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Oxford University Press eBooks

There goes the neighbourhood...

Frank Arntzenius

Published Jan 1, 2012• 3 citations
Publisher

Abstract

In this chapter it is argued that one cannot specify the state of the contents of a classical world, i.e. the state of matter and fields which is governed by classical physics, space-time point by space-time point, i.e. that classical worlds are not separable. But, it is argued, one can give such a state neighbourhood by neighbourhood, where these neighbourhoods can be as small as one likes. Moreover, it is argued, one can also give the geometric structure of classical space-times neighbourhood by neighbourhood. The upshot is that the structure of the content of classical worlds is less local than traditionally thought, while the geometrical structure of classical worlds is more local than traditionally thought.

DOI
10.1093/acprof:oso/9780199696604.003.0002

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In this chapter it is argued that one cannot specify the state of the contents of a classical world, i.e. the state of matter and fields which is governed by classical physics, space-time point by space-time point, i.e. that classical worlds are not separable. But, it is argued, one can give such a state neighbourhood by neighbourhood, where these neighbourhoods can be as small as one likes. Moreover, it is argued, one can also give the geometric structure of classical space-times neighbourhood by neighbourhood. The upshot is that the structure of the content of classical worlds is less local than traditionally thought, while the geometrical structure of classical worlds is more local than traditionally thought.
The surprising aspects of quantum information are due to two distinctly non-classical features of the quantum world: first, different quantum states need not be orthogonal and, second, quantum states may be entangled. Non-orthogonality leads to the blurring of classical distinctions. On the other hand, entanglement leads via non-locality to teleportation and other ``entanglement-assisted'' forms of communication that go beyond what is classically possible. In this article we attempt to understand these new possibilities via an analysis of the significance of entanglement for the basic physical concepts of a ``particle'' and a ``localized physical system''. Classical particles can be individuated on the basis of qualitative differences in their sets of properties. But in entangled states the ``particle labels'' of the quantum formalism usually do not pick out such sets of individuating particle properties. It is sometimes nevertheless possible to think in terms of individual particles, which may be localized; but we argue that in general the structure of quantum mechanics is at odds with such a particle interpretation. This finally leads us to the conclusion that quantum mechanics is best seen as not belonging to the category of space-time theories, in which physical quantities are functions on space-time points. The resulting picture of the quantum world is relevant for our understanding of the way in which quantum theory is non-local, and it sheds light on the novel aspects of quantum information.
This introductory chapter outlines the differences between classical physics and quantum physics by positing three imaginary worlds. World 1 is governed by the laws of classical Newtonian mechanics, World 2 by classical field physics, and World 3 by quantum physics. The point is that classical physics deals with well-defined, continuously-existing physical quantities, including trajectories, while quantum physics is fundamentally about physical quantities defined in terms of probability functions.
Classical physics is built upon the concept of classical information. The classical bits of information can be read, copied, multiplied, processed, stored or erased. Determinism is the main characteristic feature of classical physics. Determinism implies that the knowledge of the mathematical structure of the physical laws and the current state of a physical system is sufficient for calculating, at least in principle, the state of the system at any other future time. Maxwell's equations axiomatize neatly classical electrodynamics. Matter is made of massive elementary particles such as the proton, neutron and electron. Einstein's axioms imply that space and time are no longer independent but are interwoven into a single continuum known as spacetime. All irrational numbers are represented by infinite aperiodic decimal expansions. The collection of all rational and all irrational numbers forms the set of real numbers. A convenient way to geometrically represent the real numbers is to use points on the real line.
In a physical theory we typically describe a system in terms of its state . We specify the relevant physical quantities or variables and then use dynamical laws to find the time evolution of these variables to predict their values in the future. For example, in classical particle mechanics, the state of a system is specified by the positions and velocities (or, equivalently for us, the momenta) of all of the particles in the system (that is, r ( t ) and v ( t ) for each particle). Newton's second law, F = ma , then determines the time evolution of the variables, or of the state, of the system. For electrodynamics, the state variables are the electric and magnetic fields, E and B , and Maxwell's equations govern the time evolution of these. In classical physics (including relativity here), the state variables that are the central entities of the dynamical equations (typically the positions and momenta of the particles) are also the directly observable physical quantities. That is, the state itself is specified in terms of the observables of the theory. The essential features of the classical world view (as modified by relativity) are that the present state of a system in principle determines its future state (as in the flight of a baseball under the influence of gravity) and that the agency responsible for this event-by-event causal structure must propagate from the cause to the effect at some rate no greater than the speed of light.
A physical picture of macroscopic and microscopic worlds is presented. In the space-time description, both the macroscopic and microscopic worlds are described simultaneously in space-time being a single boundless four dimensional continuum in which the motion of the observer is represented as a world line. In our picture, the space-time description is valid only for the macroscopic world and the microscopic world is local in the sense that it is described in a finite space associated with each point of the macroscopic space-time. In this case, space-time does not exist in the microscopic world in which the observer can not move. For the description of our picture, the language of the non-space-time description is necessary in which space-time is not presupposed. We can utilize the space-time description for the macroscopic world and, at the same time, we have a theory of finite degree of freedom describing creations and annihilations of elementary particles in the microscopic world free from the divergence difficulty, instead of the divergent quantum field theory in the space-time description which is considered in our picture a limit of an approximation like the ideal gas in gas theories. The finite degree of freedom is given by a one-particle state-vector space of a finite dimension from which the second quantization leads to a state-vector space of a countable dimension (a Hilbert space) for the theory of creations and annihilations of elementary particles describing a local microscopic world. The relation between microscopic worlds associated with different macroscopic space-time points is given by a dynamical principle up to a phase factor due to possible fluctuations of phases of local state-vector spaces. The experiment on interference of light performed recently by Franson and Potocki is interpreted in terms of fluctuation of the local phases and is considered to support the theory of finite degree of freedom which leads to the idea of local microscopic worlds.
We amalgamate three seemingly quite different fields of concepts and phenomena and argue that they actually represent closely related aspects of a more primordial space-time structure called by us wormhole spaces. Connes' framework of non-commutative topological spaces and ``points, speaking to each other'', a translocal web of (cor)relations, being hidden in the depth-structure of our macroscopic space-time and made visible by the application of a new geometric renormalisation process, and the apparent but difficult to understand translocal features of quantum theory. We argue that the conception of our space-time continuum as being basically an aggregate of structureless points is almost surely to poor and has to be extended and that the conceptual structure of quantum theory, in particular its translocal features like e.g. entanglement and complex superposition, are exactly a mesoscopic consequence of this microscopic wormhole structure. We emphasize the close connections with the ``small world phenomenon'' and rigorously show that the micro state of our space-time, viewed as a dynamical system, has to be critical in a scale free way as recently observed in other fields of network science. We then briefly indicate the mechanisms by which this non-local structure manages to appear in a seemingly local disguise on the surface level, thus invoking a certain Machian spirit.