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December 29

Potential energy vs. kinetic energy. Why not also "potential velocity" vs. "kinetic velocity"? E.g. in the following case:

In a harmonic oscillator, reaching the highest point involves - both a minimal kinetic energy - along with a maximal potential energy, whereas reaching the lowest point involves - both a maximal kinetic energy - along with a minimal potential energy. Thus the mechanical energy becomes the sum of kinetic energy + potential energy, and is a conserved quantity.

So I wonder if it's reasonable to define also "potential velocity" vs. "kinetic velocity", and claim that in a harmonic oscillator, reaching the highest point involves - both a minimal "kinetic velocity" (i.e. involves what we usually call a rest) - along with a maximal "potential velocity", whereas reaching the lowest point involves - both a maximal "kinetic velocity" (i.e. involves what we usually call the actual velocity) - along with a minimal "potential velocity". Thus we can also define "mechanical velocity" as the sum of "kinetic velocity" + "potential velocity", and claim that the mechanical velocity is a conserved quantity - at least as far as a harmonic oscillator is concerned.

Reasonable?

Note that I could also ask an analogous question - as to the concept of "potential momentum", but this term is already used in the theory of hidden momentum for another meaning, so for the time being I'm focusing on velocity.

HOTmag (talk) 12:26, 29 December 2024 (UTC)

'kinetic velocity' is just 'velocity'. 'potential velocity' has no meaning. Andy Dingley (talk) 13:56, 29 December 2024 (UTC)

Per my suggestion, the ratio between distance and time is not called "velocity" but rather "kinetic velocity".

Further, per my suggestion, if you don't indicate whether the "velocity" you're talking about is a "kinetic velocity" or a "potential velocity" or a "mechanical velocity", the very concept of "velocity" alone has no meaning!

On the other hand, "potential velocity" is defined as the difference between the "mechanical velocity" and the "kinetic velocity"! Just as, this is the case if we replace "velocity" by "energy". For more details, see the example above, about the harmonic oscillator. HOTmag (talk) 15:14, 29 December 2024 (UTC)

You could define the potential velocity of a body at a particular height as the velocity it would hit the ground at if dropped from that height. But the sum of the potential and kinetic velocities would not be conserved; rather $v_{\mathrm {tot} }={\sqrt {v_{p}^{2}+v_{k}^{2}}}$ would be constant. catslash (talk) 18:54, 29 December 2024 (UTC)

Thank you. HOTmag (talk) 20:07, 29 December 2024 (UTC)

'Potential velocity' has no meaning. You seem to be arguing that in a system where energy is conserved, but is transforming between kinetic and potential energy, (You might also want to compare this to conservation of momentum.) then you can express that instead through a new conservation law based on velocity. But this doesn't work. There's no relation between velocity and potential energy.

In a harmonic oscillator, the potential energy is typically coming from some central restoring force with a relationship to position, nothing at all to do with velocity. Where some axiomatic external rule (such as Hooke's Law applying, because the system is a mass on a spring) happens to relate the position and velocity through a suitable relation, then the system will then (and only then) behave as a harmonic oscillator. But a different system (swap the spring for a dashpot) doesn't have this, thus won't oscillate. Andy Dingley (talk) 00:00, 30 December 2024 (UTC)

Let me quote a sentence from my original post: Thus we can also...claim that the mechanical velocity is a conserved quantity - at least as far as a harmonic oscillator is concerned.

What's wrong in this quotation? HOTmag (talk) 07:52, 30 December 2024 (UTC)

It is true, not only for harmonic oscillators, provided that you define v_pot = − v_kin.  --Lambiam 09:07, 30 December 2024 (UTC)

• You have defined some arbitrary values for new 'velocities', where their only definition is that they then demonstrate some new conservation law. Which is really the conservation of energy, but you're refusing to use that term for some reason.

As Catslash pointed out, the conserved quantity here is proportional to the square of velocity, so your conservation equation has to include that. It's simply wrong that any linear function of velocity would be conserved here. Not merely we can't prove that, but we can prove (the sum of the squares diverges from the sum) that it's actually contradicted. For any definition of 'another velocity' which is a linear function of velocity.

Lambiam's definition isn't a conservation law, it's merely a mathematical identity. The sum of any value and its additive inverse is always zero. Andy Dingley (talk) 14:04, 30 December 2024 (UTC)

It is a law of conservation of sanity. Lacking a definition of potential energy, other than by having been informed that kinetic energy + potential energy is a conserved quantity, there is not much better we can do.  --Lambiam 11:20, 31 December 2024 (UTC)

We have a perfectly viable definition of potential energy. For a pendulum it's based on the change in height of the pendulum bob against gravity. For some other oscillators it would involve the work done against a spring. Andy Dingley (talk) 16:33, 31 December 2024 (UTC)

Oops, I mistyped. I meant to write:

"Lacking a definition of potential velocity, other than by having been informed that kinetic velocity + potential velocity is a conserved quantity, there is not much better we can do."

