Brian Trumbore tells me you all are dying for the word on black
holes. I have to admit right up front that I personally have never
even seen a black hole. The closest I”ve come is the vacuum-
flush toilet found on the Maasdam on our recent Baltic cruise.
You can”t see how the toilet works because you have to lower the
lid to push the flush button and you are strongly advised never to
push that button while seated on the device. Obviously, the fear
is that you”ll be sucked in, never to be seen again. That seat is an
event horizon.
To those who hesitate to ask about black holes, let me quote from
the late Carl Sagan”s introduction to Stephen Hawking”s book “A
Brief History of Time”. “We go through our daily lives
understanding almost nothing of the world. …Except for children
(who don”t know enough not to ask the important questions), few
of us spend much time wondering why nature is the way it is.
…There are even children, and I have met some of them, who
want to know what a black hole looks like….” (Would you
believe that our 11-year old granddaughter asked that very
question recently?) In his introduction, Sagan tells of attending a
meeting in England sponsored by the Royal Society of London.
The meeting dealt with how to search for extraterrestrial life,
another pretty deep question! During a coffee break, Sagan
wandered into a larger meeting in another room, where he saw a
man in a wheelchair slowly signing his name in a book that, on
an earlier page, contained the signature of Isaac Newton. The
occasion was the induction of new fellows into the Royal
Society. The wheelchair-bound Hawking received an ovation
then and, 25 years later, is still active, even hosting a TV science
series, though reduced by Lou Gehrig”s disease to speaking via a
computer. Hawking”s chair at Cambridge was once occupied by
Newton, of the famed falling apple and the “father” of the laws
of gravity. It is only fitting that Hawking”s sophisticated
theoretical work has dealt with gravity in its most concentrated
form. What follows is derived from Hawking”s 1987 book and
other more recent sources. One source is an article in the May
issue of Scientific American by a fellow named Jean-Pierre
Lasota, whose father had known Einstein personally.
The term “black hole” was coined in 1969 by John Wheeler, but
the concept dates at least back to John Mitchell at Cambridge, of
course. He wrote a paper in 1783 suggesting that if a star were
sufficiently massive, light could not escape its gravitation and
that nobody would ever see the star. The idea went out of favor
during the 19th century and, of course, it remained for the usual
culprit, Einstein, to explain in 1915 how gravity affects light in
his general theory of relativity. We discussed earlier how light is
bent by gravity.
To see how a black hole is formed, let”s consider the life and
death of three stars, a little one, a big one and a really big one.
Stars are formed when gas, mostly hydrogen, accumulates and as
it does, its gravity pulls in more and more gas. As the gas
condenses, the hydrogen atoms bang into each other more and
more and faster and faster. This heats up the gas until it becomes
so hot that the hydrogen atoms don”t bounce off each other but
fuse together to form helium, as in the hydrogen bomb. We
know that the bomb and the sun, a mediocre star, give off lots of
heat when this hydrogen “fusion” occurs. The heat increases the
pressure of the gas. (I remember being horrified when, on a Cub
Scout overnight, the bonfire was started and someone decided to
heat a can of baked beans without opening the can! A near-
catastrophic example of heat increasing the pressure.) Well, the
increased pressure in a star builds up until it just balances the
contraction of the gas due to gravity. The star then becomes
stable in the sense that it just keeps burning up hydrogen to form
helium and, like our sun, can perk away for billions of years
Eventually, the hydrogen is pretty well exhausted. The star
cools, the pressure goes down and the star starts to contract again
after all those years. Now things get interesting. Back in 1928,
an Indian fellow named Chandrasekhar was sailing from India to
England to work with the famous astronomer Sir Arthur
Eddington, who had a wry sense of humor. When someone
suggested to him that only three people in the world understood
Einstein”s relativity theory, Eddington reportedly replied, “I”m
trying to think who the third person is.” Well, on the ship,
Chandrasekhar worked out how big a star had to be to support
itself against its own gravity. He came to the conclusion that the
star could be only about 50 percent bigger than our sun. For a
star about the size of our sun, the star would collapse to roughly
a few thousand miles in diameter. Such “white dwarfs” were
indeed observed. These white dwarfs would still have electrons,
protons and neutrons, although at huge densities compared to
matter on earth. A Russian, Lev Landau, at about the same time
proposed that stars 1 to 2 times the size of our sun could have
another fate. They would collapse so far that all the electrons
would react with the protons to form neutrons, thus forming
“neutron” stars only 10 miles or so in diameter. Neutron stars
have also been detected. In the white dwarfs a shot glass of
dwarf can weigh hundreds of tons, while a cubic inch of a
neutron star can weigh hundreds of millions of tons! Now that”s
real density!
