Monday, June 1, 2009

Uncertainty and Schrodinger's Cat

Werner Heisenberg (5 December 1901 – 1 February 1976) was a German theoretical physicist who made fundamental contributions to quantum mechanics and is best known for asserting the Uncertainty Principle of quantum theory. In very simple terms, this states that for a particle such as an electron, the more precisely you know one physical property such as momentum, the less precisely you are able to state another property such as position. This is not a statement regarding the limitations of measurement, but rather a philosophy on the nature of the Universe (at the quantum level) - that events are essentially probablistic. This means that a particle could exist in one position, another, or even both points at the same time. See my earlier post (The Bell Experiment) for a better explanation.

Other physicists such as Albert Einstein and Erwin Schrodinger refused to believe in a probabilistic Universe, and defended the classical view of determinism (all events are determined by what has gone before - that every event has a cause). In fact, in his later years, Einstein became embittered by what he percieved as the vandalism of classical physics by theoreticians such as Heisenberg, Niels Bohr and Wolfgang Pauli.

In order to defend determinism, Schrodinger attempted to show that Uncertainty was, at heart, ridiculous by coming up with his famous thought experiment, concerning a cat in a box (no real cats were ever experimented on). This is explained in the following clip:

However, despite Einstein's and Schrodinger's objections, quantum mechanics continued to go from strength to strength and has survived many attempts to discredit or falsify it. It even provides an explaination for radioactive decay and the Big Bang (which determinism is unable to do), in the sense that quantum mechanics provides a framework in which an event occurs spontaneously, without a deterministic cause.

And Schrodinger's Cat? Again, in simple terms, a cat does not itself exist at the quantum level, although the particles it is made up of do. Therefore we cannot resonably expect a cat (as a whole entity) to behave in the same way as a sub-atomic particle.


David said...

Mike - I've read Schrodinger's Science and the Human Temperment aka Science Theory & Man, when he was fully supportive of Heisenberg and Uncertainty. It's a persuasive enough argument, far as it goes, but you get the feeling there is an equal probability of it being true and untrue at the same time, which is, much like Schrodinger's cat. Is there a book or article where ECS expresses with anywhere near the same degree of care, his later determinism?

Mike Smith said...

Hello David. It's a good question. I don't think I've ever read anything where Schrodinger argued explicitly in favour of the Uncertainty Principle, although I'll certainly take a look. From what I know about him, his views evolved through the 1920s and 30s in line with Einstein (and Planck)(who he would never contradict)-so perhaps in a strange way the cat experiment was more an expression of their ideas rather than his own.