Wikipedia:Reference desk/Archives/Science/2018 April 4

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April 4

Can double blind tests be done on blind people?

Can double blind tests be done on blind people? If so, would the tests still be valid if blindfolds were not used? 49.177.234.140 (talk) 09:07, 4 April 2018 (UTC)

• Assuming you are not trolling, you need to read our article about double blind trial. It is a figurative expression, it does not have a physical meaning. Tigraan^{Click here to contact me} 09:50, 4 April 2018 (UTC)

[Edit conflict] Making a supreme effort to assume good faith for this question: yes they can, because the "blind" in "Double blind test" is a metaphor and not meant literally – it means that both the person tested and (hence the "double") the person administering the test do not know whether the medicine, device or procedure used is genuine or a dummy, aka a placebo.

Actual blindfolds are not normally used in such tests: instead the dummy and real medicine, device or procedure are designed to be not distinguishable by testee or tester, and only a third party, via code numbers or similar contrivances, knows the truth and can compare the results of the dummies and the real things. {The poster formerly known as 87.81.230.195} 2.218.14.51 (talk) 09:52, 4 April 2018 (UTC)

If they perform a test on visually impaired people the test subjects would have to wear a blindfold to put all on an equal playing field. That is, if vision (or residual vision) could influence the test result. For example, if they were testing how blind people explore a room with or without a white cane. --Hofhof (talk) 12:51, 4 April 2018 (UTC)

I decided to play the ball through and typed "double-blind" blindness into PubMed, getting 173 results. Most are not really relevant, but #3 is an example of such a study: [1]. Wnt (talk) 13:35, 4 April 2018 (UTC)

Putting damp salt in the microwave

I put my porcelain salt jar into the microwave because the salt was damp. It works great.

I always notice how blazing hot it gets very, very fast. Is this because there is actually not very much moisture to heat up, or is it something to do with the salt?

(Try it right now in your microwave to see, if you like.)

Many thanks for any thoughts you have.

Anna Frodesiak (talk) 09:40, 4 April 2018 (UTC)

According to This, sodium chloride has a specific heat capacity of about 1/5th that of water, which means that it will rise in temperature about 5 times as fast, given the same input of energy. --Jayron32 10:55, 4 April 2018 (UTC)

Jayron32, hello! Ahhh, so would you say the microwave is heating up the salt as well as the moisture? Anna Frodesiak (talk) 12:04, 4 April 2018 (UTC)

It could be heating the water, which is then heating the salt. Heat can be transferred in many ways; heat transfer can occur through radiation, conduction, or convection. In this case, the salt itself probably doesn't absorb much radiation, but the water is absorbing the radiation, and transferring energy to the salt via conduction. --Jayron32 12:30, 4 April 2018 (UTC)

Sorry to not understand Heat capacity. Basically, when I look at stuff like this...

$\left({\frac {\partial U}{\partial T}}\right)_{V}=\left({\frac {\partial Q}{\partial T}}\right)_{V}=C_{V}.$$\mathrm {d} U=\delta Q+\delta W.$$\mathrm {d} U=\delta Q-P\,\mathrm {d} V.$

...it could be just art or interesting drawings or something. It could be upside down and in a mirror and be the same to me. :) Anna Frodesiak (talk) 12:07, 4 April 2018 (UTC)

Heat capacity is just how fast the temperature changes in response to the same input of energy. If you put 1 joule of energy into different substances, the temperature rises faster in substances with a low heat capacity, and slower in those with a high heat capacity. Salt is about 0.8-ish and water is about 4-ish in terms of J/degrees C, which means that 1 joule will raise the temperature of water about 0.25 degrees C, and the temperature of salt about 1.25 degrees C. --Jayron32 12:30, 4 April 2018 (UTC)

Heat capacity is the relationship between heat and temperature. How much heat (energy) does it take to raise the temperature (a variable property of the material)? This varies for different materials - it's unusually high for water, which is why it takes a lot of energy to boil a kettle, but why a bathtub stays hot for such a long time. Andy Dingley (talk) 12:13, 4 April 2018 (UTC)

• It's not great for the microwave, if the salt dries out. It's always best when doing such things to "ballast" the microwave with a cup of water (covered in vented cling film to avoid steam everywhere) and give the microwave some load to drive. Otherwise all that microwave energy has to go somewhere, and it might be as arcing around the frame of the oven. I always do this if it's on for more than a minute (and I sometimes melt glass in my workshop microwave!).

