INVISIBLE STARS
by Dr Jim Al-Khalili
 
The idea of black holes goes back over two hundred years to the end of the eighteenth century when scientists first realised they might exist. Back then black holes were known as 'invisible stars' and were nowhere near as strange as we believe them to be today.
Until fairly recently the first person attributed to predicting their existence was the French mathematician and astronomer Pierre Laplace, in 1795. However, English geologist and rector of Thornhill Church in Yorkshire, John Michell, presented his ideas about the formation of black holes to the Royal Society in London in 1783. Michell's main claim to fame is as the 'father of seismology' and his black hole predictions are often overlooked.
Both Michell and Laplace based their quite similar arguments on the idea of escape velocity. This is the speed an object, say a rocket, needs to start off with if it is to be able to escape the gravitational pull of a planet and get into space. (Escape velocities apply to stars too even though we could not land a rocket on one.) The escape velocity on the surface of the Earth is 11 kilometres per second (or 40,000 kilometres per hour). On the Moon it is a little over 2 kilometres per second, which is why the Apollo missions' Lunar Modules did not need such large rocket engines to leave the Moon's surface and return to Earth. The escape velocity on the Sun is 620 kilometres per second.
Michell worked this number out based on the size and density of the Sun. He also knew with some accuracy another figure: the speed of light, which had been determined a century earlier and which has a value 500 times bigger than the Sun's escape velocity. He followed the conventional wisdom of the time, that light was composed of particles with mass affected by gravity. This differs subtly from our current view of the nature of light particles (or photons) which, while still affected by gravity, are in fact massless. Michell argued that a star more than 500 times the size of the Sun, but with the same density, would have an escape velocity exceeding that of light. So light leaving the surface of such a star would not be able to escape its gravitational pull and would fall back in on itself. To the outside world such a star would be invisible.
This sounds reasonable until we remember that it relies on Newton's law of gravity. And while the Newtonian view may be fine for the purposes of sending rockets to the Moon, it is simply wrong when applied to massive stars. We must instead use Einstein's Theory of General Relativity to which Newton's law is a crude approximation. We now know that black holes are much more than just large invisible stars. In fact they differ from Michell's ideas in some astonishing ways. Black holes are almost completely empty space that is forever cut off from our own Universe. In addition, general relativity says that the gravitational force at the event horizon becomes infinite, and so a collapsing star must continue to collapse until its entire mass has been squashed down to a point of zero size.
Michell and Laplace came nowhere near describing this. In the past few years black hole research has developed into a highly respectable field. Not only have astronomers found mounting evidence that large stars must, and do, eventually collapse into black holes at the end of their lives, but that black holes can form in other ways. We are now quite confident that there are supermassive black holes at the centres of most large galaxies, including our own Milky Way.
Such beasts have masses millions of times greater than that of a typical star like the Sun. In fact, they feed on any nearby stars that wander too close. But before you start to panic, our Solar System is situated on the outskirts of the Galaxy and is therefore not in danger of being sucked in. The first evidence for supermassive black holes came from the study of very faint objects called quasars. These shine like stars (hence the derivation of their name from 'quasi-stellar') but because they are so far away they must be incredibly energetic. Some are over 10 billion light years away, which is two thirds of the way to the edge of the visible Universe.
Quasars are now thought to be young 'active' galaxies with supermassive black holes in their core. Only very large galaxies would go through a quasar phase in their evolution before they settle down. But others, such as the Milky Way, would still have formed a supermassive black hole despite never being a quasar. We expect that all black holes should be spinning rapidly about their axis. This causes space-time in their vicinity to be literally dragged around them in a gravitational vortex, forming a region around their event horizon called the ergosphere, in which it would be impossible to stand still. If we ever come across a black hole in the future, and provided we are sufficiently technologically advanced, we may be able to utilise the ergosphere of a black hole to extract energy.
The idea is based on what is known as Hawking radiation. If an object enters the ergosphere and splits into two with one part falling through the event horizon, the other part could escape again with more energy than it had before. This energy would have come from the black hole itself. Another feature of a spinning black hole is that the singularity at its centre is not a point but a ring. Theory suggests that this singularity might be a window to another part of our Universe, or even to another universe entirely, in a way reminiscent of Lewis Carroll's Alice Through the Looking Glass.
At the moment this is all still highly speculative since in practice the singularity is likely to be too unstable to be used as such a stargate. But who knows?! Finally, if we ever do come across a black hole, its strong gravitational field, even outside the horizon, could be used as means of slowing down time. The effect gravity has on the rate of flow of time is a well-known consequence of Einstein's general theory of relativity and has even been measured on Earth.
If an astronaut were to orbit (at a safe distance of course) around a black hole in a spaceship then the time she would measure as having elapsed would be less than the time elapsed far from the black hole - say back on Earth. On returning to Earth she would, in effect, find that she has travelled into the future!
Dr. Jim Al-Khalili is a theoretical nuclear physicist and lecturer in the Department of Physics at the University of Surrey, England and Visiting Professor at Michigan State University.
|
