Black holes are like cockroaches: Once you find
one or two,
you know there are hundreds, thousands....
– Joseph Dolan, NASA Goddard Space Flight Center,
in a radio interview for PBS Talk of the Nation, 19 Jan 2001.
An isolated black hole as seen from 1000 and 10 times
its Schwarzschild radius
What Are They?
- In general: Regions of space
where gravity is so strong that not even light can escape (the
escape velocity is the speed of light); These regions (but not
the hot matter around them!) will then be black.
- Is light affected by gravity?
Not according to Newton's theory, but we have lots of evidence
(from stars as well as from distant galaxies) that gravity causes
light bending, and redshift; To account for this, you need Einstein's
theory of gravity, general relativity.
- Can light actually be trapped?
Yes; The escape velocity increases if a body is compressed, and
at some point an event horizon forms around it
at a distance called the Schwarzschild radius, an imaginary
surface within which everything is trapped, including light itself;
Around it, light can even orbit around in the photon sphere!
Stellar Black Holes
- Star collapse: Most black holes
are believed to be relics of very massive stars, whose cores
alone have more than 3 solar masses; Their supernova remnants are too massive to
become neutron stars and undergo
the ultimate form of gravitational collapse, possibly with a flash that can be observed.
- How massive are
they? Since the exploding star blows off most
of its mass, these "small" black holes probably are no
heavier than 15–20 solar masses.
- How large are they? Usually a few miles
across or so, depending on their mass (if the Sun could become a black
hole, its Schwarzschild radius
would be 3 km, and for
the Earth it would be 1 cm!).
of Black Holes
- How they may differ: Only by
their mass, rotation rate, and electric charge, if they have
one; All other features they may have had are crushed
away or hidden inside.
- Tiny black holes? "Primordial"
ones could have formed in the very early universe, and in that
case may or may not still be around (small black holes evaporate fast).
- Medium size black holes: Hundreds
to thousands of solar masses; They may form inside star clusters
(two have possibly been seen at the centers of globular clusters).
- Supermassive black holes: At
the cores of most galaxies, with up to billions of solar masses, formed
from pileup of matter and stars; We are just beginning to understand
their role in galaxy evolution.
What Effects Does a Black
- On nearby objects: Outside the
event horizon its gravity is like that of a star, only stronger–it
does not reach out and "suck you" in like a vacuum cleaner!
But it does produce extreme cases of tides (the smaller the black hole,
the stronger the stretching and squeezing), and an apparent slowing
down of all motion.
- On surrounding matter and space: The
accretion disk gets heated up and emits radiation (X-rays, visible
light) as well as jets of matter (and accompanying radio waves...); These
can be seen and can also have long-range effects in a galaxy; If the
black hole rotates, it also drags space along in its rotation.
- Inside the black hole: There
is no return after the horizon is crossed; Our current theory
predicts that there is a singularity inside...
Have We Actually
Seen Black Holes?
- The main problem: Black holes
cannot be seen directly; We have to rely on being able to see
their effects on a companion star or a disk of matter.
- Small: We know a few dozen
stellar black holes in binaries in our galaxy, like Cygnus X-1 (from
their estimated masses and sizes), and 20 in M31.
- Mid-size: Some globular
clusters (Omega Cen or NGC 5139, M15 and G1) may have black holes
with thousands of solar masses at their centers.
- Supermassive: We know beyond
much doubt that the core of our galaxy is a supermassive black
hole, and it is likely that the cores of most or all galaxies
are black holes, with masses in the millions of solar masses
or more (depending on galaxy size); We know of many examples,
including one galaxy with two supermassive black holes.
Confinement to the Black Hole ... to be reserved
for cases of Drunkenness, Riot, Violence, or Insolence to Superiors.
- Formation of massive black holes: Did
most of them form gradually, going through smaller sizes, or from
a sudden collapse of material, as in galaxy mergers?
- Black hole radiation: Black
holes in principle can emit particles and radiation, as if they
had a temperature, by tearing them out of the surrounding vacuum;
as Stephen Hawking put it, "Since black holes behave like black
bodies, they are not black."
- Can we see this? The radiation
is significant only for tiny black holes; but the SETI people
are searching for radio waves from the regular-size ones...
– British Army regulation (1844)
page by luca bombelli <bombelli at olemiss.edu>,
modified 15 may 2013