Direct detection of gravitational waves

Settled in 2015 by LIGO Scientific Collaboration and Virgo Collaboration; Nobel Prize 2017 to Weiss, Barish, Thorne.

Ginzburg listed gravitational-wave detection as a separate, 'technically complicated' problem (#22) from GR verification in general, describing LIGO as then under construction. On 14 September 2015, LIGO's twin detectors observed GW150914, a signal from two merging black holes about 1.3 billion light-years away; the discovery was announced 11 February 2016 and published in Physical Review Letters, opening the field of gravitational-wave astronomy. The 2017 Nobel Prize in Physics went to Weiss, Barish and Thorne for this achievement, and dozens more mergers, including the neutron-star merger GW170817 (2017), have been detected since.

LIGO measures a change in distance a tiny fraction of a proton's width over 4 kilometers of vacuum, turning the whole universe into a drum that rings whenever two black holes collide.

References

• What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
• Observation of Gravitational Waves from a Binary Black Hole Merger, B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters 116, 061102, 2016
• The Nobel Prize in Physics 2017, NobelPrize.org press release, 2017