 --Lambiam 23:32, 31 December 2024 (UTC)

December 30

Saltiness comparison

Is there some test one might easily perform in a home test kitchen to compare the saltiness (due to the concentration of Na⁺ cations) of two liquid preparations, without involving biological taste buds?  --Lambiam 09:22, 30 December 2024 (UTC)

Put two equally sized drops, one of each liquid, on a warm surface, wait for them to evaporate, and compare how much salt residue each leaves? Not very precise or measurable, but significant differences should be noticeable. {The poster formerly known as 87.81.230.195} 94.1.223.204 (talk) 10:21, 30 December 2024 (UTC)

The principle is sound, but the residue from one drop won't be measurable using kitchen equipment -- better to put equal amounts of each liquid in two warm pans (use enough liquid to cover the bottom of each pan with a thin layer), wait for them to evaporate and then weigh the residue! Or, if you're not afraid of doing some algebra, you could also try an indirect method -- bring both liquids to a boil, measure the temperature of both, and then use the formula for boiling point elevation to calculate the saltiness of each! 2601:646:8082:BA0:BD1B:60D8:96CA:C5B0 (talk) 18:22, 30 December 2024 (UTC)

Presumably the liquid preparations are not simple saline solutions, but contain other solutes - or else one could simply use a hydrometer. It is unlikely that Lambian is afraid of doing some algebra. catslash (talk) 18:57, 30 December 2024 (UTC)

Assuming the liquid preparations are water-based and don't contain alcohols and/or detergents one can measure their rates of dispersion. Simply add a drop of food dye to each liquid and then time how rapidly droplets of each liquid disperse in distilled water. Materials needed: food dye, eye dropper, distilled water, small clear containers and a timer. Modocc (talk) 21:09, 30 December 2024 (UTC)

The colligative properties of a solution will indicate its molarity, but not identify the solute. Liquid preparations that might be found in a kitchen are likely to contain both salt and sugar. Electrical conductivity is a property that will be greatly affected by the salt but not the sugar (this does not help in distinguishing Na⁺ from K⁺ ions though). catslash (talk) 22:23, 30 December 2024 (UTC)

That's what I'm thinking too -- use an ohmmeter to measure the electrical conductivity of the preparation, and compare to that of solutions with known NaCl concentration (using a calibration curve-type method). 73.162.165.162 (talk) 20:18, 31 December 2024 (UTC)

Quantitative urine test-strips for sodium seem to be available. They're probably covering the concentration range of tens to hundreds millimolar. DMacks (talk) 00:58, 2 January 2025 (UTC)

Thanks, test strips seem more practical in the kitchen setting than an ohmmeter (why not call it a "mhometer"?), for which I'd need to devise a way (or so I think) to keep the terminals apart at a steady distance. Test strips require a colour comparison, but I expect that a significant difference in salinity will result in a perceptible colour difference when one strip is placed across the other. Only experiment can tell whether this expectation will come true. Salinity is usually measured in g/L; for kitchen preparations a ballpark figure is 1 g/L. If I'm not mistaken this corresponds to (1 g/L) / (58.443 g/mol) ≈ 0.017 M = 17 mM. I also see offers for salinity test strips, 0–1000 ppm, for "Science Education".  --Lambiam 11:40, 2 January 2025 (UTC)

Test strips surely come with a printed color-chart. But if all you are trying to do is determine which is more salty, then that's even easier than quantifying each separately. Caveat for what you might find for sale: some "salinity" tests are based on the chloride not the sodium, so a complex matrix that has components other than NaCl could fool it. DMacks (talk) 18:44, 2 January 2025 (UTC)

The (uncommon?) terms "relativistic length", and "relativistic time".

1. In Wikipedia, the page relativistic length contraction is automatically redirected to our article length contraction, which actually doesn't mention the term "relativistic length" at all. I wonder if there is an accepted term for the concept of relativistic length.

2. A similar qusestion arises, at to the concept of relativistic time: The page relativistic time dilation, is automatically redirected to our article time dilation, which prefers the abbreviated term "time dilation" (59 times) to the term "relativistic time dilation" (8 times only), and nowhere mentions the term "relativistic time" alone (i.e. without the third word "dilation") - although it does mention the term "proper time" for the shortest time. Further, this article doesn't even mention the term "dilated time" either. It does mention, though, another term: coordinate time, but regardless of time dilation in Special relativity. To sum up, I wonder what's the accepted term used for the dilated time (mainly is Special relativity): Is it "coordinate time"? "Relativistic time"?

