Wikipedia:Reference desk/Archives/Science/2016 February 12

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February 12

Magnetic hovering spheres

Is it possible to suspend a hollow ferric sphere inside another slightly larger one and have them separated by some sort of magnetic repulsion and have them spin in opposite or even dissimilar directions? — Preceding unsigned comment added by 66.87.83.70 (talk) 02:51, 12 February 2016 (UTC)

I'm going to take a punt on this and suggest No as an answer, because I don't see how to have the internal shell supported by repulsion, in a stable configuration, or by external fields, at all. The answer to the second bit is easier, yes if you can suspend the inner sphere then I'm sure you'll be able to motor it with respect to the outer one. Greglocock (talk) 03:50, 12 February 2016 (UTC)

Earnshaw's Theorem says there can be no stable configuration of stationary magnetized (or electrically charged) objects. Perhaps it's possible to do what you want if (and only if) the spheres are rotating (similar to what the Levitron does) but I'm not sure about this. It would also be possible if there were some kind of feedback that adjusts the strength of the magnets based on the position of the spheres. Mnudelman (talk) 04:15, 12 February 2016 (UTC)

If you could suspend one sphere inside another by repulsion alone (which the word "hovering" seems to suggest) I can't see any way of imparting a spin to it as it would be completely enclosed. Richerman (talk) 07:24, 12 February 2016 (UTC)

You could start spinning the outer sphere, leaving the inside sphere stationary. StuRat (talk) 07:47, 12 February 2016 (UTC)

Then they aren't spinning in "opposite or dissimilar directions". Richerman (talk) 10:27, 13 February 2016 (UTC)

From the frame of reference of either sphere they are. StuRat (talk) 17:36, 14 February 2016 (UTC)

There are several different levitation methods that circumvent Earnshaw's Theorem that are listed in the article on magnetic levitation. Also, since the term ferric simply means "iron containing", a sufficiently cooled outer sphere consisting of an iron-based superconductor can levitate or suspend an inner ferromagnetic sphere via the Meissner effect. --Modocc (talk) 08:47, 12 February 2016 (UTC)

Earnshaw's Theorem is really a corollary of the n-body problem, and levitation examples represent special cases with constraints on the objects to establish a metastable equilibrium; i.e. it holds stable for long enough on time frames humans care to stare at it, but it is not perfectly stable under long enough time frames. --Jayron32 15:32, 12 February 2016 (UTC)

Just so; I think this is the only known method by which the OP's scenario might be theoretically feasible. this] is not exactly the scenario envisaged, but it will give an impression of the kind of superconductor/ferromagnetic principles involved, for those unfamiliar with the Meissner effect and it's recent headline-grabbing "quantum levitation" applications. Snow ^{let's rap} 15:03, 12 February 2016 (UTC)

I'm going to ask the obvious question here: How would you know if the inner sphere was rotating? shoy (reactions) 15:05, 12 February 2016 (UTC)

I think the same way you can tell that a gyroscope is spinning, even when it's encased in an opaque sphere (outer sphere stationary and inner sphere rotating with respect to that frame fits my interpretation of OP's words). The crux of this question is the time frame Jayron hints at. Certainly the OP's situation is possible, if only lasting for a few nanosecends. Certainly such a configuration will not be stable for billions of years. Beyond that I think someone has to do some nontrivial physics and math. SemanticMantis (talk) 16:20, 12 February 2016 (UTC)

The thing I don't get here is that Earnshaw's theorem is only about one kind of force at once. But the sphere has weight also! Is there really no way to, I dunno, rig a hollow bar magnet so that the field is strong around the rim and a little weaker inside, and use it to repel one magnetic pole of a sphere that is weighted to keep it from inverting? Wnt (talk) 17:03, 12 February 2016 (UTC)

(Newtonian) gravity is just another inverse-square force (like electrostatics), so it adds nothing beyond what the theorem already covers. --Tardis (talk) 17:42, 12 February 2016 (UTC)

Ok. Forget the magnetic part for now just replace that with high efficiency lube and/or some strategically placed ball bearings. Point is The least friction as possible. What if any physics implications are present w this system? Does this change the weight/ density of the system? any other observations that a lay person might not consider?66.87.81.187 (talk) 19:54, 12 February 2016 (UTC)

Think about the two dimensional equivalent, say two flywheels on the same axle, one larger than the other. They really aren't a very interesting system to investigate. Greglocock (talk) 22:51, 12 February 2016 (UTC)

