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What is the strongest magnetic field possible? Is there a limit?
Category: Physics
Published: August 21, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
There is no firmly-established fundamental limit on magnetic field strength, although exotic things start to happen at very high magnetic field strengths.
A magnetic field exerts a sideways force on a moving electric charge, causing it to turn sideways. As long as the magnetic field is on, this turning continues, causing the electric charge to travel in spirals. Once traveling in spirals, an electric charge acts like a small, oriented, permanent magnet and is therefore repelled from regions of high magnetic field gradient. Therefore, electric charges tend to spiral around magnetic field lines and be pushed away from regions where magnetic field lines bunch up. These two effects cause electric charges to get trapped along magnetic field lines that are strong enough. Examples of this effect include ions trapped in earth's ionosphere, radiation trapped in earth's radiation belts, hot plasma looping over the sun's surface in solar prominences, and plasmas contained in the laboratory using magnetic traps.
The stronger the magnetic field gets, the more violently an electric charge is pushed sideways by the magnetic field, the faster and tighter it therefore spirals around in circles, and the stronger it gets pushed away from regions of high magnetic field gradient. Interestingly, all normal objects are made out of atoms, and all atoms are made out of electric charges: electrons and protons. Therefore, strong enough magnetic fields have the ability to deform and even break objects. When a magnetic field gets stronger than about 500,000 Gauss, objects get ripped to pieces by the intense forces. For this reason, scientists cannot build a machine that creates a magnetic field stronger than 500,000 Gauss and survives longer than a fraction of a second. Strong enough magnetic fields therefore destroy objects as we know them. Note that the magnetic fields used in medical MRI scanners are much weaker than 500,000 Gauss and are perfectly safe when used properly.
While the destructive nature of strong magnetic fields places a practical limit on how strong of a field earthlings can create, it does not place a fundamental limit. Magnetic fields that surpass about a billion Gauss are so strong that they compress atoms to tiny needles, destroying the ordinary chemical bonds that bind atoms into molecules, and making chemistry as we know it impossible. Each atom is compressed into a needle shape because the electrons that fill most of the atom are forced by the magnetic field to spin in tiny circles. While such extremely strong magnetic fields are not possible on earth, they do exist in highly-magnetized stars called magnetars. A magnetar is a type of neutron star left over from a supernova. The intense magnetic field of a magnetar is created by superconducting currents of protons inside the neutron star, which were established by the manner in which the matter collapsed to form a neutron star.
In a review paper presented at the Fifth Huntsville Gamma-Ray Burst Symposium, Robert C. Duncan summarized many of the theoretically-predicted exotic effects of magnetic fields that are even stronger:
"In particular, I describe how ultra-strong fields
- render the vacuum birefringent and capable of distorting and magnifying images ("magnetic lensing");
- change the self-energy of electrons: as B increases they are first slightly lighter than me, then slightly heavier;
- cause photons to rapidly split and merge with each other;
- distort atoms into long, thin cylinders and molecules into strong, polymer-like chains;
- enhance the pair density in thermal pair-photon gases;
- strongly suppress photon-electron scattering, and
- drive the vacuum itself unstable, at extremely large B."
At the most extreme end, a magnetic field that is strong enough could form a black hole. General Relativity tells us that both energy and mass bend spacetime. Therefore, if you get enough energy in one region, then you bend spacetime enough to form a black hole. The black hole does not destroy the magnetic field, it just confines it. Even stronger magnetic fields create larger black holes. It is currently not known whether this is actually possible, as there may be unknown mechanisms that limit a magnetic field from ever getting this strong.
Certain unconfirmed extensions of current theories state that there is a fundamental limit to the strength of a magnetic field. For instance, if a magnetic field gets too strong, it may create magnetic monopoles out of the vacuum, which would weaken the magnetic field and prevent it from getting any stronger. However, since there is currently no evidence that magnetic monopoles actually exist, this purported limit is likely not real. We may someday discover a fundamental limit to the magnetic field strength, but there is currently no experimental evidence or well-established theoretical prediction that a limit exists.