Via the Zeitgeister, a fun panel discussion at the Perimeter Institute between Seth Lloyd, Leonard Susskind, Christopher Fuchs and Sir Tony Leggett, moderated by Bob McDonald of CBC Radio’s Quirks & Quarks program. The topic is “The Physics of Information,” and as anyone familiar with the participants might guess, it’s a lively and provocative discussion.
A few of the panel members tried to pin down Seth Lloyd on one of his favorite catchphrases, “The universe is a computer.” I tackled this one myself at one point, at least half-seriously. If the universe is a computer, what is it computing? Its own evolution, apparently, according to the laws of physics. Tony Leggett got right to the heart of the matter, however, by asking “What kind of process would not count as a computer?” To which Lloyd merely answered, “Yeah, good question.” (But he did have a good line — “If the universe is a computer, why isn’t it running Windows?” Insert your own “blue screen of death” joke here.)
So I tried to look up the definition of a “computer.” You can open a standard text on quantum computation, but “computer” doesn’t appear in the index. The dictionary is either unhelpful — “a device that computes” — or too specific — “an electronic device designed to accept data, perform prescribed mathematical and logical operations at high speed, and display the results of these operations.” Wikipedia tells me that a computer is a machine that manipulates data according to a list of instructions. Again, too specific to include this universe, unless you interpret “machine” to mean “object.”
I think the most general definition of “computer” that would be useful is “a system that takes a set of input and deterministically produces a set of output.” The big assumption being that the same input always produces the same output, but I don’t think that’s overly restrictive for our present purposes. In that sense, the laws of physics act as a computer: given some data in the form of an initial configuration, the laws of physics will evolve the configuration into some output in the form of a final configuration. Setting aside the tricky business of wavefunction collapse, you have something like a computer. I suppose you could argue about whether the laws of physics are “the software” or the computer itself, but I think you are revealing the limitations of the metaphor rather than learning something interesting.
But if we take the metaphor at face value, it makes more sense to me to think of the universe as a calculation rather than as a computer. We have input data in the form of the conditions at early times, and the universe has calculated our current state. It could have been very different, with different input data.
And what precise good does it do to think in this way? Yeah, good question. (Which is not to imply that there isn’t an answer.)
It seems to me that if we take the concept of quantum gravity seriously the Universe is not just any old computer but is a digital computer. If we say the Universe is a digital computer, it raises of computability and the halting problem, does a Universe have to be computable to be real.
“And what precise good does it do to think in this way?”
It makes people with Computer Science degrees feel more important.
One of my cats takes deterministically produces a set of output about 5 feet away from his litterbox. Is he a computer?
We have for a long time used the most advanced and rapidly developing technology of an era as a metaphor for science to interpret the world.
In the seventeenth century it was clocks and we ended up with Newtonian mechanics. In the nineteenth century it was the steam engine and we ended up with thermodynamics. In the twenty first century it is computers and we we end up with …
>And what precise good does it do to think in this way?
So we can figure out how to crash it. [insert evil geek laugh]
There’s a science fiction story for you.
So, what if the universe is not a computer? Or a calculation? What if it’s a free entity, constrained only in some ways that humans have learned to focus on, and not in others that remain beyond the ken of our logic? What if the evolution of the human brain has left us incapable of perceiving or understanding what “the universe” really consists of, and we merely perceive, and analyze, Plato’s shadows on the wall?
“What is now proved, was once only imagin’d.” What if we aren’t constructed so as to be able to even imagine it?
Pretty far out, huh? That’s what metaphors like “The universe is a computer” are, too. That’s my story, & I’m sticking to it.
Mark S wrote
It’s been done, in Arthur C. Clarke’s classic short story “The Six Billion Names of God.” 🙂
Good points Sean. I used to have a similar view of the universe as a computation. But I could never articulate it.
Specifically, how does the universe calculate things exactly?? Physicists have to make all sorts of approximations and assumptions!
Any thoughts anybody?
thank you.
I disagree with your definition that the computer should determine the output deterministically. This excludes many important real algorithms, and it also excludes quantum computers.
What is interesting to me is if the universe is a computer, what is the rate of processing and how has that rate changed over time since the big bang. Is computation (cpu cycles/bits of output) increasing with time?
The reason I find this is an interesting question is the potential relationship to the Bekenstein Bound and the holographic principle.
Any thoughts or comments welcome
e.
A process is something that evolves over time. A computer runs a program. A program is a state to state mapping. And a state is a name to value mapping.
The universe is trivially a computer.
Elliot, you’re thinking too specifically. You’re talking like the universe was Intel Inside, when really Sean’s definition was closer to that of a Turing Machine.
Maybe you are considering the concept of computer in a too narrow sense. Turing, Von Newman and others considered the idea of computation as a general algebraic description of processes. Turin machines, for example, are supposed to be “capable of performing any conceivable mathematical problem” (http://en.wikipedia.org/wiki/Alan_Turing). This pretty much includes any meaningful process.
Actually, those basic ideas of computation lead later to Maturana to propose the autopoiesis or self-production as the basis of life. So, it might be argued (and have extensively been so) that any living entity is a “computer”.
As a consequence, it might also be argue that the universe is alive.
