![]() ![]() To verify this thought experiment quantitatively, Rezzolla and his colleagues ran a computer simulation of this head-on collision and included another simplification. When the merged and kicked black hole emits gravitational waves from its forward-pointing bulge, the effect is equivalent to tapping on the brakes. Such black holes are said to “ring” like a bell emitting sound waves. Studies from the 1970s showed that a black hole with a perturbation on its horizon can emit gravitational waves to dissipate energy and remove the bulge. “Black holes, when they are isolated, they long to be perfect, without any deformation,” Rezzolla says. ![]() The region where the smaller black hole arrived will bulge out, a situation that can’t last. But Rezzolla realized that the new black hole’s event horizon–the boundary in spacetime that even light cannot pass–is not perfectly spherical because of the mismatch in the original black hole masses. This radiation generates the initial kick. Researchers already know that the smaller black hole will accelerate more than the larger one and send more radiation in the direction the small one was moving. They ignored the pre-merger “death spiral” and treated the two black holes as mutually attracting billiard balls of different sizes that simply collide and stick together. Luciano Rezzolla of the Max-Planck Institute for Gravitational Physics in Potsdam, Germany, and his colleagues, realized that they could understand the slow-down if they simplified the problem. (Click above for the original, higher-resolut. There is no kick or anti-kick in this high-symmetry case, but the pair does execute the typical “death spiral” before merging. (Click above for the original, higher-resolution version.) Gravitational radiation for a merger of equal-mass, non-spinning black holes. Gravitational radiation for a merger of equal-mass, non-spinning black holes. Koppitz, Albert Einstein Institute and Zuse Institute Berlin. Rezzolla, Albert Einstein Institute, and M. Researchers haven’t had a clear physical explanation for the anti-kick. But some simulations have shown an “anti-kick” following the initial kick–the new black hole shoots away but soon slows down. ![]() ![]() If the gravitational waves radiate mostly in one direction at the time of the merger, they “kick” the new black hole in the opposite direction. Two black holes that are close enough will mutually orbit and eventually spiral inward toward each other, sending off ever-stronger gravitational waves (ripples in spacetime), until they collide and merge into a larger black hole. The paper provides an intuitive explanation for the slow-down with a much-simplified simulation that the team believes can be generalized. That slows the black hole like a spacecraft firing retro-rockets. They suggest that the moving black hole can radiate gravitational waves preferentially in the forward direction as a result of asymmetry in the curvature of spacetime around it. In the 4 June Physical Review Letters, a team offers an explanation for this puzzling deceleration. When two black holes merge, the resulting larger black hole usually shoots away from its birthplace, but it immediately slows down in some cases, according to computer simulations. The new object will shoot off to the left but will quickly slow down as the highly curved region smoothes out and emits gravitational waves. A pair of black holes in the process of merging generates high curvature (red) in the merged horizon on the side of the smaller black hole. ![]()
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