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July 11

How hot can CCFLs get before breaking?

How hot can a CCFL get before sustaining damage? (note that CCFL is different from a CFL). I'd like to use about 50 of them (from broken laptop monitors) in a project as close together as possible and expect ~200 W waste heat. --145.255.245.88 (talk) 07:48, 11 July 2017 (UTC)

Same as a CFL - are you talking about the tubes, or their drive electronics? The electronics generate most of the heat (especially for CFLs, less so for CCFLs) but the tubes are prety much insensitive to this. Although they're a "cold cathode", that only means that they're not a directly heated cathode, it doesn't mean that theyir cathodes have to be kept cold. Andy Dingley (talk) 10:25, 11 July 2017 (UTC)

If this project is intended to be viewed by the public, then simply use an electric fan or two so as to avoid roasting the public. If the public feel comfortable so will the monitors. Also, it would help if you explained the spacial arrangement of said monitors. 75 degrees gives max efficiency. Ie. the most light for the energy converted into light. Aspro (talk) 10:39, 11 July 2017 (UTC)

In general, the CCFLs I've seen have operating temperature ranges that go up to 50 or 60 C. If you know what parts you are using, you can often find a specification online. That will tell you want the manufacturer recommends. As to how hot your installation will get, that will depend on the airflow, the mounting materials (a metal backing would remove heat more effectively than a wood one, for example), and other aspects. If 200 W is the total dissipation for 50 monitors, that is actually not very much energy, and unless they are completely enclosed and lacking in airflow, you are probably fine. Dragons flight (talk)

Thanks for everyone's responses. I'm talking about taking the CCFLs out of the monitors and lining them all up next to each other for maximum density. I expect to be able to fit all 50 CCFLs in a space about 25 cm x 35 cm. I was hoping to passively cool them by attaching to a thin (~4 mm) aluminium plate but the way each tube is terminated precludes physical contact between the tube and the metal plate. I don't want to use a fan because I hope to use this light for (non-professional obviously!) video recording. 145.255.245.88 (talk) 17:03, 11 July 2017 (UTC)

Hotel body soap, hand soap, and shampoo

I looked through the archives and didn't find what I was looking for.

In hotels, they have all three. I'm guessing they're all pretty much the same, but difference smells and colours. Is this about right? Anna Frodesiak (talk) 07:48, 11 July 2017 (UTC)

Soap and shampoo have a couple of differences: manufacture and use.

Their use is that "soap" ought to feel soapy and should lather well. A "good" soap is perceived as being the one that feels "luxuriant" on this basis. A difference with body soap (as a rarely made distinction) is that hands are washed quickly, bodies more leisurely. So a hand soap isn't quite so richly lathering, as you have to get it on and off without waiting around for it. Shampoo is different - you want the minimum amount of lather, as that has to be rinsed clean in turn. It takes longer to rinse hair than to wash it clean. Also, if conditioning the hair afterwards, any remaining shampoo will make the conditioning ineffective, so there has to be a four stage wash-rinse-condition-rinse cycle.

Their manufacture is that they can be either soaps or detergents. Most are increasingly detergents rather than soaps. Local product labelling laws may require non-soap detergents to be labelled as "handwash" rather than soap. It's rare to see a bath "soap" that's a detergent, as they just don't feel like luxury. There are though plenty of in-shower bodywashes based on detergents. These are mostly pitched as "invigorating" rather than "luxurious" and they sell on the basis of fragrances and spicy tingles (don't miss the 'flaps on fire' post (non-FB link) - I have this stuff, they're not joking). Liquid hand "soaps" are almost all detergents, because they're quicker to rinse. Shampoo is all detergent based, not soap, because although soap is usable for hair washing (and used to be, as there was little else) it takes a huge amount of rinsing.

Incidentally, the "two in one" shampoo and conditioner formulas work by mixing an easily rinsed shampoo with a lot of conditioner, and making this conditioner persistent against washing and rinsing. By the time you've rinsed the shampoo out and the conditioner can get to work, there's still enough left to do it. Andy Dingley (talk) 10:36, 11 July 2017 (UTC)

Andy, I don't know how you know all this, but I am well impressed. I had no idea. I cannot view 'flaps on fire' because facebook is blocked here in China. Thank you so much for the very educational answer! I am grateful. Anna Frodesiak (talk) 11:40, 11 July 2017 (UTC)

Note that soap is made from fats, via saponification, whereas detergents are not. StuRat (talk)

Back in the old days, chemists and pharmacists weren't as protective of their trade secrets as one might think. Sure, they'd have their own tonic recipe that they kept secret, but for the hundred mundane things they had to make and sell anyway, there were standard industry formularies like The Chemical Formulary and Spons' Workshop Receipts. These listed a great many standard perfumes, soaps and shampoos. Often bulk soap would be made in large factories (a smelly process, not popular in towns) and large blocks at different grades (from faces to nits to floors) would be sold to a retail chemist who would blend them and package them.

These days I sell at craft markets, which often involves talking to high-end soap makers. To sell artisan soap today, you have to find a way to make it special.

Really I don't know much about this stuff at all, but I keep a house chemist who does. There's little other work for an industrial chemist. Andy Dingley (talk) 12:20, 11 July 2017 (UTC)

and "flaps on fire" is a cautionary tale, unaccountably omitted from Struwwelpeter, about bathing in Mint and Tea Tree Shower Gel. The warning is addressed to women but, from the replies, is also applicable to men. Hopefully it isn't on sale in China. Thincat (talk) 12:35, 11 July 2017 (UTC)

Ah, I found news items about the facebook page. I will avoid such products.

Thank you all again. And I guess all soaps in liquid form are detergent here, but am not sure. Anna Frodesiak (talk) 15:40, 11 July 2017 (UTC)

Almost all liquid "soaps" that are transparent will be detergents. It is possible to make liquid soaps (that are soaps), but they're rare these days, because detergents are cheaper and also liquid soaps store badly, having a tendency to set in the nozzle or pump. Detergents are more efficient (less mass needed for a wash) and so are much cheaper to manufacture in the same apparent quantity. Most people use much more detergent than they really need.

Transparent soap has an interesting history of its own, but I think it's solid, not liquid (I don't know of any transparent, liquid, true soaps). They're made from glycerine (although not all glycerines are transparent). Pears soap was the first such soap, the first branded soap and one of the world's first major retail brands. At a time when product adulteration was rife and some badly-made soap was still caustic, the transparency, brand identity and general high quality of Pears' were factors in making it an easily recognised premium brand. Andy Dingley (talk) 16:04, 11 July 2017 (UTC)

Yes, when you say "...much cheaper to manufacture...", it convinces me that hotel body soap, hand soap, and shampoo are all just detergent. Anna Frodesiak (talk) 18:14, 11 July 2017 (UTC)

For liquid "soap", yes, but I'm not even sure if it's possible to make a "detergent bar" that resembles a soap bar. StuRat (talk) 21:45, 12 July 2017 (UTC)

(Caution: Do not try this at home. Lye and potassium hydroxide are dangerous).American backwoods folk of a bygone era made lye from dripping water through wood ashes, then used that with rendered animal fat to make soap. The lye tended to be weaker than commercially made lye, and if so the resulting soap was likely to be liquid rather than solid. I think this latter advice was in one of the Foxfire books. The weak lye liquid soap was still said to clean. Another site says potassium hydroxide is used in place of sodium hydroxide to make liquid soap. The example is certainly not clear. Soap does not work well in hard water for cleaning clothes or hair, in that it does not lather well without a softening agent or the use of water caught in a rainbarrel. Edison (talk) 03:45, 14 July 2017 (UTC)

Rotating an object with circularly polarized light

Suppose you have a beam of circularly polarized light, and it strikes an object. If absorbed, it should tend to rotate the object, though only to a very small degree as the Planck constant of momentum per photon is small: 6.6E-34 J*s (= N*m*s). Even a meter-long microwave photon carries relativistic mass-energy of 1.2E-6 eV = 2E-25 J, so I think to apply the equivalent of a middling 1 N*m torque screwdriver for one second (i.e. 1 N*m * s) you need 3E+9 J of energy, which our joule article describes as the energy of a one-ton vehicle moving at 100 mph. Times three, plus another factor of two because photons have *half* a Planck unit. Does that sound right to you?

However... photons don't have to be absorbed. They could reflect, and mirrors can be up to 99.999% reflective. That means that in concept you could recycle a photon 100,000 times, getting double the angular momentum transfer each time ... provided, that is, you can come up with a really good mirror, perhaps of some metamaterial, that works at the right (long) wavelength and which reverses the polarization of the photon striking it. Though I think this is actually the norm - not reversing it would actually be unusual! That would knock down the required photon energy to 1.5E+4 J, more like the solar radiation striking the Earth each minute.

I also get curious about the linear momentum (or impulse (physics) ... hmmm, is there a meaningful difference?) transferred in this system: photon says h/lambda (i.e. wavelength); for those one-meter microwaves that is 6.6E-34 N*s per photon. In the mirror system we needed 1.5E+4 J/2E-25 J = 0.8E+29 photons, so that would be still something like 5E-5 N*s of momentum, not much... wait, no... the mirror reduces the amount of energy needed, but not the force of each impact -- so actually it's the full 3E+9 J/2E-25 J = 1.5E+34 photons (that should be twice the inverse of the Planck number) that hit, carrying 2 N*s of kick, certainly not negligible. It could be made smaller with radio wave photons...

Anyway, the questions:

• a) can you point to any good lab demonstrations of turning an object with circularly polarized light?
• b) how good do low-frequency mirrors (microwaves, etc.) get?
• c) with perfect lens technology or the like, could you confine the microwaves to a cavity without completely enclosing it? How far could you get? In other words, could you use them to transmit torque between two objects at a distance from each other?
• d) the linear momentum issue is also interesting -- with that, the energy for "kick" is constant, so I think that any sufficiently "perfect" set of mirrors (any frequency, and not much better than those presently available!) should be able to physically levitate objects... at least, until a mote of dust gets in. Is anyone trying to do this?

Wnt (talk) 13:16, 11 July 2017 (UTC)

• Couple should suggest why this is a tiny effect and hard to measure - but it is real. I think Poynting was the first to recognise this, around 1909, a couple of years after Einstein's annus mirabilis. It took until the 1930s before it was successfully measured though. Andy Dingley (talk) 13:31, 11 July 2017 (UTC)

Easier to measure the effects at microwave wavelengths, BTW. Andy Dingley (talk) 13:35, 11 July 2017 (UTC)

You are basically describing some sort of movement-out-of-nowhere mechanism: a single photon strike a mirror A, makes it rotates, and restart the other way with reverse polarization, strike another mirror B just in front of A, makes it rotates, too, than restart again toward A with just the same rotating power than the first time, so it increase the rotation, etc.

congratulation for your new perpetual motion device, first time i read about this one.

Gem fr (talk) 13:45, 11 July 2017 (UTC)

@Gem fr: The "perpetual motion" issue is something I thought about, but from the other end: it is clear that the energy for the rotation has to come out of the light (which as I explain above, contains considerable energy) but how? For one thing, bear in mind that the energy gained by the target object is not going to be simply proportional to the angular momentum - they're two different quantities. I was still thinking about that part, perhaps for a follow-up (I have to work out how energy and moment of inertia relate, etc.), but first I was hoping to hear more about the state of the art for photon energy transfer. Note that bouncing photons between mirrors is often addressed in the linear sense - there it is clear that bouncing off a receding mirror redshifts the photon, deducting the energy fee. Real ordinary mirrors do reverse circular polarization ... hmmm. I just ran into an issue thinking about that.

If I visualize the photon as a rotating disk striking a wall, the direction that the disk rotates relative to its forward motion does reverse when it hits the wall ... but the angular momentum doesn't change. So can a photon change angular momentum in order not to give up angular momentum? Would a mirror somehow rigged (metamaterials and handwaving...) to keep the same circular polarization actually be what gets angular momentum transferred to it? I realize I am quite confused about this. But I know that some angular momentum sure as hell has to go somewhere if you bounce the photon at a 90-degree angle, because it can't keep the same plane it came in with. So the question is valid, I just don't know what I'm asking. ;) Wnt (talk) 16:06, 11 July 2017 (UTC)

I think you need to check redshift. The energy of a photon is tied with the frequency of the photon (energy = h*frequency). So, since Planck's constant h cannot change, when energy is lost, frequency is reduced. Visible light becomes shifted towards the red end of the spectrum. The photon will keep shifting to a lower and lower frequency, losing energy. Since you can't have a mirror that reflects every possible frequency of electromagnetic radiation, the photon will eventually be absorbed (or pass right through the material). 209.149.113.5 (talk) 16:55, 11 July 2017 (UTC)

I didn't intend to disagree with any of that. There are, nonetheless, some things I'm presently confused about. Bear in mind that linearly, kinetic energy is obtained by integrating momentum, by which I suppose I mean impulse as I described before. It makes intuitive sense that the faster something is moving away, the more redshifted the photon is, because in the plane of the mirror the interaction needs to be symmetrical. A bouncing photon should be redshifted by the same amount - which depends only on that difference in velocity - so it will lose just as much energy to any mirror receding at a velocity v, regardless of its mass; it will also give just as much momentum to the receding object as any other mirror moving at v hit by a similar photon. And yet ... the object's energy is 1/2 mv^2, and it doesn't seem like that increases the same for large and small m given an equal change in momentum from the photon. Now rotational energy is obtained by integrating angular momentum in a very similar manner; the velocity along the radius of gyration changes in the same manner, and I have the same confusion. Wnt (talk) 20:02, 11 July 2017 (UTC)

It isn't the case that a photon will lose the same amount of energy to any mirror that is receding at velocity v - that is only an approximation for large mirror masses. In the general case, the velocity of the mirror changes as well, by a different amount depending on the mirror mass; and that change in velocity occurs at the same time that the frequency of the photon changes.