It's not clear, without experimenting, just what is getting hot here. It could be the moisture in the salt, it could be the salt, it could be the ceramic jar. Maybe try an experiment with a pile of loose salt, and a pile of loose damp salt, both on a glass plate, and also the empty jar. It's noticeable that some kitchenware absorbs microwave energy (and gets hot), others don't. I eat porridge most mornings, which involves oats and cold milk in my glass porridge bowl, then two minutes in the microwave. If I use one of the ceramic bowls instead, the bowl would be too hot to hold. Andy Dingley (talk) 12:10, 4 April 2018 (UTC)

Ballast, eh? Not a bad plan. However, I only leave it on for about 30 seconds, then it gets incredibly hot, and that does the trick.

You melt glass in a microwave?

I've put salt in a paper cup. It gets incredibly hot just the same, and can burn the cup.

The porcelain jar alone only gets a little bit hot.

I'm afraid of nuking a pile of salt. It gets so hot, it might break the glass plate below.

I think I'll try dry salt in one paper cup and moist salt in another paper cup and see.

Interesting about the porridge. I notice food in a paper cup will get hot way faster than the same food in a porcelain bowl. It's like the bowl blocks it, but without itself getting very hot. I wonder where all that energy goes.

Oh, and try whole oats. Cook them for dinner like rice, but with a little less water. Fantastic! Plus, you'll poop great. Mind you, you will get a bit windy and maybe a bit of a stomach ache the first time. It's basically a fiber bomb, and your lower intestine may protest. Anna Frodesiak (talk) 12:23, 4 April 2018 (UTC)

Melting temperature of glass is about 1500 °C or higher depending on type. That's gas mark 99. SpinningSpark 12:32, 4 April 2018 (UTC)

This formula explains how glass melts in a microwave. Also, if you stare at it, like really through it, and relax your eyes, you will see a dolphin. Anna Frodesiak (talk) 12:38, 4 April 2018 (UTC)

• That is a very complex question, and the topic of how efficiently microwave energy is absorbed and converted into heat (compared e.g. to the same mass of water) is beyond me. However the topic of "it feels hot" is not.

If I assume that you touch the salt (not the cup) to check how hot it gets, you must know that we feel heat, not temperature. If we make brutal simplifying hypotheses, how "hot" it feels will vary as the square root of the thermal diffusivity. General idea: Q = deltaT*eff = (E_microwave/ (rho cp))*sqrt(k rho cp) prop to sqrt(k/(rho cp)) where eff is the thermal effusivity.

A quick estimation tells us that the thermal diffusivity of salt is about 36 times higher than water, so for the same efficiency of microwave absorption, it would feel about 6 times as hot. Equivalently, neglecting thermal losses, it would feel as hot after 1/6th of the heating time.

Needless to say, the above involves very rough estimates and there is absolutely no guarantee that we are even in the correct ballpark, though a posteriori the answer does not look so stupid. Details of the quick estimation: all data taken from our articles; density is 2 times larger (from sodium chloride), k is 10 times larger (from list of thermal conductivities), massic cp is 0.15 times as big (from heat capacity tables for water, computed from the molar value in sodium chloride for salt); k/(rho cp) rounded to 36 because it is a square number. Tigraan^{Click here to contact me} 13:56, 4 April 2018 (UTC)

IIRC, Microwave energy is primarily absorbed in the rotational mode through dielectric heating; basically the microwaves are able to set polar molecules spinning faster. It is best absorbed by assymetric polar molecules such as water and fats. Salt, being composed of spherical ions, does not have a rotational mode, and the individual particles are uniform spherical ions and not dipoles, so probably does not absorb any microwaves by itself. --Jayron32 14:05, 4 April 2018 (UTC)

Regarding the equations: These mathematical symbols are simply an abbreviation that compactly states how heat-energy relates to the other properties of the material.

