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Black
Holes
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(Section Not Complete)
Photons always travel at the
speed of light, but they lose
energy when travelling out of a gravitational field
and appear to be redder to an external observer. The stronger the gravitational
field, the more energy the photons lose because of this
gravitational
redshift. The extreme case is a black hole where photons from within a
certain radius
lose all their
energy and become invisible. Indeed, light in the vicinity of such
strong gravitational fields exhibits
quite bizarre behavior. Here are links to
some
movies
illustrating
virtual trips to black holes and neutron stars.
Event Horizons
The event horizon is the point outside the black hole where the gravitational
attraction becomes so strong that the escape velocity (the velocity at which an
object would have to go to escape the gravitational field) equals the speed of
light. Since according to the relativity theory no object can exceed the speed
of light, that means that nothing, not even light, could escape the black hole
once it is inside this distance from the center of the black hole. A more
fundamental way of viewing this is that in a black hole the gravitational field
is so intense that it bends space and time around itself so that inside the
event horizon there are literally no paths in space and time that lead to the
outside of the black hole: No matter what direction you went, you would find
that your path led back to the center of the black hole, where the singularity
is found.
Black Holes and the Speed of Light
Black holes almost certainly exist, and one of their basic properties is that
they trap light. However, it is also true that nothing exceeds the speed of
light. In fact, the theoretical prediction of black holes is due to the
General Theory of Relativity, which is built on the principle that the speed of
light in a vacuum is constant. The analogy of a cannonball falling back to
Earth with the trapping of light in a black hole is only a crude and suggestive
one that is not correct at a fundamental level (for one thing, the cannonball
has mass, but light does not; it turns out that this difference is critical,
because massless particles MUST travel at light velocity, but massive particles
CANNOT travel at light velocity).
To understand fully why a black hole can trap light but the light still always
travels at constant velocity requires an understanding of the General Theory of
Relativity, but the essential point is that the black hole curves spacetime
back on itself, so that all paths in the interior of the black hole
lead back to the singularity at the center, no matter which direction you go
(an analogy in two dimensions is that no matter which direction you go on the
surface of the Earth in a "straight line" (what mathematicians call a
"geodesic" or a "great circle"), you never escape the Earth but instead return
to the same point. Imagine extending that analogy to the 4 dimensions of
spacetime and you have a rough explanation for why light travels at light
speed, but cannot escape the interior of a black hole.
Singularities Clothed and Naked
The singularity is the point of infinite density thought to exist at the center
of a black hole. We have no way of understanding what would happen in the
vicinity of a singularity, since in essence nature divides our equations by
zero at such a point, and you probably learned some time in math class that you
cannot divide by zero and get sensible mathematics. There is an hypothesis,
called the "Law of Cosmic Censorship" that all singularities in the Universe
are contained inside event horizons and therefore are in principle not
observable (because no information about the singularity can make it past the
event horizon to the outside world). However, this is an hypothesis, not
rigorously proven, so it is conceivable that so-called "Naked Singularities"
might exist, not clothed by an event horizon. If such were the case, we can
only guess at this point what that would imply for physics near such an object.
Violence in the
Cosmos Black Holes
Black Holes in Binary Star Systems
It is thought that in some binary
systems one of the stars is a
black hole. Although the black
hole cannot be seen directly, it
can signal its presence if matter
accretes from the other star into
the black hole. The matter falling
into the black hole is likely to
form an accretion disk.
As the matter in the accretion
disk loses energy and spirals
downward into the black hole it is
heated to very high temperatures
and emits X-rays.
Generally, any binary star
system in which there is a strong
X-ray source and in which one of
the stars is not seen but is very
massive is a good candidate for a
black hole.
Identification of Cygnus X-1
Black Hole Accretion
The compact star in an accreting binary system may also be a black hole.
Accretion onto a black hole will look similar in many respects
to accretion onto a neutron star or white dwarf.
The adjacent figure is a
ROSAT X-ray image of LMC X-1, a binary system in the Large Magellanic Cloud
in which
one star is a more normal star and one is estimated to have a mass of 5 solar
masses or more and therefore is likely to be a black hole
(Source).
The diffuse glow is X-ray emission in the vicinity of the binary (which isn't
seen in the image).
X-rays from the accretion disk of the binary
knock electrons off atoms in a volume of space that may be light years in
diameter.
These atoms emit X-rays when the electrons re-combine, causing the observed glow.
