Wikipedia:Reference desk/Science

Welcome to the science section
of the Wikipedia reference desk.

skip to bottom

Select a section:

• Computing
• Entertainment
• Humanities
• Language
• Mathematics
• Science
• Miscellaneous
• Archives

Want a faster answer?

Main page: Help searching Wikipedia

   

How can I get my question answered?

• Select the section of the desk that best fits the general topic of your question (see the navigation column to the right).
• Post your question to only one section, providing a short header that gives the topic of your question.
• Type '~~~~' (that is, four tilde characters) at the end – this signs and dates your contribution so we know who wrote what and when.
• Don't post personal contact information – it will be removed. Any answers will be provided here.
• Please be as specific as possible, and include all relevant context – the usefulness of answers may depend on the context.
• Note:
  • We don't answer (and may remove) questions that require medical diagnosis or legal advice.
  • We don't answer requests for opinions, predictions or debate.
  • We don't do your homework for you, though we'll help you past the stuck point.
  • We don't conduct original research or provide a free source of ideas, but we'll help you find information you need.

Ready? Ask a new question!

How do I answer a question?

Main page: Wikipedia:Reference desk/Guidelines

• The best answers address the question directly, and back up facts with wikilinks and links to sources. Do not edit others' comments and do not give any medical or legal advice.

See also:

• Teahouse
• Help desk
• Village pump
• Help manual

October 14

Schroedinger's mathematician

I was just glancing at [1] - apparently you can entangle millions of atoms with a single photon. I'm not entirely sure, but I think they are boasting that the atoms are entangled in independent groups in this case, i.e. it is not a Schroedinger's cat state, but one where many pairs can be separately read without disturbing the others (I think!). But it intrigues me that they thought a Schroedinger's cat state was a possibility.

1) what is the closest we've actually come to Schroedinger's cat?

2) is it conceivable to genuinely make a macroscopic Schroedinger-cat state in space, with a well shielded capsule that would seem far from interacting with Earth? (Even if some bulk parameters remained observable, there might be multiple internal ways to explain them, for something less than a true superposition of all states but still useful?)

3) Is this a valid analogy for quantum computing? You send a mathematician up in one of those space capsules with a generous library, tell him to use a genuinely random number generator to decide what general methods to use to go about proving an unknown problem he has some interest in and what papers to read about it. The mission might be two months but he is instructed to break radio silence and phone home early if he has come up with a good proof. The theory is that the mathematician should be exceptionally lucky in his choices and phone home with unexpectedly high frequency, as viewed from Earth at least. OTOH I wonder if they could be an actual hazard for a mission to Mars if the probe called home early with bad news. But can the odds of calling home be changed by quantum effects at all? (I think they must be in QM computing?) Wnt (talk) 10:11, 14 October 2017 (UTC)

• 1,2) I'm no physics whiz but seems to me that Schroedinger's cat is more of a philosophical concept (Copenhagen interpretation) than a physical one, and that it's out of fashion these days. Certainly the classic experiment with a cat and a particle detector is buildable, though keeping it entangled with the whole world without quantum decoherence is beyond imagining. OTOH there is a developing theory[2] that entanglement explains spacetime, gravity, etc.

  3) I (maybe wrongly) think of QC like this: flip 1000 coins so each lands in a classical state <Heads|Tails> with 50-50 probability. The # of heads for the whole ensemble will follow a bell-shaped probability distribution (binomial distribution to be precise). Now quantum "bits" (qubits), as opposed to coins, rather than real-valued probabilities like 0.5, will have complex-valued probability amplitudes, so combining them will have wave-like behaviour showing constructive and destructive interference. A quantum computation is basically an experiment concocted to transform a state representing a problem, into a state where the potential solutions interfere in a way that the incorrect ones cancel each other out, leaving a measurable peak at the correct solution. Shor's algorithm for integer factoring is the most famous example.

  Scott Aaronson's book "Quantum Computing Since Democritus" should be a good semi-popular-level introduction (disclaimer: I haven't seen the actual book, just online excerpts, but I liked them). You might also like his blog, "Shtetl-Optimized", which discusses these subjects a lot. 173.228.123.121 (talk) 20:44, 14 October 2017 (UTC)

For what it's worth, I think that our current understanding of the collapse of superposition necessitating an observer is a load of codswhollop, at the very least fundamentally flawed. Think about it, if one of the central tenets of quantum mechanics - particle-wave duality, is correct, then every particle is acting as an "observer" of every other particle, and no part of the universe can be completely isolated from any other part (perhaps singularities are an exception). This means that superposition should be impossible in the first place, which is contrary to empirical evidence. The only reasonable conclusion that I can draw, is that collapse is not an absolute result, but is instead determined by statistical factors. Perhaps, the probability of collapse is determined by the butterfly effect - the greater the potential influence of a particular state on the environment, the more likely the collapse. However, I can't figure out how potential influence would be determined, unless the system somehow exchanges information with the future of every possible outcome, much like how a driver watches the road at night, only able to see the road directly in front, lit up by the headlights, deciding where to turn to keep on the road. Plasmic Physics (talk) 23:41, 14 October 2017 (UTC)

Ah, now you're getting to the advanced aspect of the model. The putative outcome is that we have a mathematician who comes up with a result thanks to many years of work ... done in a much shorter period of time. Did he experience this time? As who? The many-worlds interpretation is an obvious way to go, yet the premise is that somehow we get to pick the "right" world when the signal is sent, which seems absurd, if it weren't already being done by quantum computers.

The most accessible approach to conscious quantum parallel computing presently would seem to be precognition, a phenomenon that is at best difficult to control or study systematically. I would throw out an anecdote that "technical" precognition, like selecting which of a thousand files will be found to contain a keyword, or doing a web search using unrelated terms, seems to cause significant pain related to blood flow at the past end somewhere vaguely near Broca's area (this not being correlated to the level of detail or the time differential) ... but how to prove such a thing, and what collateral damage is done by the witch in the meanwhile? I suspect the essence of free will and qualia involve the choice of which solution to a causal loop "really" applies; it is thus an external interface for the universe. So ... if the mathematician is conscious, does this selection of a reality mean that he breaks the quantum parallel computing scheme? Well I just don't know. That's the fun stuff past the edge of the world we know. Wnt (talk) 03:45, 15 October 2017 (UTC)

Wnt, re the mathematician, I believe you're thinking of quantum postselection. Aaronson's informal description is: 1) write down your question (anything in NP. 2) Generate some random bits to guess a possible answer. 3) Check whether the answer is right. 4) If the answer is wrong, kill yourself! There will be a branch of the many-worlds interpretation in which you survived, which means in that universe you guessed the answer on the first try, lucky you! He joked about opening a crisis hotline for depressed complexity theorists, where he would explain to them that if suicide was really the answer to life's problems, that would give a way to solve NP-hard problems in polynomial time, which is widely thought to be impossible, so they shouldn't give up.

