"The rapid progress true Science now makes, occasions my regretting sometimes that I was born too soon. It is impossible to imagine the height to which may be carried, in a thousand years, the power of man over matter. We may perhaps learn to deprive large masses of their gravity, and give them absolute levity, for the sake of easy transport. Agriculture may diminish its labour and double its produce; all diseases may by sure means be prevented or cured, not excepting even that of old age, and our lives lengthened at pleasure even beyond the antediluvian standard. O that moral science were in as fair a way of improvement, that men would cease to be wolves to one another, and that human beings would at length learn what they now improperly call humanity!"
-- Letter from Benjamin Franklin to Joseph Priestly (8 Feb 1780), quoted in "How Mumbo Jumbo conquered the World" by Francis Wheen
> All you need a chunk of type 2 superconductor, a strong magnet, and some liquid nitrogen.
This isn't entirely accurate. A "chunk of type 2 superconductor" would just show the Meissner Effect, which is different than "flux pinning". The Meissner Effect is how superconductors essentially repel magnetic fields, resulting in levitation; it wouldn't necessarily pin the levitating body in-place such that it could follow some track for example.
Flux pinning occurs when some magnetic fields penetrate the superconductor in discrete "tubes" through the imperfections (along the grains) of the superconductor. In order for flux pinning to happen, you must have an extremely thin superconductor (in the case of the video, it's actually a sapphire crystal wafer with a 1-micron thick coating of superconductor ceramic material).
EDIT: Technically, you could also get flux pinning if you were to supercool the superconductor (i.e. make it a superconductor) while it's in the magnetic field of the magnet.
I played quite a bit with the Meissner and the Flux pinning effect. But I was never able to change a flux pinning "on the fly". You always had to reheat the superconductor to something over the transition temperature and then cool it down again inside a magnetic field (in the desired position). But then, it was quite strongly locked in that position and would snap back to where it was when moved.
So, I'm quite curious on how this works ;) Is it because it's a very thin superconductor in this video?
Thanks. I'm not sure I understand the part about "quantum flux tubes". I take it that they're leaks through flaws in the superconductor (which should exclude the field entirely). Is the superconductor is being gripped or pinned by these lines of force running through it, like a piece of cheese on a toothpick? But that doesn't quite explain how it can move along a field.
I'm trying to figure out some practical applications for frictionless movement and positioning of small, light and cold objects without changing the scale of the system, but each of my ideas implies some heating that would probably negate the phenomenon a bit too fast (ballistics for instance).
I can't resist posting one of youtube's comments on this last video: "so... you have created the worst paperweight ever"
The stuff of which awesome party tricks are made. I have myself put a very small amount of liquid nitrogen in my mouth and blown it out, although I then heard that the guy who taught me that trick (Jearl Walker) cracked a tooth doing that, so I never tried it again. And for the love of god do not swallow or you will literally kill yourself very painfully -- see footnote 3 of the above Wikipedia article.
I think I'll stick with dipping fingers briefly into LN, which is way less stupid. Done that many times. Do not get it on your clothing, though, because then you can't retreat quickly and the frost is going to bite!
"What was really astounding about Michael's case is that the liquid nitrogen instantly expanded from a volume of about 3 or 4 cc's to about 3 or 4 liters and then dissected into five separate body compartments"
I am aware of that effect, and I have seen it in action. I am trying to reconcile what I see in the video with what I have seen of Leidenfrost in the past.
He actually grips the object, and I don't see extra smoke. When I have observed the Leidenfrost effect in the past, I see a noticable increase in outgassing, and the liquid (as it usually is) skitters about freely, so it seems like it might be difficult to grasp an object through the vapor layer.
Actually, are we sure the liquid nitrogen is actually hitting his hand? The point of contact is obscured by vapour. I mean, I'm familiar with the effect, but can also believe that a big stream of liquid nitrogen is gonna hit you and it's gonna hurt.
That does not look like liquid nitrogen. It's solid so that tells me it is carbon dioxide.
Believe it or not you can dip your hand in liquid nitrogen for 10 seconds without it freezing immediately and breaking off. I think carbon dioxide is solid at higher temperatures than nitrogen but I could be wrong on that one.
Completely irrelevant. It's a theory explaining how Type II superconductors work, published five years ago, in a Chinese journal that has an impact factor of 0.512 (Nature scores 30.98, for comparison.)
What's the relevance of "impact factors"? I understand the basic idea behind them, but I don't think publishing research in a low impact factor journal should automatically make said research "completely irrelevant".
It's completely irrelevant because it has little to nothing to do with the original post, much like how you wouldn't link to a paper on algorithmic complexity in a post about Facebook, because "they're both about computers."
Additionally, the author of the paper displays some worryingly crackpot-like symptoms. He claims that it's "Nobel worthy", (!!) but Citebase can find no citations.
I know nearly nothing about condensed matter physics, but skimming the paper reveals that his theory doesn't really make any testable predictions, much less the "grand slam" predictions that would result in a Nobel, like room-temp superconductivity.
Impact factor is tremendously flawed, but as a quick sanity check, when you're not sure if something makes any sense at all, it is of great use.
So, the electrical engineer in me says that this is because any movement of the magnet would induce an electric potential in the superconductor. But because such a potential would create an effectively infinite amount of current requiring an infinite amount of work, the magnet is unable to move.
Or from a "cause-and-effect" viewpoint, movement of the magnet induces a current loop in the superconductor, the creation of which creates an opposing magnetic force.
Either way I believe this is the same principle behind electric motor braking (e.g. when you short-circuit the inputs of a motor). I believe you would also see a similar effect by dropping a magnet down a tube encircled with many (or perhaps one spiral) loop of wire -- the magnet's descent will be slowed by eddy currents in the loops.
Edit: Aha, this is just half the story. The superconductor is prevented from spinning due to flux pinning: http://en.wikipedia.org/wiki/Flux_pinning (or what the researchers call quantum trapping / quantum locking)
Quoting a comment in the video linked to by fybren:
"This levitation is NOT due to the Meissner effect. It is negligible since we use thin films. If it were the Meissner effect the field would get distorted on a length scale of the diameter (~cm) and then two discs hovering above and below each other would affect it other. Which is clearly not the case. The discs are actually trapped in constant field contours rather than levitating."
Agreed. If the "floating" (I know its not actually floating in the traditional sense) object could carry its own light-weight cooling system then you wouldn't need to chill the entire unit to super cold temperatures. You might only need to cool the superconducting layer.
Loads of up to 500 grams on this test rig, on what look to be about n40 magnets, according to the presenter's comment on one of the lab videos. The liquid nitrogen is a problem but that payload seems like something that could already have industrial applications.
As a thought experiment, could the effect demonstrated here be used for a transport system on a much colder planet with a much stronger magnetic field (I do realize that these two properties tend to correlate in opposite directions)? What about in our arctic regions, potentially over a magnetic track to interconnect enclosed "settlements"? I'm guessing it would be less cost-effective than other types of transport if even possible, but many times cooler.