Changing From Subatomic to Macroscopic Worlds

Why do the laws of physics change from the macroscopic world to the subatomic world?
- Don Giovannni (age 16)
Burundi

So far as we know, the deepest laws of physics don't change from the macroscopic world to the subatomic world. In the macroscopic world, treating objects as if classical mechanics were correct rather than using a quantum description often works well, but that approximation fails badly on a small scale. Directly measuring quantum effects on a large scale, however, is so hard that we can't rule out the possibility that there are some fundamental effects that are only important for large objects. Some attempts to deal with what's called the "quantum measurement problem" postulate that there might be such new fundamental effects. These attempts are called "macro-realist".

Here's some brief background as to why that transition from quantum small-scale behavior to our familiar classical-looking large-scale world might inspire people to look for some extra ingredients. The fundamental quantum law for how states change in time is strictly linear. That means that quantum states that are combinations of ones that lead to different large-scale outcomes should lead to states which combine those large-scale outcomes. The familiar example is Schrödinger's cat, a combination of live cat and dead cat states. Yet we never see anything like that. Either somehow there's more to the story than the simple quantum laws imply, or our experience is only of states representing little pieces (live cat or dead cat) of the overall state. That Many Worlds interpretation of quantum mechanics implies that the complete state has many slightly different you-ish beings, all of whom were you a few moments ago. Some see the live cat, some see the dead cat. In addition to feeling creepy, that interpretation has some problems accounting for quantum probabilities. Still, among people who worry about this topic Many Worlds interpretations are much more popular than they used to be.

So one not very successful subset of the desperate ways we try to make sense of this situation involves inventing new laws that only become important on a large scale. Most quantum interpretations don't require such effects, or at least don't admit to requiring them.

Mike W.

(published on 08/31/2016)