Wikipedia:Reference desk/Archives/Science/2019 January 23

= January 23 =

Entangled particles and the double-slit experiment
(I googled for this and didn't find an answer.)

Suppose you have a device producing entangled pairs of particles. One beam goes to the left and one beam goes to the right. At 2 meters to the right, you have a double-slit experiment set up. You get the normal interference pattern. (Right?) Now, 1 meter to the left, you put in a particle detector. From what I understand, when a particle hits the detector, its entangled partner suddenly has definite properties. (Right?) But now the particles going to the right reach the double-slit experiment after its entangled partner has been measured. Do you still get the double-slit interference pattern?

(My guess is that you don't get the interference pattern in the double slit. Excuse me for asking "right?" so many times, but I want to make sure my layman's understanding is right up to that point.) Bubba73 You talkin' to me? 06:58, 23 January 2019 (UTC)
 * This is one of many examples of thought experiments which are to test what is known as the EPR paradox, which Einstein derisively called "spooky action at a distance" (he's the "E" in EPR). I don't know your experiment directly, but it's of a class of experiments called the Bell test experiments, which were used to conduct tests of the EPR paradox. -- Jayron 32 11:59, 23 January 2019 (UTC)
 * Bell's experiments are not fully relevant here, I think. They are a smartly-designed refinement on the EPR situation, disproving the local hidden-variable theory, which means the cheap way to get out of the paradox does not work; but they do not really inform about the outcome of the standard EPR situation. Tigraan Click here to contact me 16:22, 23 January 2019 (UTC)
 * Yes, of course you're right. So corrected.  -- Jayron 32 21:30, 23 January 2019 (UTC)
 * In that experiment you'll still get the interference pattern on the right. And in general you can't detect any entanglement effects without bringing the results together. Dmcq (talk) 13:39, 23 January 2019 (UTC)
 * That is not what I would have thought. I thought that the reason for the interference pattern in the double-slit experiment was because of the wave nature of the particle.  And when one of the entangled particles is measured, that fixes the properties of the other particle, so it now has definite properties and would act like a particle instead of a wave.  That's not right, then?  Bubba73 You talkin' to me? 17:30, 24 January 2019 (UTC)
 * No. Both particles are in an entangled state, it is not possible to measure that by looking at only one of the particles. And in fact if one could do what you said we could send messages faster than light. Which we can't. Dmcq (talk) 18:55, 24 January 2019 (UTC)
 * OK, from what I've seen in popular accounts, they say that when one of the entangled particles is measured, now the state of the other one is determined, no matter how far away it is. Bubba73 You talkin' to me? 19:00, 24 January 2019 (UTC)
 * Yes, I think I see that now. If the interference pattern went away, you would know that the other particle had been measured, which you can't know, so the interference pattern remains.  Bubba73 You talkin' to me? 19:06, 24 January 2019 (UTC)
 * I think Quantum pseudo-telepathy is about the best way of showing how very strange the whole business is. Dmcq (talk) 19:13, 24 January 2019 (UTC)


 * If you have not read Double-slit_experiment yet, you should. (Not that it answers your questions!) Tigraan Click here to contact me 16:22, 23 January 2019 (UTC)

Question about cold weather and water/ice
When I look at my weather reports, it will say (for example): ''The temperature is 37 degrees Fahrenheit. But, it "feels like" only 27 degrees Fahrenheit, due to the wind chill factor'' (and other factors, I assume). My question is: If water freezes at 32 degrees Fahrenheit, does the actual temperature have to be 32 degrees Fahrenheit (or lower)? Or would a "feels like" temperature that is below 32 degrees Fahrenheit (but with an actual temperature of above 32 degrees Fahrenheit) -- as in my example -- also freeze water? Or not? Thanks. Joseph A. Spadaro (talk) 17:00, 23 January 2019 (UTC)


 * The water freezes when the actual temperature gets below 32 degrees. Wind chill is largely bullshit; it is defined as the cooling effect on bare skin facing the wind, while walking into the wind at 3.1 mph, at two meters above the ground in an open field, with no warming from sunlight. Unless you spend a lot of time walking upwind naked on stilts over open fields at night, the numbers are too low. The reason that it is so popular is that it allows the TV weather report to be more exciting and alarming. --Guy Macon (talk) 17:14, 23 January 2019 (UTC)


 * Thanks. Back to my example.  So, are you saying that, to a human being (who fulfills all of your other criteria), the 37-degree temperature would feel like only 27 degrees on their bare skin?  But, to a glass of water (or some such), the 37-degree temperature would still "feel" like 37 degrees (and, hence, the water would not freeze)?    Joseph A. Spadaro (talk) 17:21, 23 January 2019 (UTC)


 * No, see Wet-bulb temperature for details of evaporative cooling.  Dbfir<i style="color: #4a4">s</i>  17:32, 23 January 2019 (UTC)


