How was quantum physics discovered?
Quantum physics began with perceived inadequacies in classical theory, the basis of all of physics up until the late nineteenth century. One of those problems is the stability of matter. (Why do solid objects retain their shapes when banged?) Another problem was equilibrium of matter with radiation at some constant temperature. If you look at those issues in classical physics, it seems that no such stability and no equilibrium is possible.
So what exactly is quantum physics?
It began as the theory of the very small. So quantum physics is about molecules, atoms, elementary particles, like electrons and protons, and whatever they are made out of – quarks, perhaps superstrings. But the equations are completely different from the equations of classical physics. Moreover, you can’t just contain the difference to the micro world. It really gets out and contaminates large molecules; it contaminates collections of large molecules and then biological organisms and ordinary objects and then the planet and then the universe as a whole. This in itself isn’t surprising. In physics, when we have a new theory, we push it to its limits. If you think of what Newton did with his theory of gravity, he didn’t stop with the motions of the planets around the star but extended it to include every motion of every atom in the entire universe. That was essential to all the later discoveries. But when you try to do that with quantum theory you end up with something that seems like nonsense.
There is a famous experiment in quantum physics called Schrödinger’s cat, which finds that a cat can be simultaneously both dead and alive. How is this possible?
That is an example of what I mean. It seems like nonsense. In the face of this, what seems to follow from the equations, it was just declared that something else takes over when you have macroscopic things involved, and especially in ‘experiments’. But many physicists found this unacceptable. They think the equations have to be applied whether or not something is an experiment, or involves macroscopically many particles, but they are modified equations – they are not the usual ones. Or else you have to have extra equations, for new variables – either way, that dictate one or other of the dead cat or live cat. There is another solution, discovered by an American PhD student, Hugh Everett. At the time the idea was too radical, but nowadays, with other theories in physics calling for similar ideas, it has to be t aken more seriously. E verett’s idea was that you’ve got this superposition of the cat alive and the cat dead which extends in time (as the equations evolve in time) to a superposition of the experimenter looking at the cat dead and another experimenter looking at the cat alive, and the equations continue to evolve and then give you a collection of colleagues of the experimenter all agreeing that the cat is dead, superposed with a collection of colleagues all agreeing with one another that the cat is alive, so the fact that you’ve got the superposition cannot be observed, and this keeps going until in the end it extends to the entire universe. This is the ‘manyworlds’ theory. It is an extremely lively branch of quantum foundations. Some of the most interesting questions on this interpretation concern probability and its relationship to decision theory.
Do any of the discoveries made in quantum physics have any application to the ‘real world’?
Oh my gosh! It has transformed almost all the technology you use in everyday life: mobile phones, laptops, media, medical instrumentation, and almost anything else that uses lasers.
Simon Saunders is a Professor of Philosophy of Physics and Fellow Linacre College, Oxford
