"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].)

Trip to a Black Hole

Introduction. 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!"

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!)

  1. 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!
  2. 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 . . . )
  3. 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?

  1. 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!)!
  2. 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?

  1. 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.

QUESTIONS:

Why do you think that you might spiral towards the hole?

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?)

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.)

  1. 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 click on the above URL. [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?

  1. 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?

  1. 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.

QUESTIONS:

Why do you think your image appears streaked or mirrored behind you?

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?

  1. Approaching the surface of the black hole:

QUESTION: How much do you think that the sky will shrink behind you?

  1. Impacting the surface of a black hole:

QUESTION: Do you think that Einstein's equations still work when nuclear forces no longer can hold up an atom's nucleus?

  1. 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? (For more discussion of this see the information on black holes and travellilng to the singularity at the University of Toronto's page, http://www.upscale.utoronto.ca/PVB/Harrison/BlackHoles/BlackHoles.html.)

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 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!

(This page last updated May, 2010. Background on this page from Fun Utilities.com's lightning wallpaper.)