HOTmag (talk) 09:32, 30 December 2024 (UTC)

Are you reading these things as "contraction of relativistic length" etc.? It is "relativistic contraction of length" and "relativistic dilation of time". --Wrongfilter (talk) 09:37, 30 December 2024 (UTC)

When I wrote: The page relativistic time dilation is automatically redirected to our article time dilation which...nowhere mentions the term "relativistic time" alone (i.e. without the third word "dilation"), I had already guessed that the term "dilation of relativistic time" (i.e, with the word "dilation" preceding the words "relativistic time") existed nowhere (at least in Wikipedia), and that this redirected page actually meant "relativistic dilation of time". The same is true for the redirected page "relativistic length contraction": I had already gussed it didn't mean "contraction of relativistic length", because (as I had already written): the article length contraction...doesn't mention the term "relativistic length" at all.

Anyway, I'm still waiting for an answer to my original question: Are there accepted terms for the concepts, of relativistic length - as opposed to proper length, and of relativistic time - as opposed to proper time? HOTmag (talk) 10:12, 30 December 2024 (UTC)

A term that will be understood in the context of relativistic length contraction is relative length – that is, length relative to an observer.^[1][2][3]  --Lambiam 10:55, 31 December 2024 (UTC)

Thank you. The middle source uses the term "comparative length", rather than "relative length". I couldn't open the third source. HOTmag (talk) 08:04, 1 January 2025 (UTC)

The text under the graph labelled Comparative length on page 20 of the middle source reads:

Graph of the relative length of a stationary rod on earth, as observed from the reference frame of a traveling rod of 100cm proper length.

A similar use of "relative length" can be seen on the preceding page.  --Lambiam 10:23, 2 January 2025 (UTC)

What did Juan Maldacena say after "Geometry of" in this video?

I was watching this video Brian Greene and Juan Maldacena as they explore a wealth of developments connecting black holes, string theory etc, Juan Maldacena said something right after "Geometry of" Here is the spot: https://www.youtube.com/live/yNNXia9IrZs?si=G7S90UT4C8Bb-OnG&t=4484 What is that? HarryOrange (talk) 20:46, 30 December 2024 (UTC)

Schwarzschild solution. --Wrongfilter (talk) 21:05, 30 December 2024 (UTC)

Thank you, its the Juan Maldacena's accent which made me post here. HarryOrange (talk) 21:18, 30 December 2024 (UTC)

December 31

Brightest spot of a discharge tube

Neon is brighter in the middle.

Xenon is brighter at the edges.

What causes the discharge tubes to have their brightest spots at different positions? Nucleus hydro elemon (talk) 13:12, 31 December 2024 (UTC)

See also the pictures at Gas-filled tube #Gases in use. --CiaPan (talk) 13:26, 31 December 2024 (UTC)

January 1

Two unit questions

1. Is there any metric unit whose ratio is not power of 10, and is divisible by 3? Is there any common use for things like "2⁄3 km", "5⁄12 kg", "3+1⁄6 m"?
2. Is a one-tenth of nautical mile (185.2 m) used in English-speaking countries? Is there a name for it?

--40bus (talk) 10:41, 1 January 2025 (UTC)

1 not that I know of (engineer who has worked with SI for 50 years)

2 not that I know of (yacht's navigator for many years on and off)

Greglocock (talk) 11:35, 1 January 2025 (UTC)

In Finland, kaapelinmitta is 185.2 m. Is there an English equivalent? --40bus (talk) 18:11, 1 January 2025 (UTC)

Cable length. --Wrongfilter (talk) 18:26, 1 January 2025 (UTC)

Good article. I was wrong Greglocock (talk) 22:26, 1 January 2025 (UTC)

The answer can be found by looking up kaapelinmitta on Wiktionary.  --Lambiam 00:14, 2 January 2025 (UTC)

What is more physiological (for a right-hander) left-hand drive or right-hand drive?

Has anyone determined whether it is better for a right-hander to have the left hand on the steering wheel and the right hand on the gear shift stick, or the other way round? Are there other tests of whether left-hand drive or right-hand drive is physiologically better (for a right-hander at least)? 178.51.7.23 (talk) 12:03, 1 January 2025 (UTC)

Supplementary question: I've only driven right-hand-drive vehicles (being in the UK) where the light stalk is on the left of the steering column and the wiper & washer controls are (usually) on the right. On a l-h-drive vehicle, is this usually the same, or reversed? {The poster formerly known as 87.81.230.195} 94.6.84.253 (talk) 12:12, 2 January 2025 (UTC)

Modern cars are designed for mass production in RH- and LH-drive versions with a minimum difference of parts. Steering columns with attached controls are therefore unchanged between versions. Philvoids (talk) 12:29, 2 January 2025 (UTC)

