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get Essie to rewrite Albert's program a little less idiosyncratically.
"Sure thing, Robin," he said, cheerfully ignoring my temper. He wrinkled his
furry eyebrows. "Ah-ha," he said. "Uh-huh. Well, let's see."
"Is that a hard question for you?" I asked, more surprised than sarcastic.
"Of course not, Robin. I was just thinking how far back I should start. Well,
let's start with light. You know that light is made up of particles called
photons. It has mass, and it exerts pressure-"
"Not that far back, Albert, please."
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"All right. But the way a black hole begins starts with a failure of light
pressure. Take a big star-a blue Class-O, say. Ten times as massive as the
sun. Burns up its nuclear fuel so fast that it only lives about a billion
years. What keeps it from collapsing is the radiation pressure-
call it the `light pressure'-from the nuclear reaction of hydrogen fusing into
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helium inside it.
But then it runs out of hydrogen. Pressure stops. It collapses. It does so
very, very fast, Robin, maybe in only a matter of hours. And a star that used
to be millions of kilometers in diameter is all of a sudden only thirty
kilometers. Have you got that part, Robin?"
"I think so. Get on with it."
"Well," he said, lighting his pipe and taking a couple of puffs-I can't help
wondering if he enjoys it!-"that's one of the ways black holes get started.
The classical way, you might call it. Keep that in mind, and now go on to the
next part: escape velocity."
"I know what escape velocity is."
"Sure thing, Robin," he nodded, "an old Gateway prospector like you. Well.
When you were on Gateway, suppose you threw a rock straight up from the
surface. It would probably come back, because even an asteroid has some
gravity. But if you could throw it fast enough-maybe forty or fifty kilometers
an hour-it wouldn't come back. It would reach escape velocity and just fly
away forever. On the Moon, you'd have to throw it a lot faster still, say two
or three kilometers a second. On the Earth, faster than that-better than
eleven kilometers a second.
"Now," he said, reaching forward to tap coals out of his pipe and light it
again, "if you-
" tap, tap, "if you were on the surface of some object that has a very, very
high surface gravity, the condition would be worse. Suppose the gravity were
such that the escape velocity were up real high, say around three hundred and
ten thousand kilometers a second. You couldn't throw a rock that fast. Even
light doesn't quite go that fast! So even light-" puff, puff, "can't escape,
because its velocity is ten thousand kilometers a second too slow. And, as we
know, if light can't escape, then nothing can escape; that's Einstein. If I
may be excused the vanity." He actually winked at me over his pipe. "So that's
a black hole. It's black because it can't radiate at all."
I said, "What about a Heechee spaceship? They go faster than light."
Albert grinned ruefully. "Got me there, Robin, but we don't know how they go
faster than light. Maybe a Heechee can get out of a black hole, who knows? But
we don't have any evidence of one of them ever doing it."
I thought that over for a moment. "Yet," I said.
"Well, yes, Robin," he agreed. "The problem, of going faster than light, and
the problem of escaping from a black hole, are essentially the same problem."
He paused. A long pause. Then, apologetically, "I guess that's about all we
can profitably say on that subject, right now."
I got up and refreshed my drink, leaving him sitting there, patiently puffing
his pipe.
Sometimes it was hard to remember that there was really nothing there, nothing
but a few interference patterns of collimated light, backed up by some tons of
metal and plastic. "Albert,"
I said, "tell me something. You computers are supposed to be lightning-fast.
Why is it that you take so long to answer sometimes? Just dramatic effect?"
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