In 1934, Gregory Breit and John Wheeler—American physicists who would go on to help create the atomic bomb—developed a theory: that, under very particular circumstances, two photons of light could combine to produce an electron and a positron.
It was a daring idea. Electrons form the outer shells of atoms, so the Breit-Wheeler theory suggested that there's a way to turn light into ... stuff. Matter.
It was also an idea, however, that—as with so much else when you're talking about particle physics—was based on calculations alone. Breit and Wheeler didn't have much hope of having their theory proved in a lab setting. They published their work on the, er, matter, noted that it would be “hopeless to try to observe the pair formation in laboratory experiments,” and then went on with their careers.
Eighty years later, however, another generation of scientists claims to have taken the hopelessness out of the equation. A team of researchers at Imperial College London has developed a system, they say, for evidencing the Breit-Wheeler theory. It goes by the delightfully descriptive name of the "photon-photon collider," and it would, as that name suggests, offer a way to smash photons (light particles) together to test whether electrons do indeed result from the collision.