In the UK nowadays, are cars still mostly manual transmission, or has automatic become the norm? ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:38, 2 January 2025 (UTC)

In the UK, sales of new automatics have just recently overtaken manuals - so probably still more manuals than automatics on the road. catslash (talk) 14:37, 2 January 2025 (UTC)

This may be tied to the rise of EVs, since they have automatic transmissions by default. {The poster formerly known as 87.81.230.195} 94.6.84.253 (talk) 05:29, 3 January 2025 (UTC)

In Australia, we drive on the left, and the indicator and wiper stalks are the opposite way to the UK. Having moved back from the UK after 30 years, it took me a while to stop indicating with wipers. TrogWoolley (talk) 05:08, 3 January 2025 (UTC)

This depends more on where the car came from I think. For European or American cars it tends to be in the UK direction. For Asian cars or I guess those odd Australian made cars which are out there, it tends to be in the other. See e.g. [4]. The UK being a bigger market I think most manufacturers have come to follow the new UK norm for cars they intend to sell there [5] [6] [7] [8] although I suspect to some extent it's still true in the sense that I think most Asian car brands, at least assemble their cars in the EU or maybe the UK if they're destined for the UK (made a lot of sense pre-Brexit) [9]. It sounds like the new UK norm is fairly recent perhaps arising in the 1980s-1990s after European manufacturers stopped bothering changing that part of the production for the reasons mentioned by Philvoids. As mentioned in one of the Reddit threads, the UK direction does make it difficult to adjust indicators while changing gear which seems a disadvantage which is fairly ironic considering the the UK has much more of a preference for manuals than many other RHD places with the other direction. Nil Einne (talk) 04:31, 6 January 2025 (UTC)

For further clarity, AFAICT, LHD vehicles generally have their indicators on the left and wipers on the right. As mentioned, assuming the gear stick is in the middle which AFAIK it is for most cars by now, this seems the better positioning especially on manual cars since you're much more likely to want to need to indicate while changing gear than you are going to want to adjust your wipers even in the rainy UK. The UK being LHT/RHD especially with their own manufactured cars tended to have the indicators on the right and wipers on the left in the more distant past so again the positions that made most sense.

While I don't have a source for this going by the history and comments, it sounds to me like what happened is European manufacturers who were primarily making LHD vehicles, with the UK and Ireland their main RHD markets but still small compared to the LHD market stopped bothering changing positions for RHD vehicles as a cost saving measure. So they began to put wipers on the right and indicators on the left even in their RHD vehicles no matter the disadvantage. I'm not so sure what the American manufacturers did or when and likewise the British but I think they were a fairly small part of the market by then and potentially even for them LHD was still a big part of their target market.

Meanwhile Asian manufacturers however still put their indicators on the right and wipers on the left in RHD vehicles, noting that Japan itself is LHT/RHD. I suspect Japanese manufacturers suspected, correctly, that it well worth the cost of making something else once they began to enter the LHD markets like the US, to help gain acceptance. And so they put the indicators on the left and wipers on the right for LHD vehicles even if they did the opposite in their own home market and continued forever more. Noting that the predominance of RHT/LHD means even for Japanese manufacturers it's generally likely to be their main target by now anyway.

Later I assume South Korea manufacturers and even later Chinese felt it worth any added cost to increase acceptance of their vehicles in LHT/RHD markets in Asia and Australia+NZ competing against Japanese vehicles which were like this. And this has largely continued even if it means they need to make two different versions of the steering column or whatever. It sounds like the European and American brands didn't bother but they were primarily luxury vehicles in such markets so it didn't matter so much.

This lead to an interesting case for the UK. For the Asian manufacturer, probably many of them were still making stuff which would allow them to keep putting the indicators on the right and wipers on the left for RHD vehicles as they were doing for other RHD markets mostly Asian. And even if they were assembling them in the EU, I suspect the added cost of needing to ship and keep the different components etc and any difference it made to the assembly line wasn't a big deal.

So some of did what they were doing for the Asian markets for vehicles destined for UK. If they weren't assembling in the EU, it made even more sense since this was likely what their existing RHD assembly line was doing. But overtime the UK basically adopted the opposite direction as the norm no matter the disadvantages to the extent consumers and vehicle enthusiast magazines etc were complaining about the "wrong" positions. So even Asian manufacturers ended up changing to the opposite for vehicles destined to the UK to keep them happy. So the arguably better position was abandoned even in cases where it wasn't much of a cost saving measure or might have been even adding costs.

