Last week I started a new series of micro-posts touching on the different ways quantum physics is weird. The motivation comes from my summer project of reviewing old notes and reacquainting myself with the mechanics of quantum mechanics. But no matter how many theorems on Eigenstates and Unitary operators I crank out, I am still bothered by how strange the quantum world is compared with our common sense expectations (of course the world cares not a whit about our expectations).
So today's weirdness can be summed up in a single word: probability. We are all familiar with probability. You flip a coin and before it lands there is a 50 percent chance it will come up "heads" and a 50 percent chance it comes up "tails."
The reason we don't know which side we'll get before it lands is because of ignorance.
From a classical physics perspective, if we knew the initial location and motion of every atom in the coin and the air and our hand we could, in principle, exactly and explicitly predict the coin's fate. Thus the probabilities we usually deal with in life come from not knowing everything there is to know.
Probabilities in quantum mechanics are not like this.
Not at all.
Not even a little.
In quantum mechanics probabilities are inherent. They don't come from ignorance. They are an intrinsic to reality. While I can speak quite accurately about half-lives relevant for a large collection of radioactive atoms, there is no way to predict exactly when an individual radioactive atom will decay.
Why is this weird?
Well, we like to think that every event we see in the universe has a cause. Vases don't fall from shelves on their own. Something, or someone, has to knock them off the shelf. Not so in quantum physics. Individual events like a radioactive decay just happen when they damn well want. They are inherently probabilistic or, better yet, "a-causal."
That is very weird indeed.
You can keep up with more of what Adam Frank is thinking on Facebook and Twitter. His latest book is About Time: Cosmology and Culture at the Twilight of the Big Bang.