Feature: October 11, 1995
Nobel Physicist Joseph Taylor Takes the "Pulse" of Dying Stars
BY CHARLES SEIFE '93
"I certainly wasn't expecting it. we got the phone call early in the morning-not a call from Sweden, actually, but a call from the Associated Press." Joseph Taylor smiles at the recollection of that lucky October 13, 1993. "Marietta, my wife, told me, 'There's a reporter on the line who says that you've won the Nobel Prize.' " Within minutes, they were deluged by calls. "I had to unplug the phone so that I could brush my teeth, have a cup of coffee, and talk to my wife." In the confusion, the Swedish Academy was unable to get through to Taylor, so it wound up sending a fax to the physics department at Princeton. The official notification that Taylor, the James S. McDonnell ['21] Distinguished University Professor of Physics, and his former colleague Russell A. Hulse had won the Nobel Prize was sitting in the shipping department in Jadwin Hall.
Two years later, Taylor's life has mostly settled down, and he looks back with bemusement on the chaos that followed his winning the most prestigious prize in science. "I was beset by requests to give talks and make appearances-often, it was no more than 'Wouldn't you like to show up and be an ornamental plant?' It's a statement to the skill and dedication of my graduate students that the work got done just the same."
Taylor's "work" is on a class of celestial objects called pulsars. A rare type of star unknown to astronomers before the 1960s, pulsars have become key tools for understanding how the universe works, and Taylor has devoted his professional life to studying them.
A pulsar begins life as an ordinary large star, a ball of hydrogen held together by gravity. Inside the star, the hydrogen fuses to make helium, releasing vast amounts of energy. Near the end of its life, when a star begins to run out of hydrogen, it expands rapidly and then abruptly collapses. If the star is massive enough, the collapse triggers a tremendous explosion-a supernova. The star sheds its outer layers into space, leaving behind a dense core. In extreme cases, gravity crushes the core's protons and electrons, forming neutrons. The result is a "neutron star," some 10 miles in diameter but so dense that a marble-sized piece of it would weigh millions of tons on earth. Because of their compactness, neutron stars spin rapidly-some in excess of 100 times a second. Some neutron stars also emit a beam of radiation. As the star spins, the beam traces a cone in space, and an observer in the path of it sees the star pulsing-apparently blinking on and off, like the beam of a lighthouse as seen from a ship.
In August 1967 Jocelyn Bell, a graduate student in astronomy at Cambridge University, and her supervisor, Antony Hewish, became the first to observe these pulsing stars. (Hewish, although not Bell, would subsequently win a Nobel for the discovery.) The precise timing of the pulse suggested it might be coming from some extraterrestrial civilization, and the press was quick to label the object an LGM-for Little Green Men. Within months, however, several other celestial blinkers were found. Alien intelligence was ruled out, and the objects were dubbed "pulsars."
Taylor was at Harvard at the time, completing his doctoral work in radio astronomy, and like others in the field he was caught up in the excitement. Later, as a postdoctoral fellow at Harvard, he helped confirm the existence of pulsars, and he continued his research on these enigmatic objects after becoming an assistant professor at the University of Massachusetts, in 1969. Three years later, Taylor enlisted Hulse, a graduate student in his department, to help him refine a method he'd developed for speeding the discovery of pulsars (only a few dozen were known at the time). Thus began the collaboration that, 21 years later, resulted in their sharing a Nobel Prize.
Taylor and Hulse set up shop at the world's largest radio telescope, a dish a fifth of a mile in diameter built into a natural depression in the hills near Arecibo, Puerto Rico. Working around the clock, they collected data on magnetic tapes for a few hours a day, then spent the other 18 or 20 hours processing the tapes with a computer. "Then I had to go back and teach classes," Taylor recalls. "Russell spent many months at Arecibo, and I was back and forth every so often to check on progress." This routine went on for more than a year. Then, in September 1974, "Russell called and told me that he was onto something interesting, and I was on the next plane down with a bunch of equipment."
