There are many theories about how the worlds interact. All of them may be true or none of them may be true. Debates about such things often last late into the night.
One day, while riding the exercise bike at the gym, another theory came to Billy Boustany—the carousel theory. Imagine that our world is a carousel spinning around. When we are on it, it seems to be perfectly still and stable. We can happily sit on a painted reindeer as it pumps up and down. Now, imagine that there is a second carousel, spinning in the opposite direction. It almost touches ours. Most of the time, we have no idea that the second carousel is there, but it is just as real as the one we are on. When our painted reindeer comes around to the place closest to the other carousel, we may, if we pay attention, catch a whisper of breeze from it.
Then, imagine that we climb off our reindeer and stand on the floor of our carousel, toes right at the edge. The next time we come around to the point close by the other carousel, the floors of the two carousels almost touch. They are moving at the same speed, so, for a moment, they are stationary relative to each other.
If we are standing on the edge at that moment, and we have the nerve, all we have to do is take one small step across to the other carousel. It’s a tiny distance, but it feels like a leap into the void. Our foot lands on the other carousel and we join that world as it spins away. Our world disappears.
Billy thought that the carousel theory was pretty slick. It perfectly captured the feeling he had at the moments when he crossed over into HD. He imagined he had somehow developed the knack for knowing when he was at the right place on the spinning carousel where the other, invisible, carousel was right there waiting for him. But this theory didn’t account for the way that HD St. Louis seemed to overlay the regular city. Would there have to be millions of carousels all over the place?
So, for a while, Billy was torn between the membrane theory and the carousel theory. One or the other of them must be wrong. Then he remembered back to his college physics class. The professor had talked about Einstein’s general theory of relativity, which perfectly explained gravity, motion, and other large-scale phenomena.
The professor also described quantum mechanics, developed by Max Planck, Niels Bohr and other buddies of Einstein, which explained light—either a particle or a wave, depending on who’s measuring—and the behavior of subatomic particles.
The problem was that the two theories, which both worked well at their own scale, created nonsensical predictions when applied to the other scale. This discrepancy bedeviled Einstein, who didn’t like the randomness and uncertainty that were central principles of quantum mechanics. He spent much of his life trying to reconcile the theories, without success.
So, Billy decided that he didn’t have to choose between his two theories. He could accept both of them as true. If Einstein couldn’t figure these things out, then Billy Boustany wasn’t going to worry about them.