I recently saw a show that had this guy lift with one hand a 200 lb. or some such extraordinary weight completely above his head for at least 3 seconds. He used a drill gun to get a giant flywheel spinning really fast. That enabled him to lift the very heavy wheel. He was not able to control it largely (I believe) because it was spinning in one direction. If there was another equal mass wheel inside that to counter act the imbalance could he hold it longer than 3 seconds?66.87.81.187 (talk) 02:57, 13 February 2016 (UTC)

As far as I know, the only way a flywheel would help you hold up the weight (assuming it wasn't actually acting as a propeller, of course) is that it helps keep it stable. Probably the guy was just uncommonly strong, and he could hold 200 lb above his head with one hand for three seconds — that's not unbelievable for a serious weight lifter.

If you cancelled the gyroscopic effect as you suggest, then I don't see how you would get any benefit at all from it. It would cancel out the stabilization, and be as hard to hold as if the flywheels were stopped. --Trovatore (talk) 04:13, 13 February 2016 (UTC)

He couldn't lift it with 2 hands when it was not spinning. Let alone with one and well over his head for extended time. I wish I could locate the clip. 66.87.81.187 (talk) 06:50, 13 February 2016 (UTC)

Word of warning: Video clips showing amazing "sciency" things like that are very, very frequently faked - sometimes cleverly, other times not so cleverly. Easily more than half of these things on YouTube are faked. Gyroscopes seem almost magical - but their effect is only on the resistance of the system to rotation - physically moving (technically 'translating') any arrangements of so such contraptions is no easier or harder than if they aren't spinning. If what you think you saw was possible, we'd be using them as propulsion systems in spacecraft rather than just for attitude control. SteveBaker (talk) 17:40, 13 February 2016 (UTC)

'preciate the concern. But this was on a reputable science show on TV. It was rather new episode so it will no doubt be on again. 66.87.80.61 (talk) 22:05, 13 February 2016 (UTC)

THIS appears to be an example of that kind of thing. It's not a 200lb flywheel, it's only 40lb - but I think the effect is as we'd expect. Specifically, note that he's not holding the apparatus stationary above his head - it's on a trajectory that he appears to have little control over. Still, it's not exactly trivial to explain what's going on...sadly, that's rather typical of gyroscopes. The math ain't pretty! SteveBaker (talk) 04:11, 14 February 2016 (UTC)

Any disadvantage of graphite graphene batteries?

Is the whole hype around the graphite graphene battery warranted? Is there any reasonable known and maybe insurmountable disadvantage to it? --Scicurious (talk) 19:30, 12 February 2016 (UTC)

May be that it does not exist? Ruslik_Zero 20:38, 12 February 2016 (UTC)

I think OP may be talking about Dual_carbon_battery, in which case the cited references are a decent place to start. SemanticMantis (talk) 20:47, 12 February 2016 (UTC)

I meant actually graphene, and graphene batteries.--Scicurious (talk) 21:12, 12 February 2016 (UTC)

The cynic in me is saying: you could do anything with graphene, if you could do anything with graphene. (It used to be 'nanotubes'...) Wnt (talk) 22:07, 12 February 2016 (UTC)

Sure, but there is a debunking reaction when a technology is not living up to its expectations. I am not seeing this happening for graphete (yet). --Scicurious (talk) 22:52, 12 February 2016 (UTC)

What's graphete? Anyway in terms of graphene, this source says "Graphene-based solutions have so far been notoriously difficult to manufacture on a large scale, thanks in part to the difficulty of isolating high-quality graphene" [1].

From what I can tell, there isn't really such a thing as a graphene battery (or cell). There are multiples proposals to use graphene in chemical cells, but these are usually just to coat the anode or cathode, perhaps to enable the use of different anodes or cathodes [2] [3] [4] [5] [6] [7] [8] most commonly for some variant of lithium cells although there are also other proposals like sodium and some use them in superconductors.

In the real world, any commercial usage of graphene in batteries is probably several years away at a minimum (e.g. [9]). Most of the sources seem to be in the form of "this battery may be better than what we have currently, if we can produce it commercial at a decent price and resolve any problems and doing all that still have something better than the current state of the art which includes doing it all before someone finds something better". There are a few in the form of "perhaps we can use this to produce such batteries commercially" [10].

