At the Cosmos Club, Washington, DC
May 6, 2016
President Larry Millstein called the 2363rd meeting of the Society to order at 8:04 p.m. He announced the order of business and welcomed new members. The minutes of the previous meeting were read and approved. President Millstein then introduced the speakers for the evening, Dr. Peter S. Shawhan, Associate Professor of Physics at the University of Maryland; Dr. John G. Baker, Astrophysicist at the NASA/Goddard Space Flight Center; and Julie McEnery, ([pronounced “Mc-N-ry” or “McHenry”, basically]) Astrophysicist and Fermi Project Scientist at the NASA/Goddard Space Flight Center. Their lecture was titled “LIGO, Gravitational Waves, and Colliding Black Holes”.
Dr. Shawhan began by explaining that General Relativity describes gravity as a consequence of the curvature of space time caused by the presence of mass. Mass in turn responds to this curvature. The effect of gravity can be carried as waves, which manifest as distortions in the geometry of spacetime—stretching and squeezing as these waves pass through an object.
All objects with mass affect spacetime, and so a massive object in motion creates the distortion over time that we call a gravitational wave. Compact, massive objects—such as neutron stars or black holes—that are orbiting each other are a good source of gravitational waves because the changing mass configuration creates a pattern of distortion each time they orbit.
Because the amplitude of the distortion effect falls inversely proportional to the distance from the source, any distortion is quite small when it reaches us. The typical distortion that we expect to see on Earth is 1 in 10-21. In other words, the diameter of the Earth would change by 10-14 meters, or a few times the width of a proton. Einstein predicted the effect, but had no expectation it would ever be observed.
The Large Interferometer Gravitational-Wave Observatory, or LIGO, involves two L-shaped observatories, one in Hanford, Washington and one in Livingston, Louisisana. Each LIGO detector consists of a pair of 4km tubes, each containing a long vacuum chamber. A single laser beam is split and directed down each tunnel, to be reflected by a mirror at the far end. When they arrive back at the LIGO detector, these separate beams are recombined. When recombined, the phases of the waves should match up and the light output remain the same. Distortions in space time manifest as discrepancies in the arrival time of light waves, which would knock the beams out of phase. Thus, LIGO can detect gravity by measuring an increase or decrease in the light intensity caused by constructive or destructive interference between the mismatched phases of the laser beams.
In September 2015, when LIGO was activated for a test run, it captured a split-second “Coherent Wave Burst Event”. The rising frequency of the signal was consistent with the pattern of a binary black hole merger event, in which two black holes would orbit faster and faster until they merged.
The event radiated energy three times the entire mass of our sun. In that moment the power output was fifty times the total output of all the stars in the universe. Dr. Shawhan noted that once LIGO is at full strength, it will be able to observe events over a volume 27 times larger than this early run, resulting, hopefully, in many more events and much better understanding.
Dr. Baker explained that if two black holes exist in proximity to one another, the gravitational waves generated by their orbit sap energy and angular momentum away from the system, causing them to drift closer and thus spin faster, which generates still greater gravitational waves. This feedback loop accelerates until the black holes merge. The speed of this decay scales directly with the mass of the black holes: The faster the system evolves, the more massive it must be. Once we have the mass, we can work backward from the observed amplitude to determine the distance. We can even infer the relative masses of the two black holes by analyzing any asymmetry in the gravitational wave signal
Dr. Baker noted that higher frequency gravitational wave signals are easier to detect, but they arising from (relatively) unusual events. There are many low frequency events that have a great deal to tell us about the universe but must be observed at greater sensitivity and on a longer time scale. Future space-based LIGO programs will extend our capabilities dramatically.
Dr. McEnery explained that, about once a day, we detect a gamma ray burst, which for a brief period outshines everything else in the gamma ray sky. These phenomena was first discovered in the 1960s by satellites designed to detect nuclear tests, but for 30 years we didn’t know what they were or even where they were.
Because they were so intense, it was assumed that they originated relatively close by, on a cosmic scale. In 1997, we were able to measure the redshift of the x-ray afterglow of a burst and determine that they in fact originated very far away…and therefore had to be one of the most energetic phenomenon in the known universe.
Short gamma ray bursts are hypothesized to be produced by the merger of two massive objects, the same processes that LIGO detects. Thus, if LIGO sees an event at the same time we observe a gamma ray burst, this will provide empirical confirmation of this 50 year old mystery.
Dr. McEnery noted that co-observations between gamma ray burst observatories and LIGO will provide other valuable data as well, because gamma ray bursts can be localized much more accurately than gravitational waves, allowing cosmologists to fill in critical context about the LIGO data.
After the conclusion of the talk, President Millstein invited questions from the audience.
One questioner asked whether gravity waves were expected to display light-like behaviors such as being scattered or refracted by concentrations of mass. This would be the case if gravity was conveyed by gravitons, but the early LIGO results do not indicate such distortion, which limits the potential for a particle-like model of gravity.
Another questioner asked whether there was a delay between gravitational observations and gamma ray burst observations, and whether this delay could be used alert radio astronomy telescopes to catch more gamma ray bursts. Dr. McEnery indicated that some pf the visible and X-rays associated with gamma ray bursts would likely develop discernible delay, and thus telescopes could potentially be positioned to observed these emissions after receiving notification from LIGO. Telescopes would need to slew fast, however, because the delay would be mere minutes.
After the question and answer period, President Millstein thanked the speaker, made the usual housekeeping announcements, and invited guests to join the Society. At 9:55 p.m., President Millstein adjourned the 2363th meeting of the Society to the social hour.
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