Catching the Wave: LIGO Validates GR’s Last Big Prediction

“Ladies and gentlemen, we have detected gravitational waves. We did it!” – David Reitze, Executive Director of LIGO

[Updated February 14 and 21, 2016 to include additional links and references. Original posting on February 11, 2016.]

Last year, I wrote of the centenary of Einstein’s theory of general relativity. This morning, it was announced that the last remaining prediction of general relativity had been experimentally validated.

Courtesy Caltech/MIT/LIGO Laboratory Courtesy Caltech/MIT/LIGO Laboratory

At a press conference this morning, the Laser Interferometer Gravitational-Wave Observatory (LIGO) Collaboration announced the detection of gravitational waves, the validating the last major prediction of general relativity. What’s more, the detected signal precisely matched theoretical predictions of the waves which would be produced by the merger of a binary black hole system. And, as if that were not enough, the waves are in the acoustic frequency range, making it possible to translate the signal into sound such that we can actually “hear” two black holes merging. (Okay, it is more of a “boop,” but it came from over a billion light years away.)

Rumors of this discovery had been circulating for a few months. Today’s press conference was timed to coincide with the publication of their results in a peer-reviewed paper in Physical Review Letters. The twin Advanced LIGO detectors (located at Hanford, WA and Livingston, LA) had officially come online on September 18, 2015, but this particular detection was made by both detectors (see graphic above) on September 14 during a “shakedown” run. Some of the early rumors regarding the detection had been called into question since it was known that the detectors had been designed to randomly inject false test signals into the experiment (in a manner to which the experimenters are blind) to evaluate their handling of the data. But, as it turned out, this signal was real. And now, after a whirlwind of rumors, the official announcement is out, along with the paper.


Here is a video of the press conference:


Background: What Are Gravitational Waves?

Einstein predicted the existence of gravitational waves as a direct result of general relativity in a paper published in 1916 (English translation here). Just as the acceleration of electrical charges causes the propagation of electromagnetic waves, Einstein predicted that the acceleration of mass would cause ripples in space-time. However, these predicted ripples would be so minute, even for powerful events, he despaired of them ever being detected.

As it turned out, this is one of a handful of things Einstein turned out to be wrong about. Gravitational waves were first indirectly detected back in 1974 by radio telescope observations of the binary pulsar PSR B1913+16 by Hulse and Tayler, work which netted the pair a Nobel Prize in 1993. (Sabine Hossenfelder has more of that story here.)

How LIGO Works

But that was an indirect observation, obtained by studying the decaying orbits of a binary pair of neutron stars. Direct observation is far more challenging, and is accomplished by means of laser interferometry.

Here is what happens in LIGO’s twin interferometers. Light from a laser is divided into two beams by a beam splitter, with the two beams going out at a 90 degree angle to one another. The beams travel along a four kilometer long path, strike mirrors at the end of the path, and return to the source location. There, the reflected light beams are re-combined into a single path, where constructive or destructive interference takes place depending upon the relative phases of the two beams, which in turn depends upon how far the beams have travelled. Subtle changes in this interference are used to measure minute changes in the distances that the two beams have travelled.

How minute?  In the case of the LIGO results being discussed, the difference in the travel path is a mere fraction of the diameter of a proton!

Of course, the biggest challenge for the LIGO team is to prevent ambient vibrations from adding noise to the data.  One of the most intriguing aspects of the experimental design of the LIGO detectors is the use of glass fiber quadruple-pendulums to suspend the optical elements, thus isolating them from local ambient vibrations.

It has been a long road since the first iteration of LIGO came online back in 2002, filled with hard work and innovation. Now, champagne corks are a-popping.

Einstein’s prediction:

Indirect detection via Hulse-Taylor binary pulsars:

  • Weisberg, J. M.; Taylor, J. H.; Fowler, L. A. (October 1981). “Gravitational waves from an orbiting pulsar”. Scientific American 245: 74–82. Bibcode: 1981SciAm.245…74W. doi: 10.1038/scientificamerican1081-74.
  • Taylor, J. H.; Weisberg, J. M. (1982). “A new test of general relativity – Gravitational radiation and the binary pulsar PSR 1913+16”. Astrophysical Journal 253: 908–920. Bibcode: 1982ApJ…253..908T. doi: 10.1086/159690.
  • Taylor, J. H.; Weisberg, J. M. (1989). “Further experimental tests of relativistic gravity using the binary pulsar PSR 1913 + 16”.Astrophysical Journal 345: 434–450. Bibcode: 1989ApJ…345..434T. doi: 10.1086/167917.
  • Weisberg, J. M.; Nice, D. J.; Taylor, J. H. (2010). “Timing Measurements of the Relativistic Binary Pulsar PSR B1913+16”.Astrophysical Journal 722: 1030–1034. arXiv: 1011.0718v1. Bibcode: 2010ApJ…722.1030W. doi: 10.1088/0004-637X/722/2/1030.

Direct detection by LIGO/Virgo:


For More Information:

About Glen Mark Martin

MCSE-Messaging. Exchange Administrator at the University of Texas at Austin. Unrepentant armchair physicist.
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1 Response to Catching the Wave: LIGO Validates GR’s Last Big Prediction

  1. akutz says:

    Arrrr, time to set sail on the gravitational waves in them skies yonder! Avast ye Spyglass, be thee my matey?

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