Plasmic Physics, the conscious observer theory is the Copenhagen interpretation which I think is now mostly thought of as quaint. See: interpretations of quantum mechanics. 173.228.123.121 (talk) 04:48, 15 October 2017 (UTC)

Precognition. Yes, however, in this instance it does not require sentience/conciousness. A superpositioned particle simply follows the timeline which results in the largest change in entropy of the system, as long as the change in entropy associated with that specific timeline is different enough. If the associated change in entropy is not sufficiently unique, then the particle has a higher chance of remaining in a superposition. Howver, the particle doesn't "know" which timeline gives the largest change in entropy, without actually having followed that timeline, which means that it has, is and will have travelled them all. Please forgive the confusing wording, I don't think that the correct paralance may have been invented yet. Plasmic Physics (talk) 05:45, 15 October 2017 (UTC)

Regarding 3), it won't quite work that way (the probability of a phone call won't increase by merely isolating the mathematician). But you should have a look at Grover's algorithm. The random number generator would initialize the system to the state $|s\rangle$ and the work of the mathematician corresponds to the operator $U_{\omega }$. Then a quantum circuit implementing $U_{s}$ should be applied to the mathematician's output (all while keeping the system, including the mathematician, isolated). You'd need to repeat that $O({\sqrt {N}})$ times as opposed to $O(N)$ times for the classical case.

Icek~enwiki (talk) 08:52, 15 October 2017 (UTC)

@Icek~enwiki: This is the real deal, but I have to admit I don't necessarily understand this operation or how to apply it to the mathematician. I would suppose N is all the ways that he can be given random cues to start the work. But I am not clear what $U_{s}$ is when you apply it to his output. Is it conceivable to apply it to an entire written proof he might generate, or is this just some kind of flag to indicate which initial conditions made him say he had the answer? And if you process the output this way, then reestablish contact with the mathematician, is the mathematician you contact the one who came up with the proof? Wnt (talk) 03:03, 16 October 2017 (UTC)

N is the number of possible random inputs as you say.

Let's write the initial state similar to the article:

$|s_{M}\rangle ={\frac {1}{\sqrt {N}}}\sum _{x=0}^{N-1}|x\rangle |M\rangle .$

Here $x$ is the random cue and $M$ is the mathematician (together with his library and everything he needs) in his initial state. The initial state is created by sending entangled photons (different polarization states stand for 0 and 1, and the bits make up the number $x$) to optical detectors attached to the isolated capsule.

Now the mathematician reads the random cue from a display inside his isolated capsule. Then he starts working on the problem, using the random cue as a guide. To make it simple, he has a clock that tells him when it's time to stop. If he has solved the problem within the time limit, he pushes a button which causes a mirror that causes a phase shift of 180 degrees to be placed in an optical path; if he hasn't solved the problem, he pushes a different button which causes a mirror that doesn't cause a phase shift to the placed in the same spot. A certain time after the mathematician got the stop signal from the clock (enough time to let the mathematician push the right button), the devices attached to the isolated capsule create photons with the same polarizations that have been measured just before the number $x$ was displayed for the mathematician. One of the photons is bounced off the mirror that is either phase-shifting or not. Then the state looks (slightly simplified; now there are actual photons somewhere representing $x$ while before the information was in the detectors or already on the display of the isolated capsule) like this:

$U_{\omega }|s_{M}\rangle ={\frac {1}{\sqrt {N}}}\sum _{x=0}^{N-1}\phi _{x}|x\rangle |M_{x}\rangle .$

Here $\phi _{x}=1$ if the mathematician hasn't been able to solve the problem with random cue $x$ and $\phi _{x}=-1$ if the mathematician has been able to solve it. $M_{x}$ symbolized the mathematician and his study after the experience of attempting to solve the problem with random cue $x$.

Now, applying the operator $U_{s}$ basically leaves the mathematician's state as it is and works only on the photons. We send the photons through the quantum circuit that implements $U_{s}$ and then we have as the new state:

$U_{s}U\omega |s_{M}\rangle ={\frac {1}{\sqrt {N}}}\sum _{x=0}^{N-1}\phi _{x}(2|s\rangle {\frac {1}{\sqrt {N}}}\sum _{x'=0}^{N-1}\langle x'|x\rangle -|x\rangle )|M_{x}\rangle ={\frac {1}{\sqrt {N}}}\sum _{x=0}^{N-1}\phi _{x}({\frac {2}{N}}\sum _{y=0}^{N-1}|y\rangle -|x\rangle )|M_{x}\rangle$

After this quantum circuit, the photons are sent to the optical detectors again, the mathematician gets a random cue again, performs his work and we get the state

$U_{\omega }U_{s}U_{\omega }|s_{M}\rangle ={\frac {1}{\sqrt {N}}}\sum _{x=0}^{N-1}\phi _{x}({\frac {2}{N}}\sum _{y=0}^{N-1}\phi _{y}|y\rangle |M_{xy}\rangle -\phi _{x}|x\rangle |M_{xx}\rangle )$

Here $M_{xy}$ symbolizes that the mathematician has attempted to do his work with cue $x$ and cue $y$, in that order.

After sending the photons through the quantum circuit again, and let the mathematician do his work again, we have the following state:

$U_{\omega }U_{s}U_{\omega }U_{s}U_{\omega }|s_{M}\rangle ={\frac {1}{\sqrt {N}}}\sum _{x=0}^{N-1}\phi _{x}({\frac {4}{N^{2}}}\sum _{y=0}^{N-1}\phi _{y}\sum _{z=0}^{N-1}\phi _{z}|z\rangle |M_{xyz}\rangle -{\frac {2}{N}}\sum _{y=0}^{N-1}\phi _{y}^{2}|y\rangle |M_{xyy}\rangle -{\frac {2}{N}}\sum _{y=0}^{N-1}\phi _{x}\phi _{y}|y\rangle |M_{xxy}\rangle +\phi _{x}^{2}|x\rangle |M_{xxx}\rangle )$

Going on with this iteration, we reach a point when the state of the photons is very close to $|a\rangle$, assuming there is only a single $a$ for which $\phi _{a}=-1$. The state state of the mathematician however is a superposition of having tried to solve the problem with various random cues.

