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Comment by amluto | Hacker News Reader

The actual picture of (poor) levitation in the paper you linked is pretty compelling. This isn’t a complex, noisy measurement showing something that’s related to superconductivity — this is a magnet and a supposed superconductor repelling each other.

As far as I know, that’s possible with permanent magnets (and it would be weird, but not impossible, if the group instead synthesized a novel ferromagnet and didn’t notice), electrets (seems pretty unlikely here), very extreme amounts of static charge (again, seems unlikely), and actual superconductivity (would be awesome).

Random bits of cooked oxides, ceramics, and such don’t float on a magnet.

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Believe it or not, the levitation effect can be found in non-superconducting materials with a high diamagnetic constant such as pyrolytic carbon. Induced magnetic fields are created by "effective currents," which can occur in zero-resistance systems that are not called superconductors (because they can't conduct across a significant distance, only around a tiny loop) like molecular or atomic orbitals.

https://en.wikipedia.org/wiki/Pyrolytic_carbon

Diamagnetism was my first reaction when I saw lead, and the addition of copper makes me think eddy current reactivity.

...and this is Eddy's sofa, is it?

Their other data doesn't line up with ferromagnetism, though. It's either the real thing or a big fraud. Guess we'll find out soon enough.

ETA: the video referenced is apparently available at https://www.youtube.com/watch?v=EtVjGWpbE7k . Interestingly, posted on Feb 26, 2023.

Yeah, that thing does really want to stay in a constant level on the magnetic field. That would dispel every other explanation on the GP, as it's not simply being repelled or attracted.

I found that video very compelling. If it was eddy current, the float standoff distance would have decayed. It sure looked to me like there was no decay at all, and wow! if true.

What is float standoff distance?

Eddy currents can be induced in non-superconducting materials that make it look like levitation, but the catch is that there has to be relative motion between the magnet and the object 'levitating' to generate the currents in the first place.

I've linked to a relevant example in a Veritasium video here: https://youtu.be/g0amdIcZt5I?t=543

But in this sample video, the standoff distance doesn't appear to be slowly dropping at all, which would rule out eddy currents as a source of the behavior. If you continue watching the linked video to 13:27, he talks about how and why superconductors levitate.

Electromagnetic solenoid designer here...

Copper and a magnet can certainly interact. Drop a magnet through a copper pipe and the eddy currents will induce a field that's opposed to the magnet causing a damping effect. Maybe something like this is going on where movement of the magnetic field is inducing an opposed magnetic field in the copper, and thus interacting.

Anyhow it will be interesting. if It can generate a field of 1.5-2 Tesla you could have more efficient solenoids and probably motors.

I would say that this type of levitation where it sort of half levitates is quite common. I taught YBaCuO superconductor experiments for a few years. That Meisner effect would get full marks in my institution!

>As far as I know, that’s possible with permanent magnets (and it would be weird, but not impossible, if the group instead synthesized a novel ferromagnet and didn’t notice)

As far as I know a stable arrangement of permanent magnets levitating is impossible without a baring surface to keep them aligned. (i.e. free floating levitation is not possible without active control)

Just so everyone is on the same page, static passive diamagnetic levitation is possible with materials like pyrolytic graphite.

https://en.wikipedia.org/wiki/Diamagnetism

https://www.kjmagnetics.com/blog.asp?p=diamagnetic-levitatio...

...and superconductors are usually perfectly diamagnetic.

Every substance is one of

* ferromagnetic - attracted to one pole of a magnet but not the other (in a given orientation), this is what everybody thinks of when they think of "magnets"

* paramagnetic - attracted to both poles, i.e. stuff that sticks to magnets

* diamagnetic - repelled by both poles, except in superconductors, this effect is very weak compared to the forces experienced involving ferro-ferro or fero-paramagnetic materials.

There isn't another category, everything fits in to one of those buckets.

Saying

>Just so everyone is on the same page, static passive diamagnetic levitation is possible with materials like pyrolytic graphite.

is a bit deceptive, as what people know as "magnetic" materials are ferromagnetic.

You literally missed the most common magnetic phase - antiferromagnetism

That's not quite true. There is a Halbach array with a bunch of compensation coils that will nicely center as long as it is moving, no active control or bearing required.

https://www.sciencedirect.com/science/article/abs/pii/S03048...

And many others besides. Halbach arrays are fascinating.

https://en.wikipedia.org/wiki/Earnshaw%27s_theorem

stationary. Hence the 'as long as it is moving' bit above. Because the motion allows for the coils to generate enough of a current to drive the compensation. So you need a support system to bring the assembly up to a certain minimum speed above which it will stably levitate.

Right. Everyone should just read up on Earnshaw's theorem to know what all the boundary conditions are.

https://en.wikipedia.org/wiki/Levitron

I wonder how long you could get one of those to spin in a vacuum.

Halbach arrays with compensating coils have been proposed for some interesting applications, such as low loss flywheels for electrical storage. I don't know if that ever got commercialized but I do recall that some prototypes were made by a US company. I can't find a reference to it though.

>I wonder how long you could get one of those to spin in a vacuum.

Not too much longer apparently...

https://www.youtube.com/watch?v=mn7IedCgva0

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