Nil Einne (talk) 05:43, 6 January 2025 (UTC)

One thing I didn't consider when writing above is how often the steering column or whatever for Asian manufacturers is actually produced in the EU rather than simply shipped there after production elsewhere. That would likely mean producing two would likely incur more additional cost even if the same thing in two versions is produced elsewhere for use in the Asian market. I still think the main reason Asian manufacturers stopped using the opposite location/direction in the UK is primarily one of consumer demand, but it's true that it's fairly complicated. Nil Einne (talk) 10:28, 10 January 2025 (UTC)

I've driven different (automatic) left-hand-drive vehicles with the light stalk on each side, but left side has been more common. Perhaps because the right hand is more likely to be busy with the gear shift? (Even in the US, where automatic has been heavily dominant since before I learned to drive.) -- Avocado (talk) 17:32, 2 January 2025 (UTC)

It's better for a right-hander to have both hands on the steering wheel regardless of where the gear lever is. See Rule 160. I suspect the same goes for a left-hander. Bazza 7 (talk) 14:39, 2 January 2025 (UTC)

I suppose that the question is whether right-handers have an easier time operating the gear stick when changing gears in manual-transmission cars designed for left-hand traffic, with the steering wheel on the right (like in the UK) or right-hand traffic, with the steering wheel on the left (like in most of continental Europe). Obviously, drivers will use their hand at the side where the gear stick is, so if it is in the middle and the driver, behind the wheel, sits in the right front seat, they'll use their left hand, regardless of their handedness. But this may be more awkward for a rightie. Or not.

--Lambiam 16:30, 2 January 2025 (UTC)

In my personal experience (more than 10 years driving on each side of the road, in all four combinations of car handedness and road handedness) the question which hand to use for shifting gears is fairly insignificant. Switching from one type of car to the other is a bit awkward though. —Kusma (talk) 18:33, 2 January 2025 (UTC)

My first car, a Hillman Minx, had the gearstick on the left and the handbreak on the right, which was a bit of a juggle in traffic. Alansplodge (talk) 19:13, 3 January 2025 (UTC)

Distinguishing a picture of a sunset from the picture of a sunrise?

Is there a way (if you don't know which way is west and which way is east in a particular location) to distinguish a picture of a sunset from the picture of a sunrise? 178.51.7.23 (talk) 12:08, 1 January 2025 (UTC)

Generally, no, but there are a few tricks that sometimes work. In dry sunny weather, there's more dust in the air at sunset (due to thermals) than at sunrise, making the sky around the sun redder at sunset. But in moist weather, mist has the same effect at sunrise. If the picture is good enough to see sunspots, comparing the distribution of sunspots to the known distribution of that day (this is routinely monitored) tells you where the North Pole of the sun is. At sunset, the North Pole points somewhat to the right; at sunrise, to the left. If you see any cumulus or cumulonimbus clouds in the picture, it was a sunset, as such clouds form during the day and disappear around sunset, but absence of such clouds doesn't mean the picture was taken at sunrise. A very large cumulonimbus may survive the night. Cirrus aviaticus clouds are often very large, expanding into cirrostratus, in the evening, but are much smaller at dawn as there's more air traffic during the day than at night, making the upper troposphere more moist towards the end of the day. Cirrostratus also contributes to red sunsets and (to lesser extend, as there's only natural cirrostratus) red sunrises. Dew, rime, flowers and flocks of birds may also give an indication. And of course human activity: the beach is busier at sunset than at sunrise. PiusImpavidus (talk) 13:41, 1 January 2025 (UTC)

Supposing the photograph has high enough resolution to show Sunspots it can be helpful to know that the pattern of spots at sunrise is reversed left-right at sunset. Philvoids (talk) 13:21, 3 January 2025 (UTC)

At the equinox, the disk of the Sun with its pattern of sunspots appears to rotate clockwise from sunrise to sunset by 180 degrees minus twice your latitude (taking north positive). At my place, that's 75 degrees. Other times of the year it's less; at the start and end of polar day and polar night, there's no rotation. Sunset and sunrise merge then.

And I forgot to mention: cirrostratus clouds will turn red just after sunset or just before sunrise. At the exact moment of sunrise or sunset, they appear pretty white. PiusImpavidus (talk) 17:06, 3 January 2025 (UTC)

I differ: the same rotation is involved everywhere on Earth. If you stand on tiptoe at a N. or S. pole to take a picture of the Sun it is you who must pirouette 15 degrees per hour to keep facing the Sun. The Earth rotates you at this rate at all non-polar locations. If you stand within the arctic or antarctic circles, for parts of the year the 24-hour night or 24-hour daylight seem to prevent photographs of sunrise or sunset. However the terms "sunrise" and "sunset" can then be interpreted as times that are related to particular timezones which are generally assigned by longitude. In photographing the 24-hour Sun the equatorial rise and set times for your own longitude are significant elevation maxima worth mentioning even though the minimum elevation remains above the horizon. I maintain that the sunspot pattern observed from any location on Earth rotates 360 degrees per 24 hours and that "night", the darkness from sunset to sunrise, is when the Earth's bulk interrupts one's view of the rotation but not the rotation itself which is continuous.

