## Even if you knew the entire past history of the universe, this would not contain the information about what the particles will do in the experiment

Quantum physics is famously weird, counterintuitive and hard to understand; there’s just no getting around this. So it is very reassuring that many of the greatest physicists and mathematicians have also struggled with the subject. The legendary quantum physicist Richard Feynman famously said that if someone tells you that they understand quantum mechanics, then you can be sure that they are lying. And Conway too says that he didn’t understand the quantum physics lectures he took during his undergraduate degree at Cambridge.

The key to this confusion is that quantum physics is fundamentally different to any of the previous theories explaining how the physical world works. In the great rush of discoveries of new quantum theory in the 1920s, the most surprising was that quantum physics would never be able to exactly predict what was going to happen. In all previous physical theories, such as Newton’s classical mechanics or Einstein’s theories of special and general relativity, if you knew the current state of the physical system accurately enough, you could predict what would happen next. “Newtonian gravitation has this property,” says Conway. “If I take a ball and I throw it vertically upwards, and I know its mass and I know its velocity (suppose I’m a very good judge of speed!) then from Newton’s theories I know exactly how high it will go. And if it doesn’t do exactly as I expect then that’s because of some slight inaccuracy in my measurements.”

Instead quantum physics only offers probabilistic predictions: it can tell you that your quantum particle will behave in one way with a particular probability, but it could also behave in another way with another particular probability. “Suppose there’s this little particle and you’re going to put it in a magnetic field and it’s going to come out at A or come out at B,” says Conway, imagining an experiment, such as the Stern Gerlach experiment, where a magnetic field diverts an electron’s path. “Even if you knew exactly where the particles were and what the magnetic fields were and so on, you could only predict the probabilities. A particle could go along path A or path B, with perhaps 2/3 probability it will arrive at A and 1/3 at B. And if you don’t believe me then you could repeat the experiment 1000 times and you’ll find that 669 times, say, it will be at A and 331 times it will be at B.”

{ The Free Will Theorem, Part I | Continue reading | Part II | Part III }