The following table lists candidates black holes in some binary systems.
Black Hole Candidates in Binary Star Systems
|
Name of
Binary System |
Companion
Star
Spectral Type |
Orbital
Period
(days) |
Black Hole
Mass
(Solar Units)
|
|
| Cygnus
X-1 |
B
supergiant |
5.6 |
6-15
|
| LMC
X-3 |
B main
sequence |
1.7 |
4-11
|
| A0620-00 (V616
Mon) |
K main
sequence |
7.8 |
4-9
|
| GS2023+338 (V404
Cyg) |
K main
sequence |
6.5 |
> 6
|
| GS2000+25 (QZ
Vul) |
K main
sequence |
0.35 |
5-14
|
| GS1124-683 (Nova
Mus 1991) |
K main
sequence |
0.43 |
4-6
|
| GRO J1655-40
(Nova Sco 1994) |
F main
sequence |
2.4 |
4-5
|
| H1705-250 (Nova
Oph 1977) |
K main
sequence |
0.52 |
> 4
|
|
| |
| |
SOURCE: Fraknoi,
Morrison, & Wolff, Voyages through the Universe
|
|
Here is a table from a different source:
|
Stellar Black Holes in the Milky Way
|
| X-Ray Source
Name |
Mass of
Companion |
Mass of Black Hole
|
| Cygnus X-1 |
24 - 42 |
11 - 21
|
| V404 Cygni |
~ 0.6 |
10 - 15
|
| GS 2000+25 |
~ 0.7 |
6 - 14
|
| H 1705 -
250 |
0.3 - 0.6 |
6.4 - 6.9
|
| GRO J1655 -
40 |
2.34 |
7.02
|
| A 0620 -
00 |
0.2 - 0.7 |
5 - 10
|
| GS 1124 -
T68 |
0.5 - 0.8 |
4.2 - 6.5
|
| GRO
J042+32 |
~ 0.3 |
6 - 14
|
| 4U 1543 -
47 |
~ 2.5 |
2.7 - 7.5
|
|
|
All masses in Solar masses.
Source: "Revisiting the Black Hole",
R. Blandford & N. Gehrels, Physics
Today, June (1999)
|
Bipolar Mass Ejection
Some of the material accreting onto a black hole may get ejected at very high
velocities along the directions defined by the black hole
rotation axis; this is called
bipolar flow. The adjacent image shows a
Nebula
produced by possible bipolar flow from a binary system.
Such mass ejection might also be produced by accretion onto neutron stars. Thus
additional information, such as an estimate of the mass of the unseen compact
object,
is generally required to show that the mass ejection is probably
associated with a
black hole.
Supermassive Accretion Disks
Accretion into black holes is not
limited to binary star systems.
The following image shows a
composite of ground based
optical and radio telescope
images of the galaxy NGC 4261,
and a high resolution Hubble
Space Telescope image of the
core of this galaxy.
NGC 4261 has enormous jets
shooting from its core and very
strong radio frequency emission.
It is thought that the jets are
powered by a gargantuan black
hole of perhaps a billion solar
masses, and that the ring in the
Hubble image is an accretion
disk feeding the black hole.
The black hole itself presumably
lies inside the bright spot at the
center. Even a billion solar mass
black hole would be too small to
see in this image.
Black Holes Signature From Advective Disks
Black Holes in Galactic Centers
Virtual trips to black holes & neutron stars
Black hole FAQ
Special Relativity
General Relativity
The Light Cone: An Illuminating Introduction to Relativity
Schwarzschild black holes (from The Light Cone -- source for Schwarzscild photo also ...).
Java Applet: Orbits in
Strongly Curved Spacetime
C-ship: Relativistic ray
traced images (special relativity illustrations)
|
Radius for Black Hole of a Given Mass
|
| Object |
Mass |
Black Hole Radius
|
| Earth |
5.98 x 1027 g |
0.9 cm
|
| Sun |
1.989 x 1033 g |
2.9 km
|
| 5 Solar Mass Star |
9.945 x 1033 g |
15 km
|
| Galactic Core |
109 Solar Masses |
3 x 109 km |
|
|
|
|
|
Where Might We Find Black Holes?
It is impossible to observe a black hole directly and so any black hole
candidates have to be identified by their effect on the matter surrounding
them. If no other explanation for the observed phenomena is valid then it is
likely that a black hole is present.