So in the end, we get the answer $a$ for the right cue, and in the mathematician's history there will be in general various cues, but they include $a$. In fact, while doing his work, if the mathematician receives a cue that he received before and for which he solved the problem, he can use his time in other ways and just press the right button at the end.

Icek~enwiki (talk) 20:37, 16 October 2017 (UTC)

@Icek~enwiki: This is a great explanation, very clear. One thing that kind of amazes me about it though is that the mathematician sends out a known, non-quantum set of bits for a, which leaves his enclosure, then they get mirrored and come right back to him as qubits. And the qubits after a few iterations are (usually) going to provide the right conventional bits by incredible luck - even from the mathematician's point of view, I think! Now... is there anything about this process that requires that the mathematician is the one in the small enclosure and the $U_{s}$ part is in the rest of the world? Could you have a tiny perfectly isolated quantum device that implements U_s and an ordinary mathematician in an ordinary university sends his findings into it and gets oracular answers back based on the superposition of all possible worlds out here in the "normal" universe? Wnt (talk) 20:12, 17 October 2017 (UTC)

The point is that the photon detectors, the display, the mathematician and his library, the mirror and the photon emitter are in a superposition of states (maybe I wasn't clear about the last point when I wrote "the devices attached to the isolated capsule" - they should rather be inside, isolated as well). From inside it looks as if the photon emitter creates non-entangled photons with definite polarizations, and to the mathematician it must look like he's pretty lucky indeed (though he still needs to do $O({\sqrt {N}})$ attempts on average). But without the isolation, the wave function collapses and the probability of getting the right cue is just 1/N.

If there is spontaneous collapse of wave functions like in Ghirardi–Rimini–Weber theory, the system wouldn't work (unless the parts in superposition are small enough - if the spontaneous collapse is much rarer even than what Ghirardi, Rimini and Weber probably had in mind, then it still works).

Icek~enwiki (talk) 08:15, 18 October 2017 (UTC)

@Icek~enwiki: I was thinking the bit that does the U_s algorithm has to be outside the superposition. If everything is in the superposition then we can say the whole universe is an isolated system and may be superposed right now per the many-worlds idea. But the thing is, if the U_s calculator is outside the superposition, I'm thinking, what if the "outside" is a really really small area, a tiny electronic device, and the rest of the world is the superposition? Essentially we could use this design you proposed to communicate with parallel universes who are doing different work with different random seeds as they diverge. Indeed, I'm not sure this needs to be mathematical: if we could set the bits of a to be a message and phase-shift it depending on whether it is 'really important' (I am still a little iffy on the phase shift when I think of it) could we simply exchange random greetings through the device as our histories diverge? Wnt (talk) 11:53, 18 October 2017 (UTC)

colds and endogenous opiates

One of the painful lessons I learned about colds when I was young is that they seem -- for me -- to involve production of a fairly large amount of some kind of endogenous opiate during the "sniffle" phase. I concluded this because back then I suffered some very sore throats after colds on account of burning myself with hot soup without realizing it. More recently, though exerting extra care with hot food, I managed to do something to a back muscle that left it sore for a couple of days when that phase of a cold ended. And, of course, there is the observation that for a few days it is possible to sniffle huge amounts of crap and feel/hear wheezing in the lungs with no instinctive reaction to cough, which suggests a powerful cough suppressant activity.

But for something that seems so important to the course of the cold, I didn't quickly find much discussion of it (mostly gene therapy papers with endogenous opiates in adenoviruses!), though I understand endorphins are part of a generalized stress response, so I just wanted to check if there is a medical term for this sort of suppressive effect I'm not thinking of. I mean, do other people even have this response? Wnt (talk) 22:24, 14 October 2017 (UTC)

When you feel pain in one place, it does seem to make less severe pains elsewhere even less noticeable, but I'm not sure of the mechanism. It might be entirely within the brain, which has a limited ability to pay attention to different pains. I suppose that makes sense, as there's little survival advantage to being worried about your aching back whilst being mauled by wild animals. Best to only concern yourself with the most immediate threat, which presumably is causing the most severe pain. (Note that there's no reason to assume that this phenomenon is exclusive to humans.) StuRat (talk) 23:47, 14 October 2017 (UTC)

Stress-induced analgesia sounds like what you're describing, a form of hypoalgesia. StuRat is suggesting counterstimulation (a page that could do with some work!). Klbrain (talk) 23:58, 14 October 2017 (UTC)

I think I've experienced this - I remember being momentarily freaked out by a prank back at an undergrad and not noticing I'd misplaced a bit of skin on one malleolus the size of a dime until I happened to spot the blood. But the sort of hypoalgesia during a "fight or flight" seems hard to relate mentally to the first days of a common cold type infection. Hypoalgesia seemed like a great keyword ... but didn't bring anything up with rhinovirus or coronavirus or "common cold". I should point out though that none of the generic mechanisms seem to match what happens with a cold as I experience it -- because the cough suppression and hypoalgesia stop early on during the cold, even though there are still plenty of distracting, unpleasant stimuli, and not obviously less stress. Wnt (talk) 03:01, 15 October 2017 (UTC)

References

October 15

Confirmation of Special relativity

General Relativity had the famous confirmation with the bending of light during a solar eclipse. Was there any similar moment for Special Relativity? Bubba73 ^{You talkin' to me?} 00:47, 15 October 2017 (UTC)

Is the Wikipedia article titled Tests of special relativity insufficient for your purposes? The most important and earliest such experiment is probably the Michelson–Morley experiment, which while it predates special relativity by some decades, provided the results to verify it. --Jayron32 00:55, 15 October 2017 (UTC)

Well, it looks like there was already some experimental evidence in favor of it when it came out, but it wasn't until the 1930s that there was a confirming experiment. Bubba73 ^{You talkin' to me?} 03:51, 16 October 2017 (UTC)

Do you mean the Ives–Stilwell experiment? I thought all those outcomes could have been predicted from the Lorentz transformation, which however wasn't really an explanatory theory. Maybe I'm wrong about that though. The SR chapter[3] of the Feynman Lectures on Physics has a brief historical treatment if that's of any interest. 173.228.123.121 (talk) 06:34, 16 October 2017 (UTC)

