IT IS the most famous case of animal cruelty in physics. Or is it?
When, back in the 1930s, physicist Erwin Schr枚dinger dreamed up his notorious thought experiment about a cat that is simultaneously dead and alive, he could hardly have imagined how it would enter the popular consciousness. Or how many terrible jokes it would spawn.
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How鈥檚 this for a punchline, though: we still don鈥檛 know exactly what Schr枚dinger鈥檚 cat means. What you make of it will depend on where you stand on the fundamental question of where reality comes from.
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In the basic set-up you take a cat and stick it in a box rigged up with a radioactive atom, a hammer and a vial of poisonous gas. The atom decays, and this triggers the hammer to fall and break the vial, suffocating the cat.
Or not. Radioactive decays are random processes described by quantum theory, so we can鈥檛 say when one will happen. And quantum theory strongly suggests that before you observe or measure an object, it exists in a 鈥渟uperposition鈥� of all its possible states. Before we open the box, the atom is both decayed and undecayed 鈥� and the cat both dead and alive.
Two ways at once
For Schr枚dinger, this situation highlighted the absurdity of the dominant 鈥淐openhagen鈥� interpretation of quantum theory, which permits things to be two ways at once until a measurement kills off the ambiguity (and possibly the cat). 鈥淗e was trying to find a hyperbolic example that brought to light conceptual difficulties he was struggling with,鈥� says at Yale University.
鈥淲e don鈥檛 live in a superposition of putting milk on our cereal and not鈥�
It鈥檚 not just him: the notion is profoundly alien to our everyday experience. 鈥淲e don鈥檛 live that way,鈥� says at the University of Oxford, 鈥渋n a superposition of having poured the milk on our cornflakes and not.鈥�
And there are more fundamental problems. The Copenhagen interpretation implies that an observer 鈥渃ollapses鈥� underlying quantum uncertainty into a concrete reality just by observing it. Must that observer be human? Would any other organism with conscious thought do? Or a camera on a stick? And how did anything even become definite in the early universe, with nothing around to observe anything? 鈥淔or me, it is still a profound mystery,鈥� says Briggs.
The alternatives certainly have their own wrinkles. The quantum many worlds hypothesis, for example, suggests that at the very moment an observer has enough information to conclude whether Schr枚dinger鈥檚 cat is dead or alive, the world sloughs off a parallel universe that contains the alternative outcome (see 鈥�How to think about鈥� The multiverse鈥�).
Then there is quantum Bayesianism. According to this idea, the cat鈥檚 state isn鈥檛 uncertain, our state of mind is 鈥� in which case we must accept fundamental limits on what we know about reality. Vlatko Vedral at the University of Oxford, meanwhile, plumps for objective collapse theory. In this picture, superpositions aren鈥檛 destroyed by observation; instead, they naturally leak into the surroundings and eventually disappear. Fair enough 鈥� but what does that mean for the cat?
We might eventually find out the true state of this indeterminate moggie. Today, experiments suggesting we鈥檝e put molecules and electrical circuits into superpositions are commonplace. The scale of the delicate quantum superpositions we can maintain is growing all the time. Researchers have even proposed trying out the same thing on viruses, organisms which teeter on the edge of life. 鈥淢aking macroscopic superpositions could just be a question of money,鈥� says Vedral. Then curiosity really might kill the cat.
This article appeared in print under the headline 鈥淗ow to think about鈥� Schr枚dinger鈥檚 cat鈥�
