The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [132]
Predictions and Understanding
For all these controversies, quantum mechanics itself remains as successful as any theory in the history of ideas. The reason, as we’ve seen, is that for the kinds of experiments we can do in the laboratory, and for many of the observations we can make of astrophysical processes, we have a “quantum algorithm” that produces testable predictions. Use Schrödinger’s equation to calculate the evolution of the relevant probability waves and use the results—the various wave heights—to predict the probability that you’ll find one outcome or another. As far as predictions are concerned, why this algorithm works—whether the wave collapses upon measurement, whether all possibilities are realized in their own universes, whether some other process is at work—is secondary.
Some physicists argue that even calling the issue secondary accords it more status than it deserves. In their view, physics is only about making predictions, and as long as different approaches don’t affect those predictions, why should we care which is ultimately correct? I offer three thoughts.
First, beyond making predictions, physical theories need to be mathematically coherent. The Copenhagen approach is a valiant effort, but it fails to meet this standard: at the critical moment of observation, it retreats into mathematical silence. That’s a substantial gap. The Many Worlds approach attempts to fill it.11
Second, in some situations, the predictions of the Many Worlds approach would differ from those of the Copenhagen approach. In Copenhagen, the process of collapse would revise Figure 8.16a to have a single spike. So if you could cause the two waves depicted in the figure—representing macroscopically distinct situations—to interfere, generating a pattern similar to that in Figure 8.2c, it would establish that Copenhagen’s hypothesized wave collapse didn’t happen. Because of decoherence, as discussed earlier, it is an extraordinarily formidable task to do this, but, at least theoretically speaking, the Copenhagen and Many Worlds approaches yield different predictions.12 It is an important point of principle. The Copanhagen and Many Worlds approaches are often referred to as different “interpretations” of quantum mechanics. This is an abuse of language. If two approaches can yield different predictions, you can’t call them mere interpretations. Well, you can. And people do. But the terminology is off the mark.
Third, physics is not just about making predictions. If one day we were to find a black box that always and accurately predicted the outcome of our particle physics experiments and our astronomical observations, the existence of the box would not bring inquiry in these fields to a close. There’s a difference between making predictions and understanding them. The beauty of physics, its raison d’être, is that it offers insights into why things in the universe behave the way they do. The ability to predict behavior is a big part of physics’ power, but the heart of physics would be lost if it didn’t give us a deep understanding of the hidden reality underlying what we observe. And should the Many Worlds approach be right, what a spectacular reality our unwavering commitment to understanding predictions will have uncovered.
I don’t expect theoretical or experimental consensus to come in my lifetime concerning which version of reality—a single universe, a multiverse, something else entirely—quantum mechanics embodies. But I have little doubt that future generations will look back upon our work in the twentieth and twenty-first centuries as having nobly laid the basis for whatever picture finally emerges.
*For simplicity, we won’t consider the electron’s position in the vertical direction—we focus solely on its position on a map of Manhattan. Also, let me re-emphasize that while this section will make clear that Schrödinger’s equation doesn’t