The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [125]
When Is an Alternative a Universe?
Besides the loaded words “splitting” and “cloning,” we’ve freely invoked two other grand terms in our type-two stories—“world” and, interchangeably in this context, “universe.” Are there guidelines for determining when this usage is appropriate? When we consider a probability wave for a single electron that has two (or more) spikes, we don’t speak of two (or more) worlds. Instead, we speak of one world—ours—containing an electron whose position is ambiguous. Yet, in Everett’s approach, when we measure or observe that electron, we speak in terms of multiple worlds. What is it that distinguishes the unmeasured and the measured particle, yielding descriptions that sound so radically different?
One quick answer is that for a single isolated electron, we don’t tell a type-two story because without a measurement or an observation there’s no link to human experience that’s in need of articulation. The type-one story of a probability wave evolving via Schrödinger’s math is all that’s needed. And without a type-two story, there’s no opportunity to invoke multiple realities. Although this explanation is adequate, it proves worthwhile to delve a little deeper, revealing a special feature of quantum waves that comes into play when many particles are involved.
To grasp the essential idea, it’s easiest to look back at the double-slit experiment of Figures 8.2 and 8.4. Recall that an electron’s probability wave encounters the barrier, and two wave fragments make it through the slits and travel onward to the detector screen. Inspired by our Many Worlds discussion, you might be tempted to think of the two racing waves as representing separate realities. In one, an electron whisks through the left slit; in the other, an electron whisks through the right slit. But you promptly realize that the intermingling of these supposedly “distinct realities” profoundly affects the experiment’s outcome; the intermingling is why an interference pattern is produced. So it doesn’t make much sense, nor does it yield any particular insight, to consider the two wave trajectories as existing in separate universes.
If we change the experiment, however, by placing a meter behind each slit that records whether or not an electron passes through it, the situation is radically different. Because macroscopic equipment is now involved, the two distinct trajectories of an electron generate differences in a huge number of particles—the huge number of particles in the meters’ displays that register “electron passed through left slit” or “electron passed through right slit.” And because of this, the respective probability waves for each possibility become so disparate that it’s virtually impossible for them to have any subsequent influence on each other. Much as in Figure 8.16a, the differences between the billions and billions of particles in the meters cause the waves for the two outcomes to shift away from each other, leaving negligible overlap. With no overlap, the waves don’t engage in any of the hallmark interference phenomena of quantum physics. Indeed, with the meters in place, the electrons no longer yield the striped pattern of Figure 8.2c; instead, they generate a simple, non-interfering amalgam of the results in Figure 8.2a and Figure 8.2b. Physicists say that the probability waves have decohered (something you can read about in more detail, for example, in Chapter 7 of The Fabric of the Cosmos).
The point, then, is that once decoherence sets in, the waves for each outcome evolve independently—there’s no intermingling between the distinct possible outcomes—and each can thus be called a world or a universe of its own. For the case at hand, in one such universe the electron goes through the left slit, and the meter displays left; in another universe the electron goes through the right slit, and the meter records right.
In this sense, and only in this sense, there’s resonance with Bohr. According to the Many Worlds approach, big things made of many particles do differ from small things made from one particle