Thought Experiments


After a Golden Age

Are physicists victims of their own success? They have strived to find the fundamental laws of matter, and in recent years they’ve done it. The so-called standard model of physics provides us with a thorough census of the subatomic particles that combine to make everything we see, and its equations define a complete mathematical explanation of how they behave. The golden age, it seems, has come and gone.

Or has it?

A millennium from today, historians will look back at the twentieth century primarily as the age of a rich flowering of science. Within a few decades molecular biology unveiled the body and soul of the genetic code, cosmology reconstructed the history of the universe, and geophysics disclosed a home planet more dynamic than ever previously imagined. Yet the biggest revolution of all, from which all those others drew, was ironically the smallest: the conquest of microphysics.


Three icons of twentieth century science. Clockwise: The double helix structure of DNA, revealed by analysis of its x-ray diffraction pattern (credit: Richard Wheeler via Wikimedia); anisotropies in the microwave background radiation, providing a picture of the very early universe (credit: WMAP Science Team, NASA); motion of the continents, proved and then mapped by analysis of magnetic field reversals at mid-ocean ridges. In each case, the enabling sensitive instruments and technologies depend upon profound understanding of the properties of matter, based on microphysics (credit: EMAG2)

History does not come with time-stamps affixed, but two epochal experiments roughly bracket the High Golden Age of microphysics. In 1912, Ernest Rutherford decoded atoms experimentally, revealing that each has a tiny nucleus containing all its positive charge and almost all its mass. That nucleus is surrounded by electrons, which are bound to it by electric forces. Just a year later, Niels Bohr introduced strange new ideas about the laws of motion in the microworld. His breakthrough matured into quantum mechanics.

Quantum mechanics broke the code of the microworld, but it took decades to master its text. Finally, in the “November Revolution” of 1974, two separate experimental groups announced the discovery of a striking set of new particles, the charmonium system. Their discoveries provided a brilliant confirmation of, and a fertile proving ground for, the collection of theoretical ideas we now call the Standard Model. The charm quarks (and their antiquarks) that make the charmonium particles rounded out the theory of the weak interaction, and the forces that bind them were just right for the theory of the strong interaction. Those theories tamed nuclear physics, and together with electromagnetism and gravity they complete the description of matter.

The Standard Model provides, we believe (after very thorough, rigorous, quantitative testing!), a complete mathematical explanation of how subatomic particles combine to make atoms, atoms to make molecules, and molecules to make materials, and how all this stuff interacts with light and radiation. Its equations are comprehensive yet economical, symmetrical but spiced with interesting detail, austere yet strangely beautiful. The Standard Model provides a complete, secure foundation for astrophysics, materials science, chemistry, and physical biology. Good stuff!

The Standard Model marks the ultimate triumph of reductionism. As Isaac Newton put it, we analyze matter by finding complete and simple laws governing the behavior of its elementary components, and then use those laws to synthesize the properties of macroscopic objects.

Triumph on that scale has a dark side: It’s a tough act to follow. By the late 1980s, articles and books with titles like “The End of Physics” began to appear. At the same time, “Theory of Everything” hyperbole erupted.

Neither reaction, however unseemly, was entirely baseless. The achievements of this golden age did mark the end of a certain special—and especially wonderful—kind of physics. After plumbing the bottom of ordinary matter (that is, physical material that’s reasonably accessible and usefully stable), where do you go? As physicists deciphered the atom, they revolutionized chemistry and enabled microelectronics; as they deciphered the nucleus, they revolutionized not only astrophysics and physical cosmology, but also bomb technology and medicine. There is no realistic prospect that the sort of frontier physics explored at the Large Hadron Collider, as esoteric and expensive as it is marvelous, will yield practical fruit. (This is not to say that the indirect value of this work, which serves as “the moral equivalent of war” for many talented, enthusiastic, creative young seekers, will not repay the money invested in it. It will, handsomely.) Its application in the natural world is likely to be restricted to the extremely early universe, and (maybe) a few super-extreme astrophysical situations, like Hawking’s black hole explosions.

But lamenting the passing of a golden age, or professing to reanimate it, are exercises in nostalgia. A healthier attitude, and an attitude that is truer to the unselfconscious exploratory spirit of the golden age itself, is to engage with its legacy of unanswered challenges and new opportunities. What a legacy it is, and what opportunities there are!

For the Standard Model, despite its practical success in describing ordinary matter, leaves many loose ends and unanswered questions. One of its ingredients, the Higgs particle, has not yet been observed directly. That embarrassment may soon be remedied, but other flaws run deeper. Its equations remain lopsided in peculiar ways. They beg to be embedded in a larger, still more symmetric theory. There are, in fact, excellent ideas for advancing toward such unification. Those ideas suggest new lines of experimental investigation, notably the search for proton decay and for supersymmetric particles. The other interactions, and indeed quantum mechanics itself, have not yet been organically united with gravity. String theory might help with those problems, but it’s clear that crucial ideas still await discovery.

We’d also like to understand why the laws of microphysics appear so nearly unchanged if we run time backwards. The only known explanation predicts the existence of a remarkable new class of particles called axions. These wraithlike cousins of photons, more elusive even than neutrinos, plausibly provide the astronomical dark matter. And if axions don’t—what does?

These and other unanswered challenges amply refute the notion that physicists are, in any meaningful sense, close to having a “Theory of Everything” (or that we’ve reached “The End of Physics”).

Yet the biggest challenges, I think, are of a different kind. The art of using our comprehension of microphysics is an open-ended invitation to creativity. Music-making doesn’t end when you’ve learned how your instrument works—it begins.

Can we engineer quantum computers, and through them fashion truly alien forms of intelligence? Can we tune in to the messages the universe itself broadcasts in gravity waves, in neutrinos, and in axions? Can we understand the human mind, molecule by molecule, and systematically improve it? To ask these questions is to discover, in the ripeness of one golden age, the seeds of new ones.

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Frank Wilczek

    Frank Wilczek has received many prizes for his work in physics, including the Nobel Prize of 2004 for work he did as a graduate student at Princeton University, when he was only 21 years old. He is known, among other things, for the discovery of asymptotic freedom, the development of quantum chromodynamics, the invention of axions, and the exploration of new kinds of quantum statistics. Frank is currently the Herman Feshbach professor of physics at MIT. His latest book is A BEAUTIFUL QUESTION: Finding Nature’s Deep Design.