Professor Harrison began by explaining the origins of particle physics and how ancient Greek philosophers questioned what happens when you keep slicing something up – if you eventually reach an elementary level at which further cutting of it would fundamentally change the substance or whether it just gets infinitely smaller. Leucippus and Democritus proposed that the substance would not get infinitely smaller but that you would eventually reach an ‘atom’, meaning ‘indivisible’. 2000 years on we now know much more about what happens when we cut an object into continually smaller objects. Bulk matter is anything greater than 10-9m, atoms and molecules are between 10-8m and 10-10m. Within an atom’s inner structure there is a nucleus of size 10-14m which contains nucleons (protons and neutrons) that are 10-15m containing quarks which are 10-20. Quarks are a type of elementary particle – a particle which has no known structure but makes up larger things. There are different types of elementary particles: quarks, leptons, gauge bosons and scalar bosons. Within these categories there are different types of elementary particles, for example up, down, charm, strange, top and bottom are all types of quark. Professor Harrison then went on to explain how there are 4 guiding principles of Particle Physics – these are: Simplicity, Composition, Unification and Symmetry. With Simplicity Ockham’s Razor is used and this is a philosophical principle that if there are multiple explanations for something then the simplest one is best. Unification is used when trying to explain different physical phenomena using a single idea or law of nature, for example with Coulomb’s Law of electric field around a charge, Amperes Law of Magnetism and Faraday’s Law of Induction, Maxwell was able to show that these were all linked by ‘fixing’ one – allowing him to predict electromagnetic waves. With Symmetry, Noether’s Theorem (1915) is used, ‘Every symmetry of the laws of Nature implies an associated Conservation law’. Next Professor Harrison spoke about the tools of Particle Physics, both theoretical – Quantum Mechanics, Einstein’s Theory of Special Relativity – and things like the Large Hadron Collider and ATLAS. He finished by mentioning some of the unanswered questions such as, ‘Is the discovered Higgs boson the only Higgs boson?’, ‘Why is the Higgs boson mass stable?’, ‘Is the dark matter of the Universe really a new undiscovered particle?’, ‘Is there a way to unify the strong interaction with the weak and electromagnetic?’ and ‘Why do the masses of the elementary particles take the values they do?’.
Overall, this was a really interesting talk on a fascinating area of Physics that clearly has much more research yet to be carried out.
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June 2020
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