Just because it happened before the theory was formalized does NOT mean that the results could not be used to confirm it. --Jayron32 10:55, 16 October 2017 (UTC)

History of special relativity may also be of interest. --47.138.160.139 (talk) 01:45, 17 October 2017 (UTC)

October 16

Holographic interactions

In several sci-fi tropes, like Blade Runner 2049 or Halo holograms of humans can interact with people both visually and audially, despite having any organs or sensors to receive and analyze visual and audio cues. How is that? Unlike modern virtual assistants, it seems holograms per se can't process any data due to their nature. 212.180.235.46 (talk) 07:58, 16 October 2017 (UTC)

You're talking about works of fiction. If the author doesn't give an explanation, you're free to make up your own. --69.159.60.147 (talk) 10:48, 16 October 2017 (UTC)

The image of a person on Skype has no actual eyes or ears and yet one can talk to the image as if it were the actual person. Dmcq (talk) 12:01, 16 October 2017 (UTC)

In the classic science fiction series Red Dwarf, the holograms could not interact with real matter: hologram Arnold Rimmer could not hold objects, could walk through walls, and (if memory serves), due to his nature, he was a complete and utter smeg-head. Of course, it would be wholly unfair to compare Red Dwarf to other science fiction, in terms of scientific accuracy, literary merit, and cultural impact; it’d be a no-contest win.

Nimur (talk) 15:24, 16 October 2017 (UTC)

I'm pretty sure that Arnold Judas Rimmer BSC was a total smeg-head before he ever became a hologram. Iapetus (talk) 16:22, 16 October 2017 (UTC)

There are already virtual CAD systems that even allow limited interaction with a hologram-like augmented and/or virtual, visual representation of objects. Developers also try to implement haptic/tactile feedback into these virtual systems! Everything is still in an early stage tho and you always need interactive bridge-devices like VR-googles, -pointers, -gloves and alike, to use these systems. Its highly doubtful that there will ever be a "holodesk" which will not need such "adapters" and on top these systems certainly will have allot more limitations then their imagination in sci-fi. So allot of sci-fi-"products" are actually branded wrong since they contain somuch clearly pure letsmakesometing up fantasy elements and mechanics. Warpdrives, Wormholes, artificial gravity, teleportation... never gona happen! --Kharon (talk) 16:38, 16 October 2017 (UTC)

Some of those seem doable:

1) Artificial gravity just requires spinning the ship, but it needs to be a large ship to avoid nausea induced by a noticeably variable (apparent) gravity field. Also spinning the ship introduces lots of new problems with docking, solar panels and communications arrays and telescopes tracking their targets, external maintenance, etc. Doable, but tricky. (One solution is a rotating part and a stationary part, but linking those parts together isn't easy.)

2) Teleportation seems possible, although again not in the way they show. It would require scanning an object down to the molecular level, transmitting that info, creating a copy in that exact configuration at the remote site, then destroying the original, if you want to avoid having a clone. See Think Like a Dinosaur for a realistic treatment of the problem. Certainly there could never be transporters with equipment at one side only.

3) The time it will take to get humans to other stars does seem to be a profoundly unsolvable problem. Even if you have a ship that can go close to light speed, it would still take around a year to get to that speed at 1 g and another year to slow down at the other end. So, add almost 2 years to the travel time for that, and then we have the nearest stars being over 4 years away at the speed of light (although little time would pass for them during this period). So now we're up to maybe 6 Earth years to get there, and another 6 to come back. Would people really want to sign up to not see their loved ones for 12 years, minimum, then being a decade younger than those loved ones when, and if, they came back ? I'd have to say robotic ships seems a lot more practical. They can accelerate faster, and not worry about coming back. Even in our own solar system we've sent robotic ships all over the place but never sent people further than the Moon. Maybe if we ever found a planet we could easily colonize then sending people might make sense, but I'm skeptical of that, too, considering how none of the planets, dwarf planets, and moons in our solar system seem particularly close to being able to support a self-sustaining colony. StuRat (talk) 20:06, 16 October 2017 (UTC)

The visual and audio interaction just requires cameras, microphones, and speakers. In Star Trek: Voyager, they had a "portable holographic emitter", and as long as it had those items, and the ability to display a hologram, that seems possible. However, actually manipulating objects in the real world is another matter ("photonic matter", to be specific). For that, you would need a robot, or at least a robotic arm. A more realistic version of the holographic doctor on that show might have had him do all the "bedside manner" human interactions (except touching), like asking patients about their symptoms, while robotic arms do all the physical operations, like surgical procedures.

As for Blade Runner, we could give them the benefit of a doubt and assume that the locations were all hooked up with microphones, cameras, speakers, and holographic emitters. StuRat (talk) 20:41, 16 October 2017 (UTC)

...never gona happen! --Kharon (talk) 01:41, 17 October 2017 (UTC)

We're already headed towards every public place being filled with microphones, cameras, and speakers. StuRat (talk) 02:02, 17 October 2017 (UTC)

I was going to link smart dust. That said, it seems conceivable to me that "holograms" (sensu lato) could have direct sensory capabilities by some more integrated means, since interfering lasers are inherently capable of measuring distances very precisely and have been used to detect very small vibrations (i.e. spying by bouncing off windows). But this depends on the specifics of how you make a seemingly 3D free floating hologram far from an emitter, which is the more difficult technical question. Wnt (talk) 19:55, 17 October 2017 (UTC)

October 17

Kilonova and superheavy elements

The kilonova from neutron star merger was just announced on Monday as part of gravitational wave detection. I read the article on astronomy.com and said the kilonova produced Earth masses worth of gold, platinum and uranium, which are superheavy elements. It makes me think if the neutron star merger could also produce transactinide elements or even transoganesson elements like moscovium (element 115) and feynmanium (element 137). Based on the amount of gold being produced in that kilonova at over 10 Earth masses, it could produce asteroid-mass worth of moscovium for example. You will strike me gold if you think so. PlanetStar 00:16, 17 October 2017 (UTC)

Some points:

1) Those huge masses are likely mixed in with even huger masses of "junk" elements, so it's not like there will be solid gold planets spit out. After all, there's enough gold in seawater to pave the streets with gold, but it's diluted by a huge amount of water, etc., and hence useless to us.

