The laws of physics distill our observations about what can and cannot happen in the physical world. Powerful conservation laws tell us that certain physical attributes are immune to change. Energy, for example, can take many different forms, but it can be neither created nor destroyed.

In particle physics, conservation laws result from symmetries in the forces that govern how nature works. A symmetry of the electromagnetic force tells us that electric charge is exactly conserved, that all electrons are created equal, and that the photon—the particle of light—is exactly massless. Such symmetries have provided some of the most dramatic insights in particle physics.

An understanding of all the symmetries of nature would be equivalent to knowing how the world works at its most basic level.

There are many reasons to believe that we have not found all the symmetries of nature. String theory, for example, requires a new symmetry called supersymmetry. It introduces new quantum dimensions to spacetime and gives meaning to many of the particles we know. It predicts that every elementary particle has a superpartner, just as every particle is accompanied by an antiparticle. Scientists have not yet discovered the supersymmetric partners; fi nding them is among the greatest challenges for the current and upcoming generations of particle accelerators.