Book Review: The Fabric of Reality, by David Deutsch

In college, my favorite physics professor pointed out an interesting trend — esteemed scientists who ascend beyond mere research in their own field of expertise to build Grand Theories of Everything that combine seemingly unrelated disciplines. For example Roger Penrose, the famous physicist who attempted to explain human consciousness using quantum mechanics. My prof dismissed these theories as just attempts to sell books after these scientists got tenure and ran out of fun things to do in their own disciplines. I started picking up these books out of curiosity so I could tally up the common threads in each, and maybe get a few blog posts out of the endeavor. David Deutsch is in the club of scientists building Interdisciplinary Theories of Everything (I’m capitalizing to Make It More Official), so I picked up the first of his two books, “The Fabric of Reality“.

Turns out, reading a bunch of books from world experts (and Nobel laureates) is hard! (Maybe I should turn off the TV?) Flipping through Deutsch’s attempt when I first picked it up in the early oughts, I must have had the vague sense his book would be a harder slog than most, and put off reading it. It’s been so long since I bought the book that the retirement-aged Deutsch is described as “a young physicist of unusual originality” on the back cover blurb. Well, covid quarantine hit and I finally got some time free, and Deutsch, I’m ready to review your book! And I needn’t have feared a dry, technical slog — Deutsch’s book is certainly ambitious and challenging, but is an addictive read.

Deutsch is definitely a worthy member of the Interdisciplinary Theory of Everything club — he’s clearly aiming for a book like Douglas Hofstadter’s Godel Escher Bach (another member of the club), but where Hofstadter is melodic and metaphorical, sometimes conveying an idea through sheer literary ambiance, Deutsch is a straightforward lecturer, not in the habit of presenting his ideas with much poetry. There are nods to Hofstadter here and there (like his “Cantgotu” virtual reality environments — very cute), and their Interdisciplinary Theories of Everything (let’s call them ITOE’s — I can coin a cute term too!) share much common ground, particularly in computation and meta-mathematics. But in the end Hofstadter’s tour of many disciplines is a pedagogical tool, a metaphor for the topic actually at the center of his book (consciousness), not actually a unified theory of everything. Deutsch, on the other hand, boldly welds together four disparate fields of science into a unified theory of everything — this is not metaphor, this is a claim of an entirely new meta-science that subsumes all other fields of study. And not just in physics (where yer typical run-of-the-mill theories of everything are usually confined) — Deutsch starts the tour in quantum mechanics (as these books so often do), plows directly into epistemology (the theory behind knowledge itself), then swallows biology, virtual reality (!), time travel, the entirety of mathematics (!!), and by the end of the book is speculating on how his theory can explain morality. And of course wraps up the book with a prediction of the last few moments of intelligent life at the end of the universe.

So Who is David Deutsch?
You’re looking at Dr. David Deutsch in all universese

Yeah, who is he? He’s a physics professor at Oxford in England, a Certified Big Shot ™ in quantum physics, specifically in the field of quantum computing. He’s one of the pioneers of this field, the goal of which is to exploit quantum mechanical principles in the operation of a computer. Typically, people who design computers are fighting against quantum effects — they want the behavior of the computer to be predictable and deterministic, a.k.a. “classical”. As we shrink transistors to build ever more powerful computers, we start to see quantum effects eating away at the efficiency of our circuits. Back in the 80’s, researchers (including Deutsch) started exploring a different approach — why not simply bake in quantum effects into your computer in the first place? Rather than continually patching the bulwarks against the quantum effects threatening to flood our precious circuits, let’s set up a purely quantum scenario (e.g. a set of particles entangled with each other), and use that to run our computations. If done right, you reap the benefits of quantum superposition to accomplish much more calculational power than an equivalent classical computer.

Deutsch actually came to the field of quantum computing from research into the foundations of quantum physics — while his “day job” has led to tantalizing opportunities in advancing computers, his real passion is to understand. Understand what? Understand everything. His book starts with an anecdote from his childhood, when he learned that the scope of human knowledge is so vast that one person can’t possibly learn everything. Little David refused to believe what he was hearing, and adult David agrees — in fact, Deutsch believes we can in fact understand everything using his unified theory of the universe, and this book lays out his argument why.