But Chandrasekhar”s work had another more startling
implication. What if the original star was more than 1-1/2 to 2
times the size of our sun? Would it keep collapsing to
infinite density, i.e., to a point? Eddington, and even Einstein,
thought this impossible and Eddington was so violently opposed
to the idea that Chandrasekhar was persuaded to abandon this
work. In 1939, however, J. Robert Oppenheimer solved
Einstein”s relativity theory for such a really big star and found
that its gravity would bend the light rays back so strongly that
light indeed could not escape and the star would be invisible.
Since Einstein had shown that nothing could move faster than the
speed of light, it followed that nothing else could escape from the
condensed star. Unfortunately, World War II and the little
matter of leading the scientific development of the atomic bomb
distracted Oppenheimer. His work was rediscovered in the
1960s, when the “black hole” became a respectable concept for
speculation and people like Hawking and Penrose showed that in
the black hole there had to be a “singularity” of infinite density
(that sucker is really, really dense!). This is sort of like the big
bang in reverse, which created the universe from such a
“singularity”.
With the acceptance of the idea of a black hole, there arose a new
term, the “event horizon”. Hawking likens the event horizon to
Dante”s entrance to Hell – “All hope abandon, ye who enter here.”
The event horizon is like a one-way skin, or membrane
surrounding the black hole that lets anything in but nothing out.
Let”s imagine what it would be like for Sylvester, an astronaut
who got curious and decided to investigate a black hole up close
and personal. Let”s assume that he was somewhat of a Superman
type and decided to do a space walk while Cynthia, his fellow
astronaut, stayed behind in the space shuttle. Remember that
they can”t see the black hole, but may suspect something unusual.
As Sylvester comes near the event horizon, he sees nothing
unusual, although looking back to the shuttle, he may notice
things rather distorted due to distortion of the light rays. At any
rate, he”s humming “Fly Me to the Moon” as he blissfully passes
through the event horizon. He looks back at the shuttle and it”s
still there. Cynthia, however, can no longer see Sylvester, whose
future is sealed. He begins to feel the pull of gravity and feels
elongated and within seconds is torn to shreds as he is sucked
into the “singularity”. All the rocket power in the world cannot
help him escape since he would have to go faster than the speed
of light and Einstein has decreed that”s impossible! For
Sylvester, it”s the end of time in the region of infinite density.
Just as time began with the Big Bang from a singularity, so it
ends for Sylvester. Meanwhile, back in the shuttle, Cynthia is
totally unaware of Sylvester”s fate. She saw him perfectly well
up to just before the event horizon and then he just vanished.
Having bid a fond farewell to Sylvester, let”s consider how big a
black hole can be. Since we can”t know anything about what”s
inside the event horizon, we have to define the size of the hole as
the diameter of the event horizon. It turns out that the diameter
is determined by the mass of the black hole. For example, the
Hubble Space Telescope has detected in the center of one galaxy
a black hole (more conservative scientists call it a black hole
“candidate”) which weighs more than a billion suns and has a
diameter about the size of our solar system. On the other hand a
black hole weighing a mere 10 suns would have a diameter of
less than 50 miles.
How do we know that black holes really exist? One way is to
measure how fast the material around a black hole candidate is
spinning. Two examples, including the one above, are cited on a
website that I believe to be a Cambridge University website:
[ http://www.amtp.cam.ac.uk/user/gr/public/bh-obsv.html ]
In the galaxies cited, the speed and size of the rotating discs are
used to calculate the weights of the object that has to be there to
hold the material in orbit. It turns out in both cases the objects
are more than a billion times the weight of our sun and are
roughly the size of our solar system. There is now evidence that
our own Milky Way galaxy contains a black hole in the center.
Another, very informative detailed exposition on black holes can
be found on what I assume is a University of California at
Berkeley website: [ http://cfpa.berkeley.edu/Bhfaq.html#q2 ]
Do we have to worry about being sucked into a black hole? The
answer is no, as long as we stay outside the event horizon, or at
least far enough out. A black hole weighing one sun would not
suck us in any more than our own sun. If our sun became a black
hole, the planets would continue to orbit as before. Of course,
before we all froze to death without the sunlight we”d be mighty
puzzled that the sun just plain vanished without a trace.
To sum up, don”t worry about black holes but if perchance you
should be in the vicinity of an event horizon get out of there as
fast as possible! Incidentally, if you”re feeling sorry for poor
Chandrasekhar not getting credit for his great ideas, he did
manage to pick up a Nobel Prize in 1983, 55 years after his
journey from India to England.
Allen F. Bortrum
