Question: how do you predict the outcomes of super-high-energy protons in the LHC and in comparison to your own data to make theories?

Andy Buckley answered on 7 Mar 2019:

Excellent question. We’ve had more than a century of building up the theory tools for this: it’s one of humanity’s greatest conceptual accomplishments, so a real shame that people don’t know much about it! The mathematical framework is called “relativistic quantum field theory” (QFT), which is quite a mouthful: it’s a combination of quantum mechanics with Einstein’s relativity, which through “E=mc^2” allows for massive particles to be created and annihilated from/to kinetic energy. QFT also tells us that empty space — “the vacuum” — is not really empty at all, but a sort of bubbling ground-state of a collection of quantum “fields”. What we call particles are less bubbly excitations of those fields, like waves on a pond surface, but tiny and usually whipping along at close to the speed of light.

Using QFT as a mathematical structure and set of rules, we’ve then built up a particular set of quantum fields which we call the Standard Model, by a mixed process of experimental measurements and theory ideas. It turns out that *everything* we’ve ever seen (so far!) can be explained by 12 matter particles, and three kinds of symmetry that the theory has to obey. As if by magic, those symmetries make the fundamental forces — electromagnetism and two kinds of nuclear force — pop into mathematical existence.

The only problem with the Standard Model is that it’s very hard to make calculations that really look like what we see in the LHC detectors: there are far too many particles for our maths techniques to handle. But we’ve developed ways to deal with that, starting with full, high-accuracy calculations for the deepest bits of the proton collisions, and then wrapping them in more approximate models until we get something that looks (mostly) like reality. A lot of my work has been on building and improving those approximate models as computer codes. We then use these computing tools to both design our experiments and to test theories — both the details of the Standard Model itself, and answering the question “I wonder what would happen if…?” for all sorts of modifications, e.g. adding new quantum fields into the model and making new predictions.