He recently described his dream physics experiment to Gizmodo: an unfathomably large atom smasher—an Ultra Hadron Collider—around the outer edge of the Solar System. Particles interact through forces, of which there are four known ones: electromagnetism; the “weak” force; the “strong” force; and gravity. But experiments have shown that, at high enough energies, electromagnestism and the weak force merge together into the same force. But to know for sure, we have to get to the size of the Solar System. But to add gravity to the fray, “by some naive estimates, we’d need a collider around the orbit of Neptune.