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| Correction: flux pinning; Wallpapers?, Comets | Oct 19, 2011 10:34 AM PDT | url |
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Added 1 new A* page:^ This *would* have been a relatively disaster-free page, except that I realized just before I uploaded it that apparently I don't know how mirrors work:
Riiiiiiight. >_<
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If you tried signing up for the A* forum since about mid-May, you never got the confirmation email that the forum was supposed to send you; apparently phpBB's php mail function is notoriously finicky, and I suspect some email reconfiguration by my ISP earlier in the year displeased it. Fortunately I found a way to get it working by using gmail to send the forum mails, so you *should* actually be able to sign up and get going on the forum now, and I re-sent confirmation links to all those new accounts that had been stuck in limbo all this time. Sorry about that!
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The video I posted yesterday, despite being called "Quantum Levitation," is actually showing--according to an excellent reply I received on Google+ from physics student +Rick Leijssen--a phenomenon known as "flux pinning." Rick did a great job of explaining both quantum levitation (aka "the Meissner effect") and flux pinning (much better than Wikipedia's flux pinning article did), so I'm just going to step aside and let his words do the talking:
The effect demonstrated in this video is absolutely NOT the meissner effect (though this is a commonly made mistake). The meissner effect is the effect that a superconductor that is entirely in a superconducting state does not allow a magnetic field to penetrate at all, which would result in repulsion of any magnet brought close tot the superconductor. This does not explain the effects seen in the video, especially not suspending the superconductor below the magnets at the end.
Instead what's shown here is called flux pinning. Basically, superconductors like the one used in this video (probably YBCO, YttriumBariumCopperOxide) have an extra 'mixed' state (or 'vortex' state), where some of the material is not superconducting, but most of it is. The small, wire-like areas where the material is not superconducting is where the magnetic field is allowed to penetrate the material. In principle, these areas are free to move through the material, however, due to intrinsic contaminations or faults in the material, these "flux lines" (or "vortices") become trapped; once they are 'attached' to such a contamination, it's harder to move it around (think of it like this: a contaminated area disturbs the superconductivity anyway, so having the non-superconducting area at that spot has a lower energy than otherwise). Actually, since moving flux lines induce a resistance to current, contamination is usually added to these materials on purpose, so superconducting electromagnets (like the ones in MRI scanners) stay superconducting at higher magnetic fields.
What's happening in the video then, is that the superconductor is moved toward some (strong) magnet, and some of the magnetic field enters the superconductor in these flux lines, which are trapped in the superconductor. There is some resistance to changing the magnetic field, which is enough to suspend the superconductor, but not enough to completely resist moving the superconductor around with your hand, when applying enough force. The second and third demonstration, where the superconductor moves by turning above a circular magnet or by traversing a circular track, is possible because the magnetic field in the superconductor does not change when moving in that way (the field is symmetric). So "locking" the superconductor in place is really trapping a bunch of flux lines, induced by a certain magnetic field, in place inside of the superconductor.
Finally, some remarks about what we do and don't understand about superconductors. The simple kind of superconductor is called a "Type I" superconductor. These are metals that are superconducting at really low temperatures, usually lower than 20 K, and include tin, mercury, copper, lead, etc. These were discoverd in 1911 and are well-understood with BCS-theory, published in 1957. Other superconductors are "Type II" superconductors, high-temperature superconductors, or "unconventional" superconductors (these categories do not fully overlap, see http://en.wikipedia.org/wiki/Superconductor_classification). In 1987 a new class of superconductors was discovered that worked at much higher temperatures than before (up to ~130 K), ao YBCO. These types of superconductors are still not understood; there is really complicated interaction between magnetic spins in layers in this material that already has interesting properties before it is superconducting, and we do not understand why superconductivity occurs here. However, we do have a pretty good grasp of what happens with flux pinning since this can be observed directly, so there are pretty good descriptive models of this. In 2008 another interesting class of superconductors was found, based on iron compounds (called ferropnictides). It is hoped that these can shed some more light on exactly what kind of interactions lead to superconductivity in these and other unconventional superconductors.
EDIT: a nice source at a somewhat introductory level can be found here: http://www.phys.uu.nl/~3154181/ELE2/pe6102.pdf , though this does not really go into flux lines or levitation. |
Thank you very much for the corrections and the great explanation, Rick!
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Also on my/A*'s Google+ today, a reader asked me if I'd given much thought to selling desktop wallpapers of A* art. I had to admit that no, I hadn't (although I did do this 1080p kinda desktoppy thing once). Now that I think about it, though, my recent work is certainly high enough resolution that I could output it as a nice-looking desktop image even at the ridiculously huge desktop sizes some people are sporting these days (who's even ever heard of "QFHD" resolution? :o -- well not me, obviously =p).
So I'm thinking that as I need to get "buy the original art" purchasing links going next to the new painted work anyway, and revamp the "buy hand-signed print" links to make them more user-friendly, I may as well throw in a "as wallpaper" buying link as well--once you've clicked it on an image you want done up as a wallpaper, and entered your desktop resolution and whether the art should be cropped or given side bars to fit your aspect ratio, I'd just go ahead and whip up the wallpaper according to your specifications (hum and I suppose I should add a small A* logo to it somewhere too) and email it to you. I haven't managed to find a good example of a webcomic selling their stuff as wallpapers like that, but I suspect I could only charge a couple bucks a pop. Still it might be something people would like. So if you'd be interested or not interested, or know more about the desktop wallpaper biz than I do, lemme know what you think of this idea.
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At the beginning of September I wrote about findings that the comet Elenin, which was due to pass sorta in the vicinity of Earth on its way back into the outer reaches of the solar system from the Sun, appeared to have disintegrated. And apparently it did; its tiny remains made their closest approach--22 million miles--to Earth this past Sunday. The particles will come back around for another visit in about 12,000 years.
The article notes that about 2% of discovered comets disintegrate as they near the Sun, so Elenin's fizzle wasn't particularly surprising.
And just to show how much stuff is out there whizzing around, the very day after Elenin's fly-by--Monday--a small asteroid, 2009 TM8, passed far closer to the Earth: 212,000 miles out (341,000 km), or just inside the orbit of the Moon. That's still really far away, though, and given that there's no easily found mass estimate for 2009 TM8, it was probably pretty tiny.
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At the other end of the space threat spectrum, scientists studying observations made by Mexican astronomer Jose Bonilla in August 1883 are saying that the 450 objects he reported crossing the face of the Sun were the remains of a mammoth comet that barely missed wiping out life on Earth; their calculations from his notes--and I think we have to take these with a huge grain of salt, of course--are that the fragments ranged from 164 feet to 2.5 miles across (those larger fragments being individually as large as comet Elenin was before it broke up), and that the comet they came from "must originally have tipped the scale at a billion tons or more"--in other words, it would have been on a scale similar to the asteroid thought to have wiped out the dinosaurs 65 million years ago.
Who's got their comet insurance paid up? =D
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