Perpetual Motion Explaining quantum magnetic levitation from a scientific perspective

The idea of levitation above the ground has been a subject of science fiction and human imagination since ancient times. While we do not yet have hoverboards, we do have the very real phenomenon of quantum levitation. Under the right circumstances, a special material is cooled to a low temperature and placed on a properly configured magnet, where it will levitate indefinitely. If you make a magnetic track, it will levitate above or below the track and remain in motion forever.

 

But isn't perpetual motion supposed to be physically impossible? True, you can't violate the law of conservation of energy, but you can make the drag in any physical system as small as possible. In the case of superconductivity, a special set of quantum effects can indeed bring the resistance down to zero, enabling all sorts of strange phenomena, including what you see below: quantum levitation. Today we will describe the physics of it.

 

That the special material used for levitation is very cold.

that it can levitate above or below a magnet: that it is held in a certain position.

if you put it on a magnetic track, it doesn't lose any speed over time.

 

It's really counterintuitive stuff, not the way traditional classical physics works. The permanent magnets you're used to - ferromagnets - can never levitate like this. Let's see how they work and then see what's different about this quantum levitation phenomenon.

 

Magnetic field lines, as shown by iron filings arranged around a bar magnet: magnetic dipoles. These permanent or ferromagnetic magnets remain magnetized even after the disappearance of any external magnetic field.

(Image credit: Newton Henry Black & Harvey N. Davis, Practical Physics, 1913)

 

Every material we know is made up of atoms, which may or may not combine themselves into molecules. The atoms and molecules become part of the internal structure of the material. When you apply an external magnetic field to that material, these atoms or molecules are also magnetized internally and in the same direction as the external magnetic field.

 

The special property of a ferromagnet is that when you remove the external magnetic field, the strength of the internal magnetization remains the same. This is what makes it a permanent magnet. But almost all materials are not ferromagnetic. Most materials return to a non-magnetized state once the external magnetic field is removed.

 

In the absence of a magnetic field, the antimagnetic and paramagnetic materials remain, on average, non-magnetized, while the ferromagnets have net magnetization. In the presence of an external magnetic field, the antimagnetism will be in the opposite direction of the magnetic field, and the paramagnetism and ferromagnetism will be in the same direction as the magnetic field. All materials exhibit some degree of antimagnetism, but those that are paramagnetic or ferromagnetic can easily overwhelm the antimagnetic ones.

(Image credit: V. Iacovacci et al., Magnetic Field-Based Technologies for Lab-on-a-Chip Applications in Lab-on-a-Chip Fabrication and Application, eds. M. Stoytcheva & R. Zlatev, 2016)

 

So, what happens inside these non-ferromagnetic materials when you apply an external magnetic field? In general, these materials are.

 

antimagnetic, they magnetize to become antiparallel to the external magnetic field.

or paramagnetic, they magnetize parallel to the external magnetic field.

 

It turns out that all materials exhibit antimagnetism, but some are either paramagnetic or ferromagnetic. Anti-magnetism is always weak, so if you have a material that is paramagnetic or ferromagnetic, this effect can easily overwhelm the effect of anti-magnetism.

 

So when you turn the external magnetic field on or off - which is physically the same thing as moving a material closer to or away from a permanent magnet - you change the strength of the magnetization inside the material. What happens when you change the magnetic field inside a conducting material? There is a law of physics: Faraday's Law of Electromagnetic Induction.

 

When you move a magnet into (or out of) a coil, it causes a change in the magnetic field around the conductor, which exerts a force on the charged particles and induces them to move, thus producing an electric current. If the magnet is stationary and the coil is in motion, the phenomena are very different, but the current produced is the same. By changing the magnetic field inside a conductor, an electric current can be induced.

(Image credit: OpenStaxCollege, CCA-by-4.0)

 

This law tells you that changing the electric field inside a conducting material will cause it to produce internal currents. These small currents you generate are called eddy currents, and they counteract the internal changes in the magnetic field. At normal temperatures, these currents are very short-lived, as they encounter resistance and decay away.

 

But what if you remove the resistance? What if you turned it all the way up to zero? Believe it or not, you can reduce the resistance of almost any material to zero; all you have to do is reduce it to a low enough temperature until it becomes a superconductor!

 

 

Inside a material subjected to a changing external magnetic field, small currents called eddy currents are generated. Normally, these eddy currents disappear quickly. But if the material is superconducting, there is no resistance, and they not only can, but need to, continue indefinitely.

 

These suspended materials are actually made of special materials - superconducting at very low temperatures, or their resistance drops to zero. In principle, any conducting material can be superconducting at sufficiently low temperatures, but the interesting thing about these special superconductors is that they can be superconducting at 77 K (-196.15 °C): the temperature of liquid nitrogen! These relatively high critical temperatures make it easy to fabricate low-cost superconductors.

 

Each material has a critical temperature (labeled Tc below), and when you cool that material below the critical temperature, it no longer has any resistance to electric current. But what happens when you lower the temperature of a material below the critical temperature so that it is superconducting? It expels all of its magnetic field from the inside! This is the well-known Meissner effect, which turns superconducting materials into perfectly antimagnetic materials.

