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The Gravity of the Situation

U of G prof studies gravitational waves to help decipher laws of the universe

BY ANDREW VOWLES

Gravity has long fascinated newly arrived physics professor Luis Lehner, who is studying the theoretical physics of gravitational waves at U of G and the Perimeter Institute for Theoretical Physics. Photo by Martin Schwalbe
Gravity has long fascinated newly arrived physics professor Luis Lehner, who is studying the theoretical physics of gravitational waves at U of G and the Perimeter Institute for Theoretical Physics. Photo by Martin Schwalbe

Drop a pencil on Earth and, unless the pencil point stabs your toe, you risk little damage. Drop that pencil on a neutron star, and you could cause the equivalent of an atomic explosion. That’s among the many weird properties of gravity that have brought Prof. Luis Lehner, Physics, to his studies of gravitational waves.

He’s now studying that field here at Guelph, having arrived last month from a faculty position in the southern United States. Lehner will spend roughly half his time at U of G. For the rest of the week, look for him at the Perimeter Institute for Theoretical Physics in Waterloo. Last fall, the institute added him to its growing roster of scientists, whose affiliate members already include four U of G faculty.

“I’ve always found gravity and its effects fascinating,” says the Argentinian native, who moved to Waterloo this summer with his wife, Lucy Beltramo, and their two daughters.

Take that pencil-sized “atomic bomb.”

A neutron star is what’s left over after a massive star runs out of fuel and collapses. It’s so dense that its gravity is many orders of magnitude greater than Earth’s. Formed in a similar way, a black hole is so dense that its gravitational collapse causes space-time to curve so strongly that not even light can escape from it.

Lehner studies the theoretical physics of gravitational waves, predicted by Einstein in his 1916 theory of general relativity. Many of these waves are believed to emanate from binary systems containing neutron stars and black holes, especially as these structures collide and merge.

To explain gravitational waves, he evokes the common analogy of a bowling ball on a rubber sheet. A ping-pong ball travelling across the sheet will be drawn to the dent created by the larger object.

Now imagine that the bowling ball is not rigid but is wobbling. Those wobbles send out ripples in space-time like waves travelling through water. Even from far galaxies, those ripples can be felt here on Earth — or so physicists believe.

The trick is to catch them. That requires special detectors able to pick up the waves’ vanishingly small signal.

Three LIGO (Laser Interferometer Gravitational-Wave Observatory) detectors are located in the United States — two in Washington state, the other in Louisiana. These massive L-shaped instruments send laser light down two long arms and compare the beams’ arrival time at the detector.

Gravitational waves would theoretically stretch and compress the arms relative to each other, just as those waves would cause a pencil to shrink and expand by an infinitesimally small amount, says Lehner.

These detectors are specially built to cancel out noise from other kinds of oscillations, including earthquake tremors, and to pick up the space-time ripples.

Another instrument in Germany called the GEO600 is operated by a German-British collaboration; yet another interferometer in Italy is run by a separate Italian-French collaboration. Scientists using these detectors co-operate in a larger network.

There are also plans to build an even larger detector in space that could “see” much further out. That plan would tie to research by Lehner and departmental chair Prof. Eric Poisson, who also studies gravitational waves.

Poisson says Lehner “is a ‘name’ in the field. He’s already developed a leading international reputation for his work.”

Since the LIGO instruments began running, no gravitational waves have been detected. But in a Nature paper published this summer, a research team reported that it has further tuned the instrument’s “ear” to catch gravitational waves that could have come from the Big Bang.

That’s important, says Lehner. Besides telling us more about such entities as neutron stars and black holes, this work is intended to help us “hear” or “see” conditions immediately following the universe’s birth. That will help us understand the laws that govern the universe and perhaps help in cracking a key puzzle — the relationship of gravity to the other basic forces of the universe.

One of those other forces — the strong nuclear force that holds atoms together — is the research focus of Guelph physics professors Carl Svensson and Paul Garrett, who work at another kind of detector based in British Columbia. On our Earth-sized scale, gravity is the weakest force, says Lehner, “but on a larger scale, it’s the only one that matters. The cosmos as a whole is dominated by gravity.”

As detectors become more sensitive, scientists should begin picking up their hoped-for signals within about a decade, he says.

“We should be routinely using gravitational waves to understand our universe.”

He says this work will probably deepen our understanding of matter and even help refine or revamp Einstein’s theory.

Lehner will run complex computer simulations to help model gravitational waves, including the results of collisions involving neutron stars and black holes.

Until this summer, he worked at Louisiana State University, within an hour’s drive of the LIGO detector in Baton Rouge. He explains that his theoretical work can occur anywhere he has access to a computer and interferometer data.

Here at U of G, he’s teaching physics for biological sciences this fall. Early in 2010, he will begin teaching in a new graduate program at the Perimeter. That program, called Perimeter Scholars International, will provide intensive training to prepare students for doctoral research in theoretical physics.

Lehner hopes to establish a similar training program in South America. He attended the National University of Cordoba in Argentina before pursuing PhD studies at the University of Pittsburgh. He held post-docs at the University of Texas at Austin and the University of British Columbia before joining Louisiana State.

At Guelph, he plans to work with the Canadian Institute for Theoretical Astrophysics and the Canadian Institute for Advanced Research.


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