Physicists Detect Gravitational Waves

Today’s major story in the scientific world is an announcement from Laser Interferometer Gravitational Wave Observatory (LIGO) on the detection of gravitational waves, long hypothesized by Albert Einstein. The New York Times has the most comprehensive coverage that I’ve seen:

The discovery is a great triumph for three physicists — Kip Thorne of the California Institute of Technology, Rainer Weiss of the Massachusetts Institute of Technology and Ronald Drever, formerly of Caltech and now retired in Scotland — who bet their careers on the dream of measuring the most ineffable of Einstein’s notions.

Dr. Thorne of Caltech and Dr. Weiss of M.I.T. first met in 1975, Dr. Weiss said, when they had to share a hotel room during a meeting in Washington. Dr. Thorne was already a renowned black-hole theorist, but he was looking for new experimental territory to conquer. They stayed up all night talking about how to test general relativity and debating how best to search for gravitational waves.

Dr. Thorne then recruited Dr. Drever, a gifted experimentalist from the University of Glasgow, to start a gravitational wave program at Caltech. Dr. Drever wanted to use light — laser beams bouncing between precisely positioned mirrors — to detect the squeeze and stretch of a passing wave.

The two LIGO observatories (one in Washington State and the other in Louisiana) showed a similar response to the gravitational waves from two colliding black holes, as seen in the below graphic:


The sensitivity to detect these gravitational waves is extraordinary:

Lost in the transformation was three solar masses’ worth of energy, vaporized into gravitational waves in an unseen and barely felt apocalypse. As visible light, that energy would be equivalent to a billion trillion suns.

And yet it moved the LIGO mirrors only four one-thousandths of the diameter of a proton.

The actual abstract LIGO is below. The full paper is here (with author citations at the end of the paper that number ~1,000 scientists!).

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0 × 10-21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160-180 Mpc corresponding to a redshift z = 0.09+0.03−0.04. In the source frame, the initial black hole masses are 36+5-4 M and 29+4-4 M, and the final black hole mass is 62+4-4 M, with 3.0+0.5-0.5 Mc2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Incredible. Don’t miss the video at the top of The New York  Times article. It’s worth ten minutes of your time.

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