What did Schrodinger's Cat experiment prove?

Category: Physics
Published: July 30, 2013

"Schrodinger's Cat" was not a real experiment and therefore did not scientifically prove anything. Schrodinger's Cat is not even part of any scientific theory. Schrodinger's Cat was simply a teaching tool that Schrodinger used to illustrate how some people were misinterpreting quantum theory. Schrodinger constructed his imaginary experiment with the cat to demonstrate that simple misinterpretations of quantum theory can lead to absurd results which do not match the real world. Unfortunately, many popularizers of science in our day have embraced the absurdity of Schrodinger's Cat and claim that this is how the world really works.

Public domain image, source: CDC.

In quantum theory, quantum particles can exist in a superposition of states at the same time and collapse down to a single state upon interaction with other particles. Some scientists at the time that quantum theory was being developed (1930's) drifted from science into the realm of philosophy, and stated that quantum particles only collapse to a single state when viewed by a conscious observer. Schrodinger found this concept absurd and devised his thought experiment to make plain the absurd yet logical outcome of such claims.

In Schrodinger's imaginary experiment, you place a cat in a box with a tiny bit of radioactive substance. When the radioactive substance decays, it triggers a Geiger counter which causes a poison or explosion to be released that kills the cat. Now, the decay of the radioactive substance is governed by the laws of quantum mechanics. This means that the atom starts in a combined state of "going to decay" and "not going to decay". If we apply the observer-driven idea to this case, there is no conscious observer present (everything is in a sealed box), so the whole system stays as a combination of the two possibilities. The cat ends up both dead and alive at the same time. Because the existence of a cat that is both dead and alive at the same time is absurd and does not happen in the real world, this thought experiment shows that wavefunction collapses are not just driven by conscious observers.

Einstein saw the same problem with the observer-driven idea and congratulated Schrodinger for his clever illustration, saying, "this interpretation is, however, refuted, most elegantly by your system of radioactive atom + Geiger counter + amplifier + charge of gun powder + cat in a box, in which the psi-function of the system contains the cat both alive and blown to bits. Is the state of the cat to be created only when a physicist investigates the situation at some definite time?"

Since that time, there has been ample evidence that wavefunction collapse is not driven by conscious observers alone. In fact, every interaction a quantum particle makes can collapse its state. Careful analysis reveals that the Schrodinger Cat "experiment" would play out in the real world as follows: as soon as the radioactive atom interacts with the Geiger counter, it collapses from its non-decayed/decayed state into one definite state. The Geiger counter gets definitely triggered and the cat gets definitely killed. Or the Geiger counter gets definitely not triggered and the cat is definitely alive. But both don't happen.

Roger Penrose, a Nobel Prize winner and one of the most brilliant physicists of the last sixty years, wrote about Schrodinger's Cat in his book The Road to Reality as follows: "So the cat is both dead and alive at the same time! Of course such a situation is an absurdity for the behavior of a cat-sized object in the actual physical world as we experience it... There is a 50% chance that the cat will be [definitely] killed and a 50% chance that it will [definitely] remain alive. This is the physically correct answer, where 'physically' refers to the behavior of the world that we actually experience." Penrose goes on to explain that any physical theory or philosophical interpretation of quantum physics that leads to the cat being both dead and alive at the same time must be a faulty theory or interpretation, because that is not what happens in the real world.

Despite that fact that the Schrodinger's Cat story is not a real experiment, does not prove anything, does not match physical reality, and was intentionally designed to be absurd, this line of thinking does indeed lead to a meaningful question. Why does the cat in this setup not end up in a state of dead and alive at the same time in the real world? In other words, why does a measurement collapse a quantum object from a superposition of states to a single definite state? This question has not yet been fully answered by quantum physics. This is known as the "measurement problem" of quantum physics. Note that the measurement of a quantum object in a superposition of states collapsing down the object to a definite single state is very well predicted by the mathematics of quantum physics. Therefore, the "measurement problem" is more a problem of philosophical interpretation and incomplete scientific explanation, than a problem of the theory being incorrect.

In summary, quantum state collapse is not driven just by conscious observers. Unfortunately, many popular science writers in our day continue to propagate the misconception that a quantum state (and therefore reality itself) is determined by conscious observers. They use this erroneous claim as a springboard into unsubstantial and non-scientific discussions about the nature of reality, consciousness, and even Eastern mysticism. To them, Schrodinger's Cat is not an embarrassing indication that their claims are wrong, but proof that the world is as absurd as they claim. Such authors either misunderstand Schrodinger's Cat, or intentionally misrepresent it to sell books.

Christopher S. Baird is a physics professor at West Texas A&M University and author of the book, The Top 50 Science Questions with Surprising Answers.

Topics: Schrodinger's Cat, observation, quantum, superposition, wavefunction collapse