The Four Threads

Deutsch’s theory combines four major topics from science, math, and philosophy — or as he calls them, four ‘threads’ that together weave the Fabric of Reality. The four threads are:

Quantum physics: Specifically, the many-worlds interpretation of quantum mechanics. No surprise, that’s his expertise. (Why don’t these brilliant physicists — and they usually are physicists — ever dream up a Theory of Everything with, say, botany at the core?) Postulating the existence of an infinity of parallel universes [] is an intriguing way out of some of the sticky philosophical points of quantum mechanics — while it’s not the accepted dogma, the theory has a small minority of supporters among physicists, of which Deutsch is one of the most strident and vocal. According to Deutsch, this thread is the most important of the four.

Theory of computation: OK, so this is a little hard to explain — this is the abstract theory behind the idea of “computation”. Like what computers do (like when you have 83 tabs open in your browser on your phone, grandpa), but more generally any systematic process that follows an algorithm to derive some result. Could be a computer, or a calculator, or someone on an abacus. What exactly is a “computation”? What are its limits? For a while we thought that perhaps mathematical theorems could be automatically derived through computation (via systematic manipulation of math symbols), which if true would mean we could polish off all of mathematics once computers became sufficiently powerful — just turn on the computer and wait for it to crank out theorems. (Spoiler alert, it turns out not to be true.) Deutsch, being one of the pioneers of quantum computing, is thus responsible for tying together these first two threads.

Epistemology (Theory of knowledge): He doesn’t state so, but this feels to me like the 2nd most important thread in his collection. If you thought “theory of computation” was abstract and meta, get a load of epistemology, the theory of knowledge. What exactly is “knowledge”? How do we acquire knowledge? Are we born with innate knowledge, or do we acquire everything we know through how we experience the environment (through our senses)? More specifically, how does science gain knowledge? You probably learned about the scientific process back in Junior High, and you might not be surprised to hear that the textbook version is a dramatic oversimplification. A rigorous debate has raged in the halls of philosophy departments for hundreds of years now, centered around defining how science actually works. In the early part of the 20th century, Karl Popper famously rejected outright the “scientific method” that you had to memorize for that quiz in 8th grade — I guess news hasn’t percolated down to the junior highs yet. Deutsch has very particular views on how science actually accrues new knowledge, and how strongly we should believe in that knowledge.

Evolution: OK, this one’s not so abstract as the last two threads — everyone’s heard of evolution, right? The hard part for this thread will be explaining what the hell it has to do with the other three — in my opinion, Deutsch is stretching this thread pretty far to fit in his Fabric of Reality. But for the sheer audacity, I’m glad he did — if he stayed with just the other three he’d have himself a nice tidy statement on the future of science and information in the quantum era; but now pull in evolution, and you’ve got yourself a real Interdisciplinary Theory of Everything! That might be why evolution gets comparably less airtime in the book than the other threads, not even appearing until the middle of the book.

The Wildcard: Deutsch weaves together four threads over the course of this book, but I think there’s actually a fifth thread at the center of his Interdisciplinary Theory of Everything. The most important concept in Deutsch’s vision is that of an explanation. In other words, someone’s reasoning (be it scientific, logical, or otherwise) to explain some phenomenon. Most books of this ilk would use the word ‘theory’, which I think Deutsch avoids for good reason (more on this later). Maybe it’s no surprise that an “explanation” would be important to understanding how science works — Deutsch dismisses the more arcane theories of scientific progress (like Kuhn’s paradigms, or various black-box interpreations) for a straightforward one: the best explanation wins. But scientific explanations also course through the other threads, such as during a discussion of time travel, Deutsch ponders how different parallel universes might be able to share knowledge. The knot that Deutsch uses to tie these four threads together is knowledge, and the tool we will use to disentangle the knot will be the explanation.

Quantum Mechanics
The two-slit electron experiment. Chances are, if you’re reading this far down, you’ve already read about this

Detusch starts the book with one of the better explanations of the “Many Worlds” interpretation of quantum mechanics that I’ve ever read. No surprise, as Deutsch has likely had a lot of practice giving this lecture as the current flag-bearer for Many Worlds in the physics community. Support for the theory has risen since it was first proposed by Hugh Everett in his doctoral thesis; Everett left academia soon after completing his PhD for industry, after utterly failing to convince the physics community of his views. Since then, Many Worlds has risen in status arguably as much because of the shortcomings of other interpretations as its own benefits. While it isn’t (yet) the de facto mainstream interpretation of quantum mechanics, it’s probably in second place at this point.

