When does a physical system compute?

Clare Horsman, Susan Stepney, Rob C. Wagner, Viv Kendon

Research output: Contribution to journalArticlepeer-review

95 Citations (Scopus)
18 Downloads (Pure)


Computing is a high-level process of a physical system. Recent interest in non-standard computing systems, including quantum and biological computers, has brought this physical basis of computing to the forefront. There has been, however, no consensus on how to tell if a given physical system is acting as a computer or not; leading to confusion over novel computational devices, and even claims that every physical event is a computation. In this paper, we introduce a formal framework that can be used to determine whether a physical system is performing a computation. We demonstrate how the abstract computational level interacts with the physical device level, in comparison with the use of mathematical models in experimental science. This powerful formulation allows a precise description of experiments, technology, computation and simulation, giving our central conclusion: physical computing is the use of a physical system to predict the outcome of an abstract evolution. We give conditions for computing, illustrated using a range of nonstandard computing scenarios. The framework also covers broader computing contexts, where there is no obvious human computer user. We introduce the notion of a 'computational entity', and its critical role in defining when computing is taking place in physical systems.
Original languageEnglish
Pages (from-to)1-25
Number of pages25
JournalProceedings of the Royal Society A : Mathematical, Physical and Engineering Sciences
Issue number2169
Publication statusPublished - 8 Sept 2014


  • computation
  • computer
  • physical computation
  • calculations
  • computers
  • mathematical models
  • biological computers
  • computational devices
  • computational entities
  • computational level
  • experimental science
  • formal framework
  • non-standard computing
  • physical computing
  • biology


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