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May 1

Changing the speed of light, when the medium is not a perfect vacuum.

Is it possible to exert forces, on light moving in a medium that is not a perfect vacuum, for example in water, in order to change the speed of light in that medium?

If it's impossible, then why ? After all, as we know, it is possible to apply forces that slow down - or accelerate - massive bodies, so why is it impossible to apply such forces also to the light when it moves in water etc., so that the forces will make the light move (in water) at a slower speed, for example at the speed at which light moves when it's in glass? Or so that the forces will make the light move (in water) at a faster speed, for example at the speed at which light moves when it's in the air? 2A06:C701:747E:800:D855:A0F7:59BA:AFAB (talk) 18:35, 1 May 2023 (UTC)[reply]

So, the thing about all of the physics involved is that the speed of light is invariable, full stop. That's part of what general relativity says; any "acceleration" supposedly experienced by light (whether changing direction or speeding up/slowing down) is not an acceleration, it's spacetime curvature. Light being "slowed" in a material is due to interactions with the photons with particles of the medium, see Speed of light, to wit "This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light..." Light still travels in the space between, say, atoms and molecules, at the speed of light, however interactions between photons and the atoms themselves results in the apparent speed of light to slow, photons move in straight lines at the speed of light; however the apparent speed at which light waves move through a medium is slowed due to quantum interactions between the photons and the particles of the medium. When you say "the speed of light in water" what you are saying is "the speed light appears to move in water due to quantum interactions between photons and atoms" or some such, the light itself, insofar as it is actually traveling, still moves at "c". --Jayron 32 18:54, 1 May 2023 (UTC)[reply]

It is not completely true as speed of light in matter can exceed the speed of light in vacuum. Ruslik_Zero 20:13, 1 May 2023 (UTC)[reply]

What? it seems you confuse photons with electrons (in Cherenkov radiation). 2A06:C701:747E:800:D855:A0F7:59BA:AFAB (talk) 20:50, 1 May 2023 (UTC)[reply]

Well, that depends on your definition of "speed" and "light". At the most basic level, the photon can only travel through the quantum field at c. No faster, no slower. What we call light is, yes, photons, but it is also a more complex phenomenon that has several definitions of "velocity", such as phase velocity or group velocity. As a quantum particle (a disturbance in a quantum field), it can only move at the speed of c. --Jayron 32 11:08, 2 May 2023 (UTC)[reply]

Right, if you adopt the "path integral" or "sum-over-histories" interpretation of QM, you can think of photons being absorbed, emitted, and in between always traveling at exactly c. I think this is mostly of philosophical importance in this particular case; you would never actually calculate anything that way.

Alternatively, you can take the view that light in matter is not composed of photons but of polaritons, which do travel at sub-c speeds. --Trovatore (talk) 17:37, 2 May 2023 (UTC)[reply]

Fizeau experiment may be relevant. catslash (talk) 21:34, 1 May 2023 (UTC)[reply]

My point was only that you should be very careful when claiming that "the speed of light is invariable, full stop". It is only true in a sense that there is a fundamental constant called "speed of light in vacuum", which is of course is invariable. As to the real speed of electromagnetic waves in a medium - everything depends on the definition. Ruslik_Zero 20:23, 2 May 2023 (UTC)[reply]

Yes, but the OP is asking about exerting a force on light. What thing are we exerting a force on? A photon? Photons don't accelerate in any meaningful sense, they just move at c in a straight line. --Jayron 32 11:17, 3 May 2023 (UTC)[reply]

The speed of light (viewed as a wave phenomenon) in a medium (or in a vacuum) is set by the electric permittivity and magnetic permeability of the medium. To change the speed of light in water, you have to change those properties. It doesn't work by pushing it (although a change in pressure could change them, so that the index of refraction of ice is different from that of liquid water). You can push stuff which has mass and you could say that wet water has mass, but the matter is complicated as the mass of wet water depends on the energy. This speed of light in a medium, or more exactly the phase velocity of light, can be larger than the speed of light in a vacuum. The group velocity can't be larger than the speed of light in a vacuum.

How would you push light? When you push something to make it move faster, you perform work to add kinetic energy. We can do that with light by hitting it with charged particles; it's called inverse Compton scattering. Not easy to do in a dense medium. It leads to a change of frequency for the light, which may, in a medium, result in a change of velocity. For visible light in water, increasing the energy leads to a lower velocity. PiusImpavidus (talk) 09:33, 2 May 2023 (UTC)[reply]

However, see Superluminal group velocities and single waves wherever v=c carry energy and information. Modocc (talk) 11:54, 2 May 2023 (UTC)[reply]

May 3

Hum in headphone amplifier

I have a headphone amplifier that has some hum in it. If I put my hand on the metal case, the hum goes away - or at least so low that I can't hear it. Does it need to be grounded?