2) Isotopes of moscovium lists all half-lives less than a second, so I wouldn't expect any to be left, say, a day after the event.

3) Island of stability might mean there are some stable heavy elements created by such an event. If so, this could be quite interesting. We should look for their atomic spectra in light coming from the event. StuRat (talk) 01:27, 17 October 2017 (UTC)

There are likely more stable isotopes of moscovium with more neutrons than the ones we know. As for looking for their atomic spectra: this is a bit difficult considering that we do not currently know what they are, though there have been some theoretical calculations, such as this one for copernicium. Double sharp (talk) 10:26, 18 October 2017 (UTC)

Nucleosynthesis#Explosive nucleosynthesis is a good start for your reading, I would then follow the links from there for more details on various questions you may have. --Jayron32 01:30, 17 October 2017 (UTC)

Pretty much anything that can exist, would be expected to be created in some quantity in such events. However, as a general rule the superheavy elements are also very unstable. Many will radioactively decay within very short periods of time, preventing them from being seen or used elsewhere. Dragons flight (talk) 11:16, 17 October 2017 (UTC)

This article in Sky and Telescope magazine addresses some of these issues. The section Striking gold includes a periodic table coded to show what elements are made in what cosmic events, including merging neutron stars: unfortunately, it doesnt go beyond Uranium. {The poster formerly known as 87.81.230.195} 94.0.129.189 (talk) 10:12, 18 October 2017 (UTC)

You might want to consider that:

(1) there is a "cycling factor" in the r-process, since if a nucleus doesn't capture enough neutrons after A = 209, it will simply decay back to Pb and Bi; and the same thing happens right after A = 238, with nuclei decaying back to Th and U, so that the shorter-lived superheavies would get depleted;

(2) neutron capture, especially high-energy neutron capture, above A = 209 has a tendency to result in fission instead of continuing up the neutron drip line.

Both of these will cut your yield of superheavies severely. Assuming a generous half-life of about a millenium for the superheavy isotopes produced, the amount you find has been calculated to only about 10⁻¹² times that of the Pb produced. Double sharp (talk) 10:29, 18 October 2017 (UTC)

Moving from warfarin to heparin + surgery

Why would a surgeon move a patient from heparin from warfarin (which was the usual treatment) pre and post surgery? As far as I know, both would increase bleeding (and also work as blood thinners).--Dikipewia (talk) 00:37, 17 October 2017 (UTC)

The patient should ask. If the doctor can't give a good reason, it might be good to get a 2nd opinion. I've seen doctors change meds "for no apparent reason" way too often. Any change in medication should be discussed with the patient, and a reason given. I wonder if there's a "patient bill of rights" item somewhere that lists "The patient has the right to be informed of any change in medication, given a reason for the change, and refuse the change, if they so choose".StuRat (talk) 01:19, 17 October 2017 (UTC)

It's not a real ongoing case. It just appear to be normal praxis, see [[4]]. I just want to know the rationale behind this.--Dikipewia (talk) 01:48, 17 October 2017 (UTC)

Did you mean "praxis" or "practice" ? StuRat (talk) 02:05, 17 October 2017 (UTC)

Yes. Indeed.Dikipewia (talk) 15:24, 17 October 2017 (UTC)

Warfarin is generally discontinued for surgery due to the bleeding risk. There is a lot of literature about "bridging" the period when warfarin is discontinued with heparin or other anticoagulants. One fairly recent study [5] of atrial fibrillation patients was of the opinion it was unnecessary to have any anticoagulant. But this is a big topic and it would really take a lot more effort than I'm willing to give it to see how general and agreed-upon that conclusion actually is. Wnt (talk) 11:39, 17 October 2017 (UTC)

I know that some doctors are against or don't see the necessity of this bridging treatment.

However if they choose a bridging mechanism, why would another anticoagulant be different? During a surgery, what makes the anticoagulant warfarin unsafe and the anticoagulant heparin safe? Both seem to act in the same way, a blood thinners that reduce coagulation to avoid blood clots. Wouldn't this imply that both increase bleeding risk? Dikipewia (talk) 15:24, 17 October 2017 (UTC)

Warfarin is a vitamin K antagonist while heparin activates antithrombin on binding. Warfarin's effect should be more long lasting (I think) and heparin's can rapidly be reversed with protamine sulfate. Again, this is an area where a great deal is known but I don't know much at all, but I think this is at least part of the answer. Wnt (talk) 19:58, 17 October 2017 (UTC)

Clean air in the UK

Is it just a question of cars? Couldn't it be that the air on a really small place is contaminated by a local industry? Could that be more unhealthy than London?--Hofhof (talk) 00:53, 17 October 2017 (UTC)

See air pollution in the United Kingdom. Unfortunately, that doesn't seem to consider any source other than vehicles. Here's a more even treatment of the source, but alas a bit dated (2001): [6]. Note that vehicles are somewhat unique in that they pollute most where the most people are, whereas factories or power stations can be located where the prevailing winds will blow the smoke clear of the cities. StuRat (talk) 00:55, 17 October 2017 (UTC)

• There are many sources of air pollution. Historically, London had life-threatening pollution ("killer fogs", mostly from coal fires) prior to the advent of automobiles. As otehr sources were addressed, car pollution became relatively more important. Modern monitoring methods do a fairly good job of identifying sources, -Arch dude (talk) 00:58, 17 October 2017 (UTC)

But the question remains: air pollution monitoring covers things like nitrogen dioxide and ozone. But what if I'm close to a chemical plant. Could this chemical plant contaminate more than anything that you find in London?--Hofhof (talk) 01:07, 17 October 2017 (UTC)

A UK chemical plant shouldn't release many chemicals into the air normally, due to regulations, but there's always the risk of a Bhopal disaster event. StuRat (talk) 01:17, 17 October 2017 (UTC)

"Shouldn't" is not an exact synonym for "doesn't", mind you. --Jayron32 01:27, 17 October 2017 (UTC)

List of active coal fired power stations in the United Kingdom does show they are rapidly reducing reliance on this dirty energy source. StuRat (talk) 01:17, 17 October 2017 (UTC)

Thats why there are so many record high Chimneys in industrial areas! As long as anyone pollutes in a save distance from any detector, nature, livestock or human population, they can "contaminated" almost as much as they want without direct, local consequences. Cars emit right where they are, so there are direct, local consequences. --Kharon (talk) 01:40, 17 October 2017 (UTC)

