If he had worked it out a few years before the experiments of Stern and Gerlach, he could’ve predicted their results! His mathematics automatically included a description of spin. One of those implications of uniting quantum mechanics with special relativity was - you guessed it - spin.
And unlike his buddy Erwin, he was able to crack the mathematical code and figure out its implications.
The formulation of the quantum world that most folks are familiar with - say, the famous Schrodinger wave equation the allows us to compute probabilities of particle locations - doesn't naturally include the concept of spin.īut around the same time, a certain theoretical physicist named Paul Adrien Maurice Dirac was also puzzling out the quantum world and went full bore with an approach to quantum mechanics that included special relativity. That right there is the bedeviling nature of quantum mechanics: It fundamentally limits our ability to measure things at small scales.Īfter enough experimentation, the "rules" of spin were added to scientists' knowledge of quantum physics, concurrently being developed in the 1920s. It's like the most useless GPS navigation in the world: Instead of giving you accurate directions, you're only told, "Go north 500 steps," or "Go south 500 steps." Good luck finding that restaurant. If it's a little bit down or very much down, it doesn’t matter, we get -1/2. If the arrow is pointing even slightly up it will register in any experiment as +1/2. The length of that arrow is fixed for each kind of particle, but we're only ever allowed to measure a limited number of directions. Keep in mind that the actual direction of the spin could point anywhere - imagine a little arrow tagged onto each and every particle. I know it’s confusing notation, but you’re going to have to blame the physicists who first describing it a hundred years ago. A spin 1 particle, such as a photon, can be measured to have directions +1, 0, or -1, and that's it. And the magnitude of a particle's spin determines what directions of the spin we can actually measure.įor example, a spin 1/2 particle like an electron can only ever be measured to be +1/2 or -1/2, corresponding to the up and down deflections of the Stern-Gerlach experiment. Other particles might have spin of 1, 3/2, 2 or even 0. By definition, electrons have a spin equal to 1/2. And it turns out that spin has some pretty weird properties indeed.įor one, the magnitude of a particular particle's spin is fixed. Taking it out for a 'spin'Īnd just like mass and charge, we can perform experiments to discover the nature of the spin property and how it interacts with the other forces and particles in the universe. In this case, quantum effects were in full force, and researchers soon realized that atoms (or more precisely, the particles that comprise atoms) have a previously unknown property that only reveals itself in the presence of a magnetic field.Īnd since those atoms kinda-sorta behaved as spinning balls of electrically charged metal, this new property was dubbed "spin." And so particles like electrons suddenly had three properties: mass, charge and spin.
The experimenters were witnessing one of the first in-your-face clues that the subatomic realm operates on rules that are far from the familiar ones.