Yesterday, while giving a lecture on the philosophical implications of quantum mechanics, I was asked, for the second time in this kind of context:
So is this quantum weirdness like the question: if a tree falls in the forest and there’s no one there to hear it, does it make a sound?
So I think it’s probably worthwhile explaining the difference.
Sure, you can ask that kind of question even in a classical world. The answer obviously depends on how you interpret it. In an obvious sense it does make a sound. The air molecules around the tree would vibrate in exactly the kind of way which would be sufficient to produce the sensation that we call sound if those vibrations were to meet a human ear. Or you can choose to define sound as being the human sensation itself. The air vibrations would be there, surely, but with no ear around to catch them, no sound would be produced. Or you can choose to go really philosophical and wonder, à la Bishop Berkeley, whether the world really exists independently of anyone’s sensations, so that not only the tree does not make a sound, but there’s no tree or even a forest there if no one’s around to see or hear it.
Far from me to depreciate the fun of this kind of questioning. But if that was all there was to it, the problem with quantum mechanics wouldn’t be nearly as interesting. The thing is, in a classical world you can always imagine, even if you are not able to convince the good old Bishop, that there is in fact a tree there, and that its fall generates as momentous a vibration in the surrounding atmosphere whether or not you’re there to witness it. Yeah, you could speculate about all sorts of philosophical minutiae, but that’s the best way to think about it.
Even further, we could say that there is someone there. There’s the animals, the insects around the tree. They would hear or feel the fall. If you go back to the scene later, you may be able to find that there’s a crack in a spiderweb which was caused by the vibrating air, or some displaced leaves. Or you can imagine that you could, if only you had the technology, find traces of the sound produced by the tree fall in the distribution of air molecules in the atmosphere. Heck, maybe this one tree might even be the cause of the next hurricane in Florida!
Now with quantum mechanics, the trouble is that there really is no one there. No air to vibrate, no leaves to displace, no footprints. When we gather all information about a certain physical system (and it so happens that usually we can only gather such complete information for small systems, like an atom or a photon), that’s when things get interesting. Then we find that when we ask what it’s doing in-between our observations, when absolutely no-one or nothing else is getting information about it, we find that we can’t even imagine that it was really here or there independently of our looking. “I can imagine all sorts of things”, you say, but what’s more, we can prove mathematically, via Bell’s theorem or the Kochen-Specker theorem, that no one (yes, not even you!), can imagine it either!
Well, at least not with the old kind of imagination. You need to allow some sort of nonlocality, backwards causality, multiple universes, or what-not, if you want to have that picture of the world independent of observers. And this is when it gets messy. With quantum mechanics we can prove that our old conceptions of reality can’t work, but it seems that we can pick and choose what we want to replace it with.