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Ask Ethan: Why Did Light Arrive 1.7 Seconds After Gravitational Waves In The Neutron Star Merger?

4 Nov 2017, 14:01 UTC
Ask Ethan: Why Did Light Arrive 1.7 Seconds After Gravitational Waves In The Neutron Star Merger?
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Artist’s illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). Image credit: NSF / LIGO / Sonoma State University / A. Simonnet.With a journey of 130 million light years, both signals should move at the speed of light. So why did one get here first?On August 17, after a journey taking 130 million years, the gravitational wave signal of two neutron stars, spiraling inwards in the final stages of a merger, finally arrived at Earth. As the surfaces of the two stars collided, the signal abruptly ended, and then there was nothing. Although these stellar corpses, perhaps only 20 kilometers in diameter, were moving at some 30% the speed of light, we saw nothing in the immediate aftermath. Only 1.7 seconds later did the first signal arrive: light in the form of gamma rays. Why did this delay happen? It’s an incredible question, and what Joel Mills wants to know:Please discuss significance of the 1.7 sec. difference in arrival time between GW and Gamma Ray burst ...

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