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 equation of the wave function, and 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 picture below. Which does not make sense. As we know, as is obvious, a thing is always one way or the other.
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.
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 Superworld Solution
The key point is that Schrödinger, before he makes the observation, is in both these versions of the ordinary world. His superworld includes both 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.
This diagram symbolises the class of all the worlds in Schrödinger’s superworld. The two lines stand for specific lifelines in specific worlds.
The diagram represents the class of worlds where the cat dies on the left, and the class of worlds where it lives on the right. 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, namely the class of worlds in the superworld. I believe this is exactly what he meant. As he states in emphasis, the mind is wholly present in an entire continuum of ordinary worlds:
One should now think of one’s mind as being wholly present at each of the points on a line laying at right angles to any given point on the time axis, where these points represent simultaneous maximal experiences. (1996, 183)
The “points on the line at right angles to the time axis” are the parallel realities of worlds in this universe. The “maximal experience” is another term he uses for the conscious point of view, the world hologram.
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 ordinary 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 cat really is alive and dead at the same time, and it still is afterwards.
The next section is Probability.