What Is Schrodinger's Cat?

Understand Schrodinger's Cat Thought Experiment

Schrodinger's cat is in a box in a situation where it's both dead and alive, yet becomes one or the other once you open the box to observe the cat.
Schrodinger's cat is in a box in a situation where it's both dead and alive, yet becomes one or the other once you open the box to observe the cat. Andrzej Wojcicki, Getty Images

Question: What is Schrodinger's Cat?

Answer: Erwin Schrodinger was one of the key figures in quantum physics, even before his famous "Schrodinger's Cat" thought experiment. He had created the quantum wave function, which was now the defining equation of motion in the universe, but the problem is that it expressed all motion in the form of a series of probabilities -- something which goes in direct violation to how most scientists of the day (and possibly even today) like to believe about how physical reality operates.

Schrodinger himself was one such scientist and he came up with the concept of Schrodinger's Cat to illustrate the issues with quantum physics. Let's consider the issues, then, and see how Schrodinger sought to illustrate them through analogy.

Quantum Indeterminancy

The quantum wave function portrays all physical quantities as a series of quantum states along with a probability of a system being in a given state. Consider a single radioactive atom with a half-life of one hour.

According to the quantum physics wave function, after one hour the radioactive atom will be in a state where it is both decayed and not-decayed. Once a measurement of the atom is made, the wave function will collapse into one state, but until then, it will remain as a superposition of the two quantum states.

This is a key aspect of the Copenhagen interpretation of quantum physics - it's not just that the scientist doesn't know which state it's in, but it's rather that the physical reality is not determined until the act of measurement takes place.

In some unknown way, the very act of observation is what solidifies the situation into one state or another ... until that observation takes place, the physical reality is split between all possibilities.

On To The Cat

Schrodinger extended this by proposing that a hypothetical cat be placed in a hypothetical box.

In the box with the cat we would place a vial of poison gas, which would instantly kill the cat. The vial is hooked up to an apparatus which is wired into a Geiger counter, a device used to detect radiation. The aforementioned radioactive atom is placed near the Geiger counter and left there for exactly one hour.

If the atom decays, then the Geiger counter will detect the radiation, break the vial, and kill the cat. If the atom does not decay, then the vial will be intact and the cat will be alive.

After the one-hour period, the atom is in a state where it is both decayed and not-decayed. However, given how we've constructed the situation, this means that the vial is both broken and not-broken and, ultimately, according to the Copenhagen interpretation of quantum physics the cat is both dead and alive.

Interpretations of Schrodinger's Cat

Stephen Hawking is famously quoted as saying "When I hear about Schrodinger's cat, I reach for my gun." This represents the thoughts of many physicists, because there are several aspects the thought experiment that bring up issues. The biggest problem with the analogy is that quantum physics typically only operates on the microscopic scale of atoms and subatomic particles, not on the macroscopic scale of cats and poison vials.

The Copenhagen interpretation states that the act of measuring something causes the quantum wave function to collapse. In this analogy, really, the act of measurement takes place by the Geiger counter. There are scores of interactions along the chain of events - it is impossible to isolate the cat or the separate portions of the system so that it is truly quantum mechanical in nature.

By the time the cat itself enters the equation, the measurement has already been made ... a thousand times over, measurements have been made - by the atoms of the Geiger counter, the vial-breaking apparatus, the vial, the poison gas, and the cat itself. Even the atoms of the box are making "measurements" when you consider that if the cat falls over dead, it will come in contact with different atoms than if it paces anxiously around the box.

Whether or not the scientist opens the box is irrelevant, the cat is either alive or dead, not a superposition of the two states.

Still, in some strict views of the Copenhagen interpretation, it is actually an observation by a conscious entity which is required. This strict form of the interpretation is generally the minority view among physicists today, although there remains some intriguing argument that the collapse of the quantum wavefunctions may be linked to consciousness. (For a more thorough discussion of the role of consciousness in quantum physics, I suggest Quantum Enigma: Physics Encounters Consciousness by Bruce Rosenblum & Fred Kuttner.)

Still another interpretation is the Many Worlds Interpretation (MWI) of quantum physics, which proposes that the situation actually branches off into many worlds. In some of these worlds the cat will be dead upon opening the box, in others the cat will be alive. While fascinating to the public, and certainly to science fiction authors, the Many Worlds Interpretation is also a minority view among physicists, though there is no specific evidence for or against it.

Edited by Anne Marie Helmenstine, Ph.D.