The multiverse theory is a clever but ambitious way around the puzzling questions in the traditional interpetation of quantum mechanics. What exactly is going on when a wavefunction collapses? Instead of hand-waving theories about observers, the multiverse theory proposes that all possible outcomes of every wavefunction collapsing actually do happen, albeit in different parallel universes. So when we force Schrodinger’s cat into the box and set the timer on the quantum poison-releasing mechanism, our universe splits into two parallel universes, one with a living cat and the other with a deceased cat. For every possible quantum state of anything in the universe (every electron, photon, molecule, short-haired cat, etc.), a universe exists with that state. It shows the magnitude of the difficulties with the traditional interpretation that proposing an infinite number of parallel universes actually seems simpler and more elegant. If Occam’s Razor had arms, they’d be shrugging at this point. (I go into more detail in my review of another ITOE book, “Quantum Enigma” by Rosenblum and Kuttner.)

When defending Many Worlds, Deutsch is on the most solid ground in this book; his explanation is clear and compelling. While I might take issue with some of the logical leaps that he insists are unassailable, he clearly writes from a position of expertise. He follows Feynman’s tradition of introducing quantum concepts to the reader via a tour of a single simple scenario, the two-slit electron interference experiment — Feynman found this experiment a great pedagogical tool to open the reader’s mind to the strangeness of quantum phenomena. Deutsch goes even further — by describing the outcome of this experiment and following a series of logical deductions to interpret those observations, he concludes there must be an infinite number of parallel universes, all interacting with each other through quantum mechanics.

Theory of Knowledge
Karl Popper is disappointed in your junior high science class

Then we come to a large section of the book devoted to a theory of how science works. Officially the strand is Epistemology, or the theory of how all knowledge is acquired. But it’s pretty clear that Deutsch is only interested in scientific knowledge — in other words, how exactly do we create scientific knowledge? How does science work to create such knowledge? Deutsch’s description of this thread is really a long and strenuous defense of Karl Popper’s theories against all-comers. Popper is probably the Einstein of the Philosophy of Science — his views are probably the mainstream view of most scientists today, although other views hold sway (such as David Hume’s “paradigm” concept). While it may seem obvious that over time, the industry of science creates actual, objectively-true knowledge, there’s actually a lively debate within the philosophy of science community about whether anything in the scope of science can actually be proven true.

Deutsch strongly dismisses any theory of science that doubts the veracity of the output — he urges us to accept that our scientific explanations are real (i.e. they describe what’s actually going on, when we find the right one). Other philosophers contend we shouldn’t 100% believe in any explanation — it might match up with all observations, it might let you make correct predictions about the future, but it still might be wrong. (See Newton’s laws, for example — they’re fabulously accurate for day-to-day macroscopic physics, but they’re still wrong). So taken to extreme, this view holds that no explanations are “right” — they’re fictions we humans invent, trying many until we find one that fits reality sufficiently well. We don’t know if we’re converging on truth, so some would say we shouldn’t assume any science theory is true. No matter how tempting it would be to believe in your theory, it’s better to hold it at arm’s length as a model of the universe that conveniently happens to give good predictions. Deutsch says this is nonsense — if it’s a good explanation, we should take it seriously.

The extreme counterargument to Popper’s (and Deutsch’s) concrete view on explanations is called instrumentalism. Instrumentalism contends that if you can’t directly put your hands on it and test it, don’t put it in your theory. There’s no point in including features in your theory that you’ll never be able to directly experience — you might as well treat anything you can’t directly observe as a “black box”, rather than positing some mechanism that you’ll never be able to verify. (Though if you do have a theory about what’s inside the black box, you might be right — instrumentalism doesn’t say you’re always wrong, instead it says you can never prove you’re right.) But Deutsch asks, we don’t really not believe our theories might be objective truths, do we? Newton’s description of gravity is more plausible than “a magic fairy pulls things down to the ground”, even though both explanations can equally explain the observations. Deutsch stresses that we should take our scientific theories seriously — if it passes muster as a good explanation, we should take it at face value, that the theory is describing what really is happening in our universe. At least until a better explanation comes along (e.g. relativity replacing Newton’s theories). Focusing on the predictions of your theory over the actual theory itself is focusing on the wrong thing, Deutsch says — checking predictions is how you test a candidate theory, so they’re important, but they’re not the be-all-end-all that instrumentalists would contend. Several times throughout the book, Deutsch stresses that we should “take our theories seriously” — i.e. believe they are objective truths, when they hold up to scientific scrutiny.

