Q:

I have heard it stated by renowned scientists, for example Stephen Hawking, that the macroscopic world is completely deterministic from a theoretical if not practical perspective, while the quantum realm is probabilistic. My question concerns the interaction of atomic radiation with the macroscopic world. The emission of a particle from a particular nucleus at a particular time is, as I understand it, purely probabilistic. If that particle hits a DNA molecule and causes a mutation resulting in cancer how can that cancer be said to be theoretically deterministic?

- Jack Stephens (age 70)

Neptune Beach, Florida, USA

- Jack Stephens (age 70)

Neptune Beach, Florida, USA

A:

Hi Jack,

Quantum mechanics (our extremely successful description of the microscopic realm) is indeed probabilistic, and is generally believed to be exact (in which case it must reduce to classical mechanics in some macroscopic limits). Exactly *how *these two descriptions intersect is still being worked out, both on paper and in the lab.

The experiment you have described is essentially Schrödinger's cat: a probabilistic event governed by quantum mechanics is entangled to the fate of a macroscopic entity, which would thereby be expected to behave similarly (i.e. exist in a superposition of being both dead and alive, or having cancer and not having cancer). In practice, we don't observe these superposition states in macroscopic systems because of interactions with the environment ("measurements"), which "collapse" the object's state.

This collapse, as you suspected, is probabilistic, and not influenced by any local variables. (See .)

Therefore, I think it is safe to say that quantum mechanics introduces probabilities into all branches of physics, including those governing the macroscopic events in our lives.

David Schmid

*(published on 03/12/2013)*