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Third Gravitational Wave Detected From Black-Hole Merger 3 Billion Light Years Away (bbc.com)
sycodon quotes a report from The New York Times (Warning: may be paywalled; alternate source): Astronomers said Thursday that they had felt space-time vibrations known as gravitational waves from the merger of a pair of mammoth black holes resulting in a pit of infinitely deep darkness weighing as much as 49 suns, some 3 billion light-years from here. This is the third black-hole smashup that astronomers have detected since they started keeping watch on the cosmos back in September 2015, with LIGO, the Laser Interferometer Gravitational-Wave Observatory. All of them are more massive than the black holes that astronomers had previously identified as the remnants of dead stars. The latest detection was made at 10:11 GMT on January 4, and is described in a paper accepted for publication in the journal Physical Review Letters. "The analysis suggests the two black holes that coalesced had starting masses that were just over 31 times and 19 times that of our Sun," reports BBC. "And when they finally came together, they produced a single object of a little under 49 solar masses. It means the unison radiated a simply colossal quantity of pure energy."
"I felt a minuscule disturbance in the gravitational force. I fear something terrible has happened."
The difference in distance to the event that caused the waves on different places on Earth is indeed much smaller than the measurement uncertainty of the distance, but the detectors are receiving the same waves with different delays, so the relative distance can be measured very precisely. In addition, the detectors are not equally sensitive to signals from all directions, so the relative amplitudes measured by different detectors provides some information too.
"pure energy" = anything other than the energy due to the rest mass of a particle. So kinetic energy, gravitational field energy, and electromagnetic field energy (including photons) are "pure energy". An electron is not a form of "pure energy". A proton isn't pure energy either (although strictly speaking, most of the mass of a proton is due to the energy in its gluon field, which really is pure energy by the definition I gave. But just ignore that.)
In the end, pure energy is just to distinguish between matter and everything else. There is no deep meaning behind it. Consider an atomic bomb explosion. The light, heat, and sound produced is pure energy. The fallout is not pure energy.