Occasionally, Deutsch uses the “we should take theories seriously” as a logical tool itself to reinforce or extend his ideas, much like you’d use Occam’s razor as a logical argument in places where experimental evidence is not feasible. For example, why is the universe comprehensible to humans? It’s not self-evident a priori that we should be able to make sense of the universe — why does science seem to work so well? Is the universe ultimately 100% comprehensible to a sufficiently advanced civilization, or will there turn out to be phenomena that we will just never understand? Is there a pocket of pure, indescribable chaos out past the Andromeda galaxy that will keep us from hitting 100%? Deutsch argues that it only makes logical sense that the universe is either entirely comprehensible, or entirely incomprehensible, not (say) 73% comprehensible — such an arbitrary amount of comprehensibility begs to be justified, and in fact introduces a new physical constant (why 73% and not 28%?) for us to explain. But so far we’ve managed to figure out a thing or two about a thing or two, so the universe is at least partially comprehensible, therefore it must be 100% comprehensible.

I find this fairly convincing from a gut-feeling perspective, but not a logical perspective — Deutsch is simultaneously asking us to take our explanations seriously (i.e. believe they really do describe physical reality), then assuming the validity of our explanations to claim the universe is partially comprehensible. But how did we get to claim success for our explanations? After all, our explanations might be entirely wrong, might just appear to explain the universe (either coincidentally or approximately), and the further we dig the less we understand. We might still be at the starting line of 0% comprehension. I would probably agree with Deutsch’s point of view, but more from being swayed by Leibniz’s question “Why is there something and not nothing?” rather than from Deutsch’s reasoning.

Deutsch spends quite a large chunk of the book discussing the Problem of Induction, and how thread #2 solves it. I have to be frank, I don’t really understand it. Possibly because of Deutsch’s writing style, but more likely I just don’t have sufficient background to appreciate the debate. The basic problem of induction was first explained to me with a question: is it true that the sun will rise tomorrow? Can that be proven as a fact? Scientific theories can’t be proven in the way that mathematical theorems can be, given a set of axioms. Of course we could posit some “axioms” of our scientific theory and then deduce “theorems” from them, like deriving the flight of a baseball given Newton’s Laws. But the proof is always contingent on the veracity of the axioms, and in science we aren’t given axioms a priori. We have to make educated guesses about the axioms (laws of physics), then derive theorems from those axioms (make predictions about an experiment), then check against the real world to verify the theorem (run the experiment and find your beautiful theory turns out to be completely wrong). So in that sense, a scientific theory that is inferred from observations can never be proven by pure logic, begging the question whether such theories can really be called knowledge. Interesting, eh? That’s what I thought too — then I tried to understand Deutsch’s discussion, particularly an invented dialogue Deutsch includes between himself and a “crypto-inductivist”. Well I mean, I still found it interesting, but about 2 pages in I was LOST. I had to conclude that I probably had better preparation when it comes to physics than for epistemology. I felt a little guilty at my relief to end that chapter and start a fresh topic in the next chapter, the Evolution thread.

“Ladeeees and Gentlemen… approaching the Octagon, weighing in at 135 lbs, The Darwin Bulldog, RIIICHARRRDD DAWWWWWKINNNNNNSSS!!!!”

Regarding evolution, Deutsch is strongly on the side of Richard Dawkins’ precisely sharpened view of evolution, that the fundamental unit of life is the gene, or more generally the meme (in its original definition). Deutsch goes further than the Darwin Pitbull to say that the physical embodiment of the gene is not as important as the information contained — the specific sequence of DNA is what’s important, not the molecules themselves. Deutsch agrees with Dawkins that “life” is really centered in the information content of our genomes, and the physical substrate (DNA molecules, proteins, organs, teeth, etc.) is merely the implementation or “rendering” of the underlying information. And genes who evolve to be well-adapted for survival are inherently carrying quite a bit of information about their environment. In a sense, life is nothing more than information, and the information is: “here’s how to keep me alive in this environment.” Here’s where it gets very meta — the information in our genomes is detailed instructions on how to protect and propagate these very instructions! Reproduction (or “replication” in unromantic meme terminology) is simply a mechanism to maintain survival of this information. You can think of life as a specific computer program running on biological “hardware”, and Deutsch urges us to consider this is more than just a metaphor — life is information, is computation, and thus is subject to the theory of computation (our 2nd strand).