It's not so easy to "debunk" something which is actually a lot of different proposals and where all most of them are saying is "we may be able to do this one day if we overcome all the obstacles". You may be able to find some informed discussion about the chances of success but really it can be quite difficult to know at such an early stage. Actually a lot of these sort of things are never really debunked as debunking doesn't make much sense if the published studies are real and accurate but what they are reporting is something still quite far from a commercial product. If you're lucky, and it seems there is sufficient hype and talk about graphene that this may happen there will be a fair amount of future analysis of why something never really achieved success (which is different from debunking). But there are a lot of technologies where all that well happens is there's a bit of hype and talk but they are never really able to overcome the obstacles to commercial production or usage and you never really hear about them again.

There is this review [11] although it's a bit old now.

Of course it's difficult to know for sure what's going on in the various commercial/private labs and there is one company who are talking of a graphene battery or supercapacitor of fuelcell or something [12] [13] [14] [15] but they don't seem to have a commercial product and frankly such hype gets boring. (And it's also fairly difficult to debunk when they have provided so little info and no products to test except for possible flaws or lies in their demonstrations.)

Hobbyking claim to be selling Turningy graphene batteries, I don't think anyone really believes there's any significant use of graphene (which I guess you could call debunking) [16] [17]. While Hobbyking and Turningy aren't quite as bad as many other Chinese sellers and tends to have batteries with capacities that normally aren't that far from advertised, they still can do bullshit marketing.

[18] actually says pretty much the same thing as Wnt, graphene seems more similar to carbon nanotubes (i.e. much hype but not actually used for much) than it does to silicon (i.e. one of the most important materials of the modern age). It seems someone in China was also claiming to produce a smartphone with several major components including the battery using graphane [19] last year but whatever happened to this it didn't seem to receive much attention.

Nil Einne (talk) 07:48, 13 February 2016 (UTC)

What is graphane? Anyway, thanks for your answer.Scicurious (talk) 13:20, 13 February 2016 (UTC)

See our articles: Graphane and Graphene - it's not just a typo. Graphene is a mono-atomic sheet of pure carbon - Graphane has hydrogen atoms in the two-dimensional lattice. (Oh - and there is also a theoretical material called Graphyne...similar deal). SteveBaker (talk) 17:21, 13 February 2016 (UTC)

OK, graphane exists and it could also store energy in the form of hydrogen. But this is only a potential use and not a reality (yet). Scicurious (talk) 21:34, 13 February 2016 (UTC)

Although in this case it was just a typo, sorry for any confusion. Nil Einne (talk) 14:19, 14 February 2016 (UTC)

Elemental composition and electrons in a white dwarf star, etc

What is the elemental composition of a white dwarf? I figure there is at least some hydrogen left. How fast are electrons traveling in a white dwarf star? How greatly does their mass increase due to relativistic effects? Does time dilation cause any interesting things? (I have heard that electrons are believed absolutely stable, so time dilation wouldn't affect decay rates, but I wonder if minor or subtle things can happen that are interesting.) In a white dwarf, could electrons be under so much pressure that they occupy shells Inside a nucleus, instead of occupying electron shells that "orbit" the nucleus? Are interesting chemical compounds or crystals formed due to the increased electron mass?155.97.8.168 (talk) 23:50, 12 February 2016 (UTC)

did you read White dwarf, whech tells a bit aboug the elemental composition. The material is electron-degenerate matter. This is a gas or plasma, and you would not expect molecules or crystal structures to be present. The surface, which we see is not under such pressure, but atomic gases are what are observed. Graeme Bartlett (talk) 01:47, 13 February 2016 (UTC)

Yes I did read it, it tells a bit. I was hoping to learn more.155.97.8.169 (talk) 03:15, 13 February 2016 (UTC)

A white dwarf is the leftover core of a star that was more massive than a red dwarf but not massive enough to go supernova. So, the composition is that of the star's core, which is the "ash" produced by the fusion reactions during the star's lifetime. See Stellar evolution#White and black dwarfs for details. You were on to something with the speculation about electrons being "forced" into the nucleus. This does happen in even more massive stars. But when it does, the electrons and nucleons react, in electron capture. This is a "bad thing" for the star, because the electrons are helping to support the star against collapse. Stars that wind up as white dwarfs aren't massive enough for their gravity to cause electron capture, and so you wind up with a big ball of plasma supported by electron degeneracy pressure. More massive stars overcome this, and you get either a neutron star or black hole. You might find Crash Course Astronomy informative. --71.119.131.184 (talk) 11:55, 14 February 2016 (UTC)

