He continued to pace, thinking.
“Here’s more support for it,” Runyan said. “Look at the holes drilled in solid concrete with no sign of searing or scorching. That’s one of the singular pieces of evidence and very hard to understand any other way. It’s just what a small black hole would do. A black hole will pull in matter from a volume much bigger than itself as it moves, the gravitational force sucking the material in from the immediate vicinity.” He made a crushing motion with his fist. “A black hole will carve a tunnel as it goes, but leave no other sign of its passage, not like a laser beam or any other such device, as Dr. Danielson was quick to see.” He smiled at her for a moment. “In fact, from the size of the holes left behind, I can estimate the mass of the thing.”
Runyan paused and dug into a pocket of his cutoffs and brandished the napkin. The numbers blotched irregularly where the ink from Danielson’s pen had run in the porous material. He did this more from a sense of drama than from a need to refresh his memory. He recalled the result perfectly well. He made an abbreviated OK sign with index finger and thumb and peered through the small hole at his audience.
“The holes drilled are about this size,” he said, “a few millimeters to a centimeter. Depending on the tensile strength of the material through which the hole passes, I would guess the mass to be comparable to a small mountain and its size to be about that of an atomic nucleus.”
“But would a small black hole do what we are observing?” Gantt asked. “That is, if it knifes through the Earth as if it were butter, how does it generate the acoustic signal?”
Runyan pondered for a moment. “Well,” he began, “as I’ve said, it would exert a force sucking in matter from the immediate vicinity. It would carve a tunnel as it went. Does that suggest anything?”
“I suppose,” replied Gantt. “At least in subsurface rock the ambient pressure would prevent such a tunnel from existing except momentarily. I can imagine the collapse of such a thing generating acoustic waves, depending on the size.”
“That’s a good point,” Runyan aimed a blunt finger at him, “the size of the tunnel is related to the mass of the object and the rate at which the tunnel forms and collapses should give an estimate of the acoustic power— which we know! Can we check to see if the picture is self-consistent?”
Gantt joined Runyan at the board and they began a crude, but rapid calculation. They stood in front of their figures and symbols to the consternation of those in the room trying to follow the arguments. After a few minutes of gesticulation and occasional cursing, Gantt returned to his seat.
“With some uncertainty,” Runyan announced, “the acoustic signal is consistent with the idea of a small tunnel continuously being drilled at the orbital velocity and then collapsing.”
“I’m sorry,” Isaacs said, his voice polite, but firm. “This is very important because you’re talking about the basic data that led us to this thing.” If Runyan were off base, Isaacs wanted to nail him quickly. He also recognized that the notion of a black hole and its implications were too foreign to him to be absorbed rapidly. If it turned out to be more than a crackpot idea, he didn’t want to miss details that would aid his ultimate understanding. “Could you explain to me a bit more clearly what you just did.” Isaacs gestured at the board.
“Oh, sure.” Runyan was loathe to halt the flow of ideas, but recognized his responsibility to Isaacs. “The picture is that a small black hole will move without resistance through the rock of the Earth’s core. It’s like a little vacuum cleaner, sucking up particles that it gets too close to. The mass of the black hole dictates the strength of the gravitational pull it exerts. Close to the black hole that gravitational force is overwhelming, but at larger distances the tensile forces of the rock that make it solid are stronger than the gravity of the black hole. The quantitative question is to determine the distance from the black hole at which the internal forces in the rock are stronger than the gravitational pull of the hole. Further than that, the rock remains intact. Closer than that, the suction of gravity is dominant. If you were somehow to hold the hole still, it would eat out a cavity the size of which is proportional to the gravity of the black hole and hence to its mass. If the hole has a mass comparable to a small mountain, as I said, then it will carve a hole of about the diameter that you’ve reported in the foundations of those buildings.”
“Okay,” Isaacs replied, “I guess I see that. And you get a tunnel rather than single hole if this black hole moves along a path sucking up everything out to a certain distance.” He pinched an imaginary particle between thumb and forefinger and moved it methodically in a line at arm’s length.
