There is not just a single universe, there are a huge number of them, and new ones are constantly splitting off. In his impressive book, David Wallace argues persuasively that it is. The first is the heliocentric revolution Copernicus and Galileo ; the second is dinosaurs. The book opens with a thought-provoking quote from Wittgenstein: what would it have looked like if it had looked like the Earth went round the Sun? Stop and consider that for a moment.
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There is not just a single universe, there are a huge number of them, and new ones are constantly splitting off. In his impressive book, David Wallace argues persuasively that it is. The first is the heliocentric revolution Copernicus and Galileo ; the second is dinosaurs.
The book opens with a thought-provoking quote from Wittgenstein: what would it have looked like if it had looked like the Earth went round the Sun?
Stop and consider that for a moment. The answer, of course, is that it would have looked exactly the same.
Every piece of factual evidence people had, which convinced them that the Sun went round the Earth, could equally well have been interpreted in the opposite direction. At the end of the day, the main reason why people were so slow to agree with Copernicus was a simple one. Similarly with the Many-Worlds Interpretation. The math represents this as the sum of two algebraic terms: one stands for the live cat, plus everything else in the world; the other stands for the dead cat, plus everything else in the world.
The two terms rapidly "decohere", in other words cease to influence each other. The basic claim of the Many-Worlds Interpretation is that this is best conceptualized as saying that the universe splits into two copies. Yet, somehow, most people seem reluctant to take this final step. What do they do instead? Indeed, throw out the question as irrelevant and positively distracting. So over to the dinosaurs. As Wallace says, suppose people applied the same kind of reasoning to paleontology. There are fossils; everyone agrees on that.
Fossils are bits of rock which you can touch. There are consistent patterns in many of these bits of rock, and the only sensible way of explaining these patterns is to say that their appearance is as it would have been if there once had been dinosaurs.
Just about everyone agrees on that too. You would have a hard time refuting this argument. Our hypothetical geologist would agree with everything you said about the links between fossils and dinosaurs, and in fact she would probably know rather more about it than you did, since it was part of her job. Stylistically, Copernicus and Galileo were polar opposites: Copernicus was a dry, technical writer, and Galileo was an entertaining polemicist. Copernicus was extremely conservative, and worked entirely within the Ptolemaic system.
As Rovelli remarks in his recent book on Anaximander, no one could have loved Ptolemy more than Copernicus did. Galileo, in contrast, wanted to shake things up and introduce genuinely new ideas.
Wallace has daringly attempted to mix these two very different styles. Rather more than two-thirds of the book is Copernican, and consists of lengthy technical proofs; the most important ones have to do with the concept of rational behavior in the quantum multiverse, where it is easy to become confused and think that, since everything is going to happen in some branch, it makes no difference what you do. Wallace shows that this is absolutely not true.
In fact, the concept of "branch weight" plays a role exactly analogous to that of probability in a classical theory, and rational agents end up doing what they would have done in a classical universe.
Establishing this apparently trivial conclusion unfortunately requires over fifty pages of difficult mathematics. If all the book were like this, it would have been unreadable; despite its honored place in the history of science, it is notorious that hardly anyone has ever read De revolutionibus orbium coelestium. Wallace has addressed this problem by adding a parallel thread written in an engagingly Galilean style, where he explains the intuitive consequences of the ideas in everyday language.
The layman will no doubt want more Galileo; on the other hand, the Copernicus is necessary to convince the many sceptical experts, none of whom appear yet to have detected obvious holes. Should you buy The Emergent Multiverse? You will just have to decide for yourself. The author makes a strong case for the reality of the quantum multiverse; if his reasoning becomes generally accepted, it is impossible to imagine how fundamentally it will change the way we view the world.
At the moment, the evidence is of an indirect nature, as it was when the pioneers of the heliocentric revolution first proposed their idea. The math works out more sensibly when you posit that the Earth goes round the Sun; also, as Aristarchus had pointed out seventeen hundred years earlier, the Sun is evidently much bigger than the Earth, and it seems odd to have the big thing circle the small thing.
The accumulation of indirect evidence was overwhelming. In the case of the multiverse, Wallace suggests that the next tranche of indirect evidence will probably come from quantum computing. If things progress a little further along the directions that are currently being explored, it will soon be possible in practice to solve problems with quantum algorithms that cannot be solved at all on conventional computers.
People will routinely be writing quantum software and thinking about debugging and improving it. As Wallace says, the natural way to conceptualize some of these algorithms is that the computation is parallelized by sending subtasks into enormous numbers of parallel worlds, then retrieving the answer from the branch which succeeded.
Wallace appears reluctant to delve too deeply into the moral and ethical aspects. But in cases like the notorious quantum suicide thought experiment, it is not as clear that things are still the same.
Wallace notes that death is "philosophically difficult", and explicitly advises philosophers not to discuss these matters in popular works. There is a striking resonance with the last chapter of Time Reborn , where Smolin expresses concern that belief in multiple universes may lead people to value less the one universe which we can directly perceive around us.
But, as Eve said to Adam, those apples just looked so tasty
The Emergent Multiverse: Quantum Theory according to the Everett Interpretation
The point of science, it is generally accepted, is to tell us how the world works and what it is like. But quantum theory seems to fail to do this: taken literally as a theory of the world, it seems to make crazy claims: particles are in two places at once; cats are alive and dead at the same time. So physicists and philosophers have often been led either to give up on the idea that quantum theory describes reality, or to modify or augment the theory. But really, the interpretation is not sensationalist at all: it simply takes quantum theory seriously, literally, as a description of the world. Once dismissed as absurd, it is now accepted by many physicists as the best way to make coherent sense of quantum theory. David Wallace offers a clear and up-to-date survey of work on the Everett interpretation in physics and in philosophy of science, and at the same time provides a self-contained and thoroughly modern account of it--an account which is accessible to readers who have previously studied quantum theory at undergraduate level, and which will shape the future direction of research by leading experts in the field.
The Emergent Multiverse: Quantum Theory According to the Everett Interpretation
Emergent Multiverse: Quantum Theory According to the Everett Interpretation
The Emergent Multiverse