Edwin J. Beggs

Computational complexity with experiments as oracles

We discuss combining physical experiments with machine computations and introduce a form of analogue–digital (AD) Turing machine. We examine in detail a case study where an experimental procedure based on Newtonian kinematics is combined with a class of Turing machines. Three forms of AD machine are studied, in which physical parameters can be set exactly and approximately. Using non-uniform complexity theory, and some probability, we prove theorems that show that these machines can compute more than classical Turing machines.

Experimental computation of real numbers by Newtonian machines

Following a methodology we have proposed for analysing the nature of experimental computation, we prove that there is a three-dimensional Newtonian machine which given any point x∈[0, 1] can generate an infinite sequence [pn, qn], for n=1, 2, …, of rational number interval approximations, that converges to x as n→∞. The machine is a system for scattering and collecting particles. The theorem implies that every point x∈[0, 1] is computable by a simple Newtonian kinematic system that is bounded in space and mass and for which the calculation of the nth approximation of x takes place in O(n) time with O(n) energy. We describe variants of the scatter machine which explain why our machine is non-deterministic.

Embedding infinitely parallel computation in Newtonian kinematics

First, we reflect on computing sets and functions using measurements from experiments with a class of physical systems. We call this experimental computation. We outline a programme to analyse theoretically experimental computation in which a central problem is: Given a physical theory T, explore and classify the computational models that can be embedded in, and abstracted from, the physical systems specified by the physical theory T. We consider the embedding of arbitrary sets, functions, programs, and computers into designs for systems that can be specified in subtheories or fragments T of Newtonian kinematics in order to explore some of the physical assumptions of T that allows its systems to qualify as hyper-computers, i.e. physical models that compute sets and functions that cannot be computed in classical computability theory. In designing systems we work strictly within the chosen theory T and do not concern ourself with whether or not T is valid of the world today. We are interested in exploring the subtheory from a computational point of view and especially in restrictions on the assumptions of T that allow us to return from hyper-computation to classical computation. Secondly, we give a construction of an infinitely parallel machine that can decide all the arithmetical sets of natural numbers. We embed this hyper-computer as system in 3-dimensions obeying the laws of a fragment of Newtonian kinematics. In particular, the example shows that communication allowable in Newtonian kinematics is especially powerful. We conclude with further reflections and open problems.

Limits to measurement in experiments governed by algorithms

We pose the following question: If a physical experiment were to be completely controlled by an algorithm, what effect would the algorithm have on the physical measurements made possible by the experiment? In a programme to study the nature of computation possible by physical systems, and by algorithms coupled with physical systems, we have begun to analyse: (i) the algorithmic nature of experimental procedures; and (ii) the idea of using a physical experiment as an oracle to Turing Machines. To answer the question, we will extend our theory of experimental oracles so that we can use Turing machines to model the experimental procedures that govern the conduct of physical experiments. First, we specify an experiment that measures mass via collisions in Newtonian dynamics and examine its properties in preparation for its use as an oracle. We begin the classification of the computational power of polynomial time Turing machines with this experimental oracle using non-uniform complexity classes. Second, we show that modelling an experimenter and experimental procedure algorithmically imposes a limit on what can be measured using equipment. Indeed, the theorems suggest a new form of uncertainty principle for our knowledge of physical quantities measured in simple physical experiments. We argue that the results established here are representative of a huge class of experiments.

Computational complexity with experiments as oracles. II. Upper bounds

Earlier, to explore the idea of combining physical experiments with algorithms, we introduced a new form of analogue–digital (AD) Turing machine. We examined in detail a case study where an experimental procedure, based on Newtonian kinematics, is used as an oracle with classes of Turing machines. The physical cost of oracle calls was counted and three forms of AD queries were studied, in which physical parameters can be set exactly and approximately. Here, in this sequel, we complete the classification of the computational power of these AD Turing machines and determine precisely what they can compute, using non-uniform complexity classes and probabilities.

