The Amazing Story of Quantum Mechanics by James Kakalios Page A

angle away from the sun, results in a circular trajectory (the actual orbit is an ellipse—a distorted circle). The stable orbits of the planets in our solar system are possible only through the continual falling toward the sun. The Earth maintains its orbital motion, as there is nothing to slow it down (collisions with space particles provide a very small frictional drag that we can neglect), while the conditions that led to the Earth’s original velocity about the sun are, as they say, a subject of current research.
    As the force of electrical attraction between the electrons and protons in an atom is mathematically similar to the attractive force of gravity, a completely analogous argument suggests that the electrons should move in circular or elliptical orbits about the nucleus, not unlike the way the planets orbit the sun in our solar system. The main difference is that planets do not emit energy as they orbit the sun, but orbiting electrons do lose energy—in fact, quantum mechanics was developed in part to explain why all atoms don’t suffer a death spiral to oblivion.
    It was known from the days of the American Civil War that whenever an electric charge changes its direction, as in an elliptical orbit about an atomic nucleus, it emits electromagnetic radiation—that is, light. Since light carries energy, the electrons should lose energy as they orbit, and the slower they move, the less they are able to resist the attractive pull of the positively charged protons in the nucleus. In a short time (actually, less than a trillionth of a second), they should spiral into the nucleus. However, atoms form chemical bonds with other atoms, by which materials such as table salt, sand, and DNA are possible. The chemical bonds holding molecules and solids together involve interactions of the orbiting electrons among neighboring atoms, which would not be possible if the electrons were sitting on the nuclei. Something in the picture had to be wrong. If accelerated electric charges did not emit electromagnetic radiation, then radio and TV would not be possible. 21
    An important step in reconciling this puzzle was Niels Bohr’s suggestion in 1913 that the electrons in an atom can assume only particular trajectories about the nucleus. That is, only certain planetary-like orbits are allowed. Electrons can jump from one orbit to another, but they may not follow any arbitrary path around the nucleus. An analogy: The city of Minneapolis in Minnesota contains a series of lakes that can be circumnavigated by paved paths. There are several paths encircling each lake, one intended for pedestrians, another for bicyclists, and a third for automobiles, and each pathway is separated from the others by a grassy median. Bohr’s electronic orbits were analogous to these pathways, where electrons were free to travel but were forbidden from walking on the grass, as shown in Figure 12. The closer the orbit was to the nucleus, the more tightly bound the electron would be, so that more energy would have to be supplied to remove an inner-orbit electron than would be required to remove one from an outer ring. The electron could jump from an outer pathway to an inner loop, with the emission of the appropriate amount of energy, say by emitting light. Alternatively, by absorbing just the right amount of energy, it could be promoted from an inner orbit to a higher-energy, outer orbit (provided that the path had an open, available space for the electron). Bohr proposed that, for some reason that he could not explain, the electron would not emit light during its orbit, despite the requirements of Maxwell’s equation for a charge that is constantly changing direction, but rather give off light only when moving from one orbit to another.
    Bohr’s proposal that only certain discrete orbits were possible was an attempt to account for the spectrum of light emitted by different atoms. Why are neon signs red, while the light from sodium lamps has a yellow tint?