Six Impossible Things: The Mystery of the Quantum World

by John Gribbin

According to the CI that I was taught as a student, and that too many students are still taught today, as “the” way to “understand” quantum mechanics, an electron is emitted from a source—an electron gun—on one side of the experiment as a particle. It immediately dissolves into a “probability wave” that spreads through the experiment and heads toward the detector screen on the other side. This wave passes through however many holes are open, interfering with itself, or not, as appropriate, and arrives at the detector as a pattern of probabilities, higher in some places and lower in others, spread across the screen. At that instant, the wave “collapses” and turns back into a particle, whose position on the screen is chosen at random, but in accordance with the probabilities. This is called “the collapse of the wave function.” The electron travels as a wave but arrives as a particle. The wave, however, carries more than just probabilities. If the quantum entity has a choice of states it can be in, such as an electron that may be spin up or spin down, both states are somehow included in the wave function, the situation called a “superposition of states,” and the state the entity settles into at the point of detection, or interaction with another entity, is also determined at the moment the wave function collapses. In a lecture at the University of St Andrews in 1955, Werner Heisenberg said “the transition from the ‘possible’ to the ‘actual’ takes place during the act of obser...