Taking the Earth as reference frame, the Sun rotates around the Earth's spin axis. The observer rotates around his own vertical axis. The better both axes are aligned, the smaller the wobble of the Sun. In the northern hemisphere, it rotates clockwise from about 6 till 18 by 180 degrees minus twice your latitude and counterclockwise at night, in the southern hemisphere it's the opposite. Try a planetarium program if you want to see it. Stellarium shows some sunspots, does things right and is free and open source. PiusImpavidus (talk) 10:27, 5 January 2025 (UTC)

Relationship between Earth's axial tilt (ε) to the tropical and polar circles

We deprecate the obselete Geocentric model and suggest Wikipedia references that are free and just one click away (no extra planetarium software needed). The axes of rotation of the Sun and Earth have never in millions of years aligned: the Ecliptic is the orbital plane of Earth around the Sun and Earth currently has an Axial tilt of about 23.44° without "wobbling" enough from this to concern us here. Philvoids (talk) 14:56, 5 January 2025 (UTC)

This isn't my field but sunspots aside, if you know the location and date, I assume the appearance of other astronomical objects like the moon or rarely another star probably Venus, in the photograph should be enough to work out if it's a sunset or sunrise. That said, to some extent by taking into account other details gathered from elsewhere's I wonder if we're going beyond the question. I mean even if you don't personally know which is east or west at the time, if you can see other stuff and you know the location or the stuff you can see is distinctive enough it can be worked out, you can also work out if it's sunset or sunrise just by working out if it's east or west that way. Nil Einne (talk) 03:54, 6 January 2025 (UTC)

In my experience (Southern England) they tend to be pinker at dawn and oranger(!) at dusk. {The poster formerly known as 87.81.230.195} 94.6.84.253 (talk) 03:23, 4 January 2025 (UTC)

Pink clouds must result from blending of reddish clouds with the blue sky behind. There's actually more air between the observer and the clouds than behind the clouds, but for that nearby air the sun is below the horizon. PiusImpavidus (talk) 10:27, 5 January 2025 (UTC)

The questioner asks for interpretation of a single picture. It is beside the point that more would be revealed by a picture sequence such as of changing cloud colours. Philvoids (talk) 12:41, 4 January 2025 (UTC)

Recalling Leonard Maltin's comment about the Green Berets movie, which was filmed in the American state of Georgia: "Don't miss the closing scene, where the sun sets in the east!" ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:37, 5 January 2025 (UTC)

Which you can only tell if you know which way is east in the image. Maltin, or his writer, appears to have assumed that Vietnam has a seacoast only on the east, which is wrong. --142.112.149.206 (talk) 03:46, 6 January 2025 (UTC)

Georgia has only an eastern seacoast. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:31, 6 January 2025 (UTC)

Black seas matter! Philvoids (talk) 14:18, 6 January 2025 (UTC)

So what. Bugs? The claim is about the setting, not the filming location. --142.112.149.206 (talk) 07:30, 7 January 2025 (UTC)

But as it was filmed in (The US State of) Georgia, it must actually show a sunrise, regardless of what the story line says – how do you know that wasn't what Maltin actually meant? {The poster formerly known as 87.81.230.195} 94.6.84.253 (talk) 10:35, 7 January 2025 (UTC)

• Because things filmed for movies often are not actually what they are shown as being, so that wouldn't be interesting and Maltin's guide wouldn't waste space on it. If what they show it as — for example, in Krakatoa, East of Java — is wrong or impossible, that could be interesting. --142.112.149.206 (talk) 17:43, 8 January 2025 (UTC)

I assume (not having seen the film) that, in the story line of The Green Berets , the closing scene takes place in the late afternoon, which means it shows a sunset. The plot section of our article on the film places the closing scene at or near Da Nang, which is on the east coast of Vietnam. This means that Maltin did not make an unwarranted assumption; he was just seeking an excuse to bash the film.  --Lambiam 13:55, 7 January 2025 (UTC)

I've seen The_Green_Berets and confirm that the closing scene with End title is an offshore sunset. Philvoids (talk) 20:12, 7 January 2025 (UTC)

January 6

Does the energy belonging to an electromagnetic field, also belong (or is considered to belong) to the space carrying that field?