Deutsch then brings out the Many Worlds interpretation as a nice tool to help define “information”. Let’s say someone stumbles across a good idea for something. To pick a concrete example, let’s say it’s how to tie your shoes. It’s immediately obvious it’s a good idea, so it rapidly becomes adopted as the standard for shoe tying. Deutsch proposes that if you could somehow look across the multiverses, you would see this shoe-tying idea pop up in many universes. Whereas some phenomena that doesn’t matter much (e.g. the patterns of molecules in your living room, right now) would vary greatly from universe to universe, any useful information would necessarily look similar across universes. Or to choose a better example (the one Deutch picked, actually), really important stretches of DNA in your genome would look similar (or “conserved”) across parallel universes, because any small change to them will likely result in death. So if you lined up the human geome from many multiverses together, you’d be able to see which stretches of DNA must be really important and which are probably “junk” by how much variation you see. (Not too different than what is currently done for genomes across species — if humans and bananas have a particular gene in common, that gene is probably pretty fundamental to life.) The important stretch of DNA can be said to contain more information than junk DNA, even though there’s no physical difference between them. This now gives us a convenient way to detect “knowledge-bearing matter” — “Within one universe it looks irregular; across universes it has a regular structure, like a crystal in the multiverse”. This gives us a physical way to define and quantify “information”, a difficult-to-define concept if we stick to just our universe.

Thus Deutsch has started tying together the threads — he has defined the concept of “information” employing his Multiverse interpretation of quantum mechanics, anchored the concept of “life” to this bedrock idea of “information”, and framed evolution as the process of acquiring and refining knowledge (in this case, inherent knowledge about the environment that the organism lives in). Note how the Multiverse provides us a convenient tool for getting out of a philosophical pickle here — without the concept of multiple parallel universes, defining what we mean by “information” becomes tricky. For DNA we have the luxury of looking across genomes of similar species, but you can’t really do that for, say, Shakespeare’s plays. Scientific formulations of information usually rely on mathematical probability, and probability itself can be a hard concept to define. Sometimes mathematicians will resort to postulating what would happen in alternative universes or “possible worlds” to craft a definition of probability. It sure is a lot more convenient for this effort if those alternative universes are actually real.

Time and the Brain
You could do worse than to discover the same theory as Dr. Emmett Brown

Later on, Deutsch again employs the Multiverse theory to propose a clever but somewhat confusing explanation for time travel. (I told ya, this book is not short on ambition!) His theories are not too far from what Doc drew on the chalkboard in Back To The Future II. He first established that the laws of quantum physics imply the existence of parallel universes, each with their own copies of everything we see in our universe (including ourselves). Given there’s an infinite supply of parallel universes, where anything that is physically possible will actually happen in some universe. Of course everything’s quantized, this being a quantum land. Time is also quantized — while it may seem to continuously flow into the future, we actually step forward in discrete steps, jumping from static moment to moment. And those moments always exist — instead of materializing into existence when needed, each moment of time (of each universe) exists always and forever. We conscious beings just experience one moment at a time, without the ability to observe the rest of the moments. Deutsch encourages thinking of these moments of time as a deck of cards, where each card contains the entire universe at a single moment. We step through these cards, one at a time, but the deck of cards exists in its entirety the whole time. And other parallel universes have their own decks of cards.

Time travel would constitute simply jumping ahead to a future “card” in your universe’s deck, or to a card in the past. For that matter, you could also jump to some other universe’s card, at any point in their timeline. Deutsch proposes that time travel is simply jumping to another universe’s set of cards — so if you go backwards in time, it’s not necessarily your own universe’s past that you jump to. In fact, if you go back in time and somehow change something in your past, you are by very definition in a different universe — the only way you can go back and visit your own past would be if you carefully made sure to not change anything. So the usual paradoxes of time travel are resolved — if you go back and kill your grandfather, the parallel-universe you will cease to be born, but you still exist. And if you bring your Collected Works of Shakespeare with you back in time, impersonate the Bard and re-publish all of his famous works yourself, you haven’t created a circular paradox where the great works were created from nothing. Shakespeare did exist in your past, he did actually sweat over writing every couplet. You’ve traveled to another universe’s past, and brought his works with you. So you made a loop, but the ends of the loop don’t connect into a circle.

