This journey has thirteen stages! Each stage is numbered!
ACTIVITY: Print out a copy of this journey. Then, look through
the links for "Amazing Space" and "NASA's Hubble Space Telescope"
for pictures that might illustrate different things you see during
this journey. Print the pictures and paste them next to what
they illustrate!
Try also looking at the pictures collected in the "Black Hole Trip
Links!"
"Gift of a star's furnace, it grew heavier
when brought here . . . "
--John Peck (1978),
describing the key to the
glass mountain where the
seven ravens were imprisoned
(in the tale of "The
Seven Ravens").
(From John Peck, "Redemption,"
in "The Broken Blockhouse Wall:
Poems" [Boston: David
R. Godine Publishers]).
QUESTION: Why do you think that the black hole's MASS pulls
more strongly than an ordinary star of the same MASS--that
is, why is GRAVITY so strong near a black hole?
(The MASS of something is the "total amount of matter" in it.)
1. Viewing the black hole from a distance:
* You see a black spot in space, but strange things might
be happening nearby.
* You might see gasses streaking past at nearly the speed of light!
* You might also see lightwaves emitted from the black spot's ends.
* You might see nearby stars suddenly become brighter as they
get closer to the black hole's image in space, and then
fade again as they move away.
QUESTION:
Why do you think matter might travel past a black hole at speeds
close to that of light? (You'll think about this again at stage
10 of your journey!)
2. Approaching the black hole:
* As you approach the black hole, you would feel much as
you would approaching any other star of a similar mass.
If you stay the same distance from the black hole's center
as you would stay from the center of another star of the
same size, you'll feel just fine, and not fall into the black
hole at all.
* As you travel closer to the black hole, you'll see the light
from stars around the hole and the hole's gravity increasing
at about the same rate!
3. Getting closer to the black hole:
* As you get to the point where the normal star's surface would
be, you'll feel the same gravitational pull on your ship that
you would from the normal star's surface--even though there
is nothing there.
* However, if you could travel to the interior of a normal star,
the star's gravity would decrease its pull on you, not
increase it. This is because, as you enter the star, you
leave some of its mass behind you. Thus, you no longer
feel the gravity caused by that mass pulling you down.
(You might, of course, feel pressure from that mass
as gravity pushes it down on you--unless you create
a tunnel through the star that resists the pressure.)
The black hole is different. The closer you get, the more
gravity you feel, because none of the mass of the black hole
has been entered yet! The entire mass of the
black hole is still pulling you to it!
(Or is there a way to tunnel through it too? It's so small
and concentrated--but, of course, I wonder if we can really see
all of the black hole, since it's so far away, and light does
not escape . . . )
4. Taking another look at the black hole:
* You'll see a lot of light from orbiting light waves,
super-heated gasses moving at the speed of light, and
electrical fields created by or drawn to the black hole's
super high gravity. The super-heated gasses will look
like streaks in the sky.
* In the center of all this will be a black area of the sky.
QUESTION:
Why do you think the image of heated gasses which are
travelling at about the speed of light appears
streaked across the sky?
5. Experiencing the surrounding radiation:
* An ordinary space ship could not withstand the surrounding
radiation of the black hole--from the light and electromagnetic
waves (if it is really as concentrated as everyone says!)!
6. Trying to orbit the black hole:
* You'll feel gravity pulling you down in a sort of spiral.
To stay in orbit, and not fall, you'd have to travel very
fast and create lots of CENTRIFUGAL FORCE to pull you away
from the black hole.
* The two opposing pulls--that of the centrifugal force
you create and that of the black hole gravity--will
cause TIDAL PULLS on your body. (TIDAL PULLS are the same kind
of changing gravitational pulls that cause the Earth's tides
--in one place of the Earth, water is pulled
one way, and in another, it may be pulled another way.)
This means that part of your body will be pulled in one
direction and part will be pulled in another direction.
The closer to the black hole that your orbit is, the stronger
the tidal pull.
QUESTION:
What do you think very strong tidal pulls might do to
the human body?
7. Spiralling towards the black hole:
* Ultimately, you will not be able to stay in orbit very long--
due to all the tidal pulls on you!
The shortest distance (or at least the natural way to travel)
to the center of a gravitational pull for some reason is not
a straight line, but a spiral down. Your ship will start
spiralling down.
QUESTION:
Why do you think that you might spiral towards the hole?
QUESTION:
What do you think you will 'weigh?' Remember, your mass stays
constant, but your 'weight' varies with changing gravity.
