Now You See Me

In his book on quantum physics and reality, Schrödinger's Cat, John Gribben observes that,

"Very few things in physics 'care' which way time flows, and it is one of the fundamental puzzles of the universe we live in that there should indeed be an 'arrow of time,' a distinction between past and future."

In the quantum world, the very act of observing a thing changes it, and that we, as observers, are in a very real sense part of the experiment. We can look at an atom and see an electron in energy state A, then look again and see an electron in energy state B. We guess that the electron jumped from A to B, perhaps because we looked at it. In fact, we cannot say for sure that this is even the same electron, and we cannot make any statement about what it was doing when we were not looking at it, what Schrödinger referred to as that 'damned quantum jumping.'

Equations tell us nothing about what particles do when we do not look at them. If we cannot say what a particle does when we are not looking at it, neither can we say if it exists when we are not looking at it, and it is therefore reasonable to extend the observation to state that nuclei and positrons did not exist before 1900 because no one before that time had seen them.

Gribben adds, "In the quantum world, what you see it what you get, and nothing is real; the best you can hope for is a set of delusions that agree with one another."

The mythical Cat in the book's title was invoked by Schrödinger to make clear the differences between the quantum and everyday worlds. The Cat is enclosed in a box with a phial of radioactive material. In the everyday world there is a fifty-fifty chance that the Cat will be killed by radioactive decay as a result of the phial breaking. Without looking in the box, we can say that the Cat is definitely alive or definitely dead. In the quantum world, neither possibility has any reality unless it is observed. The cat has neither been killed nor not killed, until we look inside the box to see what has happened.

There's an old joke about this. A traffic cop stops a car because one of its brake lights is out. He  asks the driver for his licence, and observes, "Ah... Dr. Schrödinger... I'll need to have a look around your vehicle." A few minutes later he returns. Handing the licence back to the good doctor, he says, "Did you know there's a dead cat in a box in the boot of your car?" Schrödinger shrugs, "Well, there is now."