 

At temperatures above the critical superconductor temperature, the magnetic flux is free to pass through the atoms of the conductor. However, below the critical superconducting temperature, all fluxes are expelled. This is the essence of the Meissner effect.

(Photo credit: Piotr Jaworski, in Classic and Advanced Ceramics, 2010)

 

Materials like aluminum, lead or mercury behave like superconductors when you cool them below a critical temperature that eliminates all internal magnetic fields. However, if you mix multiple types of atoms together to produce various compounds, most superconducting materials will superconduct at much higher and more easily achievable temperatures, and these compounds can have different properties at different locations in the material. This takes us a step further than simply making superconductors.

 

Let's imagine that we have an antimagnetic material with impurities in it, rather than a homogeneous, perfect antimagnetic material.

 

If you cool the material below a critical temperature and change the magnetic fields inside it, these internal magnetic fields will still be discharged, with one exception. Anywhere there is an impurity, the magnetic field is retained. Because it cannot enter the region where it was expelled, those field lines are held within the impure region.

 

Top and side views of a type II superconductor exposed to a strong magnetic field. Note that the side view shows where the impurities are created and where the magnetic flux is fixed, while the top view shows the resulting eddy currents, which do not decay due to superconductivity.

(Image credit: Philip Hofmann, Aarhus University)

 

Impurities are the key to producing this magnetic quantum levitation phenomenon. The magnetic field is expelled from the pure region of superconductivity. But the magnetic field lines penetrate the impurities, changing the internal magnetic field and creating vortices.

 

This is the key: these vortices are moving charges that do not encounter any resistance, because the material is superconducting!

 

Therefore, as long as the material remains superconducting and the temperature is below the critical temperature, the current does not decay, but continues indefinitely.

 

This is an image taken with a scanning SQUID microscope of a very thin (200 nm) layer of yttrium barium copper oxide film subjected to liquid helium temperature (4 K) and a strong magnetic field. The black dots are the eddy currents generated by the vortices around the impurities, while the blue/white areas are where all the magnetic flux is discharged.

(Image credit: F. S. Wells et al., Nature Scientific Reports, 2015)

 

In summary, we have two different things happening in two different regions.

 

In the pure superconducting region, the magnetic field is discharged, giving you a perfect antimagnet.

In the impure regions, the magnetic field lines are concentrated and fixed, passing through them and causing continuous eddy currents.

 

It is the currents generated in these impure regions that hold the superconductors in place, creating the levitation effect! A strong enough external magnetic field can disrupt this effect, but there are two types of superconductors. In type I superconductors, increasing the strength of the magnetic field destroys superconductivity everywhere. But in type II superconductors, superconductivity is only destroyed in impure regions. Because there are still regions where the magnetic field is discharged, type II superconductors may experience this suspension phenomenon.

 

By creating an external and an internal magnetic track pointing in opposite directions, a type II superconductor will levitate, be fixed above or below the track, and move along it. In principle, if room-temperature superconductors are realized, scaled up, resistance-free motion over a large area could be achieved.

(Image credit: Henry Mühlpfordt/TU Dresden/Wikimedia Commons)

 

As long as you have an external magnetic field - which is usually provided by a series of well-placed permanent magnets - your superconductor will continue to levitate. In fact, the magnetic quantum levitation effect can only end when the temperature of the material rises above a critical temperature.

 

This gives us the confidence to pursue the holy grail that if we can create a material that is superconducting at room temperature, then it will remain in that levitated state indefinitely.

 

When cooled to sufficiently low temperatures, certain materials exhibit superconducting properties: their internal resistance drops to zero. When exposed to strong magnetic fields, some superconductors exhibit a levitation effect.

(Image credit: Peter nussbaumer/Wikimedia Commons)

 

If we design and build a magnetic track for it, create this superconductor with high impurity content, leave it at room temperature, and let it start moving, it will stay in motion unconstrained. If we do this in a vacuum chamber, eliminating all air resistance, we will create a perpetual motion machine: a device that can keep moving forever, losing no energy as it continues to move.

 

What does all this mean? This levitation is real and has already been achieved on Earth. We would never have been able to do this without achieving the quantum effect of superconductivity, but with the quantum effect, it is simply a matter of designing the right experimental setup.

 

It also gives us a huge science fiction dream of the future. Imagine a road made up of these correctly configured magnetic tracks. Assume that pods, vehicles and even shoes have the right room temperature superconductors in them. Imagine gliding along at the same speed without consuming a single drop of fuel until deceleration.

 

All of this could become a reality if we can develop room-temperature, atmospheric-pressure, type II superconductors. If you count from absolute zero, we're already a long way toward room-temperature superconductors at atmospheric pressure. Science has the potential to turn the "holy grail" of low-temperature physics into reality in the near future.

 

The original BIG THINK article "How quantum levitation works" is linked below.

https://bigthink.com/starts-with-a-bang/quantum-levitation/