General Meeting Report

14/11/2001 BLACK HOLES

Dr. Leo Brewin, Mathematics Dept Monash University

The theory of Black Holes has come a long way since the term was coined by John Wheeler in 1967. From an abstract concept, an obscure oddity of infinity calculations, like the advanced-wave solution to Maxwell=s equations on electromagnetism, it has now moved into reality as part of cosmological evolution. The possibility was first suggested by John Mitchell in 1783, who, in his >Dark Star= proposal, took Isaac Newton=s theories on gravity and light to their ultimate conclusion. Shortly after, in 1796, Pierre Laplace independently worked out a similar proposal that, from a sufficiently large concentration of mass even light will no longer be able to escape. But the subject remained a mathematical curiosity until 1916 when Karl Schwarzschild analysed the implications of Albert Einstein=s Theories and calculated the exact mathematical geometry of space-time around a spherical mass. He showed that for any given mass there is a critical size below which space-time closes around and pinches it off from the rest of the universe. This dimension, now called >Schwarzschild Radius= is for our Sun 2.9km, for the Earth 0.88cm and for the Universe its cosmological horizon.

What brought Dr. Leo Brewin to black holes is his life-long fascination with trying to understand how things work. It led him from a self-professed ‘electronic junkie= to become an expert on cosmological mysteries. There is a wealth of information about him on the Internet. A quick search on “Google” for ‘Black Holes - Brewin= came up with 23 references. From his support of the >Gifted Student Program= in 1995 (why does an apple fall?) to citations at International Conferences, Leo Brewin=s life makes for interesting reading. He is a lecturer in Mathematics and Statistics at Monash University. “My goal is to understand - by combining mathematics, physics and computers - how black holes might form and how they might interact with other stars”.

A "Black Hole" is defined by just three parameters: Mass, charge and angular momentum. In that sense a black hole is not completely cut off from our Universe, and interaction through those properties with the visible world, is now being closely studied by astronomers and mathematicians around the world. The first candidate to fit the bill was an X-ray source known as Cygnus X-1, identified with star HDE 226868. Other possible prospects are LMCX-3, V404 Cygni, M84 and A0620-00. There has also been speculation of a super massive black hole at the centre of galaxy M87. The telltale signs of a black hole are gravitational interaction with a visible companion star and high-energy radiation. In binary star systems the two effects are often found together: a wobble of the visible star as the two bodies orbit each other and powerful X-ray emission from the accretion disk of matter being drawn into the black hole. Up to 50 percent of the mass of this in-falling material can theoretically be radiated away as energy.

Our Galaxy contains some 100 billion stars, and it has been around for 10 billion years. Through this statistic, and by comparative reasoning with known Pulsars and neutron stars, the number of stellar-mass black holes is estimated at about 100 million, scattered around the Galaxy. Other methods of detecting them are being investigated, such as gravitational lensing, in which the light from a source behind a black hole appears concentrically focussed around the hole; or rapid brightness changes in a star through occultations. A PowerPoint presentation helped the meeting to focus on the wide-ranging subject, with animated sequences and cartoon characters Marvin and Duffy Duck as hapless helpers on some of the abstract aspects.

How do black holes fit into our cosmological picture? Are they the vacuum cleaners that sweep up the mess and debris after the party is over, or are they the seeds for future universes? “These are fascinating questions that most of us have probably mused over at some time in our lives. Yet nobody knows all of the answers - which is a good thing, otherwise my colleagues and I would be out of a job”. Alfred Klink