In the early 1900s, physicists Niels Bohr and Hendrika Johanna van Leeuwen separately discovered something shocking. According to every known rule of physics, magnetism was impossible. This finding made no sense.

Humans had relied on magnetic compasses for a thousand years, and anyone who had ever held two magnetized metals could tell you that some invisible force was acting on them. Bohr and van Leeuwen knew they were missing something – but what?

Invisible magnetic fields

By the 1920s, physicists had learned quite a bit about the fundamentals of matter. They knew that every material is made of atoms, each comprised of a nucleus containing protons and orbited by electrons. They also knew that both of these particles have a few fundamental properties, including a small amount of mass, an electrical charge, and some amount of magnetism.

The origin of these intrinsic properties remains an ongoing mystery. But the result is that every electron and proton is a tiny magnet that works just like their full-size counterparts. Both have invisible magnetic fields that radiate out in closed loops from one end to the other. These magnetic poles are attracted to the dense field lines of the opposite pole of other magnets, and repelled by fields flowing in the opposite direction.

Of course, researchers still had questions. What determines the direction of these magnetic fields? And if every atom is composed of tiny magnets, why isn’t everything magnetic? The answers emerged with the birth of quantum mechanics: a new branch of physics exploring how the laws of nature work differently at the sub-atomic level. In 1926, a pair of Dutch physicists discovered that – in addition to mass, charge, and magnetism – particles had another fundamental property: quantum spin. mThis is a confusing label because while nothing is actually spinning, spin does refer to a kind of innate direction that influences a particle’s behavior. And in the 1930s, Nobel Prize laureate John H. Van Vleck discovered that this property held the answer to countless questions about magnetism – including Bohr and van Leeuwen’s.

Basically, the direction of a particle’s spin determines the direction of its magnetic field. And since electrons have much stronger magnetic fields than protons, the direction of their spin determines whether or not something is magnetic. If enough of a material’s electrons spin in the same direction, their many tiny magnetic fields will align, turning the material into a magnet with a field pointing in that direction. The more aligned electrons, the stronger the magnet.

On the other hand, if a material is full of electrons with competing spins their fields cancel each other out, leaving you with something non-magnetic. Most elements have electrons that come in pairs and spin in opposite directions, which is why most materials aren’t magnetic. But iron, nickel, cobalt, and a few rare metals prefer to leave some of their electrons unpaired.

Laid the groundwork for MRI machines

This doesn’t automatically make them magnetic – just because their electrons are unpaired doesn’t mean they always spin in the same direction. However, they can be nudged into alignment by interacting with neighbouring atoms. And researchers can kick-start this process by applying a strong magnetic field or by heating and cooling the material inside a weaker magnetic field. These metals will stay magnets permanently unless their electrons are scrambled by intense heat or an extremely strong magnetic field. But this magnetization process can make anything magnetic temporarily. Felix Bloch and Edward Mills Purcell took advantage of this in 1945 when they invented nuclear magnetic resonance spectroscopy. This technology uses a strong magnetic field to temporarily align a material’s protons, then a weaker field to knock specific molecules out of alignment. The machine then tracks the electromagnetic energy expended by the jostled protons as they realign, allowing researchers to map the material’s molecular structure. This invention laid the groundwork for MRI machines, which use a similar approach to magnetize the water in our bodies and map our tissues. Of course, NMR and MRI are just two of countless modern technologies that rely on magnetism. Computer hard drives, electric motors, speakers, wireless communication – all these inventions prove that not only does magnetism exist, but studying this phenomenon remains endlessly attractive.