Constructor theory is a proposal to extend quantum information theory beyond both quantum theory and computation, to cover more general machines than programmable computers – called constructors. It consists of newly conjectured physical principles that can be expressed as constraints on what tasks are possible, what are impossible, and why. These principles also determine the repertoire of the universal constructor, which is a programmable machine that can perform all physically possible tasks. The principles of constructor theory have novel physical content that supplements current dynamical laws, leading to new predictions for experimental tests. In this paper, we review the main experimental proposals to test the principles of constructor theory and discuss their implications for existing theories of physics and for their successors.

Constructor theory asserts that the laws of physics are expressible as specifications of which transformations of physical systems can or cannot be brought about with unbounded accuracy by devices capable of operating in a cycle ('constructors'). Hence, in particular, such specifications cannot refer to time. Thus, laws expressed in constructor-theoretic form automatically avoid the anomalous properties of time in traditional formulations of fundamental theories. But that raises the problem of how they can nevertheless give meaning to duration and dynamics, and thereby be compatible with traditionally formulated laws. Here we show how.

Everett's relative-state construction in quantum theory has never been satisfactorily expressed in the Heisenberg picture. What one might have expected to be a straightforward process was impeded by conceptual and technical problems that we solve here. The result is a construction which, unlike Everett's one in the Schrödinger picture, makes manifest the locality of Everettian multiplicity, its inherently approximative nature and its origin in certain kinds of entanglement and locally inaccessible information. (By Everettian, we are referring not only to Everett's own work, but also to versions of quantum theory that elaborate and refine his. The notion of relative states first appeared in Everett (Everett 1973 In The many worlds interpretation of quantum mechanics (eds BS DeWitt, N Graham)). We are proposing a formalism for relative states that is more detailed and more illuminating than Everett's.) Our construction also allows us to give a more precise definition of an Everett 'universe', under which it is fully quantum, not quasi-classical, and we compare the Everettian decomposition of a quantum state with the foliation of a space-time.

Claims that the standard methodology of scientific testing is inapplicable to Everettian quantum theory, and hence that the theory is untestable, are due to misconceptions about probability and about the logic of experimental testing. Refuting those claims by correcting those misconceptions leads to various simplifications, notably the elimination of everything probabilistic from fundamental physics (stochastic processes) and from the methodology of testing ('Bayesian' credences).

Probability is as much use for explaining how the world really works as the flat-Earth theory, says physicist David Deutsch

We propose a theory of information expressed solely in terms of which transformations of physical systems are possible and which are impossible-i.e. in constructor-theoretic terms. It includes conjectured, exact laws of physics expressing the regularities that allow information to be physically instantiated. Although these laws are directly about information, independently of the details of particular physical instantiations, information is not regarded as an a priori mathematical or logical concept, but as something whose nature and properties are determined by the laws of physics alone. This theory solves a problem at the foundations of existing information theory, namely that information and distinguishability are each defined in terms of the other. It also explains the relationship between classical and quantum information, and reveals the single, constructor-theoretic property underlying the most distinctive phenomena associated with the latter, including the lack of in-principle distinguishability of some states, the impossibility of cloning, the existence of pairs of variables that cannot simultaneously have sharp values, the fact that measurement processes can be both deterministic and unpredictable, the irreducible perturbation caused by measurement, and locally inaccessible information (as in entangled systems).