There are some objects that are good candidates for the presence of a black hole.
- Any star shines and survives because the pull of gravity, which is
trying to compress it, just balances the pressure generated by the
nuclear furnace at its centre, which is trying to expand it. Once the
furnace runs out of fuel, which must eventually happen, the pressure
decreases, loses its battle with gravity, and the star collapses.
Astronomers believe that one of only three things can happen to a star
in this situation, depending on its mass. A star less massive than the
Sun collapses until it forms a `white dwarf', with a radius of only a
few thousand kilometers. If the star has between one and four times
the mass of the Sun, it can produce a `neutron star', with a radius of
just a few kilometers, and such a star might be recognised as a
`pulsar'. The relatively few stars with greater than four times the
mass of the Sun cannot avoid collapsing within their Schwarzschild
radii and becoming black holes.
So, black holes may be the corpses of massive stars.
- Most astronomers believe that galaxies like the Milky Way were formed
from a large cloud of gas which collapsed and broke up into individual
stars. We now see the stars packed together most tightly in the
centre, or nucleus. It is possible that at the very centre there was
too much matter to form an ordinary star, or that the stars which did
form were so close to each other that they coalesced to form a black
hole.
It is therefore argued that really massive black holes, equivalent to a
hundred million stars like the Sun, could exist at the centre of some
galaxies.
Evidence for galactic size black holes may be found in the section on
active galaxies.
Based on information in:
Science and Engineering Research Council
Royal Greenwich Observatory
Information Leaflet No. 9: `Blackholes'
webman@mail.ast.cam.ac.uk
How Might We See Black Holes?
Because black holes are small, and no signals escape from them, it might
seem an impossible task to find them. However, the force of gravity
remains, so if we detect gravity where there is no visible source of light
then a black hole may be responsible. This type of argument, by itself, is
not very convincing, and so we must look for other clues. If there is other
material around a black hole which might fall into it, then it will. There
is then a good chance that as it falls it will produce some detectable
signal not from the black hole itself, but from just outside it.
Most stars are not single, like the Sun, but are found in pairs, small
groups or large clusters. If a pair of stars have different masses then the
more massive one will burn up its nuclear fuel and may become a black hole,
whilst the other remains a normal star consuming its fuel more slowly. Gas
can then be sucked from the star into the black hole. The gas becomes very
hot, with a temperature of millions of degrees, and will shine not with
visible light but with X-rays. These X-rays will have an observable effect
on the light output from the ordinary star. Since the star and black hole
go round each other every few days, we might expect to see regular
variations in the brightness and X-ray output.
There are some X-ray sources which have all the properties described above.
Unfortunately it is impossible to distinguish between a black hole and a
neutron star unless we can prove that the mass of the unseen component is
too great for a neutron star. Strong evidence was found by Royal Greenwich
Observatory
astronomers that one of these sources called Cyg X-1 (which means the first
X-ray source discovered in the constellation of
Cygnus) does indeed contain
a black hole. Things are
rather different if there is a massive black hole in the centre of a galaxy.
It is possible there for a star to be swallowed by the black hole. The pull
of gravity on such a star will be so strong as to break it up into its
component atoms, and throw them out at high speed in all directions. Some
of the fragments will fall into the hole, increasing its mass, whilst others
could produce an outburst of radio waves, light and X-rays. Evidence for
such behavior may be found in the section on
Active Galaxies.
This is just the behaviour which is observed in galaxies of the type called
`Quasars' and may well be happening in a milder way in the centre of our own
Milky Way.
Based on information in:
Science and Engineering Research Council
Royal Greenwich Observatory
Information Leaflet No. 9: `Blackholes'
webman@mail.ast.cam.ac.uk
Frame Dragging
One of the strangest predictions of the general theory of relativity concerning black holes is called
frame dragging. For a rotating black hole, the theory predicts that space and time itself can
be dragged by the rotating black hole. The adjacent figure shows an artist's conception of this idea
(J. Bergeron, Sky & Telescope: get permission;
Ref).
Some recent data has been interpreted as supporting evidence for frame dragging around a black hole
(Ref).
Summary
These ideas are very bizarre, and yet there is rather strong evidence that
black holes exist, as predicted by the theory. If that is true, then
singularities and event horizons probably exist too. It is less likely that
naked singularities exist, and we have no experimental evidence for them, but
they can't be ruled out based on our present understanding.
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