Introduction to Particle Physics Part 2: What holds the world together?
How many forces can you think of? At school you might have heard about pushing, pulling, lift, drag, friction and perhaps the more exotic electric and magnetic forces. But what if I told you that all of these were the same? In fact, everything we see and experience in our day to day lives, except for gravity, can be attributed to a single force of nature – electromagnetism.
In the early 1800s, a number of ground-breaking experiments carried out by people such as Michael Faraday discovered that electricity could make magnetism, and – incredibly important for our modern world – magnetism could be used to make electricity. In 1861, a brilliant Physicist called James Clerk Maxwell unified the two forces together, and in doing so explained that light itself is nothing more than a dancing intertwined mixture of changing electric and magnetic fields. When we push on a wall, the electrons in our hands repel the electrons in the wall – creating the force we feel. All of Chemistry, Biology, even life itself, were understood to be due to a single force of nature. This was one of the greatest triumphs in Physics; the first theory of almost everything.
But all was not well with just electromagnetism and gravity alone, can you guess why? If we examine the nucleus of any atom, then we see a mixture of positively charged and neutral particles. But wait, if positive charges repel then why don’t they fly away from each other? This thought baffled physicists for decades, leading to the proposal of an entirely new force of nature: the strong force. When we say strong, we mean really strong: the force between just two protons is enough to hold the entire weight of a small child!
Excellent, reality is complete once again! Oh wait, there’s the minor problem that in the 1930s neutrons were seen turning into protons during radioactive decay… No worries, Enrico Fermi came along and proposed a new force of nature, the weak force, to explain the observation. Whilst it may sound feeble, it’s actually this force that allows the Sun to change hydrogen into helium (helium has two neutrons whilst the hydrogen nucleus is just made of a proton, so something must turn some protons into neutrons). Importantly, the weak force makes this process go slow enough for life on our planet Earth to be possible – if it was much stronger then the Sun would have long since used all its fuel.
Physics had reached a turning point; everything in the universe was understood to be made of 3 sub-atomic particles (protons, neutrons and electrons), interacting with each other by the 4 fundamental forces of nature. But things are not so simple, as the universe had other plans in store for us…
(Next time: Order from chaos – the Standard Model arrives)