Energy from black holes merging

News stories say that a gravity wave detector, LIGO, picked up Gravitational waves when 2 black holes merged 1.3 billion light years away. The merged black hole was reported to have less mass than the sum of the 2 original black holes, to the extent of "three Earth solar masses" which was released as energy in the form of gravity waves. If the Sun's mass is about 2 x 10 ³⁰ kilograms, then per E=mc squared this would represent a release of about 1.8 x 10 ⁴⁷ joules or 4.3 x 10 ³¹ megatons of TNT per a site which does the calculation for lazy types like me. This was said to cause a barely detectable vibration in ultrasensitive detectors here, but what would it have looked like/felt like for an observer to the disturbance who was much closer? That is a hell of a lot of energy, but electromagnetic waves like light or heat would seem to have trouble escaping the combined gravitational field. ( I understand that there is electromagnet radiation from stuff falling into a black hole without there being a merger, but I'm more interested in the gravity wave's effects)The observed waves caused a change of about 1 part in 10²⁰ at a distance of 1.3 billion light years, so by the inverse square law it would seem that at 13 light years distance, the effect should be about 1 part in 10 (please check the math). What would that feel like? If ones head and feet were changing their distance by one tenth many times per second, would the observer be smushed into jelly? Would the wave be doing work and transferring energy to everything around it? Or, since a measuring stick checking the distance from head to feet would also be changing its dimensions, would there be no work done and no energy transferred? Edison (talk) 23:57, 12 February 2016 (UTC)

Note : edited above to say 3 solar masses rather than 3 earth masses. Edison (talk) 00:07, 13 February 2016 (UTC)

I don't know if they would be turned into jelly, but it seems to me that the molecules aren't moving because of it, it is the space itself that moves (gets rippled). Tgeorgescu (talk) 00:36, 13 February 2016 (UTC)

I'm also not sure that the gravity wave itself would have done that. It's warping all of space - so everything that's not massless shouldn't really notice. My feeling it that you'd see an abrupt red-shift and then blue-shift of light - then nothing special...and even that would be so brief, I'm not sure we'd notice it. I could be way off on this though - it's a hard thing to think about. SteveBaker (talk) 02:52, 13 February 2016 (UTC)

I believe you are way off on this. Changes in the curvature of spacetime are felt as tidal forces, so my first instinct is to suggest that a sufficiently close observer would be ripped to shreds, like a high frequency version of spaghettification. Though I'm no physics expert, so I'm not sure if Edison's estimate holds for this phenomenon, though it's the same calculation that occurred to me when I read the paper. Someguy1221 (talk) 05:25, 13 February 2016 (UTC)

The Sticky bead argument confirms that energy is transferred during the process - and energy generally results in damage - so I believe you are correct. (In my defense, I did say I wasn't sure!) SteveBaker (talk) 17:17, 13 February 2016 (UTC)

Here is the press release from Caltech, and here is the paper in Physical Review Letters: Observation of Gravitational Waves from a Binary Black Hole Merger. These are the authoritative primary sources of information on the event. I'm not sure the answer to User:Edison's question is actually known to the scientists who published the finding. They did not discuss the effects of the event on objects close to the source. Probably the best answers will be found in The Astrophysical implications... , which is cited as the best discussion of the astrophysical implications of the source of the event, consistent with the best numerical models available. Nimur (talk) 03:16, 13 February 2016 (UTC)

The merged black hole does not lose mass compared to the rest mass of the two merging black holes. The energy that is radiated comes from gravitational potential energy which just before they merge will have been turned mostly into kinetic energy with them whizzing round each other at a very high speed. Energy and mass are equivalent by Einstein's famous equation. Dmcq (talk) 10:43, 13 February 2016 (UTC)

The power flux coming from the merger (as measured in joule per second per square metre) falls off according to the inverse square law, as it has to for conservation of energy. This quantity however is proportional to the square of the amplitude of the wave. Amplitude is what is measured (in dimensionless units) and reported as 10^-21 and falls off inversely proportional to distance. At 1 million kilometres from the source, amplitude is still only 10^-5. That would feel like sitting on a large speaker box. At 1000 km, the amplitude is about 1%, which may be enough to break your bones. Static tidal acceleration (which goes as da/dr=2GM/r³) at this distance would be about 16,000 s^-2, or 1600 g per metre, which will rip you apart. An ant might survive. Anyway, X-rays from gas falling into the black hole(s) would have killed you before reaching that point.

