Physicists have a habit of referring to "the four fundamental forces in Nature." Other people, quite reasonably, ask, why four? Why couldn't there be just two fundamental forces, or ten, or twenty?

Certainly a larger number than four seemed appropriate to our remote ancestors. They were born into the world with nothing; no fire, no tools, no shelter, no recorded learning, no understanding of what humans were or how they had appeared on earth; no guarantee, even, of where the next meal was coming from. They had just one thing that no creature on earth had ever before possessed: a brain capable of sorting objects into categories, then dreaming up links between those categories. Three trees share, in some inexplicable but definite way, a property with three buffalo. A salmon is more like a catfish than either is like a rabbit. Humans began to put the world into logical boxes and try to make sense of it.

Despite their best efforts, your distant forebears still found many things that would not fit with anything else. What was fire, what was lightning, what was thunder, what was a rainbow, why did rain fall, why did rivers flood, why did dropped objects always fall, why did a seed grow in the spring, where did the sun go at night, what were stars, what were clouds, what controlled the complicated movements of planets in the sky, and how did living things differ from dead or never-living things?

A reasonable first approach was to invent - and this was a great intellectual achievement - a class of super-powerful beings associated with and controlling the inexplicable. Abbreviate that lengthy expression to a word, call it a "god," and you had a rain god, a river god, a god of thunder and of lightning, gods of different living things, and individual gods for the moon, the sun, and each planet.

Humans, with the possible exception of teenagers, are naturally tidy, so as time went on our forebears tried to see connections and reduce the number of necessary gods. Once you realized that clouds gave rise to rain, and that heavy rain in mountains hundreds or even thousands of miles away could cause your local river to burst its banks, then you didn't need a cloud god as well as a rain god and a river god. When you learned that thunder always followed lightning, by a time that could be related to distance, one god took care of both events. The gods became an endangered species, dwindling in their number with every advance in understanding of the world.

A huge step forward was made in the seventeenth century, when Isaac Newton's theories united in one intellectual structure the gravity that made objects fall, and the force that controlled the movement of the planets and moon in their orbits. Not only that, Newton showed how with his theories you could make precise calculations of where objects would move in the future. With that understanding, a huge number of pieces in the puzzle of nature fell into place. Another group of gods slid into obscurity.

There were, however, still more than enough mysteries to go 'round. No one could explain lightning; no one knew why a sliver of a material known as lodestone, left to rotate freely about an axis, would point north-south to provide a compass. In the eighteenth century, electricity and magnetism were considered quite different from each other and quite inexplicable. Ben Franklin, who understood electricity as well as anyone of his time, said, "If there is no other purpose for the electricity than this, it may serve to make a vain man humble."

Electricity and magnetism were placed into a single framework in the middle of the nineteenth century, thanks largely to Michael Faraday and James Clerk Maxwell; a second force, the electromagnetic, was added to gravity as fundamental.

One by one, the other mysteries were being disposed of. At the beginning of the seventeenth century, de Dominis used the theory of optics to provide a first explanation of the rainbow. About the same time, Galileo's telescope revealed that the planets were worlds resembling the earth in shape and sometimes in size. With the rise of chemistry in the late eighteenth century, fire was understood to be nothing more than a combining of oxygen with other elements. The metabolism of living things could be described as a "slow burning" process, carefully controlled by enzymes. Science was reducing the number of essential and distinct components - forces - in the world. The old gods were in full retreat.

They were not, however, in retreat everywhere. Until about 1930, we seemed to be heading for a universe that could be explained with just two fundamental forces: gravity and electromagnetism. Exploration of the nucleus of the atom then revealed a need for two more, the so-called "weak force" and the "strong force." These, together with gravity and electromagnetism, comprise the four forces referred to in the first sentence.

We are far from finished. Since the 1930s, every great physicist from Einstein onward has itched to see the four forces "unified," explained in a single theory from which all will naturally arise. There has been major progress. Abdus Salam, Sheldon Glashow, and Steven Weinberg received the 1979 Nobel Prize in Physics for showing how the weak force and the electromagnetic force can be regarded as two facets of a single force. The strong force has been more recently incorporated, and an abstruse field known as superstring theory may bring gravity into the same framework.

Will there then be just the one fundamental force, with four different manifestations? That's not clear at all. One unified force may explain what we know today, but we certainly don't know everything. As we explore the universe at smaller and smaller or larger and larger scales, we may be obliged to introduce totally new forces, just as in the nineteenth century it was necessary to invoke the electromagnetic force, or in this century to add the strong and the weak forces.

How many fundamental forces are really needed to describe Nature? One, two, four, seven? Take your pick. No one can prove you wrong.

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