To see a black hole

You may be thinking by now that all this talk of time slowing
down, travelling at the speed of light, being stretched like spaghetti
then crushed to zero size and infinite density is purely the stuff of
science fiction. After all, no one has ever come face-to-face with
a real black hole and all these conclusions have been reached by
studying their properties theoretically.
Until the 1960s, most astronomers found it hard to believe,
despite the theoretical predictions, that there could really be
black holes out there. But with the advances in radio and x-ray
astronomy and a number of exciting discoveries during the 1960s
such as the cosmic background radiation (which confirmed the Big
Bang theory), quasars and pulsars, suddenly black holes no longer
seemed so outrageous. Coupled with this, many of the important
theoretical advances in black hole physics were made during the
1960s and ’70s and by the ’80s I would guess that astronomers
would have been something like 90% sure that black holes existed.
You may consider a 90% confidence level not enough so,
thankfully for black hole fans like me, the 1990s have seen a further
accumulation of evidence and we are no longer in any real doubt.
I would put the current confidence level at 99%. What is this
evidence then? After all, if by definition a black hole is black, how
can it be picked out against the blackness of space? Even if one
happened to have a luminous nebula as its backdrop, you must
remember that black holes are so small on an astronomical scale
that they would be far too tiny to be seen even by the most powerful
telescopes.
The secret to their possible detection (which, amazingly, was
pointed out two hundred years ago by John Michell) lies in the way
they influence visible matter nearby. Recall that binary stars orbit
round each other or, more correctly, around their combined centre
of gravity (an imaginary point in space which is the mid-point of
their masses). If they have the same mass then they will have the
same orbital radius since their centre of gravity will be half way
between them, but if one star is much heavier than the other then
it will only wobble slightly while the lighter one does most of the ‘running’ around it. This is because the centre of gravity is now
much closer to the larger star.
If one of the stars is large enough to collapse into a black
hole then, even though it is now invisible, its gravitational effect
on its partner will be the same. Note that they would not be so
close together that the other star could get swallowed up by the
black hole since the stars would have been attracted together long
before if they were that close (but they may still be close enough
for the black hole to suck off some of the gas from the surface of
its partner).
We should in theory be able to observe the ‘wobble’ in the
motion of the remaining visible star and hence deduce how much
mass would be required to cause an object as big as a star to move
about like this. After all, single stars do not wobble about for no
reason and such motion must be the result of a tremendous nearby
concentration of mass. It has been discovered recently that a tiny
wobble in a star’s position might be due to planets in orbit around
it that cannot be seen directly. However, from the mass of the
star and the amount of wobble we can deduce how massive the
invisible partner is. If it is more than, say, ten solar masses (to be
on the safe side) then it would have to be a black hole.
The way the wobble is detected is not, as you might expect,
by measuring the sideways motion (the to and fro motion of the
star at right angles to our line of sight) but from the change in
the wavelength of light that leaves the star when its orbit takes it
towards us and when it is receding from us. This is just a Doppler
effect again. The wavelength of the light gets compressed (towards
the blue end of the spectrum) when it is moving towards us and
stretched (to the longer wavelength red end) when it is moving
away. It does not matter in what direction the binary system as a
whole is moving since it is a change in the observed wavelength
that we require. From the rate at which this change in wavelength
occurs, plus some other pieces of information, we can deduce the
period of the orbit and hence the mass of the invisible partner.
This all sounds very clever in theory, but in practice it turned
out not to be quite so straightforward. There are other reasons
why we would only see one of the stars in a binary system. The simplest explanation is if the other star was just too small and dim
and is thus outshone by its larger, brighter, companion. It may
be that the invisible partner is a white dwarf or even a neutron
star. What is needed is proof that it has a mass that is well above
the critical value for a black hole (say five or ten times the mass
of the Sun). However, even this is not proof that it is a black hole.