Basically it's just a totally elastic collision of a photon and a mirror (what is conserved are the relativistic versions of energy and momentum). Icek~enwiki (talk) 20:24, 11 July 2017 (UTC)

@Icek~enwiki: That's true - the math is gone into here. But what troubles me is that if you bounce a photon off a 1-ton mirror, the delta v will be twice what it is if you bounce it off a two-ton mirror, which means that the "delta 1/2 mv^2" will be twice what it is for the two ton mirror. But the term in the paper I link for the change in photon frequency, as you'd expect, is a small inverse term for mass that is negligible compared to the velocity term. So I'm not convinced this actually is relevant to the issue. Wnt (talk) 21:31, 11 July 2017 (UTC)

For the change in kinetic energy of the mirror, we have

$\Delta {\frac {1}{2}}Mv^{2}={\frac {1}{2}}M(v+\Delta v)^{2}-{\frac {1}{2}}Mv^{2}=Mv\Delta v+{\frac {1}{2}}M(\Delta v)^{2}\approx Mv\Delta v$

neglecting the small quadratic term.

This gain in energy, linear in $\Delta v$, is balanced by the loss in energy described by the middle term of the right hand side of equation (6) in the article that you linked to.

But your question pertains to this neglected quadratic term, I guess. It is cancelled by the rightmost term of the same equation, just that in the next step the author neglects that term as well.

Icek~enwiki (talk) 22:17, 11 July 2017 (UTC)

@Icek~enwiki: I shudder to get into math markup, but that equation 6 is:

Change in photon energy = $\hbar \omega -\hbar \omega '={\frac {\hbar v}{c}}(\omega cos\alpha +\omega 'cos\beta )+{\frac {\hbar ^{2}}{2Mc^{2}}}(\omega cos\alpha +\omega 'cos\beta )^{2}$ ... for our purposes, $\alpha$ and $\beta$ can be 90 degrees; they are the angles light moves relative to the mirror taking into account that the incidence = reflection only in the mirror's rest frame. Now, there's a problem with this equation that it's obviously not solved for $\omega -\omega '$. Still, we know that its limit as M tends to infinity is $\omega '=\omega {\frac {1-\nu cos\alpha /c}{1+\nu cos\beta /c}}$. Now your expression $Mv\Delta v+{\frac {1}{2}}M(\Delta v)^{2}$ can, with a slow-moving heavy mirror, be written under the assumption that the momentum is conserved - without angles this is ${\frac {\hbar (\omega +\omega ')}{c}}=M\Delta v$ ... Well, I can't say phooey, because now it seems like it works out, and I'm not sure why it didn't before. I am not getting top marks in math today. Wnt (talk) 23:45, 11 July 2017 (UTC)

I scanned to see if anyone had made this point; apologies if I missed it.

User:Wnt, assuming you're right that photons flip their circular polarization when they reflect, that actually means they don't transfer angular momentum.

That's because the circular polarization of light is defined by whether the angular momentum is in the same direction as the velocity (applying the right-hand rule), or the opposite direction.

So if you shoot, let's call it a "right-handed photon" headed at your mirror, that means that it's "spinning clockwise" (of course there's nothing that really spins AFAIK) as you look at it in its direction of motion.

Now it reflects, and its direction of motion is reversed, but it's still spinning in the same direction as you look at it. But now it's coming back at you rather than going away from you, so now that same spin is counter-clockwise for someone looking towards it in its direction of motion, and hit has become a left-handed photon.

And yet no angular momentum has been transferred. --Trovatore (talk) 20:19, 11 July 2017 (UTC)

@Trovatore: Well, as I said above, you can surely... maybe... make them transfer some angular momentum by bouncing them at a 90-degree angle. Because then the angular momentum out isn't what came in, but in a whole different plane. True, that seems to be transferring partial Planck units, which is a no-no, but what I guess it to mean is that you must end up racemizing the photons to a random mix, with the momentum necessarily going into the mirror. Wnt (talk) 20:25, 11 July 2017 (UTC)

@Wnt: Angular momentum can be a little painful to deal with at the quantum level. The problem is that the operators corresponding to the components of angular momentum in different directions do not commute with one another. So in general, a particle won't have well-defined x-, y-, and z-components all at the same time. I think maybe for a photon they can, if z happens to be the direction of motion, because the other two are zero so it doesn't matter that they don't commute. (For particles with half-integral spin, like the electron, they can basically never have well-defined angular momentum components in all directions simultaneously.)

See Clebsch–Gordan coefficients for more information. Not exactly a light read, and I don't have time right now, though I would like to get this straight in my head. --Trovatore (talk) 23:39, 11 July 2017 (UTC)

@Trovatore: As I recall, photons actually have angular momentum of sqrt(2) Planck constants. Like in a lot of systems, a smaller amount (1 Planck unit) can be measured in a given direction. Intuitively I assume that if you know a photon has a spin along its axis, it must have a "topspin" also in some unknown direction, sort of like an electron in a p orbital that really has an angular momentum of sqrt(l(l+1)) = sqrt(2) in that case. You know it is going round and round the nucleus in the complex solution, but it's also doing something else you don't know (there's a picture in magnetic quantum number). Wnt (talk) 23:53, 11 July 2017 (UTC)

I think Wnt has made this point himself already. As both of you say, an ordinary mirror does not change the direction of rotation of the circularly polarized wave, i.e. it does change the polarization (the relation between the rotation and the direction of propagation).

But if you have an interface between two materials of which at least one is birefringent, you can have a reflection without a change in polarization: The circularly polarized photon can be represented as superposition of 2 linearly polarized states, say a vertically polarized state and a horizontally polarized state, with a phase shift of π/2 between them.

When light would pass from a material A to a material B and is instead reflected, the phase of the light changes by π if the refractive index of B is larger than that of A.

What we need is that for the vertically polarized light, the refractive index of B is larger than that of A, and the other way around for the horizontally polarized light. Then the direction of rotation of the reflected light is inverted, and thus the polarization stays the same.

Because we want to make it easy to measure the angular momentum transfer, material A should be a vacuum; so we need a material that has a refractive index larger than 1 for one linear polarization and smaller than 1 for the other.

Icek~enwiki (talk) 20:24, 11 July 2017 (UTC)

bottom line: as stated above by @Trovatore:, NO momentum is transfered in the reflection process. The photon just reverse course, but keeps the very same angular momentum. Same if reflected with an angle.

there is no real energy / momentum issue, anyway: a mirror is a macroscopic object with close to infinite energy(thermal, etc.) relative to a photon. The reflected photon is just a minute contribution to the regular emission of this macroscopic object, that is, it makes no process difference in energy /momentum for the macroscopic object, whether it reflects incoming light (white body), or absorbs it then emits it as per Planck's law ([[black body}}), or anything in between (grey body): in any case, Radiation pressure applies, but the special photon considered is just one in the ocean of photons that the mirror floats in/contributes to.

Basically, you cannot cope with your question with the particle avatar of the Wave–particle duality of incoming light.

Furthermore, entropy prevents the small photon to rotate the whole macroscopic mirror

Gem fr (talk) 21:45, 11 July 2017 (UTC)

@Gem fr: I don't think physics lets you get away with "small" accounting errors. ;) Wnt (talk) 00:04, 12 July 2017 (UTC)

statistical physics. But actually the "small" accounting error would be to consider the photon, as if it mattered. it doesn't. Mirrors do not exist at the particle level, it is a wave/macroscopic conceptGem fr (talk) 07:30, 12 July 2017 (UTC)

There is some truth to what you say, in the sense that AFAIK a line of particles in the mirror bounce photons off them in random directions, and constructive interference from a large number of particles is needed to convert this from being more like a diffraction grating to being something that sends all the light out in one single direction. That said, a single photon with a poorly known position might reflect off many particles and interfere with itself in this same way, much like a double-slit experiment. Alternatively, I assume that a QM analysis of the mirror with many photons whose localization is well known would find that the interference between them ends up working out that the individual photon has a high chance of leaving on the route it is supposed to leave from a mirror, with its angular momentum still being meaningful. Wnt (talk) 12:25, 12 July 2017 (UTC)

OK, I'm convinced the math works out for linear momentum of a photon bounce. The way it works out is that the mirror yields more when there is low mass, causing more of a change in the photon energy. But ... how does the mirror yield more because it has a low moment of inertia, when a quantum of angular momentum is transferred? And there are still a bunch of leftover questions above... Wnt (talk)

Suppose we have a mirror that reverses the z component (along the direction of propagation) of the angular momentum of the photon. It has to absorb z angular momentum amounting to $2\hbar$.

If the mirror wasn't rotating before the reflection, it is going to begin to rotate. Because the photon is circularly polarized, that changes the frequency of the photon and in this way reduces the energy of the photon.

If we have another mirror, just like the one before except with a lower moment of inertia, the z angular momentum transferred will be the same, but the resulting rotation rate of the mirror will be higher, and thus the frequency shift of the photon will be higher.

Neglecting the transfer of linear momentum, and assuming that the inertial tensor is diagonal in our coordinate system (the z direction is the direction of propagation of the photon), the math looks like this:

Angular momentum of incoming photon:

$L_{z\gamma i}=\hbar$

Angular momentum of outgoing (reflected) photon:

$L_{z\gamma o}=-\hbar$

Angular momentum of the mirror after the reflection:

$L_{zM}=2\hbar$

Angular velocity of the mirror after the reflection:

$\Omega _{zM}={\frac {L_{zM}}{I}}={\frac {2\hbar }{I}}$

The relevant energy (here I use $\omega$ meaning $2\pi$ times the frequency of the photon; $\omega '$ is the frequency after the reflection) before and after reflection:

$E=\hbar \omega =\hbar \omega '+{\frac {L_{zM}^{2}}{2I}}$

From this, we get

$\omega '=\omega -{\frac {2\hbar }{I}}$

Icek~enwiki (talk) 13:46, 12 July 2017 (UTC)

@Icek~enwiki: Your math is impeccable, and yet ... it is saying that the photon, in the act of bouncing "instantaneously" off a mirror, is able to measure the size of the mirror, which could be meters, before recoiling from the surface. Wnt (talk) 22:33, 12 July 2017 (UTC)

@Wnt: You are right; consider the macroscopic analog of a plate that reflects a ball in an elastic collision. There is a spring on the backside of the plate connecting it to a heavy mass. If the natural oscillation time of the system (the spring, the plate, and the heavy mass) is much longer than the interaction time of the ball bouncing, then the speed of the reflected ball is to a good approximation the same as after the reflection from a free plate. The "mirror" then gains some vibrational energy.

In the case of a photon being reflected by a mirror, the angle between the path of the reflected photon and the surface normal being the same as between the incoming photon and the surface normal, it is always a coherent interaction between the photon and many atoms/electrons of the mirror, but of course not necessarily all of them. I guess that the effective mass from which the photon bounces off is in general the higher the larger the coherence length of the photon is. And the formula resulting from the simple elastic collision model is a good approximation if the whole mirror interacts significantly with the photon, like in the case of the reflection from an uncoated glass plate. Icek~enwiki (talk) 09:49, 13 July 2017 (UTC)

That makes sense, though I doubt the photon can couple with all or most of the mirror. If it turns a smaller part, the energy it gives up is greater, and as you say, that energy ends up in little vibrational/rotational whirlwinds that move about in the atomic lattice of the mirror. But some of the rest of the mirror can make itself known to some degree in the rigidity of the bonds where the torque is actually applied. I suppose the process should be quite inefficient for light with a wavelength much smaller than that of the entire mirror. Wnt (talk) 15:34, 13 July 2017 (UTC)

@Icek~enwiki: your math says nothing about this being real or not. Let's take a macroscopic equivalent: if you launch against some rubber wall a cylinder, its axe being parallele to its motion, and rotating clockwise from your point of view: it will bounce and come back to you, but still rotating clockwise relative to you. Write all the math you want stating that it comes back rotating counterclockwise, it won't make it happen. Gem fr (talk) 02:10, 13 July 2017 (UTC)

It is straightforward to construct a mirror that reflects circular polarized microwave radiation with the same handedness (opposite absolute rotation) as that incident. A planar screen consisting of fine parallel wires, each several wavelengths long and spaced about a tenth of a wavelength apart will reflect microwaves plane-polarized parallel to the wires while being transparent to microwaves plane-polarized perpendicular to the wires. The mirror consists of such a screen placed one quarter of a wavelength infront of, either a similar screen rotated 90 degrees with respect to the first, or else in front of a flat metal sheet. A circularly polarized wave striking the front screen is split into two plane-polarized components. The component polarized parallel to the wires is reflected by the front screen, the other component is reflected by the rear screen. The component reflected by the rear screen has a round-trip which is half a wavelength longer than the other, so it suffers a relative 180 degree phase-shift - or equivalently (an additional) reversal of polarity. The superposition of the two reflected waves constitute a circularly polarized wave of the desired handedness.

Because the mirror depends on the λ/4 spacing of the screens, it will only work at one frequency, but it is not beyond the wit of microwave engineers to construct a relatively broadband mirror.

Classically the mirror feels no torque within the incident beam, but would feel a transverse force at the periphery of the beam (because there is some longitudinal magnetic field that there, which results in a Lorentz force on the current-carrying wires). However, the total torque depends on both the length of the periphery of and on the lever-arm (the radius of the beam), and so is proportional to beam the cross-sectional area. --catslash (talk) 23:01, 12 July 2017 (UTC)

nice. i won't call it "straightforward", but nice

however, neither the first nor the second screens would be applied any torque, nor any lateral force, just a push rearward as per radiation pressure, and so would the pair of them as a whole. So i don't see where the torque is supposed to come from is this clever arrangement.

Gem fr (talk) 02:10, 13 July 2017 (UTC)

A torque comes from a force on the charges that accumulate at the ends of the wires due to being pushed there by the component of parallel polarization. For a given half-oscillation, positive charge accumulates at one end and negative charge at the other. Now the electric field of the component of perpendicular polarization acts on those charges.

