The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [121]
A Tale of Two Tales
In describing how quantum mechanics may generate many realities, I used the word “split.” Everett used it. So did DeWitt. Nevertheless, in this context it’s a loaded verb with the potential to grossly mislead, and I’d intended not to invoke it. But I gave in to temptation. In my defense, it’s sometimes more effective to use a sledgehammer to break down a barrier separating us from an unfamiliar proposal about the workings of reality, and to subsequently repair the damage, than it is to delicately carve a pristine window that directly reveals the new vista. I’ve been using that sledgehammer; in this and the next section I’ll undertake the necessary repairs. Some of the ideas are a touch more difficult than those we’ve so far encountered, and the explanatory chains are a bit longer as well, but I encourage you to stay with me. I’ve found that all too often, people who learn about, or are even somewhat familiar with, the Many Worlds idea have the impression that it emerged from speculation of the most extravagant sort. But nothing could be further from the truth. As I will explain, the Many Worlds approach is, in some ways, the most conservative framework for defining quantum physics, and it’s important to understand why.
The essential point is that physicists must always tell two kinds of stories. One is the mathematical story of how the universe evolves according to a given theory. The other, also essential, is the physical story, which translates the abstract mathematics into experiential language. This second story describes how the mathematical evolution will appear to observers like you and me, and more generally, what the theory’s mathematical symbols tell us about the nature of reality.6 In the time of Newton, the two stories were essentially identical, as I suggested with my remarks in Chapter 7 about Newtonian “architecture” being immediate and palpable. Every mathematical symbol in Newton’s equations has a direct and transparent physical correlate. The symbol x? Oh, that’s the ball’s position. The symbol v? The ball’s velocity. By the time we get to quantum mechanics, however, translation between the mathematical symbols and what we can see in the world around us becomes far more subtle. In turn, the language used and the concepts deemed relevant to each of the two stories become so different that you need both to acquire a full understanding. But it’s important to keep straight which story is which: to understand fully which ideas and descriptions are invoked as part of the theory’s fundamental mathematical structure and which are used to build a bridge to human experience.
Let’s tell the two stories for the Many Worlds approach to quantum mechanics. Here’s the first.
The mathematics of Many Worlds, unlike that of Copenhagen, is pure, simple, and constant. Schrödinger’s equation determines how probability waves evolve over time, and it is never set aside; it is always in effect. Schrödinger’s math guides the shape of probability waves, causing them to shift, morph, and undulate over time. Whether it’s addressing the probability wave for a particle, or for a collection of particles, or for the various assemblages of particles that constitute you and your measuring equipment, Schrödinger’s equation takes the particles’ initial probability wave shape as input and then, like the graphics program driving an elaborate screen saver, provides the wave’s shape at any future time as output. And that, according to this approach, is how the universe evolves. Period. End of story. Or, more precisely, end of first story.
Notice that in telling the first story I did not need the word “split” nor the terms “many worlds,” “parallel universes,” or “Quantum Multiverse.” The Many Worlds approach does not hypothesize these features. They play no role in the theory’s fundamental mathematical structure. Rather, as we will now see, these ideas are called upon in the theory’s second