What Are They?
Black holes are the evolutionary endpoints
of stars at least 10 to 15 times as massive as the Sun. If a
star that massive or larger undergoes a supernova explosion, it
may leave behind a fairly massive burned out stellar remnant.
With no outward forces to oppose gravitational forces, the
remnant will collapse in on itself. The star eventually
collapses to the point of zero volume and infinite density,
creating what is known as a " singularity ". As the density
increases, the path of light rays emitted from the star are bent
and eventually wrapped irrevocably around the star. Any emitted
photons are trapped into an orbit by the intense gravitational
field; they will never leave it. Because no light escapes after
the star reaches this infinite density, it is called a black
hole.
But contrary to popular myth, a black hole is not a cosmic
vacuum cleaner. If our Sun was suddenly replaced with a black
hole of the same mass, the earth's orbit around the Sun would be
unchanged. (Of course the Earth's temperature would change, and
there would be no solar wind or solar magnetic storms affecting
us.) To be "sucked" into a black hole, one has to cross inside
the Schwarzschild radius. At this radius, the escape speed is
equal to the speed of light, and once light passes through, even
it cannot escape.
The Schwarzschild radius can be calculated using the equation
for escape speed.
vesc = (2GM/R)1/2
For photons, or objects with no mass, we can substitute c (the
speed of light) for Vesc and find the Schwarzschild radius, R,
to be
R = 2GM/c2
If the Sun was replaced with a black hole that had the same mass
as the Sun, the Schwarzschild radius would be 3 km (compared to
the Sun's radius of nearly 700,000 km). Hence the Earth would
have to get very close to get sucked into a black hole at the
center of our solar system.
If We Can't See Them, How Do We Know
They're There?
Since black holes are small (only a few to
a few tens of kilometers in size), and light that would allow us
to see them cannot escape, a black hole floating alone in space
would be hard, if not impossible, to see. For instance, the
photograph above shows the optical companion star to the
(invisible) black hole candidate Cyg X-1.
However, if a black hole passes through a cloud of interstellar
matter, or is close to another "normal" star, the black hole can
accrete matter into itself. As the matter falls or is pulled
towards the black hole, it gains kinetic energy, heats up and is
squeezed by tidal forces. The heating ionizes the atoms, and
when the atoms reach a few million degrees Kelvin, they emit
X-rays. The X-rays are sent off into space before the matter
crosses the Schwarzschild radius and crashes into the
singularity. Thus we can see this X-ray emission.
Binary X-ray sources are also places to find strong black hole
candidates. A companion star is a perfect source of infalling
material for a black hole. A binary system also allows the
calculation of the black hole candidate's mass. Once the mass is
found, it can be determined if the candidate is a neutron star
or a black hole, since neutron stars always have masses of about
1.5 times the mass of the sun. Another sign of the presence of a
black hole is random variation of emitted X-rays. The infalling
matter that emits X-rays does not fall into the black hole at a
steady rate, but rather more sporadically, which causes an
observable variation in X-ray intensity. Additionally, if the
X-ray source is in a binary system, the X-rays will be
periodically cut off as the source is eclipsed by the companion
star. When looking for black hole candidates, all these things
are taken into account. Many X-ray satellites have scanned the
skies for X-ray sources that might be possible black hole
candidates.
Cygnus X-1 is the longest
known of the black hole candidates. It is a highly variable and
irregular source with X-ray emission that flickers in hundredths
of a second. An object cannot flicker faster than the time
required for light to travel across the object. In a hundredth
of a second, light travels 3000 kilometers. This is one fourth
of Earth's diameter! So the region emitting the x-rays around
Cygnus X-1 is rather small. Its companion star, HDE 226868 is a
B0 supergiant with a surface temperature of about 31,000 K.
Spectroscopic observations show that the spectral lines of HDE
226868 shift back and forth with a period of 5.6 days. From the
mass-luminosity relation, the mass of this supergiant is
calculated as 30 times the mass of the Sun. Cyg X-1 must have a
mass of about 7 solar masses or else it would not exert enough
gravitational pull to cause the wobble in the spectral lines of
HDE 226868. Since 7 solar masses is too large to be a white
dwarf or neutron star, it must be a black hole.
However, there are arguments against Cyg X-1 being a black hole.
HDE 226868 might be undermassive for its spectral type, which
would make Cyg X-1 less massive than previously calculated. In
addition, uncertainties in the distance to the binary system
would also influence mass calculations. All of these
uncertainties can make a case for Cyg X-1 having only 3 solar
masses, thus allowing for the possibility that it is a neutron
star.
Nonetheless, there are now about 10 binaries for which the
evidence for a black hole is much stronger than in Cygnus X-1.
The first of these, an X-ray transient called A0620-00, was
discovered in 1975, and the mass of the compact object was
determined in the mid-1980's to be greater than 3.5 solar
masses. This very clearly excludes a neutron star, which has a
mass near 1.5 solar masses, even allowing for all known
theoretical uncertainties. The best case for a black hole is
probably V404 Cygni, whose compact star is at least 10 solar
masses. With improved instrumentation, the pace of discovery has
accelerated over the last five years or so, and the list of
dynamically confirmed black hole binaries is growing rapidly.
What makes it impossible to escape from
black holes?
Popular accounts commonly try to explain
the black hole phenomenon by using the concept of escape
velocity, the speed needed for a vessel starting at the surface
of a massive object to completely clear the object's
gravitational field. Using Newton's law of gravity it is
straight forward to show that if you take a sufficiently dense
object its escape velocity will equal or even exceed the speed
of light. Citing that nothing can exceed the speed of light they
then infer that nothing would be able escape such a dense
object. Of course, this argument has a flaw in that it doesn't
explain why light would even be affected by a gravitating body,
let alone why it wouldn't be able to escape. Some argue that in
general relativity light is affected by gravity and that indeed
the energy required to escape a black hole is infinite. This
makes the argument for the attraction of light stronger but
still leaves needed explanation.
Two concepts introduced by Albert Einstein help us understand
this situation. The first is that time and space are not two
independent concepts, but are interrelated forming a single
continuum, spacetime. This continuum has some special
properties. An object is not free to move around spacetime at
will, instead it must always move forwards in time. In fact, not
only must an object move forwards in time, it also cannot change
its position faster than the speed of light. This is the main
result of the theory of special relativity.
The second lesson is the main message of general relativity,
mass deforms the structure of spacetime. Loosely speaking, the
effect of a mass on spacetime is to slightly tilt the direction
of time towards the mass. As a result, objects tend to move
towards masses; we experience this as gravity. As you get closer
to a mass this tilting effect becomes stronger. At some point
close to the mass this effect becomes so strong that all the
possible paths an object can take lead towards the mass. That
is, you can no longer get further away from the black hole no
matter how much you try; you are trapped. This is precisely what
happens at the event horizon of a black hole.
So, to put it succinctly, the reason you cannot escape a black
hole is because you cannot move backwards in time (or faster
than the speed of light).
What about all the Wormhole Stuff?
Unfortunately, worm holes are more science
fiction than they are science fact. A wormhole is a theoretical
opening in space-time that one could use to travel to far away
places very quickly. The wormhole itself is two copies of the
black hole geometry connected by a throat - the throat, or
passageway, is called an Einstein-Rosen bridge. It has never
been proved that worm holes exist and there is no experimental
evidence for them, but it is fun to think about the
possibilities their existence might create.
Source:NASA,Wikipedia
