# Special day for physicists' cats

-August 12, 2013

When we graduate from college or graduate school with a degree in physics, we are required to sign an oath that if we ever get cats, at least one of them must be named Schrodinger.

Even physicist characters on TV have a cat named Schrodinger. Fans of StarGate SG1 will recall episode 116, where Samantha Carter introduces her cat, named Schrodinger, to Narim. She uses this as a way of starting a discussion on quantum mechanics, which he describes as Kulivrian Physics, in his world.

I have a cat name Schrodinger, who manages to get into more trouble than most cats and, I suspect, has borrowed a few extra lives from his brother, Maxwell. He also tends to be too critical. The figure below shows Schrodinger’s commentary on the first draft of my science fiction novel, Shadow Engineer. This was the last time I asked his opinion.

Figure 1. My cat Schrödinger’s reaction to the first draft of my science fiction novel Shadow Engineer.

The reason cats named Schrodinger are of special interest today is because 125 years ago today, on August 12, 1887, Erwin Schrodinger was born. Even the Google Doodle is honoring Schrodinger today.

Schrödinger’s famous story about a cat goes like this. Put a cat in a box with one radioactive atom, a detector, a hammer and a vial of poisonous gas. When the atom decays, the detector detects it and triggers the hammer to fall on the deadly vial.

Before you open the box, the cat is either dead or alive. In one interpretation of quantum mechanics, the nucleus of the atom is in a superposition state of partially stable and partially decayed. The cat, then, is in a superposition state of being partially dead and partially alive. By opening up the lid of the box, you force the atom to be in either the stable or the decayed state. Likewise, you force the cat to be either dead or alive. Did you kill the cat by opening the lid?

Schrodinger, though one of the great fathers of quantum mechanics, was on the skeptic side of the fence, along with Neils Bohr and Albert Einstein who questioned the probabilistic interpretation of the wave function.

He created this bizarre thought experiment to illustrate how absurd the interpretation of the wave function was in describing the superposition of multiple outcomes. How could the cat exist in both an alive and a dead state? And how did the act of opening the lid, observing, collapse the wave function into a definite state? His camp did not believe in the probabilistic interpretation of quantum mechanics, which is what prompted Einstein to famously say, “I do not believe that God plays dice with the Universe.”

Schrodinger is most famous for what we call today the Schrödinger equation, which he published in 1926. He won the Nobel Prize in Physics in 1933 for establishing this equation, one of the pillars of modern physics. This equation launched the field of wave mechanics and describes the dynamic properties of the wave function, that the time derivative of the wave function equals the total energy of a particle. Leveraging the ideas of Louis DeBroglie, who connected the momentum of a particle with the wavelength of the particle, Schrödinger expanded the interpretation of the energy of a particle in terms of the Laplacian of the wave function.

To this day, every physicist when first learning about quantum mechanics asks the question, “but what is the thing that waves in the wave function?” It is still uncomfortable to think of the thing that waves as a “probability amplitude” that has a phase. The wave function is not a probability, the square of the wave’s amplitude is actually the probability of finding an object somewhere. So what does that make the thing that waves? And how does the process of observing actually collapse the probability into a definite state?

As bizarre as the interpretation of the wave function is, which Schrödinger never felt comfortable with, in every experiment that has been performed, the interpretation of the wave function as a probability amplitude matches the measurements to as many digits as can be measured.

But it doesn’t mean we should not still question the interpretation. In honor of Schrödinger, today is a good day to rethink what is it that waves, or at least to pet your cat.

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