Quote:
Originally Posted by julianh72
Unless I am misunderstanding the situation (which is entirely possible; probable even!), I'm not surprised that we didn't see a corresponding change in the Pulsar timing yet.
The gravitational waves reached us by travelling in a straight path 1.3 billion light years long (if "straight" means anything when we are talking about distortions of space itself!). Unless the pulsar we are monitoring happens to lie on the same "straight" path (VERY unlikely!), the path length from the Black Hole Merger to the Pulsar to Earth is longer than the "straight" path from the Black Hole Merger to Earth.
If the Pulsar happens to lie close to the straight path, between Earth and the merged Black Holes, it would have felt the gravitational waves before us, but as the cumulative path length is longer, the disruption to the regular pulsar frequency won't reach us for some time yet. If the Pulsar lies 100 light years beyond us, the gravitational waves won't reach it for another 100 years, and then it will take another 100 years for the Pulsar fluctuation signal to reach us.
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To expand on Ray's post.
Gravitational waves are emitted in all directions and continuously up to the time of the Black Hole merger.
The Pulsar timing array is designed to measure fluctuations in pulsar arrival times caused by gravitational waves which alter the Earth-Pulsar distance.
The Pulsar fluctuation times are long period events, and are expected to be caused by gravitational waves of very low frequency (long period).
These very low frequency gravitational waves are generated by orbiting supermassive black hole binaries in galaxies.
GW150914 on the other hand is an example of a Black Hole merger producing gravitational waves in the high frequency range which would not impact on pulsar fluctuation time.
http://nanograv.org/assets/img/GWBigPicture.png
Regards
Steven