 * Windchill isn't bullshit; it is useful for knowing how the combination of temperature and wind will affect your body, which is useful information. Your body cools off at a different rate depending on whether or not there is wind blowing on it.  Wind chill is about how fast your body cools off, which when trying to decide what I will wear and what to expect in terms of my safety (how long until I start to have negative health effects), is very useful information to have, presumably more useful than knowing when water would freeze.  Presuming still air, an object will cool off to the temperature of the atmosphere according to Newton's law of cooling.  Let's do a little thought experiment.  Imagine you died right now, and your corpse happened to be in a 10 degree C room.  Imagine I died at the same time, and my corpse was in a 0 degree C room.  Which body would cool off faster?  Mine would: in 5 minutes my body would be at a lower temperature than yours would.  A 0 degree room cools off your body faster than a 10 degree room.  Now, run a second experiment.  Pretend your mom dies in a 10 degree C room, and my mom also dies in a 10 degree C room, but my mom's room has a fan blowing.  Now, which body would cool off faster?  My mom, of course, since my mom's room has a wind in it.  After 5 minutes, my mom's body will be colder than your mom's body, because of the presence of wind.  Wind makes things cool off more.  What is the most convenient way to quantify the effect of that wind?  Easy: just figure out what ambient temperature difference would be equivalent to that wind; that is how much would I have to cool your mom's room to so that her rate of cooling exactly matches that of my mom, in her room with the fan.  The temperature of the room with your mom's corpse in it is called the "wind chill factor" temperature; the temperature that still air would have the same cooling effect as moving air.  I certainly hope that makes sense, because I already killed 4 people to explain it.  I'd hate to try to find more people to kill.   Knowing that will tell a person things like: how long can I stay outside before I get frostbite?  What thickness coat do I need?  How comfortable will I be if I need to run out and check the mail but can't be arsed to put on a coat?  Wind chill factor is more useful to me trying to answer those questions than "will water freeze".  -- Jayron <b style="color:#090">32</b> 17:36, 23 January 2019 (UTC)


 * Actually, wind chill is about how fast your body cools off if you are walking upwind naked on stilts over open fields at night. If you are wearing clothes, sitting down or standing still or the sun is shining, the wind will still cool you, but the actual effect will be less that the TV weather person says it is. --Guy Macon (talk) 17:43, 23 January 2019 (UTC)
 * Jayron32 is almost entirely correct, excepting that he didn't provide an exactly precise equation to compute wind-chill - it's not exactly equal to the Newton's Law of Cooling "equivalent" ambient temperature, but it's close. Guy Macon, you are not quite correct: the wind-chill is based on a human face at five feet above ground level, and not on a "naked" human on ... "stilts."  The National Weather Service Wind Chill Brochure explains that the wind-chill is an experimentally-derived model, based on details of thermal physics and incorporating details about human physiology.  They also provide the equation, and an easy-to-read chart for windchill.  If you want to learn more about the research that drives these models, here's The new wind chill equivalent temperature chart, (Journal of the American Meteorological Society, 2005), which details the research that backs the equation, and cites a few dozen works of prior research.  Nimur (talk) 21:30, 23 January 2019 (UTC)
 * [EC] That is correct. The wind chill factor assumes that the object being cooled by the wind is at human skin temperature. A strong wind at 40 degrees F will definitely cool human skin at 90 degrees F, but it will have zero effect on water at 40 degrees F, and it will melt ice that is at 32 degrees F faster than it would melt with no wind.


 * Dbfirs talks about evaporative cooling, but human sweat glands pretty much shut down when they are at 37 degrees F. Evaporative cooling would be a major factor on a windy 120 degree F day with low humidity. --Guy Macon (talk) 17:39, 23 January 2019 (UTC)

Jayron32's "how long can I stay outside before I get frostbite" question is especially interesting. If It is, say, 25 degrees F outside, unprotected skin is at risk of frostbite. And any wind at that temperature will cause you to get frostbitten sooner. If it is 40 degrees F outside with a wind chill factor of 25 degrees F, frostbite is impossible. No amount of wind will cool your skin bellow 40 degrees F if that is the actual temperature outside, and your skin needs to go below 32 degrees F for you to get frostbite. --Guy Macon (talk) 18:02, 23 January 2019 (UTC)
 * If you're really a Californian, it's understandable why you would pooh-pooh the wind chill factor. Spend a month or two in North Dakota in the dead of winter and see if you still think it's meaningless. ←Baseball Bugs What's up, Doc? carrots→ 19:27, 23 January 2019 (UTC)


