There Is No Classical World

Caltech’s Institute for Quantum Information and Matter is a fun place. It’s led by people like John Preskill, Jeff Kimble, and Alexei Kitaev — some of the world’s great scientists — so you know the physics is going to be top-notch. But it’s the youngsters, such as postdoc Spiros Michalakis, who are bringing the fun. Suff like the IQIM blog (where you should read John’s recent post on the Maldadcena/Susskind wormhole proposal) and a successful Kickstarter campaign for science-inspired fashion.

The fun is now being ratcheted up even higher, as IQIM is teaming with Jorge Cham of PhD Comics fame to make a series of animated web videos about quantum mechanics. I ask you, who doesn’t love some good videos about quantum mechanics??

Sensibly, they’ve kicked off by spotlighting an interesting experimental result, rather than diving right into the realms of esoteric theoretical speculation. Of course, this is quantum mechanics we’re talking about, so even the experiments get pretty wild in their implications. The work is by Amir Safavi-Naeini and Oskar Painter, who take a small mirror and put it into a quantum state where its center of mass is as cold as it is possible to be. Classically, of course, the mirror can be perfectly still; quantum-mechanically, there is a ground state wave function that still shows “fluctuations” (i.e. the fact that observations won’t always show zero motion).

Now, the mirror is tiny — microscopic, it’s fair to say — but it’s not that tiny. It’s a piece of metal, non just an atom or two. (I didn’t catch what the actual size was.) So the implication here is that things don’t miraculously “become classical” when they are made of many atoms rather than just a few. We don’t notice the quantum-ness of the universe in our everyday lives, but that’s because the systems we encounter are noisy and constantly jostled by their environments, leading to rapid decoherence; not because there is a magical transition to classicalness once you get above a certain number of atoms, or a truly distinct “classical realm.”

Of course, no right-minded person really believes that there is a hard and fast transition to a classical realm once objects get big; rather, there is a sense in which the classical approximation becomes more and more accurate, but it’s always just an approximation. The experimental results here are simply affirming the truth of quantum mechanics. Nevertheless, you can still meet people (the wrong-minded ones) who are willing to believe that electrons and photons are governed by quantum mechanics, but not that they are governed by quantum mechanics. Have them watch this video, and hope that the implications sink in.