Programming the Universe: A Quantum Computer Scientist Takes On the Cosmos
- Seth Lloyd
A little less than 14 billion years ago, a huge explosion gave birth to the Universe, and once it sprang into existence, the Universe began computing. The positions, velocities and internal states of every elementary particle, every atom and molecule, indeed every single physical entity register bits of information. Those bits are continually altered by physical interactions that act like sequences of logic gates — given a sufficient supply of bits and enough time, they can compute just about anything that is computable. Thus, the Universe is a computer. It is not a metaphor, it really is. More than that, the fundamental laws of physics that govern any interaction are quantum; hence, the Universe is a huge quantum computer that computes its own behaviour. It started in a very simple state initially, but in time, as the number of computational steps increased, the computing quantum Universe spun out more complex patterns, including galaxies, stars and planets, and then life, humans, you and me, and Seth Lloyd and his book Programming the Universe.
Like many other good stories of this type, Lloyd's book will puzzle and even irritate as much as it persuades. Lloyd writes in a lively style, weaving jokes and personal anecdotes into more technical narrative. He shares his views on cosmology, computation, quantum physics, complexity, sex, life, the Universe and all that, and he does it well. Despite this proliferation of topics, the main message stands out and is reiterated several times — the Universe is a quantum computer programmed by quantum fluctuations, and the computational capability of the Universe explains how complex systems can arise from fundamentally simple physical laws.
Lloyd tells the story of the evolving Universe in terms of interplay between energy and information. In the conventional history of the origin and the evolution of the Universe, the story usually told by cosmologists and astronomers, energy plays the central role. First there was a singularity and there was no past for it to emerge from. Then expansion. As the Universe expanded, it cooled down and various forms of matter condensed out because the disruptive thermal energy gradually dropped below the binding energies that hold constituent parts of protons, nuclei and atoms together. Tiny quantum fluctuations made some regions of the Universe slightly denser, and gravity amplified this effect, which resulted in gas clouds, stars and galaxies. Stars exploding to supernovae produced heavier elements, then our Sun and Solar System formed, and about 4 billion years ago, life emerged on Earth. But this story leaves many questions unanswered. How did life arise? Why is the Universe so complex? Could such complexities have arisen from total randomness?
Now, enter computer science. Algorithmic information theory shows that there are short, random-looking programs that can cause a computer to produce complex-looking outputs. Lloyd illustrates this with a popular story attributed to the French mathematician Émile Borel. Imagine a bunch of monkeys typing randomly into typewriters. Given enough time, it is certainly possible that one of these monkeys will type the first million digits of π or the first act of Hamlet. Possible, but very unlikely. Now, take the typewriters away and give the monkeys computers that recognize any random inputs not as text but as a computer program. When the computers try to execute random programs, most of the time they will crash or generate garbage, but every now and then just a few lines of random code typed by monkeys will give interesting outputs — for example, the successive digits of π, or intricate fractals. Or perhaps much more interesting patterns if the computer is the Universe itself.
This vision of a computational Universe is not new: it was proposed in the 1960s by Konrad Zuse and Ed Fredkin, and revived more recently by Stephen Wolfram. However, unlike his predecessors, Lloyd stresses the quantum nature of computation. This distinction is important because, to the best of our knowledge, it seems impossible to simulate the evolution of a quantum system in an efficient way on a classical computer.
A classical computer simulation of quantum evolution typically involves an exponential slowdown in time. This is because the amount of classical information needed to describe the evolving quantum state is exponentially larger than that needed to describe the corresponding classical system with similar accuracy. However, instead of viewing this intractability as an obstacle, today we regard it as an opportunity — if that much computation is needed to work out what will happen in a quantum multi-particle interference experiment, then the very act of setting up such an experiment and measuring the outcome is equivalent to performing a complex computation. Since Richard Feynman and David Deutsch pointed out this opportunity in the 1980s, the hunt has been on for interesting things for quantum computers to do, and at the same time, for the scientific and technological advances that could allow us to build quantum computers. The field is flourishing, and Lloyd provides a good popular introduction to the subject. However, he does not stop at the level of building quantum computers, he takes on the biggest quantum computer there is — the Universe.
The Universe is a quantum computer, and quantum mechanics supplies the Universe with 'monkeys' in the form of ubiquitous random quantum fluctuations — the same fluctuations that provided the seeds of galaxy formation and of all that followed. The Universe has pockets of complex behaviour because, Lloyd claims, the monkeys have been working very hard. He estimates that the visible Universe, programmed by quantum fluctuations, has performed about 10122 operations on 1092 quantum bits. No wonder we are here!
I think this is a delightful book, but some parts are patchy and many details are brushed under the carpet. For example, anyone trying to work out numerical estimates of the physical limits to computation or the computational capacity of the Universe is much better off consulting Lloyd's original paper on the subject (see Nature 406, 1047–1054; 2000). It is clear that Lloyd has forsaken accuracy for snappiness in several places, but then this is a popular exposition.
Seth Lloyd is a good storyteller, but is the story convincing? Well, I was convinced, but when I tried a nice line from the book — “programmed by quanta, physics gave rise to chemistry and then to life, programmed by mutation and recombination, life gave rise to Shakespeare, programmed by experience and imagination, Shakespeare gave rise to Hamlet” — on a colleague of mine, an English literature fellow, he only shook his head in disbelief and walked away.
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Ekert, A. The Universe's quantum monkeys. Nature 445, 366–367 (2007). https://doi.org/10.1038/445366a