 * That's why wind chill is only reported during cold temperatures; the only thing a person watching the weather really wants to know is two things: what kind of coat and hat do I need to wear? Do I need to bring an umbrella today?  That's it.  I need nothing else out of my weather report.  Windchill is very useful in answering the first question.  A calm day at 0 C requires a less bulky coat than a breezy day at 5 C.  That's all I need to know; so give me a number that tells me what kind of coat to wear. -- Jayron <b style="color:#090">32</b> 21:28, 23 January 2019 (UTC)
 * That's precisely it. It can be 20 below in the upper Midwest on a calm day, and it's not bad. But it can be 20 above and a breeze feels like knives cutting through any exposed area, such as your face - or anywhere on your body, if your covering is too light. (And by the way, that "cutting" feeling is why the wind in the "canyons" of Chicago is called "the hawk".) ←Baseball Bugs What's up, Doc? carrots→ 21:40, 23 January 2019 (UTC)
 * Or, it can be 39 degrees Fahrenheit in the morning, and I might ride my bike to work at 18 miles per hour (true airspeed, of course); so I need a winter jacket and face cover, because with my wind-chill factor, I've been exposed to air that "feels" several degrees below freezing for an hour. So when I show up to work in Sunny California wearing bulky winter jackets and hats and gloves, I might get some weird looks, especially if by mid-day it's back to blue skies in the mid-sixties.  Car people really don't realize how sheltered they are from the forces of nature.
 * Nimur (talk) 21:50, 23 January 2019 (UTC)
 * Maybe that's the case in the US, although even there I have my doubts. In NZ, a lot of people also look at the weather to work out when might be a good time to do the washing, or whether to hang up their washing outdoors, or use the dryer or whatever. Increasingly people also look at it for other things like working out whether they can be bothered cycling to work, although yes sometimes this is also what to wear when cycling. Of course there are also a lot of other things depending on context e.g. is it likely a good day to visit the beach, botanical gardens, Waitakere ranges, or maybe some indoor activities instead. Should I fire up the barbeque this weekend? Etc etc. See e.g. [//blog.metservice.com/BBQIndex] [//www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11672625] In fact, you can see MetService provides 2 additional bits of information which may not be strictly associated with a weather report, sun protection warnings and pollen levels. (But yes, they do also provide clothing suggestions.) [//www.metservice.com/towns-cities/auckland/waitakere]. This was never implemented [//www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10834313] but anecdotal OR I know at least 2 people who check the weather reports at least in part in relation to drying clothes. (A drying index is part of the commercial service [//blog.metservice.com/washingweather] and that and other details could probably be used by third parties e.g. the TV channels but although I think it's common for the presenter to comment on drying clothes, I don't think they generally show any specific detail although it's been a while since I've paid much attention.) Nil Einne (talk) 07:30, 24 January 2019 (UTC)


 * It is entirely possible for water to get a lower temperature than the air with a wind. Evaporative cooling does work. That is the difference between a dry bulb and wet bulb thermometer, and a higher wind will cause faster evaporation. The effect is lowered at low temperatures because the vapor pressure is low. There is not the same thing as wind chill though. Dmcq (talk) 21:54, 23 January 2019 (UTC)
 * This thread has concentrated mostly on wind chill, but I am under the impression that the "feels like" temperature is also affected by humidity. Whether or not human sweat glands shut down at low temperatures (as stated above), surely if the air is really damp, a human body—including one that is clothed—will cool down more quickly than if the air is dry, as per the wet bulb thermometer? PaleCloudedWhite (talk) 22:44, 23 January 2019 (UTC)
 * The very low humidity at minus-20 is what makes a calm day tolerable in the upper Midwest. However, low humidity is of little or no help when the wind kicks up. ←Baseball Bugs What's up, Doc? carrots→ 23:26, 23 January 2019 (UTC)
 * In the United States, our National Weather Service does not use the key-phrase "feels like" in a standard weather report; however, this phrase is commonly used by commercial weather forecast retailers, so they may use it in any of a variety of ways.
 * In some documents, National Weather Service describes both the wind chill and the heat index as what the temperature "feels like," but this is more properly called the "apparent temperature."
 * Nimur (talk) 23:11, 23 January 2019 (UTC)
 * In the UK, the Met Office uses the term "feels like temperature" in its weather forecasts (e.g. here). This Met Office page states that "Feels like temperature takes into account wind speed and relative humidity to give a more representative indication of how the temperature will feel". PaleCloudedWhite (talk) 23:41, 23 January 2019 (UTC)
 * Likewise in NZ, MetService does publish a "feels like" temperature. The details of how it is calculated are here [//blog.metservice.com/FeelsLikeTemp]. But simplistically Nil Einne (talk) 06:47, 24 January 2019 (UTC)
 * Thanks guys! I do appreciate the feedback with a greater perspective on weather services around the world!  I updated my comment to reflect that I was referring to the USA only!  Insofar as there is any international standardization - in organizations like ICAO or the UN's World Meteorological Organization, I can't find guidance on the use of these terms ("feels like," or even "wind chill" or "heat index,") - so it appears there is room for national variation!   Nimur (talk) 15:41, 24 January 2019 (UTC)


 * This would be a good time for Guy Macon to fly to Chicago or Minneapolis and see if he still thinks wind chill is no big deal. ←Baseball Bugs What's up, Doc? carrots→ 23:49, 29 January 2019 (UTC)
 * You need to be dressed properly. Count Iblis (talk) 00:04, 30 January 2019 (UTC)