measured property whose measurement depends on (purely) physical [dimensions usually measured by physical] units. A few examples of physical properties include: momentum, energy, electric charge, magnetic charge, velocity, and the like (actually the elementary particles carry plenty of purely physical properties).

However, by purely (physical property), I mean that it's not also a mathematical or geometric property, i.e. excluding: numeric value (size) of a physical property, density of energy ("density" is also a mathematical concept - e.g. in density of primes), center of mass ("center" is also a geometric concept), and the like. But I do consider velocity to be a purely physical property, because its description invloves (e.g.) the temporal dimension (which actually "flows" - whereas the way time "flows" can't be described by any mathematical equation. Anyway this "flow" is another issue I don't want to discuss in this thread).

So, for finding a purely "physical property of a physical property" (of a body), I've thought about one example so far: the physical units dimensions of any physical property.

I'll be glad for any additional examples. 2A06:C701:746D:AE00:ACFC:490:74C3:660 (talk) 11:22, 9 December 2024 (UTC)

The physical units in which physical quantities are expressed (such as erg, eV, foe, joule, therm) are somewhat arbitrary social constructions. The dimension of a physical quantity is a much more purely physical property. It is a point in an abstract vector space. One may argue that there is some arbitrariness in the choice of the basis of this space. The SI standard uses time (${\mathsf {T}}$), length (${\mathsf {L}}$), mass (${\mathsf {M}}$), electric current (${\mathsf {I}}$), absolute temperature (${\mathsf {\Theta }}$), amount of substance (${\mathsf {N}}$) and luminous intensity (${\mathsf {J}}$) as the basis, but other choices for the base physical dimensions span the same vector space.  --Lambiam 12:42, 9 December 2024 (UTC)

Yes, I really meant "dimensions" of a physical property, thank you. 2A06:C701:746D:AE00:ACFC:490:74C3:660 (talk) 14:24, 9 December 2024 (UTC)

December 10

Proton decay and cosmic expansion

A friend's physicist father opined that the phantom energy causing more and more rapid cosmic expansion will never be as strong as the attraction of the strong force, so protons will not be ripped apart in the big rip. Be that as it may, if the phantom energy is counter to the strong force, however weakly, wouldn't protons, consisting of quarks held together by the strong force, have an increased rate of decay in the far future? I have heard that the theories that protons do undergo decay at all have not yet been supported by experiments, though. Rich (talk) 13:41, 10 December 2024 (UTC)

We have to suppose quite a few things to get to the question: suppose there is some form of proton decay, suppose there is phantom energy, and suppose that the phantom energy reaches some plateau before getting to an energy scale high enough to create a quark-gluon plasma. Would protons then decay at a faster rate? I don't think that's necessarily the case. Proton decay is not the same kind of process as making a quark-gluon plasma. I believe the answer depends on what kinds of operators lead to the hypothetical proton decay. --Amble (talk) 22:49, 10 December 2024 (UTC)

Thanks, nice clarification of the issues. You've thought through the issues more clearly and knowledgeably than I did. That's a valuable answer. But having said that, is there more information available about current speculations and theoretical work by physicists concerning proton decay interacts with cosmic expansion? I can't be the only one wondering about it and many of the people wondering about it would be physicists.Rich (talk) 07:30, 13 December 2024 (UTC)

The nearest paper I came across is [3], but there "proton decay" actually means p⁺ → n + e⁺ + ν and not p⁺ → e⁺ + 2γ. --Amble (talk) 20:22, 13 December 2024 (UTC)

December 13

What is the most iconic tornado photo

Request for opinions
The following discussion has been closed. Please do not modify it.
What photo of a tornado would you say is the most iconic? I'm researching the history of tornado photography for an eventual article on it and I've seen several specific tornadoes pop up over and over again, particularly the Elie, Manitoba F5 and the "dead man walking" shot of the Jarrel, Texas F5. Which would be considered more iconic? ApteryxRainWing🐉 | Roar with me!!! | My contributions 17:21, 13 December 2024 (UTC) At the top of this page is a bullet point stating "We don't answer requests for opinions, predictions or debate": this reads to me like a request for subjective opinions. Perhaps you would like to consider what quantifiable and referenceable metric would answer what you want to know? Presumably you also want only real tornadoes considered? Otherwise some might nominate the the twister from The Wizard of Oz, or from more recent tornado-related movies – Sharknado, anyone? :-). {The poster formerly known as 87.81.230.195} 94.1.223.204 (talk) 18:07, 13 December 2024 (UTC) "Swegle Studios" has a couple of YouTube videos dedicated to the backstories of famous tornado photos and video; you might find them useful in your research. Photos, Videos. Matt Deres (talk) 18:40, 13 December 2024 (UTC) I googled "most iconic tornado photo" and a bunch of different possibilities popped up. I don't see how you could say that any given photo is the "most iconic". ←Baseball Bugs What's up, Doc? carrots→ 18:57, 13 December 2024 (UTC)