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James Cameron's Avatar featured floating mountains which stayed aloft due to the Meissner effect; they contained large quantities of the room-temperature superconductor unobtainium, which, due to their strong electromagnetic fields, were suspended above the ground.

Would this really work though? I've seen the concept criticized - see here - and I was wondering if you could help me overcome the flaws in the movie's floating islands; namely the fact that

• A superconductor can't just give off a magnetic field without a current


• The magnetic field would have to be so big, dense and strong that it could be uninhabitable for humans


• I'd rather not just magic an RC superconductor onto the periodic table unless it theoretically could exist on my planet but not Earth

I would also like to know how such a huge magnetic field could be produced naturally. Some requirements:

• The floating boulders I want have to be able to exist as large as houses


• The planet must be habitable to non-extremophilic animal life


• It is a planet, not a moon of a gas giant - so no help from Jupiter-like planets with metallic hydrogen outer cores

1. Only those pieces of rock which contain a current can produce a magnetic field adequate to float in the planetary magnetic field.




2. Rocks will float at different distances from the surface depending on the strength of their internal current. Some might not float, but just be lighter than would be the case without the current. Some will be very high.




3. Some rocks obtain their initial current through lightning.




4. Rocks can maintain their current through induction. A conductor moving (falling or rising) thru a magnetic field will have a current induced within it. This induced current will produce a magnetic field opposing the one that produced it.




5. I conclude from videos of small animals being levitated that a magnetic field adequate to oppose gravity is not itself lethal.

ADDENDUM: Maybe Pandora floating rocks are very light?

I recently had opportunity to see the simulated world of Pandora in Orlando. They have some cool "floating" rocks.

I conclude that the "rocks" are a lot lighter than they look. Pumice is a tenth the mass of the same volume of limestone. Mayne the levitating rocks are light too.

A do not think there is an upper limit to magnetic fields. Magnetars have a lot. Questions about how much magnetic field can a given volume of mass host, or how much can a given magnetic field (or interacting magnetic fields) lift can be answered. One would need to understand magnetic flux density and I do not.

Avatar was a fun movie to watch, but a lot of it was fantasy

Can you float a superconductor without an external current? Sure you can. What you need is a strong enough magnetic field to induce the necessary current in the superconductor. for example, in that Youtube video, a neodymium magnet floats atop a super cold superconductor.

The problem you have is that planetary magnetospheres are insanely weak. The magnets holding your child's drawings on your fridge are 200X stronger than Earth's magnetosphere and you can't float a superconductor with a fridge magnet.

Does this stop you?

Not necessarily. You simply need to justify a strong enough magnetosphere that it would magnetize the fillings in a visitor's teeth.

Or do what Cameron did: ignore the details and tell a good story.

But, according to your question you don't want a magnetosphere of that strength. That leaves one answer: sorry, it can't be done based on the science we know today.

BTW, for fascinating insight into the idea of lethal magnetism, check out this post from our sister site, Physics.SE.

And then my memory kicked in

After answering this question, I realized it's a duplicate of Floating cities with a new superconductor v2. But since I've already answered this question, I'm not going to VTC.

The magnetic force is much stronger than the gravitational force. Even a small magnet will easily hold a metal object a few times its mass.

Here is a video of common household objects being levitated:

https://www.youtube.com/watch?v=1gMMM62NC-4

On top of that the gravity on Pandora might be lower than that on Earth, and the rock less dense.

You can imagine layers of magnetic unobtanium separated by layers of ordinary rock. When the rock erodes, the like charged layers on unobtanium repel each other, thus resulting in floating rocks.