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of different-size red marbles and green marbles, with one marble of each color in each size. Suppose you are playing a game with a friend, and you get the red marbles and your friend gets the green ones. If the marbles were exactly paired, neither color choice would give you an advantage. However, if there weren’t an equal number of red and green marbles of any given size, it wouldn’t be an even playing field. It would matter if you chose red or green, and the game would proceed differently if you and your friend were to switch colors. For there to be a symmetry, every size of marble must come in both red and green, and there must be the same number of marbles of each color of any given size.

Similarly, supersymmetry is possible only if bosons and fermions are exactly paired. You need the same number of boson and fermion particle types. And just as the marbles that were interchanged had to have identical sizes, the paired bosons and fermions must have the same mass and charges as each other, and their interactions must be controlled by the same parameters. In other words, each particle must have its own superpartner with similar properties. If a boson experiences strong interactions, so does its supersymmetric partner. If there are interactions involving some number of particles, there are related interactions involving their supersymmetry partners.

One reason physicists find supersymmetry so exciting is that if it is discovered in our world, it will be the first new spacetime symmetry to be found in almost a century. That’s why it’s “super.” I won’t give the mathematical explanation, but just knowing that supersymmetry exchanges particles of different spin is enough to deduce a connection. Because their spins are different, bosons and fermions transform differently when they rotate in space. Supersymmetry transformations must involve space and time in order to compensate for this distinction.23

But don’t think that this means you should be able to picture what a single supersymmetry transformation looks like in physical space. Even physicists understand supersymmetry only in terms of its mathematical description and its experimental consequences. And these, as we’ll soon see, could be spectacular.

Superhistory

You can skip this if you like. It’s a historical section that won’t introduce any concepts that will be essential later on. But the development of supersymmetry is an interesting story, in part because it nicely demonstrates the versatility of good ideas and the way string theory and model building sometimes have a productive, symbiotic relationship. String theory motivated the search for supersymmetry, and the superstring—the best string theory candidate for the real world—was identified only because of insights from supergravity, the supersymmetric theory that includes gravity.

The French-born physicist Pierre Ramond put forward the first supersymmetric theory in 1971. He wasn’t working with the four dimensions that we (used to) think we live in, but in two: one of space and one of time. Ramond’s goal was to find a way to include fermions in string theory. For technical reasons, the original version of string theory contained only bosons, but fermions are essential to any theory that hopes to describe our world.

Ramond’s theory contained two-dimensional supersymmetry and evolved into the fermionic string theory he developed with André Neveu and John Schwarz. Ramond’s theory was the first supersymmetric theory to appear in the Western world: Gol’fand and Likhtman in the Soviet Union had simultaneously discovered supersymmetry, but their papers were hidden from the West behind the Iron Curtain.

Since four-dimensional quantum field theory was on much more solid footing than string theory, the obvious question was whether supersymmetry is possible in four dimensions. But because supersymmetry is intricately woven into the fabric of spacetime, it was not a straightforward task to generalize from two to four dimensions. In 1973, the German physicist Julius Wess and the Italian-born physicist Bruno Zumino developed