“I believe in an external physical reality beyond my own experience,” says of the Max Planck Institute of Quantum Optics in Garching, Germany. “The world would be there without me, and was there before me, and will be there after me.”
Given what we know about quantum physics, that seems a bold statement. The assaults that this most fundamental theory of reality makes on our intuition are legion: particles that exist as probabilistic wave functions in “superpositions” of multiple states or places, or at least seem to as long you don’t look at them; “entangled” particles that influence each other over vast distances of space when you measure one of them.
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For at the University of Toronto in Canada, dealing with such troubling concepts is a matter of retraining our brains. “As much as we talk about ‘counter-intuitiveness’ of quantum mechanics, we just mean that it’s counter to the intuitions we have before we learn quantum mechanics,” he says. After all, we aren’t that great at second-guessing aspects of classical reality, either: how many of us would naturally say that feathers and bricks fall at the same rate under gravity?
With quantum physics, though, it doesn’t help that the quantities used to describe objects seem to exist only mathematically. Visualising a wave function as a real thing is fine for a single particle, but things rapidly get more tricky. “Once you’re talking about more than one particle, the wave function lives in some high-dimensional space I don’t know how to visualise,” says Steinberg. He ends up having to break a complex quantum system down into parts. “But they’re all merely ways of chipping away at the abstract mathematical object I know provides the complete description.”
More fundamentally, though, if you accept quantum physics at face value then at least one of two dearly held principles from the classical world must give. One is realism, the idea that every object has properties that exist without you measuring them. The other is locality, the principle that nothing in the universe can influence anything else “instantaneously” – faster than the speed of light.
For most quantum physicists, it’s realism that has to give, given all the evidence that the cosmic speed limit is never broken. The Copenhagen interpretation is the most widespread of the approaches that result, says Kofler. This demands that at least some properties of microscopic objects don’t exist prior to and independent of measurement. Alternatively, physicists resort to the many worlds interpretation, in which all possible results of a measurement happen, each spawning a different universe – whatever that means.
It’s fair to say that no one really gets all this. That means practitioners of quantum physics need to guard against relying too heavily on new intuitions and imagery, says Steinberg. “That’s exactly the point at which one develops a dangerous self-confidence.”
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This article appeared in print under the headline “How to think about quantum reality”
