The great theories of physics are just five in number:

**1. Mechanics** (sometimes called Newtonian mechanics or classical mechanics). The theory of the motion of material objects.

**2. Thermodynamics**. The theory of heat, temperature, and the behavior of large arrays of particles.

**3. Electromagnetism**. The theory of electricity, magnetism, and electromagnetic radiation.

**4. Relativity**. The theory of invariance in nature, and the theory of high-speed motion. (General Relativity encompasses also the theory of gravitation.)

**5. Quantum mechanics**. The theory of the mechanical behavior of the submicroscopic world.

(This theory is still evolving.)

Even these five are closely interrelated. Every one of the myriad phenomena in the physical world that is understood is explained in terms of one or more of these few theories. The behavior of a single atom, for example, is governed by quantum mechanics, relativity, and electromagnetism. A collection of many atoms requires also the theories of mechanics and thermodynamics for its description.

About these five theories we can say with some confidence that none will ever be completely overthrown. If history is a reliable guide, we can say with equal confidence that none will prove to be entirely correct. Consider mechanics, already known to be “incorrect.” Relativity “overthrew” mechanics when it showed Newton’s laws of mechanics to be incorrect for describing ultra-high-speed motion. Then quantum mechanics showed classical mechanics to be incorrect for describing internal motions within atoms. Each of these twentieth-century developments proved mechanics to be wrong. Yet we doggedly list mechanics as one of the great theories of physical science. Why? Because over what is still a vast domain of sizes and speeds, mechanics is so extremely accurate that it is for all practical purposes completely correct. It is the best and simplest tool for describing nature in a certain domain. It is better to say that relativity and quantum mechanics have chipped away at the boundaries of mechanics, reducing it from an infinite to a finite domain, than to say that either theory has overthrown mechanics. The situation is analogous to the “overthrow” of the automobile by the airplane. Using quantum mechanics to describe a phenomenon, such as the trajectory of a satellite, that is accurately described by classical mechanics is like taxiing an airplane on the ground a few miles from home to office in order to avoid using an old-fashioned vehicle, the automobile.

Electromagnetism, relativity, and quantum mechanics are theories with no known flaws. No scientist believes this situation will persist indefinitely. Undoubtedly new discoveries will show that these theories, too, have a limited domain of validity. But when that happens—when the theories are “overthrown”—they will very likely also refuse to disappear and will remain in the scientist’s bag of tools as the most satisfactory description of those parts of nature where they have already been well tested.

Are there more “great theories” awaiting discovery? To this writer it seems likely. Here we are, nearly a hundred years since quantum mechanics arrived on the scene, ripe for something revolutionary. The current probing in search of “new physics” is “downward” (particle and sub-particle physics) and “outward” (cosmology, the study of the whole universe—or universes). So far, both the downward probing and the outward probing are conducted in the framework of already known theories. So-called supersymmetry is an extension of quantum mechanics. Gravitational radiation finds its explanation within relativity theory. Will string theory require a new window to be opened? Will quantum foam become more than a vague term for the super-small? Will we gain a new and deeper understanding of the origin and history of the universe we inhabit and new reason to believe in other universes? Perhaps so, in this twenty-first century.