“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 have come from space somehow, so it must be massive enough not to have evaporated before it got caught in the Earth. Isn’t that right?”
Runyan beetled his brows at Noldt and paced along the narrow corridor in front of the blackboard a couple of times. Then he turned to face him again.
“No,” he said, “I’m not sure that is right. It’s true that the cosmologists have told us about the possibility of such mini-black holes created in the turbulence of the Big Bang. But there are two problems. In the first place, though my estimates are crude, I don’t believe this object is massive enough to have survived since the beginning of time. Secondly, there is a great difficulty with the curious fact that it moves with the Earth.”
“What’s that?” Noldt was puzzled.
“If this were a black hole born in space,” Runyan explained, “there is little chance that it could get trapped in the puny gravity of the Earth. For that to happen, it would have to be moving very slowly with respect to the Earth. But what with the Earth’s motion around the Sun and the Sun’s motion around the Galaxy and the Galaxy’s motion off to god knows where, the relative speed between the Earth and any random astronomical body would be much greater than the escape velocity from the Earth. The Earth could not possibly attract and hold anything moving past it so rapidly.
“Do you remember the Tungus event?” He asked Noldt.
Noldt had to think for a second. “Tungus? Russia. Siberia! Big explosion?”
“Right,” Runyan replied. “Still rather mysterious. Some explosion in Siberia in 1919. Burned and flattened trees for miles around. But no crater. That ruled out a large meteorite. Any piece of space rock big enough to do the damage done would have to have left a crater rivaling the old one in Arizona. The best idea seems to be a comet. Comets are thought to be very loose filamentary icy structures. Such a thing could deliver a hell of an impact but be sufficiently diffuse not to gouge a crater.”