Nonassociative Riemannian geometry by twisting

Many quantum groups and quantum spaces of interest can be obtained by cochain (but not cocycle) twist from their corresponding classical object. This failure of the cocycle condition implies a hidden nonassociativity in the noncommutative geometry already known to be visible at the level of differential forms. We extend the cochain twist framework to connections and Riemannian structures and provide examples including twist of the S7 coordinate algebra to a nonassociative hyperbolic geometry in the same category as that of the octonions.

Quantization by cochain twists and nonassociative differentials

We show that several standard associative quantizations in mathematical physics can be expressed as cochain module-algebra twists in the spirit of Moyal products at least to O(ℏ3), but to achieve this we twist not by a 2-cocycle but by a 2-cochain. This implies a hidden nonassociativity not visible in the algebra itself but present in its deeper noncommutative differential geometry, a phenomenon first seen in our previous work on semiclassicalization of differential structures. The quantizations are induced by a classical group covariance and include enveloping algebras U(g) as quantizations of g∗, a Fedosov-type quantization of the sphere S2 under a Lorentz group covariance, the Mackey quantization of homogeneous spaces, and the standard quantum groups Cq[G]. We also consider the differential quantization of Rn for a given symplectic connection as part of our semiclassical analysis and we outline a proposal for the Dirac operator.

The impact of models of a physical oracle on computational power

Using physical experiments as oracles for algorithms, we can characterise the computational power of classes of physical systems. Here we show that two different physical models of the apparatus for a single experiment can have different computational power. The experiment is the scatter machine experiment (SME), which was first presented in Beggs and Tucker (2007b). Our first physical model contained a wedge with a sharp vertex that made the experiment non-deterministic with constant runtime. We showed that Turing machines with polynomial time and an oracle based on a sharp wedge computed the non-uniform complexity class P/poly. Here we reconsider the experiment with a refined physical model where the sharp vertex of the wedge is replaced by any suitable smooth curve with vertex at the same point. These smooth models of the experimental apparatus are deterministic. We show that no matter what shape is chosen for the apparatus: the time of detection of the scattered particles increases at least exponentially with the size of the query; and Turing machines with polynomial time and an oracle based on a smooth wedge compute the non-uniform complexity class P/log* ? P/poly. We discuss evidence that many experiments that measure quantities have exponential runtimes and a computational power of P/log*.

Computational Models of Measurement and Hempel’s Axiomatization

We have developed a mathematical theory about using physical experiments as oracles to Turing machines. We suppose that an experiment makes measurements according to a physical theory and that the queries to the oracle allow the Turing machine to read the value being measured bit by bit. Using this theory of physical oracles, an experimenter performing an experiment can be modelled as a Turing machine governing an oracle that is the experiment. We consider this computational model of physical measurement in terms of the theory of measurement of Hempel and Carnap (see Fundamentals of Concept, Formation in Empirical Science, vol 2, International Encylopedia of Unified Science, University of chicago press, 1952; Philosophical Foundations of Physics, Basic Book, New York, 1928). We note that once a physical quantity is given a real value, Hempel’s axioms of measurement involve undecidabilities. To solve this problem, we introduce time into Hempel’s axiomatization. Focussing on a dynamical experiment for measuring mass, as in Beggs et al. (Proc R Soc Ser A 464(2098): 2777–2801, 2009; 465(2105): 1453–1465; Technical Report; Accepted for presentation in Studia, Logica International conference on logic and the foundations of physics: space, time and quanta (Trends in Logic VI), Belgium, Brussels, 11–12 December 2008; Bull Euro Assoc Theor Comp. Sci 17: 137–151, 2009), we show that the computational model of measurement satisfies our generalization of Hempel’s axioms. Our analysis also explains undecidability in measurement and that quantities are not always measurable.

Solitons in the Chiral equation

The purpose of this paper is to give a geometric description of the solitons in the principal chiral equation in 1+1 dimensions in terms of Grassmannians, and a qualitative description of their behaviour in terms of Morse functions. Additionally it shows how a soliton can be “added” to an arbitrary solution of the chiral equation.