Can Black Holes be seen?
Black Holes conjure up a variety of images for people. To some, they are beautiful, celestial objects that drift through the cosmos whilst to others they are fierce, destructive forces that seek out and destroy other worlds in an attempt to satisfy an insatiable appetite. To yet others, they are imaginary; made up by astronomers to fill gaps in what we think we know. To those people that came to the December Birmingham Café Scientifique, a new understanding of Black Holes was formed, that brought together current research from a variety of fields, in an evening that was entertaining and fascinating in equal measure.
Prof Somak Raychaudhury, an astrophysicist from the University of Birmingham, led us through an evening of intrigue and mystery as he talked about what we do and don't know about Black Holes, and why. Let's start from the beginning and discuss colour. I would hazard a guess and say that if you were to ask the average person what colour a Black Hole was, they would answer you fairly confidently that they are black. This might be a bit obvious, given that there is a fairly significant clue in the title, but for a scientist (or a pedant), this doesn't really answer the question, given that black is not really a colour; it is what is left when there is no colour. One of the things that the general population seem to know about Black holes is that they have such a massive gravitational field; that once in their grasp, nothing can escape - not even light. All objects exert a gravitational effect on all other objects and the strength of this field depends on the mass and the size of the objects. Another way to think of this is in terms of density; the more dense an object is, the more gravity it has. Gravity is also a very weak force so you are generally unaware of it unless you are working with large objects like planets. Having said that, I am sure I have heard of an experiment where a weight was hung off the side of a mountain and found to be deflected a tiny amount due to the gravitational field of the mountain itself!
Black Holes are the highly dense and compact remnants of dead stars, so they are both very heavy and very small relatively, giving them a huge gravitational potential, which is so significant that it can indeed prevent light from escaping. You may have heard of Neutron stars, which I believe are that next most dense thing that we know of. A normal atom is made from protons, neutrons and electrons, but the reality is that most of the volume of an atom is in fact empty space - the standard analogy being that if the nucleus of the atom is the size of a football, the electrons would be the size of peas racing around in the stands of the pitch and beyond. Everything else is a vacuum. In a Neutron star, the atom becomes so compacted that the electrons are forced into the protons, forming neutrons and all of these neutrons are condensed into a lump of neutrons with no vacuum between them, the forces being so intense that the strong forces of repulsion in atoms that normally give them their structure, are completely overpowered. This means that a star the size of our Sun, might be condensed down into a mass of neutrons measuring only kilometres in diameter and with a very high gravitational field. However, Neutron stars are often seen as blue because blue light can still escape them, so a Black Hole must be even more dense than this.
We also associate colour with the visible spectrum that forms the familiar colours of the rainbow. These are the colours that can penetrate our atmosphere from the Sun (the source of our light before artificial lighting was invented) and as such these are the wavelengths of light that most lifeforms have evolved to work with. Somak joked on the night that rattlesnakes might be able to see ultra-violet light, but because there is not much of it around, they wouldn't make very good astronomers. There are a wide variety of other forms of light though, which human eyes are not sensitive to, but which modern technology has allowed us to explore within the cosmos. Space based telescopes have X-ray vision, infra-red vision, gamma ray vision and a whole host of other amazing tools to allow us to see things that are invisible to the naked eye. By launching them into Space, we can also see all of the wavelengths of light that Earth's atmosphere normally screens out. If you knew where a Black Hole was in Space, bearing in mind that this would effectively be a black body on a black background (in the visible spectrum), but were viewing at other wavelengths, you may well see incredible activity in the X-ray and infra-red spectra as matter is crushed and collided as it journeys towards the Black Hole, emitting incredible amounts of energy. This energy and light is not coming from the Black Hole itself (because it is still not powerful enough to escape the gravitational influence of the Black Hole, but beyond the event horizon of the Black Hole, there would be an incredible display of energy.
We are now fairly certain that there is a Black Hole at the centre of our galaxy (and all others). This Black Hole was discovered by observing the orbits of a group of stars and pinpointing what and where the centre of those orbits was. Much like the planets of our Solar System orbit the Sun, these stars in turn were in orbit around something massive and this turned out to be a Black Hole. If you were to look at the centre of our galaxy with X-ray and infra-red goggles on, you would see a tempestuous and angry place spewing out matter, heat and energy. What we don't know (yet), is how these galactic super-massive Black Holes were formed.
One of the most interesting questions of the night was posed right at the end of the evening: 'Do we know why the Universe is the way it is?' This was asked by one of our longstanding supporters and I will be honest when I say that with about 5 minutes left of the evening, I was hoping for a question that was a little less open-ended, something like 'How many Black Holes do we know of?'. The answer though was one of the most wonderful that I have ever heard. Somak talked for a while about observations in science and hypotheses, fair testing and the possibility of parallel universes where the laws of physics as we know them did not exist but were instead replaced with an entirely different set. This was all summed up in one simple phrase though - 'It's the way it is, because it isn't the way it isn't'. How very true!