—Adapted from Matzner, Rothman, and Unruh
With this background, let us now imagine a free-float journey into a billion-solar-mass black hole (M = 109 MSun = 1.5 x 109 kilometers = 1.6 x lO-4 light-years = about ten times the radius of Earth's orbit around Sun). The horizon radius of this hole—double the above figure—is about the size of our solar system. We begin our journey at one-tenth the velocity of light at a distance of r = 20 000 M from the center of the black hole. We record each stage in the journey by giving both our wristwatch time remaining before we finally reach the singularity and our radius r.
The beginning of the journey, 30 years before the end. At this point, the black 30 years to the end hole is rather unimpressive. There is a small region (about 1 degree across—i.e., twice the size of Moon as seen from Earth) in which the star patterns look slightly distorted and within it a disk of total blackout. Careful examination of the stars shows that a few of those nearest the rim of the blacked-out region have second images on the opposite side of the rim. Had these images not been pointed out to us, we probably would have missed the black hole entirely.
Ten days before the end, atr = 32 M. The image has grown immensely. There 10 days is now a pure dark patch ahead with a radius of about 10 degrees (approximately the size of a dinner plate held up at arm's length). The original star images that lay near the direction of the black hole have been pushed away from their original positions by about 15 degrees. Further, between the dark patch itself and these images lies a band of second images of each of these stars. Looking near the darkness with the aid of a telescope, we can even see faint second images of stars that actually lie behind us! This light has looped once around the black hole on its way to our eye.
Four hours before the end. We are now at the horizon, and thus our shell 4 hours speed is near that of light. Aberration effects are now extremely important. Anything we see from this moment on will be a secret taken to our grave, because we can no longer send any information out to our surviving colleagues. Although we are now "inside" the black hole, not all of the sky in front of us appears entirely dark. Because of our high speed, aberration causes light rays to arrive at our eyes at extreme angles. In fact, only the patch immediately in front of us is fully black, subtending an angle of 90 degrees—a substantial fraction of the forward sky.
Behind us we see the stars grow dim and spread out, moving around forward to meet the advancing edge of the black hole. This apparent star motion is again an aberration effect (Figure 15). But there is a more noticeable feature of the sky: We can now see second images of all the stars in the sky surrounding the black hole. These images are squeezed into a band about 5 degrees wide around the image of the black hole. These second images are now brighter than were the original stars because of the blue-shifting of light falling into the hole. Surrounding the ring of second images are the still brighter first images. The rings encircle our direction of motion as the Arctic Circle surrounds the south-to-north axis of the Earth. The band of light caused by both the primary and secondary images now shines with a brightness ten times that of Earth's normal night sky.
4 minutes Four minutes before oblivion; r = M/7 yet to cover. The black hole now sub tends an angle of 150 degrees—almost the entire forward sky. Behind us stars are dimming and rushing forward. Only 20 percent of the stars are left in the sky behind us. In a 10-degree-wide band surrounding the outer edges of the black hole, not only second but also third and some fourth images are now visible. This band running around the sky now glows 1000 times brighter than the night sky viewed from Earth.
Final seconds The final seconds. The sky everywhere except in that rapidly thinning band is dark. The luminous band—glowing ever brighter—runs completely around the sky perpendicular to our direction of motion. At 3 seconds before oblivion it shines brighter than Earth's Moon. New stars rapidly appear along the inner edge of the shrinking band as higher- and higher-order images become visible from light wrapped many times around the black hole. The stars of the universe seem to brighten and multiply as they are compressed into a thinner and thinner ring transverse to our direction of motion.
Ring bisecting the sky Only in the last tenth of a second do the tidal forces become strong enough to end our journey and our view of that awesome ring bisecting the sky.
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