Bromley reviews a century of physics
The American Physical Society (APS) celebrated its centennial with its biggest meeting ever in Atlanta, 20-26 March 1999. They had a century of physics to celebrate and had prepared an eleven-poster wall chart edited by Hans C. Von Baeyer under direction of Brian Schwartz. And to kick off their week-long meeting, they asked D. Allan Bromley, Science Advisor to former President Bush and now Dean of Engineering at Yale University, to review "A Century of Physics."
Bromley, who also hosted a panel of seven presidential science advisors the following morning, dipped back into the preceding century to begin his talk with J. J. Thomson's discovery of the electron in 1897, which ushered in the age of subatomic physics, marked by Planck's hypothesis of the quantum in 1900, Einstein's three celebrated papers of 1905, Rutherford's discovery of the atomic nucleus in 1911, and Bohr's theory of the hydrogen atom in 1913. The Bohr-Rutherford atom was superseded in 1925 with the quantum mechanics of Heisenberg and Schrdinger, the equivalence of which was shown by Dirac, who also predicted the existence of antimatter from his relativistic generalization.
Bromley singled out the year of 1932 for three important particle discoveries -- the neutron, positron, and deuteron. That decade also saw the development of machines to accelerate the particles which were electrically charged, and at its end came the discovery of nuclear fission, applied to the military and followed by Vannevar Bush's Science-The Endless Frontier and Vern Ehlers' Unlocking Our Future. World War II efforts showed that basic research and development of new technologies were closely intertwined.
The period after World War II saw research on solids and liquids, Bromley pointed out, leading to superconductivity, fullerines and nanotubes, the scanning tunneling microscope, fluid mechanics, the transistor, computers, and optical transmission of information. The Internet, once the province of the Defense Advanced Project Research Agency, has now been opened to all. Held out for the future is the quantum computer.
Turning to the spectrum of electromagnetic radiation, Bromley observed that increased X-ray intensity from synchrotron light sources has fostered more small science with a big machine. Laser technolgies have found a number of uses, among them the trapping of individual particles -- including a positron long enough to measure its electromagnetic properties. And the long-sought Bose condensate was also achieved.
Bromley cited two frontiers in nuclear structure -- approaching the nuclear continuum of a quark-gluon plasma and creating elements in an "island of stability," presently being approached with elements 112 and 114. Hyper- as well as superdeformed nuclear states have been found, including a state of a dysprosium isotope with 60 units of angular momentum.
Physics has also led to applications in medical diagnosis -- magnetic resonance imaging, positron emission tomography, and DNA (whose structure was deduced by Crick and Watson from the work of Franklin and Wilkins, characterized by Bromley as one of the three greatest discoveries of the twentieth century).
Finally, Bromley noted that two disparate fields are now coming together -- elementary particles and cosmology. This derives, he said, from Gell-Mann and Zweig's quark model, which has led to the "standard model" of quarks and leptons. Unfortunately, he lamented, gravity has remained outside this model. In 1974 John Schwartz proposed incorporating both gravity and the standard model with string theory.
At the cosmological level, supernova SN 1987a was found to emit neutrinos in accordance with predictions by Hans Bethe and Gerry Brown (both of whom also gave papers at the meeting, the former at the age of 93). To explain fluctuations observed by NASA's COBE satellite in the temperature of cosmic radiation 300,000 years after the Big Bang, Alan Guth has invoked a rapid expansion called "inflation" at the beginning of the universe. This "inflation" also explains the apparent geometric flatness of the universe.
As Bromley introduced the past century of physics with the discovery of the electron, APS President-Elect James Langer said in introducing Steven Hawking at the Atlanta Civic Center the following Wednesday evening, "In a hundred years we have gone from just discovering the electron and not being sure whether atoms exist to being able to see individual atoms and forcing electrons to go wherever we want them to." But even at the end of a century of great progress physics still faces many unanswered questions, and Bromley concluded with his own "top ten" list, headed by "What is the origin of mass?" and "Why matter rather than antimatter?" Others, raised in the final poster of the APS wall chart include "How did time begin?" "How did life begin?" and "Are we alone in the universe?"
The APS "Century of Physics" wall chart is being provided free to schools, universities, and science museums, and at a cost of $35 to all others. Ordering details are available at www.aps.org/timeline. An on-line version, sponsored by IBM, will also be available at www.timeline.aps.org.
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