The Structure of the Multiverse David Deutsch Centre for Quantum Computation The Clarendon Laboratory University of Oxford, Oxford OX1 3PU, UK April 2001 Keywords: multiverse, parallel universes, quantum information, quantum computation, Heisenberg picture. The structure of the multiverse is determined by information flow. 1. Introduction The idea that quantum theory is a true description of physical reality led Everett (1957) and many subsequent investigators (e.g. DeWitt and Graham 1973, Deutsch 1985, 1997) to explain quantum-mechanical phenomena in terms of the simultaneous existence of parallel universes or histories. Similarly I and others have explained the power of quantum computation in terms of ‘quantum parallelism’ (many classical computations occurring in parallel). However, if reality – which in this context is called the multiverse – is indeed literally quantum-mechanical, then it must have a great deal more structure than merely a collection of entities each resembling the universe of classical physics. For one thing, elements of such a collection would indeed be ‘parallel’: they would have no effect on each other, and would therefore not exhibit quantum interference. For another, a ‘universe’ is a global construct – say, the whole of space and its contents at a given time – but since quantum interactions are local, it must in the first instance be local physical systems, such as qubits, measuring instruments and observers, that are split into multiple copies, and this multiplicity must propagate across the multiverse at subluminal speeds. And for another, the Hilbert space structure of quantum states provides an infinity of ways David Deutsch The Structure of the Multiverse of slicing up the multiverse into ‘universes’, each way corresponding to a choice of basis. This is reminiscent of the infinity of ways in which one can slice (‘foliate’) a spacetime into spacelike hypersurfaces in the general theory of relativity. Given such a foliation, the theory partitions physical quantities into those ‘within’ each of the hypersurfaces and those that relate hypersurfaces to each other. In this paper I shall sketch a somewhat analogous theory for a model of the multiverse. The quantum theory of computation is useful in this investigation because, as we shall see, the structure of the multiverse is determined by information flow, and the universality of computation ensures that by studying quantum computational networks it is possible to obtain results about information flow that must also hold for quantum systems in general. This approach was used by Deutsch and Hayden (2000) to analyse information flow in the presence of entanglement. In that analysis, as in this one, no quantitative definition of information is required; the following two qualitative properties suffice: · Property 1: A physical system S contains information about a parameter b if (though not necessarily only if) the probability of some outcome of some measurement on S alone depends on b. · Property 2: A physical system S contains no information about b if (and for present purposes we need not take a position about ‘only if’) there exists a complete description of S that is independent of b. I shall assume that an entity S qualifies as a ‘physical system’ if (but not necessarily only if) it is possible to store information in S and later to retrieve it. That is to say, it must be possible to cause S to satisfy the condition of Property 1 for containing information about some parameter b. It is implicit in this, and in Properties 1 and 2, that b must be capable of taking more than one possible value, so there must exist some suitable sense in which if S contained different information it would still be the 2 David Deutsch The Structure of the Multiverse same physical system. This condition raises interesting questions about the counter- factual nature of information which it will not be necessary to address here. It is also necessary that S be identifiable as the same system over time. This is particularly straightforward if S is causally autonomous – that is to say, if its evolution depends on nothing outside itself. 2. Classical computers Consider a classical reversible computational network containing N bits B B . A 1K N specification of the values b (t), , b (t) of the bits just after the t’th computational 1 K N step constitutes a complete description of the computational state of the network at that instant. Given the structure of the network (its gates, and how the carriers of the bits move between them), this also determines the computational state just after every other computational step. We are not interested in the network’s state during computational steps, nor in its non-computational degrees of freedom, because we know that the computational degrees of freedom at integer values of t form a causally autonomous system, and it is that system which we shall regard as faithfully modelling, with some finite but arbitrarily high degree of accuracy, the flow of information in a classical system or classical universe. Information flow in the network is local in the sense that if some information is confined to a set of bits C at time t, then at time t + 1 that information is confined to bits that have passed through the same gate as some member of C during the ( t +1) ‘th computational step. In particular, if a network consists of two or more sub- networks that are disconnected for a period, then information cannot flow from one of those sub-networks to another during that period. Where a system S has local dynamics – for instance, if it is a field governed by a differential equation of motion – and we want to draw conclusions about information flow in S by studying networks that model S to some degree of approximation, we must consider only models with 3 David Deutsch The Structure of the Multiverse the property that local regions of S correspond to local (in the above sense) regions of the network. If we were to construct such a network in the laboratory, then each of the 2 N possible bit-sequences b , , b would specify a physically and computationally 1 K N different state of the network. But if reality consisted of such a network, that would not necessarily be so, because there would then be no external labels, such as spatial location, to distinguish one bit from another. So, for instance, if the network consisted of two disjoint sub-networks with identical structures, containing bits B , , B and B , , B respectively, then any two bit-sequences of the form 1 K N 2 N 2+1 K N b , ,b and b , ,b ,b , ,b would refer to the same physical state. The same 1 K N N 2+1 K N 1 K N 2 applies when we are considering a hypothetical network that models information flow in reality as a whole: if the structure of such network is invariant under some permutation P of its bits, then any two bit-sequences that are related by P refer to the same state of reality. Let us refer to a bit-sequence b (t), , b (t) collectively as b(t) (which can be thought 1 K N N- 1 of as the binary number 2 b (t)+ + 2b (t) + b (t) Î Z N ). During each N K 2 1 2 computational step, the values of the bits in the network change according to ( ) ( ) b t +1 = ft(b t ), (1) where each f is some invertible function from Z N to itself, which characterises the t 2 action of all the gates through which the bits pass during the (t+1)’th computational step. 4 David Deutsch The Structure of the Multiverse Fig. 1: History of a classical computation The course of such a computation with initial state b(0) = b is shown schematically in Fig. 1. The parts of the graph in the shaded regions (i.e. during computational steps), and the non-integer values of b, have no significance except to indicate that the motion of a real computer would interpolate smoothly between computational states. 3. Ensembles of classical computers Consider a collection of M classical networks of the kind described in Section 2, all with the same structure in terms of gates, but not necessarily all starting in the same initial state. One way of describing such a collection is as a single network consisting of M disconnected sub-networks. The network has NM bits B B , where B B 1K NM 1K N belong to the ‘first’ sub-network, B B to the ‘second’, and so on. But since the N+ 1K 2N structure of the network is invariant under any permutation of the sub-networks, we must regard any pair of bit-sequences of length NM that are related by such a permutation as referring to physically identical states. In other words, when such sub-networks are in identical states, they are fungible. The term is borrowed from law, where it refers to objects, such as banknotes, that are deemed identical for the purpose of meeting legal obligations. In physics we may define entities as fungible if they are not merely deemed identical but are identical, in the sense that although they can be present in a physical system in varying numbers or amounts, permuting them does not change the physical state of that system. 5 David Deutsch The Structure of the Multiverse Fungibility is not new to physics. Many physical entities, such as amounts of energy, are fungible even in classical physics: one can add a Joule of energy to a physical system, but one cannot later extract the same Joule. In quantum physics some material objects – bosons – are fungible too: it makes sense to ask how many identical photons there are in a cavity, and it makes sense to add one more of the same kind and then to remove one, but it does not make sense to ask whether the photon that has been removed is or is not the photon that was previously added (unless there was exactly one photon of that kind present).