Even in preindustrial times, people knew about static electricity, lodestones (or magnetized rocks), and light. From a modern point of view, this means that one of the fundamental forces of nature—electromagnetism—was observed without any modern technology. Of course, preindustrial people did not know that static electricity, magnetism, and light are different aspects of the same thing. This only became clear when James Clerk Maxwell combined electric and magnetic forces into the theory of electromagnetism in the mid-19th century. Maxwell’s equations—together with the discovery of the first elementary particle, the electron, in 1897—led to the invention of radio and, ultimately, to today’s electronic technologies.

One other fundamental force was known before the 20th century—gravity. Gravity is vastly weaker than the other forces—so weak that the gravitational forces between individual elementary particles are too small to observe. Yet the gravitational effects of many particles are cumulative. Thus for everyday objects gravity is clearly observable, and gravity is the dominant force for galaxies and in the universe as a whole today.

The advanced technology of the 20th century was required to discover and understand the two other forces that influence the behavior of particles. Some atoms decay radioactively by emitting electrons and neutrinos. In the 20th century these decays were shown to be the product of weak force interactions. The weak force—which is critically important in stellar processes, the formation of the elements beyond iron, and the evolution of the early universe—is just as fundamental as electromagnetism or gravity, but it is far less obvious in everyday experience.

Recognition of the strong, or nuclear, force resulted from research into the atomic nucleus. The nucleus consists of protons and neutrons that are bound together in a tiny ball. Protons have a positive electric charge, which makes them repel each other. However, something keeps the nucleus from flying apart. This something is the strong force.

Understanding the strong and weak forces depends centrally on quantum mechanics. In the 1920s, physicists began studying the properties and behaviors of particles, in part to understand the forces between them. This process culminated a half century later with the emergence of the Standard Model. The Standard Model, in a remarkably concise way, describes and explains many of the phenomena that underlie particle physics and captures with astonishing precision an incredible range of observational data.

The Standard Model has another important feature. It reveals a deep analogy between the four forces, in keeping with Einstein’s goal of unifying all of the

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