Schrödinger’s Cat

The famous paradox of Schrödinger’s cat is a thought experiment by the eminent physicist Erwin Schrödinger. He discovered the fundamental mathematics of quantum mechanics, the wave equation or wave function. He was awarded the Nobel Prize in 1933.

He thought what he had discovered could not be true because the implications could not make sense. It was in order to demonstrate the absurdity of the wave function being the basic reality that he described this idea. It means the cat has to be alive and dead at the same time.

Essentially, a cat in a closed box has a fifty-fifty chance of being killed by a quantum device. According to standard quantum mechanics, all possibilities must come true. So the cat is then in a superposed state of dead and alive at the same time, as in the first picture below. Which does not make sense. As we know, as is obvious, in the ordinary world such a thing is always one way or the other.

Schrödinger's cat cartoon with 2 panes. First shows cat alive. Second shows cat alive and dead superposed.
Schrödinger’s cat

Then, according to the physics, when Schrödinger observes the state of the cat, it thereby becomes determinate. And this too of course makes no sense. How can looking at something change what is physically real.

No Collapse

The first step in the unravelling of this paradox is that the cat is indeed both alive and dead, but in different parallel versions of an ordinary world. In the many-worlds universe of Everett’s Many-Worlds theory, both versions of the outcome become real. This was Everett’s solution to the measurement problem. It was a stroke of genius.

Illustration of Schrödinger's cat showing parallel realities in which the cat is dead and alive
Schrödinger’s cat in two parallel realities

This was confirmed by the pioneering physicist Bryce DeWitt. He was the first to take a serious interest in Everett’s work. He challenged Everett on various points but eventually confirmed his theory. As he stated:

Schrödinger felt that the wave mechanics that led to this paradox presented an unacceptable description of reality. However, Everett, Wheeler and Graham’s interpretation of quantum mechanics pictures the cats as inhabiting two simultaneous, noninteracting, but equally real worlds. (1970, 31)

The key point is that Schrödinger, before he makes the observation, is in both these versions of the ordinary world, the parallel worlds.

But how does the strange collapse dynamics actually operate? What exactly changes at the point of observation? The superworld makes the solution crystal clear.

The Superworld Solution

This diagram symbolises the class of all the worlds in Schrödinger’s superworld. The two lines stand for specific lifelines in specific worlds.

The class of lifelines (Lockwood calls them biographies) in the Schrödinger's cat thought experiment. (Lockwood, 1989)
The class of lifelines (Lockwood calls them biographies) in the Schrödinger’s cat thought experiment. (Lockwood, 1989)

The green section at the bottom is before he makes the observation. At this point he only sees the outside of the box. And this means he lives in all the worlds where there is this box with a cat in. Dead in some, alive in others. So in his superworld, the cat is indeed both alive and dead.

Then, as he makes the observation of the state of the cat, say the cat is alive, he exists only in worlds in which the cat lives, the yellow class of worlds. And, of course, a different version of him finds himself in the other class of worlds, the blue one.

This diagram, by Michael Lockwood, makes clear what changes on observation – the class of worlds in the superworld. I believe this is exactly what Lockwood meant. As Lockwood states in emphasis, quoted earlier, the mind is “wholly present” (1996, 183) in an entire continuum of ordinary worlds.

So before the observation, the cat really is alive and dead at the same time. Not however, of course, because there is an ordinary world that is somehow a mish mash of dead and alive. It is because he is in both versions of the world. This resolves the central paradox of quantum theory, the measurement problem. The problem is that the world seems to change when an observation is made. The resolution is that the world itself does not change. What changes is the class of worlds in which the individual exists.

The next section is Probability.