I went to a lecture by Sir Anthony Leggett at Otterbein University tonight. His question was this: where does the weirdness of quantum mechanics go in the real world?
He started by describing the double-slit experiment as the classic example of quantum weirdness. I actually prefer Richard Feynman’s much simpler example of quantum weirdness in everyday life. Aim a photon gun at a pane of glass. To make it easy, suppose there is a 50% chance of the photon getting through. If your photon gun shoots one photon at a time, say, one every hour, then you can capture each photon event. Either the photon gets through or it bounces off.
Here’s the problem. Each photon is absolutely identical. Why does one get through and the other not? There’s no reason, and no way of predicting which photon gets through and which doesn’t. According to quantum mechanics, until someone observes the photon it has neither passed through nor bounced off. Experiments of similar situations confirm that the photon really is in this weird middle world.
In the world of big things, we never see such strange behavior. Why? The traditional answer is that the real world is so complicated that quantum effects get smeared out. Dr. Leggett sneered at this answer, saying that just because evidence disappears (for instance, an axe used in a crime) doesn’t make the perpetrator any less guilty. In a bit of a twisted argument, Dr. Leggett said then that just because quantum effects aren’t visible in the world of big things is no proof that quantum effects are simply hidden by messiness.
OK, that’s a little twisted, so let’s try it again. Dr. Leggett is hoping to do experiments with several billion helium atoms. These several billion helium atoms could potentially display quantum behavior. Dr. Leggett says one of three things might happen.
First, the experiment might not work. Quantum effects are hidden by the noise, just as conventional wisdom says happens for any other event in the world of big things.
Second, the experiment might show quantum effects. If so, it would be the largest system yet to show quantum weirdness, bringing quantum weirdness one step closer to the world of people, turtles, and peanut butter sandwiches. That would be an achievement.
Third, the result Dr. Leggett hinted strongly that he was hoping for, the experiment might show no quantum effects, yet clearly not be swamped by noise. If this happens, Dr. Leggett says, then reductionism is dead. There are additional laws of physics, not just fuzzy smeariness resulting from complexity, that prevent quantum weirdness from showing up in the macroscopic world. There are laws of physics, in other words, that work only when lots of things are brought together. One hundred monkeys.
Dr. Leggett is excited by this idea. I hope it is the wrong idea. Why do I hope that? Surely new laws of physics, new things to discover, opens an exciting new world, much like the world that suddenly opened to scientists at the beginning of the last century as quantum mechanics, radioactivity, and relativity burst into the world.
Maybe I’m just too old. But I would mourn the loss of reductionism. Reductionism gets a bad rap, because people feel constrained by it. But reductionist ideas have been so very powerful. Atomic theory in physics and chemistry. Evolution in biology. Plate tectonics in geology. Big Bang cosmology in astronomy. These great ideas, these “theories of everything” have been so useful in all these fields.
Also, to me, reductionism is the essence of understanding. To understand something is to see its inner workings in terms of simpler, more fundamental somethings. I understand an engine when I reduce its parts to the laws of thermodynamics and motion. I understand the gas laws when I reduce them to the behavior of atoms. If, instead, the gas laws only made sense when you considered the collective, emergent behavior of 100 or 200 or 10 million atoms, we lose an elegant piece of the explainable universe.
Emergent properties are certainly real things (Why does water feel wet?), but my hunch, and my hope, I have to admit, is that as we understand more and more about emergent properties we’ll see that they “emerge” directly from the fundamental laws underlying everything, and not, as the non-reductionists hope, from new laws that only kick in as complexity grows.
The great thing about science is, someday we’ll know!