The top scientists of your era agreed on one explanation. It was first suggested by the work of a grad student named Hugh Everett, in the late nineteen-fifties. As it was developed it got a fancy name—the Many-Worlds Interpretation of Quantum Mechanics. Our universe, the physicists theorized, was just one of an infinite number of universes continually branching out every second like new limbs from a tree, to form a multiverse. So if someone went back in time and killed his grandfather, no paradox would occur because that person would be creating a whole new universe.

Is the gondola whirling more quickly, or are Gisco’s explanations making my head spin? I try to concentrate. So if that’s right, I ask, my parents couldn’t change their polluted, unhappy world by sending me back in time? If I did change something to try to fix their problem, I would just be creating a new branch of the multiverse tree. But their own world would remain as bad and polluted as it was.

Correct, Gisco agrees.

I mull it over. That seems pretty nutty, I finally tell the dog. To think that there are an infinite number of universes, with new ones being generated all the time.

If you polled the leading cosmologists of your era, the vast majority of them were convinced it was so, Gisco says. It was a fascinating theory. Complex and completely unprovable. Of course, it turned out to be as misguided as the notion five hundred years earlier that the world was flat and explorers who sailed too far would fall off the edge.

So if it was wrong, what was right?

A Russian mathematician thirty years after Everett pointed in a direction that turned out to be much closer to the truth. His name was Igor Novikov, and he said that Einstein and all the other theorists who worried about time-travel paradoxes had gotten it completely wrong. According to Novikov, you couldn’t travel back in time and kill your grandfather before he met your grandmother precisely because that would create a paradox.

I don’t get it, dog, I confess. Suppose I traveled back in time and met my grandfather and wanted to kill him. What would stop me?

Suppose you wanted to walk on the ceiling? What would stop you?

Gravity.

And according to Novikov, an invisible cosmic force would prevent you from killing your grandfather.

What force?

Novikov explained it this way. In the absence of time travel, the unfolding present is influenced only by the past. So if you were trying to figure out what to do at three in the afternoon, and you had made a dentist’s appointment for then, when three o‘clock came you would go to the dentist. Got it?

So far.