No Resting on Laurels For Physicists Testing Theories of Matter
BOSTON — However well established their theories seem to be, physicists keep on testing them. Results of two experiments reported this fall illustrate this. They probed basic aspects of physicists' concepts of the nature of matter.
Einstein's insight that mass and energy are equivalent probably is the most famous of these concepts. The sun shines by converting some of the mass of hydrogen atoms into energy. Matter turns into energy all the time in particle accelerator laboratories. Now, for the first time, a research team has turned the energy of light into material particles in experiments with the Stanford Linear Accelerator in Palo Alto, Calif.
A 20-member team from Stanford University, Princeton University in New Jersey, and the University of Tennessee in Knoxville bounced photons in a laser beam off a beam of electrons from the Stanford accelerator. Just as golf balls would gain energy when hitting a speeding freight train, the head-on interaction of the photons with the electrons boosted the photons from the energy range of visible light to the high-energy range of gamma rays. Some of these gamma ray photons then collided with incoming laser photons. These latter interactions concentrated enough energy at one spot for that energy to turn directly into matter.
That matter appeared as pairs of negatively charged electrons and positively charged positrons, just as theory predicts it should. Positrons are the so-called antimatter twins of electrons. Their appearance was the inverse of the process in which particles of matter and antimatter annihilate each other, turning their mass into pure energy. As reported last September in Physical Review Letters, this experiment not only validates a prediction of present theory, it also demonstrates a new technique for using photons to test the theory of matter's basic nature more extensively.
Meanwhile, other physicists are probing the electron itself. Textbooks will tell you that electrons are basic material particles. There is no evidence that something even more fundamental underlies their structure. These texts also explain that the charge of an electron is the fundamental unit of electric charge. But experiments reported in September in Nature and Physical Review Letters validate physicists' suspicions that this "ain't necessarily so."
Physicists have known that entities called quarks have amounts of electric charge that are only fractions of the charge on the electron. However, quarks are locked up inside nuclear particles such as the protons and neutrons that make up atomic nuclei. All the outside world sees is the overall charge of these nuclear particles. The charges of the underlying quarks add up in such a way that the charge on these nuclear particles, and on the atomic nuclei they construct, is always a whole multiple of that supposedly "fundamental" unit - the charge of the electron itself.
Now Michael Reznikov and colleagues at the Weizmann Institute in Rohovot, Israel, and a team led by D. Christian Glattli at the Commission of Atomic Energy facility in Saclay, France, have directly measured entities in semiconductor devices that carry one-third of the electron charge.
This validates a theory proposed in 1982 by Robert Laughlin, now at Stanford University. He suggested that the experimental data known at that time made sense if the electrons interact in ways that create these fractional charges. Although this flew in the face of one of the physicists' cherished beliefs, his far-out suggestion turns out to be right.
By continuing to test their cherished theories, physicists strengthen their understanding of how nature works. That understanding underpins the technology that has transformed our world. Without it, the computer on which this column was written would not have been invented.