A far-fetched idea that might compute
In the popular science fiction film The Matrix, reality as perceived by humans in the future is actually not real but a virtual, simulated world created by machines to subdue and control the human race. The film plays with the strange idea that we humans may have become an integral part of a purely fictional computed reality and that we exist inside a computer simulation.
Surprisingly, a number of serious scientists have recently suggested an even more far-fetched theory along the lines of The Matrix: they claim that everything in our universe is just the output of a giant digital computer - including us humans.
Such ideas and suggestions may be appropriate for Hollywood movies, but how can this be a serious scientific approach?
The supporting arguments for such counter-intuitive theories come from modern quantum physics. Dr Seth Lloyd, a professor at the Massachusetts Institute of Technology in Boston and one of the inventors of quantum computers, thinks of atoms as containers of bits of information. For example the charge of a particle may be positive or negative. This can be considered as equivalent to digital values like one or zero. Whenever two atoms interact, they may or may not change the value of their bits from one to zero or vice versa (for example change the positive to negative charge). According to quantum theory these properties only change in discrete quanta, not in fractions of quanta. Therefore, this changing of bits caused by the interaction of atomic or nuclear forces can be considered as a form of digital computation.
That covers the 'hardware' of the universe, the underlying particles that perform the computations. Is there an equivalent to 'software' as well in our universe? Yes, according to Lloyd, there is. The software or 'program' that runs the actual digital universe is simply the laws of physics dictating the way atoms interact with each other.
So far, so good, but what does the universe actually compute? The answer is again simple and straight forward: it is the universe itself. The universe is constantly computing each particle's new state and hence continually evolving according to the laws of physics and will do so until the end of time and along the way it is creating the expanding universe as we know it. The number of computational steps the universe has already executed/calculated since the big bang has been estimated to be approximately 10,120 steps in total so far; that would be - give or take - 1,020 to 1,030 steps/computations for each atom in the universe on average.
Stephen Wolfram, another quantum physicist and the famous inventor of the Mathematica program, goes even further with this idea. In a recent speech given at a major science conference earlier this year he conceptualised our universe to be basically just a specific example of actually a relatively simple type of computing device, a so-called cellular automata.
A cellular automata can easily be visualised as a plane with equidistant horizontal and vertical lines dividing the plane into squares like a chessboard. Each square can be either black or white. Depending on the colour of its neighbour cells, each cell will update its colour in discrete steps. For example, if the centre cell is white and all cells adjacent to it are black, then the centre cell may turn black as well. If there are clear rules like these that define the update of each cell given any combination of the colours of its neighbour cells, then a cellular automata is defined.
Despite their simplicity, it was proven that such cellular automata can compute anything that even the most powerful computer could ever compute. They are in a true sense universal machines. Cellular automata can also easily be generalised. The cells may assume more colours, not just black and white and the whole automaton may have three or many more dimensions. A three-dimensional cellular automata with multicoloured cells would look pretty much like a Rubik's cube.
Now how can Wolfram justify that our universe might be such a simple cellular automata? The answer again comes from quantum theory. According to quantum theory our world is built up from minute particles of finite size. Nowadays we know that the nucleus of each atom is made up of many even smaller particles (some 60 of them are known today). We don't know yet how small the smallest particles are as more particles may still be discovered, however the axiom of quantum theory is that this process has a limit and that all particles have a finite, minimum size. The size of the smallest particles kind of defines the nature and granularity of space in a sense. It doesn't make much sense to assume that space can be divided any further than the size of the smallest particles as there would be nothing left to further divide.
So space on the most minute scale (the so-called Planck scale) is considered to be chunky and concrete rather than smooth and infinitely subdividable.
Now, this links us back to the cellular automata theory. Just imagine the whole universe as being divided into discrete space chunks of this minimum Planck-scale size of the smallest particles. Each of these cells can be viewed as filled with a particle or no particle and, depending on the laws of physics, particles in neighbouring cells may influence each other and change one another's quantum state. This is even consistent with higher dimensional universes. Wolfram thinks of space as a loose 11-dimensional network of cells. So his cellular automata universe looks more like a higher dimensional foam of connected quantum cells. Who would have thought that relaxing in a nice foam bath watching The Matrix on your iPad could get you so close to the modern concepts of quantum physics?
Eberhard Schoneburg, a computer scientist, is the CEO of Artificial Life