There is a post on the back for grounding it, but I don't know what to ground it to. Years ago I had my turntable grounded to a water pipe, when the pipes were metal. My current house has plastic pipes, so what can I ground it to? Run a wire to the actual ground? Bubba73 ^{You talkin' to me?} 04:03, 3 May 2023 (UTC)[reply]

It is most likely a ground loop problem. What are the headphones plugged into? USB? If it's USB, then the device itself needs to be grounded; if it uses a 2-prong adapter or something, then there's your problem. 136.56.52.157 (talk) 06:29, 3 May 2023 (UTC)[reply]

They are Audeze CRBN electrostatic headphones. Bubba73 ^{You talkin' to me?} 06:31, 3 May 2023 (UTC)[reply]

At that price point, you should contact customer service for tech support: [1]. 136.56.52.157 (talk) 06:40, 3 May 2023 (UTC)[reply]

P.s. this query from Audeze might be helpful, it suggests that it most likely relates to RFI. Not mentioned however, is that there are RFI filters for such problems. 136.56.52.157 (talk) 17:25, 3 May 2023 (UTC)[reply]

The hum goes away when I touch the metal case of the headphone amp, which is a Stax SRM-400S. Bubba73 ^{You talkin' to me?} 23:59, 3 May 2023 (UTC)[reply]

A couple a days ago I had this exact same issue with two guitar amps. Relocating the new amp from the campfire table to the bare ground stopped the 60HZ buzzing. Indoors, placing it on the floor reduced the hum only some, so I clamped a tape measure to it and wrapped that around a quart can of varnish I have. :-) Modocc (talk) 00:06, 4 May 2023 (UTC)[reply]

I'll try moving the cables and the amp around. Bubba73 ^{You talkin' to me?} 01:28, 4 May 2023 (UTC)[reply]

I think I'm going to try a ground-loop isolator, for the XLR inputs into the amp. Bubba73 ^{You talkin' to me?} 00:05, 4 May 2023 (UTC)[reply]

How did the ancients deal with mold and mildew in ancient aqueduct systems?

Modern drinking water systems rely on chlorine etc. to prevent mold from growing in water systems. Ancient aqueduct systems must have been exposed to the outside elements. Surely they would have accumulated mildew over time? Or did enough sunlight do the trick? Yanping Nora Soong (talk) 12:08, 3 May 2023 (UTC)[reply]

Does lead piping suffer from mildew? One just hopes it kills the bugs quicker than the humans! Martin of Sheffield (talk) 13:13, 3 May 2023 (UTC)[reply]

Pethaps continuously moving water is less prone to this (biologist required). A second hypothesis is that they didn't know or care about what was growing in their water supply. Dr John Snow was the first to realise that drinking water with sewage in it was a bad idea, and that wasn't until 1854. Alansplodge (talk) 13:56, 3 May 2023 (UTC)[reply]

Molds aren't the problem, bacteria are. And they aren't a problem within the system, but on either end. Abductive (reasoning) 15:36, 3 May 2023 (UTC)[reply]

Not an issue. As an example Plymouth's drinking water was supplied via leats which are large drainage channels that drain Dartmoor. They are effectively canalised streams. The water in them is entirely drinkable, if a bit tannic. Greglocock (talk) 20:38, 3 May 2023 (UTC)[reply]

One may discover, just upstream and out of site from where you quenched your thirst, a dead sheep in the leat. DuncanHill (talk) 20:47, 3 May 2023 (UTC)[reply]

Correct, which is why you walk downstream before drinking. Survival101. Greglocock (talk) 00:35, 4 May 2023 (UTC)[reply]

Images in Egyptian tombs from the 15th to 13th century BCE show various water treatment devices. An ancient Sanskrit text on medicine the Sushruta Samhita specified various methods such as boiling water under the sun and filtering it through sand and coarse gravel. Hippocrates c. 460 – c. 370 BC designed a crude water filter to “purify” the water he used for his patients. Later known as the “Hippocratic sleeve,” this filter was a cloth bag through which water could be poured after being boiled. Diophanes of Nicaea of the first century BC advised putting macerated laurel into rainwater, Paxamus proposed that bruised coral or pounded barley, in a bag, be immersed in bad tasting water and the eighth century Arabian alchemist, Gerber, described various stills for purifying water. Sir Francis Bacon in his compilation "A Natural History of Ten Centuries" 1627 discussed desalination and began the first scientific experimentation into water filtration. Philvoids (talk) 00:33, 4 May 2023 (UTC)[reply]

