Hiding a heresy can be hard. I’ve been doing a few science-faith talk here – and somehow or other the shocking news slides out, that I don’t believe in black holes. This may seem strange as black holes frequently appear in the science news (for example here and here). So I thought I should explain why.
It’ll probably help to start by focussing on a prime example of a ‘black hole’ system. The image below is an artist’s impression of what’s happening in Cygnus X-1: there’s a large, luminous, blue star that produces most of the visible light – but the x-rays largely come from an ultracompact object which is orbiting it. This object draws material off from the blue star, which swirls in a disk before eventually being dragged into the centre – some of which is then fired outwards via jets.
The importance of this particular system is that the mass of the compact object can be measured (Kepler’s laws of planetary motion enable one to do this straightforwardly). It is about 15 times the mass of the Sun [ref]. This mass is essential to understanding what the object might or might not be: it means, for example, that it can’t be either a white dwarf or a neutron star – both of which would be much lighter. Let me explain.
A compact star is formed at the end of the life of a normal star. All normal stars, like the Sun, are giant nuclear fusion reactors: most burn hydrogen into helium, others (the red giant stars) burn helium into carbon, or carbon into other heavier elements; but whatever the fuel, it will eventually run out. At that point the core of the star will collapse in on itself. What happens next depends on how big it is.
Most stars will end up as white dwarfs. The Sun certainly will, and the faint companion of Sirius (see opposite) is a nearby example. In this type of object, a star the mass of the Sun is compressed into an object the size of the Earth (which has one-millionth the volume), so that a thimble-full of material will weigh several tonnes.
However, because of the quantum properties of white dwarf material, there is a maximum mass for such a star: it’s 1.4 times the mass of the Sun. If it’s bigger than this, it will form a neutron star, an ultracompact object in which a star the mass of the Sun is compressed into a volume which is about ten miles across. The best known examples of these are the pulsars, such as the one in the Crab Nebula [here]. Astronomers know most about those which appear as pulsars, or those which are in binary star systems and whose effects can be seen in other ways. But there’s a maximum mass here, too: it’s about three times the mass of the Sun.
So what about a compact object like in Cygnus X-1 which, at 15 times the mass of the Sun, is comfortably bigger than these limits? The standard answer is that it becomes a black hole. But there is a hidden assumption: this is that nothing denser than neutron star material can exist, so if an ultracompact star is heavier than a neutron star, it must be a black hole. This assumption was excusable in the 1960s, when there really weren’t any other, denser forms of matter known – but not any longer.
For example, there is a substance called Bose-Einstein condensate, which has been manufactured in laboratories [ref]. Unlike the material that makes up white dwarfs and neutron stars, this substance does not have a maximum mass. Consequently, a small number of theorists have considered this material as the basis for an alternative to black holes, which they call gravastars.
Particle physicists get excited about a substance called quark-gluon plasma, which has been produced at particle accelerator laboratories [ref]. It’s believed that this state of matter existed for a few microseconds after the Big Bang, before the universe cooled enough to form protons and neutrons. One Indian physicist, Abhas Mitra – a self-proclaimed heretic who is a bit too gifted to be easily ignored [ref] – has developed a theory that quark-gluon plasma balls form instead of black holes [ref].
In both cases, the main problem is that it would be exceptionally difficult to be able to show observationally that a compact star is a gravastar or a quark-gluon plasma ball as opposed to a black hole (or vice versa). But perhaps the burden of proof is the wrong way round: given the existence of such material, shouldn’t it be necessary to demonstrate that black holes do exist, rather than to assume that they do because objects exist which are larger than neutron stars?
So that’s why I’m a heretic about black holes!