I thought about that, but for the other half-oscilation, it works just the opposite (negative where positive were, and vice versa), for a null average effect, doesn't it?Gem fr (talk) 10:33, 13 July 2017 (UTC)

For the other half-oscillation, the locations of negative and positive charges are swapped, but the electric field of the component of perpendicular polarization now points the other way, so the torque is still the same. Icek~enwiki (talk) 12:01, 15 July 2017 (UTC)

By the way, conserved quantities like energy and angular momentum etc are (at least as far as all successful theories go) conserved exactly, not just in an approximate or macroscopic way as you seemed to suggest earlier (it reminds of an anecdote from about 1930 when the energy spectra of beta decay seemed puzzling and some famous physicist (Niels Bohr, in a different version Ernest Rutherford) suggested that the conservation laws aren't exact, but Wolfgang Pauli objected and suggested an invisible particle (later called neutrino) instead). Icek~enwiki (talk) 09:49, 13 July 2017 (UTC)

Of course conserved quantities are conserved quantities, i didn't suggested otherwise. My point was exactly of the same nature of your anecdote: there has to be something forgot, as the suggested device (a pair of mirror and a photon bouncing in between) would generate rotational movement out of ... where?Gem fr (talk) 10:33, 13 July 2017 (UTC)

@Gem fr: That's the easy part - the energy comes out of the photons, which is why they get redder with every bounce. This is not conceptually much different than running a motor on the Space Station and watching the one end start spinning one way while the back end spins the other, except here the photons are our battery and drive shaft. Wnt (talk) 17:01, 13 July 2017 (UTC)

@Wnt:indeed. But the energy is the easy part, too (it is still an issue, as this means a "rotating" photon would redshift more than a "non rotating" similar one that just bounce --radiation pressure--. WTH?). The main issue is the "rotation" of the photon, which basically switch side in the scenario: why, how? I surely can imagine a scenario where the whole of a particle's energy is turned into rotationnal energy of a macroscopic object, but it seems to me that entropy doesn't agree (since you require a hell of negentropy to coordinates the mirror molecules into a nice rotation).Gem fr (talk) 06:57, 14 July 2017 (UTC)

It's interesting, isn't it? I'm not sure if the angular momentum of photons seems neglected out of disinterest or if the experiments are just too hard -- I'm still hoping someone can tell me more about state of the art experiments actually being done. First, my assumption is that all photons are "rotating", and a plane polarized photon is merely one in a quantum superposition of states. So there's no distinction between them except which way they're rotating. We have discussed what happens one way -- the other is interesting though, since if a photon decreases the object's rotational energy, it should actually be blueshifted by this to some extent. (Bear in mind though that as described above there is potentially a lot of inefficiency by getting small portions of a mirror to rotate independently as a vibrational energy) Now bouncing a beam of racemic photons at a rotating object and getting back two different frequencies of reflection with opposite circular polarization would be a very persuasive experiment, and one that seems like it might be doable given how awesome the spectroscopy studies of exoplanets etc. seem to be. I even wonder ... can a rotating object spontaneously create a pair of circularly polarized, very long wavelength photons de novo to carry away its energy and momentum of rotation? It seems like in the sort of ridiculously far future scenarios people look at that something like that could spin down the surviving supermassive black holes and the like. Wnt (talk) 13:30, 14 July 2017 (UTC)

I cannot point you to relevant publications (at least not at the moment), but an experiment to detect the frequency change of circularly polarized light reflected from a rotating mirror should be easy to do even with relatively small rotation rates; there are laser spectroscopy systems achieving a frequency resolution of something like 1 Hz or better.

By the way, if you bounce linearly polarized light off a rotating mirror, you get what you describe with racemic photons, just that the phase shift between the 2 circular polarizations is known and constant at the mirror. The reflected light can be thought of as a clockwise polarized beam with a frequency ω+Ω and a counterclockwise polarized beam with a frequency ω-Ω. Assuming that the mirror's angular frequency Ω is small compared to ω, it is still approximately linearly polarized light of circular frequency ω; but because you have a beat between the 2 circular polarization modes, the plane of polarization slowly changes as you move away from the mirror.

If you can make your mirror rotate 10⁷ times per second, the plane of polarization should make a full 360 degree turn per 30 meters (speed of light divided by frequency). I.e. the polarization would be perpendicular to the original one after 7.5 meters. If the mirror is only a few micrometers in diameter, the mechanical stress is low enough; there should be a way of magnetically levitating it and also spinning it up magnetically.

Icek~enwiki (talk) 12:01, 15 July 2017 (UTC)

@Icek~enwiki: I really like this "macroscopic circular polarization" where the apparent plane polarization of two oppositely circularly polarized light beams with different frequencies slowly changes. It reminds me of Rydberg atoms in a vague way. It seems like both very small differences between the light, accomplished by rotating a mirror on the way out, and much larger differences, ?perhaps? accomplished by splitting a pulse laser and bouncing the halves off a mirror that is synchronized to recede or approach the pulse, and circularly polarizing them before recombining them?, might be very useful.

Picture a spacecraft orbiting, oh, Eris. It wants to know exactly where it is now, but bouncing a round trip signal to Earth would take a while. So it has a pair of very sensitive detectors for a small frequency band, one behind a filter linearly polarized one way and one behind a filter polarized the other. On Earth, you have a big laser very precisely pointed, and send out a distinctive timing pulse, then a bunch of other pulses with this macroscopic circular polarization, same base frequency (or more than one if the satellite has more than one detector) but with different beat frequencies for each pulse in the series. One filter or the other on the spacecraft will light up most depending on how many meters it is away, and the succession of pulses each can provide different beat wavelengths, so even if it has interrupted viewing. By using these sort of like digits (more like a Mayan calendar, maybe) the satellite can quickly run through them all and determine precisely how far it is to the base station - possibly down to the meter, provided the Earth-based beat production can be made accurate enough. Set up three base stations (each with its own allotted places in the series after the timing pulse; they'd have to time their broadcasts to match up when they meet at the spacecraft's approximate position) and now it can triangulate. I'm daydreaming the thing could be so accurate that it could measure the stretching of space by the gravity wells of passing asteroids. You might need to program a good GR model of the Solar System just to get maximal accuracy out of its position finding. (probably not, but I can dream!) Does anyone do anything like this? Wnt (talk) 17:08, 15 July 2017 (UTC)

That's a nice positioning system, though I don't think you could do it with pulsed lasers unless you do something fancy with mode-locking. That's because a short pulse means a broad frequency spectrum. Then you have different frequencies rotating the plane of polarization. E.g. if you have a frequency separation between the 2 circular polarizations of 10 MHz, and a spectral width of 1 Hz, then the different frequency components of each of the 2 circular polarization will mess up the helical pattern after 10 million rotations or earlier - and as one rotation is 30 meters, that would be 300,000 km, less than the distance to the Moon. For Sedna you need the a pulse last a day or so and be very stable in its frequency. And a separate frequency for each "digit".

The number of such digits needed depends of course on how accurate you can measure the angle of polarization. There is a limit to the accuracy in terms of photon number (for a single photon, you can only see if it does or doesn't pass through a polarization filter and then conclude that its polarization was likelier to have been closer to the horizontal or vertical direction of the filter).

If there are N photons per digit (given a certain distance from the transmitters, a certain collecting area, and a certain time limit for determining the position), the uncertainty of the polarization angle should be proportional (correct me if I am wrong here) to $N^{-{\frac {1}{2}}}$, at least if N isn't too small. So, if you have a constant factor of polarization rotation lengths between adjacent digits, and you consider leaving out every other digits by increasing the power of the transmitter in order to increase the number of photons, then you'd now need N² photons instead of N photons. That makes it energetically favorable to rather have many digits with lower power than few digits with high power. Icek~enwiki (talk) 19:43, 15 July 2017 (UTC)

Phooey! I knew there'd be something ... should have remembered that a precise frequency implies something happening over and over enough to have one. And making a day-long pulse is no solution because the probe keeps moving, Eris keeps moving for that matter - the hope was to get an "instantaneous" distance reading. Unless there's some fancy math thing to do to figure out the polarization of interfering copies of a pulse despite the lack of a clear wavelength, I think I'm sunk here. Wnt (talk) 20:17, 15 July 2017 (UTC)

Energy consumption in different contexts

You walk for an hour on a certain piece of land in temperatures that require little thermoregulation by your body. Some days later, when you're in the same physical condition, you perform an identical walk: same route, same duration, etc., but it's significantly warmer, and you're sweating a good deal. Are you likely to consume the same amount of energy, or does the extra thermoregulation require additional energy?

Related question: you walk for a much longer period of time on the cooler day, doing that same route ten times without significant interruption. Of course, by the end, you're more tired and haven't stopped for more than bathroom breaks and drinks. Since you're more exhausted, does it require a greater expenditure of energy to move your body the same distance, especially if you push yourself to walk the same speed? It feels much more difficult, but difficulty is fatigue-related; it doesn't have a 1:1 correlation with energy expenditure. Nyttend backup (talk) 13:54, 11 July 2017 (UTC)

human power is about ~100 W ;

osmotic power, the one used in drinking and sweating, is about ~1 kWh/m³, that is, ~1 W if you sweat 1 liter in a hour.

so thermoregulation does require some energy, but not an amount significant enough to be felt.

What you will feel, however, is muscle fatigue, lack of water if you run out of water, overheat if thermoregulation is overrun, and other health risks

Gem fr (talk) 14:44, 11 July 2017 (UTC)

However, generating heat burns lots of energy, with 10 minutes of shivering burning as much as an hour's exercise: [1]. Also note that there's the myth that high temps burn lots of calories, based on the immediate weight loss due to dehydration. But that's absolutely the wrong way to lose weight. StuRat (talk) 15:40, 11 July 2017 (UTC)

Feynman Lectures. Exercises PDF. Exercise 8-4 JPG. Lecture 8 & 9

. .

Here is my solution png. But I do not fully understand the resolution into components. Feynman speaks about this after eq. 8.11. At a moment of Lecture 8 we do not know about vectors. According to Feynman we can get directly only the components for $ds$. I have shown in solution that the velocities obey an analogous resolution with cosines of the same angle. But where is the guarantee that accelerations do obey the resolution? We should prove the possibility of acceleration resolution and the formula $dv_{x}=a_{x}dt$. How to do that? Username160611000000 (talk) 15:10, 11 July 2017 (UTC)

hell, you make it so complicated... it is not, since you have no horizontal acceleration ($dv_{x}=a_{x}dt=0$ ; BTW this is just definition of acceleration, no need to "prove" it), and constant (g) vertical acceleration.

Your answer is obviously not stupid since you find 0 distance traveled for both horizontal and vertical shot, and distance traveled proportional to V² and inverse of g, as dimensional analysis suggest.

Indeed it is correct (but you knew it)

Gem fr (talk) 15:57, 11 July 2017 (UTC)

In the ex. 8-4 we need only components for velocity (so there is no problem yet, but the problem will appear in Lect. 9 with components for acceleration). In the ex. 8-4 the angle Θ is given. Between what is this angle measured ? I think between path (i.e. ds) and X-axis, so we should prove that it is same angle between v and its components.

I have a general question , not directly connected with ex. 8.4: If we have acceleration direction and magnitude and XYZ rectangular coordinate system, then we can project magnitude on axes. Do these 3 projections coincide with the acceleration 3 components received from table of x, y positions? Username160611000000 (talk) 16:48, 11 July 2017 (UTC)

$\definecolor {myorange}{rgb}{0,0.52,0.37}\color {myorange}dv_{x}=a_{x}dt=0$ ; BTW this is just definition of acceleration, no need to "prove" it -- a = dv/dt is the definition, but Feynman have said nothing about components. E.g. we have a table with columns t, x, y and infinite number of rows for each particle position. Then from this table we know dx and dy, therefore we can find v_x, v_y (organize 2 more columns). Then we can find dv_x, dv_y and therefore a_x, a_y. We know only $ds={\sqrt {dx^{2}+dy^{2}}}$ from assumption that trajectory can't be very curved over small $ds$ and $v={\sqrt {v_{x}^{2}+v_{y}^{2}}}$ as proved, but we know nothing about dv, dv_x, dv_y. How can we calculate resultant $a$ and angle $\angle (a,a_{x})$ and check these values with directly measured ones by the accelerometer? If such formula as ${\text{magnitude}}={\sqrt {{\text{x-component}}^{2}+{\text{y-component}}^{2}}}$ is axiom, then why does Feynman prove it for $v$?Username160611000000 (talk) 20:34, 11 July 2017 (UTC)

Again, you turn very simple things into horrible. dv = dv_x + dv_y, that's a definition, too. Not a definition of v, but of a coordinate system. This answers your "general question" above.

there is no such thing as an assumption that "trajectory can't be very curved over small $ds$"; physicists are simple-looking, pragmatic, persons, that first try simple things and are happy with them as long as they works. Here, the idea is just to try to find a solution in the realm of the simplest, that is, "not very curved". If it works (and it does, it eventually appears), why look for complicated solution in the realm of erratic, brownian, fractal bang-around kind of motion, where the word "trajectory" lose relevance? If, however, it appeared that so such solution could be found, or if it didn't match the observations, well, some other, more complucated solution would be looked for. happily this is not needed here.

An accelerometer is not a mean to measure angle. It will just give you g, not the current angle of the trajectory relative to the flat ground. This you'll have to calculate with usual trigonometry.

Gem fr (talk) 22:35, 11 July 2017 (UTC)

dv = dv_x + dv_y, that's a definition, too. -- it is a vector form, which we officially do not know. If v = (v_x² + v_y²)^0.5 is definition, then why does Feynman prove it by formulae 8.11 - 8.15?Username160611000000 (talk) 03:56, 12 July 2017 (UTC)

facepalm. Feynman is teaching, for god sake. Teaching implies saying things so that even the dumbest get it. That's why. Gem fr (talk) 07:09, 12 July 2017 (UTC)

First, please use dry scientific voice. Second, such answer does not agree with Feynman's approach. Feynman gives some mathematics subjects but not in a style of writing down redundant data.