That’s a compelling and tidy theory, but Deutsch doesn’t explain how we move through these discrete units of time. If each moment always exists, including us in that universe’s moment “card”, then why do our consciousnesses experience hopping from moment to moment? Whatever “we” are, we seem to be some entity that jumps through the cards of the multiverse deck in a particular order. Why aren’t we aware of all moments of our universe simultaneously? Deutch answers that just as there is a parallel-universe you, experiencing their universe just like yours, there are other-time yous. Every card in the deck contains a “you”, conscious and alive, experiencing that moment. They exist always — there is no overarching “you” jumping from moment to moment, there are merely a bunch of parallel yous inhabiting all times and parallel universes. Each of them are conscious at their given moment, static and frozen forever. The moment itself is unchanging, existing always:

“We do not experience time flowing, or passing. What we experience are differences between our present perceptions and our present memories of past perceptions. We interpret those differences, correctly, as evidence that the universe changes with time. We also interpret them, incorrectly, as evidence that our consciousness, or the present, or something, moves through time.”

A Boltzmann Baby, just created out of a random fluctuation of the universe, is already finding this book review tedious

Presumably then, each of us has our memories of the past by virtue of physics, not because “we” actually experienced those past events — we’re static and unchanging in our moments, but we remember a past. There is no single “I” that jumps from moment to moment, experiencing each and saving memories into “I”‘s brain, building up memories of a life being lived. Each moment actually contains its own unique “I”, trapped in that moment, but each “I” happens to contain memories in its brain that provide the illusion of a past. And it’s not random chance — presumably the laws of physics constrain the sequence of cards in the deck in the right way. This is reminiscent of the “boltzmann baby” idea — through sheer improbable (though not impossible) happenstance, matter could randomly coalesce together into the form of an intelligent being (say, a human), who materializes into existence with an intact brain, consciousness, and memories of a past. The past is fictional, of course, but to this unfortunate creature, it feels real. After all, if I were to make an exact atom-by-atom copy of you, the copy would have an identical copy of your brain, and hence would have all of your memories. Similarly, the “us” who are each trapped in a card in the multiverse deck are experiencing memories of the past that “we” didn’t actually experience. A disembodied supernatural observer who could peek into the brains of all of us would see the telltale neural signals that correspond to the experience of time (experiences turning into memories) if they step from one card to the next.

It holds together as a physical theory, but feels implausible. If this is true, then who am “I”? “I” have a sense of a continuous flow of time, of my past receding into the distance and my future in front of me. I’ll grant you that it’s possible my memory of my past is illusory, since I can’t distinguish between a real past and a brain that just happens to contain memories of a past. But why do “I” feel the sensation of the present turning into the future? It sure feels like something is experiencing the cards in the deck, one by one, in a particular order, as the sequence steps into the future. What is that sensation? And why does it move forward, not backward? If time is an illusion and all slices exist simultaneously forever, why do the stack of cards have a built-in order? Why can’t I “remember” future events? I’m not saying Deutsch is wrong (which I’m sure is of great relief to him), but I feel like an explanation for the illusory concept of time would be a welcome addition to the strands on his loom.

In fact, consciousness is notably absent from Deutsch’s theory of everything. Most of his colleagues in the ITOE club take a swing at explaining consciousness. If we did a survey (and one of these days I will), consicousness is probably second only to quantum physics in most popular “threads”. Deutsch on the other hand breezes past consciousness towards the end of the book, stating that he guesses the brain is a mere “classical computer”. “Classical” in the sense that the brain doesn’t depend on any esoteric quantum effects, and “computer” in the sense of obeying the same laws as other computational devices. And “mere” in the sense that Deutsch believes consciousness requires no new science beyond the four threads we’ve already explored — certainly it’s an interesting topic, but he sees it as not fundamental. But I feel like it’s an important omission given the lack of explanation on how multiverses work.

