Relativity Lite

Gravity Lite | 65 Figure 13. A pair of black holes orbiting each other would stir the spacetime they reside in, sending out an expanding ripple of gravitational waves that move at light speed. Only two spatial dimensions are shown, with the third being the time dimension. * Again, we have taken the additional liberty of showing just one slice of time. Had we let time flow upward as the third dimension here, the two holes would form a lovely collapsing spiral climbing upward, but the ripples would have been less easy to visualize. The first clue that gravitational waves might be real was Russell Hulse and Joseph Taylor Jr.’s 1975 discovery of a pulsar orbiting a neutron star that was slowing down, apparently due to the emission of gravitational radiation waves, † work that later earned them the 1993 Nobel Prize. As the pulsar and neutron star lose energy via gravitational waves, they spiral inward over billions of years and will eventually crash into each other. Explicit confirmation of the existence of gravitational waves from such a source came on September 14, 2015, thanks to the twin Laser Interferometer Gravitational-Wave Ob- servatory (LIGO) detectors located in Livingston, Louisiana, and Hanford, Washington, which each detected a passing gravitational wave. Figure 14 shows the characteristic sound of spacetime ringing as two black holes collide. In this final fraction of a second, the two black holes collide at nearly half the speed of light to form a single black hole and convert some of the mass of the black holes into wave energy. Only this peak energy is strong enough for LIGO to detect. The signals came from two merging black holes lying 1.3 billion light-years away, one about 29 solar masses (29 times the mass of our Sun) and the other about 36 solar masses. The final, merged black hole was 62 times heavier than the Sun, and the remaining 3 solar masses were emitted as the energy of the massive gravitational wave crest that was detected. * Modified from a Mathematica notebook by Jeff Bryant at https://​community​.wolfram​.com/​groups/​-/​m/​t/​790989. He notes, “The function being plotted is not derived from any general relativity equations. It’s a very similar surface to the one used on the Wikipedia page for eLISA, and in fact was designed to try to emulate this.” † R. A. Hulse and J. H. Taylor, Ap. J. 195 , L51 (1975).