The Eq. (9.4) and the same in Lect.8 are also redundant information for poor students, are not they? Username160611000000 (talk) 09:36, 12 July 2017 (UTC)

IMHO, saying that v_x is the horizontal component of v is a definition. Coming up with a Pythagorean Theorem solution for the relationship of its magnitude with v is a calculation. Wnt (talk) 12:44, 12 July 2017 (UTC)

It seems in Ch. 8-5 we do not know at first that v²=v_x²+v_y², because all these values are calculated independently (it could be that v²≠v_x²+v_y² as it is for Δs, Δx and Δy). Username160611000000 (talk) 15:14, 12 July 2017 (UTC)

In the limit, as Δs, Δx and Δy → 0, then Δs² → Δx² + Δy² and so (Δs/Δt)² → (Δx/Δt)² + (Δy/Δt)². But the limit of Δs/Δt is v, the limit of Δx/Δt is v_x and the limit of Δy/Δt is v_y (by definition). So v² = v_x² + v_y². Gandalf61 (talk) 15:34, 12 July 2017 (UTC)

@Gandalf61: I think we are finally on the right track. So Feynman proves v²=v_x²+v_y² not because he is teaching, but because it is necessary step. Then how can we prove the analogous for acceleration? For trajectory we have used the graph and ds² = dx² + dy². Is it possible to plot v_x - v_y graph? E.g. if X-Y graph is like this then the v_x - v_y graph will be png and we can use the same approach, can't we?Username160611000000 (talk) 16:24, 12 July 2017 (UTC)

copyright

Point of order: the link above is actually to "The Swiss Bay", which apparently is a The Pirate Bay knockoff of some sort (I recognize the ship). I don't think there's any need to resort to this when the text is readily available at Caltech, and those links might be more stable. In any case, there is no earthly reason not to simply reprint Question 8-4 here in its entirety for the reader's convenience: "8-4. A projectile is fired over level terrain at initial speed V, at an angle $\theta$ with the horizontal. (Neglect air resistance) Find the maximum height attained and the range." Wnt (talk) 12:35, 12 July 2017 (UTC) Thank you. "The Pirate Bay" was a torrent-files hash base, but not a file - server. If The Exercises are available on Caltech site, please write the link. I'm still in suspense about copyright, Medeis said that typing the exercise was bad. I think if I give a link to a public resource, it is not copyright violation. Username160611000000 (talk) 14:03, 12 July 2017 (UTC) I'm not going to argue Wikipolicy here; it's off topic, though I'll note some people are fanatic about it. But from a practical standpoint, you're generating a database of calculations that could lose the associated questions if the local shamans put out a fatwa against Swiss Bay. The individual exercises are short, straightforward questions and you can use Fair Use to simply write them here as I did, and it will make your questions much more usable long term. Wnt (talk) 14:10, 12 July 2017 (UTC)

I think we might solve the height by determining the kinetic energy at the beginning, subtracting the kinetic energy of the horizontally moving round at its highest point, then determining where that amount of potential energy puts it. Or we might divide components entirely, then solve them separately. You seem to be pushing for a parametric approach, which is possible but more complicated. So if we take that all the way:

theta(t) is the angle of the round; it starts at a known positive value at t=0, has the same negative value at t=T when the round lands, and has a value of 0 at t=T/2.

v(t) is the speed (scalar) of the round; it starts at a known positive value at t=0 and has the same value at t=T.

We expect dv/dt = -g sin theta(t), i.e. gravity pulls back to the extent the round is going up.

Dimensional considerations make me think dtheta/dt is something like -g cos theta(t) v(t)^-2 (we want inverse seconds). Unfortunately I actually have to think about this one. ;) If we can get through this, and the diffy-q's that follow, then we can have a true parametric solution. Wnt (talk) 14:07, 12 July 2017 (UTC)

... OK... after a short delta t with perpendicular gravity g cos theta(t), the object will be moving at its original velocity, but also downward by g delta t cos theta(t); this divided by v is the sine of delta theta(t), which for small values is delta theta(t). So delta theta(t)/delta t = g cos theta(t)/ v(t), which has the appropriate dimensions at least (i.e. none).

Now I have to dust off what I know about solving these, which is in a sorry state of decay. I'm thinking the first equation is a nonlinear separable differential equation that can be rewritten dv = -g sin theta(t) dt and integrated... erm, not so easily as I thought. Hrm. Well, if I put the sines and cosines together I know that (dv/dt)^2 + (v dtheta/dt)^2 = g^2. Which means ... well, what it means is I ought to go relearn differential equations. Meanwhile, this exercise demonstrates that the only way I can see to do this "without vectors" is to 'intuitively' make so many vector assumptions (like perpendicular velocity and splitting gravity into components) that you're just fooling yourself. The interesting part in all this is I keep feeling that there is some angular momentum like property about the moving shell, despite its mass and orientation not being an issue, as evidenced in the second term in that last equation above. That equation is sort of like a KE + PE calculation, with the mass cancelled out, but instead of PE we're looking at how much the trajectory turns. I'm wondering if there's some physics concept I've overlooked here. Wnt (talk) 12:18, 15 July 2017 (UTC)

Energy loss, planes and ships

How much energy planes and ships lose by generating a vortex behind them? — Preceding unsigned comment added by Tyief (talk • contribs) 17:24, 11 July 2017 (UTC)

See lift-induced drag. It isn't a simple "how much" question. There are a lot of variables. 209.149.113.5 (talk) 17:49, 11 July 2017 (UTC)

see also

Drag coefficient

finesse: if it is 12, for instance, the plane travelling a distance L lose the same energy as it gains by falling 1/12 L, that is, mgL/12 (where m is its mass and g the acceleration of gravity). The higher the finesse, the lower energy loss.

Reynolds number

Gem fr (talk) 23:06, 11 July 2017 (UTC)

Boats, ships and aircraft all leave wakes consisting of vortices and turbulence which represent kinetic energy of the water and air. That kinetic energy is all eventually transformed to heat as the kinetic energy subsides to zero. Dolphin (t) 21:29, 12 July 2017 (UTC)

But if another vehicle is riding in the wake, or draft, it can reclaim some of that energy. StuRat (talk) 21:42, 12 July 2017 (UTC)

Yes, and not only other vehicles. If a ship's wake washes sand or debris higher up a beach it represents kinetic energy transformed to potential energy. Vortices and turbulent wakes can also transform kinetic energy to sound energy. Dolphin (t) 07:59, 13 July 2017 (UTC)

Relative calorie burn in air and water

Hello. How many calories would a person standing in water burn per minute, compared with the same person standing on dry land at the same air temperature? Twice as many? Thrice as many? Thanks.--90.69.12.160 (talk) 18:00, 11 July 2017 (UTC)

Assuming that the air and water are the exact same temperature and there is absolutely no air or water motion (impossible), then the only difference would be weight change due to buoyancy. You weigh less in water (mass is the same, weight changes) because you float a little. Therefore, your muscles have to hold up less weight. You should burn less calories, but not much less. How much less is dependent on the person. Some people are more buoyant than others and body mass is highly variable. 209.149.113.5 (talk) 18:07, 11 July 2017 (UTC)

Assuming that the temperature of the environment (water or air) is less than body temperature, the body loses heat. The loss of heat is larger in water, as the heat transfer coefficient from skin to water is larger than from skin to air, and the thermal conductivity of water is larger. The heat has to be produced by the body, so more energy is converted to heat in the case of water (the picture is made more complex by many factors, e.g. the constriction of blood vessels close to the skin as the skin is colder in the case of water). Icek~enwiki (talk) 20:32, 11 July 2017 (UTC)

you need to consider not just temperature, but also speed of the air/water, and Thermoregulation in humans.

I remember (no ref at hand, sorry) that confort temperature (that is, when thermoregulation is mostly in idle state, the loss being more or less equal to basal production, around 100W) is around 20-22°C in the air, but 26-28°C in water

also, you can easily stand -40°C in the air (with no wind) for as long as enough food is available , while a mere 2°C in water will kill you in few minutes (hypothermia).

Gem fr (talk) 23:23, 11 July 2017 (UTC)

I think that being correctly dressed would also assist in surviving at -40C. I don't think -40 in a t-shirt and jeans is going to do it. I would like to see a source for the idea that you could survive at -40 provided you had food and were correctly dressed. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 00:04, 12 July 2017 (UTC)

I seldom if ever try to argue from experience/authority. But User:CambridgeBayWeather I think hails from Cambridge Bay, and so I respect their thoughts on human survival in cold weather and climates ;) SemanticMantis (talk) 00:38, 12 July 2017 (UTC)

Turns out this is hard to search for. I suspect because nobody has tried it. I just realised that without water you aren't going to live too long. Melting snow in your mouth will just cause hypothermia to set in quicker. This also means that you are putting frozen food in your mouth and that's going to lower your body temperature. By the way it is possible to survive at least 18 minutes in −1.7 °C (28.9 °F) water, see Lewis Pugh#North Pole. I uploaded a couple of pictures of people swimming in the Arctic Ocean, File:Swimming in the Arctic Ocean 01.JPG and File:Swimming in the Arctic Ocean 02.JPG. CambridgeBayWeather, Uqaqtuq (talk), Sunasuttuq 04:22, 12 July 2017 (UTC)

I remember a documentary about these things, picturing a man swimming in cold water (without swimsuit). But the man was trained, and the usual "don't do this, you may kill yourself" message was clear, as the body of the man had adapted in very interresting ways (hence the point of the documentary). He could swim and hence generate heat in conditions the normal person just dies. Wasn't this one [2]; may be this one [3] (i didn't check)

Henri Guillaumet was not trained, but he nonetheless survived 1 week in harsh climate above 3000m

Gem fr (talk) 10:42, 12 July 2017 (UTC)

July 12

Intelligence difference between animals and humans

So... Why is it that I notice that many animals don't even really want to try to do what we do? For instance, why will a cat not see a human pick up a pencil to write, and see that same thing often, and then try to pick up the pencil themselves. (though I don't believe it'd work, I've never seen a cat try) What sort of lack of intelligence is this really? To make the decision "I will try to pick up a pencil like that human is" doesn't sound like it takes that much thought, especially if you can catch a mouse so easily. What field or term in neurology or psychology refers to this? Something that an animal could possibly think of, but that they won't?

To expand my question, in a similar manner, what is one of the things that a human would pretty much never decide to do that we probably could at least think of trying to do? I really can't think of any examples, because whenever I think of an example of one of these things I can almost definitely imagine some human doing that thing, no matter how crazy or stupid it sounds; like trying to jump to the moon with his or her own two feet. Yeah I think some human would be crazy enough to try that.

But this seems to be because humans are so diverse in thought, and really maybe a human could think of or perceive or try anything? Philmonte101 😊😄😞 (talk) 01:30, 12 July 2017 (UTC)

The term is observational learning. Spitballing, curiosity is also probably important here (sadly we have no article on animal curiosity that I can find). What if the cat totally understands what you're doing, but just doesn't want to copy you? Someguy1221 (talk) 03:05, 12 July 2017 (UTC)

It has puzzled me for some time how the dog, despite being in close proximity to humans for thousands of years, has never learned to raise one leg and point to something, such as its food bowl when it wants food. Instead, all it can do is bark and run back and forth from the object. Akld guy (talk) 07:44, 12 July 2017 (UTC)

Akld guy based on the excellent post by SteveBaker here, the answer may be that dogs equate humans' hands with extra mouths, rather than with paws. Adrian J. Hunter^{(talk•contribs)} 08:26, 12 July 2017 (UTC)

It may also puzzle dogs how the human, despite being in close proximity to dogs for thousands of years, has never learned to walk on all fours, raise a leg and.... Proximity learning only works between animals of the same species; the dog or cat does not identify with the human or their wish to pick up a pencil. Similarly you don't train a dog to do tricks by doing them yourself.--Shantavira|^{feed me} 08:02, 12 July 2017 (UTC)

As anyone who has interacted much with dogs knows, dogs 'point' by attracting a human's attention and then directing their gaze, which any dog-familiar human readily understands. This natural, mutually understood interaction has been refined by artificial selection in the particular dog breeds known as 'pointers', but the unaugmented version works well because over the thousands of years that dogs became self-domesticated by their associations with humans, humans also Coevolved to better interact with dogs, in what might be termed "co-domestication" (see Section 3.6 Convergent evolution between dogs and humans). {The poster formerly known as 87.81.230.195} 90.204.181.91 (talk) 08:38, 12 July 2017 (UTC)

Dogs can learn to point, it's just they normally they do so with their face and eyes rather than their paw (after all, dog anatomy is different to ours, and their "wrist" is effectively just a second knee - they lack the ability to point as precisely as we can.) Smurrayinchester 09:28, 12 July 2017 (UTC)

Pointing requires, apart from a faculty for cooperative behavior, a certain recursive way of thinking that (trigger warning) may well be unique to humans. Chimps are generally bad at understanding finger pointing (apparently, worse than dogs.) Some actually believe the neurocircuitry for pointing, creating joint attention, shared intentionality etc had to be already in place for speech to evolve. I once worked in a manual job with people whose language I didn't speak. You won't believe how well one can get along by just pointing at stuff, showing things etc, they instantly knew what the problem was and could help me Asmrulz (talk) 13:03, 12 July 2017 (UTC)

Our cat does like to grab pens, but not to write . Interestingly they have their own limited language, which we can learn and exploit to our advantage. I'm not a cat or a dog, but I'm pretty sure that they simply find pointless that we mysteriously gaze at paper and computers, except for the fact that those objects steal our attention from them... But it can be fun to watch their amazed reaction and wondering when we do peculiar things or manipulate new objects. They probably are trying to understand what occurs for a moment (the aforementioned curiosity is also a factor here) and to evaluate threat and safety, if it can be used as a toy or food, etc. Their needs are simple and they are not very interested in anything beyond those, we can trick them into doing things they would not normally do, but they don't do them for the same reasons we do (except perhaps for the rewarding factor)... Animal cognition is probably a good starting point to read more about the topic. —PaleoNeonate - 12:56, 12 July 2017 (UTC)

Oh, a point I was thinking about before posting, but forgot to include: humans are capable of abstract symbolic thinking. This is what allows us to develop complex language, to analyze, classify, develop theories and technology, etc. We are even capable of metacognition and we seek answers to transcendental questions... —PaleoNeonate - 13:28, 12 July 2017 (UTC)

You guys are hopefully outdated. About 300 years ago a man named Gulliver in one of his numerous travels came across a race of Houyhnhnms. If you don't know how to pronounce it, ask your horse if you happened to have one. They were horses, but highly intelligent. Needless to say they had language and a developed culture. They also dominated another race, Yahoo. Yahoos still survive in Silicon Valley but their future is questionable. Jonathan Swift is the man who wrote the account about this remarkable adventure. He-he. --AboutFace 22 (talk) 13:49, 12 July 2017 (UTC)

Reference deskers try to provide references. Houyhnhnms are a race of intelligent horses described in the last part of Jonathan Swift's satirical Gulliver's Travels in 1726. Blooteuth (talk) 15:35, 12 July 2017 (UTC)

Mirror neurons are important here. They are involved in the "learn by copying" process. For animals which lack those, they don't seem to "copy for copying's sake" (also known as "aping", because primates are particularly into this behavior). However, they do still do "goal based copying". For example, if a dog wants out, and has observed that the doorknob must be turned to get out, he may very well try turning it with his mouth (and, if large enough, he may succeed). StuRat (talk) 16:26, 12 July 2017 (UTC)

Re the last observation: it can depend greatly on the intelligence (in the broad sense) of the individual dog. Friends of mine owned two sibling English springer spaniels: one of them figured out (on his own) how to open a particular door they both frequently wanted to pass through (by pulling down with his forepaws on the lever-style door handle), and did so many times every day; his brother, despite observing this many times, never even attempted the same feat and had to wait until someone opened it for him. {The poster formerly known as 87.81.230.195} 94.9.9.203 (talk) 09:09, 13 July 2017 (UTC)

I once observed two cats in a room with the door closed. Many times they had seen someone turn the round doorknob and open the door. After an extended time in the room one cat climbed on a table by the door and started rotating the doorknob with its paws, while the other stood on the floor with its claws in the gap at the edge of the door and pulled on the door. No one had spent time teaching them to do this and in the end it was unsuccesful, except that after a while I opened the door and let them out (so maybe it was successful).Edison (talk) 04:01, 14 July 2017 (UTC)

We're fortunate we have opposable thumbs that we can leverage to outwit our pets... - Nunh-huh 04:06, 14 July 2017 (UTC)

How does polysiloxanes effect scar development?