On the computer printout of potential pulsars Hulse had noticed an anomaly: one of the sources was emitting bursts that were slightly less regular than the others. He and Taylor deduced correctly that the signal was coming from a pulsar in co-orbit with an unseen companion star. (As the pulsar circled in its orbital embrace, it moved toward and away from an observer on earth. This relative movement produced the irregularities in the recorded signal.) They had found a binary pulsar, the first known to science. Actually two neutron stars locked in a gravitational dance around a mutual axis, the binary pulsar was an amazing discovery in its own right, and it would prove a literally stellar laboratory for testing Einstein's general theory of relativity.
Physicists love to find ways to test general relativity on the off chance that Einstein might be proved wrong. Unveiled in 1916, the theory implies the existence of gravity waves, and its predictions about the motion of celestial objects are slightly different from those derived from the classical physics of Isaac Newton. The differences are so subtle that testing general relativity is next to impossible in "normal" celestial systems. But the extreme masses, gravitational fields, and rotational speeds of binary pulsars make them ideal for observing relativity in action.
"We saw that we would be able to measure ways that Einstein's theory differs from Newton's, including the existence of gravitational waves," Taylor recalls. The implications unfolded quickly. By mid-October, Taylor and Hulse had written a paper about their discovery and its significance. Their calculations showed the unseen companion star affecting the pulsar's signals as predicted by relativity theory.
Subsequent observations of the binary pulsar led in 1978 to an indirect proof of the existence of gravity waves. Taylor's research team measured a tiny decrease-just 75 millionths of a second a year-in the interval between the pulses. As gravity waves carried away energy from the binary system, the pulsar and its companion were drawing closer together. This in turn altered the timing of the pulsar's signal as observed from earth, and the change in the signal was just as Einstein's theory said it should be.
A year after the publication of Taylor's paper on gravity waves, Hulse obtained his PhD and became a postdoctoral fellow at the National Radio Astronomy Observatory. In 1977 he took a position at the Princeton Plasma Physics Laboratory. He continues to follow developments in the astrophysics of pulsars, but his professional focus has long been on furthering the laboratory's mission to harness fusion (the source that powers the sun and other stars) to generate electricity. Hulse, who is now one of the lab's principal research physicists, moved into plasma physics because it seemed to offer better career opportunities. "I had concerns about the long-term prospects of getting a permanent position in astronomy, and I didn't want to go from post-doc to post-doc all over the place," he says. "It certainly wasn't because I didn't like astronomy anymore."
Taylor's interest in astronomy goes back to his boyhood. Born in Philadelphia in 1941, he grew up on a peach farm in Cinnaminson, New Jersey, that has been in his family for more than two centuries-"a plot of green," he recalls, in the industrial belt along the Delaware River north of Camden. "My older brother, Hal, and I once built an optical telescope. We used it more for spying on the neighbors than looking at the stars, though." Young Joe and Hal (who also became a physicist; he teaches at Stockton State College, in Pomona, New Jersey, and still lives on the family farm) were ham-radio enthusiasts, and they both credit this hobby for whetting their interest in science. "We erected large, rotating antennas on the roof of the farmhouse," says Taylor. "One time we managed to shear off the brick chimney, flush with the roof. My parents didn't appreciate it."
As a high school student at Moorestown (N.J.) Friends, Taylor excelled in mathematics, a subject he pursued at Haverford College before switching to physics. A particular influence was his senior adviser, Thomas Benham, a blind professor. "He would lecture from his notes in Braille," Taylor recalls, "and we had to report our homework assignments orally."
Taylor enjoyed electricity and radio, so with Benham's guidance he combined the two subjects and built a radio telescope for his senior project. For about $100 he bought enough equipment to build two separate detectors in a field along the edge of campus. "They were made out of wire, string, and sealing wax," he says, smiling. The biggest expense was a coaxial cable several hundred feet long to connect the telescopes. By wiring together two small antennas this way, Taylor made them function like one large telescope. He buried the cable a foot underground. The instrument was accurate enough to pick up a few of the major radio sources in the sky, including the stars Cassiopeia A and Cygnus A. Taylor refers to the former as "Cass A," as if it were an old girlfriend. Taylor says he heard that, several years ago, workers digging the foundation for a new building at Haverford were annoyed to find a long coaxial cable that seemed to run on forever.
