Wikipedia:Reference desk/Archives/Science/2018 May 19

= May 19 =

What could a human notice if his antipodal hemisphere instantly disappeared?
Without the help of technology or other humans (i.e. Internet, seismometers, Internet feeds of seismometers, grandma tweeting you Internet feeds of seismometers) What's the new (extremely temporary) gravity strength? How close to spherical would the hemisphere centered on you get before you're unconscious? Sagittarian Milky Way (talk) 04:17, 19 May 2018 (UTC)
 * This is a bit hypothetical! How would this happen? Blasted by a stream of neutron star fragments? Anyway the gravity would drop a bit so you would notice that first. Next you would be hit with massive seismic waves. Air would disappear at about the speed of sound. You could expect that the remaining hemisphere would absorb so much energy from planetary reconfiguration that it would be turned into a magma ocean. Graeme Bartlett (talk) 06:45, 19 May 2018 (UTC)


 * The one thing we can be sure of is you get a 1/30 second grace period due to the speed of light, though a stickler will insist that during that period the Event hadn't happened "yet". Relativity sticklers will also note that the faster you get rid of the mass, by any means, the more you mess with space making gravitational waves; i.e. gravitomagnetic effects become relevant.
 * Removing one hemisphere implies that material at the center, formerly under no gravitational field, would suddenly be pulled to one side, and hence could release energy by falling. On the other hand, the pressure would reduce from insane to zero in an instant, so it would also push outward.  The inner core surface is estimated at 5430 K and the boiling point of iron at atmospheric pressure is 3134 K, so a rather impressive explosion is to be expected.  However, note that the core of a half-sphere is still a lot lower than the outer surface.  I don't know how you'd begin trying to calculate if the explosion reaches the lip of the crust.  To take a wild guess, I'd say figure the heat capacity of liquid iron, i.e. (5430K - 3134K)*heat*core volume, figure out how much iron that can vaporize in moles, figure out how much volume that takes at 1 atm, and see if it fits in the flat side -- but I know that's an equilibrium, but an explosion isn't.
 * To find the center of a hemisphere, you set up some nested integrals in polar coordinates ... lolno, you go and do a web search and let your skills rust a little longer.  It's 3/8 of the way in.  That means that if West Berlin is just on the good side of the Event boundary, the gravity that was formerly straight down is now coming from R down and 3/8 R over, i.e. it is off by tan-1 (3/11), or 15.2 degrees.  This would be a bad time to have a house on the west side of a lake, though then again, the water might get there just in time to put out the fire from that burning core material, I dunno. ;)  But actually that's just an approximation - a planet's gravity seems to come from its center of gravity because it's a sphere, but gravity doesn't come from the center of gravity of the Cavendish apparatus or any other un-planet-like shape.  I suppose we could do a set of integrals ... but this one would not be in polar coordinates.  But using the approximation, we know that the two halves of the earth pulled equally, summing the cos(+-15.2deg) components of each but cancelling the sin(+-15.2deg) bits, so gravity is at 51.8% or so of what it was.  Sounds like a great time to launch a rocket ship, but it better be a fast one.  Oh, but if you're at the center of the hemisphere, let's say, somewhere between El Dorado and Cayambe Coca Ecological Reserve in Ecuador, then you are 3/8 closer to the "center of gravity", so by an increasingly dubious approximation you should get 51.8% * 121/64 = 97.9% gravity.  Hmmmm. ;) Wnt (talk) 16:55, 19 May 2018 (UTC)
 * Well if you remove half the mass and have the Earth settle into a smaller sphere of the same density, the new gravity is (1/2)^(1/3) ~ 4/5 g. So the 98% for the top of the lone hemisphere isn't unrealistic at all IMO. 93.142.87.187 (talk) 05:35, 22 May 2018 (UTC)

What to call it?
I'd like to know what to call this condition so I can do some research to understand it better. I'm not asking for medical advice.

I have trouble remembering things I want to do. For some time I've relied on visual clues, like placing objects in appropriate places. I put things in my calendar but sometimes forget to look there (I don't have many calendar events). Sometimes I remember the day but not the time.

On a cruise recently the problem was magnified. The ship each day provided a schedule with a few dozen entertainment opportunities, and I would pick three or four I was interested in. But I had a hard time remembering not only the time and place, but even what the event was. (To tell the truth, I wasn't very committed to most of them.)

I don't experience any other memory problems.

--Halcatalyst (talk) 17:26, 19 May 2018 (UTC)
 * I think you've answered your own question: You weren't committed. Or what could be called "invested". Now, when you are invested but still have trouble remembering, talk to your doctor. ←Baseball Bugs What's up, Doc? carrots→ 19:39, 19 May 2018 (UTC)


 * I didn't ask for medical advice. --Halcatalyst (talk) 21:12, 19 May 2018 (UTC)
 * Nor did I give you any. ←Baseball Bugs What's up, Doc? carrots→ 23:22, 19 May 2018 (UTC)