How does polysiloxanes, common in the scar-treatment gel "Kelo Cote" effect scar development? Are these polysiloxanes become a permanent part of the scar tissue, or what? 109.66.156.177 (talk) 01:54, 12 July 2017 (UTC)

According to the best sources I can find, "the mechanism of action is unknown". That said, scar treatment gels were preceded by solid scar treatment sheets that most likely were not diffusing into the skin. There are a number of theories as to how scar treatments work that you can read about here, if that's not behind a paywall for you. Someguy1221 (talk) 03:02, 12 July 2017 (UTC)

My plastic surgeon says that it keeps the scar tissue hydrated. That is true of the silicon sheets, as the skin underneath gets distinctly clammy. Whether that really improves scarring is a whole nuther thing. Greglocock (talk) 06:53, 12 July 2017 (UTC)

Animal intelligences.

In the view of evolution, humans brains started to get smarter when we learned how to make fire and cook meat. They say that put a big speed on human brains intelligence. Then like at carnivores (like wolves and cats) are in general more intelligent than herbivores (like giraffes and hippos). I don't think anyone knows, but who do you guys think are the dumbest carnivores, vs. the smartest herbivores? Heh. (In this case, omnivores are still compared as carnivores).

2ndly, all carnivoresly expected evolved from a former herbivore. Are there are species that are herbivores that evolved from a carnivore? Thanks. 50.198.21.45 (talk) 14:00, 12 July 2017 (UTC).

Who says carnivores are more intelligent? And who says they evolved from herbivores? ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:10, 12 July 2017 (UTC)

The first carnivore must have evolved from a herbivore, perhaps with a scavenger being the first step. As for being more intelligent, the theory is that this is needed to catch prey. (While intelligence also helps at avoiding predation, some prey animals just overwhelm their predators with numbers, such as locusts, or spend much of their time some place safe, like penguins.) Of course, not all predators are intelligent, as in the case of predatory plants. StuRat (talk) 16:43, 12 July 2017 (UTC)

the theory is that... - which theory? Link? Tigraan^{Click here to contact me} 16:54, 12 July 2017 (UTC)

Don't think it has a specific name, but here's a source at the University of California listing many factors that control animal intelligence, and this is number 1: [4] (it's about 2/3 down the page, do a CTRL F on "carn" to find it quickly.) StuRat (talk) 18:22, 12 July 2017 (UTC)

• Apart from humans, the most intelligent* animals are generally thought to be other great apes, and these include gorillas which are herbivores (see Gorilla#Diet) and very intelligent (see Gorilla#Intelligence). (* Really, it's hard to say whether one animal is more intelligent than another, because there are so many ways to be intelligent. Who can tell if a dolphin is cleverer than a chimp, for instance?) Smurrayinchester 15:08, 12 July 2017 (UTC)

• Baleen whales are quite intelligent, and, while technically carnivores, since they eat zooplankton, there's not a huge difference between hunting them or phytoplankton. StuRat (talk) 16:39, 12 July 2017 (UTC)

Typically fish have smaller brains (relative to body mass) than reptiles, which are again smaller than mammals. While imperfect, brain size often correlates with proxy measures of intelligence. So off-hand, I would look for carnivorous fish for relatively low carnivore intelligence and herbivorous mammals for relatively high herbivore intelligence. However, you'd have to find specific studies to get anything concrete and measuring intelligence across species is fraught with difficulties. Not sure about pure carnivores giving rise to herbivores, though there probably are some, but bears are typically omnivorous, while the Giant Panda has evolved to be a very specialized herbivore. Actually, the Panda is interesting in part because it isn't very well-adapted to its diet and doesn't obtain nearly as much nutrition per kilogram of food as most herbivores. Dragons flight (talk) 17:01, 12 July 2017 (UTC)

Yep, giant pandas had carnivorous (or at least omnivorous) ancestors, and there's the famous (almost totally) vegetarian spider Bagheera kiplingi, who's snuck in to an ant-plant mutualism, and steals beltian bodies, which are basically nature's veggie burger. All the herbivorous bees are derived from carnivorous wasps. See here [5] for some nice discussion of evolution of intelligence. It's not so simple as carnivores are generally smarter. Omnivory and social structure also have a lot to do with animal cognition. SemanticMantis (talk) 17:20, 12 July 2017 (UTC)

Here's a good accessible peer-reviewed article that summarizes a lot of our current understanding on how intelligence evolves, with lots of good references for further reading [6]. SemanticMantis (talk) 20:03, 12 July 2017 (UTC)

A highly developped brain requires a lot of energy and long supervised development time. However when environment pressures require a certain level of intelligence and fast-paced adaptation to survive, species that cannot rapidly cope with changes can become extinct by selection. When larger territories are needed, more complex food-processing or hunting needed to eat, more skill and organization required to achieve mating and raise offspring, larger group cohesion needed to cope (including for task assignment and specialization), culture transmission needed to keep and improve acquired skills, etc... It becomes more plausible for this extra energy to be offset by the advantages more complex brains provide. Evolution of human intelligence summarizes some of those aforementioned aspects. —PaleoNeonate - 21:41, 12 July 2017 (UTC)

Such explorations as above should start with brain anatomy. Human brains are much more complicated than a cat's brain for instance and seeing what my cat actually can do always marvels me, she is so intelligent and in many instances she even mewing appropriately. She is able to jump on the door handle and open the door. Cats are unbelievable. It's been said that domestic cats and dogs can learn and understand 250 words. I think my cat is already over that limit. :-) Human brains aside form massive cerebral cortex, many gyri (folds) existing to increase the cortex surface even further, have two unique areas that are absent in all other species. They are Wernicke's area (receptive understanding of language) and Broca's that controls motor language function. Sure there is no direct correlation between the brain size and intelligence, Frederick Gauss, the Prince of Mathematics, reportedly had a very small brain, but still his brain was probably larger than that of many animals. Exceptions are dolphins and elephants, but those quite intelligent in their own right. They still lack those specialized areas. --AboutFace 22 (talk) 00:49, 13 July 2017 (UTC)

Yep, and other animals presumably have specialized brain areas we lack, like for echolocation, or greatly expanded areas, like for processing scents. However, we don't normally consider this type of brainpower "intelligence". StuRat (talk) 00:57, 13 July 2017 (UTC)

It's hard to tell how big a brain is needed for intelligence. Portia (spider) with 60,000 neurons for instance is considered quite intelligent. It is also indubitably a carnivore. Dmcq (talk) 09:40, 13 July 2017 (UTC)

connect opamp outputs together?

as a poor man's mixer, instead of a proper summer circuit (the outputs go to either sides of a pot and the taper contact is the output of the circuit) - is this kosher? 80.171.94.156 (talk) 17:27, 12 July 2017 (UTC)

You'll need to connect them through a couple of resistors. This is the standard circuit for summing two signals. Take a look here [7]

Mostly you need to just follow the rule of following a low impedance output with a high impedance input (and that "input" can be as simple as a couple of resistors). This avoids the following stage loading the preceding stage, and especially for avoiding that loading changing if you move a pot. Andy Dingley (talk) 21:14, 12 July 2017 (UTC)

The circuit "[Opamp] outputs go to either sides of a pot and the taper contact is the output" can be used as a simple crossfader (it does not sum the signals). The potentiometer should be a linear-taper type with resistance R ohms noting that each op-amp must be capable of driving a load of R/2 ohms and the following stage should be a buffer with high input impedance. Blooteuth (talk) 16:10, 13 July 2017 (UTC)

July 13

Do midwives have knowledge of abortifacients and induce abortions?

Who actually performs abortions? A surgeon, physician, midwife, or a pharmacist/apothecary? Is the physician traditionally for rich folks but the apothecary is for poor folks and the midwife is for women? 50.4.236.254 (talk) 01:16, 13 July 2017 (UTC)

Off topic. The gist is the OP should use the encyclopedia more, and a lot of people like Wiki-policy too much.
The following discussion has been closed. Please do not modify it.
Did you actually try to look any of this up in Wikipedia or anywhere else, or are you just rambling ideas without making any effort to look things up? Wikipedia has articles on the topic, which are quite detailed. Read them first, then ask questions about what you don't understand. Your manner of asking random questions whose answers are easily findable with a minimum effort are starting to wear the patience of many people quite thin. --Jayron32 01:47, 13 July 2017 (UTC) Eh? I thought this was the reference desk, and that I'm supposed to ask for references, even if they are "easily findable". 50.4.236.254 (talk) 02:15, 13 July 2017 (UTC) See those boxes at the top of the page? Have you ever tried using them? --jpgordon𝄢𝄆 𝄐𝄇 05:01, 13 July 2017 (UTC) What boxes? 50.4.236.254 (talk) 11:49, 13 July 2017 (UTC) Reference Desk is for those that have all ready done some basic inquiry, but without result and thus require help from a greater cohort of editors here on RF. Read : [8]. Aspro (talk) 12:21, 13 July 2017 (UTC) Cute. And the kind of thing that could cause a lengthy row at the talk page. But I too am curious: What boxes are you talking about? ←Baseball Bugs What's up, Doc? carrots→ 20:44, 13 July 2017 (UTC) I'm quite sure the search boxes for wikipedia and for the reference desk. (Particularly the former.) Nil Einne (talk) 09:02, 14 July 2017 (UTC) You're welcome to ask for any reference you're having trouble finding yourself, but the reference desk is staffed by real people who are not being paid for this. We're happy to help if you need help, but It's rude to ask volunteers for something that you could have gotten yourself very easily. For this question, I think you could find your answers from the article abortion. "Apothecary" is a job description that doesn't really exist any more in developed nations. The modern equivalent is a pharmacist. ApLundell (talk) 15:56, 13 July 2017 (UTC) I would have thought the reverse. That it is rude to ask paid employees. It's like a guy who says, "Please volunteer to cut my lawn." If I don't want to cut off his lawn, then I'd not cut his lawn. I would probably not say, "Stop being lazy! You are able-bodied, so you should cut your own lawn." Instead, I would just not reply to the request. A similar example is receiving a phone call from a telemarketer or scammer or phone prankster. I'd just not reply. By not replying, I can pretend that I never saw the request in the first place. 50.4.236.254 (talk) 23:29, 13 July 2017 (UTC) I think you're missing the point. If someone is paid to do a service and you're asking for that service, then they tend to be more patient when people waste their time with stupid question. So for example, a real life librarian staffing a reference desk who is paid to (probably amongst other things) staff that reference desk is going to be more willing to put up with a dumb troll like you simply because it's their job. Likewise a if there is a menu right in front of you, wait staff will be more patient if you ask them what's on the menu than if you ask some random person. If you want to stick with the silly lawn example, if you ask someone who owns a lawn mowing business to mow your lawn be fully willing to pay them whatever the cost is, then they may say no if they have too many customers or whatever but, they're not likely to get annoyed if you only ask once. If you ask some random person, even if you're willing to pay them, they're likely to say no but may or may not get annoyed depending on the circumstance. If you're Mark Zuckerberg and today you go to a community group who offers free lawn mowing for the poor elderly, and ask them to mow your lawn for free they may very well get annoyed and ask you to bugger off. If you keep asking them in different ways they may end up asking you to leave, threatening to trespass you if you keep refusing. Nil Einne (talk) 08:58, 14 July 2017 (UTC) Nil, I have to say that I think you crossed over the line into personal attacks here. Dragons flight (talk) 09:20, 14 July 2017 (UTC) Don't think it so much a personal attack on User:Nil Einne's part, more an expression of growing frustration, that many of us are getting from this single purposes and anonymous account that just keeps asks questions on our help desks instead of doing some simple searching themselves. One might well ask: what did his last slave die of. Rather than ban, lets send him to Coventry. For he is delaying us to getting around to addressing and considering fully, other questions from people that really derive some useful benefit from our help desks. The OP has proven himself to be a butterfly that flits from one flower to another. This is not what our helps desks are for. Aspro (talk) 11:53, 14 July 2017 (UTC) I think selective ignoring of the OP is better than total ignoring of the OP. On rare occasions, the OP does answer questions on the Reference Desk. It's really just the questions that get annoying. I say we ignore the OP's stupid questions. 50.4.236.254 (talk) 23:14, 14 July 2017 (UTC) 50.4, I understand why you would think of that, but I'm afraid you're wrong. This is not equivalent to asking random friends if they'll help you out. This is a service provided by volunteers. People here are happy to help you, but if it's clear you haven't put in even thirty seconds of effort on your own, your request will be seen as rude. (After all, you're implying that your time is too valuable to waste, but ours isn't.) ApLundell (talk) 14:21, 14 July 2017 (UTC) You are assuming that the person actually wants an answer. There are people who go to forums like this and try to think of stupid questions just to be annoying. They don't actually want an answer. They just want to be annoying. 209.149.113.5 (talk) 18:39, 14 July 2017 (UTC) Yes. I am assuming good faith. ApLundell (talk) 19:19, 14 July 2017 (UTC) Without disagreeing in the slightest with any particular point that Jayron, jpgordon, ApLundell, Nil Einne, or Aspro have said, it's also worth noting that there's no reason not to simply ignore the poster in circumstances such as these. Jayron's initial AGF comment was perhaps advisable, insofar as there's no harm in making the point he chose to make to the IP (who may have taken the advice on board). But as soon as the IP started getting into this entitled defense of their refusal to do even the most basic search of information (that is already painstakingly gathered and organized per the entire point of this project), the regulars here should know to wash their hands. Not double down on trying to explain to the IP why it was not a smart first move to not at least look at our article on abortion first. Because if this individual is a troll (and I agree it looks like a very distinct possibility, given their responses), then you are simply biting hard on the lure. And even if they aren't a troll--even if they are just that lazy--then ignoring them is still the best way to send the message that we're only interested in helping those whose interest in a subject is beyond the idle level needed to post an inquiry about whatever random thought happens to be floating through their head at the moment--or at least when the answer to said basic question is easily found within 60 seconds of using our encyclopedia. Which this community was kind enough to build for them for that exact purpose. If ignored in such circumstances, they either take the hint and next time wait until they've truly hit a roadblock before inquiring here...or they don't, but at least our contributors didn't waste their time on the affair and just blew by the thread without comment (in an ideal world). WP:DENY all around, is my advice--especially if this has been going on for a while with this particular user. Snow let's rap 21:14, 14 July 2017 (UTC) I think it is best to ignore the stupid questions. 50.4.236.254 (talk) 23:14, 14 July 2017 (UTC)

