Ken Tapping, October 25, 2017
About 130 million years ago, two neutron stars collided. Their impact created waves in space-time – gravity waves, which radiated offinto space at the speed of light. Here, 130 million light years from the event,our gravity wave detectors have just picked it up. A light year is the distancelight travels in a year, just under 1E13 (1 followed by thirteen zeros)kilometres. This event is especially interesting in that it was first detected through gravity wave observations, and then monitored with other telescopes, providing a wealth of data on an unusual event. Moreover, we might have for the first time obtained multiple observations of the formation of a new, small black hole.
The idea of gravity waves was first raised by Albert Einstein in 1916, when he proposed gravity is not a force, as proposed by Newton, but is in fact due to distortions of the fabric of space-time bymaterial objects. The more massive or more compressed a body is, the greater the distortion. Imagine dropping balls of different sizes and weights onto stretched rubber sheet. Just as the impacts make ripples run over the sheet, explosions and other dramatic cosmic events make ripples in space-time, and bodies moving through it make a bow wave and a wake. However, Einstein thought gravity waves would remain of academic interest only, being too weak to observe. On the other hand, successfully detecting them would be the ultimate vindication of Einstein’s General Theory of Relativity, so the search was on,and it was a long one.
In 2015 the Laser Interferometer Gravitational WaveObservatory (LIGO), built in 2002, which had to that point detected nothing, was upgraded with new, more sensitive detectors. In 2016 it successfully detected the collision of two black holes and then went on to detect gravity waves from two more black hole collisions. Einstein was right. However, in August this year, something really interesting turned up. On 17 August LIGO detected a “chirp” of gravity waves. Two seconds later the Fermi Space Telescope received a short burst of gamma rays. These are the highest-energy electromagnetic waves and an indicator of a really dramatic event. The LIGO and Fermi observations identified the patch of sky the waves came from, so around the world telescopes got to work searching. Eleven hours later, a telescope in Chile found it, in the galaxy NGC 4993, in the constellation of Hydra. Lots of telescopes monitored the object over the following days as the explosion faded. Observers realized that what they had seen was a collision between two neutron stars, and possibly the birth of a new black hole.
Neutron stars are formed in the explosive deaths of large stars. Shock waves compress the cores of the stars so hard that the atoms collapse. Atoms are mostly empty space, with a number of electrons orbiting around a nucleus consisting of a number of protons and neutrons. The shock wave pushes the electrons right into the nucleus, where the electrons and protons combine to form more neutrons. The result is a ball of neutrons, a few kilometres across, with the mass of a star. A teaspoonful of neutron star material would weigh around 100 million tonnes.
A collision between two neutron stars can produce even greater compression. There is a point in the compression where anobject’s own gravity overcomes its ability to resist. Theoretically the object will then shrink without limit. The idea that something can achieve zero size and infinite density is questionable, but if the object shrinks enough, it will distort space-time enough to close itself off behind a one-way, no return frontier called an event horizon. It has become a black hole.
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