Into the fifth dimension

摘要

Particles such as the proton can be imagined as vibrating strings. We also know that protons contain smaller, point-like particles, going against the string theory. But in five dimensions, the contradiction disappears. In fundamental physics, our description of nature involves four forces: gravitational, electromagnetic, weak and strong. The strong force is responsible for binding protons and neutrons inside the atomic nucleus. Two different theoretical approaches have been taken in describing the workings of the strong force and the structure of particles such as the proton and neutron. The theories are seemingly at odds with each other, but steps are gradually being taken to reconcile the two. Writing in the Journal of High Energy Physics, Polchinski and Strassler now dispel worries over an apparent contradiction between the theories, by showing that it isn't necessarily a contradiction at all. In the 1960s, experiments on high-energy collisions between protons revealed a plethora of other short-lived, strongly interacting particles. Shortly afterwards, a theory emerged that proposed that all of these different particles were particular excitation modes of a string: as a violin string can vibrate with different frequencies, these strings could oscillate in different ways, corresponding to the 'zoo' of particles that was observed. This 'string theory' proved useful in explaining some aspects of the masses and spins of the particles.