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As you might well know, all matter in the universe consists of small particles called atoms and each atom contains electrons that circle around a nucleus. This is how the world is made.
If one places an atom (or a large piece of a matter containing billions and billions of atoms) in a magnetic field, electrons doing their circles inside do not like this very much. They alter their motion in such a way as to oppose this external influence.
Incidentally, this is the most general principle of Nature: whenever one tries to change something settled and quiet, the reaction is always negative (you can easily check out that this principle also applies to the interaction between you and your parents). So, according to this principle, the disturbed electrons create their own magnetic field and as a result the atoms behave as little magnetic needles pointing in the direction opposite to the applied field*.
As you probably saw many times when playing with magnets, magnets push each other away if you try to bring together their like poles, for example, two north or two south poles. Similarly, the north pole of the external field will try to push away the “north poles” of magnetized atoms.
Our magnet creates a very large magnetic field (about 100 to 1000 times larger than school or household magnets).
In this field, all the atoms inside the frog act as very small magnets creating a field of about 2 Gauss (although very small, such a field can still be detected by a compass). One may say that the frog is now built up of these tiny magnets all of which are repelled by the large magnet. The force, which is directed upwards, appears to be strong enough to compensate the force of gravity (directed downwards) that also acts on every single atom of the frog. So, the frog’s atoms do not feel any force at all and the frog floats as if it were in a spacecraft.
Diamagnetic levitation was first demonstrated as long ago as in 1939 when small beads of graphite and bismuth were levitated in an electromagnet (for historic details, read Physics Today). It took scientists another 50 years to rediscover levitation when physicists from Grenoble lifted several organic materials by the diamagnetic force. They were not aware of the earlier experiment. Although Grenoble's research was published in Nature, a few scientists noticed it.
The water and the frog are but two examples of magnetic levitation. We have observed plenty of other materials floating in magnetic field - from simple metals (Bi and Sb), liquids (propanol, acetone and liquid nitrogen) and various polymers to everyday things such as various plants and living creatures (frogs, fish and a mouse). We hope that our photographs will help many – particularly, non-physicists – to appreciate the importance of magnetism in the world around us. For instance, it is not always necessary to organize a space mission to study the effects of microgravity– some experiments, e.g. plants or crystal growth, can be performed inside a magnet instead. Importantly, the ability to levitate does not depend on the amount of material involved, V, and high-field magnets can be made to accommodate large objects, animals or even man. In the case of living organisms, no adverse effects of strong static magnetic fields are known – after all, our frog levitated in fields comparable to those used in commercial in-vivo imaging systems (currently up to 10T). The small frog looked comfortable inside the magnet and, afterwards, happily joined its fellow frogs in a biology department.
There is one important aspect in which the diamagnetic levitation differs from any other known way of levitating or floating things. In the case of diamagnetic levitation, the gravitational force is compensated on the level of individual atoms and molecules. This is, in fact, as close as we can - probably ever - approach the science-fiction antigravity machine
In September 2009, NASA's Jet Propulsion Laboratory in Pasadena, California announced they had successfully levitated mice using a superconducting magnet, an important step forward since mice are closer biologically to humans than frogs. They hope to perform experiments regarding the effects of microgravity on bone and muscle mass.