To understand them will require a little bit of memorization: for example, you must know that "∂x" is a short abbreviation that means "the change in the value of x." And you must memorize that "Q" is the abbreviation for "heat energy," which is a fact that must be taught to you explicitly at some time - you can hardly guess that one!

But this is commonplace for any "abbreviated shorthand." In the same manner, you must memorize that "N.Y." is an abbreviation for "New York"; and you must memorize 49 other abbreviations for the states - some of them are non-obvious! - and you must "intuit" that the ambiguous sequence "ME" refers to "Maine" when we are discussing in this context; and if you should ever be so unfortunate as to combine US Postal Service abbreviations for state mailing addresses with the US Coast Guard abbreviations for state boat registrations, where the abbreviation conventions are different and overlapping, you can easily become confused. There is a little ambiguity in the standards - and a lot of prerequisite knowledge about what is standardized. But, if you write a lot of post-cards, it's a lot faster to write the short-hand form - so you put up with the hazards and hassles!

Here's a paper on How Students Understand Physics Equations, by some physics education researchers at Northwestern University.

Nimur (talk) 16:53, 4 April 2018 (UTC)

Thank you all! I had no idea that it was such a complicated thing.

I still haven't done the damp salt vs. dry salt experiment. I'm getting a objections. After telling them of the melted glass, some here think the experiment could fold spacetime. I'll let you know if I'm permitted to do it. Anna Frodesiak (talk) 01:14, 6 April 2018 (UTC)

Critical temperature

At http://science.jrank.org/pages/2922/Gases-Liquefaction-Critical-temperature-pressure.html it says:

"For example, the critical temperature for carbon dioxide is 304K (87.8°F [31°C]). That means that no amount of pressure applied to a sample of carbon dioxide gas at or above 304K (87.8°F [31°C]) will cause the gas to liquefy. At or below that temperature, however, the gas can be liquefied provided sufficient pressure is applied. The corresponding critical pressure for carbon dioxide at 304K (87.8°F [31°C]) is 72.9 atmospheres. In other words, the application of a pressure of 72.9 atmospheres of pressure on a sample of carbon dioxide gas at 304K (87.8°F [31°C]) will cause the gas to liquefy."

Suppose I have some CO2 just below the critical temperature, and I apply enough pressure to liquefy it. Then, without allowing any change in volume, I increase the temperature to just above the critical temperature. What is the essential physical difference in the CO2 that means the former can be called a liquid but the latter can't? 86.191.58.187 (talk) 13:53, 4 April 2018 (UTC)

• Not much, that is exactly the crucial part of understanding what the critical point means.

The simple definition of a liquid is that it will take the shape of any container but not change volume. The simple definition of a gas is that it follows the ideal gas law (well, not exactly, but "particles are independent except for occasional collisions" gives you that (possibly with minor corrections) after pages of equations). Neither of those is correct near the critical point - from our article critical point (thermodynamics):

In the vicinity of the critical point, the physical properties of the liquid and the vapor change dramatically, with both phases becoming ever more similar. For instance, liquid water under normal conditions is nearly incompressible, has a low thermal expansion coefficient, has a high dielectric constant, and is an excellent solvent for electrolytes. Near the critical point, all these properties change into the exact opposite: water becomes compressible, expandable, a poor dielectric, a bad solvent for electrolytes, and prefers to mix with nonpolar gases and organic molecules.

Below the critical point, there is latent heat when converting between the two phases (gas and liquid), but that latent heat decreases to zero as you get closer to the critical point (that is essentially the definition of the critical point). Tigraan^{Click here to contact me} 14:16, 4 April 2018 (UTC)

Thank you for your reply. So, if I pressurise the CO2 at just below the critical temperature, at some point "something happens" and it becomes a liquid, whereas at just above the critical temperature this "something" never happens, so there is never a point where we can say that it has turned into a liquid. Is the best way of defining this "something happens" to do with latent heat then? 86.191.58.187 (talk) 17:52, 4 April 2018 (UTC)

There are many videos on YouTube showing CO₂ being heated past its critical temperature - some are clearer than others. Try this one [2] --catslash (talk) 22:52, 4 April 2018 (UTC)

Well, the "something" is a first-order phase transition. This means many properties go through a discrete jump (e.g. density and thermodynamic activity), and depending on your use case those may be easier to measure than latent heat. If you intend to experiment on CO2 we must obviously warn you that 73atm is quite a lot of pressure and you should take safety precautions in your setup. (Now of course, the closer to the critical point you are the smaller the jumps, but some of them might become small faster than others; the relevant article here is critical exponent, but I must warn you that the underlying theory is quite a bit math-y.)

