Decoding the Universe by Charles Seife

Author:Charles Seife
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2010-03-01T05:00:00+00:00

Einstein’s theory brought Planck’s quantum hypothesis into the mainstream, and over the next three decades Europe’s best physicists developed a theory that did a beautiful job of explaining the behavior of the subatomic world. Werner Heisenberg, Erwin Schrödinger, Niels Bohr, Max Born, Paul Dirac, Albert Einstein, and others built up a set of equations that explained with stunning precision the behavior of light and electrons and atoms and other very tiny objects.3 Unfortunately, though this framework of equations—quantum theory—always seemed to get the right answers, other consequences of those equations seemed to contradict common sense.

The dictates of quantum theory, at first glance, are ridiculous. The strange, seemingly contradictory properties of light are par for the course. Indeed, they come directly from the mathematics of quantum theory. Light behaves like a particle under some conditions and like a wave under other conditions; it has some of the properties of each, yet is neither truly particle nor wave.

It is not only light that behaves this way. In 1924, the French physicist Louis de Broglie suggested that subatomic matter—particles like electrons—should have wavelike properties as well. To experimentalists, electrons were obviously particles, not waves; any half-competent observer could see electrons leave little vapor trails as they streaked from one end of a cloud chamber to the other. These trails were clearly the tracks of little chunks of matter: particles, not waves. But quantum theory trumps common sense.

Though the effect is much harder to spot with electrons than it is with light, electrons do show wavelike behavior as well as their more familiar particle-like behavior. In 1927, English physicists shot a beam of electrons at a crystal of nickel. As electrons bounce off regularly spaced atoms and zoom through the holes in an atomic lattice, they behave as if they have just passed through the slits of Young’s experiment. Electrons do interfere with each other, making an interference pattern. Even if you ensure that only a single electron at a time strikes the lattice, the interference pattern persists; the pattern cannot be caused by electrons bouncing off each other. This behavior is not consistent with what you would expect of particles: the interference pattern is an unmistakable sign of a smooth, continuous wave, rather than discrete, solid particles. Somehow, electrons, like light, have both wavelike and particle-like behavior, even though the properties of waves and of particles are mutually contradictory.

This twofold wave-particle nature is true of atoms and even molecules just as it is true of electrons and light. Quantum objects can behave like waves as well as particles; they have wavelike properties and particle-like properties. At the same time, they have properties inconsistent with being a wave and with being a particle. An electron, a photon, and an atom are both particle and wave, and neither particle nor wave. If you set up an experiment to determine whether a quantum object is a particle, 1, or a wave, 0, you will get a 1 sometimes or a 0 sometimes, depending on the experiment’s setup.

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