Omega Point

I’m skipping over many fascinating topics in my review of Deutsch’s book for the sake of time, for example where he proposes that logic and mathematics are a branch of physics (instead of the other way around, like almost all physicists believe). I’m eager to skip to the end, because the end of the book contains a delightful surprise — Deutsch evaluates his theory of everything in terms of Frank Tipler‘s Omega Point theory of the end of the universe. Tipler proposed this theory in his 1994 book The Physics of Immortality, in which he contends that as the universe ages, intelligent life will grow more and more technologically advanced, eventually learning to harness all matter and energy of the universe, growing in computational power without bounds, right up to the last moments of the universe before its destruction. The limitless growth in power leading up to the universe’s destruction will mean time will seem to slow down for them, so much that to them it will appear they can fit in an infinite lifetime crammed into the last few moments of the universe. (Tipler also proposes these future uber-beings will have the computational power to simulate other universes, will in fact simulate the past of this universe, thereby simulating us, which will in effect resurrect us from the dead. They will decide the only fair thing to do is to resurrect all sentient life from the universe’s past, and place us into a utopian paradise we can enjoy. i.e. this is where Tipler uses physics to not just explain the universe but also religion.)

Deutsch generally agrees with Tipler’s concept, but disagrees that these final days of the universe will be a harmonious utopia of super-intelligent beings that spend their time tending to their zoo of resurrected past beings. As the universe ages, society doesn’t just get extra computational power for free, we have to earn it. In other words, science won’t stop — we’ll have to continue pushing the frontiers of science to increase computational power, particularly near the end of the universe. As the universe breaks down, society will be faced with a set of “deadlines” where they need to develop a sufficiently-advanced technology to stave off being destroyed or dissipated by the end of the universe. This doesn’t come about organically, but must be achieved by fighting and clawing, urgent (and possibly frantic) scientific research in a more advanced but essentially same manner we do now. In other words, as the universe comes to an end, the advancement of science will become a critical matter of self-preservation:

“Their culture will presumably be peaceful and benevolent beyond our wildest dreams, yet it will not be tranquil. It will be embarked upon the solution of tremendous problems and will be split by passionate controversies. … it will be a vast number of people intraacting at many levels and in many different ways, but disagreeing.”

In other words, the future will converge to a typical research symposium. So the pursuit of science will eventually be paramount — the arguing, convincing, experimenting, in the end, finding and judging explanations, will be the primary goal of life as the universe ends. Deutsch’s threads converge at the end of the universe, and once again explanations are tying the knot. (I’m not 100% sure this metaphor really works, but it’s poetic, ain’t it?)

Final Opinions

So now I’ve summarized (and slightly critiqued) maybe 20% of the topics Deutsch brought up in this wide-ranging book — this guy covers a lot of ground, and I’m running out of thread metaphors. Let’s get to my quibbles!

I have a fairly minor complaint about Deutsch’s writing style that, to be fair, is applicable to pretty much all books written for laypeople by Esteemed Scientists. At many points I get the vague feeling of him talking about the theory, but not precisely stating the theory itself. (Albeit this could very well be from my lack of understanding the big picture. Despite having summaries at the end of each chapter, it was quite hard to keep the entire construction in my head at one time). He’s not a professional writer, of course, and like other scientists who write for the unwashed heathens of the general public, Deutsch sometimes has a hard time calibrating the depth of detail. Some points are belabored, restated again and again over the span of several pages, while others are breezed by too quickly as though they should have been self-evident to us (and likely are to a mind like Deutsch’s). (Though Deutsch’s writing skill is much better than Stephen Wolfram, whose “A New Kind of Science” was the nonfiction literature equivalent of driving with someone who is just learning how to use the clutch.)

My other small complaint is that Deutsch is somewhat indebted to the bold thinkers before him — not just Newton and Einstein and all them, but the fellow members of the Interdisciplinary Theory Of Everything club who published before him. I’ve mentioned the debt to Hofstadter, but Deutsch also spends time discussing Roger Penrose’s merging of quantum physics and consciousness (which Deutsch admires but doesn’t agree with). And after all, for each individual strand his main contribution is a stout defense of someone else’s formulation (Everett’s Many Worlds hypothesis, Dawkins’ evolution, Popper’s epistemology, Turing’s computation). On the one hand this makes Deutsch’s book not quite self-contained if you don’t read the works of these other big thinkers. On the other hand, Deutsch is admirably free of egotistical territory-defending — he knows (and readily tells the reader about) a good idea when he sees one in someone else’s book. It gives him credence — you get the sense he truly wants to understand the universe, not buff his own ego. (Not that he shows any modesty when he thinks he’s right, such as when he airily dismisses all mathematicians as a teaser for chapter 10.)