HOTmag (talk) 18:41, 6 January 2025 (UTC)

It would be unusual to express the situation in such terms. Since the notion of energy "belonging to" some entity is not itself a physical concept – any practical approach to energy bookkeeping that satisfies the law of conservation of energy will do – this cannot be said to be wrong. It is, however, (IMO) not helpful. Does an apple belong to the space it occupies? Or does that space belong to the apple?  --Lambiam 23:37, 6 January 2025 (UTC)

First, I let you replace the notion of energy "belonging to" some entity, by the notion of energy "attributed to" some entity, or by the notion of energy "carried by" some entity, and the like. In other words, I'm only asking about the abstract relation (no matter what words we use to express it), between the energy and the space carrying the electromagnetic field, rather than about the specific term "belong to".

Second, I'm only asking about what the common usage is, rather than about whether such a usage is wrong or helpful.

The question is actually as follows: Since it's accepted to attribute energy to an electromagnetic field, is it also accepted to attribute energy to the space carrying that field?

So, is your first sentence a negative answer, also to my question when put in the clearer way I've just put it? HOTmag (talk) 03:28, 7 January 2025 (UTC)

The answer remains the same. It would be a highly unusual use of language to "attribute" electromagnetic energy to a volume of space, in quite the same way as it would be strange to "attribute" the mass of an apple to the space the apple occupies. But as long as an author can define what they mean by this (and that meaning is consistent with the laws of physics), it is not wrong.  --Lambiam 13:21, 7 January 2025 (UTC)

An electromagnetic field that we may (even tenuously) conceive to have the form of a massless photon has, like the aforementioned apple (a biological mass) its own unique history, that being a finite path in Spacetime. I reject apparent effort to give spacetime any kind of identity capable of owning, or even anticipating owning or remembering having owned anything at all. Concepts of owning¹², attributing³ or whatever synonymous wordplay one chooses all assume identification that can never be attached to the spacial location of an em field. The energy of the photon is fully accounted for, usually as heat at its destination, when it is absorbed and no lasting trace remains anywhere. I am less patient than Lambian in my reaction to this OP who under guise of interest in surveying "what is commonly accepted" returns in pursuit of debate by patronisingly "allowing" us to reword his question in abstract "words that don't matter" to make it purportedly clearer and worth responders' time. Philvoids (talk) 14:55, 7 January 2025 (UTC)

Thank you Lambiam for your full answer. I always appreciate your replies, as well as your assuming good faith, always. HOTmag (talk) 15:08, 7 January 2025 (UTC)

January 8

Australian for double-decked bridge?

On a topographic map (or on any other kind of map, like a track diagram), what symbol represents a railroad bridge which is directly above and collinear with another railroad which is either on a lower deck of the same bridge, or else is at grade (as in, for example, a narrow-gauge line on a coal trestle above a standard-gauge one)? 2601:646:8082:BA0:48AA:9AA4:373D:A091 (talk) 06:35, 8 January 2025 (UTC)

Our List of multi-level bridges#Australia article only lists two multi-level bridges in Australia, neither of which seem to fit your criteria. Alansplodge (talk) 19:16, 8 January 2025 (UTC)

Clarification: in this case, "Australian" is meant figuratively (as in that Fosters ad) -- what I was really asking was the representation of such a bridge on a map. 2601:646:8082:BA0:48AA:9AA4:373D:A091 (talk) 01:03, 9 January 2025 (UTC)

What Fosters ad? That link doesn't help, and Australians don't drink Fosters, so won't have seen any ad for it. HiLo48 (talk) 01:15, 9 January 2025 (UTC)

Nonsense. I have it on good authority—Fosters own ads on TV in the US two decades ago—that all Australians do nothing but drink Fosters all day because it is the one true Australian beer. DO NOT ARGUE WITH YOUR CAPITALIST OVERLORDS' CULTURAL APPROPRIATION! Um, I mean, Foster's Lager had a bunch of ad campaigns promoting their image as being Australian. See its article for details. Search youtube for fosters australian to see some examples. DMacks (talk) 01:28, 9 January 2025 (UTC)

Nit pick, at grade means at the same height, you mean grade separated. Greglocock (talk) 05:32, 9 January 2025 (UTC)

It's all grade-separated (rail-line vs rail-line). I assume they mean one rail-line is on the ground (in contrast with being on a bridge as the first example). The term is annoying, but we're stuck with terms like at-grade railway. DMacks (talk) 05:38, 9 January 2025 (UTC)

Yes, in this case "at grade" means at ground level -- with the narrow-gauge line on the trestle directly above it! 2601:646:8082:BA0:48AA:9AA4:373D:A091 (talk) 06:25, 9 January 2025 (UTC)

Only example of a multi-level bridge or viaduct I've found so far in the world having a WP article is Highline Bridge (Kansas City, Kansas). DMacks (talk) 06:32, 9 January 2025 (UTC)

