In the years since Einstein published his theory of general relativity -- in which he proposed that gravity, traditionally considered a force, is actually a manifestation of curved space and time -- the theory has been tested and affirmed in some of the most rigorous regimes nature could provide.
But even Einstein speculated that the theory might falter under extreme conditions. And if it does, scientists are intent on finding out what those conditions are.
Cornell professor of astronomy Jim Cordes is looking at some of the most extreme places in the universe to probe the limits -- if there are any -- to Einstein's central theory. By finding and studying pulsars in binary systems (in which two such objects orbit each other), researchers can measure gravitational effects that the ultra-dense objects exert on each other and make comparisons with the predictions of general relativity. Pulsars are rapidly spinning neutron stars that produce pulses locked to the spin, providing an astrophysical clock for measuring relativistic effects.
"[When] the system is compact, the gravity is intense -- so it ends up mattering what theory of gravity you use to describe the orbit," Cordes said. "Another way of saying that is: If you assume a particular theory of gravity that predicts what the orbit should look like, and you monitor and compare it with the prediction, then as you see departures from the model you can infer something about the gravity."
Using that strategy, in 1974 University of Massachusetts astronomers Russell Hulse and Joseph Taylor discovered a binary system with two neutron stars in an orbit about the size of the sun and with an orbital period of eight hours. Yearly observations over the next two decades indicated that the orbit shrank by about 1 centimeter per day. That result can't be explained by Newtonian dynamics, but it is in almost perfect agreement with Einstein's theory of general relativity, which predicts the shrinkage because the binary should be emitting gravitational waves that carry away energy from the system.
Hulse and Taylor earned the Nobel Prize in 1993 for their work after both had moved to Princeton University. Now, Cordes and others are pushing the envelope even further.
"Gravity has to be tested in much stronger regimes," he said. For binary systems that are more compact than the Hulse-Taylor binary, or ones with more mass (such as a massive black hole companion to a pulsar), the effects of gravity are even bigger and easier to measure, said Cordes. By measuring orbits and the propagation of the pulsar signal past the companion, especially near to the event horizon (the boundary, or point of no return) of a black hole, astronomers can detect "effects that disagree with general relativity and signify that a different theory of gravity is needed."
Since compact objects in binary systems are in a process of spiraling inward, Cordes said, such systems must exist -- possibly in distant galaxies, or possibly near the center of our own. And a number of highly sensitive operating and planned observatories are poised to make the search easier by scanning larger areas of the sky, providing data that are processed with better algorithms and using more powerful computer systems for identifying pulsar signals. (Because data management for large-scale surveys requires considerable resources, Cordes and colleagues are collaborating with the Cornell Theory Center to archive and process the data and provide survey results to the broad astrophysics community.)
With this array of new and planned instruments, Cordes said, collaboration between astronomers specializing in different areas will increase dramatically. And from that cooperation (between, say, radio astronomers and researchers looking for direct evidence of gravitational waves with such instruments as the Laser Interferometer Gravitational Wave Observatory), he hopes for new insights into the nature of gravity, how neutron stars and such exotic compact objects as magnetars form from supernovae, and into what the universe looked like in its first moments of existence.
Currently, Cordes is using the Arecibo L-Band Feed Array (ALFA) receiver at Arecibo Observatory to survey the Milky Way in search of new and interesting compact objects. And he is also hoping one day to use the Square Kilometer Array, a proposed observatory with about 1-square-kilometer of collecting area -- large enough to complete a full galactic survey for neutron stars.
"The next major discovery to be made is a pulsar orbiting a black hole," Cordes said. That system would be valuable for several reasons: Black holes are ultra-massive; the orbit of such a system would probably be very small because of the extreme compactness of the black hole; and for the radiation that just grazes the event horizon or close to it, its actual path depends on the properties of gravity. Thus, by monitoring such an orbit it might be possible to probe gravity in an intensely strong field.
"And, who knows?" Cordes added. "We might see a departure from general relativity -- or we might not. It's interesting either way."