On the same subject, our article about critical opalescence is short but quite good. Correlation function (statistical mechanics) is the underlying theory, and one of the rigorous ways to define a liquid (by comparing the correlation length against the range of the Lennard-Jones potential) but there again it is quite a bit math-y. Tigraan^{Click here to contact me} 07:42, 5 April 2018 (UTC)

• (OP) Thanks for the helpful replies. Thanks for the link to the video. 86.143.78.218 (talk) 19:41, 7 April 2018 (UTC)

Quick answer that I don't see above: Below the critical temperature, if you have some liquid CO2 in the container, but not enough to fill it up, there'll be liquid CO2 in the bottom, then a surface, then CO2 vapor above it.

When you raise it above the critical temperature, the surface goes away, and it's all the same throughout the container.

In some sense, it's not so much that the CO2 becomes a gas, as it is that the distinction between liquid and gas goes away, and it becomes a supercritical fluid.

By the way, supercritical CO2 is used in some forms of dry cleaning. It behaves enough like a "liquid" that you can dissolve the detergent in it and use it to wash clothes. --Trovatore (talk) 20:02, 7 April 2018 (UTC)

Blue zebra stripes

Here you can see that for some reason this part of zebra coat has bluish or dark blue stripes instead of black. Is it light playing tricks or something else? 212.180.235.46 (talk) 18:59, 4 April 2018 (UTC)

• In mammals the black skin often has a bluish cast. Perhaps this is showing through? Heaviside glow (talk) 20:13, 4 April 2018 (UTC)

  

• I think it is just a trick of the light or camera. It reminds me of that wretched "what colour is this dress" thing. 86.191.58.187 (talk) 20:42, 4 April 2018 (UTC)

The hair seems to have a shiny reflection over it, which may be of a somewhat bluish sky? Guard hair may be relevant here, as it is typically shinier, and more to the point, it can have a somewhat complicated structure of pigment bands along the shaft so that superficial grazing light might hit less of it, and I don't know if zebras have a dark down that can really soak up light that hits more perpendicularly. Wnt (talk) 12:04, 5 April 2018 (UTC)

BTW, we actually have an article on that wretched "what color is that dress" thing. 107.15.152.93 (talk) 18:24, 5 April 2018 (UTC)

It's a satisfying article. Not merely does the dress look blue and black, it updates us that it really is blue and black as confirmed by the manufacturer. Which means that those other lunatics online can now be discounted and we can go back to our porn internet browsing with confidence that we are seeing people in their true colors. Wnt (talk) 00:15, 7 April 2018 (UTC)

A study carried out by Schlaffke et al. reported that individuals who saw the dress as white and gold showed increased activity in the frontal and parietal regions of the brain. These areas are thought to be critical in higher cognition activities.

(Personally, to me it appears to be old gold and bluish-white, a combination that isn't really mentioned, though "brown and blue" is called out separately. I think of the recognized categories it's closest to white and gold. It's also very close to the literal color of the pixels.) --Trovatore (talk) 23:53, 7 April 2018 (UTC)

Well, I would guess if you just look at something it takes your visual cortex, but if you're going to daydream it into existence you'll need something more... Wnt (talk) 21:14, 8 April 2018 (UTC)

The way I see it is pretty much the color of the pixels. The people who manage to see black and blue seem to be somehow correcting the white balance based on the background. Off the top of my head, I would expect that to take more processing power. Honestly I don't understand how they do it at all; to me, the background is so far from anything reasonable that I can't recover any useful information from it, and my brain seems to fall back to a literal-minded what-color-is-the-actual-light analysis. --Trovatore (talk) 21:35, 8 April 2018 (UTC)