There is one on the Driving Creek Railway (no photo of this detail in the article, but a few in c:Category:Driving Creek Railway). I've seen mentions of some others that are long-gone (or have one or both levels now used for other modes). Lots of pictures of old New York City have an el with rails in the street under it, but nothing still existing or in-use. DMacks (talk) 07:25, 9 January 2025 (UTC)

Right, so how would one show such a bridge on a map? 2601:646:8082:BA0:48AA:9AA4:373D:A091 (talk) 22:51, 9 January 2025 (UTC)

Exactly the same as a map would indicate a railway under a roadway or a roadway under a railway (or anything under anything), of which there are numerous examples on maps, i.e. the lower railway disappears under the upper railway and then reappears at the other end of the bridge. Shantavira|^{feed me} 10:27, 10 January 2025 (UTC)

Thanks! Which would actually make it easier if the two railroads are of different gauges and one of them is at grade, as in my (fictional) example (I'm currently mapping the station layouts on the North Western Railway for a possible scenario pack for Train Sim Classic and/or Train Sim World, and there's a setup just like I describe at Arlesburgh West -- the narrow-gauge Arlesdale Railway goes up on a coal trestle above an at-grade siding of the North Western) -- in that case, the standard-gauge line goes under the ends of the bridge lengthwise and disappears, while the narrow-gauge line remains continuous on the bridge deck, and because they have different symbols there's no confusion! 2601:646:8082:BA0:48AA:9AA4:373D:A091 (talk) 22:11, 10 January 2025 (UTC)

January 11

Pork belly and microwaves

Why does pork belly always seem to pop in a microwave whenever I cook it in there? It also splatters, too, which creates a mess I have to clean up. Kurnahusa (talk) 02:53, 11 January 2025 (UTC)

Boiling of intracellular fluid? 2601:646:8082:BA0:48AA:9AA4:373D:A091 (talk) 07:10, 11 January 2025 (UTC)

I agree with the IP. Also food in a microwave should always be covered. Microwave plate covers are widely available. Shantavira|^{feed me} 09:52, 11 January 2025 (UTC)

Which bird species?

Bird from Brenman Park, Alexandria, Virginia, February last year.

I found this picture on Commons. Is this really a mallard (Anas platyrhynchos)? We have lots of mallards here in Sweden where I live, and nor male or female looks like that.

I'm sure it belong to Anseriformes, yes... but what kind of bird species?

// Zquid (talk) 21:48, 11 January 2025 (UTC)

A female gadwall seems most likely, although a lot of female dabbling ducks are rather similar. Mikenorton (talk) 23:31, 11 January 2025 (UTC)

Which primate species?

Info from Flickr images says this is purple-faced langur...

I found this picture on Commons. Description says Purple-faced langur, and so did the category. I changed the category to Semnopithecus vetulus, but I'm not sure the picture shows Purple-faced langur/Semnopithecus vetulus.

Can someone tell me what kind of primates?

// Zquid (talk) 21:59, 11 January 2025 (UTC)

Going by the long nose and concave facial profile, that looks to me like a macaque. In fact, based on the ludicrous hairstyle, the first second last on the list, Toque macaque, is indicated. It is endemic to Sri Lanka like the Purple-faced langur. These individuals in the picture do have very purple faces, I must admit. Perhaps it was mating season and they go like that? But monkeys tend to send that kind of signal via the butt, not the face. Our article says "With age, the face of females turns slightly pink. This is especially prominent in the subspecies M. s. sinica", so I suppose that could be it.

It was convenient that this species was wrongly sorted to the top of the alphabetical list.  Card Zero  (talk) 01:30, 12 January 2025 (UTC)

Flying off to infinity in a finite time

In "Newton's law of motion", chapter Singularities we find this text: "It is mathematically possible for a collection of point masses, moving in accord with Newton's laws, to launch some of themselves away so forcefully that they fly off to infinity in a finite time."

How can one write such a thing, when by definition infinity has no limit and whatever the speed of a point mass, it will therefore never reach infinity, that is to say a limit that does not exist? Malypaet (talk) 22:07, 11 January 2025 (UTC)

Did he actually refer to his own work as "Newton's laws"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:16, 11 January 2025 (UTC)

Looking at the citation, we find an article entitled "Off to infinity in finite time".[10] I didn't find it at all answers your question, though. What does it mean? --jpgordon^{𝄢𝄆𝄐𝄇} 02:48, 12 January 2025 (UTC)

I would assume it means there's some finite time $T$ in the future such that, for any natural number $n$, there's a time $t<T$ such that the object is more than $n$ meters away at every time between $t$ and $T$.

What happens to the object after time $T$ seems to be unspecified. Maybe it's just gone? --Trovatore (talk) 05:36, 12 January 2025 (UTC)