Glory Road by Robert A. Heinlein

“No, I would not! Nosy.”

“It is a long nose, isn’t it? As you wish, milord. In a less personal vein the fact of crossbreeding among humans of different universes–and some animals such as dogs and cats–is a most interesting question. The only certainty is that human beings flourish only in those universes having chemistries so similar that elements that make up deoxyribonucleic acids are so alike as not to matter. As for the rest, every scholar has his theory. Some hold to a teleologic explanation, asserting that Man evolves alike in all essential particulars in every universe that can support him because of Divine Plan–or through blind necessity, depending on whether the scholar takes his religion straight or chases it with soda.

“Some think that we evolved just once–or were created, as may be–and leaked across into other universes. Then they fight over which universe was the home of the race.”

“How can there be any argument?” I objected. “Earth has fossil evidence covering the evolution of man. Other planets either have it or not, and that should settle it.”

“Are you sure, milord? I thought that, on Earth, man’s family tree has as many dotted lines as there are bastards in European royal lines.”

I shut up. I had simply read some popular books. Perhaps she was right; a race that could not agree as to who did what to whom in a war only twenty years back probably didn’t know what Alley Oop did to the upstairs maid a million years ago, when the evidence was only scattered bones. Hadn’t there been hoaxes? The Piltdown Man, or some such?

Star went on, “Whatever the truth, there are leakages between worlds. On your own planet disappearances run to hundreds of thousands and not all are absconders or wife-deserters; see any police department’s files. One usual place is the battlefield. The strain becomes too great and a man slides through a hole he didn’t know was there and winds up ‘missing in action.’ Sometimes–not often–a man is seen to disappear. One of your American writers, Bierce or Pierce, got interested and collected such cases. He collected so many that he was collected, too. And your Earth experiences reverse leakage, the ‘Kaspar Hausers,’ persons from nowhere, speaking no known language and never able to account for themselves.”

“Wait a minute? Why just people?”

“I didn’t say ‘just people.’ Have you never heard of rains of frogs? Of stones? Of blood? Who questions a stray cat’s origin? Are all flying saucers optical illusions? I promise you they are not; some are poor lost astronauts trying to find their way home. My people use space travel very little, as faster-than-light is the readiest way to lose yourself among the Universes. We prefer the safer method of metaphysical geometries–or ‘magic’ in the vulgar speech.”

Star looked thoughtful. “Milord, your Earth may be the home of mankind. Some scholars think so.”

“Why?”

“It touches so many other worlds. It’s the top of the list as a transfer point. If its people render it unfit for life–unlikely, but possible–it will disrupt traffic of a dozen universes. Earth has had its fairy rings, and Gates, and Bifrost Bridges for ages; that one we used in Nice was there before the Romans came.”

“Star, how can you talk about points on Earth ‘touching’ other planets–for centuries on end? The Earth moves around the Sun at twenty miles a second or such, and spins on its axis, not to mention other motions that add up to an involved curve at unthinkable speed. So how can it ‘touch’ other worlds?”

Again we rode in silence. At last Star said, “My Hero, how long did it take you to learn calculus?”

“Why, I haven’t learned it. I’ve studied it a couple of years.”

“Can you tell me how a particle can be a wave?”

“What? Star, that’s quantum mechanics, not calculus. I could give an explanation but it wouldn’t mean anything; I don’t have the math. An engineer doesn’t need it.”

“It would be simplest,” she said diffidently, “to answer your question by saying ‘magic’ just as you answered mine with ‘quantum mechanics.’ But you don’t like that word, so all I can say is that after you study higher geometries, metaphysical and conjectural as well as topological and judicial–if you care to make such study–I will gladly answer. But you won’t need to ask.”