Color samples from "the dress". Aside from (3,1) from top left, which is arguably a "golden" gray, all the pixels are black, gray, or blue except for background data in the right column. Right? ;^)
Huh? No. There are no black pixels in that image, not even one. --Trovatore (talk) 21:13, 9 April 2018 (UTC)
This lists 105-105-105 as "dim gray", one step up from black. The first four in row 5 and also two others in column 2 fall short of that brightness. Wnt (talk) 01:56, 10 April 2018 (UTC)

@Trovatore: I copied the image in GIMP and moved around the eye-dropper tool and looked at the color on the color picker and all I see there, outside of the dress context, are blacks and greys and blues and just a hint of a goldish grey in some places. I mean, I have 121-135-175 blue and 92-93-105 blue and 62-49-29 black and 81-68-47 gray-brown and 109-93-64 brown at the very goldenest in the one top stripe there. The whitest bit I see is 161-179-223 robin's-egg blue ... and that's not white. Is it possible we are disagreeing about how the individual pixels of the image look in terms of color? I hadn't seen such a low level disagreement given as an explanation. P.S. I can make it look like a white-and-gold dress to me, if I set the brightness on GIMP to +70 and the contrast to +100. But that is right on the verge of destroying the image entirely. Wnt (talk) 02:08, 9 April 2018 (UTC)

What image were you trying? The alleged PNG image in the article has something wrong with it; when I download it and try to open it in GIMP, it says it isn't a PNG at all. It apparently comes from a JPEG published in The Independent.

When I look at that JPEG, I see for example an 81-61-36 in the darkish area a bit above here left shoulder blade, and 124-110-71 in the middle of her back, where it's brighter (but still in the dark stripe). Both of those could easily be "old gold" or "russet" I think — very much not "black".

If I move down to the lighter stripe, I see a 134-147-191, which is white with a bluish tinge. Well, maybe "powder blue". --Trovatore (talk) 05:45, 9 April 2018 (UTC)

Wind gusts

Hello, I am curious to know if it is common to speak of a measure of the percentage of wind gust. In other words, if from a weather app I have that the wind speed is 15mph, with gusts of 24 mph, how much of the time on average it is gusting, or what the average wind speed is including gusts. What percentages are common in areas such as upper Minnesota?

As an aside, the reason I ask this is that I am an amateur drone pilot. I have a UAV forecast app that I use and my drone states that it can handle 20mph winds. I will fly in conditions at 7mph with gusts at 30mph, for instance, reasoning that the drone could still make it back because "there's no way it's gusting more than 50% of the time!" (7 + 30) / 2 = 18.5 mph, so I'm safe! Although I feel like I am likely correct and this reasoning is comforting (and so far it hasn't bitten me), I would like to confer with actual scientific data points and not just gut feeling. Thanks in advance!

2600:387:B:902:0:0:0:12 (talk) 21:21, 4 April 2018 (UTC)

According to our wind article a gust is "a short burst of high speed wind...one technical definition of a wind gust is: the maxima that exceed the lowest wind speed measured during a ten-minute time interval by 10 knots (19 km/h)". A speed that is sustained for more than a minute is a squall, not a gust. Theoretically, under that definition, the gust speed could be present for more than 50% of the time. For instance, if there were nine gusts in the period of just under one minute, that would be gusting for 90% of the time. But somehow, I think that's rare. SpinningSpark 23:14, 4 April 2018 (UTC)

Thanks for the definitions. This certainly helps clarify what a gust is... But it seems recorded info about frequency of gusts is still absent. 216.173.144.190 (talk) 03:40, 7 April 2018 (UTC)

Aircraft carriers in WWII

What I want to know is if my perception how they operated on takeoff and landing of the aircraft is correct. I want to limit myself to 3 carrier: Hornet, Lexington and Yorktown. My understanding is that when they needed to launch the planes they would turn into the wind, develop maximum speed and the planes would roll from stern to bow, then take off. Is it correct?

When they needed to collect the planes they would likewise turn into the wind, but will stand still, and the aircraft will fly in to the stern and come to the rest close to the bow. Is it correct?

