A Physics Digest—Part 7: Atomic and Nuclear Physics

Up to this point my physics digest has taken the classical or Newtonian viewpoint. Now it’s time to pass beyond and enter quantum theory. This is a term that’s been severely misused in recent years, as we’ve heard of everything from quantum Zen to quantum force—whenever someone wants to sound esoteric.

Around the end of the 19th century, experimental physicists were first able to explore elementary particles of matter: electrons, protons, and even smaller entities. The first surprise they met: while energy and radiation at the macroscopic level appear continuous, when you get down to the microscopic level, energy comes in discrete bundles, called quanta. The study of nature’s workings at wee scales is called quantum mechanics. It extends and modifies Newtonian mechanics into this realm.

A second surprise physicists discovered was that you cannot nail down both the speed and position of a tiny particle. If you constrain its location, you lose information on its speed. If you can accurately observe its speed, you’re not really sure just where it is. This vagueness is called Heisenberg’s Uncertainty Principle.

A third surprise: photons of light can behave either as a stream of particles or as a continuous wave. It all depends on how you observe them… literally. Before you observe them they exist only as possibilities. The observer impacts the observation. There is no separate, objective reality at the quantum level.

These findings threw many scientists (including, and especially, Einstein) for a loop. The quantum world is counterintuitive to us macroscopic beings. Physicists are still trying to come to terms with these bizarre results.

These findings also brought about a reconsideration of the structure of the atom. We now understand that the nucleus is surrounded, not by individual electrons in orbit, but by a cloud of electrons in some probabilistic state somewhere between a wave and a particle. Either? Both? Yes.

This uncertain nature of electrons is described by a wave equation—the microscopic world’s equivalent of Newton’s laws—which describes the probabilities of the behavior of electrons and other elementary things. We can only state what’s likely to be—not precisely what is—until we look at it. We then sort of make it come into being. Isn’t that weird?

If we move inward from the atom’s electron probability cloud, we find a lump of positively-charged protons—jammed tightly together in the atom’s nucleus. To keep the electrical repulsive force between them from breaking the nucleus apart, something called the strong (attractive) nuclear force was discovered. But when the nucleus gets rather big—like for uranium—an opposing force called the weak nuclear force comes into play and makes the nucleus rather unstable. Science is still trying to bring the description of these various forces under one umbrella (in the so-called Grand Unified Theory, or GUT).

The counteracting forces in a large nucleus like uranium causes it to be rather unstable and so it will slowly break down, emitting high-energy radiation. A uranium atom decays to the metal lead in some 14-15 steps, passing through nine different elements along the way. In nature this decay is slow—maybe on the order of thousands of years.

Each time a nucleus splits a little mass disappears, becoming transformed into pure energy. Einstein’s famous equation E = mc2 then comes into play. “E” is the energy released, “m” is the tiny bit of mass that is lost, and “c” is the speed of light. In English units c2 is about 2 followed by 20 zeroes, so the energy is large, even though the lost mass is small.

If we gather enough uranium together (far more concentrated than ever occurs in nature) we can reach a critical mass. Now the atoms decay almost instantaneously and achieve the explosive power of the atomic bombs dropped on Hiroshima and Nagasaki. But we can also slow down this process of fission by controlling it with graphite rods, and create a nuclear reactor. The released energy is used to boil water and spin an electrical turbine.

There’s a process even more powerful than the fission (splitting) of uranium atoms: the fusion (merging) of hydrogen atoms. It’s more powerful partly because we can jam together a lot more hydrogen than we can scarce uranium. (Hydrogen is the most plentiful element in the universe.) The hope is that we may soon create a peaceful use for nuclear fusion—in the generation of electricity. Its byproducts are far cleaner than the radioactive remains of current nuclear reactors.