Black holes are good candidates for being the weirdest things in nature. There are lots of them out there; some of them are the remains of giant stars, others, with millions or even billions of times the mass of the Sun, sit at the centres of galaxies. There is one at the centre of ours. 鈥淏lack Holes Colliding鈥 could be a topic for a disaster movie.
Albert Einstein predicted their existence back in 1915, when he formulated his General Theory of Relativity, which is basically an explanation of the force of gravity. He suggested that gravity is a distortion of the fabric of space-time by massive objects. His theory states that if a body is massive and/or compressed enough, its gravitational pull could become so strong it closes off the fabric of space-time around the body so that nothing, not even light, can get out.
This boundary is known as the 鈥渆vent horizon.鈥
However, that strong gravity reaches out beyond the event horizon, and can pull things in, on a one-way trip, which led to these objects becoming known as 鈥淏lack Holes鈥. Forming black holes requires very special circumstances. Our Sun, the Earth and other planets in the Solar System are highly unlikely to become black holes. However, there are cosmic situations where those extreme conditions occur.
If we compress the matter in a star or planet enough, either by piling more stuff on top or applying high-energy shock waves, we reach a point where the gravitational force becomes irresistible. According to our current understanding of physics the body will then shrink indefinitely, into a body of infinite density and gravitational attraction. This is almost certainly not the case; it is likely that as the conditions become more extreme, some new force we don鈥檛 know about comes into action. Even so, the density and gravitational attraction can become high enough to fold the fabric of space-time around itself, closing it off, as Einstein predicted, forming a black hole. This is a one-way door for incoming traffic, marked 鈥淣o Exit鈥. We have found lots of these bizarre objects scattered around in space. Most of them have star-like masses and are the remnants of collapsed supergiant stars. Others, having masses millions of times the mass of the Sun, sit at the centres of galaxies. Our galaxy has a big one.
Black holes can grow in two ways. They can pull in nearby gas clouds, stars and planets, pulling them apart into a hot disc of material slowly spiralling in, across the event horizon. Our infrared and radio telescopes show our black hole to be feasting on nearby stars and gas clouds. There are almost certainly planets being eaten too. The other way they can grow is by assimilating other black holes.
One black hole merging with another is a very different thing from gorging on nearby stars. The gravitational attraction between them as they spiral into one another is so intense it makes ripples in space-time, like the ripples we get on a pond if we move our fingers in circles. These waves, called gravitational waves, take away huge amounts of energy, allowing the black holes to spiral in closer and closer.
As they get closer, the energy radiated as gravitational waves gets higher, accelerating the inward spiral. At some point they start moving through each othersa国际传媒 disc of star debris, which dissipates more energy.
Finally the two event horizons merge, and those tiny balls of incredibly dense material sitting in their centres combine, forming a single, more massive black hole.
Almost any event in space-time generates gravitational waves. However, only big events such as black hole mergers produce gravitational waves strong enough for us to detect at our huge (and safe) distance, and we have now detected some. As the black holes spiral in closer and closer, orbiting faster and faster, the pitch of the gravity waves increases, until they suddenly stop.
Venus is very low and hard to see in the sunset glow. In the late evening Saturn lies low in the south-west. Brilliant Jupiter is high in the south and Mars, dimmer but distinctly reddish, high in the east. The Moon will be Full on the 15th.