Thanks, - AboutFace 22 (talk) 22:16, 4 April 2018 (UTC)

They certainly turn into the wind to launch and collect planes, but I doubt that they come to a stop for collection. I can't see the advantage of it. Sailing into the wind at speed helps the plane to land because it has greater airspeed for the same ground speed relative to the ship deck. Also, in wartime coming to a standstill could be extremely dangerous if there was a risk of submarine or air attack. The ship needs to be moving to be able to take avoiding action. SpinningSpark 22:53, 4 April 2018 (UTC)

I think sailing into the wind while collecting aircraft will negate the advantage of flying into the wind and conceivably can drop the wind effect to zero. AboutFace 22 (talk) 00:49, 5 April 2018 (UTC)

You can visit an aircraft carrier of the 1940s vintage - if you're in California, the USS Midway is in San Diego; the USS Hornet is in Alameda; on the East Coast, the USS Intrepid is in New York City; the USS Yorktown is in South Carolina; if you're in Texas, the USS Lexington is in Corpus Christi.

The tour guides will be happy to tell you all about catapult launches, landing traps, and deck operations.

Generally, the operating regime for an aircraft carrier conducting air operations (take-offs and landings) would be to develop 25 to 30 knots of airspeed over the deck, by turning directly into the wind and providing engine propulsion.

For example, when the Midway was configured for jet aircraft operations, it had two catapults toward the forward part of the ship; those would be used to launch aircraft off the "front." Landing aircraft would land at the rear of the ship. Take-off and landing- operations did not, by convention, occur at the same time; equipment related to aircraft launch (the JBD for catapult 1) "fouled" the landing area. I'm less familiar with the operations of other carriers during the 1940s in the pre-jet-era - but that's exactly the sort of information you might find by perusing our articles - Modern United States Navy carrier air operations is a good place to start!

Nimur (talk) 01:02, 5 April 2018 (UTC)

I agree with Nimur about visiting one of the historic aircraft carriers that they mention. I had a wonderful and informative time on the USS Hornet and have also toured a battleship, a submarine and Liberty ship. Cullen³²⁸ Let's discuss it 05:47, 8 April 2018 (UTC)

"...sailing into the wind while collecting aircraft will negate the advantage of flying into the wind..." No, just the opposite happens, it adds to it. The landing aircraft has to fly even faster through the air to "catch" the moving carrier. This increases the aircraft's airspeed, but the groundspeed, or perhaps we should say deckspeed, remains low. SpinningSpark 06:53, 5 April 2018 (UTC)

...keeping in mind that high air speed and low deck speed/ground speed is a Good Thing. and that the opposite is a Bad Thing. I can see the approach path to LAX from my house. Or I should say what is normally the approach path given the normal onshore flow. When we have an offshore flow the approach path is over the ocean and I see the departure path. I wonder how long it takes them to switch over and have the planes land and take off from the other direction? "OK, everybody, make a 180 degree turn...NOW!" --Guy Macon (talk) 07:06, 5 April 2018 (UTC)

I'm not extremely familiar with LAX, but I know that SJC has a great explanatory website on switching runways: here's "South Flow Operations" from the official website of Mineta San Jose International Airport. It takes seconds for the tower at Palo Alto to switch runways - I've even had the direction change while on short final approach; but when the runways switch at the big airports, it is a complicated procedure coordinated with all nearby airports (SFO, OAK) and with air traffic control across Northern California; the change-of-runway is nearly instantaneous, but because so many aircraft are inbound and outbound, the runway switch is only a small part of a lengthy air traffic flow switch-over. It's reasonable to believe that LA Center and SoCal coordinate similarly among the numerous airports in the Los Angeles area.

The change-over process is gradual and takes around a half hour to "complete," meaning that every aircraft at every nearby airport can resume a "standard" published arrival or departure; there are fuzzy edges to that number.

The standard procedures are available for LAX: d-TPP publications; and the far more cryptic MVA chart shows you everywhere in 3 dimensions that "non-standard" vectoring can take place when Air Traffic Control needs to move around traffic during a switch of flow (or for any other normal operational reason).

If you're really interested - and I mean really interested - here's a great video game: Sector 33, an educational game produced by NASA, that lets you simulate the operations of an Air Traffic Control radar operator in Sector 33 of NORCAL TRACON. The game is not hyper-realistic; you don't have to operate the radios or phones, nor coordinate with the ARTCC; the game pilots never err; there's no need to vector (rather, no requirement to compute unexpected course adjustment vectors); and every flight flies the standard routing; and one player only manages one airport (KSFO); and there's never any change of runway at SFO nor any other airport; (it's a kid's game for math-enrichment, after all, even though it is from NASA); but it gives you some great conceptual insight into how air routing works. A real, "pro" ATC operator can do all those things simultaneously and still fill the radio channel with pleasant banter.

