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Authors: Mitchell Begelman
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disk below and ahead of me. From
this height the magnetic flares seemed less significant, little more than minor viscous eruptions like the bubbles and cheese-spouts that erupt from the surface of a simmering fondue pot or, given the diskâs thinness, a bubbling pizza (my harrowing encounter with the black holeâs gravity had unaccountably given me an appetite). I could see the underlying large-scale structure of the disk, stretching away toward the black hole. The disk was luminous everywhere, and the more so the closer one got to the hole. With increased radiance came the expected increase in temperature, manifested in the progression of âcolors.â I have to put this word in quotation marks, because the progression far surpassed the sequence from red to yellow to blue-white that a human eye perceives when a metal bar is heated to thousands of degrees. Here the colors went off the human scale at violet proceeding through the ultraviolet, far ultraviolet, extreme ultraviolet, and on into the X-rays. Apart from these gradations of temperature and radiance, the disk was overwhelming in its flatness and featurelessness.
To get my bearings, I focused on the hottest and most luminous region of all, the area surrounding the black hole. I could see the blackness in the center and the disk continuing beyond. There the flatness appeared to be broken by a curious asymmetry. Just the other side of the hole, the disk seemed to warp upward, forming a distorted band that folded over on top of the gap that signified the location of the black hole. I moved around toward the far side to get a better look, but the distorted region of the disk seemed to compensate for my motions, always remaining exactly on the opposite side of the hole. It dawned on me that I was witnessing an illusion caused by the wrapping of light rays around the black hole as they crossed the intense gravitational field on their way to my instruments. I should have expected this. I knew that radiation, as well as matter, responds to gravity, though it was shocking to see the effect so forcefully displayed. There was another asymmetry as well, but the origin of this one was easier to visualize. I noticed that the disk was a little bit brighter, the X-ray luminescence just a little bit harsher, to
the right of the hole than to the left. This, I immediately recognized, must be due to the âheadlight effect,â the forward beaming of the radiation emitted by moving matter. In this case, the light would be beamed toward the direction of the diskâs spinning motion, and the brighter side had to be the side that was coming toward me. According to the theory of relativity, an asymmetry this pronounced could mean only one thing: The matter that I was viewing swirled around the black hole at nearly the speed of light. These were all well-known theoretical predictions, but by this point I was so awed by the spectacle that I was disinclined to trust the intuition I had developed through years of painstaking study. As if to pinch myself mentally, I double-checked that the disk was spinning in a clockwise direction, as it would have to be if light from the right-hand side were really boosted by its oncoming motion. It was.
I tried to follow the gas as it disappeared through the black holeâs horizon. The horizon was a sphere with a radius of 45 kilometers. Was I right that the matter in the disk would break its slow inward spiral and plummet into the black hole from 100 kilometers farther out? I saw what looked like evidence of a dramatic change in the diskâs state of motion near the predicted place, but I couldnât be sure; the hot atmosphere above the innermost parts of the disk became so thick that it all but obscured my view. At the other end of the death-plunge I could barely make out the gas fading just outside the horizon. The textbooks appeared to be right. The X-ray radiation regressed through the colors as it fought its way out through
this height the magnetic flares seemed less significant, little more than minor viscous eruptions like the bubbles and cheese-spouts that erupt from the surface of a simmering fondue pot or, given the diskâs thinness, a bubbling pizza (my harrowing encounter with the black holeâs gravity had unaccountably given me an appetite). I could see the underlying large-scale structure of the disk, stretching away toward the black hole. The disk was luminous everywhere, and the more so the closer one got to the hole. With increased radiance came the expected increase in temperature, manifested in the progression of âcolors.â I have to put this word in quotation marks, because the progression far surpassed the sequence from red to yellow to blue-white that a human eye perceives when a metal bar is heated to thousands of degrees. Here the colors went off the human scale at violet proceeding through the ultraviolet, far ultraviolet, extreme ultraviolet, and on into the X-rays. Apart from these gradations of temperature and radiance, the disk was overwhelming in its flatness and featurelessness.
To get my bearings, I focused on the hottest and most luminous region of all, the area surrounding the black hole. I could see the blackness in the center and the disk continuing beyond. There the flatness appeared to be broken by a curious asymmetry. Just the other side of the hole, the disk seemed to warp upward, forming a distorted band that folded over on top of the gap that signified the location of the black hole. I moved around toward the far side to get a better look, but the distorted region of the disk seemed to compensate for my motions, always remaining exactly on the opposite side of the hole. It dawned on me that I was witnessing an illusion caused by the wrapping of light rays around the black hole as they crossed the intense gravitational field on their way to my instruments. I should have expected this. I knew that radiation, as well as matter, responds to gravity, though it was shocking to see the effect so forcefully displayed. There was another asymmetry as well, but the origin of this one was easier to visualize. I noticed that the disk was a little bit brighter, the X-ray luminescence just a little bit harsher, to
the right of the hole than to the left. This, I immediately recognized, must be due to the âheadlight effect,â the forward beaming of the radiation emitted by moving matter. In this case, the light would be beamed toward the direction of the diskâs spinning motion, and the brighter side had to be the side that was coming toward me. According to the theory of relativity, an asymmetry this pronounced could mean only one thing: The matter that I was viewing swirled around the black hole at nearly the speed of light. These were all well-known theoretical predictions, but by this point I was so awed by the spectacle that I was disinclined to trust the intuition I had developed through years of painstaking study. As if to pinch myself mentally, I double-checked that the disk was spinning in a clockwise direction, as it would have to be if light from the right-hand side were really boosted by its oncoming motion. It was.
I tried to follow the gas as it disappeared through the black holeâs horizon. The horizon was a sphere with a radius of 45 kilometers. Was I right that the matter in the disk would break its slow inward spiral and plummet into the black hole from 100 kilometers farther out? I saw what looked like evidence of a dramatic change in the diskâs state of motion near the predicted place, but I couldnât be sure; the hot atmosphere above the innermost parts of the disk became so thick that it all but obscured my view. At the other end of the death-plunge I could barely make out the gas fading just outside the horizon. The textbooks appeared to be right. The X-ray radiation regressed through the colors as it fought its way out through
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