The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [176]
I’m glad. Had Newton’s equations enjoyed unlimited reach, accurately describing phenomena in any context however big or small, heavy or light, fast or slow, the subsequent scientific odyssey would have taken on a distinctly different character. Newton’s equations teach us much about the world, but their unlimited validity would have meant that the cosmic flavor was vanilla through and through. Once you understood physics on everyday scales, you’d be done. The same story would have held all the way up and all the way down.
In continuing Newton’s explorations, scientists have ventured into realms far beyond the reach of his equations. What we’ve learned has required sweeping changes in our understanding of the nature of reality. Such changes are not made lightly. They are closely examined by the community of scientists, and they are often sharply resisted; only when the evidence reaches a critical abundance is the new view embraced. Which is just as it should be. There’s no need to rush to judgment. Reality will wait.
The central fact, most forcefully emphasized by the last hundred years of theoretical and experimental progress, is that common experience fails to be a trustworthy guide for excursions that wander beyond everyday circumstances. But for all the radically new physics encountered in extreme conditions—described by general relativity, quantum mechanics, and, should it prove correct, string theory—the fact that radically new ideas would be required is not surprising. The basic assumption of science is that regularities and patterns exist on all scales, but as Newton himself anticipated, there’s no reason to expect the patterns we directly encounter to be recapitulated on all scales.
The surprise would have been to find no surprises.
The same is undoubtedly true regarding what physics will reveal in the future. A given generation of scientists can never know whether the long view of history will judge their work as a diversion, as passing fascination, as a stepping-stone, or as having revealed insights that will stand the test of time. Such local uncertainty is balanced by one of physics’ most gratifying features—global stability—that is, new theories generally do not erase those they supplant. As we’ve discussed, while new theories may require acclimation to new perspectives on the nature of reality, they almost never render past discoveries irrelevant. Instead, they incorporate and extend them. Because of this, the story of physics has maintained an impressive coherence.
In this book we’ve explored a candidate for the next major development in this story: the possibility that our universe is part of a multiverse. The journey has taken us through nine variations on the multiverse theme, which are summarized in Table 11.1. Although the various proposals differ widely in detail, they all suggest that our commonsense picture of reality is only part of a grander whole. And they all bear the indelible mark of human ingenuity and creativity. But determining whether any of these ideas goes beyond mathematical musings of the human mind will require more insight, knowledge, calculation, experiment, and observation than we’ve so far achieved. A final reckoning on whether parallel universes will be written into the next chapter of physics’ story must therefore also await the perspective that only the future can bring.
Table 11.1 Summary of Various Versions of Parallel Universes
PARALLEL UNIVERSE PROPOSAL: Quilted Multiverse
DESCRIPTION: Conditions in an infinite universe necessarily repeat across space, yielding parallel worlds.
PARALLEL UNIVERSE PROPOSAL: Inflationary Multiverse
EXPLANATION: Eternal cosmological inflation yields an enormous network of bubble universes, of which our universe would be one.
PARALLEL UNIVERSE PROPOSAL: Brane Multiverse
EXPLANATION: In string/M-theory’s braneworld scenario, our universe exists on one three-dimensional brane, which floats in a higher-dimensional