"The Government announced in November 2015 that the UK will phase out coal-fired power generation by 2025" UK COAL PLANT CLOSURES - A STRUCTURAL SHIFT AWAY FROM COAL. Alansplodge (talk) 12:53, 17 October 2017 (UTC)

The mayor of London has recently called for a ban on domestic wood-burning stoves - [7] - and there have been concerns about the amount of methane produced by cows - [8]. Pollution is a highly complex issue, with no easy answers. Wymspen (talk) 10:21, 17 October 2017 (UTC)

Stand next to a wood burning grill and you are breathing more dangerous particulates than you would ever encounter in normal London air. So, yes, local pollution can be intense. However, humans rarely spend much time near intense pollution sources (e.g. wood fires) but many people breathe city air all the time, so the cumulative effect of the latter is often more important. Beyond a certain scale, all industries have regulations for the quantity and type of pollutants they can legally emit into the air. Often there are inspections to show that they have the right kind of mitigation procedures (e.g. the right type of burners, smokestacks, etc.) to mitigate any expected air pollution. For very large scale industries there is also routine monitoring of local air quality. Power plants and industrial activity are a source of air pollution in the UK. (So is agriculture, for some pollutants like ammonia.) However, cars get a lot of attention in the UK because they are a major source of pollution, and they operate in close proximity to people. The growth of relatively more-polluting small diesel engines (roughly 50% of UK transport) and the relatively less stringent emissions standards (compared to, for example, the US) has made air pollution from the transportation sector a more prominent problem in the UK than in most other developed countries. Dragons flight (talk) 11:09, 17 October 2017 (UTC)

The Department for Environment, Food and Rural Affairs web page, Causes of air pollution , says that "In all except worst-case situations, industrial and domestic pollutant sources, together with their impact on air quality, tend to be steady or improving over time. However, traffic pollution problems are worsening world-wide". A more detailed breakdown linked from that page is What are the causes of air Pollution. Alansplodge (talk) 12:47, 17 October 2017 (UTC)

• One of the issues to consider here is the difference (in modern terms) between Point source pollution and non-point-source pollution. Point source pollution means "We can identify a single source for where this pollution is coming from". Of course, with zero environmental controls you get runaway pollution regardless (such as with the Great Stink, or modern Beijing). If you want to introduce controls, the easiest target is where it is most concentrated: individual industrial sources and power plants. What remains is all of the small pollution sources which add up to a lot. One "scrubber" can clean a LOT of pollution out of a fossil-fuel smoke stack, where the released pollution can all be captured, but put 10,000 cars on the road for a year and you may end up generating just as much pollution as that one stack used to. Stopping each one of those cars from polluting is a lot harder of a logistical problem, and why in modern, developed countries automobile traffic is considered the biggest issue: the former "large single industrial polluter" problem has largely been solved (for a certain definition of "solved"), but the problem of all those cars continues to be an issue. This page looks at one factor (CO2 emissions) in one place (California), but the issues in any developed country are likely similar: It reports that 58% of pollution comes from road transport. To solve pollution most effectively that is the issue to fix, but to get at that problem is not as simple as "clean up a few power plants" it's "install effective public transportation" and "better city planning" and "get people to buy electric cars". Those are MUCH more daunting problems. --Jayron32 12:32, 18 October 2017 (UTC)

Voltage drop

I am experimenting with conductive ink, which I applied to a strip, at the ends of which I placed terminals (nuts and bolts) and I measure a resistance of about 45 Ω over those terminals. But I also want to measure the voltage drop at different points.
So I stacked two 3 V batteries, resulting in about 6.5 V. When I connect two copper wires to the poles I measure a slightly lower voltage at the other ends of those wires. But then when I connect those ends to the terminals I measure only 0.37 V over the terminals. That is about 1/20 of what it should be.
If I place the leads of the multimeter further in, I get an increasingly lower voltage, which is as expected (the voltage drop), but the initial voltage just can't be right, can it?
To make sure, I measured the resistance over a terminal, but that is 0.2 Ω or less. And the wires are well connected to the terminals (a firm pull doesn't pull the wires out). So what may cause this? DirkvdM (talk) 15:55, 17 October 2017 (UTC)

• This would be much simpler with a diagram, because while I think I understood it (and find it as puzzling as you) I may have missed something; or better yet, a photograph. The best guess I have that matches all the symptoms is that one of the copper wires, or its connection with the battery, has a resistance ~ 10kΩ, but that sounds unlikely (this is too low for a broken cable or faulty connection). You could try measuring the resistance of those.

BTW: if you do any kind of experiment, take a lot of photographs - do not spend time choosing good angles/lighting or sorting them out afterwards, just take tons of crappy shots with your cell phone and dump in into a date-named folder on a hard drive. In this day and age it is pretty much free to do that, and once in a blue moon you will be able to retrieve the one photograph from two years ago that shows a crucial point of the setup that you did not realize was crucial back then. This is of course in addition to keeping a lab book, but the amount of lab-book writing required to capture as much information is just enormous. Tigraan^{Click here to contact me} 16:12, 17 October 2017 (UTC)

You should check the voltage of the pair of batteries when connected. If it is still 6.5 V, then measure the voltage drop on your power supply wires. There could be a problem in those wires. If the voltage from the battery pair is very low it could mean that the battery is flat, or has a very high internal resistance. Or perhaps they are connected back to front. Graeme Bartlett (talk) 21:11, 17 October 2017 (UTC)

Ah, I think I found the cause. It appears to indeed be internal resistance.
Yesterday, after I disconnected the wires, the voltage over the battery stack was a lot less than what it was at first, just over 3 V if I remember correctly. So the low resistance of the condcutive ink (45 Ω) was a huge drain on the batteries. But this morning it was 6.12 V. So the batteries had drained a bit overall, but during the experiment the 'surface voltage' (what is that called?) was a lot less.
So I tried it again, and now measured the voltage over the battery stack, and it was about 1 V, although it fluctuated quite a bit. Over the terminals it started at 0.42 V and over half a minute it dropped and then seemed to stabilise at about 0.3 V.
So it seems the batteries (CR2032) just couldn't keep up, so to say (they were meant to power a little LED lamp).
But then there is still the difference between the battery stack (about 1 V) and the terminals (0.42 V dropping to 0.3 V). The resistance over a wire is 0.1 Ω (the multimeter can't go any lower). Times two is 0.2 Ω at most. That is a fraction of the resistance of the conductive ink, so the voltage drop should also be a fraction, right? DirkvdM (talk) 10:58, 18 October 2017 (UTC)

