The gravitational pull of the moon upon the Earth produces tides, and because the Earth rotates, there are two tides a day. Friction between the ocean and the Earth's crust slows down the Earth's rotation and lifts the moon into a higher orbit with a longer period. Now, a team of CENTRA researchers has shown that an analogous mechanism works with orbiting bodies around black holes. The team is composed of Drs. Cardoso, Chakrabarti, Pani from CENTRA, Berti from Mississippi and Gualtieri from Rome.Einstein's theory is one of mankind's greatest achievements, yet it is thought to be an incomplete theory, incompatible with the Standard Model of particle physics. Attempts at a unified theory (such as string theory) add so-called "scalar fields" to Einstein's original description of gravity. Scalar fields are now being hunted at the LHC and are good candidates to explain the mysterious dark energy which accelerates the expansion of the Universe. Compact stars can raise tides on the event horizon of a black hole (which behaves like a flexible, viscous membrane) and increase the distance between the compact star and the black hole. However the motion of the star also generates gravitational waves -- ripples in spacetime that carry energy away from the system, so that the compact star gradually spirals into the black hole. In Einstein's theory gravitational-wave emission always dominates over tidal effects, so that all orbiting bodies should spiral into the black hole and eventually be swallowed. The new study shows that scalar fields could enormously amplify the tidal effects. This amplification means that the compact star will tap rotational energy of the black hole in a sustained way, transforming it to gravitational waves, and the star will "float" for a long time around the black hole instead of being swallowed by it. This study predicts a new steady source of gravitational waves. If observed, gravitational waves from "floating orbits" would provide important clues to unifying general relativity with the Standard Model of particle physics.
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