After graduating from Haverford in 1963, Taylor entered the astronomy department at Harvard. He knew from the beginning that he wanted to work in radio astronomy. His dissertation involved locating radio sources by a technique called lunar occultation. (The radio telescopes of the day lacked the resolution to pinpoint small-diameter radio sources. But if such a source lay in the moon's orbital path, it blinked out whenever the moon moved between it and the earth. The moon's location thus revealed the source's.) Says Taylor, "In January 1968 I finished up my thesis. I was tired of occultations, and I was looking for something new to do." A month later, Bell and Hewish published their paper about the pulsar they had discovered the previous August. Taylor had found his new project, and he has never looked back.
Taylor spent 11 years at the University of Massachusetts, and by 1980, when he accepted an offer from Princeton, he was regarded as one of the nation's leading radio astronomers. In 1984, he and several Princeton students began a systematic search for "millisecond" pulsars-those that flash in excess of 100 times a second. (The first millisecond pulsar had been discovered in 1982. Of the few dozen now known, Taylor's group has found about half. Most of the known millisecond pulars are binary.)
A pulsar slows down rapidly after its creation. But a binary pulsar has a second life. When the companion star swells to great size towards the end of its life, it loses a great deal of stellar material. The pulsar sucks in the effluvium. This added mass causes the pulsar to spin faster and faster, and in some cases it becomes a millisecond pulsar. Because such rapidly spinning pulsars are old, they are relatively cool, so very stable, making them extremely accurate celestial clocks. "This has some interesting cosmological implications," says Taylor. "Imagine that the earth and all other celestial objects are corks floating on a sea of gravitational waves. The waves affect the time it takes for pulses from a pulsar to reach the earth. It would seem as if the period were changing slightly"-he wiggles his hands, simulating the influence of imaginary gravitational fields. "If you time these variations-these wiggles-by comparing the pulses with the ticks of an atomic clock here on earth, you can get an upper limit on the amount of gravitational radiation rattling around in the universe. It turns out that there isn't all that much. As a result, several cosmological theories which had looked interesting have been ruled out."
Today, Taylor's routine is divided between research and teaching, tasks he regards as integral to each other. At his Nobel news conference in Jadwin Hall, he thanked his students, whom he called "bright, eager, and pushy," for stimulating his research. "I tend to enjoy lecturing undergraduates more than graduate students, so most of my graduate teaching is one-on-one in the laboratory or the observatory," he says. Taylor often teaches introductory physics, and in 1992, when he won the Wolf Prize, one of the most prestigious in physics, he and his wife donated the $100,000 that came with it to his department to support graduate fellowships.
Not surprisingly, such generosity, along with his enthusiasm and outgoing style, have won Taylor many friends in his department. When P. James Peebles *62, the Albert Einstein Professor of Science, introduced him at the news conference, he noted, "One of the best things about this prize is that such a nice person could be involved."
His research often takes him and his graduate students to Arecibo and another large radio telescope at Greenbank, West Virginia. The observational part of radio astronomy, he says, is "mostly just sitting around a display screen and control panels with knobs and buttons. The telescope itself is computer controlled-typically from a file of instructions prepared in advance." But the unexpected can disrupt what is ordinarily a dull routine. In 1988, Andrew Fruchter *89, at the time one of Taylor's graduate students, was observing a binary pulsar when its signal abruptly vanished. Taylor and other members of his team present were astonished. "We saw the signal coming in, and then all of a sudden it disappeared. There was great consternation, and a heated argument began as we tried to figure out what had gone wrong. Two or three of us almost came to blows as we argued about what to do next." They had mistakenly assumed that the instruments had broken down. Then the pulsar's signal reappeared 50 minutes later. As they soon figured out, the signal had been blocked by the companion star as it passed between the pulsar and earth-an eclipsing pulsar, the first ever observed.
"I've spent my entire career doing pulsar research," says Taylor. "About half a dozen times I thought that I should move on and do something else. But every time I thought about switching, some new discovery has fired me up again. I don't see the field getting stagnant-there's always something interesting and exciting to do." He pauses for a moment, then smiles. "Besides, ordinary stars, galaxies, quasars, and nebulae never vary in a man's lifetime. They seem lifeless and cold. Pulsars have personality." After 25 years of pulsar research, Taylor shows no signs of winding down.
Charles Seife '93, who worked this summer as a science writer at The Economist, of London, is a graduate student at the Columbia School of Journalism.