What does quantum probability really mean?

Most people are at least vaguely aware that quantum mechanics has elements of probability associated with it. The implication is that the classical, pre-quantum, deterministic universe we once knew is dead in the water.

But there are a number of ways you could look at quantum probability:

  1. The universe is genuinely probabilistic, by which I mean it’s the very nature of the universe at root.
  2. Quantum mechanics isn’t the final say on things. It’s phenomenally accurate over its domain of applicability, but it’s actually just an approximate theory of some underlying, possibly deterministic theory that’s yet to be discovered.
  3. The universe may or may not be deterministic but, either way, we can only ever know things about it as probabilities. The lack of determinism — if it is in fact deterministic — we see is actually a measurement problem or, perhaps, a limit quantum mechanics leaves us with.

Whichever one of those (or combination thereof, or whatever else) is true, what we have for now is a probabilistic theory.

One of the strange things about quantum mechanics is that the act of measurement is intricately woven in to things. ‘Measurement’ may or may not mean a human performing some experiment, depending on what philosophy you subscribe to. A small, self-contained system may in a sense be ‘measured’ when it comes into contact with the rest of the universe.

Either way, the very act of measurement changes the underlying system we’re measuring. In some quantum philosophies the underlying system is simply undefined until it’s measured, maybe existing in all possible configurations at once, and then the act of measurement forces it to do something definitive.

It’s hard to explain but consider this analogy. If you shuffle a pack of cards thoroughly, you have no idea what the top card will be. You know some probabilities: there’s a 1 in 4 chance it’s a heart, there’s a 1 in 13 chance it’s a jack and there’s a 1 in 52 chance it’s the jack of hearts. But it’s only when you turn over the top card — make a measurement, in a sense — that you find out what it actually is. The probabilities you initially had become realities after measurement.

As with most things quantum, analogies leave a lot to be desired. With my analogy we’d be inclined to think the top card actually has a real value before we measure it and that measuring it doesn’t change anything but simply reveals it.

The distinction is that in quantum mechanics that may or may not be true depending on what interpretation you subscribe to. It may be that it doesn’t genuinely have a value (or perhaps has all possible values) before it’s measured.

But perhaps you get the idea.

Anyway, we’re stuck with probabilities and the thing is there are different sorts of probabilities and different ways to interpret them. Scientists are keen to find out which interpretations are best and the article I link to expands on that subject.