In both popular and technical literature in subjects such as cosmology or theoretical physics, one can often find authors referring to the universe as a “computer”. I have found, however, that frequently non-specialists (including, sometimes, authors of popular literature) are misled by such characterizations. This is quite understandable. Inaccurate computer metaphors abound (particularly when it comes to the mind, brain, and consciousness), as do computers. Computer no longer brings up images of mechanistic calculations, and we no longer think of machines that carry out computations (like calculators) as “computers”, because “computers” are the laptops and desktops we are familiar with.
But when physicists call the universe a computer, they are asserting it has more akin with a pocket calculator than with your iMac or PC. Actually, in a very real sense by equating the universe with a computer physicists are saying it has more in common with your wristwatch or clock than your computer. Before the advent of quantum physics, it seemed as if the universe evolved deterministically according to some surprisingly simple dynamical laws (Newton’s equations of motion described just about everything, and what couldn’t be described by Newtonian mechanics could be by Maxwell’s equations, or Hamilton’s, or some other mechanics). The future state of any system is “computed” based upon current states entirely deterministically, like clockwork.
Quantum mechanics changed the deterministic part of this universe of computed states, but not the computation part. In quantum mechanics, the evolution of a system is still governed by entirely deterministic evolution (the famous Schrödinger equation, for example, is a linear partial differential equation, such that given any particular input we can compute, or the universe can, a single determined outcome). How indeterminism comes into the theory is not important here (especially because it is not exactly intuitive and involves going into the manner of preparation and specification of systems and measurements). What’s important is that even in quantum mechanics the future states of systems are given by dynamics so precise and “regular” they can be encapsulated by a few equations.
In the “universe as computer” concept, this encapsulation is really more of a description: the mathematical models are what the “universe” computes in order to evolve in time (i.e., to arrive at future states). This concept doesn’t entail a universe which is conscious, or that we are programs stored on some cosmic hard drive, that the universe has memory (at least no more than sandpiles or solitons), that we are living “in” a “computer”, or any other of the more exotic notions I’ve heard or read that result from comparing the universe to the things you use to check Facebook, create PowerPoint presentations, or visit this blog.