So what sort of gravitational fields do you think you might
feel? Will you weigh a lot--or a little--as you fall? (HINT:
there are several ways to cause a g-field: first, gravity can be
caused simply the pull of mass on mass, which is of course
affected by the closeness of the two masses pulling on each other
--for example, the closer you are to a certain mass, the greater
the gravitational pull of that mass on you; a second is caused by
acceleration--for example, the pull back you feel when you accelerate
a car suddenly is the result of a g-field caused by acceleration; the
third kind of gravitational field--which is really related to the
second--is the pull caused by spinning around something--in this
case, you keep changing the direction of your motion in order to circle
or spin, and so you are pulled in the previous direction as you
change--this pull is known as CENTRIFUGAL FORCE. Consider the
first two kinds of G-fields to decide what gravity you will feel.
Also, look up the term "FREE FALLING." Will this feel like
FREE FALLING? Or not?)
QUESTION:
Do you think that there might be a way for people interested
in long distance space travel to warp space (and the gravity
that helps determine the way people can travel through
space) so that astronauts can travel to places along a
more direct path than gravity normally allows? (This was
proposed in Marcus Chown's article, "Planes, trains, and
wormholes"--which was published in 1996 in New Scientist.)
8. Looking back at the universe behind you as you fall rapidly:
* You'll see rings in the sky where the background stars--
at a certain distance from the black hole--suddenly
have double images--so first one ring of stars and then
another will appear as doubles. These rings of double
images are called "Einstein rings." To see these, go to:
http://antwrp.gsfc.nasa.gov/htmltest/gifcity/fig2f.gif
(Just type the above address into your
web browser window--then use the back button to
come back to finish the rest of your trip! [This link
is also at "Black Hole Trip Links."])
(The above image is from Robert Nemeroff of NASA's site!)
QUESTION:
Why do you think the images behind you start to double?
ACTIVITY WITH MULTIPLE IMAGES:
For this you'll need two mirrors, probably a hand mirror and
a wall mirror. Stand in front of the wall mirror with your
back to it, and hold the hand mirror in front of you. Look
through the hand mirror, positioning it so that you can see--reflected
in the hand mirror--(1), images of your back reflected in the wall
mirror; and (2), images of yourself looking into the hand mirror
also reflected in the wall mirror. Start tracing the
images. How many images do you see? Is the number finite?
Were all those images reflected at the same time? What do you think?
9. Approaching and passing through the black hole's EVENT HORIZON:
* Everything beyond the EVENT HORIZON of the black hole
looks totally black, with only a little radiation escaping.
This is because the EVENT HORIZON is the point at which
gravity is so great that nothing that we know of can get
out--not even your radio signals which you try to send back to Earth--
everything is 'infinitely red-shifted,' because it is travelling
away from us so fast (due to "THE DOPPLER EFFECT," which you
will want to learn more about--read more below about how light
moving away from us vibrates less frequently!). Radiation near
a black hole, such as light waves and radio waves becomes too
faint and weak for us to detect it.
(Of course, most known black holes are so far from the
Earth that it takes years for light waves, radio waves, and other
RADIATION to travel between the part of the universe where
the black hole is and Earth, anyway!)
It seems then that no one outside of the black hole's
event horizon will ever hear what you had
to say over your radio as you crossed the event horizon.
(The EVENT HORIZON is a sphere of high gravity that surrounds
the black hole, and that is 'powered' by the matter in the
black hole.)
QUESTION:
What is a horizon on earth? When ships sail over the horizon, can
we still see them?
10. Accelerating through the PHOTON layer:
* The black hole's gravity will accelerate you more and
more rapidly.
* You will fall to a point where not even matter that can
spin away from something at the speed of light can escape.
Even PHOTONS (particles with no mass and no charge that
make visible images; the particles in light waves) that
enter this layer cannot escape. They start orbiting the
black hole at the speed of light and possibly losing their
energy to the black hole's gravity, thus vibrating less and
less frequently (light waves that vibrate less frequently
than visible light are INFRARED waves), or may even
accelerate into the black hole completely, and be absorbed
by it.
(Remember, the speed of light is relative to the speed of
the matter that light comes from; all matter in the universe
is moving relative to each other; there is no way
to decide that some matter is moving and that some matter
is standing still; the speed of all matter must be measured
relative to what other matter is doing; likewise, the speed
of light must be measured this way--this is called
RELATIVITY!--remember Einstein!)
* As you pass through this photon layer and start accelerating towards
the hole, you will see your image streaking behind you.
QUESTION:
Why do you think your image appears streaked or mirrored
behind you?
QUESTION:
SUPPOSE gravity travelled in waves, much like light, but at a much
faster speed. (We really do not know exactly how gravity
travels.) IF you accelerated away from a point, toward another
point, at the speed at which the gravity waves travelled, what
would you experience?