Flu vaccine and disease prevention (US vs. Europe)

Having lived in both US and Europe, I am acutely aware of their differing policies regarding flu vaccines. The US recommends a flu vaccine for all healthy adults (not otherwise excluded by allergies or other concerns). In Europe, the flu vaccine is only recommended for at risk populations (e.g. children, elderly, etc.) As a consequence, the US vaccinates ~50% of the population each year, while the coverage in European countries is much, much lower. I was wondering if there was any research comparing the effects of these diverging policies in terms of the relative incidence of flu-related disease, lost productivity, death, etc. from the US and Europe. I would be particularly interested to know if the much higher vaccination rates in the US can be shown to have appreciable herd immunity related benefits, or is ~50% not high enough to see benefits in the unvaccinated populations. Dragons flight (talk) 16:01, 17 October 2017 (UTC)

Something else you might want to look at is what happens with strains which the vaccinations don't cover. That is, do those strains spread more when vaccinations occur for the other strains, because people who would have contracted another strain and stayed home now go out and catch the unvaccinated strains ? StuRat (talk) 16:10, 17 October 2017 (UTC)

• The thing about herd immunity is that it is an abrupt transition between "everyone infectable will get it" and "herd immunity works" (based on a few more or less realistic assumptions - large population (often an OK assumption), probability that person A will catch the disease from person B if infected more or less the same for all A and B (pretty much never the case) - but still a good first approximation). Our article cites [9] (which I have not checked) and says that the herd immunity threshold (= level of vaccination, basically) to stop influenza from propagating is 33 to 44%, meaning a 50% vaccination rate would indeed provide herd immunity, but that it would not take a large drop in the vaccination rate to lose it. I will note that this press article leaves one with the impression that the threshold for herd immunity for flu would be somewhere in the 80-90% range, but I would rather trust the NCBI source. Tigraan^{Click here to contact me} 16:27, 17 October 2017 (UTC)

The mathematics of these types of functions are actually fairly well studied; there's math like bifurcation theory or even the famous Mandelbrot set which is based on iterative functions that have two states, a "stable" state that collapses back to a single, small value, and an "unstable" state that runs away to infinity. The Mandlebrot set is just the limit between the stable state of the function and the "runaway" state of the function. Disease immunity follows similar behavior: at some value of vaccination, the infection rate always drops back to a small, stable value, whereas at any vaccination rate below that threshold, the infection rate skyrockets to essentially "everybody". While each disease has its own characteristic function that describes its transition, there's usually some "tipping point" between "herd immunity" and "everyone gets sick". LOTS of natural systems obey this kind of mathematics, such as population dynamics. --Jayron32 16:40, 17 October 2017 (UTC)

I have been imagining that within the US there might be enough state-to-state or city-to-city variation in vaccination rates that herd immunity was not necessarily an all-or-nothing proposition for the whole US. Also, with flu, the immunity rate should be higher than the vaccination rate if past exposure to similar strains provides some protection. Which is of course is all a way of saying "it's complicated". Dragons flight (talk) 17:41, 17 October 2017 (UTC)

The U.S. should be vaccinating more widely than it is, for example with universal free vaccinations. I mean, when you antagonize a country that likely has ready resort to pandemic flu strains, and other fun creative projects, it would be a good idea to practice eradicating flu on a yearly basis. Wnt (talk) 20:03, 17 October 2017 (UTC)

Yes, that's where we get localized outbreaks ("outbreak" is medical speak for "where the infection-rate function tends towards infinity" in my discussion above) of dieseases where the vaccinate rates are too low. This paper discusses the matter. --Jayron32 13:33, 18 October 2017 (UTC)

See also here. Count Iblis (talk) 20:35, 17 October 2017 (UTC)

How many grams of food caused each gram of blattella germanica in a someone's home?

No, this is not professional advice. Are periplaneta americana similar? Sagittarian Milky Way (talk) 23:31, 17 October 2017 (UTC)

Potentially none:

1) They can get their food elsewhere and then come into your home.

2) They can also eat things we don't consider food. See German_cockroach#Diet and American_cockroach#Diet. StuRat (talk) 06:09, 18 October 2017 (UTC)

Well then how many adult roach body weights of nutritious possibly human food substance does a roach pair have to eat to grow from newborn to their first eggs hatching? Sagittarian Milky Way (talk) 07:03, 18 October 2017 (UTC)

October 18

How (un)likely are these science fiction inventions to ever exist in this universe?

If the universe is real, non-supernatural and the things physicists think are ruled out really are.

1. Affordable cars like in the Fifth Element or Star Wars.

2. Earth-like gravity in space without accelerating, spinning or having a massive enough spaceship.

3. Rayguns that exceed the utility of current handguns in the average application (i.e. if everything's similar except hits damage twice the meat and magazines hold 1 shot they might not be better for most things (also ignoring price, sunk cost of already having a gun, how much or little they're taken seriously by potential victims, bans/raygun control or that coolness could make many switch a bit before it's rational since these are either mostly transient or not actually affecting how good they are))

4. Vehicles that look like rounded boxoids or simple geometric solids and have at least the speed, range, acceleration and maneuverability of cars combined with the altitude capabilities of spaceplanes. And can do any movement in any orientation as long as it doesn't subject it to uncomfortable g-forces or density-adjusted airspeeds or compressional heating above it's speed capabilities. Means of moving/hovering/turning/flying etc not visible to the naked eye from outside are acceptable. Like I don't know, microscopic particle accelerators coating the hull. Could it also plausibly do single-stage-to-orbit and back? Without losing >10% mass? Accelerate to 0.1c at 1g, turn around and stop at 1g? If it's limited to 1g and 0.1c and has to arrive stopped, what's the furthest it could plausibly go on 1% of it's mass lost as fuel? Sagittarian Milky Way (talk) 03:43, 18 October 2017 (UTC)

Whoa. InedibleHulk (talk) 03:46, October 18, 2017 (UTC)

1) Do you mean hovercraft ? They exist, but are quite impractical for most private civilian uses, as they would tend to blow down pedestrians as they pass by, and make a lot of noise.

Some can fly at least thousands of feet high unlike hovercraft. They're also not as loud and I think they were shown to dive in Star Wars. Korben Dallas pitched his cab into a dive and parked it vertically in the smog to hide. Sagittarian Milky Way (talk) 07:19, 18 October 2017 (UTC)

2) This would seem to violate the known laws of physics.