And furthermore, his readiness to simply point to someone else’s theory (albeit by writing multiple chapters that staunchly defend said theory) suggests to me that Deutsch is more likely to be correct than other ITOE’ers. He of course has made legitimate breakthroughs in quantum computation himself, but for the other threads his contribution is to see the connections between the threads, e.g. perceiving evolution as a form of computation. He doesn’t try to shoehorn quantum weirdness directly into consciousness (like Penrose) or evolution (like McFadden) — the connections between the threads are at a higher, more abstract level than that. As I’ve said earlier I think the threads are crossing in the vicinity of the concept of “knowledge” and “explanations”, though Deutsch doesn’t say so explicitly.

And thus we get to my biggest complaint — Deutsch’s phantom fifth thread. The one he doesn’t promote to thread-ness (“thread-ification”? “threaditude”?), but shows up at every point where the threads connect: the “explanation”. My complaint is a lack of a rigorous definition — what exactly is an explanation? Sure sure, to you and I it seems common-sense and not in need of a careful definition. But this is a book that spent a chapter to define “knowledge”, two chapters to define “time”, claims to refute entire philosophies (inductivism, solipsism), and repeatedly revels in debunking our common sense intuitions. So in this book we’re not lacking for 1) extremely careful thoughts about fundamental concepts, and 2) breathtakingly ambitious leaps of reasoning. So why not spend a few pages discussing exactly what an “explanation” is? For example, what makes a good explanation? Who’s the arbiter of a compelling explanation vs. a weak one? It’s notable that he uses the plain-english word “explanation” instead of “theory” — I suspect he’s appealing to our common-sense notion of an explanation to avoid the scrutiny on his fifth strand (a scrutiny that he applies with such zeal to any criticism of the other four).

However, it feels out of character for Deutsch to intentionally skirt careful scrutiny of his fifth strand. It would be preposterous to think that it simply didn’t occur to him to precisely define the “explanation”. Maybe he’s taken a stab at it more recently, I wondered? After all, this book is over 20 years old. Maybe I should apply some due diligence and check what he’s been doing lately before I start accusing him of hiding from scrutiny. (And perhaps I shouldn’t narrate my musings in this very blog post — stop writing this you fool, you’re incriminating yourself!) As it turns out, yes he has applied his laser focus to the concept of an “explanation” in the years since writing The Fabric of Reality. In the TED talk “A New Way to Explain Explanation” from 2009, Deutsch proposes a way to define explanation:

A bad explanation is one that is easy to vary.

Meaning, if you change some of the details of your crappy theory, and the change makes no difference to the plausibility of your theory, then there’s no reason to believe your theory over someone else’s. For example, Deutsch says, the ancient Greeks believed that the Earth’s winter season is caused by the goddess Demeter being sad at the kidnapping of her daughter Persephone. If we swapped out Demeter for (say) Thor, it wouldn’t change any prediction of the theory. On the other hand, a good explanation is hard to vary. If you change a detail in a good explanation (or theory), you likely ruin the theory completely. Change the “2” in the equation E = mc^2, and
you get a physical law that won’t match experiment. There’s very little “wiggle room” in Einstein’s General Relativity, or Darwin’s Evolution, or Turing’s Theory of Computation — if you dream up an alternative by changing some aspect of the theory, almost certainly that change will
be testable, proven wrong-able, and thus rejectable. We’ve heard from Popper that the progress of science comes from falsification — require that theories make testable predictions, and weed out those that are incorrect. Deutsch is saying that the best theories will come out the other end of this gauntlet as economical, elegant, all killer no filler — if you tweak any piece of the theory, it falls apart. Deutsch concludes the talk:

That the truth consists of hard-to-vary assertions about reality is the most important fact about the physical world. It’s a fact that is itself unseen, yet impossible to vary.

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