In 1801, Thomas Young (1773-1829), an English polymath with a doctorate in physics from the University of Gottingen, decided to prove that light was a wave phenomenon, such as sound. He set up an opaque screen, in which two vertical slits were cut and spaced about an inch apart. He directed a beam of light into the slits on one side of the screen, and on the other side he observed the result­ing pattern of light on a piece of white paper.

The experiment was entirely successful, showing an interference pattern of alter­nating light and dark lines projected onto the paper. From this simple setup, there is no doubt that light travels as a wave. The wave front encounters the screen, where it is divided into two sub-waves, which recombine in space and then hit the paper with a predictable interference pattern. The experiment has been rerun innumerable times, using all known types of electromagnetic radiation and even beams of high-speed particles. In 1961, the experiment was run using a beam of electrons, and in 1989, it was run successfully using a single electron aimed at the screen between the two slits. The results are always the same.

In 1913, Niels Bohr confirmed the theories of Planck and Einstein, that light is not a wave but a particle, with his quantum theory of the orbital mechanics of

Bohr modeled the ground energy state of an electron as a sphere of hypothetical altitude above the central nucleus. These ground energies occur in integral steps, of course, and as the ground energy becomes higher, the sphere has a larger simulated diameter and can accommodate a larger number of electrons. It is possible for the first ground state to have as many as two electrons in it. Hydrogen has this ground state, but it contains only one electron, as there is only one positive charge in the hydrogen nucleus to counteract one negative charge. This ground state is only half filled. Helium had two protons in the nucleus and could accom­modate two electrons, so its ground state is filled. Higher ground state energies, or shells, can accommodate eight electrons, 18 electrons, then 32 electrons, and so on as the shells grow larger around the nucleus.

With Bohr’s orderly arrangement of electrons in an atom, the physi­cal meaning of the periodic table of the elements, as invented in 1869 by

atomic electrons. Furthermore, Arthur Compton (1892-1962), head of the physics department at Washington University in St. Louis, proved in 1922 that photons lose energy when they crash into something solid and exchange momentum with elec­trons. Waves do not do that. Only solid particles exchange momentum and scatter like balls on a billiard table. From Compton’s experiments, there is no doubt that light travels as particles. In the single-electron experiment of 1989, however, an electron, which is a particle, apparently divided in half and interfered with itself on the other side of the slit screen.

Scientists found, and still find, these experimental results profoundly baffling. Light can be a particle or light can be a wave, but it cannot be both. Set up both experiments, in tandem. First, divide the wave with the double slits, and then determine that the light coming out of one of the slits is a particle. When the light is detected as a particle, the interference pattern from the two slits disappears. The light can be determined to be a particle or a wave, but not both simultane­ously, and the nature of the measurement determines what it shall be. It seems as if light knows how it is being perceived and adjusts its identity according to the experiment.

Niels Bohr summed up the physical meaning of these findings: "Nothing exists until it is measured.” Quantum mechanics is considered counterintuitive, or outside normal perceptions of reality, for these and other reasons.

The double-slit experiment, showing a most vexing paradox of quantum mechanics: Light can be a particle or a wave, depending on how the experiment is set up. (GIPhotoStock/Photo Researchers, Inc.)

the Russian chemist Dmitry Mendeleev (1834-1907), became clear. Alkali metals, such as lithium, sodium, and potassium, are arranged in a column in the periodic table, as they all seem to have a similar chemical charac­teristic. They have this common characteristic because all elements in this column have a single electron in the outermost energy shell. It is the outer shell that determines an elements chemical interaction, and any lower ground state electrons do not contribute at all to compounding. The inert gases, such as helium, neon, argon, and krypton, all have a completely filled outer shell. With no unfilled electron stations in this uppermost ground state, there is no way one of these gases can bind with another element. Bohr went so far as to predict that when element number 72 was discovered, it would have four electrons in its outer shell and therefore behave chemically like zirconium.