Digital physics

In theoretical physics, digital physics holds the basic premise that the entire history of our universe is computable, that is, the output of a (presumably short) computer program. The hypothesis was pioneered in Konrad Zuse's book Rechnender Raum (translated by MIT into English as Calculating Space, 1970). Its proponents include Edward Fredkin, Juergen Schmidhuber, Stephen Wolfram, and Nobel laureate Gerard 't Hooft. They hold that the apparently probabilistic nature of quantum physics is not incompatible with the notion of computability. A quantum version of digital physics has recently been proposed by Seth Lloyd.

The theory of digital physics is that there exists a program for a universal computer which computes the dynamic evolution of our world. For example, the computer could be a huge cellular automaton, as suggested by Zuse (1967), or a universal Turing machine, as suggested by Schmidhuber (1997), who pointed out that there is a very short program that computes all possible computable universes in an asymptotically optimal way.

Some try to identify single physical particles with simple bits. For example, if one particle, such as an electron, is switching from one quantum state to another, it may be the same as if a bit is changed from one value (0) to another (1). There is nothing more required to describe a single quantum switch of a given particle than a single bit. And as the world is built up of the basic particles and their behavior can be completely described by the quantum switches they perform that also means that the world as a whole can be described by bits. Every state is information and every change is a change in information (one or a number of bit manipulations ). The known universe could, as a conclusion, be simulated by a computer capable of saving about 10⁹⁰ bits and manipulating them, and could very well be a simulation. Should this be the case, then hypercomputation would be impossible.

Loop quantum gravity could lend support to digital physics, in that it assumes space to be quantized.

Criticism

The critics - including a majority of professionals who work with quantum mechanics - argue, among other things, that:
* The models of digital physics are incompatible with the existence of continuous symmetries such as rotational symmetry, translational symmetry, Lorentz symmetry, electroweak symmetry, and many others. Proponents of digital physics, however, reject the very notion of the continuum, and claim that the existing continuous theories are just approximations of a true discrete theory.
* Some argue that the models of digital physics violate various postulates of quantum physics. For example, if these models are not based on Hilbert spaces and probabilities, they belong to the class of theories with local hidden variables that some think have been ruled out experimentally using Bell's theorem. This criticism has two possible answers. First of all, any notion of locality in the 'digital' model doesn't necessarily have to correspond to locality formulated in the usual way in the emergent space-time. A concrete example of this case was recently given by Lee Smolin. Another possibility is a well known loophole in Bell's theorem, known as pre-determinism. In a completely deterministic model, the experimenter's decision to measure certain components of the spins are pre-determined. Thus, the assumption that the experimenter could have decided to measure different components of the spins than he actually did is, strictly speaking, not true.

See also

* A New Kind of Science
* Cellular automata
* Holographic principle
* Digital philosophy

References

# Fredkin, Edward, "Digital Mechanics", Physica D, (1990) 254-270 North-Holland.# G. 't Hooft, Quantum Gravity as a Dissipative Deterministic System, Class. Quant. Grav. 16, 3263-3279 (1999) preprint.#S. Lloyd, The Computational Universe: Quantum gravity from quantum computation, preprint.#L. Smolin, Matrix models as non-local hidden variables theories, preprint.#J. S. Bell, Bertlmann's socks and the nature of reality, Journal de Physique 42, C2 41-61 (1981).

External links

* Petrov, Plamen, and Joel Dobrzelewski, "Digital Physics". 1998.
* "Digital physics". Mountain Math Software.
* Schmidhuber, Juergen "Algorithmic Theory of Everything, 1997-2002".
* [ftp://ftp.idsia.ch/pub/juergen/zuse67scan.pdf Scan of Zuse's paper in PDF]
* The Oxford Advanced Seminar on Informatic Structures
* Wired: God is the Machine