Gravitational waves have been detected

Accelerating masses, according to the general theory of relativity, emit gravitational waves—distortions of spacetime that propagate at the speed of light. But gravity is a feeble force; the only gravitational radiation that could be plausibly observed arises from spectacular cosmic events such as the merger of two black holes. Today at the National Press Club in Washington, DC, scientists from the Laser Interferometer Gravitational-Wave Observatory announced that LIGO has spotted a gravitational wave generated by just such a merger. As its name suggests, the heart and soul of LIGO is a precise Michelson interferometer. When a spacetime-distorting gravitational wave passes through the device, it ever so slightly strains—that is, changes the relative lengths of—the detector’s finely tuned arms. Those changes could be detected as the interference of laser light shot down the arms and reflected off their ends. On 14 September 2015, LIGO facilities in Livingston, Louisiana, and Hanford, Washington, nearly simultaneously observed the signals shown in the figure. Note that the observed strain of 10⁻²¹ implies that the distortion in LIGO’s 4-km-long arms was less than a proton width. After matching the observed data against curves generated by numerical relativity simulations for various models, the LIGO team concluded that their signal was generated by an event that occurred 1.3 billion years ago: A 29 solar-mass (M_⊙) black hole combined with a 36 M_⊙ one to yield a single 62 M_⊙ black hole and 3 M_⊙ of radiated energy. Incidentally, black hole binaries had never been seen before the LIGO observation; the collaboration discovered them along with gravitational waves. (B. P. Abbott et al. [LIGO scientific collaboration and Virgo collaboration], Phys. Rev. Lett. 116, 061102, 2016 .)