Black Holes

 

Black Holes

In describing Black holes it would be as well to first describe the structure of the universe. Our planet Earth is circling the sun along with 8 other planets. The sun is a star which may be described as a big concentration of ( mainly ) hydrogen gas which is so big that the pressure and temperature at its core enables fusion reactions to take place. These fuse hydrogen atoms together producing helium along with huge amounts of energy. This is the same process as a hydrogen bomb although happening at an immense scale and continuously.

The sun is one of many millions of stars grouped together in our home galaxy. Throughout the universe are very many millions of galaxies. The universe is very big and modern measurements suggest it is expanding. It is important to grasp the universe is not expanding into anything. There is nothing, no space, no time. The analogy often used for the expanding universe is that the universe is a bit like a balloon with galaxies on its surface. As the balloon inflates it expands and the galaxies on its surface grow further apart. This is happening in three dimensions. The effect is that we are finding other stars are becoming more distant although even at big distances the effect is still quite small.

First a largely theoretical concept Black holes became important when it was realised that a large one existed at the centre of our galaxy. Our galaxy is shaped rather like a disc with a bulged centre and spiral arms like a giant Catherine wheel. Our sun is positioned out along a spiral arm. When we look up on a clear night we see a great concentration of stars in a overhead band, the Milky Way. Indeed our galaxy is called the Milky Way and the effect arises because we are looking in from the side of the disc.

The spiral arms are rotating around the centre at high speed, so high that it became apparent that only the gravity of something very large at the centre stopped stars like the sun from flying away into space. That something is a Black hole. A Black hole is something where the gravity is so strong that even light cannot escape; it is quite literally black because nothing can escape. The Black hole has at its centre some form of matter of fantastic density, so dense that something the size of earth is only the size of a suitcase.

Before this seems impossible it is as well to know that ordinary matter, the atoms of which we are made, is almost entirely space with a small nucleus of protons and neutrons with electrons orbiting around at large distances away. This is a very simple view of an atom which is more complex but gives  the idea that an atom is mostly space. Under Black hole conditions that space disappears and a very large density is obtained. A minute bit of such matter would weigh millions of tons.

Astronomers have just seen and photographed this Black hole at the centre of the Milky Way Black holes vary in size and this has the mass of many suns. There is an immediate question- a Black hole is black, totally black, so in fact what we see is a halo of hot gas in the process of being ingested with a silhouette of nothing in the middle. In fact this halo isn’t symmetrical but rather blobby as the Black hole spins .This irregularity may be an artefact of the way the observations were made. One slight puzzle is that it was expected we would see the hole side on rather than face on as is actually observed.

The above description of a Black hole is rather limited. Einstein has shown that gravity is the result of a curvature in space-time. In a Black hole  the curvature has become so great that nothing can emerge. The above description of density of matter is a gross approximation of a phenomena which is beyond the scope of present day physics. There is a useful cop-out which says that a Black hole proceeds to a singularity; in other words to something we don’t understand. In fact what we know and can observe is that at some point in space the gravity around the Black hole is so intense that any matter will fall into the hole. This line around the hole is called the event horizon.

The observation of the Black hole was made with the Event Horizon Telescope which is several radio observatories at different places in the world working together.

It is thought that Black holes can be very large indeed. The one at the centre of our galaxy is much smaller than the only other previous visualisation which is several billions of suns in size. Very small Black holes are thought possible and there was some alarming speculation that the Large Hadron Collider could produce them. Physicists were confident this was not the case and so it proved.

For all practical purposes matter falling into a Black hole has left the universe.