If you're a super-plane-nerd in the Los Angeles area, you'll surely want to read about the event in 2016 in which vectoring did not go so smoothly. A lot of very important detail is buried in technicality here - so reserve any harsh judgement of both the pilot and the ATC specialist until you find and read the official NTSB report.

Just to hammer the point in: aircraft perform best when they take off and land directly into the wind. Pilots of big fast heavy jet planes try not to make sharp turns, so air traffic control - in both civil and military settings - tries to line the approach to the runway as close to the favored wind as possible, while simultaneously meeting every other operational need.

Nimur (talk) 15:39, 5 April 2018 (UTC)

Thanks everyone, @Nimur especially. AboutFace 22 (talk) 17:14, 5 April 2018 (UTC)

'Up to and during World War II, most catapults on aircraft carriers were hydraulic. United States Navy catapults on surface warships, however, were operated with explosive charges similar to those used for 5" guns'. See Aircraft catapult. Alansplodge (talk) 16:52, 6 April 2018 (UTC)

When a body touches another body

When a body touches another body (for example, a book on a table), what is actually touching? What happens in the boundary? Do their electrons collide? Do the electrons repel reciprocally? Do the protons repel reciprocally? --Hofhof (talk) 22:47, 4 April 2018 (UTC)

It is nearly entirely the repulsion of the outer shell of electrons. The nucleus and inner electrons have very little effect. Basically, nothing has actually touched, inasmuch as "touching" has any meaning at all in the quantum world. SpinningSpark 22:59, 4 April 2018 (UTC)

I'm not the OP, but I wonder if "touching" something really means allowing my sensory neurons to detect the repulsion between outer shells of electrons. The object is detected as "present", because something in the environment has been detected by my sensory neurons, which signals the brain, and the brain interprets that as "touching". Then, the language part of the brain vocalizes this feeling as "touching". SSS (talk) 00:52, 5 April 2018 (UTC)

I think the sense of touch registers an increased pressure extending millions of layers of atoms into your body. Touch has details but it's not my field. PrimeHunter (talk) 01:07, 5 April 2018 (UTC)

Well, sensory physiology is rather within my wheelhouse, but the issue SupersuperSmarty raises is more an ontological/epistemological one (and also to a large degree a matter of semantics, of course) than it is an empirical question. "Touch" is used both to describe a variety of qualia and to indicate the physical circumstances of two bodies being in contact with one-another. The two meanings of the word are functionally separate, even if in some contexts the word might be used to describe an event in which both meanings are occurring simultaneously, blurring the line between which meaning is being invoked so completely that in most circumstances where someone is "touching" something, we don't even pause to disentangle the meaning (which is actually a phenomena which is known to be typical of the language used when people describe their sensory experiences).

However, it is pretty plain from the OP's question that they are specifically about the physical qualities of non-exotic matter, and not the experiential phenomena that occurs within the mind, nor the neurophysiology of what happens in the brain during the sensation of touch. Of course, the take-away from contemporary particle physics models is that the other phenomena--matter actually having "contact" with other matter--is a phenomena that, in a sense at least, doesn't really exist; the matter that feels so "solid" to us as an intuitive matter in the observations we make without instrumentation is in reality mostly empty space (by truly astronomical margins) and even the particles that help define quantum fields don't have substance in the way we are used to thinking about it in classical/macro physics. Spinning actually nailed this one with the first response about as well as can be accomplished without getting into rather complicated quantum field theory. Snow ^{let's rap} 05:32, 5 April 2018 (UTC)

• This video from physics professor Philip Moriarty directly addresses the question and does a very good job explaining some of the misconceptions of the nature of objects touching (some of which are repeated above). --Jayron32 12:46, 5 April 2018 (UTC)

Happy, but also not surprised, to see that User:Jayron32 is fan of the excellent set of videos made by Brady Haran! --Lgriot (talk) 19:53, 5 April 2018 (UTC)