The Copenhagen interpretation says that before a wave function is pinned down by a measurement, things that you might potentially measure, like position or energy state, can take many possible values. Some values may be more probable than others, as determined by the details of the wave function, but each remains possible. As we have previously discussed, there is no single position for a traveling electron, only a range of probabilities.
This idea was hard for many scientists to accept, even some who were instrumental in the development of quantum physics. Among them was Erwin Schroedinger, who was so bemused that he devised a way to bring the paradoxes of the Copenhagen interpretation into the macroscopic world for all to see.
You may have already heard of his famous Gedankenexperiment, called “Schroedinger’s Cat.” The idea is to imagine a quantum system that has an equal probability of being in either of two states, then to imagine a way that the actual state can exhibit macroscopic effects. For his quantum system, Schroedinger selected radioactive decay: quantum physics does not predict exactly when a given radioactive atom will decay; it merely gives us the probability that it will decay during a certain period of time. This was useful to Schroedinger since detecting an atom’s radioactive decay is straightforward enough with a macroscopic gizmo called a Geiger counter. Next, he imagined the following:
“Confined in a steel chamber is a Geiger counter prepared with a tiny amount of [radioactive] uranium, so small that in the next hour it is just as probable to expect one atomic decay as none. An amplified relay provides that the first atomic decay shatters a small bottle of [cyanide]. This and—cruelly—a cat is also trapped in the steel chamber. According to the wave function for the total system, after an hour … the living and dead cat are smeared out in equal measure.”
We want to stress right from the start that this was just a thought experiment; it was not performed. No living animals were harmed, domesticated felines or otherwise. There is no evidence that Schroedinger even owned a real cat, so please don’t worry.
QUANTUM LEAP
The preceding quotation of Schroedinger was taken from a letter he wrote to Albert Einstein in August 1935. Schroedinger’s cat was actually the last of a series of progressively more sophisticated thought experiments intended to extend microscopic quantum effects into the macroscopic world. It was actually based in large part on a pair of thought experiments proposed by Einstein, called “Einstein’s boxes” and the “gunpowder experiment.”
Let’s examine what he meant by “smeared out in equal measure.” The box is completely closed, soundproof, and opaque, so that during the hour there is no way to tell from the outside whether the decay has happened or not, and thus no way for anyone to know if the cat is alive or dead. At the moment the box is sealed and the clock started, the probability that the decay has occurred is zero, so the cat is certainly alive. But as time passes, the probability that the decay has happened gradually grows. At the 60-minute mark, the probability has reached 50 percent. Thus at the end of the hour, if the box remains closed, it is equally likely that the cat is alive or dead.
According to the Copenhagen interpretation, when Schroedinger’s box is sealed, the cat is certainly alive. An hour later, the cat is in a 50-50 superposition of “alive” and “dead” states. The moment we peek inside the box, the cat’s wave function collapses to either “alive” or “dead.”
According to the Copenhagen interpretation, this unfortunate animal is neither alive nor dead, but simultaneously in both states while it is inside the box. Only when the box is opened and someone looks inside does the cat’s wave function collapse into one state or the other. We have a very hard time imagining what a cat is like when it is both alive and dead, and no idea at all what that would feel like to the cat! For Schroedinger and others, the absurdity of this situation highlighted a flaw within this interpretation of quantum physics.
But is there really a flaw? Maybe the only issue is figuring out when the wave function collapse actually occurs. If the critical moment is the observation of the cat by a conscious observer, like a person, then the cat is both alive and dead before the box is opened. On the other hand, the critical moment could be when the Geiger counter detects the radioactive decay. If we define this as “the measurement,” then there is never a weird semi-alive cat. The radioactive atom’s wave function collapses when the decay is detected, but from that moment on, the cat is definitely 100 percent dead.
Even among those who accept the Copenhagen interpretation, there is not a complete agreement on the issue of whether or not a conscious observer is required to trigger the collapse of a wave function. Since we who are interested in this question are all conscious beings ourselves, it is difficult to devise a real experiment that distinguishes these two different ideas regarding the nature of measurement. (We will explore the role of consciousness a little.)
One thing that followers of the Copenhagen school do agree upon is that it’s not necessary to have a classical picture of unobserved quantum states. The purpose of quantum physics is to account for the results of measurements, and it does a fine job of that no matter what state the cat is in when we can’t observe it. This is a general feature of the Copenhagen interpretation: quantum physics is not required to say anything about states which are not observed or measurements that are not made.
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