Quantum mechanics is the most successful scientific theory ever devised. It can accurately predict the outcome of sophisticated experiments to many decimal places. But trying to make sense of it outside of mathematics leads to bizarre conclusions, including the puzzling ideas that an atom can be in two places at once and that reality does not exist until we observe it.
Richard Feynman, perhaps the most brilliant of all the 20th Century physicists, dismissed the problem altogether. Don't bother trying to understand it, he said. " Nobody understands quantum theory."
John Gribbin, a British science writer who has written many informative books, won't take no for an answer. Ten years ago, he wrote "In Search of Schrodinger's Cat" about the paradoxes of quantum theory. In that book, he agreed with Feynman.
Now he returns to the subject with "Schrodinger's Kittens and the Search for Reality," and he claims to be able to make sense of it all. The problem, Gribbin says, is that the Copenhagen Interpretation, the one that has created all of the trouble, is wrong.
The "cats" and "kittens" in Gribbin's titles refer to a little story that captures the problem in all its glory:
Step 1: Suppose there is a sealed box containing one electron. According to the standard interpretation, if nobody opens the box and looks inside, there is an equal probability of finding the electron anywhere inside the box. It is only after somebody takes off the top and peeks inside that the electron's "probability wave" collapses and the electron becomes real and actually occupies a specific place.
Step 2: Now suppose that the box is equipped with a sliding door that splits it into two halves. You might think that the electron is in either one half or the other half. Not so, says the standard interpretation. Until somebody looks, its "probability wave" still fills the whole box, and there is an equal chance of finding the electron on either side of the partition.
Step 3: We put this little box into a sealed room, along with a cat. The room is equipped with an electron detector programmed to release lethal gas into the room if it senses an electron. Without looking inside the box, we open one half of it. If the electron is in that half, it escapes into the room, triggering the electron detector, which releases the gas, killing the cat.
But until somebody looks into the box, the electron is neither in one half nor in the other. Gribbin adds a new twist: Suppose we have two kittens, and we put them into two spaceships that are attached by a tube, in the middle of which is a box containing one electron and a partition across the middle. Both spaceships are attached to electron detectors, which release poison gas if they find an electron. We sever the tube down the middle, and we send the two spaceships to the opposite ends of the universe. What happens to those kittens?
Each of them is in the same dead / not dead limbo as described above.
One of the spaceships lands on an inhabited planet, and the people there open it up to see what's inside. At that moment, the "probability wave" of the electron collapses, and the electron winds up either in that spaceship, killing the cat, or in the other one, killing that cat.
Problem: How does the second spaceship instantaneously "know" what has happened at the other end of the universe?
Gribbin gives these paradoxes to show that the Copenhagen Interpretation cannot possibly be right. He offers instead the Cramer Interpretation, proposed by physicist John Cramer.
Alas, I've run out of space and cannot explain it.