Quantum physicists David Deutsch and Chiara Marletto say information is key to understanding the universe. Their constructor theory puts it centre stage

The most fundamental theories in physics, provide a meaning for information or even a way of measuring it. And it has a counterfactual character: a message cannot carry information unless a different message is also possible. Statements about information were therefore long regarded in physics as second-class, non-fundamental approximations. Information itself was considered an a priori abstraction like Euclid's perfect triangles and circles, whose physical instantiations are inevitably approximate. Here, Deutsch and Marletto talk about the current theories of physics

Constructor theory seeks to express all fundamental scientific theories in terms of a dichotomy between possible and impossible physical transformations–those that can be caused to happen and those that cannot. This is a departure from the prevailing conception of fundamental physics which is to predict what will happen from initial conditions and laws of motion. Several converging motivations for expecting constructor theory to be a fundamental branch of physics are discussed. Some principles of the theory are suggested and its potential for solving various problems and achieving various unifications is explored. These include providing a theory of information underlying classical and quantum information; generalising the theory of computation to include all physical transformations; unifying formal statements of conservation laws with the stronger operational ones (such as the ruling-out of perpetual motion machines); expressing the principles of testability and of the computability of nature (currently deemed methodological and metaphysical respectively) as laws of physics; allowing exact statements of emergent laws (such as the second law of thermodynamics); and expressing certain apparently anthropocentric attributes such as knowledge in physical terms.

In a previous paper [arXiv:quant-ph/9906007] Hayden and I proved, using the Heisenberg picture, that quantum physics satisfies Einstein's criterion of locality. Wallace and Timpson have argued that certain transformations of the Heisenberg- picture description of a quantum system must be regarded as leaving invariant the factual situation being described, and that taking this into account reveals that Einstein's criterion is violated after all. Here I vindicate the proof and explain some misconceptions that have led to this and other criticisms of it.

The idea of a doppelganger (a double of a person) is a frequent theme of science fiction.

Fatty acid microspheres have been used for taste masking purposes whereby the drug is preferentially released in the lower gastrointestinal tract, although the mechanisms involved are poorly understood. In this study, we use a combination of surface pressure measurements, Brewster angle microscopy (BAM) and neutron reflectivity measurements to study the phase miscibility and escaping tendency from mixed stearic and palmitic acid films with a view to relating this to drug dissolution behaviour. It was noted that mixed systems showed considerably greater film interaction and instability than those composed of the pure lipid, especially in alkaline media. BAM studies were able to identify a range of phase separated structures for both the pure and mixed systems. Neutron reflectivity studies indicated a marked selective dissolution of palmitic acid into the subphase as a function of time and allowed quantification of the rate of dissolution of this species. It is concluded that the fatty acids are interacting within the monolayer and in addition the palmitic acid is escaping the mixed monolayers and dissolving into the alkali subphase. These findings have strong relevance for understanding the mechanism of drug release from the associated microspheres.

Douglas Hofstadter's writing talent makes his love of paradox contagious. Reading I Am a Strange Loop inclines one to see whimsical connections, language games and self-reference everywhere. Part of Hofstadter invades one's brain and starts thinking there in its own right – a phenomenon that is itself a theme of the book. Hofstadter, therefore, is in effect co-writing this review, inclining it towards paradox. Which may be why my method of urging you to read the book will itself be paradoxical: I shall summarize why I find it ultimately unconvincing.

In 1985, David Deutsch turned physics upside down by describing a universal quantum computer, pioneering the field of quantum information science. He explains to Amanda Gefter how this relates to notions of truth and reality in our universe – and even outside it

What are the three best popular-science books? My three favourites are Longitude by Dava Sobel, The Labyrinth of Time: Introducing the Universe by Michael Lockwood and The Neptune File: Planet Detectives and the Discovery of Worlds Unseen by Tom Standage.

Abstract Along with countless other people, I had been labouring under some significant misconceptions before I relearned the theory of evolution from Richard Dawkins’ book The Selfish Gene. Many of my own interests have concerned information flow— how information gets from one place to another and how it changes from one form into another. I did not always think of it in these terms at the time, but, for example, one of the fields in which I have worked is the ‘parallel-universes’ interpretation of quantum theory. It says that the universe that we see around us is part of a much larger structure, the ‘multiverse’, which contains many such universes, some like ours, some different. And I became convinced of this theory essentially by regarding the world as a system of information flow: if one analyses this flow under quantum theory, it turns out to consist of vast numbers of sub-flows that are nearly autonomous.