Far more delicate than the tug on a spider's web: If a fifth
force is out there, its impact on our world must be nearly imperceptible.

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At the turn of the 20th century, finding a new form of radiation could put a physicist’s career on the fast track. Wilhelm Röntgen changed the world by discovering X-rays in 1895. Soon thereafter, Ernest Rutherford and Paul Villard identified three different kinds of radiation, dubbed alpha, beta, and gamma rays, emitted by radioactive compounds. In 1903 French scientist René Blondlot added to the frenzy with his announcement of N-rays, a strangely democratic form of radiation emitted by wood, iron, living organisms—just about anything at all.

Some 300 scientific papers were written about N-rays. There was just one problem: They weren’t real. A skeptical physicist named Robert Wood visited Blondlot’s lab and secretly removed a key part of his apparatus; this had no effect on Blondlot’s perception of N-rays, showing that they were purely a product of the imagination.

Blondlot’s reversal of fortune served as a reminder that the world isn’t really full of countless kinds of radiation waiting patiently to be discovered. Nature is more parsimonious than that. Even as forms of radiation seemed to proliferate, theory was driving physics the other way, toward consolidation. X-rays and gamma rays were soon recognized as different forms of electromagnetic radiation, like radio waves and visible light but more energetic. Beta rays are simply fast-moving electrons, and alpha rays are fast-moving helium nuclei. Beneath the dazzling array of new phenomena lurked just a few simple ingredients.


The trend toward unification and simplification is a major theme of modern physics. At the same time, nature has ways of surprising us, and it pays to be watchful. We know a lot about the physics of the macroscopic world, but can we be sure that we aren’t missing one of those crucial ingredients? The answer is yes: In certain well-defined cases, we can be very sure. Physicists long ago mapped the entire electromagnetic spectrum. The modern version of the search for new kinds of radiation is the search for new forces of nature. And while there may be unknown forces waiting to be discovered, we can say with great confidence that such forces must be so feeble that only a professional physicist like me would really care.

Here’s how I can justify such a grandiose pronouncement. According to modern physics, the world is fundamentally composed of particles interacting via forces. Over the course of the 20th century, researchers discovered many new particles interacting in many different ways. But it gradually became clear that the vast majority of such particles are merely different combinations of smaller ones, and the great variety of interactions boils down to just a few forces. When the dust settled in the 1970s, we were left with two kinds of elementary particles: quarks, which group into heavier composites like protons and neutrons; and lighter particles called leptons, like the electron and the neutrino, which can move freely without bunching into heavier combinations.

Amazingly, these particles interact through just four different forces. Two are familiar—gravity and electromagnetism. Gravity is the most recognizable force; we struggle against it every time we climb a flight of stairs. But electromagnetism is arguably more crucial to our everyday lives. Almost everything we experience that is not directly due to gravity is ultimately attributable to electromagnetism. A table is solid because the atoms in it are bound together by electromagnetic forces. The thinking that happens inside your brain can be traced to chemical signals passing between neurons, and those chemicals move the way they do because of electromagnetism. Radio waves, visible light, and X-rays are all different forms of electromagnetic radiation.

The other two forces are the strong nuclear force and the weak nuclear force. We don’t notice them in everyday life because they are short-range, extending over only tiny distances smaller than an atom. The strong nuclear force binds quarks into protons and neutrons and sticks protons and neutrons together to make atomic nuclei. The weak nuclear force is responsible for—well, nothing much, as far as familiar phenomena are concerned. It is too weak. But if you isolate a single neutron away from any protons, within a few minutes it will decay into other particles; that decay is caused by the weak force.