 * What you describe sounds to me like the perfectly normal performance of an ordinary memory (of someone who has not adopted special techniques of memory improvement). Some tactics to avoid the problems you mention are obvious: Forget to look in your calendar/diary? – form the habit of doing so on a regular basis, e.g. at breakfast. Forget times of events? – check those written in the calendar/diary, that you carry on you, more often instead of trying to memorize them. Become confused by many-choiced schedules of events? – Highlight the ones you're most interested in on a printed schedule and carry it with you; maybe mark back-ups in a second colour in case you're prevented from attending a first-rank choice. (Your cruise ship scenario is similar to that encountered at any large Science fiction convention, of which I have extensive experience, most of it in a less-than-sober state :-).) {The poster formerly known as 87.81.230.195} 2.221.82.140 (talk) 21:30, 19 May 2018 (UTC)
 * Perhaps long-term working memory might be the concept you're looking for. Klbrain (talk) 21:54, 20 May 2018 (UTC)


 * Thanks for the reference! I'm following up on it. --Halcatalyst (talk) 14:58, 21 May 2018 (UTC)

How do milk and eggs whilte function as anti intoxication?
I always used to hear that milk is used in case of intoxication but I didn't take it seriously and it was for me as a myth. But today I was read in allegedly academical book (pharmacology - Odessa university): "Inactivation and binding of the poison in the stomach should be performed simultaneously to rinsing.] Potassium permanganate, tannin, activated carbon, egg whites and milk are used for these purposes." (I found the same sentence in Wiki university...) then my question what is the mechanism that these things fight the intoxication? 18:23, 19 May 2018 (UTC) — Preceding unsigned comment added by 93.126.116.89 (talk)
 * My understanding has always been that they react with stomach acids and enzymes to form a jelly like substance which forms a protective layer coating the stomach lining, and thereby reducing the absorption of toxins. I think that comes from First Aid training some 50 years ago. Wymspen (talk) 19:35, 19 May 2018 (UTC)
 * Those substances have different mechanisms of action, so they're not equally useful for all poisonings. Potassium permanganate as an oxidizing agent to chemically modify (hopefully inactivate) a poison, and activated charcoal because it binds to many poisons due to its large surface-area for non-specific binding of a wide variety of toxins. I don't know the proposed mechanisms of egg white, but it may be much as for the activated charcoal, a close analogy being the very common phenomenon of plasma protein binding, an effect which greatly reduces the free concentration of many drugs in the blood plasma. As an aside, not that the main protein in the plasma is albumin, and egg white is also known as albumen. Milk also contains proteins which might serve the same function, while its fat might isolate lipophilic drugs and slow stomach emptying, which would slow toxin absorption (which largely occurs in the small intestine). There are drugs which line the surface of the stomach, like sucralfate, but they are used to protect the oesophagus/stomach in cases of ulceration rather for poisoning; they don't really help absorption because that doesn't take place to a significant extent in the stomach. Klbrain (talk) 22:25, 20 May 2018 (UTC)

Quantum (and classical!) speed limits
I started reading this news story about quantum speed limits on how fast energy distributions can change, which are said to have nothing to do with relativity. There are two independent papers about this, and. I will admit that so far I have not followed the crucial bits in the middle - there is a lot of advanced math in there, by which I mean referring to concepts named after people, like Wigner function and Moyal bracket and Bures angle and Poisson bracket and Liouville equation and Bhattacaryya coefficient and Hellinger distance; given that my command of classic Hamiltonian formalism is questionable, let alone noncommutative algebra, obviously I'm in for some trouble. We have some mini articles on the Margolus-Levitin theorem and the Lieb-Robinson bound. I am hard pressed to understand how any speed limit based on the Planck constant can be defined as classical, and outright mystified by what "semiclassical" is all about.

Yet it seems like the ability to set fundamental speed limits however you look at an evolving system has to be something with an importance comparable to the level of relativity -- with which these speed limits are said to have nothing in common by all involved. (The Bekenstein bound apparently can be used to derive general relativity from scratch, if you are very good at physics; it seems superficially related to these others but what do I know?)

At a real dumb level, I saw a mention that one of the bounds was basically the Planck constant. At a glance this kind of makes sense, in that the J * s units of that can be divided by time or energy to give (in Margolus-Levitin theorem, with a 4 thrown in) a time to make a change in energy. The idea would be that anything heavy has a de Broglie frequency, or at least a de Broglie wavelength, that limits how fast it can change, the more the lighter it is. That said, I don't get how an "average energy" as our article talks about can exist; I mean, can you have a dense blast of energy in one region of space and a weak trace of energy next to it, and when you average them, now the weak energy can beat the limit? How do you decide what to average? Etc.

Anyway, I wanted to throw this question out there, see if someone can pick out some useful epitopes for a person to latch onto, maybe you could start some kind of "quantum speed limit" article (or maybe that's a bad name!), maybe you could tell me if this is majorly important to the progress of physics or 'trivial'. Wnt (talk) 22:42, 19 May 2018 (UTC)


 * Quantum speed limit (QSL) is not a "bad name" (e.g.: Quantum speed limits set an upper bound to the rate at which a quantum system can evolve...); however, Quantum Speed Limit is Not Quantum. I haven't a clue as to what all that means, having not studied the subject since the '90s. —2606:A000:1126:4CA:0:98F2:CFF6:1782 (talk) 16:07, 20 May 2018 (UTC)
 * A technical follow-up - should Bures angle redirect to Bures metric or Fubini–Study metric? Both seem to be, from my non-expert viewpoint, suitable targets. Tevildo (talk) 18:23, 26 May 2018 (UTC)