Recommended reading: Midwife, Abortion. Blooteuth (talk) 15:51, 13 July 2017 (UTC)

Midwives don't do abortions. Their role is to assist in standard childbirth. Abortionists do abortions. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:25, 14 July 2017 (UTC)

I guess as a tautological statement, that's true, but "abortionist" is not a title or occupation used anywhere in the world, that I'm aware of, except in reference to unlicensed/"back alley" procedures, perhaps. In most of the world where the procedure is legal and regulated (and even in many nations where it is illegal) abortions are performed by a medical doctor (they could possibly be a surgeon or a general physician, but they are likely to be specialists in obstetrics).

You are actually mistaken that midwives never perform abortions; in many developed countries they do. In fact, a Swedish midwife very recently went before the European Court of Human Rights, asserting unfair dismissal from her position as midwife after she was terminated for refusing to perform abortions, for religious beliefs. But part of this is a matter of variation in nomenclature and licensing schemes; midwives are not licensed to perform abortions in the U.S., for example. And given the focus of their work, I would speculate that most would not want to (though no small number of them will have provided assistance in a miscarriage at some point in their career). Pharmacists do not perform abortions (not legally anyway), though most would have the knowledge of how to trigger one with medication, though this would be very dangerous, especially outside a controlled environment and not under the guidance of someone trained in the procedure.

They like careful answers as well as careful questions, but this is still off topic here
The following discussion has been closed. Please do not modify it.
There, now you see what you made me do? I've had to answer the OP's question in full in order to correct your fly-by response. (I understand being curt with the OP, but if you're going to answer at all, remember that other people may end up reading this). Shall we now close this as a resolved, since the OP's very basic question ahs been answered? Snow let's rap 21:48, 14 July 2017 (UTC)

Resolved

What to do if you asked two experts and they have conflicting opinions?

If you don't know much (or nothing at all) about some issue, but have to take a decision: what to do if you asked two (or more) experts and they have conflicting opinions? --Hofhof (talk) 17:56, 13 July 2017 (UTC)

Then you, yourself, can either a) seek more information or b) remain agnostic on the decision, that is, take no stance yourself. In formal statistical logic, this is called the Principle of indifference, and holds that you should believe that each person has a 50% chance of being correct, i.e. you should think that neither is more right than the other until more evidence comes along from other sources. Or, you have no reason to do anything. It is rare that exactly two, and no more, equally respected people are sufficiently knowledgeable in that subject. That is, you always have more than two opinions usually (except, perhaps, the Schleswig-Holstein Question, but I think there you had at least 3 people who knew about it). From a philosophical point of view, what you're asking about is an abstract version of Buridan's ass perhaps. Since you're speaking in the abstract, I'm not sure what help we can give you except to direct you to vague philosophical concepts. If you gave us the specific conundrum you face, we could perhaps direct you to more information on it to break your indecision. --Jayron32 18:30, 13 July 2017 (UTC)

We have a nice article on decision theory, SEP's article [9] is better. There's statistical methods, bayesian and frequentist inferential methods, etc etc. Here's [10] a freely accessible older but well-cited scholarly article on the topic of behavioral decision theory. One key distinction is to separate normative from descriptive branches of study. "deciding agents should do this" is a very different from "this is what deciding agents are observed to do." While this theory has some application in systems science and Management_science, it also has implications for AI design, control systems, expert systems, etc. If you can give us more context you might be a better answer but this is a huge field of inquiry, and there is no one correct answer, either from normative or descriptive points of view. It's all well and good to remain uncertain or seek more information, but in the real world, no decision is often worse than a bad decision, and things have to be decided with only partial information and consensus. SemanticMantis (talk) 18:38, 13 July 2017 (UTC)

Thanks for the answers so far. I was asking from a general point of view, but I have an interest in complex practical issues like the security of my PC or personal investing. Hofhof (talk) 18:55, 13 July 2017 (UTC)

I can't give a reference but I use the 3 P's. Possible, Plausible and Probable. If it fails any of these I'm dubious. 41.13.198.49 (talk) 20:00, 13 July 2017 (UTC)

You bring the two experts together, and get them to talk it through. Then you will have three opinions. Wymspen (talk) 20:05, 13 July 2017 (UTC)

In science expert opinion counts for nothing. The whole point of the scientific revolution a few centuries ago was to reject any arguments from authority (X says Y, and X is a well respected expert so Y must be true). We stopped doing that since about the 17th century and in the few centuries that have passed, the entire World got transformed into the modern technological world. But we have evolved over hundreds of thousands of years to respect authority, so the scientific method is not the way we intuitively think. Count Iblis (talk) 20:25, 13 July 2017 (UTC)

That is quite wrong. The motto of the Royal Society is indeed Nullius in verba, but that is a statement of principle, not of practical operation. Expert opinion is not sacrosanct, and ancient authorities are even less so, but of course expert opinion has significant weight. Why else hire expensive professors to do the teaching when the word of a bum of the street has the same weight? Things don't go into the scientific canon because Professor X says so. But he is probably a good candidate to tell you what is currently widely accepted, and possibly even why. --Stephan Schulz (talk) 22:00, 13 July 2017 (UTC)

Also notice that my question was about using knowledge that already exists. It was not about obtaining scientific knowledge from nil.Hofhof (talk) 02:58, 14 July 2017 (UTC)

Well I can give one bit of advice. Do not listen to the experts, and do not look at the experts, just read what they say and then make the decision. Dmcq (talk) 22:15, 13 July 2017 (UTC)

• The Delphi method is a formal way to do this. -Arch dude (talk) 00:16, 14 July 2017 (UTC)

It also depends on the nature of the question. In your example of PC security there are some well established principles that experts agree on (e.g., don't use a dictionary word as a password) while other ideas have less agreement (e.g., whether requiring frequent password changes is a help or hindrance to security). Shock Brigade Harvester Boris (talk) 03:31, 14 July 2017 (UTC)

you may use the old (Odysseus') method of dealing with "Between Scylla and Charybdis" issue: opt to pass by Scylla and lose only a few sailors, rather than risk the loss of entire ship in the whirlpool; same as minimax. This is the basis of insurance: rather incur a certain moderate loss, than a chance to lose everything. If no insurance can be given and even "moderate" loss is too heavy, just go boldly to charybdis and hope for the best. Gem fr (talk) 06:24, 14 July 2017 (UTC)

• Consult more experts, until you see a consensus emerge.

• In some cases, you should then decide for yourself, based on the evidence. For example, if you are on a jury and the case depends on a finger-print ID, and the defense has an expert claim that he's 100% certain that's not a match with the accused, while the prosecution puts forth an expert that says he's 100% sure it is a match, I'd suggest you look at it and decide for yourself. StuRat (talk) 19:06, 14 July 2017 (UTC)

Err, no. You are not an expert in the subject of fingerprints (unless you actually are). If two expert witnesses polarizingly disagree, as a juror you ignore their evidence entirely and the benefit of the doubt goes to the accused. Akld guy (talk) 22:30, 14 July 2017 (UTC)

That would mean the defense only needs to pay off a so-called expert to lie, and their man goes free. Fingerprints have the advantage, unlike DNA, that whether two prints match is something a juror can hope to decide on their own. More generally, one side will always trot out "experts" who are paid to lie for their side, so it's important NOT to just say "well, there's an argument on both sides, so let's do nothing", as in the case of cigarettes causing cancer, and now man-made global climate change. StuRat (talk) 22:40, 14 July 2017 (UTC)

Arabic doesn't use Arabic numerals? Who knew?! Sherlock freaking Holmes would not have caught this one - kudos! Wnt (talk) 16:14, 15 July 2017 (UTC)

I was serving on a jury at Crown Court and the prosecution lawyer mentioned that the accused had lined up as no. 6 in an identity parade but the witness had pointed to no. 7 on the cards in front of him. After the lunch break I passed a note to the judge pointing out that in Arabic (the witness was a Muslim) the number "six" is written "7". We were immediately sent home for the day, and the following morning were told the case was still not ready to proceed. Finally the court reconvened and we heard from an expert witness who had been called to introduce the new evidence. So it's not necessarily best to keep quiet. 92.19.185.111 (talk) 14:21, 15 July 2017 (UTC)

Temperature at which water vapour forms

I put a pot of cold water on the cooktop and turn it on. The water starts to heat up. Well before the water boils, steam (or water vapour) starts to rise from the water. At exactly what temperature does this occur, and what is the significance of this temperature? -- Jack of Oz ^{[pleasantries]} 21:54, 13 July 2017 (UTC)

It's not a single temperature. At any temperature, the molecules of water have an average kinetic energy, but the energy distribution follows a particular formula (for ideal gases it's the Maxwell–Boltzmann distribution, but I'm not enough of a physicist to know which one applies to liquids). In this distribution, some molecules will have enough energy to overcome surface tension and leave the liquid phase. The higher the temperature, the more molecules will be able to do so, but there will be some at any temperature. --Stephan Schulz (talk) 22:08, 13 July 2017 (UTC)

I see. So, there are molecules spontaneously separating out of any container of water (or any liquid?) sitting at room temperature, but we can't see them because there's not enough of them; it's only when their volume increases under the effect of heat that we can see them. I get that. -- Jack of Oz ^{[pleasantries]} 04:09, 14 July 2017 (UTC)

Almost. You can't see the molecules because water vapor is a colorless gas. You can only see water when it is a liquid or solid. The water vapor mixes with colder air above pot and condenses to form tiny liquid droplets that you can see. Shock Brigade Harvester Boris (talk) 04:30, 14 July 2017 (UTC)

One possible answer to Jack's original question would be that you can see the visible fog when you get the vapor pressure high enough that the humidity above the pot rises enough that the dew point drops below rises above the ambient temperature. However the relationship between vapor pressure and humidity may be complicated. --Trovatore (talk) 05:34, 14 July 2017 (UTC)

See vapor pressure. As the temperature rises, the vapor pressure does as well, until it reaches the ambient pressure, which is when the water boils (that's slightly simplified; see nucleate boiling).

As a side note, the visible white "steam" or "water vapor" is neither — it's actually (tiny drops of) liquid water, condensed from the water vapor as it hits cooler air. Water vapor itself is invisible, though you might be able to detect it from the change in refractive index. --Trovatore (talk) 00:50, 14 July 2017 (UTC)

It also happens below the freezing point, as water vapor will sublimate well below the freezing point from solid ice. If you've ever left an ice cube tray in a freezer for a long time, you've already done the experiment to prove this if you've ever noticed the cubes to have shrunk. Here is a table of vapor pressures of ice at temperatures down to -100C. You'll note they are small, but decidedly non-zero. The answer to your question is likely absolute zero, or near enough as to not matter.--Jayron32 03:31, 14 July 2017 (UTC)

Grr, peeve alert — "sublimate". The perfectly proper and euphonious verb form is "sublime". I don't know when people started saying "sublimate" like it was a repressed memory or something, but I do wish they would stop. --Trovatore (talk) 20:59, 14 July 2017 (UTC)

It started in 1559 (or earlier) according to the OED, long before Freudian theory. Dbfirs 21:25, 14 July 2017 (UTC)

The OED is a sublime work. -- Jack of Oz ^{[pleasantries]} 23:26, 14 July 2017 (UTC)

You can tell that some of the water in the pot becomes vapor even at room temp (provided the relative humidity in the air is less than 100%), because the water will all eventually evaporate. And, technically, some of it still evaporates even at 100% humidity, but just as much water condenses in the pot from the air, so the equilibrium is maintained. StuRat (talk) 21:21, 14 July 2017 (UTC)

Is the wrapper by Klondike ice cream made of real aluminum?

