Physics student wins acclaim for new theory of neutron-star spin
PASADENA--When you're beginning a career in cosmology, it's only fitting to start with a bang.
That's what Ben Owen will do now that he has his doctorate in physics from the California Institute of Technology. Not only did Owen win the annual Clauser Prize for the best Caltech dissertation at the June 12 commencement, but his work has also been the subject of an international symposium. In September, he'll fly to Germany for a new job at the Albert Einstein Institute (where the symposium was held) as a postdoctoral researcher.
The reason Owen's dissertation has stirred so much interest is that it solves a nagging, decades-old question in astrophysics and opens up vistas of new questions.
In particular, chapter 5 of his "Gravitational Waves from Compact Objects" shows why young neutron stars have such slow spins. The research of chapter 5, which was done with Lee Lindblom and Sharon Morsink, predicts that rapidly spinning, newborn neutron stars will pulsate wildly, throwing off their spin energy as gravitational waves. The work appears in the June 1 issue of the journal Physical Review Letters.
The new theory of Owen and his colleagues will be tested experimentally in a few years, after the Laser Interferometer Gravitational-Wave Observatory (LIGO) comes on-line.
Neutron stars are extremely compact bodies about the mass of the sun, packed into a sphere about 15 miles in diameter. They are typically formed in the supernova explosions of massive stars.
Because the fusion of lighter elements in the star has ceased, the material remaining after the explosion scrunches together so closely that the electrons and protons of most of its atoms actually fuse together to form neutrons—and thus the name.
Neutron stars are not so compact as black holes, which are regions so dense that not even light can escape. But neutron stars are still compact enough to generate some bizarre effects. If an astronaut landed on a neutron star, for example, both he and his spaceship would be smeared by gravity into an even layer of just a few atoms over the entire surface of the star.
Also, neutron stars are noteworthy for their tendency to spin like crazy. Astronomers on Earth infer this spin from a telltale "blinking" in radio signals or sometimes even in a strobelike blinking in visible light. Based on the rate of blinking, observers know that these particular neutron stars—known as pulsars—can spin as rapidly as 600 times per second.
But this is where the controversy comes in and where Owen's dissertation is stirring up so much interest. Based on the laws of Newtonian physics, there's no compelling reason why a slowly rotating normal star shouldn't speed up to the fastest rotation rate possible once it goes supernova and then collapses into a neutron star.
The same effect can be seen in an ice skater who pulls in her arms to rotate faster while spinning.
But all of the young neutron stars observed by astronomers spin at 120 revolutions per second or less—a factor of five slower than the fastest known pulsar, which is very old and is thought to have been spun up long after the supernova by other mechanisms.
Owen's theory is that a type of fluid circulation occurs on the neutron stars that creates a sort of drag in space-time. Called "r-modes" because they owe their existence to rotation, these motions look much like the ocean eddies that move currents in circular motions on Earth.
What Owen's dissertation has shown is that the r-modes of a rapidly rotating neutron star strongly emit gravitational waves. The drag effect, caused by the gravitational waves leaving the star, in turn causes the r-modes to grow when they would normally die away due to the internal friction found in young neutron stars. In the process, this forces the spinning neutron star to slow down.
Thus, newly created neutron stars can indeed start their lives spinning quite rapidly, but are quickly slowed down by the growing r-modes. Old neutron stars have much stronger friction and can be spun up again by other processes.
"The standard methods known right now say that these currents could grow very large," says Owen.
The size of the r-modes is the key, he explains. His work shows that, if an r-mode were to be so large that it sloshed material virtually from pole to pole, the neutron star should slow down to one-tenth its original rate of rotation within a year. This, in fact, conforms to the rates of rotation seen in existing pulsars.
But the effect is a self-defeating one, Owen says. The r-modes are kept going by gravitational waves, which are stronger when emitted by rapidly rotating stars. But the gravitational waves leaving the star cause it to spin down, which makes the waves weaker, which in turn means there is less power to keep the r-modes going. So the neutron star eventually reaches an equilibrium.
"If the r-modes get very large, they'll start radiating a lot of energy as gravitational waves," Owen says. "But they can't do that forever, because the rotational energy they're radiating is what keeps them alive in the first place."
So in the course of a year, Owen shows, just about any pulsar should be spun down to a rotation rate much less than the Newtonian maximum.
Owen's work is purely theoretical at this point, but could be tested when LIGO is operational. LIGO, a collaborative project between Caltech and MIT with twin detectors in southern Louisiana and central Washington, is designed expressly for the detection and detailed study of gravitational waves.
If a supernova goes off in our cosmic neighborhood-say, within 60 million light-years-LIGO should be able to detect the gravitational waves thrown toward Earth. And if the waves change at the predicted rate over the course of a year, Owen's theoretical work will be borne out by observation.
"Several supernovae should go off every year at a distance close enough for LIGO to detect the waves," he says. "So when a supernova occurs, we should first see the waves start very abruptly at up to 1,000 cycles per second, and then chirp down to about 100 to 200 cycles per second over the course of a year."
The work of Owen, Lindblom and Morsink raises a vista of new questions, with which cosmologists and gravity-wave experimenters world-wide are now struggling. Just how large does the sloshing in a young neutron star get, and what limits its growth? Can LIGO experimenters redesign their computer programs to find Owen's predicted waves in LIGO's plethora of data? What other kinds of stars will slosh wildly, like Owen's newborn neutron stars, and what will that sloshing do to them, and can LIGO be tuned to find their gravitational waves?
Owen's thesis supervisor at Caltech was Kip Thorne, a renowned theoretical physicist who is author of the popular book Black Holes and Time Warps: Einstein's Outrageous Legacy.