I read the newspaper today and I found that LIGO had detected gravitational waves. This is from their official website:
For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.
How did they know the source of the gravitational wave was a binary black-hole merging event? Wikipedia says:
Measurable emissions of gravitational waves are expected from binary systems (collisions and coalescences of neutron stars or black holes), supernova explosions of massive stars (which form neutron stars and black holes), accreting neutron stars, rotations of neutron stars with deformed crusts, and the remnants of gravitational radiation created by the birth of the universe.
There are more possible sources. Can the LIGO detect black holes as well?
Answer
Firstly, they were able to calculate the masses of the two objects involved and their sizes. This was an indicator that they were compact objects since the masses were two objects roughly a few hundred kilometres across and weighing about 30 times the mass of the sun. This is a strong indication that such an object is a black hole.
Secondly, like Gabi Gonzalez mentioned in the announcement, the waves detected actually tell a lot more than you can actually read off. In particular, the model that fits the observed data the best is the model of a binary black hole merger.
There is no direct proof that can confirm that these were black holes because a good estimate of the location is not known. If it were known, then the telescopes would be turned to view that patch of the sky and most likely will see nothing, confirming that the gravitational waves were from black holes.
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