11. Approaching the surface of the black hole:
* Looking behind you, you'll see the sky shrink into a disk
surrounded by blackness. Try looking at the images
listed below--in order--to see what happens!
http://antwrp.gsfc.nasa.gov/htmltest/gifcity/fig2o.gif
http://antwrp.gsfc.nasa.gov/htmltest/gifcity/fig2p.gif
(Just copy the first address into your web browser
window, and then use the back button to go back to
this page. Copy the second address and do the same!
To really simulate the trip, save the addresses somewhere,
and go to them in order without going back to this page!)
(Both of these images are from Robert Nemeroff of NASA!)
QUESTION:
How much do you think that the sky will shrink
behind you?
12. Impacting the surface of a black hole:
* If the black hole is not a singularity--that is, if its
radius is a little greater than 0--you'll hit a surface just below
the photon layer, and all the matter in you will collapse
under gravitational and tidal pulls. The physical matter
in you will collapse to a density greater than that of
an atom's nucleus, as even nuclear forces will not be
able to keep the particles separate.
(To see what this might look like, try going to
the "Movies from the Edge of Spacetime" at the
University of Illinois'"Spacetime Wrinkles" site--
or try going to the following "thumbnail clips" from
the "Spacetime Wrinkles" site:
http://www.ncsa.uiuc.edu/Cyberia/NumRel/Images/DistortedBH3_tn.jpg
http://www.ncsa.uiuc.edu/Cyberia/NumRel/Images/DistortedBH2_tn.jpg
http://www.ncsa.uiuc.edu/Cyberia/NumRel/Images/DistortedBH1_tn.jpg
Just copy one of the above addresses into your
browser window--then use the "Back" button to finish your
trip! [Some of these "Thumbnail Clips" are also available
at the "Black Hole Trip Links" page!])
QUESTION:
Do you think that Einstein's equations still work when
nuclear forces no longer can hold up an atom's nucleus?
13. Reaching the singularity:
* If, however, the black hole is a singularity, with a radius
of 0, all of its matter is concentrated into a point in space
approaching 0 volume, and hence, infinite density (density
remember, equals mass divided by volume; since the volume
approaches 0, you have to divide the huge mass by a number
approaching 0, and so a singularity is infinitely dense).
* If you fall to a singularity, you fall to just a single
point in space. At any moment, you take up only a point
in space, and only move along in space as the black hole
moves. So space becomes something like time--remember, we can only
move through time as time moves us. Thus we move in time like
a point moving along a line (a 'ray') or vector, and our
velocity seems to be controlled by the vector! This is how the
matter in a singularity moves through space: it is all concentrated
into that tiny point, and its movement is controlled by the movement
of the black hole's singularity!
* What happens to time? No one is sure. Gravity pulls you
to any point--within the confines of the
singularity--instantly, and all matter in a black hole's
singularity can affect all other matter in the
singularity instantly.
There is no travel time from one point in the singularity to
another.
QUESTION:
Do you think that time travel is possible in a singularity?
QUESTION:
Do you think all 'black holes' are pretty much the same?
And what are they? Furnaces that break matter down and compact
and change it? Or are they simply tunnels that suck things into
places we cannot see? Or what?
(Black holes, remember, were only recently 'discovered.'
They are a type of 'variable star,' and originally all variable
stars were grouped together by astronomers who were studying
the variable stars--stars whose light intensity
varied. Some of these stars turned out to be double stars
rotating around each other--when one star in a double was hidden
behind another, the double star appeared dimmer--Alpha
Centauri is an example of this; some turned out to be pulsars,
very dense stars that emitted light pulses periodically; and some
turned out to be stars magnified by something that appeared to
have gravity even more concentrated
than in pulsars! These were named Black Holes. But what are
Black Holes?)
* * * * * * * * * * * * *END OF THE TRIP!* * * * * * * * * * * * * *
(The information in this section comes from Robert J. Nemiroff
of NASA and the Goddard Space Flight Center (1993), Visual
distortions near a neutron star and black hole, The American
Journal of Physics, 61: 619 (published on the web
November 13, 2000);
and from the various websites--
including NASA's "Ask the Space Scientist; NASA's "Hubble Space
Telescope;" NASA's "Amazing Space;" and the University of
Illinois' "Spacetime Wrinkles."
This information has been compiled and interpreted by C. E.
Whitehead. Any errors in it are hers. It has been sent out to
several astronomers and space scientists so that its accuracy can
be reviewed; while awaiting feedback, Ms. Whitehead has checked
the information carefully herself, too.)
NOTE TO TEACHERS AND STUDENTS: Select pictures from the list above,
save in order in "favorites," and, to finish illustrating your black
hole journey, add your favorite black hole pictures to these, putting
them in the order you think they might go in on the trip
in "favorites" also. Now, click on these in order to see your trip!
