Nobel laureate lectured at MU about pulsars

Erik Hoversten

“This is a detective story of not who dunnit, but what dunnit,” said Russell Hulse of the Princeton Plasma Physics Lab Thursday night at Physics Hall.

Hulse then led a captivated audience on a tale of mystery, perseverance and luck which led to his 1993 Nobel Prize in physics. He and Joe Taylor, his Ph.D adviser, were awarded for the discovery of a binary pulsar system, two pulsars orbiting around each other.

Pulsars are a special kind of neutron star, which is the remnant of a catastrophic supernova explosion at the end of the lives of some stars. Despite having 1.4 times the mass of the sun, Hulse said, pulsars are a mere 15 miles across.

What separates pulsars from regular neutron stars is that they have powerful magnetic fields, a trillion times stronger than that of Earth, Hulse said. Because of this, they emit two beams of radio waves on opposite sides.

Pulsars also spin very fast, Hulse said. Each revolution ranges from milliseconds up to 4 seconds.

The combination of powerful beams of radio waves and the spinning pulsar creates a “lighthouse effect,” which Hulse demonstrated by waving a penlight over his head like a lariat.

Hulse said the time that pulsars take to spin is unique to each one and barely changes over time. “The really important thing to remember is that pulsars are some of the best clocks in the universe,” he said.

Hulse began his story in 1974, when he was a graduate student at the University of Massachusetts. He was working on discovering new pulsars with faster pulsation periods at Arecibo Observatory in Puerto Rico.

“I’m nuts about antennas, so you’ll have to bear with me while I give you the tour,” Hulse said as he put up an overhead of the gigantic radio telescope where he spent so much time. The dish he showed measured 1,000 feet across and was carved into a hill.

The radio waves the dish receives are bounced to a receiver suspended 450 feet above the bottom of the dish and accessible only by a long catwalk or cable car, he said.

“There are the people who refuse to go up the catwalk and those who refuse to go up the cable car. Actually, there are the people who refuse to go up at all,” Hulse quipped, admitting he was one of the few who thought it was actually cool to go up to the receiver.

What separated Hulse’s search for pulsars from previous ones was that his was the first to utilize computers. Signals from pulsars are garbled by extraneous signals and different arrival times of different frequencies of radio waves caused by interstellar clouds of gas.

In his search, Hulse found 40 new pulsars — but of particular interest was PSR 1913 +16, at the time the fastest known pulsar.

Hulse measured the pulsation period of the star twice but found that the periods differed by 27 millionths of a second. Even though this sounds very close, he said, experimental error was calculated to be one millionth of a second. “One doesn’t look at this as ‘Eureka, a discovery,’ one looks at this as ‘Oh now what’s wrong,'” Hulse said.

Instead of taking the easy way out and writing off the discrepancy as some sort of error, he measured the period one more time and got an entirely different result.

After recording more data, he had a revelation one late evening while staring at plots in his lab book, which caused him to suspect that the pulsar must actually be a pulsar and a neutron star spinning around each other.

Because pulsars form in such violent explosions, it was widely thought that binary pulsars did not exist. “Did that mean I was going to go tell other people about and look like an idiot if I was wrong? No, I wasn’t going to do that.”

After further study, the two stars were found to be twice the distance from the sun to the moon at their closest. They also reach speeds of 300 km/s as they orbit each other every 7 hours and 45 minutes.

The significance of the discovery that made it Nobel Prize-worthy was the system acts as a laboratory for testing Einstein’s General Theory of Relativity, published in 1915.

The best evidence for the theory at the time was the advance of the perihelion of Mercury, in which the elliptical orbit of Mercury rotates itself one thousandth of a degree per year which is unexplained by the actions of the other planets. For the pulsar, it is four degrees per year.

Even more impressive, because the stars are so massive “you make ripples in space-time which radiate out.” This causes the system to lose energy and the stars to slowly spiral inward.

It is estimated they will collide in 300 million years.