CHAPTER 41

Is the whole universe

a computer?

jack copeland, mark sprevak,

and oron shagrir

T

he theory that the whole universe is a computer is a bold and striking one. It is a theory

of everything: the entire universe is to be understood, fundamentally, in terms of the

universal computing machine that Alan Turing introduced in 1936. We distinguish

between two versions of this grand-scale theory and explain what the universe would have to

be like for one or both versions to be true. Spoiler: the question is in fact wide open—at the

present stage of science, nobody knows whether it’s true or false that the whole universe is a

computer. But the issues are as fascinating as they are important, so it’s certainly worth while

discussing them. We begin right at the beginning: what exactly is a computer?

what is a computer?

To start with the obvious, your laptop is a computer. But there are also computers very different

from your laptop—tiny embedded computers inside watches, and giant networked supercom-

puters like China’s Tianhe-2, for example. So what feature do all computers have in common?

What is it that makes them all computers?

Colossus was a computer, even though (as explained in Chapter 14) it did not make use of

stored programs and could do very few of the things that a modern laptop can do (not even long

multiplication). Turing’s ACE (see Chapters 21 and 22) was a computer, even though its design

was unlike that of a laptop; for example, the ACE had no central processing unit (CPU), and

moreover it stored its data and programs in the form of ‘pings’ of supersonic sound travelling

along tubes of liquid. Turing’s artificial neural nets were also computers (Chapter 29), and so are

the modern brain-mimicking ‘connectionist’ networks that Turing anticipated. In connection-

ist networks—as in a human brain, but unlike a laptop—there is no separation between memory

and processing, and the very same ‘hardware’ that does the processing (the neurons and their