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Our future, our universe, and other weighty topics


Wednesday, March 5, 2014

A Critique of Seth Lloyd's Theory of the Universe as Quantum Computer

Seth Lloyd is an MIT professor of mechanical engineering who wrote a book called Programming the Cosmos. Some of the ideas in this book have been stated in a recent scientific paper he wrote entitled The Universe as Quantum Computer. In this paper Lloyd deals with some fascinating ideas, and flirts with some promising lines of thought. But does he come up with a workable conclusion?

I will skip over the first seven sections of this paper, which mainly deal with a discussion of exotic issues in computer science such as universal Turing machines and cellular automata. In section 8 “The Universe as Quantum Computer,” Lloyd leaps to the conclusion that the universe is “observationally indistinguishable from a giant quantum computer,” but does not justify this assertion. For one thing, no one has built a giant quantum computer. For another thing, if we were to build a giant quantum computer, there is no reason to think that it would look anything like our universe of galaxies, stars, and planets. 


Lloyd then asserts, “The ordinary laws of physics tell us nothing about why the universe is so complex.” This is a very serious misstatement which is easy to disprove. In fact, the ordinary laws of physics tell us a great deal about why the universe is so complex. We have large complex objects such as galaxies, stars, and planets largely because of the law of gravitation. We have 100 different types of atoms (and many complex molecules) largely because of the laws of electromagnetism, and the laws of nuclear physics involving the strong nuclear force. We have a stable planet partially because of conservation laws that maintain various types of balances such as the balance between positive charges and negative charges. We have complex life partially because of various complicated laws that allow stable sun-like stars to produce thermonuclear fusion at a slow, steady pace. I could list numerous other examples of laws of physics that help to assure that we have a universe as complicated as ours rather than merely an unordered lifeless soup of particles.

Lloyd then asserts that the known laws of physics can be written on the back of a tee shirt, something that will come as quite the surprise to anyone studying physics in graduate school, who has to lug around 600-page books filled with the complex mathematics and equations of general relatively, nuclear physics, electromagnetism, and quantum mechanics. This page lists or gives links to more than a hundred laws of physics, a much larger list than can be written on the back of a tee shirt.

Lloyd then wonders how the universe got so complicated after the simplicity of the Big Bang, when everything was presumably packed into a simple incredibly hot and incredibly tiny superdense ball. To explain the rise of complexity in the universe (things such as galaxies, planets, and life), Lloyd offers his “quantum computational model of the universe,” which he attempts to explain in terms of typing monkeys.

The story of the typing monkeys is well-known to anyone who has read books on the origin of order in the universe. The idea is that if you have a sufficient number of monkeys typing for a sufficient length of time, they will eventually produce any imaginable literary work. Lloyd imagines monkeys typing text that will be fed into a computer. Purely by chance, Lloyd infers, some of this output would produce a working computer program. Lloyd suggests such a randomly produced program might somehow be responsible for order in our universe.

But where is this computer, and where are the monkeys? Lloyd gives this answer: “In addition, quantum fluctuations – e.g., primordial fluctuations in energy density – automatically provide the random bits that are necessary to seed the quantum computer with a random program. That is, quantum fluctuations are the 14 monkeys that program the quantum computer that is the universe. Such a quantum computing universe necessarily generates complex, ordered structures with high probability.”

Taken literally, this thesis is quite nonsensical.

First, let's look at the primordial quantum density fluctuations mentioned by Lloyd – not his speculations about them, but the basic concept of primordial quantum density fluctuations. Scientists imagine these as incredibly tiny random variations in density that occurred in the early universe. Cosmologists say that such fluctuations would have occurred in the early universe because of Heisenberg's Uncertainty Principle. But that law of nature (and its associated physical constant, Planck's constant) set a very specific limit on these fluctuations. According to Heisenberg's Uncertainty Principle, a quantum fluctuation cannot be greater than about a billionth of a trillionth of a trillionth of a joule during any second. A joule is about the energy needed to slide a brick a distance of one meter. So the maximum allowed quantum fluctuation in a second is an amount of energy billions of times smaller than the energy used in a single one-second flash of a firefly.

Given that limit, it is quite nonsensical to imagine quantum fluctuations literally being the source of some randomly produced program that might help to produce order in the universe. Even with random fluctuations occurring all over the universe, nowhere in the universe would we have for even one second some program that might be used later in producing order in the universe. If such a program were to somehow permanently pop into existence (contrary to the limitations of Heisenberg's Uncertainty Principle), there is no reason to think that it would then somehow be applied generally as the universe's computer program. Since quantum fluctuations would be occurring all over the universe, any random process producing one computer program would also produce trillions of other computer programs. If those programs were then somehow used by the universe, what we would see is not the universe we see (one in which there are physical laws the same everywhere), but some totally different hodgepodge smorgasboard patchwork-quilt universe in which every little patch of space had its own laws of nature. Our universe is totally different, and scientists have done observations tending to confirm that laws of nature behave the same at opposite ends of the universe, indicating a great uniformity of law throughout the observable universe.

I may also note that there is absolutely no reason for thinking that a particular part of space would start to use a random program that happened to pop into existence due to a quantum fluctuation. Just because a computer program pops into existence doesn't mean that a nearby computer will start using that program as its operating system. Also, if we are to explain the order needed for life by postulating something like a computer program, we need not a random computer program created by quantum fluctuations, but a highly optimized, fine-tuned program (given the huge number of anthropic requirements for observers like us, discussed here).

But perhaps Lloyd is just speaking in metaphorical terms (despite making such statements in a scientific paper). Given his completely incorrect statement that “the ordinary laws of physics tell us nothing about why the universe is so complex,” perhaps Lloyd thinks that most of the universe's order is because of some lucky quantum density fluctuations in the early universe, and perhaps he is poetically or metaphorically referring to these as a computer program. In reality, only a small fraction of the universe's order (less than 10%) is due to such quantum density fluctuations, with most of it (much more than 50%) being due to the universe's seemingly optimized laws of nature and physical constants.

To his credit, Lloyd seems to have some general idea or suspicion that programming and computation play an important part in the universe. But he's taken this promising idea, and failed to create a workable thesis from it. The truth is that the universe's order is mainly caused by a series of highly favorable laws and fine-tuned physical constants that seem to have existed from the very beginning, a seemingly goal-oriented set of laws and constants that can only be described as programmatic and conceptual. Our universe seems to have been programmed for success from the very beginning, as I discuss here and here. We understand only a small part of this programming (that part which we call the known laws of nature), Far from being some simple thing that can be written on the back of a tee shirt, there is every reason to suspect that the programming that allows a life-containing universe to evolve from the super-dense state of the Big Bang is some programming vastly more complicated and proficient than any software man has ever created. We cannot plausibly explain that cosmic programming either through a theory of typing monkeys or through a theory of quantum fluctuations occurring after the origin of the universe.