Subtitled: The Science of Parallel Universes—and Its Implications. Published Aug. 1997 by Allen Lane, Penguin Press. 390pp, of which 24pp are bibliography and index.

Here is a book that took me a year and a half to read, but was worth the trip. And that’s after it sat on my shelves for almost a quarter of a century! I bought it in a physical bookstore (a chain called Bookstar), though I don’t remember what tempted me to buy it. A review somewhere? Or did it just look like the type of general science book that appealed to me? Perhaps even that of the five blurbs on the back cover, two are by science fiction writers – Douglas Adams and James P. Hogan – and two of the others are by well-known scientists, John Gribbin and Frank Tipler. OTOH, getting endorsements for your popular science book from science fiction writers seems a tad dicey, as if we’re not supposed to take the book entirely seriously… Yet it does seem very serious, and makes some major points.

Deutsch is a scientist noted for this book and one other, THE BEGINNING OF INFINITY (2011), and seems a tad eccentric.

It took me a year and a half because while the book is fascinating, it is abstruse at times to the point that I set it down for a while to read something else. I read the book in three bursts (of a couple weeks each) beginning in Summer 2020 and ending January 2022. I took 5889 words of notes. Fortunately, while there’s lots of detail, the book is quite succinct and profound in its major themes.

Opening lines:

If there is a single motivation for the world-view set out in this book, it is that thanks largely to a succession of extraordinary scientific discoveries, we now possess some extremely deep theories about the structure of reality. If we are to understand the world on more than a superficial level, it must be through those theories and through reason, and not through our preconceptions, received opinion or even common sense. Our best theories are not only truer than common sense, they make far more sense than common sense does.

Here are his key points, with some comments from me.

  • Our current deep theories about the structure of reality have replaced many previous theories into a small number, and by understanding these, one can indeed understand everything that is understood: a theory of everything.

This is a revelatory insight. As author discusses how, at one time (centuries ago, or perhaps in antiquity) it was thought possible for a learned person to know everything that was known, but that has seemed to become impossible with the growth of knowledge, which supposedly now doubles every two years (or something like that). His point is that underlying ideas, all the theories that explain all that knowledge and observations, have in fact converged, or merged, into just a handful of “deep” theories. And all of these can be known. These comprise a “theory of everything,” not in the physics sense (involving fundamental particles and forces), but in a broader sense.

  • The four strands of explanation are: quantum physics; epistemology; the theory of computation; and the theory of evolution.

Together these form his “theory of everything.” The balance of the book is exploring how these four ideas are analogs of each other, or can be understood in terms of each other, and what they imply about reality, virtual reality, the nature of life, the limits of computation, the nature of mathematics, and even time travel.

  • Quantum mechanics implies the existence of parallel universes.

There have long been alternate “interpretations” of quantum mechanical observations, and the author chides most other scientists for resisting the most straightforward of them: that for every QM event that can happen in multiple ways, there truly are alternate universes born into existence for each of those ways. There are thus an infinite number of universes, infinite to a degree we can’t comprehend, new infinities every moment.

  • Science is not about deriving a theory from observations (the traditional idea); it’s a problem-solving process in which solutions are examined and replaced with better ones, until new problems emerge.

This is something of a sidebar claim, following from his personal observations that real science is almost never done in the “traditional” way, i.e. making observations, formulating a hypothesis, then making additional observations to confirm or deny the hypothesis. In real life, a scientist usually has a problem, e.g. an observation or hypothesis that doesn’t align with previous ones, and iterates solutions until some new problem emerges. (He challenges Popper and Kuhn much as Strevens does in his (much later) book, reviewed here.)

  • Physical reality is self-similar on several levels. If this were not true, we could not do science; since it is, we can accumulate knowledge.

This may seem obvious but is in fact profound; if the universe were not physically consistent, its parts and components self-similar – if reality were as capricious and subject to a god’s whimsy as some apparently believe – then science could not exist, and we would not have accumulated the knowledge that we’ve used to build the modern world.


Those are the big take-aways. The book explores those four big theories and shows how they are self-similar and explain each other. I will not further present my notes, exactly, but I will glance through them and note some striking observations or claims.

  • He explores the difference between whether science explains or predicts; how thinking that prediction is all that matters is “instrumentalism”. Author thinks explanation is what matters; the deep theories. Positivism, reductionism, and holism are also wrong.
  • He discusses how we can’t disprove solipsism, but it doesn’t explain anything; creationists and behaviourists are variations of solipsism.
  • As were the Inquisitionists, who believed in a reality, but rejected its being understood through observation, rather than on authority of a holy book. Solipsism, taken seriously, self-destructs. Instead we can reject roundabout explanations (like conspiracy theories) in favor of explanations that solve problems.

Page 95, a passage that echoes something I wrote here on my blog a while back:

Every part of the Earth’s surface, on every clear night, for billions of years, has been deluged with evidence about the facts and laws of astronomy. For many other sciences evidence has similarly been on display, to be viewed more clearly in modern times by microscopes and other instruments. Where evidence is not already physically present, we can bring it into existence with devices such as lasers and pierced barriers – devices which it is open to anyone, anywhere and at any time, to build. And the evidence will be the same, regardless of who reveals it. The more fundamental a theory is, the more readily available is the evidence that bears upon it (to those who know how to look), not just on Earth but throughout the multiverse.

  • The theory of computation gives limits to the idea of virtual reality. In principle, VR can render any reality that is not logically impossible. Its interaction with the user pushes the boundaries of computation. But the VR machine could always freeze the user’s awareness indefinitely, until it could render the requested scenario.
  • Quantum computers are about performing tasks in collaboration among parallel universes. Thus the limits of computation, and thus of VR, depend on QM. [[ I do not think this is what is meant by “quantum computers” in current parlance. ]]
  • He disputes the notion that “mathematics is the study of absolutely necessary truths” and claims that “mathematical knowledge is inherently derivative, depending entirely on our knowledge of physics. The comprehensible mathematical truths are precisely the infinitesimal minority which can be rendered in virtual reality.” (You see how his four big ideas inform each other and mesh…)
  • There is no such thing as the “flow” of time. A case where common sense is nonsense. All moments exist forever. If time flowed it would have to be with respect to some “exterior” time… but this leads to infinite regress. Other times are… special cases of other universes.
  • All of this informs the notion of time travel. It’s possible in the sense that VR can render any conceivable past. We can change the past by choosing which snapshot of the past to be in.
  • All four of these strands have the unusual status of being accepted (for use in practice) and ignored (as explanations of reality) by most people working in those fields, p320.3
  • The final chapter takes on Frank Tipler’s idea of an “omega point,” where in the infinite timelessness of the end of the universe, all possible existences will occur, including recreations of all of us, in VR renderings. His idea might work at infinity; but at any given point, author claims, it will still be riddled with errors. The omega point will be full of people disagreeing.
  • Finally: these four strands form the best worldview currently conceived, and should be taken seriously, before we can move on.


Takeaway impression: this book is a prime example of how ‘the universe is unintelligible to you’ and stranger than you can imagine. Humans live by skittered along the surface of a deep, vast, intractable universe.

I’ll try to get the author’s second book in less than another 15 years.

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