Molecules sitting stationary in space, without any acceleration, would vibrate relative to each other when a gravitational wave passes. They wouldn't feel any acceleration directly, but if the molecules are part of a measuring rod, they will feel the resulting elastic forces in the measuring rod. PiusImpavidus (talk) 13:20, 13 February 2016 (UTC)

Apparently, between the 1920s and the 1950s there was disagreement among physicists about whether gravitational waves actually transfer energy to physical objects. The sticky bead argument by Feynman finally convinced almost everyone that this does actually occur. Mnudelman (talk) 15:35, 13 February 2016 (UTC)

A working astrophysicist has published the answer I was seeking in a Forbes article. He says it is based on a back of the envelope calculation. Unfortunately, at the time I asked the question, all envelopes within my reach had their reverses totally covered in dense calculations. (I am not kidding). See "Could Gravitational Waves Ever Be Strong Enough To Feel?" by Brian Koberlein, astrophysicist and Senior Lecturer of Physics and Astronomy at the Rochester Institute of Technology. He says that if the observer were 1.3 light years from the event,"The entire Earth would shift in diameter by about a hundredth of a millimeter" with modest effects. He says that at 10,000 kilometers from the event, an observer would experience a variation one part in 1000. He does not mention the deadly static tidal acceleration, spaghettification, or radiation described by some above. The power being the square of the amplitude, so it varies linearly with distance rather than inverse square of distance makes sense, just as electrical power varies as the square of voltage. Edison (talk) 02:57, 14 February 2016 (UTC)

It starts to sound like the other dangers of being a few thousand kilometers away from a pair of spinning/converging black holes means that your last concern is going to be the gravity waves! SteveBaker (talk) 03:37, 14 February 2016 (UTC)

Before anyone says "AAAH! If there is a black hole ,"THE TIDAL FORCES WOULD TOTALLY SPAGHETTIFY YOU!" they should specify the distance from the black hole, and calculate the relative gravitational acceleration of the observer's head and feet. If the observer is distant enough that the head and feet accelerate about the same then there would be little "spaghettification." Edison (talk) 04:27, 14 February 2016 (UTC)

It's not just the tidal forces - consider also the radiation. SteveBaker (talk) 16:07, 14 February 2016 (UTC)

PiusImpavidus did calculate it above and got 1600 gees/meter of static tidal force at 1000 km, which is around the distance where the gravitational wave might start to damage a human body, so it's clear that the gravitational wave is not a big concern for this merger. But since the gravitational wave amplitude falls off as 1/r, the ionizing radiation as 1/r², and the tidal force as 1/r³, it seems that for a sufficiently large (maybe impossibly large) black hole merger, there might be a distance range where the gravitational wave would kill you and the others wouldn't.

PiusImpavidus and Koberlein didn't mention that the effect of the gravitational wave is not simply to stretch/squash you to some percentage of your original length. The value h ~ 10⁻²¹ that LIGO pretends to measure is actually unphysical, as pointed out in a thread below this one. It's more accurate to say that you're stretched by h''(t), which has units of s⁻² = (m/s²)/m like the static tidal force. If your natural frequency is much lower than the gravitational wave frequency, which is certainly true of LIGO, then it makes little difference and you might as well say you feel h. But the speed of sound in water is ~1500 m/s (and even higher in bone), while the maximum frequency of this chirp was only 150 Hz, so I think the effect on a human body would be smaller than you'd guess by just looking at h_max. -- BenRG (talk) 07:50, 14 February 2016 (UTC)

Another published popular science piece by an astrophysicist on the physical effects:at Gizmodo, where Dr. Amber Stuver of the LIGO Livingston Observatory in Louisiana says "...assume that we are 2 m (~6.5 ft) tall and floating outside the black holes at a distance equal to the Earth’s distance to the Sun. I estimate that you would feel alternately squished and stretched by about 165 nm (your height changes by more than this through the course of the day due to your vertebrae compressing while you are upright). This is more than survivable." Edison (talk) 13:35, 14 February 2016 (UTC)

If someone is so inclined, it might be interesting to consider the gravitational wave impact of merging supermassive black holes, i.e. 10⁵ times larger masses than the black hole merger currently observed. If you want to imagine feeling a gravitational wave, that is probably the best case for it, though I don't know if there is a distance at which a person would be close enough to feel the passing wave but far enough away to survive all the other impacts of being near a supermassive black hole. Dragons flight (talk) 21:13, 14 February 2016 (UTC)