In the late 1980s, a pair of Russian physicists put forward a new theory that just might serve to unite all physics in one model. But it's so fiendishly complicated that decades later, few physicists are willing to touch it.
The brainchild of the Mikhail Vasiliev and the late Efin Fradkin, the theory requires such brutally complex mathematics that it's only in the past few years that it's started to pick up even the slightest bit of momentum among physicists. Thanks to some truly fearless mathematicians, we now know the theory is just within the realm of human comprehension. But only just.
So just what is the theory? Scientific American editor Geoff Musser does an admirable job laying out the basics in a new blog post. The key here is spin, which we can very briefly describe as a subatomic particle's rotational symmetry. Each type of particle always has the same spin, and these vary in half-integer increments. A particle with spin-1 looks the same after being rotated 360 degrees, but one with spin-1/2 needs to be rotated 720 degrees to look the same, and a spin-2 particle needs only a 180 degree turn. As Musser explains, this theory just extends that idea up to infinity:
Vasiliev theory (for sake of a pithy name, physicists drop Fradkin's name) takes to extremes the basic idea of modern physics: that the world around us consists of fields-the electrical and magnetic fields and a handful of others that represent the known forces of nature and types of matter. Vasiliev theory posits an infinite number of fields. They come in progressively more complicated varieties described by the quantum-mechanical property of spin.
In Vasiliev theory, there are also spin-5/2, spin-3, spin-7/2, spin-4, all the way up. Physicists used to assume that was impossible. These higher-spin fields, being more symmetrical, would imply new laws of nature analogous to the conservation of energy, and no two objects could ever interact without breaking one of those laws. The workings of nature would seize up like an overregulated economy.
From this initial idea, the theory manages to bring forth a bunch of key aspects of the universe, including the emergence of spacetime itself. It's still way too early to know whether this idea of lots more spin numbers could really serve to unify physics, but it's well worth taking a look at the complete, rather fascinating explanation over at Scientific American.