The best way to purify water is to warm it with some malt, boil it with some hops, let it cool and add the yeast. A few days later your water is fit for anyone! :-) More seriously, much weak ("small") beer was drunk prior to 20C just because it was safe, unlike the water supply from wells next to cess pits (see Alansplodge above. Martin of Sheffield (talk) 07:39, 4 May 2023 (UTC)[reply]

(Insert joke based on reading "20C" as a temperature in Celsius here.) --174.89.12.187 (talk) 18:17, 4 May 2023 (UTC)[reply]

(Point out that unlike 293K it would be 20°C.) Martin of Sheffield (talk) 19:38, 4 May 2023 (UTC) [reply]

20C??? That's a lot of electrical charge! 73.162.86.152 (talk) 06:42, 5 May 2023 (UTC)[reply]

Equivalent to downing 3-1/3 six-packs of Mountain Dew in a second. DMacks (talk) 06:51, 5 May 2023 (UTC)[reply]

The ancient Greeks and Romans added wine to drinking water, which killed some of the bacteria, according to this article. Adding wine also improved the taste of stagnant water, according to this website. Cullen328 (talk) 20:08, 6 May 2023 (UTC)[reply]

So I actually found the best answer to my question - ancient Romans used a piscina limaria, or settling basin, to settle and remove crud at the intake and just before a city. They were also used in baths. I'll add that to the Roman aqueduct article with good sources soon. There is probably much also unsaid about the use of UVC-rich sunlight. Mold, odor and discoloration is easy to see (probably why the miasma theory had currency before the germ theory). Yanping Nora Soong (talk) 08:48, 7 May 2023 (UTC)[reply]

May 4

Language and laugh

Why only humans have language?

And why humans laugh properly? Does humans have special ability in their tongue, throat, vocal cords?

Humans evolved in same environment with millions of species, so why can't animals have a language not just sounds. Some animals have bigger head than humans so their brain must have more capacity. If anyone checks brains of elephant, hippopotamus, great whale, what special thing is missing from their brain that human brains have? PatricSt (talk) 17:41, 4 May 2023 (UTC)[reply]

You could start by reading Origin of language. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:57, 4 May 2023 (UTC)[reply]

There is also an article titled Laughter which should answer many of your other questions. --Jayron 32 18:06, 4 May 2023 (UTC)[reply]

"The prefrontal cortex is commonly associated with cognitive capacities related to human uniqueness: purposeful actions towards higher-level goals, complex social information processing, introspection, and language."^[2] --Lambiam 20:12, 4 May 2023 (UTC)[reply]

Rats also laugh, especially when tickled. However, they do so at frequencies higher than humans can hear, so this had gone unnoticed until quite recently. {The poster formerly known as 87.81.230.195} 90.213.18.208 (talk) 11:19, 7 May 2023 (UTC).[reply]

May 5

Coal merchant occupational diseases

I've just finished an article on coal merchants, once one of the most common businesses but now much rarer. The Dutch Wikipedia article on the topic is totally unsourced but mentions that back and shoulder injuries were common because of the backbreaking work of carrying heavy sacks. Does anyone know if there's a source on this to add? (Yes, I know about chimney sweeps' carcinoma, but I don't think it's directly relevant.) And of course any other sources to improve the article would be much appreciated. Blythwood (talk) 11:18, 5 May 2023 (UTC)[reply]

I don't have a reference but I would imagine hauling heavy sacks of anything could cause back and shoulder injuries and this is just stating the obvious. I doubt you need a reference for that. That said, black lung disease may be of interest. "It is common in coal miners and others who work with coal." (Referenced.) 41.23.55.195 (talk) 12:31, 5 May 2023 (UTC)[reply]

I know about Wikipedia:You do need to cite that the sky is blue but gimme a break here. 41.23.55.195 (talk) 12:43, 5 May 2023 (UTC)[reply]

This article may be of interest (not sure how RS but it has a named author). Alansplodge (talk) 12:55, 5 May 2023 (UTC)[reply]

Not related to your question, but some images of coal merchants here, here, here and here. Alansplodge (talk) 17:45, 5 May 2023 (UTC)[reply]

Thanks so much to both of you! Will add the photos and that source. Currently thinking about a DYK hook. Haven't found a specific source for black lung in people delivering coal but might try looking further. Blythwood (talk) 19:47, 5 May 2023 (UTC)[reply]

