When thinking of levitation, one often pictures a concentrated magician and a lady floating in the air, or a levitating guru. However, this is only an optical illusion or quackery. The physical principle of levitation is the balance of two forces: a force that acts in opposite direction and is equal to the gravitational force, which is the force that pulls an object back to earth. This means that these two forces cancel out and therefore an object levitates.
There are several forms of levitation: acoustical, electrostatic, aerodynamic, diamagnetic and superconducting. In the acoustic levitation, it is the sound waves that push against the gravitational force; in the electromagnetic case it is an electric field that pushes a conducting object against gravity; in aerodynamic levitation it is an air push, while in the diamagnetic and superconducting it is a magnetic force, which depends on the material and the strength of the applied magnetic field.
Diamagnetic levitation: the levitating frog
One might think that magnetic force is only present in ferromagnetic materials but this is not the case. As mentioned in our spintronics contribution, all atoms that form matter have electrons with spins that can be regarded as tiny magnets with north and south pole. When an atom has paired electrons, these tiny magnets cancel each other out so that the material is considered non-magnetic. However, the tiny magnets still produce a very weak magnetic field, which goes undetected. But they actually are magnetic. Even we are magnetic. So, it is possible to levitate magnetically every material and every living creature on earth. We just need a sufficiently strong magnetic field to do so. This is exactly what physicists from the Nijmegen High Field Laboratory did: They put a living frog into levitation in a 16 Tesla magnet. For comparison, a fridge magnet produces a magnetic field of about 0.005 Tesla (this is 3.000 times smaller). By the way, the frog was not harmed by this experiment. After its visit inside the magnet, it jumped away.
This is called diamagnetic levitation. A diamagnet is a material with paired electron spins, but whose atoms produce tiny magnetic fields that repel and can be repelled by a very strong magnetic field. The frog levitates because the force produced by the magnetic field of the tiny magnets – due to the huge external magnetic field – cancels out with the gravitational force. So that in diamagnetic levitation the gravitational force is compensated for each individual atom. This can be regarded as an antigravitation machine.
Levitation by superconductor
The phenomenon of magnetic levitation also occurs in another class of special material systems – the so called superconductors. As the name suggests, superconductors possess certain characteristics which differentiates them from normal conductors in a similar way as Superman is different to a common man. The only difference is that while superman is fiction, superconductors are very much real.
Inside a conductor, the electrons can move freely between two points A and B. This flow of charges in a conductor is what we call current. But, when the electrons flow inside the conductor, they inevitably collide, either with the positive ionic cores or among themselves due to the vibrations of the constituent atoms of the conductor. This collision results in the property of conductors which we call resistance, which is indeed a resistance to the flow of electrons. Thus, when connecting a copper wire between the two leads of a battery, a current flows and simultaneously the copper wire also offers some electrical resistance to the flow of current which is usually made visible in form of heat (the copper wire heats up because of this collisions). Many materials are conductors, but specially metals, for example copper and aluminum. Other materials such as silicon, germanium or gallium-nitride are called semiconductors, because they have a much higher and tunable degree of resistance to the electron flow. So, is there any conductor where the electrical resistance can vanish? The answer is: Yes, in superconductors.
A superconductor is a material with no electrical resistance, this means that it can transport an electrical current without losses nor heat dissipation. Therefore, superconductors would be perfect for electrical devices. However most of the superconductors work only at very low temperatures. There is a critical temperature, below which a superconductor has zero resistance. Above this temperature, the material behaves like a metal.
The story of superconductors started at the beginning of the last century in Leiden, Netherlands, where in 1911, the Dutch physicist Heike Kamerlingh Onnes and his graduate student Gilles Holst started studying the electrical properties of liquid metal mercury at the boiling point of liquid Helium at 4.2 Kelvin (-269 degrees Celsius). To their surprise, the electrical resistance of mercury at this temperature simply vanished. In words of Onnes, this zero resistance state of mercury was something never observed before in history and he named this unique phenomenon as "superconductivity". Soon after this discovery, a full palate of metals and alloys were found to exhibit the zero resistance state below the critical temperature. In 1933, two German physicists Walther Meissner and Robert Ochsenfeld discovered that when a superconductor is cooled down below its critical temperature, it expels the outside magnetic field. This means that no magnetic field can go inside the superconductor. This is the reason why a permanent magnet creating a magnetic field floats on top of a cooled superconductor as shown in the video.
The expulsion of magnetic flux from the superconductor occurs due to the small surface currents that are set up in the superconductor when a magnet is brought close to it. Such surface currents, also called persistent currents makes superconductors the ideal systems for many applications with low or no power. This is why superconductors are often regarded as perfect diamagnets. The effect called Meissner-Ochsenfeld effect, is of course, much more complicated than this and is, in a way, the working principle of the levitating trains in Japan. Apart from explaining the levitation, the Meissner-Ochsenfeld effect is also an answer to the existence of the so called God particle or the Higgs boson. But, we leave the explanation for this open for another blogpost. (Andrea Navarro-Quezada, Rajdeep Adhikari, 13.8.2019)
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