Speaker John Waterhouse

John said that he wanted to discuss 3 questions about Black Holes: -

• Are they real?

• How do they come about?

• What do they tell us about our universe?

In 1784 John Mitchell was the first person to think about how something could escape earth’s gravitational pull. He theorised that if you could throw ball hard enough it would not come back to earth. He thought about Newton's theory that light is very tiny particles and said “what if the velocity to overcome gravity works out at speed of light then light couldn't escape so you wouldn’t see anything”.

Using Newton's theory, based on the mass and size the escape velocity is vesc = Square root of √2GM/r, (where G is the gravitational constant, M is the mass of the attracting mass, and r is the distance from the centre of that mass. The equation for the earth works out at 11km per second and to escape from sun it works out at 618km. If the earth were to be compacted to 1/3inch then the escape velocity would increase to the speed of light and it would become a dark star.

Using Einstein theory of relativity, it is possible to work out that heavy objects deform space time creating a well in the space time continuum. The theory was tested in 1919 with eclipse of sun. By this date it was now known that light doesn't have particles so the bending of the light beam around the sun in the eclipse must be caused by this space time deformation.

The inward gravity of a star such as our sun is so great that it causes particles to fuse as in nuclear fusion. If this fusion effect stops, because the star runs out of particles to fuel the reaction, then the outward force stops and so the star collapses causing a super nova.

In 1968 Fred Howle coined the phrase a Black Hole to describe what is left when a star collapses. In 1972 Steven Hawkins proved you would have to get a Black Hole when star collapses.

In 1967 Joclyn Bell Burnell built the first radio telescope out of chicken wire. Using this she watched for signals of radio waves from space. She found a repeating signal and concluded that this was a lighthouse effect i.e. a rotating object that sends out a signal along its axis. Could this be a Black Hole Quasi-Stellar Object causing a pulsar hence named a Quasar?

The clue to the Black Holes is a very bright object in the very far distance. These are drawing matter in which circulate around the mass. The circulating matter causes friction which creates the light energy showing us where the Black Hole is located.

When a small star uses up its fuel it collapses and becomes a white dwarf. But if a bigger star (7 to 10 times the size of our sun) collapses it will become a super nova and then a black hole. Einstein predicted this effect. 

In 1994 and 1995 two Laser Interferometer Gravitational-Wave Observatory (LIGO) were built in the USA. These have two 4km legs of exactly same length down to the size of a proton at right angles to one another. A laser signal is sent simultaneously from the centre to mirror at the end of each leg. If the signals return at same time, then everything is as expected. However, if they do not, then they have been disturbed by a gravitational wave. Finally In 2012 a signal deformation caused by 2 Black Holes combining creating gravitational wave was recorded. These two Black Holes had a solar mass of 30 in one case and 35 in the second. The combine Black Hole had a solar mass of 60. The other solar mass of 5 was thrown out from the collision as a gravitational wave.

The centre of our galaxy is a Black Hole with a solar mass of 4 million. There are similar Black Holes at centre of each galaxy which is what causes spiral effect around a galaxy. Around 2 % of the matter circulating around a Black Hole gets pulled into the Hole each century.

Black Holes aren't actually black as they are throwing off a very bright light. Steven Hawkins applied quantum theory saying pairs of particles circulating around a Black Hole would get split with one getting particle getting sucked in and other being thrown off as real particles that are radiated. Therefore, we need to create a new theory combining quantum theory and the theory of relativity.