I tried to google it. But I couldn't find any results of the material of the wrapper. Is the wrapper made of real aluminum foil? Can it be used instead of buying aluminum foil? 50.4.236.254 (talk) 23:12, 13 July 2017 (UTC)

Supposing it were (and I'm not at all convinced of it), would it be worth the bother of having to wash the thing before using as if it were aluminum foil? And it's pretty small, so its application would be limited. One thing you could try (very carefully) is to put a match to it and see if it burns. I suspect it's as much paper as anything else. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:32, 13 July 2017 (UTC)

Sterile inoculating loops are used to culture plates. They can be made of plastic, which is disposable, or can be made of metal, which must be sterilized over an open flame before use. If one holds the metal loop long enough in the flame, then the loop will glow red. So, does that mean metals will glow red while paper combust? What if the wrapper is part aluminum and part paper? 50.4.236.254 (talk) 00:06, 14 July 2017 (UTC)

Note that aluminum burns; in fact, all aluminum foil is covered in aluminum oxide (Al2O3, the basis of the mineral corundum, from which gems are made). It makes it able to scratch things you probably don't want it to. Atomized aluminum is fun stuff; you can mix it with perchlorate or chlorate or the like (solid sources of oxygen, basically) for flash powder (fireworks...). Of course, I would not tell you to do so, since celebrating the 4th of July in the old fashion would make you a terrorist... Anyway, point is you can more or less dispose of aluminum foil in a fire (leaving behind some residue), making this a bad test really. Wnt (talk) 01:36, 14 July 2017 (UTC)

Good point. What I had in mind was some kind of test using the wrapper instead of foil. Like, for example, baking a (very small) potato in it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 08:24, 14 July 2017 (UTC)

Klondike does not publish what the wrapper is made of (except for the news about changing from metallic to plastic wrappers). However, there are tariffs on many items, including metallic wrappers. So, Klondike had to explain exactly what their wrappers were made of. If you check this, you will see a brief description of exactly how much aluminum is in the Klondike wrapper. 209.149.113.5 (talk) 12:41, 14 July 2017 (UTC)

Very impressive answer! But to be clear, was your approach simply to go to customsmobile.com and search for rulings that contained Klondike wrapper - is that a general enough approach to find most brand products? - or is there a higher-level search mechanism that led you to this site? Wnt (talk) 21:19, 14 July 2017 (UTC)

I knew that there was tariff issues on candy with metallic wrappers. So, I googled Klondike wrapper tariff. I hoped to see if there was a tariff and if it cell under aluminum. The first two hits were about switching to plastic wrappers to avoid tariffs. The third explained how much aluminum was used in detail. 71.85.51.150 (talk) 01:09, 15 July 2017 (UTC)

July 14

Julian lunar calendar

Is the Julian lunar calendar always Gregorian lunar minus 4 days from 2001 till this century changes? I can't find a Julian lunar calendar online besides one that only shows the luna XIV before Easter. Sagittarian Milky Way (talk) 01:06, 14 July 2017 (UTC)

Ah, here it is: [11]. Sagittarian Milky Way (talk) 08:58, 14 July 2017 (UTC)

Your source does not agree with the source quoted in Computus#Theory, which has a month beginning on 2 November (the intercalary month) followed by one beginning on 2 December at line 13). In your source there is a month beginning on 1 December with the one beginning on 31 December also having thirty days. The compilers didn't allow two thirty day months to begin in one Julian month. 92.8.217.19 (talk) 20:01, 14 July 2017 (UTC)

There's another inaccuracy in your source. The last three months on line 2 begin 4 October, 3 November, 2 December. The article says the intercalary month began on 3 December. This conflicts with its cited source, which gives 2 December. However, there's a more impressive secondary source, the printed Julian calendar.[12] This gives 4 October, 3 November, 3 December. The two tables are almost certainly copied from this: [13] — Preceding unsigned comment added by 92.8.217.19 (talk) 18:17, 15 July 2017 (UTC)

Thanks. Good catch. I wonder how that source I found screwed up. Sagittarian Milky Way (talk) 18:40, 15 July 2017 (UTC)

Is it really true that vomit doesn't affect men as much as women in the tendency to vomit when others are vomiting?

A long time ago, I had a physiology instructor who said that one would vomit when one saw others vomit for an evolutionary reason that the food might be tainted. Then, a girl suggested something along the lines of "If I vomit in front of my husband, then will he also vomit?" Then, the instructor said, "No, it doesn't affect men as much." Recently, I tried looking up the web to see if I could find any evidence. But nope, I can't find it. Is there a difference between men and women in the tendency to vomit when others are vomiting? 50.4.236.254 (talk) 03:26, 14 July 2017 (UTC)

Like you, my search for any kind of confirmation or refutation of this claim through a reliable source in medical/beahvioural science literature has turned up zilch. I don't like to speculate here, even in the absence of any kind of sourcing (actually, especially in that context), but in this case I am going to go out on a limb and say that you are right to be skeptical here: I can't see it as likely that either substantial research or even speculation has been conducted in this area by either physiologists or behavioural scientists. It's entirely possible your instructor was sharing some impressionistic opinion, based on his limited experience or even just confirmation bias about the supposedly less hardy stomachs of women working in medical fields (which assertion has been around forever, even as the minds of most people who work in that field boggle at the notion).

That said, it's not outside the realm of possibility; I've heard speculation in the past that the social vomiting reflex is best explained in evolutionary terms by the possibility that it was meant to help evacuate poisons from the systems of hunter-gatherers. I'm skeptical of that theory, because emulation-induced vomiting itself would have significant medical cost for someone vomiting needlessly (dehydration, for example, which in the hunter-gatherer context would not always be easy to immediately remedy). But it's at least possible, and if that is the evolutionary root of the instinct, its worth noting that (from what we know of hunter-gatherer societies), women did tend to have a more active role in collecting flora. That said, most all members of the few hunter-gatherer cultures remaining in the world today suggest that most all members of a band are familiar with the toxicity of most every species within their range of foraging (I recall, for example, Jared Diamond relating this observation from his discussions with the remaining bands on the Indonesian plateau.

I appreciate that is a whole lot of speculation with zero truly useful sourcing, but I think it at least gives you a fair impression of just how much certainty we can have on the matter (not much) lacking any real research. Snow ^{let's rap} 04:35, 14 July 2017 (UTC)

same. my 2 cents, or rather 4, though:

• women have more fatty reserves than men so vomitting (wasting a meal) cost them less,
• men are heavier so it takes more poison to make them ill,
• traditional culture assigne different food to different sex, and this is still true
• last but not least, poison may affect foetus much more dramatically than grown-ups, vomitting is seen as a sign of pregnancy (and women in a band tend to be pregnant at the same time)

all this translates into: it makes evolutionary sense for women to vomit more.

Gem fr (talk) 06:07, 14 July 2017 (UTC)

My 5 pence, When I was a youngster any hint or sound of someone vomiting would elicit a similar reaction in me. When I took up nurse training I witnessed patients vomiting on a regular basis and quickly became accustomed to it. The experience has put me in good stead for the rest of my 7 decades. There are many causes for vomiting, not just toxins, so I am not convinced by the "get rid of the toxins" theory. Richard Avery (talk) 07:40, 14 July 2017 (UTC)

There is no discussion that some toxins (or toxic level of, say, alcohol, that do NOT cause nausea under low impregnation) cause nausea, and that it makes sense to vomit to get rid of it. And no discussion either that training can get rid of reflex that turns into nuisance for a professional routinely exposed to circonstance usually rare for ordinary people, but this cannot be used to dismiss a fact that apply to them. Motion sickness: "The most common hypothesis for the cause of motion sickness is that it functions as a defense mechanism against neurotoxins. etc." Now, there may be others causes, for sure, but it makes sense to use a routine defense behavior when you don't know what's happening.Gem fr (talk) 10:56, 14 July 2017 (UTC)

The view alone certainly does not affect men or women. There are lots of movies that contain such "acts", some even exessively. The smell however affects most civilized humans - men and women alike i believe. However every human is capable to get used to revolting smells. Pathologists for example are by profession used to smells that probably would let normal people faint. Similar men often seem to vomit when confronted with "filled" Baby Diapers unless they get used to do (irony on) that "mom's job"(irony off). So in conclusion it doesnt seem a natural thing at all to be affected by someone else vomit or smells. Its mostly socialized (learned) behavior really. --Kharon (talk) 22:03, 14 July 2017 (UTC)

Kharon, I almost didn't respond here because I couldn't find a definitive piece of research on this, but I still think you've turned reality on its head with your assumption here. Most people do in fact have a strong gag reflex in response to vomiting; regardless of culture and whether they are ever explicitly taught to view the act as disgusting, most have a visceral reaction to the act (and to the excretion of bodily fluids generally) that is innate, not learned. There is, however, great variability in how much people react and some just are not bothered at all (I would suspect you may be one of them, given your outlook here). You are correct that conditioning can change the degree of response, but it's usually in the other direction (a person becoming habitualized to the act and thus not having a strong response, rather than people being innately not bothered and then learning to be physically upset by it). For my own part, I was never really one to get nauseous from visual or auditory stimuli, and once I had participated in top-down dissections of a cadaver (early in my study of physiology in my academic career), most everything biological seemed trivial by comparison, and I've rarely had so much as twitch of reaction (at least vomit wise), so you are somewhat correct in that this is a strange area where the psychological and the purely physically reflexive intertwine in peculiar ways.

It's worth noting that all of this interfaces with the psychology (and biopsychology) of disgust, which even more broadly is a curious area of behaviour. Many aspects of disgust are conditioned in childhood; any parent will tell you that up until a certain age, most children hoover-up almsot any food put in front of them, if hungry enough; they do have favourites and some are picky even at an early age, but they are generally very adventurous in what they will accept (or shove into their own gobs if left unattended!). Then, around the age where a child is becoming truly mobile and independent, they go through a period where their dietary preferences contract significantly (in some cases setting for life). The theory here is that, this being the period when a child in a traditional hunter-gatherer context would begin to have the ability to sometimes select their own foods, it's best that at this age they would begin to create two separate categories for "acceptable" and "unacceptable" foods, given the potential for poisoning. It is believed that this has become the basis for cultural differences in cuisine and that this has further fed into the creation of distinct cultures in general (insofar as an inability to "break bread" can create surprisingly large gaps between groups [14]; and of course, historically and even in to today, some people will go so far as to classify other people as "not human" due to their dietary choices). What's more, some research has even suggested a strong link between the degree to which an individual will classify food choices which they are not used to as "disgusting" and their broader social views; those with more closed diets also seem to be more willing to classify harmless sexual acts that they do not participate in (homosexuality, for example) as "disgusting" or abhorrent and are more likely to be conservative or fundamentalist in other social beliefs. Furthermore, this high/low level of general disgust may be somewhat inheritable. [15], [16].

I know that second paragraph is a bit of a divergence, but I think it's food for thought on the main inquiry here; disgust is neither generated entirely by culture or innate propensity alone; it's a case of neuropsychological development and epigenetics working in bizarre ways (even by the peculiar standards of human behvioural phenomena). On that one little point, though, I still think you are wrong, and making assumptions contrary to the general established consensus of whether it is innate for most people to have a reflex to vomitting; take 10 people from 10 different countries and put them in a room with someone vomiting, and I bet you most of them will be fighting against that reflex to varying degrees, even if they never explicitly thought about whether or not the act is "gross". Snow ^{let's rap} 23:21, 14 July 2017 (UTC)

Btw. Best ever "Wanna sip?" --Kharon (talk) 22:13, 14 July 2017 (UTC)

If men do tolerate vomit better than women, that just supports the theory that all men are dogs. :-) StuRat (talk) 23:40, 14 July 2017 (UTC)

Snow Rise do you know Surströmming? Its very famous for forcing anyone not familiar with it to instantly vomit! Also read Fermented fish to see the many examples in different countries. That alone is prove enough for me that this is an entirely conditioned phenomenon. --Kharon (talk) 00:08, 15 July 2017 (UTC)

The evidence you provide in your first two sentences there (aside from being highly impressionistic) stands in diametric contrast to the conclusion you raise from it, and from your previous comment. If most people have an automatic reflex towards highly fermented foods (and I agree that they do) and generally only lack this visceral response if they have "acquired the taste" for its consumption, then that is evidence that the emesis reflex is the somewhat instinctual response, and having no such response is the result of conditioning. Exactly the opposite of your original assertion. In my post, I was not saying that you were wrong that experience can qualify the baseline response; I'm saying you've turned the direction in which those conditioned responses run on their head. People learn to control the instinct to gag or vomit in the context of stimulation by certain smells--they don't learn to want to vomit in that context. Quite the opposite to your assertion that they undermine that conventional assumption, those cuisines that you cite reinforce it. Snow ^{let's rap} 01:53, 15 July 2017 (UTC)

It is true women are more prone to vomiting in response to various stimuli, plus they mirror others feelings a bit more than men. You can find out stuff on the web by googling the appropriate medical terms 'emesis gender' to get the medical texts. Dmcq (talk) 00:07, 15 July 2017 (UTC)

That only brings up very specific differences found in these medical studies. This does not prove or even hint a general gender difference. --Kharon (talk) 00:17, 15 July 2017 (UTC)

Could you name one study of anything that satisfies your requirements in that it showed up something everywhere instead of in the particular cases tested? Dmcq (talk) 13:34, 15 July 2017 (UTC)

Appetite suppressant to to point of starvation and death

Are there any substances that completely suppress appetite to the point of starvation and death? Have there been any experiments on rats/mice or other animals that I can read about? Thanks for your time.

