Schrödinger’s Cat

Steve discusses the multiple theories of Quantum Mechanics


By Steve Humphrey


“Schrödinger’s Cat” is a thought experiment invented by the German physicist Erwin Schrödinger in 1935 as a response to the then-standard interpretation of Quantum Mechanics (QM) and was intended to point out an absurdity inherent in that view. Imagine a box in which a small sample of a radioactive substance has been put, this substance having a half-life of 30 minutes (meaning that after half an hour, there is a 50/50 chance that it will have decayed.) Imagine a Geiger counter placed in the box that will detect any decay product, which is connected to a hammer that smashes a vial of poisonous gas upon a positive detection by the Geiger counter. Finally, imagine a cat being put into the box and a lid on top. Now, after half an hour, what is in the box? According to QM, the radioactive substance will be in a superposition of having decayed and not decayed. (Remember from my discussion of the Double-Slit Experiment that an electron shot toward the slits does not follow a determinate trajectory through one slit or the other but remains in a superposition of position states until reaching the detector behind the slits). That is, it doesn’t pass through slit A, or slit B, or both slits, or neither slit. A superposition is a state unique to QM and applies to many properties other than position. Since the rest of the box’s contents are also made of quantum “stuff,” this superposition should continue up the ladder. The Geiger counter will be in a superposition of having detected and not detected a decay product, and so on. Following this chain to its conclusion, the box should contain a cat that is in a superposition of being dead and alive. But when the box is opened, we see either a live cat or a dead one. (No actual cats were harmed during the performance of the experiment, and I don’t know what Schrödinger’s attitude toward cats was.)

In jargon, we say that the “wave function” describing the state of the cat has “collapsed” from a superposition of states into a real, determinate state (an “eigenstate”). Several questions naturally arise. Is the wave function a real, physical thing, or just a piece of mathematics used to calculate outcomes? Is the collapse of the wave function a real, physical process or simply a description of our transition from ignorance of the state to definite knowledge of the state? It is tempting to dismiss all this as mathematical trickeration, but what is the point of a theory of physics if it is not to describe some aspect of the physical world? Leaving aside those who think we should simply “shut up and calculate,” many theorists take the wave function quite seriously. But if it is real and collapse is real, this presents more challenges. These challenges fall under the name “The Measurement Problem,” the search for a solution to which occupies the intellectual energies of many philosophers and philosophically minded physicists. 

For example, when does the wave function collapse? When we open the box and look inside? What is it about opening the box and making an observation that causes this collapse? Eugene Wigner suggested that interaction with consciousness collapses the wave function. (I call this the “Multiplication of Mysteries.” We don’t understand QM, and we don’t understand consciousness. Maybe they are related.) But how conscious does something have to be to effect collapse? Could an earthworm do it? A cat? An undergraduate? If the student looks into the box, then before she reports the result to the professor, she might be in a superposition of having seen a live cat and a dead cat. 

Perhaps it is not the observation that collapses the wave function but something else that happens before a measurement is made. Maybe it is the interaction of the quantum system with the environment, with air molecules or light rays or gravity, that collapses the wave function very early in the process, preventing the sequence of superpositions. (This view is called “Environmental Decoherence” and is quite popular.) Or maybe as a quantum system becomes larger and more complex, some natural phenomenon causes collapse. (The GRW interpretation is of this sort.) 

One of the problems with deciding upon a solution to the Measurement Problem is that there is apparently no way to confirm it. We can’t catch a quantum system in the act of collapse. All we can actually observe are eigenstates and never superpositions.

The most interesting and provocative proposal is the Everett-Wheeler Many Worlds interpretation. Hugh Everett’s dissertation for John Wheeler suggested that collapse doesn’t happen at all. Instead, the Universe splits into identical copies, and in each one, a different element of the superposition is real. So, in the case of the cat, when we open the box, the Universe splits into two new Universes, exactly alike, except that in one, the cat is alive and in the other the cat is dead, and these Universes continue to evolve independently. (A lot of science fiction is based on this scenario. The recently released movie “Everything Everywhere All At Once” is a perfect example. One of the characters in the film uses the term “superposition.”) This view has the advantage of taking the formalism of the theory literally. (I could demonstrate this, but there is an old saw in science writing: for every equation you include, you lose half your readers.) As bizarre as this view appears, many perfectly sane physicists are adherents.

This concludes my discussion of Quantum Mechanics and its mysteries. The main takeaway is that this is a sound, highly confirmed theory of physics, which is also the basis of a vast amount of modern technology. And yet what it seems to be telling us about the ultimate nature of reality is so counterintuitive that it renders the theory unbelievable. Thus, we have a genuine conflict between our ordinary worldview and a remarkably successful theory. I hope you find that as fascinating as I do!

Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics.
Questions? Comments? Suggestions? 
Email him at steve@thevoicelouisville.com