Consider a universe encased in a boundary like a snow globe. Apart from having a big wall around it, the interior is basically like our universe: It has gravity, matter, and so forth. The boundary, too, is a kind of universe. It does not have gravity and, being just a surface, lacks depth. But it makes up for that with vibrant quantum physics, and all in all it’s exactly as complex as the interior. Different though these two universes may look, they are perfectly matched. Everything in the interior, or “bulk,” has a counterpart on the boundary.