This is the point to stop and pick a cup of coffee, I guess.
I hope Seth will weigh in, but in my understanding there are at least three other issues here. First, a computer performs a task that is computable in the Turing-machine sense. Second, a computer operates in discrete time. These are more restrictive conditions on the laws of nature than your definition implies. Third, there are times when an algorithm is a more useful conceptual paradigm than an equation because the latter doesn’t have a closed-form analytic solution; it has to be re-expressed as a difference equation for numerical solution, in which case, why not just start with the difference equation?
George
Tony Leggett hit the nail on the head. The universe is a computer is the same sense that any process is. Put another way, for the universe to qualify as a computer, the definition of that term must be so broadened as to become meaningless (or at least purely metaphorical). In real practice, the key features of a computer are its read-in and read-out, neither of which the universe has. With a true computing device, it must be possible to vary the input of its computations, either by providing a different program or by changing the “wiring” of the computer itself.
Seth Lloyd’s incessant babbling that the universe is a computer has actually, from what I’ve seen, done damage to the perception of quantum computing in the scientific community. The damage has been minor, but it’s visible. Just about every time I hear a serious talk on quantum computation, the speaker takes time early on to disclaim the notion that the whole universe is a computer.
Just because you can’t find where to plug the keyboard in… 🙂
Computers operate in discrete time, George? Digital computers do, yes, but analogue computers use continuous phenomena to do calculations.
Computing practice as it stands is a tiny subset of what is theoretically possible. For example, it’s eminently possible to build a processor that works in decimal natively, removing this silly obsession with binary; we don’t because binary signalling is much more noise-tolerant.
We can’t really call the universe a computer because we don’t really know all the universe consists of and all of it’s properties, rules, etc. We don’t even know all of the thinks to look for.
Wait, I guess that does sound like my computer.
Bob,
I was trying to use an analogy. I am talking about bits processed in the most general sense. The relationship to the holographic principle is dependent on a quantification of bits of information in the universe.
e.
The universe at the least can’t be a digitial or finite computer, because linear operators on a finite Hilbert Space can’t form Hiesenberg/Lie bracket Uncertainty relationship. You would either have to give up conservation of energy or conservation of probability in the wave function. Only inifinite Hilbert Spaces have Banach spaces of linear operators that can contain Lie Brackets that equal the identity operator, and even then the representation as Lebesgue intergrable functions is only modulo to functions defined almost everywhere.
If the universe is a computer its not like any that we have seen, quantum, digital, or otherwise.
Maybe the problem here is that we’re trying to draw the line between what is and is not a computer, but in reality it is more of a continuous quality (computerness?).
That said, I sense a critical missing element in these definitions: the meaningfulness of the information being computed.
Meaningness, being a state of mind, requires a perceiver. So, this would mean that for something to be a computer, a perceiver is required. I’m not prepared to claim that the universe has any external perceivers, but it sure does have internal ones. The universe perceives itself through us, at least.
I think we need to find unambiguous meaning in the universe’s calculations before we can rightly call it a computer (or high on the “computerness” scale, rather).
Nah, the universe is not a quantum computer, it is actually a giant organism whose genes are constantly mutating. Actually, on second thought, it is a mathematical structure obeying the cold hard rules of logic. I used to think that the universe is a marketplace, whose invisible hand guides us towards the greater good, but that is obviously just a meaningless bunch of words…
It seems to me that in order to model a universe with a computer one would necessarily have to use an algorithm with some random variables. The real question, in my mind, arises from how those random numbers are generated. For instance a typical software program uses the last couple digits of the time that the program has been running and takes the modulus of that number (e.g. using the time 12:03:04.21426713 the program would take 13mod4 to generate one of four possible outcomes and if it needed to make another calculation then it would use the next time 12:03:04.21473459 and use 59mod4 to generate the next possible outcome). This is only pseudo-random and if the universe is a computer that picks it’s random numbers in such a manner then it should be possible to determine exactly where (not just most the probable location) a particle will land once it goes past an edge, or through a slit, or what it’s spin state will be after measurement of the entangled state, etc. However, if the universe generates a truly random number then we will be forced to rely on some form of quantum mechanics forever, again assuming that the universe is a computer…sorry if this is totally incomprehensible to you physics folks I am just putting my thoughts on paper, so to say. I am just a humble molecular biologist who wishes he had taken the physics route as an undergrad.
Are any of these information theories of the universe testable? Wouldn’t it be cool if the answer really was 42.
For it to proceed as a computer it need not be deterministic, though ultimately that distinction is irrelevant. A deterministic Turing machine can and will compute the same things a NDM turing machine does though it will take longer presumably (depending on the P = NP problem).
The whole discussion is a bit of a scam, since any Turing machine can write down an algorithm for the exact laws of physics and mathematics and simply proceed as usual eg no one has learned anything.
Whether the universe can be described by a simple cellular automata unlike the basic laws we know (something Humans can write down from first principles and recursively compute) though is a better question, and I think that its pretty clear that you cannot.
Incidentally, computer science teaches us that there is no real difference between hardware and software (calculation vs device). This is true by the Church theorem.
Though there does exist examples of noncomputable problems if we assume the machine has limited resources (N states different than infinity)