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Robert Clayton
Prof. Kudelka
CCI
6 December 2005
Invisible Vacuums
Imagine a vacuum that is so powerful that it sucks everything around it in, even the
world. An object like this would certainly be interesting. The most fascinating aspect is how it
exists, without collapsing on itself. What would happen if one could travel to an object like this,
and go in it? These objects exist in space, and they are called black holes.
Roughly speaking, a black hole is a structure of mass that has such a strong gravitational
pull that nothing can escape it, not even light. Every body of mass has an escape velocity, which
is the rate at which an object needs to be propelled at in order to escape another object’s gravity.
A bigger object will have more mass; therefore the escape velocity will be greater. The Earth for
instance has an escape velocity of 11.2 kilometers per second (about 25,000 m.p.h.); however the
moon, which is much smaller, has an escape velocity of only 2.4 kilometers per second (about
5300 m.p.h.). That seems very fast, but a black hole, on the other hand, has so much mass that
not even light, which travels at 700 million miles an hour, can escape from its pull.
A dying star forms a black hole. Once the star (in this stage of the star’s life it is a red
super giant, much larger than our sun) has used up all its fuel, it simply is unable to support itself
against its own gravity. It collapses upon itself and a black hole is formed. The gravity of a black
hole is so strong that any object inside of it will be crushed.
The physical end of a black hole is called the ‘event horizon,’ a sphere-shaped border that
surrounds the black hole. It is possible for objects to cross into the horizon, but impossible for
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them to cross out. It is easiest to think of the horizon as a place where the escape velocity equals
the speed if light. Even if something is less than a millimeter away from the horizon, it can still
get away if it has enough force to. Of course the only thing that would be able to get away that
close to the horizon is light, but it is still possible. However, as soon as an object crosses the
horizon, its fate is sealed. Not even light would be able to escape.
The horizon is a complicated entity. If someone were able to get close enough without
being in danger, they would view it as an unmoving object, however; once you get into the
horizon, you would see that the horizon is actually moving outward at the speed of light. This
helps to explain why you can cross into it, but not out.
A black hole is a big object. It is big in two ways: mass and size. Mass is how much
something weighs, while size determines how much space it takes up. There is no limit to the
amount of mass that can be made to form a black hole. Black holes are formed from the death of
a massive star, but the Conservation of Energy law states that no mass can be created or
destroyed in a reaction, so it is suspected that a black hole will have the same amount of mass as
the star from which it died. Let’s say that the mass of a star was around ten times the amount of
our sun’s mass (a solar mass), which would be 1031 kilograms. Some of the heaviest black holes
lay at the center of some galaxies and those black holes weigh up to a million times the solar
mass, 1036 kilograms.
The mass a black hole contains is directly proportionate to the radius of its horizon. For
example if one black hole weighed five times as much as another black hole, its horizon’s radius
would be five times as big. It works in terms like this: if a black hole had a solar mass, it would
have a radius of three kilometers. A ten solar mass black hole would have a radius of thirty
kilometers and a thousand solar mass black hole would have a three thousand kilometer radius.
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By comparison, the radius of the Earth is 6 378.1 kilometers, so some black holes are about half
of the Earth’s size, but they are so dense they would crush the Earth in seconds.
Many people wonder what would happen if they took a spaceship into a black hole. They
wonder if it would be an instant death or if they would experience the gravity for a while. For an
example, let’s use a black hole that has a million solar mass. A million solar mass black hole is a
prime example, because a smaller black hole would crush you before you could really feel its
effects. If you were far enough away from the horizon at first, you wouldn’t feel anything
because although the hole is pulling you in, pull is not that strong so you wouldn’t feel it.
Everything that would be around (your body, your ship and everything in it) would be pulled the
same way at the same rate, so you’d feel weightless. Eventually, you’d feel a “tidal pull,” which
can best be described like this: If your feet were closer than your head (to the center of the hole)
then they would be getting more of a pull, leaving you with a stretched feeling. You can get the
same affect if you spin very fast in a circle. Although it wouldn’t be nearly to the degree of a pull
that a black hole would have, it is a decent simulation.
Your vision would be fine. You’d see some objects that are slightly distorted because the
hole’s gravity bends light. Even as you cross the horizon, you would be able to see out. This is
because there would still light coming into the hole. No one could see into the hole because the
light is not strong enough to get out. However, you wouldn’t really have time to think about that
kind of stuff, because seven seconds after crossing the horizon, the tidal pull would get so strong
on your body and ship that it would crush you both. Something like that would be sure to but a
damper on someone’s day.
If someone were watching you fall into a black hole, they would get quite a different
view. The closer and closer you get to the horizon, the slow and slower they would see you
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move. This is because the light would take longer to escape the pull because it is so close to the
horizon. The person watching would never see you actually cross the horizon, which is why the
holes were originally called frozen stars, because the object seemed to “freeze” just before it
crossed the horizon. The light you’d emit would never reach anyone because it would be
suspended at the horizon. Earlier, it was mentioned that the escape velocity at the horizon is the
speed of light. The light is not strong enough to fully get out, but not weak enough to get sucked
in. You have long since passed into the horizon and dead by now, but the light being sent to the
person who observed it, telling them that you crossed the horizon would never reach them.
Another common question that people have is that since a black hole is the remnants of a
dead star, will the sun eventually become a black hole? Scientists don’t think the sun will follow
that path. They think that the sun will just burn itself out. Before either of these happen,
however, the sun will become a super giant, which means life on Earth won’t be possible. The
sun will become so big that it will engulf Mercury and Venus, making Earth the closest planet to
the sun. The water will all evaporate and the atmosphere will crumble. There is good news in all
this though, this all isn’t supposed to happen for another five to eight billion years, so there is
nothing to worry about now.