The completion of the Large Hadron Collider and the hundredth anniversary of Rutherford’s first scattering experiments make this seem like a good time to reflect on the century of incredible changes in this field–where the arc has taken us, where we want to go, and what new challenges loom ahead.
Newton’s aphorism “If I have seen a little further, it is by standing on the shoulders of giants” is as true for today’s physicists as it was for him. The generation of Einstein, Planck, Curie, Rutherford, and Bohr, our intellectual ancestors, laid the foundations for understanding the atom. Then came the early 20th century’s young geniuses, who discovered quantum mechanics, explored the nucleus, and built the new machines. Though the divisions blur, a new generation–my own–emerged in the wake of World War II. We identified the elementary particles and the forces between them, uniting them in the standard model.
This has been a remarkable journey, but great challenges remain. How do the elementary particles acquire mass? Is there some deeper theory explaining the identities of particles and the relations between forces that also encompasses Einstein’s grand vision of gravity? Many think that string theory is a giant step in this direction, but conclusive evidence is not yet available. These are all questions for the present generation, now reaching its peak of creativity.
Nor can one omit from the story the realization that conditions fleetingly created by collisions in the highest-energy accelerators mimic those that took place, for a fraction of a microsecond, immediately after the Big Bang that marked the universe’s beginning. Because of this development, which has caused a great stir, elementary-particle physics and cosmology now frequently see their aims as parallel. Today’s young physicist might go to a conference called “Inner Space/Outer Space” or study a book called From Quarks to the Cosmos.
This convergence has also led to a revival of interest in neutrinos, for they seem to play an important role in our cosmos. They are a subject that has become near and dear to me. We now speak routinely of situations that Fermi, my uncle, and their friends could not have imagined in the winter of 1933. Experiments have shown neutrinos being emitted from the sun’s core. We even know that in the explosion following a large star’s collapse, neutrinos carry off 99 percent of the emitted energy in a single 10-second burst. This amount is comparable to all the energy radiated by the sun in its 10-billion-year lifetime. Yet three-quarters of a century after the neutrino’s existence was proposed, we still don’t know its mass. It is much, much less than that of the electron, but how large is it?
I conclude by bringing you up to date on my end of the family business. None of Emilio’s children became physicists, but one of his grandsons and a nephew of mine did. My three daughters didn’t go into the business, but the oldest married the son of a well-known theoretical physicist. So perhaps a grandchild will carry on the tradition, though I am not sure how much influence the two grandfathers have. The days when parents told their children what careers to pursue are over.
Given all this history, it will probably not surprise you that I married the daughter of a physicist, Herman Hoerlin. I know that when my wife, Bettina, told him about me, he checked me out in American Men and Women of Science before giving his approval. Fortunately, I passed. Finally, I acquired a physicist brother-in-law a little over 15 years ago, when Bettina’s sister Duscha met, fell in love with, and soon married an elderly Austrian widower. He was none other than Viki Weisskopf, the idol of my youth at MIT and CERN. At first I was a bit tongue-tied in his presence, but then I realized that it was going to be okay. He was family.
Gino Segrè is a Professor Emeritus of Physics at the University of Pennsylvania and the author of Faust in Copenhagen: A Struggle for the Soul of Physics.