“Exactly,” Runyan confirmed.
“Then where does the seismic signal come from?”
“Ah!” Runyan exclaimed. “Now picture this hole falling freely through the rock at a speed that is determined by the gravitational acceleration of the Earth. That speed determines the rate at which this little tunnel is carved.
“But what happens to the tunnel?” Runyan proceeded to answer his own question. “After the black hole moves on, the tunnel can’t just sit there. The huge pressure in the surrounding rock will crush it. So there’s a continuous process by which the hole carves the tunnel and then moves on leaving the pressure forces to collapse it. The seismic signal is very plausibly the continuous noise made by the collapsing tunnel.”
“That can’t be the whole picture,” Isaacs was thinking hard. “At the surface, in normal rock, you should just get a hole drilled, just as we’ve seen in these concrete foundations.”
“Good, good. That’s very perceptive.” Runyan was a little condescending, but he looked at Isaacs with new respect. “In the mantle the pressure forces are not as great and the wound of the tunnel should remain unhealed. I remind you that the strength of your seismic signal falls as the influence nears the surface. Pat said there was no detectable signal from the upper mantle. This could be exactly the reason!”
“What about the acoustic signal in the water?” Isaacs inquired.
“Probably a similar idea with cavitation.”
“Cavitation? You mean like with a motorboat propeller?”
“Right. The hole should consume a surrounding volume of water just as it does rock. After it moves on, the water will rush into the vacuum in its wake creating thousands of tiny popping bubbles. Cavitation, and acoustic noise.”
“It looks to me,” Fletcher pointed at the board, “as if you’ve assumed the hole moves subsonically. What if it moves faster than the material can respond. What if it moves super- sonically?”
“I don’t think that’s a problem except maybe in the liquid iron core of the Earth where the hole would be moving at its highest speed,” Runyan replied. “Whatever this is seems to move relatively slowly at the surface— fast, but slower than the speed of sound in rock, water, or even air. There could be shock waves near the Earth’s center, though. I’ll have to think some more about that.”
“Gentlemen,” cut in Phillips from the side of the room where he had been standing, “I’m impressed with the virtuosity of your arguments, but I’m still very disturbed at the nature of your conclusions. Doesn’t anyone have an alternative suggestion?”
The question was greeted with silence. Runyan stood mute. His eye rested on, but barely registered, a dollop of coffee on the desk, spilled from a cup Gantt had brought in after lunch. His fixation was broken by Ted Noldt who stirred and said, “I have a question that bears on the possibility of a black hole.”
Runyan lifted his eyes and looked at the speaker.
“I don’t know much about black holes,” Noldt said, “but I thought the small ones, about which you are talking, were supposed to radiate away their mass and energy at a great rate, causing them to evaporate and explode. Doesn’t that rule out such a black hole?”
“We’re going to have to consult a real expert on the subject, which I’m surely not,” replied Runyan. “That question has been very much on my mind.” He paused a moment and then continued. “Here’s a possibility. The theory of evaporating black holes was worked out in the context of idealized, empty space, whereas this one’s in the real world!” He caught himself. “Sorry. A grotesque pun. Unintentional. Anyway, maybe the fact that this one is surrounded by matter changes things.”
“That may be right,” mused Fletcher, picking up the argument. “If it’s consuming matter, the infall may squelch the outflow. Let’s see, didn’t you and Ellison estimate the rate of consumption just now?”
“Right,” said Runyan, turning to the board once more. “I don’t remember all the formulae for the evaporation rate, but maybe I can piece something together.” He doodled for a minute while the others looked on and listened to the scratching chalk. “Yes!” he looked up. “That’s probably it; there seems to be a comfortable margin. As long as the hole bores through the Earth, it will eat the matter and grow. You’d have to stop the consumption to get it to evaporate.”
“Wait a minute,” said Noldt, punching a finger in the air. “That’s not really relevant, is it? This thing must
