Monday marks the 126th anniversary of the birth of the Austrian-Irish scientist Erwin Schrödinger.
Schrödinger's work spanned the breadth of physics and even spilled over into biology, but he is best known for Schrödinger's cat, a thought experiment that criticized the Copenhagen interpretation of quantum mechanics.
Developed in the 1920s by the Danish physicist Niels Bohr, with input from Werner Heisenberg, Max Born, and other physicists, the Copenhagen interpretation is an attempt to make sense of some very weird phenomena observed in the subatomic realm.
Subatomic particles, it seems, are not really at all like anything we're familiar with. Electrons, for instance, sometimes behave like waves, destructively interfering with one another. And the exact position and momentum of an electron cannot be specified at any given instant.
According to the Copenhagen interpretation, that's because electrons and all other subatomic particles actually exist in multiple places at once, and can have multiple, contradictory properties at the same time. It is only when a particle's properties are measured, said Bohr, that the "wavefunction" of possibilities collapses into a specific state.
The Copenhagen interpretation is the most widely accepted interpretation of quantum mechanics. It can be used to predict – with astonishing accuracy – the behavior of the most fundamental constituents of matter.
But Schrödinger wasn't a fan. Little things like electrons, he thought, ought to follow the same rules as big things like billiard balls, planets, galaxies, physicists, and cats.
To express his displeasure at Bohr's quantum capriciousness, in 1935 Schrödinger devised a thought experiment by which the seemingly indeterminate state of a relatively little thing – in this case, the product of radioactive decay – can influence the seemingly understandable state of a relatively big thing, in this case a potentially unfortunate cat.
Imagine, wrote Schrödinger, a cat sealed in a box for one hour with a diabolical device consisting of a tiny radioactive substance, a Geiger counter, a hammer, and a flask of hydrocyanic acid. When the Geiger counter detects a high-speed electron flung from the radioactive material, it triggers the hammer, which smashes the flask, releasing the hydrogen cyanide and quickly causing all nine of the cat's lives to succumb to hypoxia.
But there's only a 50 percent chance that, in the space of sixty minutes, the substance will decay and trigger the feline-killing mechanism. There's a 50 percent chance that it won't.
So when our sadistic experimenter opens the box after one hour, there is a chance that he'd get a dead cat (along with what cat-fanciers might argue is a well-deserved whiff of cyanide). But there's an equally good chance that he'd get a living cat.
The fate of the cat all hinges on the behavior of an electron, which, according to the Copenhagen interpretation, can simultaneously exist in multiple contradictory states – both striking the Geiger counter and not striking it – until it is measured by an observer and the wavefunction collapses into one state or another.
Physicists might be comfortable with the concept of an unobserved electron smeared out into a cloud of probabilistic fuzziness, but Schrödinger's cat-murdering apparatus amplifies this subatomic indeterminacy all the way up to the cat's continuing metabolic processes, or lack thereof.
How do you feel, asked Schrödinger's thought-experiment, rhetorically, about not just an electron that in one state or another until it is observed, but about an entire cat that is simultaneously alive and dead until you observe it?
Unlike Schrödinger, you might also want to ask how the cat feels. Even if you buy into the Copenhagen interpretation, presumably it wouldn't seem at all unusual to be Schrödinger's cat. You would sit there in the box, licking your paws or maybe scratching your ear with your hind foot, blithely unaware of the nasty chemicals nearby. Either that or you'd be dead and wouldn't have a point of view at all.
That's because the act of observation happens at different times for different observers. Think of it this way: Suppose that the laboratory containing the cat-in-the-box is itself sealed for two hours. After one hour our sadistic experimenter opens the box to collapse the cat's wavefunction and observes the cat as either living or dead. Then he has to wait another hour for the door of the lab to unlock so he can tell the colleagues the result of his experiment.
As far as the colleagues are concerned, the cat is in an indeterminate state for two hours. The amplification of the electron's quantum state continues: For the world outside the lab, the wavefunction consists not just of the electron, the diabolical relay, and the cat, but now also the lab and the experimenter.
And now let's suppose that the entire building in which the lab is housed is sealed for three hours. To the clamoring media gaggle on the lawn waiting to hear of the cat's fate, the cat is both living and dead for all that time and all the scientists inside the building are simultaneously attending and not attending the cat's funeral service.
And so on, all the way up to the whole universe. Which raises an odd philosophical question: Did the entire universe exist in multiple contradictory states prior to the evolution of the first observer?