Could collimated graviton rays ever be shot out of the floor? Sagittarian Milky Way (talk) 06:53, 18 October 2017 (UTC)

3) Seems plausible. In particular, it could automatically target whoever you look at, or rapidly aim and fire at everyone in range. The lack of recoil would be quite a plus. For a power source, how about a nuclear reactor in a connected backpack ?

4) You might want to break this up into subparts. StuRat (talk) 06:02, 18 October 2017 (UTC)

As for number 4: Relativistic mass at 0.1c is 1.005 m₀, or, even at 100% efficiency, you need to spend about 0.5% of your original mass to accelerate to 0.1c. Braking has the same cost, so your minimum mass loss for going to 0.1 c and stopping is already 1% (plus a few digits I have rounded down). Since there will never be a 100% mass to kinetic energy conversion (part of the energy goes into your reaction mass), I'm pretty sure that number 4 will never happen, unless you use "cosmic energy" or invent a way to do regenerative braking relative to the cosmic microwave background (both of which, I'm sure, have been described in SF, but neither of which is possible within the scope of our scientific understanding). On the other hand, if you somehow manage to get up to 0.1c, there is basically no limit of how far you can go (well, how far your vehicle can go - you may run out of resources like air, water, food, or lifetime). --Stephan Schulz (talk) 07:53, 18 October 2017 (UTC)

And Stu, a nuclear backpack is not such a good idea. Nuclear reactors are still heat engines, and a portable version will be lucky to run at 30% efficiency. So for every Watt you fire at the enemy, you need to provide cooling to get rid of two Watts from your backpack. And nuclear reactors cannot be regulated up and down quickly (even if you use control rods to quickly interrupt the main chain reaction, this will create neutron poisons, and there still will be many unstable intermediate reaction products that keep producing decay heat), so you basically need to keep it running at full output all the time you may want to use the gun - in which case you need to get rid of even more waste energy. --Stephan Schulz (talk) 08:02, 18 October 2017 (UTC)

I agree that it would be dangerous, but then so were gasoline-filled "backpacks" use with flame-throwers. StuRat (talk) 14:16, 18 October 2017 (UTC)

Elliptical Orbit of planets

Why do the planets revolve around the sun in elliptical orbits and not in circular orbits? — Preceding unsigned comment added by Hemant1776 (talk • contribs) 11:30, 18 October 2017 (UTC)

A circle is merely a special case of an ellipse where the two foci are merged to a single point. Statistically, planets have an infinite number of ways to orbit in an ellipse, but only one circle, so it's really just a question of the math; it's basically almost impossible for it to work out to be a circle, as 1 out of infinity is basically never. --Jayron32 11:34, 18 October 2017 (UTC)

• As noted, a circle is also an ellipse.

This is due to gravity having an inverse square law behaviour. Johannes Kepler had already observed enough to work out this behaviour (but not to explain it), and that satellite's orbits "swept out equal areas in equal times". This inspired Isaac Newton to give his model of gravity an inverse square law behaviour, thus explaining Kepler's observations.Andy Dingley (talk) 12:07, 18 October 2017 (UTC)

Our article Kepler orbit, albeit a bit long and technical, has all the information - including a demonstration of why the orbits are elliptical (from Newton's second, which was discovered later as AD said, but it is conceptually simpler in that direction). Tigraan^{Click here to contact me} 12:12, 18 October 2017 (UTC)

I agree with both Jayron and Andy Dingley. An alternative perspective that appeals to me is to observe that the only force acting on a planet is its own weight (the gravitational force between the planet and the center of mass of the rest of the solar system, or even the universe.) If the only force acting on any object is its own weight, the mechanical energy of the object will be conserved. So the kinetic energy of the planet, and its potential energy, can vary up and down but its mechanical energy will remain constant, just like a pendulum swinging back and forth.

If a planet revolves around the sun in a circular orbit its potential energy will remain constant, and so will its kinetic energy, just like a pendulum hanging vertical and motionless. This is a special trivial case of all the different possibilities. There are infinite alternatives in which the planet has variable potential energy (but constant mechanical energy and therefore variable kinetic energy); and these infinite alternatives are associated with elliptical orbits. Dolphin (t) 12:19, 18 October 2017 (UTC)

• Feynman's Lost Lecture is worth a read too, for a graphical proof of Newton. This has been published as a small, cheap paperback. Andy Dingley (talk) 12:20, 18 October 2017 (UTC)
• Basically per above. But it should be mentioned that Newtonian mechanics only allows two orbital solutions to the two-body problem: the orbit of both is spherical around the barycenter; or elliptical around the barycenter. With massive bodies orbiting smaller bodies the barycenter will be typically within the massive body, so the motion of the planets around the Sun has an approximately stationery Sun. See Orbital eccentricity: spherical orbit are merely a special case with zero eccentricity, and not much likelihood as explained earlier.

Relativity also complicates things slightly, leading to apsidal precession; but this effect is usually very small. See Two-body problem in general relativity for a discussion.

There is no general solution to three or more bodies orbiting: see Three-body problem. But in practice the orbits of planets around the Sun are are approximately elliptical; perturbations (differences from the elliptical) are mostly due to the gravitational attractions of other bodies, especially the planets; see Perturbation theory. --Jules (Mrjulesd) 12:47, 18 October 2017 (UTC)

Nitpick: Newtonian mechanics only allows two solutions to the two-body problem: the orbit of both is spherical around the barycenter; or elliptical around the barycenter - only in the case of bounded trajectories. If the initial relative speeds are large enough, the kinetic energy is high enough to allow one body to escape the other's gravitational well (hyperbolic trajectories in the Kepler problem). Tigraan^{Click here to contact me} 13:10, 18 October 2017 (UTC)

True, I have amended it to "orbital solutions" --Jules (Mrjulesd) 13:13, 18 October 2017 (UTC)

It goes back to the formation of the solar system. Typically the planetary orbits have very low eccentricity (i.e. they are almost perfect circles). Their axes also have very low tilt and they revolve and rotate in the same direction. Their orbital planes are almost identical. If some other factor intervenes eccentricities can be very large - for example the orbits of captured comets and the remnants of the disintegration of the fifth planet (the minor planets or asteroids). 82.14.24.95 (talk) 15:18, 18 October 2017 (UTC)