A Brief Review of 20th Century Physics

Highlights from the opening plenary lecture at the APS Centennial meeting.

By D. Allan Bromley

D. Allan Bromley

Science and its applications - which today we call technology - has from its very beginning been an important part of the American society. As we approach the close of the 20th century, it is entirely appropriate that we celebrate the role of our particular sector of this science and technology: physics and its applications.

What, then, is physics? The best definition I have encountered is that of my old friend, the late Edward Purcell. In 1970 he wrote, "Science is knowing. What man knows about inanimate nature is physics - or rather, the most lasting and universal things that he knows make up physics." We physicists have the arrogance to believe that the laws we deduce from our measurements here on earth apply throughout the universe, and that what is true today was true throughout the entire life of the universe. Our measurements support that arrogance. Purcell goes on to say, "As he gains more knowledge, what would have appeared complicated or capricious can be seen as essentially simple and in a deep sense, orderly." Turning to applications, he said, "To understand how things work is to see how, within environmental constraints and the limitations of wisdom, better to accommodate nature to man and man to nature." Many have noted that the 20th century of science truly began in 1897 with J.J. Thompson's discovery of the electron. This reflects the enormous impact that our ability to manipulate the atom and its component electrons has had on such diverse areas of modern civilization as communications, computation, energy, and medicine. In 1905, Albert Einstein published his classic papers on Brownian motion, the photoelectric effect and special relativity, the latter providing us with one of the classic equations of all time: E=mc². And in 1911 Ernest Rutherford discovered the atomic nucleus. The next two decades saw the emergence of quantum mechanics, culminating in 1932, truly an annus mirablis in the physics of the time, with the discovery of positrons in cosmic rays; experimental confirmation of the relativity of time; the first electrostatic accelerator; and the first cyclotron.

The 1930s closed with the discovery of convincing evidence for nuclear fission, and recognition of the potential military consequences came rapidly, with the establishment of the Manhattan Project, as well as MIT's Radiation Laboratory, devoted to the development of radar. These activities ushered in a total seachange in the scientific and technical communities. Prior to World War II, basic research was directed toward the understanding of nature, while invention and technology were directed toward the mastery of nature, and the two proceeded on rather parallel and noncommunicating courses. What the wartime projects made very evident was that basic understanding could greatly facilitate the development of technology, and basic technology could facilitate whole new areas of basic research. The prewar activities that had frequently been called natural philosophy and invention, respectively, were irretrievably joined, and nowhere more so than in physics.

This 20th century in physics began with a rush of new insights and, happily, it is ending in much the same way. For example, our ability to understand, to probe, and to structure surfaces has opened up entirely new areas of catalysis and corrosion resistance, and an entirely new understanding of phenomena such as friction and adhesion. Entire optical benches and chemical laboratories are now being fabricated on single chips with nanoscale rotary and linear motors powering the necessary motions. The development of new materials has had a major impact on our ability to develop human prosthetic devices to replace both bones and soft tissue. Our understanding of chaotic phenomena and their dependence on nonlinearities and initial conditions marks one of the major achievements of the 20th century in physics.

Elementary particle physics and cosmology are slowly coming together to address some of the most fundamental questions in physics, because with ever more powerful accelerators, it becomes possible to recreate, if only for tiny fractions of a second, the conditions that were present within the first moments of the existence of our universe. Atomic and nuclear technology has found wide application in biology and medicine, and the interconnections are growing on almost a daily basis. In communications, single optical fiber bandwidths have been doubling every nine months and the actual in-the-field telephone company products now lag the research frontiers by only four years. The resulting communication and computation explosion has truly reduced our planet to a global village and changed the entire nature of our society. There are far too many other exciting developments at the frontiers of physics to attempt a complete list here.

With regard to the future, there are ten open questions in physics that strike me as being of particular interest. How does mass originate? Does nonbaryonic dark matter exist, and if so, in what form? Why are we in a matter universe? What is the ultimate fate of our universe? What is the structure of quantum gravity? Are quarks and leptons truly elementary, or composite? Do the physical constants change with time? What are the consequences of a nonzero neutrino mass? How does one build a quantum computer? And finally, is room temperature superconductivity possible?

Lord Raleigh, then president of the British Association for the Advancement of Science, was asked 115 years ago to give a review of physics in the 19th century as his presidential address. He began by noting that this was impossible, and I know only too well how he felt. But I would like to quote one of his closing comments: "Increasing knowledge brings with it increasing power, and great as are the triumphs of the present century, we may well believe that they are but a foretaste of what discovery and invention have yet in store for mankind."

We remain a vital, active and productive science. We physicists are among the most fortunate of humans; we have been privileged to engage in that greatest adventure of discovery at a time when technology has allowed us to push outward the frontiers of knowledge at unprecedented rates. And in so doing, we have also bettered the lives of humans everywhere. Physics, as the most fundamental of the sciences, will always remain a vital part of this great adventure.

Former APS President D. Allan Bromley is Sterling Professor of the Sciences and Dean of Engineering at Yale University and former advisor to the Bush Administration. The above text was heavily condensed from his address at the opening plenary session; APS Centennial Meeting; Atlanta, Georgia.