W&L's Williams Examines Gravitational Waves Discovery in Roanoke Times
The following oped by H. Thomas Williams, Edwin A. Morris Professor of Physics Emeritus at Washington and Lee, was published in the Sunday, March 13, 2016, edition of the Roanoke Times and is reprinted here by permission.
A Perfect Pass
by H. Thomas Williams
Whenever I see an NFL quarterback throw a pass to a receiver running with his back to the quarterback but who makes his cut at precisely the right place and time for the pass to be completed, I marvel at the planning and precision of such a play.
That is the image — a perfectly executed forward pass — that came to mind in February as I watched a live-stream status report from the Laser Interferometer Gravitational Wave Observatory (LIGO).
If you are not aware of the news from LIGO, you should be. Although this report might have been lost among other major media events and breaking news during February, it is far more consequential than the presidential debates, the Grammy Awards or even one presidential candidate’s dustup with the Pope.
In fact, that report is the most significant scientific news of my half-century-plus career as a scientist — by far. And there have been many stunning breakthroughs during this period: practical advances such as lasers and superconductors, revealing new views of our solar system, and the 2013 discovery of the Higgs boson, explaining the notion of mass of fundamental particles.
The LIGO announcement featured three compelling narrative threads: confirmation of Albert Einstein’s century-old theory, the remarkable achievement represented by the wave detectors themselves, and the staggering cosmic coincidence that delivered a significant piece of data to those detectors just as they were turned on.
First, Einstein: In 1915, he developed a generalized version of his iconic special theory of relativity. It explained the behavior of accelerating objects and its intimate relationship to gravitation. With the general theory, Einstein provided an answer without a question, exercising his deep intuition to develop arcane notions such as black holes and gravitational waves well in advance of any experimental evidence for their existence.
The intrinsic weakness of the gravitational force suggested that for gravitational waves to be detectable, they would have to originate in events involving huge masses experiencing extreme accelerations. The merger of two black holes, each about the mass of 30 suns, resulting from a violent gravity-driven collision would seem to be just what was needed. A century following Einstein’s insights, we have now used gravitational waves to detect for the first time a binary black hole system —one that met its cataclysmic demise 1.3 billion years ago.
Beyond the discovery itself, the LIGO detection mechanism itself is a modern wonder. It consists of two identical instruments called laser interferometers — one in Hanford, Washington, the other over 1800 miles away in Livingston, Louisiana. In each device, a laser beam is split and its pieces are sent down and reflected back along two perpendicular arms, each of which is four kilometers long. The light beams, recombined, are capable of detecting miniscule differences in the length of the arms. To separate infinitesimal gravity wave effects from more common and less interesting vibrations from a passing vehicle or a chair sliding on the laboratory break room floor, unprecedented levels of vibration isolation were developed. Despite this, local vibrational noise remains problematic.
To distinguish gravitational signals from vibrational noise, the two widely separated interferometers are monitored simultaneously. The notion is that the hiss and burble of random local vibrations will be recorded in each location, with no correlation between the two: truck traffic in Louisiana will affect the signals there, but not those at the Washington site. If, however, these two signals become correlated, even briefly, the cause must be not local, but cosmic — i.e., gravitational waves.
The side-by-side traces of signals from the Hanford and Livingston interferometers at 5:51 a.m. EDT on a September day last year show separate random wiggles evolving into a breathtaking synchrony lasting roughly two-tenths of a second before returning again into unrelated noise. The signal was unmistakable, and stunning. The modern-day watchmakers who crafted this instrument array for the sole purpose of seeing such a signal were rewarded richly, and immediately.
This takes me back to that NFL pass play, and the precision necessary to achieve a completion. Just consider: the initial gravitational wave event recorded by LIGO was triggered by the merger of two black holes 1.3 billion years ago. The resulting wave spread like a ripple caused by a stone dropped into an otherwise smooth pond, the size of the ripple diminishing with its distance from its source. Moving at the speed of light, this gravitation ripple passed through the LIGO detectors in 0.2 seconds, and then was gone. Its fleeting arrival — a pass thrown 1.3 billion years ago — appeared on LIGO’s receivers during the first week in the history of mankind when we had an instrument capable of detecting it!
Even more surprising — this tiny “chirp” was detected on September 15 of last year, a week before the instruments were scheduled to go live, in fact during its final “dress rehearsal.” To consider this mere coincidence goes beyond reason. It might be read as near-definitive proof of the existence of a benevolent God — one, in fact, who has particular interest in the success of this scientific project.
Another explanation is that this violent cosmic event is far from unique, and the advanced LIGO observatory has opened a window on the universe that will flood us with such data and new discoveries. This notion is supported by the fact that during the four months of active observation with LIGO since that initial celebrated event, scientists have recorded and are now analyzing at least four more potential gravitational wave events.
So now what? It is hard to say, but chances are that in opening this new channel of communication with the universe we will be treated to a constant stream of unexpected revelations about our cosmic home. This is a channel to which we should all stay tuned.
Thomas Williams is the Edwin A. Morris Professor of Physics Emeritus at Washington and Lee University where he taught for 37 years prior to his retirement in 2011. His research has been focused on theoretical nuclear and particle physics.