As an example, see Brain_stimulation_reward#Strength_of_drive: 'Experiments have shown rats to forgo food to the point of starvation in order to work for brain stimulation [of the Lateral hypothalamus] or intravenous cocaine'. AndrewWTaylor (talk) 11:41, 14 July 2017 (UTC)

Anorexia nervosa, anorectic Gem fr (talk) 11:51, 14 July 2017 (UTC)

Re the 'rats on cocaine' experiment: I read or heard (on radio) some time earlier this month that the original experiments in the 1960s (see your first cited article's Reference 9 here) were later discredited. The rats – members of a highly social species – were effectively placed in extended "solitary confinement" in totally featureless enclosures offering no mental stimulation whatever, and thus were in a highly abnormal mental state when offered the cocaine. When repeated by Bruce K. Alexander as part of his Rat Park study in parallel with modified experiments giving the rats a more normal environment including contact with other rats, the latter moderated their cocaine use and functioned reasonably normally. {The poster formerly known as 87.81.230.195} 90.206.219.214 (talk) 14:52, 14 July 2017 (UTC)

Pfft, extroverts. Sagittarian Milky Way (talk) 20:01, 14 July 2017 (UTC)

Note that it would need to go beyond a lack of appetite for people to stop eating. That is, knowing they would die without food, they would force it down if they possibly could, so you would need something like ipecac, to prevent people from keeping food down. See emetic. Not true of other animals, though. StuRat (talk) 14:55, 14 July 2017 (UTC)

Appetite suppressants like amphetamines have a checkered history and are ineffective in the long run. What is the long run for an appetite suppressant? Three months at the most, but this also varies with individuals. You can think of appetite suppressants which are almost universally also psycho-stimulants if you take coffee. It is a psychostimulant and an appetite suppressant if you never drank coffee in your life or had at least a year of abstinence. Many of your brain cells are covered with receptors which are small areas on the boundary of the cells (cell walls) and if you drink a cup of coffee all those receptors and there are zillions of them become excited and send signals inside the cells. The next thing they do--some of them disappear inside the cell wall, or you can say the caffeine you drank pushed them inside. Tomorrow you drink another cup and more receptors dive in. The process continues to the point that very few receptors are left to monitor what you drank. Stop drinking coffee and over time some of the receptors, perhaps all of them eventually will come up to the surface. Amphetamines which are used as attention stimulants are most likely act on the same receptors but their affinity may be different, higher or lower. You can do research on that in Wikipedia yourself. --AboutFace 22 (talk) 18:23, 14 July 2017 (UTC)

That's not how caffeine works. It's an antagonist, not an agonist or a reuptake inhibitor. Specifically, it blocks adenosine receptors. If the receptors were to be "pushed inside the cell", it would be like you were always on caffeine.

How the body adapts to caffeine I'm not really sure — it could make more adenosine, or more receptors.

I'm not sure whether it's an appetite suppressant or not. Our article mentions that kola nuts were used to suppress hunger pangs; that's the only reference I can find in our article to appetite suppression. --Trovatore (talk) 20:11, 14 July 2017 (UTC)

• To take the question literally, yes, there are definitely drugs that can suppress appetite to the point of starvation and death, namely, drugs that cause severe nausea. Many of the drugs used in cancer chemotherapy, for example, have this characteristic. None of them are addictive or rewarding, to be sure, but the question didn't say anything about that. Looie496 (talk) 03:16, 15 July 2017 (UTC)

"Smartphones Hijack Cognitive Capacity" -exact quote

In this Medscape article[17], not sure if it is accessible by everyone, you can read: "Having a smartphone nearby reduces cognitive capacity, even when the phone is turned off, new research shows." "The mere presence of their smartphone was enough to reduce their cognitive capacity." "The study was published online April 3 in the Journal of the Association for Consumer Research." I smell telepathy here. I think Mark Zuckerberg might be interested. --AboutFace 22 (talk) 23:20, 14 July 2017 (UTC)

Telepathy isn't necessary, as the phone being nearby distracts you by thinking about it. Imagine having a fire-breathing dragon nearby, wouldn't that be distracting, even when it's asleep  ? StuRat (talk) 23:36, 14 July 2017 (UTC)

Well, @StuRat, how about this: "The researchers found that the mere presence of a smartphone adversely affected available cognitive capacity, even when participants were successful at sustaining attention, were not using their phone, and did not report thinking about the phone. These cognitive effects were strongest in those who reported greater smartphone dependence. "It's not that participants were distracted because they were getting notifications on their phones," said Dr Ward in a press release. "The mere presence of their smartphone was enough to reduce their cognitive capacity."" --AboutFace 22 (talk) 00:56, 15 July 2017 (UTC)

They may not be aware of it, but still have part of their attention spent listening for notification tones. I don't think we can turn that off automatically when the device is turned off. The real test would be to have the phone close or far from them, in a double blind way. If it still takes their attention, only when near, even though they don't know it's nearby, then something strange really is happening. StuRat (talk) 01:47, 15 July 2017 (UTC)

Yes, even a cursory review of the article that the OP references demonstrates that the researchers believe their results reflect an impact upon cognitive load for participants who knowingly had their devices on their person, even when they knew these devices were turned off. There's not even the slightest suggestion of ESP or some other "spooky" explanation; the researchers clearly felt that participants treat their phone a little reflexively in that some portion of their mind is still "on reserve" to interact with the device if it is on their person (even if it is off), which in itself is not altogether surprising. Even then, it's just one study, and not published in a journal I would expect to have the most rigorous of standards for peer review of controls, so the results could be inflated and at the very least would need to be independently replicated. But again, if true, not earth-shattering and certainly not evidence for clairvoyance. Snow ^{let's rap} 02:28, 15 July 2017 (UTC)

Yes just because someone did not say they were thinking about the phone doesn't mean they weren't at some level. Nil Einne (talk) 06:49, 15 July 2017 (UTC)

July 15

What happens when the current is too strong for the electrode to measure?

Don't direct me to electrochemistry. Its scope is way too broad and does not answer why I'm not supposed to use an electrode to measure an extremely high current. How will the strong current break the electrode? Is it like a rushing stream and a stick that measures the force of the stream, but if the force is too great, then the stick will break? 50.4.236.254 (talk) 01:25, 15 July 2017 (UTC)

Sorry, but it isn't clear what you are asking. There are many types of electrodes. Pretty much all of them have some sort of limit on the current they can handle, but the specific reasons vary. It would be very helpful if you could clarify what sort of electrode you are referring to. Looie496 (talk) 03:10, 15 July 2017 (UTC)

It's likely due to deviation from ideality. An ideal multimeter should have a linear, or at least polynomial, response, but at high enough current, there's going to be a complex relationship between resistance and heat which probably bring some calculus into the situation which a standard multimeter probe probably doesn't take into account. That's the best of what I can interpret from the vagueries of the question. --Jayron32 05:33, 15 July 2017 (UTC)

I posit this to be about why a multimeter cannot be used to measure high current. The key idea here is that a multimeter does not need to pass much (theoretically, not any) current through it to measure voltage, but the only way it can measure current is to be wired into the circuit and have the entire current pass through it. This cannot be done reliably past a certain point because it only has so much conductivity, i.e. its resistance can only be so low. Because V=IR, high current with a nonzero resistance implies voltage builds up across the multimeter, which perturbs the circuit being measured in an unpredictable way making the measurement invalid. And of course eventually the power dissipation I^2 R means that the multimeter would catch on fire if a fuse didn't stop it first. That said, you could have some other kind of measuring device that determines the current flow from the magnetic field no matter how strong the current is. I'm not an expert and have no idea if this is commercially available, or how well it can image the current from careful distributed measurements of the magnetic field and computation. I suspect that SQUID devices might be used this way from great distances for military purposes? Wnt (talk) 12:30, 15 July 2017 (UTC)

• The reason I was so reluctant to answer is that the OP referred to electrochemistry, where specialized electrodes are used to measure the concentrations of chemicals. The factors that come into play there are quite different from those that come into play when ordinary metal electrodes are used to measure voltages. Looie496 (talk) 23:22, 15 July 2017 (UTC)

The Hydraulic analogy (see article) is useful in understanding basic electronics but "poking a stick in a stream" is not the way to apply it. One attaches an Electrode to a system either A) to apply or extract electricity, or B) to measure what is happening in the system. In the field of electrochemistry, A) might apply to electrolysis or a battery and B) to the measurement of cell e.m.f.. The OP asks about B) measurement. We can distinguish between measurement of B1) Voltage and B2) Current though there are many other Electrical measurements. Voltage corresponds to fluid pressure in the hydraulic analogy and can usually be measured without disturbing the system. For this one uses a Voltmeter that draws only a negligible current; if even the few milli- or micro- amps that a moving coil meter draws would disturb then there are amplified VTVMs and FET-VMs with extreme high input resistance. (The corresponding pressure sensor in the hydraulic analogy has zero leakage.) However to measure an electric current (which is fluid flow in the hydraulic analogy) in the system it is usually necessary to disturb it by diverting some current to a measuring instrument. If the system current is large it is not necessary to divert more than a small fraction of it if a shunt resistor can be introduced in the current path. See Shunt_(electrical)#Use_in_current_measuring. A shunt allows almost any magnitude of current to be measured by means of a voltmeter that connects with lightweight wires to the shunt's two electrodes. Wnt mentions that a current flow can also be measured by the magnetic field it produces, which is the principle employed by the clamp-type current probe. Blooteuth (talk) 00:26, 16 July 2017 (UTC)

What is the difference between a magnet and the thingy that deionizes water?

Please don't direct me to electrodeionization, and the picture on that page, though relevant to this inquiry, seems to be in a foreign language. Anode is anode. Kathode is likely cathode. And the lines and arrows are the movements of water ions. But how is this different from a magnet? If I put a magnet with an anode and cathode in water, then will it deionize the water? I know extremely pure water is not drinkable, but if I put fruit purée or just sodium chloride, then will the water become drinkable? 50.4.236.254 (talk) 16:53, 15 July 2017 (UTC)

It is a myth that extremely pure water is not drinkable. (Just make sure your diet has other sources of the minerals you were previously getting from tap water, like magnesium and calcium.)

[18][19]

ApLundell (talk) 19:44, 15 July 2017 (UTC)

I agree with ApLundell, although I'll also add that the taste of such water may not be optimal. —PaleoNeonate - 19:49, 15 July 2017 (UTC)

But the question is simple to answer: A magnet creates a magnetic field, while cathode and anode create an electric field. Icek~enwiki (talk) 19:55, 15 July 2017 (UTC)

Well since you don't like the electrodeionization article can I direct you to the Capacitive deionization article instead, it isn't too different a process. Or Purified water which is what you get if you put 'deionization' into the search box and which has a section on 'Health effects of drinking purified water'. I fail to see where your idea of a magnet comes from, just because a diagram has an arrow in it does not mean there is a magnet involved. Dmcq (talk) 19:56, 15 July 2017 (UTC)

The diagram is in a foreign language (German), but the gist is clear to me. You have a - voltage at one end (cathode) and a + voltage at the other (anode). Anions go ("ion" means "go" or "move") to the anode and cations go to the cathode. (for confusion, cathode rays come from the cathode and have charge to match the cathode, which is the opposite charge from cations, i.e. cathode rays are negatively charged electrons so the - charge pushes them away, while cations are things like Mg++ with positive charge that pulls them in) When you put an anode at one end of a big stack of membranes, all the - charges (anions) try to go to it. But you split up the area with anion-impermeable membranes, so the anions stop. Then, in between the anion-impermeable membranes, you put cation-impermeable membranes halfway. So the cations stop when they hit them. And when you look at the diagram, you see the geometry is that you have alternating areas where anions and cations get stopped, and form saltier solution, and then areas where they are coming from, which are desalinated to a greater degree. You then pour off less-salty water from one half of the sections of the stack and more-salty water from the other half that are shuffled between them. Really quite clever, nothing I ever imagined before just now. But -- I have no idea what AAT, KAT, MB, KR in the diagram stand for. There's a fair chance native speakers of German wouldn't have a clue. There could be wide range of membrane materials and it seems like this might be one specific type... but who knows? Wnt (talk) 20:34, 15 July 2017 (UTC)

Despite having and OK-ish reading knowledge of German I couldn't really understand much of the schematic. So I deleted it. After all this is en:wp. Shock Brigade Harvester Boris (talk) 01:57, 16 July 2017 (UTC)

The abbrevs are explained in the German language article. Apparently, AAT=Anionenaustauschmembran (anion exchange membrane), KAT=Kationenaustauschmembran, MB=Mischbettionenaustauscher (mixed bed ion exchanger), KR=Konzentratraum (concentrate container(?)) Asmrulz (talk) 02:25, 16 July 2017 (UTC)

Which US coal basin deliver which coal?

Is there any Met coal coming from Illinois (Illinois Basin)? Or is it only Thermal coal?

Is there any Thermal Coal coming from Pennsylvania, or only Met Coal?--Zen priest (talk) 20:32, 15 July 2017 (UTC) File:Appalachian map.svg

Our articles are in a sorry state, and I'm certainly not an expert. In general, anthracite coal in the Appalachian Mountains has been subjected to metamorphism that has driven off impurities and made it purer and harder, and bituminous coal is softer, more similar to lignite and lower grades of coal (see coal for the full series). However, coking coal, described as a form of metallurgical coal, can also be bituminous. The level of sulfur is apparently very relevant, which may not perfectly relate to hardness grades ... should look into this further.

I suspect this question is prompted by a recent news item: [20] My impression is that anthracite mining traditionally was a more skilled art, in smaller angled veins or deep beneath the earth rather than in flat beds, and so the opening of a new mine with only 70-100 employees seems understandable. Somerset County, Pennsylvania is right near the edge of the Valley and ridge province .. I imagine the geology would be fun to dig into. But I would suspect that mines a little further west or north are going to be more like bituminous strip mines with lower-grade coal. For example, this article suggests a mine that Scott Pruitt visited just a few counties to the west in Sycamore, Pennsylvania is being run by a company looking to get out of the business and faces decreasing demand.

Our article on anthracite lists some out of Pennsylvania reserves like Crested Butte, Colorado. I tried putting that through Google for metallurgical coal and it spat back this about a Las Animas, Colorado mine, the only metallurgical coal mine in the state. So yes, there are such mines outside of Pennsylvania. I didn't look up the orogeny but obviously there are some features in Colorado that could have caused some metamerism. Wnt (talk) 01:40, 16 July 2017 (UTC)

As for the Illinois Basin, well... I didn't quickly find any mention of metallurgical coal. The region is known for cheap, high-sulfur bituminous coal for burning [21]. My gut feeling is that even if there were the roots of a lonely mountain in the middle of the Illinois Basin surrounded by anthracite, it would by definition not be part of a geological basin. But I don't know that! Wnt (talk) 01:47, 16 July 2017 (UTC)