Gravitation By Calla Cofield
The APS Topical Group in Gravitation
reached 1,000 members at the end of 2007, making it the largest topical group in APS. But general relativity alone can’t take the credit. As the group’s Chair Dieter Brill, of the University of Maryland, explains, “The community of gravitational physicists is widely varied and very diverse, ranging from astrophysicists to mathematicians, from experimentalists to quantum gravity theoreticians, and from cosmologists to data analysts. It is united not by a shared toolbox of techniques or by a common point of view, but rather by a passion to understand the workings of gravity.”
The group’s chair-elect David Garfinkle hopes the group is on its way to becoming an APS Division. It will take a few hundred more members for the group to achieve that goal, but over 30% of the group’s members are graduate students, suggesting that the group will continue to grow in the near future.
The field of gravitation is experiencing a particularly large growth spurt as scientists begin to analyze data from the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO is searching for gravitational waves, originally predicted by Einstein. Gravitational waves are thought to arise mainly from violent events, such as two colliding black holes or supernova explosions. Until recently, the existing technology, both experimental and theoretical, was insufficient to allow the detection of gravitational waves, and even LIGO is unlikely to detect them. Advanced LIGO, an upgrade that will make the current LIGO detectors 10 times more sensitive, is set for completion in 2008, and is expected to detect gravitational wave signals.
“We are all waiting for the first confirmed detection of a gravitational wave signal. This will confirm once again…that purely by thinking about things (if you are an Einstein) you can bridge the gap over enormous distances in space, interaction strength, conceptual pictures …if, that is, these waves are discovered. If not it’s an even greater challenge, to find out why the Good Lord does not fully use this beautiful possibility (as Einstein might have said),” says Brill. If Advanced LIGO can’t find signs of gravitational waves, it will seriously call into question the understanding of how astrophysical bodies behave, but will not immediately rule out the existence of gravitational waves.
About 30 scientists work full-time for the NSF-funded LIGO Laboratory, a joint effort of MIT and Caltech, which built the LIGO detectors in Oregon and Louisiana. Over 580 scientists, engineers, and other contributors from 11 countries and 42 institutions belong to the LIGO Scientific Collaboration. Members of the Collaboration have worked to analyze LIGO’s data and interpret the results, to develop improved detector technologies for future upgrades and to contribute to building and installing the detectors.
Beyond LIGO, current questions facing gravitational physicists include how to unite gravitation with quantum theory, as is being pursued, for example, by string theory. Brill adds, “Another exciting challenge to [gravitation] theory is the new cosmological observations that give us ever better data about the early universe, about dark matter and dark energy, extra dimensions, and even more exotic constituents. This is a far cry from the old days, where the few cosmological parameters we had were only inaccurately known. Today we can build quite exact models of the universe.”
The gravitation group has a semiannual newsletter that Brill recommends to anyone interested in learning more about the field of gravitation. The newsletter began in 1995, and features original articles written by active gravitational physicists. The articles aim to be accessible to all physicists, not just specialists.
Many GGR members participate in four annual regional general relativity meetings (Pacific Coast, Gulf Coast, East Coast, and Midwest). The GGR sponsors a $200 prize at each of them for the best student presentation. Hosted by different universities each year, these meetings are free and open to all, and information on them is sent through the GGR mailing list.
The APS April Meeting also serves as an important venue for GGR. Both Brill and Garfinkle were excited about last April’s meeting when Francis Everitt of Stanford announced the results of the Gravity Probe B experiment, which, after decades of planning, successfully launched a package of four gyroscopes into orbit around Earth. The results were in accordance with Einstein’s theory, and were also an advance in precision instrumentation. Other GGR sessions are organized for the April Meeting, such as last year’s History of Relativity. The GGR supplies travel grants for graduate students and postdocs to attend the April Meeting, which is an important tool for bringing together its community of physicists. The GGR also presents certificates to its new Fellows at the meeting, and organizes the prize session for the biennial Einstein Prize, awarded by APS for outstanding work in gravitation. The 2007 prize went to Ronald Drever of Caltech and Rainer Weiss of MIT for their fundamental contributions to LIGO.
As part of its public outreach effort, GGR collaborated with the Forum on the History of Physics to establish the Las Cumbres Speakers Program. Schools and groups can apply to have specially selected speakers come and give talks on Einstein’s life and science, as well as talks on current developments in gravitation. The program was established in 2005, in conjunction with The World Year in Physics, celebrating the centennial of Einstein’s “miracle year.” Einstein’s legacy is a large part of public fascination with physics, and Brill says that’s important for the GGR to consider. He adds, “Society certainly has not been indifferent to relativity and cosmology. They are among the topics in which people show the greatest, purely curiosity-driven interest. The recent centenary of Einstein’s Annus Mirabilis has renewed world-wide interest in relativity and physics generally, an interest that deserves to be further explored in educating people, particularly young people, in the sciences.”