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Merger of Two Black Holes Detected by Gravitational Wave Observatory– A Historic First


On 2015 September 14 at 09:50:45 UTC, the merger of two black holes with masses in the range of 25-50 stellar masses was detected by a pair of advanced gravitational wave receivers, as reported in a letter to the Physical Review Letters of the American Physical Society on February 11, 2016.  This was the first confirmed detection of a “gravitational wave” by the new advanced LIGO (laser interferometer gravitational wave observatory) based on the classic Michelson-Morley interferometer first used in an attempt to detect the presence of a universal ether.

This first attempt failed, resulting in the general conclusion that there is no “universal ether” that surrounds all objects in space and thus no preferred direction for orienting us in space.   What was worse, there was no carrier on which to base the radiation of light to create “waves” (unlike the waves on the surface of a pond, which are carried by the water itself.) The new detection shows that the original interferometer was not sensitive enough; now that the sensitivity has been increased by a factor of several thousand, the extremely weak and small amplitude signals that accompany gravity waves can be “felt.”

Several of these new interferometers have been built, but so far only two were on line to record the merger of GW150914 last September.  They are the culmination of a long series of increasingly sensitive devices, which have finally been built to reach the predicted sensitivity needed for detection of the most massive gravity waves: those produced by the merger of black holes.  First, in 1975, the observation of a binary pulsar system, and later, the calculation of its energy loss, allowed the calculation of the amplitude of such waves at galactic distances and the demonstration of the definite existence of gravitational waves.  Over a period of  30 years, the development of increasingly sensitive interferometers finally produced a network that could, theoretically, detect the passage of waves if sufficiently large objects producing waves could be found.  By 2011, the upper limits for the magnitude of such waves had been established; finally, in 2015, the first definite signals of gravity waves were detected.

The gravitational waves that are associated with the merger of two medium-sized black holes are very small at this distance, although the source intensity is roughly 2 x 10 to the 56th power ergs.  In a time of less than 0.2 seconds, a signal increasing in frequency from about 35 Herz to almost 150 Herz (at maximum amplitude) was detected over a period of just eight cycles.  It was calculated that this signal resulted from the final collapse of a binary system of two black holes, their merger, and the “ringdown” or reducing intensity signal after they were unified.  A bandpass filter was used to confine the signal to these frequencies for convenient analysis.  Only two of the interferometers were turned on and making observations at the time, but this was enough to record an identical signal with a 10 microsecond delay corresponding to the different locations of the two detectors: Hanford, Washington, and Livingston, Louisiana.

Calculations placed the orbital period of the two black holes at 75 cycles per second, with a separation of roughly 350 kilometers apart, just before the final merger.  Each of the black holes must have been less than 210 kilometers in diameter.  The signal from this black hole merger traveled far enough to produce a redshift of 0.09– not very far away as galaxies go.  A hundred years after the existence of gravitational waves was predicted by Einstein and their mathematical reality was confirmed by Schwarzschild (who published a solution for the field equations that described a black hole) their existence has been confirmed to a high degree of certainty by advances in instrumentation and physical technology.

What does this mean for us?  It confirms to a high degree of confidence that Einstein’s predictions were correct and the “standard model” of physics is accurate (as far as it goes.)  It opens up a new avenue for observing the universe around us.  It makes us more comfortable in the certainty that Einstein was right.  Other than that, it’s not very exciting.  The implications for astrophysics of this observation are discussed in another technical article in Astrophysical Letters. For those of us deterred by astrophysical jargon, there is an article in Science News that clearly summarizes the findings in nontechnical language.

Physics has become fairly dull (at least, to outsiders) because all the observations made to date agree with our theories of what those observations should look like.  The exciting part will be when we find Earth-like planets circulating not-so-distant star systems and tantalizing us with the possibility of life elsewhere in the universe.  Or else, perhaps, some experiment will uncover an inconsistency in the “standard model” and we will have to start all over again.

Someday, we will solve the problem of travel to other star systems.  The evolution of the technique for observing gravitational waves from theory to confirmation in just a hundred years makes me confident that we will not have to wait too terribly long for that trip to Alpha Centauri.


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