Gravitational waves

Ripples from the past

Print edition : March 18, 2016

Two black holes merge into one. The collision of two black holes holes is seen in this computer simulation. LIGO detected gravitational waves, or ripples in space and time generated as the black holes spiralled in toward each other, collided, and merged 1.3 billion years ago. This simulation shows how the merger would appear to our eyes if we could somehow travel in a spaceship for a closer look. Photo: The SXS project (

Figure 1: How our Sun and Earth warp space-time is represented here with a green grid. According to Albert Einstein's General Theory of relativity, the gravity of massive bodies warps the fabric of space-time and those bodies move along paths determined by this geometry. Photo: T. Pyle/Caltech/MIT/LIGO Lab

A map of the sky, from the earth's perspective, showing the probable locations of the binary black hole merger. Photo: Shane Larson, Northwestern University

Figure 2: These plots show the signals of gravitational waves detected by the twin LIGO observatories at Livingston and Hanford.

Figure 3: Comparison of the reconstructed wave strain with the predictions of the best-matching waveform computed numerically from general relativity over the three stages of the event: in-spiral, merger and ringdown. Also shown are the separation (in units of about 200 km, the Schwarzschild Radius of the binary system) and velocity of the black holes (in units of c, the velocity of light.) The plot shows the black holes were within a few hundred km before merging and were moving towards each other at significant fractions of the velocity of light.

Fig. 3. A schematic depiction of LIGO's interferometric gravitational wave detector. Light from a laser is split in two by a beam splitter; one half travels down the vertical arm of the interferometer, the other half travels down the horizontal arm. The detector is designed so that in the absence of gravitational waves (top left) the light takes the same time to travel back and forth along the two arms and interferes destructively at the photodetector, producing no signal. As the wave passes the travel times for the lasers change, and a signal appears in the photodetector. (The actual distortions are extremely small, but are exaggerated here for easier viewing.) Inset: The elongations in a ring of particles show the effects of a gravitational wave on spacetime. Photo: Alan Stonebraker/American Physical Society

The first detection of the sounds of the cosmos may be the harbinger of more such signals, which may tell us new things about the universe.
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