Can someone explain how it works upside down? If the superconductor changes surface currents to expel the magnetic field, how does it still cancel out the gravitational pull?
I was a bit misleading in my last post because I focused on the problem with a ring of superconductor (in fact as we see in the video that the object is not a ring at all but a solid puck). I did this because a ring is usually the example that many students see when the first read up on superconductors (e.g. Feynman's lecture series). So the way that the expulsion of the magnetic field works is that the induced surface currents create their own magnetic field. This secondary field will work to cancel out the original magnetic field inside of the superconductor. However, outside of the superconductor there are no guarantees. So what happens is that the superconductor behaves as a magnet itself. This secondary magnetic field provides the repulsive and attractive force to keep the puck levitating in place.
A similar thing can be seen in electrostatics. If I have a conductive pith ball held on a string that is neutrally charged, I can induce a non-uniform surface charge on the ball by bringing into close proximity another charged object. The electric field from the charged object causes the charges on the pith ball to rearrange non-uniformily so that the pith's surface charge creates a secondary electric field that cancels out the applied field inside of the pith ball. The charge distribution looks like a dipole. That is, positive charges build up on one side of the ball while the negative charges build up on the opposite side. The collection of charges on the pith ball near the charged object are opposite to the net charge of the charged object. So these charges attract each other and the attraction can be strong enough that the pith ball moves towards the charged object, even though the pith ball still has no net charge.
Holy shit, that is crazy.
Doesn't the princple only work because the object is close to absolute zero? But I would think the act of him simply touching the object would add enough body heat to make it not work anymore?
Actually we now have a variety of what are called high temperature superconductors. They can achieve superconducting states using temperatures that are still very cold for us but are actually quite warm compared to the old superconductors that needed to be within a few degrees of absolute zero. Many of them can simply be cooled by liquid nitrogen which is only 77 Kelvin (-200 Centrigrad thereabouts). It still seems very cold but liquid nitrogen is cheap and easy to make. So much so that they now have industrial superconducting electrical transmission lines in use in some places in the US. These lines use liquid nitrogen for their cooling. So a high temperature superconductor, like the one that he is using, can be cooled using simple liquid nitrogen and still remain cold enough to operate despite momentary handling and exposure to room temperature.
According to a few sites I've read, (
here's one), some magnetic forces are allowed through due to the size of the wafer and account for the "quantum locking" seen in the video.
Yes. I think that these "flux tubes" are similar to having a little superconducting rings. The flux through these tubes has to be quantized and maintained. So once we set the puck above the magnet, we lock in the flux in these tubes and the superconductor keeps itself at that position. It takes a decent perturbation to pull the superconductor out of the state (ie: pulling it up with your hand).