Pendulum Motion

Suppose a pendulum consists of a rigid slender rod. As the pendulum swings downward, the points closer to the free end of the rod are moving faster than the points closer to the fixed end, but all of the points are being accelerated by gravity at the same rate. I understand that gravity provides a torque about the fixed end, but somehow I can't intuitively understand what's allowing the points to have different speeds. Puzzledvegetable^{Is it teatime already?} 21:43, 5 May 2023 (UTC)[reply]

The gravity is applying a force to each part of the pendulum. But because it is rigid, there are forces withing the material called stress. This mostly counterbalances the force of gravity. It varies in strength and direction in different parts of the pendulum at different phases of the swing. Graeme Bartlett (talk) 21:50, 5 May 2023 (UTC)[reply]

Since the pendulum is a rigid body, gravity can be simplified to acting at the centre of mass. For a uniform rod this will be in the centre of the rod. Neglecting air resistance (probably not a valid assumption for a rod), then gravity acts on this centre and will vary from a maximum when the rod is horixontal to a minimum when it is vertical. Martin of Sheffield (talk) 22:12, 5 May 2023 (UTC)[reply]

The reason, though, this simplification can be made is due to forces acting within the rigid body, as indicated above by Graeme Bartlett. These internal forces counteract the effect of external forces (such as gravity) that threaten to shear it apart. If the stress (in the pendulum case more specifically the tension) exceeds a certain limit, it will still come apart. --Lambiam 06:29, 6 May 2023 (UTC)[reply]

May 6

Why is tungsten so cheap?

Why is tungsten so much cheaper than all the other noble metals like platinum and osmium? Or is it not that much cheaper and I'm just misunderstanding all the confusing price quotes that I find on the internet? They keep using different units for everything so I might just misunderstand them. – b_jonas 14:51, 6 May 2023 (UTC)[reply]

It may partially be that it's because it's not a Noble metal. --Phil Holmes (talk) 15:38, 6 May 2023 (UTC)[reply]

It also plays a role that tungsten is found in the relatively abundant minerals wolframite and scheelite, whereas platinum and osmium are both extremely rare. --Lambiam 19:54, 6 May 2023 (UTC)[reply]

According to its article, the top three countries produce just under 90,000 tonnes of tungsten a year, for platinum it's under 150 (according to the graph in List of countries by platinum production; another source says a bit more, but still under 200 tonnes), for osmium various sources vary, but none of them are higher than 1 tonne. So tungsten is a lot more readily available. Clarityfiend (talk) 09:06, 7 May 2023 (UTC)[reply]

May 7

Domestic appliances

In ovens, washing machines, coffee machines etc. (at least those that I've needed to fix), it seems that the various heaters, motors and actuators have one terminal connected to the live rail, with the switches, thermostats and controllers on the neutral side. Why's it that way around? catslash (talk) 00:32, 7 May 2023 (UTC)[reply]

Live rail? Do these appliances run on tracks? Perhaps there is a rationale grounded in the theory of electricity, but I suspect this is an arbitrary convention, just like the colour codes of wires are an arbitrary convention. --Lambiam 09:56, 7 May 2023 (UTC)[reply]

Eagle's brow ridge

Eagle_eye#Eye_anatomy_and_physiology says the bony brow ridge "protects the eyes from protruding tree branches when it perches on trees, and also from prey which struggles to escape". Considering that some other birds of prey and generally many other animals live in a similar tree habitat and have similar hunting behavior, why that bony ridge evolved only in eagles? Apparently there are similar adaptations elsewhere that for some reason are not present among all genera living in the same habitat and sharing similar behavior. 212.180.235.46 (talk) 17:14, 7 May 2023 (UTC)[reply]

@@ Line 121: / Line 121: @@
 :[[Live rail]]? Do these appliances run on tracks? Perhaps there is a rationale grounded in the theory of electricity, but I suspect this is an arbitrary convention, just like the colour codes of wires are an arbitrary convention. &nbsp;--[[User talk:Lambiam#top|Lambiam]] 09:56, 7 May 2023 (UTC)
+== Eagle's brow ridge ==
+[[Eagle_eye#Eye_anatomy_and_physiology]] says the bony brow ridge "protects the eyes from protruding tree branches when it perches on trees, and also from prey which struggles to escape". Considering that some other [[birds of prey]] and generally many other animals live in a similar tree [[habitat]] and have similar hunting behavior, why that bony ridge evolved only in eagles? Apparently there are similar adaptations elsewhere that for some reason are not present among all genera living in the same habitat and sharing similar behavior. [[Special:Contributions/212.180.235.46|212.180.235.46]] ([[User talk:212.180.235.46|talk]]) 17:14, 7 May 2023 (UTC)