Igor Novikov puts it this way: “‘invisibility’ is a poor proof for the
existence of something”, and he quotes the old joke about the title
of the research thesis: ‘The absence of telegraph poles and wire in
archaeological excavation sites as a proof of the development of
radio communications in ancient civilizations’.
What finally clinches it in support of black holes among stellar
binary systems is something I have already alluded to. If the
two companions (the still shining star and the invisible black
hole candidate) are close enough together then the black hole’s
incredible gravitational field will slowly suck gas off the outer
envelope of the star. This gas will spiral in towards the hole’s
event horizon heating up all the time as it speeds up and forms
what is known as an accretion disc surrounding the hole. The
matter in this disc is so hot that, before it falls in, it will give off
an unmistakable signal of powerful x-ray emissions, which are
just bursts of high energy electromagnetic radiation. They will
be subtly different from the x-ray emissions produced by some
pulsars (the spinning neutron stars) due to their rapid rotation,
since in the case of a black hole’s accretion disc the timing of the
bursts is random. X-ray pulsars give off their signal at regular
intervals as they spin, a bit like a searchlight.
Do such x-ray binary systems exist? The answer is yes. The
most famous example is one that Stephen Hawking finally, and
somewhat reluctantly (due to a bet he had with Kip Thorne),
admitted must contain a black hole. It is called Cygnus X-1 and
is about six thousand light years from Earth, but within our own
Galaxy. The visible companion is a giant star about thirty times
the mass of the Sun (deduced by studying the light it gives off).
By studying the way it wobbles (from its periodic Doppler shift)
the mass of its invisible partner has been put at about ten solar masses8. The last piece in the jigsaw comes from the period of
the x-ray emissions given off with a frequency of several hundred
times per second from the accretion disc and tell us how fast the
gas is orbiting the hole. Since nothing can go faster than light, this
period gives us a maximum size for the orbit and suggests that
the hole must be much smaller than the Earth, and to squeeze ten
solar masses into such a small volume means it would have to be
a black hole. The laws of physics state that it can be nothing else.
There are several other candidates for black holes within x-ray
binary systems in our Galaxy and in nearby ones. It is estimated
that our Galaxy alone probably contains millions of black holes!
All I have discussed so far are the common-or-garden black
holes that are formed when massive stars collapse under their own
weight. There is another type of black hole that is, in a way, even
more impressive. Another of the major discoveries in astronomy
madein the 1960swasthat of quasars (which stands for quasi-stellar
radio sources). By quasi-stellar it is meant that they were thought
to be similar to stars in that they appeared as pointlike objects
rather than extended blobs of light like galaxies or nebulae. They
also gave off strong radiation in the radio frequency band—and
not because they contained their own radio stations as I used to
think as a child when I first read about them. Nowadays, only
a small fraction of all quasars discovered are radio sources, but
the name has stuck. What is more important is that it turns out
that quasars are nothing like as small as stars. They are also the
most distant objects in the Visible Universe and some are over ten
billion light years away (which means that light left them when the
Universe was very young). For objects so far away to shine with
such brightness means they must be incredibly energetic. In fact
quasars are now thought to be young ‘active’ galaxies with most of
their energy (about a thousand times more than the energy output
from all the stars in our Galaxy) coming from a tiny central core.
This core contains what is known as a supermassive black hole.
Typically such black holes would have masses millions of times
greater than the Sun. Since the discovery that quasars have lurking inside them
gigantic black holes it has been discovered thatmanylarge galaxies
probably go through a quasar phase before they settle down.
Even if they didn’t, they could still well contain a supermassive
black hole at their centre. These would have formed from the
accumulation of vast amounts of stellar gas in the dense centres
of galaxies. Andromeda’s black hole is roughly 30 million solar
masses with a radius extending out to the size of our solar system.
Our own Galaxy’s black hole is smaller with an estimated size of
just a few million solar masses. Once a supermassive black hole
has formed it will slowly feed on the surrounding stars and grow
bigger.