On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0 × 10 −21 . It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410 These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.
Gravitationswellen -ein leichtes Zittern der raumzeitVor 1,3 Milliarden Jahren: Seit langer Zeit schon haben sich in einer fernen Galaxie zwei schwarze Löcher umkreist, Gebilde von so ungeheurer Dichte, das selbst Licht ihrer Schwerkraft nicht mehr entweichen kann und von ihnen eingefangen wird. Seit Jahrmillionen haben sie bei ihrem Tanz umeinander mit ihrer masse die Raumzeit verformt und dabei Gravitationswellen abgestrahlt. ihr Abstand wurde dabei immer kleiner, ihre Geschwindigkeit immer höher, bis sie schließlich unter einem gewaltigen Ausbruch von Gravitationswellen zu einem einzelnen schwarzen Loch verschmelzen. Später werden wir diese Wellen GW150914 nennen. Für einen kurzen Augenblick wird durch sie mehr Leistung abgestrahlt als von allen Sternen im gesamten sichtbaren Universum in Form von elektromagnetischer Strahlung zusammen. Diese Gravitationswellen rasen mit Lichtgeschwindigkeit durch das Weltall und lassen auf ihrem Weg die Raumzeit erzittern.25. November 1915: GW150914 ist schon längst in unserer milchstra-
Rapid interstellar travel by means of spacetime wormholes is described in a way that is useful for teaching elementary general relativity. The description touches base with Carl Sagan’s novel Contact, which, unlike most science fiction novels, treats such travel in a manner that accords with the best 1986 knowledge of the laws of physics. Many objections are given against the use of black holes or Schwarzschild wormholes for rapid interstellar travel. A new class of solutions of the Einstein field equations is presented, which describe wormholes that, in principle, could be traversed by human beings. It is essential in these solutions that the wormhole possess a throat at which there is no horizon; and this property, together with the Einstein field equations, places an extreme constraint on the material that generates the wormhole’s spacetime curvature: In the wormhole’s throat that material must possess a radial tension τ0 with the enormous magnitude τ0∼ (pressure at the center of the most massive of neutron stars)×(20 km)2/(circumference of throat)2. Moreover, this tension must exceed the material’s density of mass-energy, ρ0c2. No known material has this τ0>ρ0c2 property, and such material would violate all the ‘‘energy conditions’’ that underlie some deeply cherished theorems in general relativity. However, it is not possible today to rule out firmly the existence of such material; and quantum field theory gives tantalizing hints that such material might, in fact, be possible.
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