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Cover of The Science of Can and Can't
The Science of Can and Can't
A Physicist's Journey Through the Land of Counterfactuals
Chiara Marletto
Read: 2022
My Rating: 5/10
In The Science of Can and Can't, physicist Chiara Marletto proposes a radical new way to look at the universe: through the lens of counterfactuals.

3 min read

The Science of Can and Can’t: A Physicist’s Journey Through the Land of Counterfactuals

Overview

Chiara Marletto introduces “constructor theory,” a framework developed with David Deutsch that reformulates the laws of physics in terms of what transformations are possible, what are impossible, and why — rather than predicting specific trajectories of particles. The book argues that traditional dynamical laws (equations of motion) are incomplete because they miss deep regularities expressible only as counterfactual statements about which tasks a physical system can or cannot perform. Marletto shows how this shift in perspective can unify information, thermodynamics, and even biological evolution under a single explanatory umbrella.

Key Concepts

Constructor Theory — A New Mode of Explanation

  • Tasks, not trajectories — instead of asking “what will happen next?”, constructor theory asks “which transformations can be performed reliably and which are forbidden?” A “constructor” is any entity (enzyme, catalyst, computer) that can cause a transformation and remain able to do so again
  • Counterfactual properties as fundamental — statements like “it is possible to copy this information” or “it is impossible to build a perpetual-motion machine” are elevated from secondary observations to primary laws of nature
  • Subsidiary theories — constructor theory does not replace quantum mechanics or general relativity but sits above them, imposing meta-constraints that any dynamical theory must satisfy

Information as a Physical Quantity

  • Information media — a physical system counts as an information medium if and only if it can be in at least two distinguishable states and a constructor exists that can switch between them — a counterfactual definition that makes no reference to observers or meaning
  • Copying and interoperability — constructor theory distinguishes between information that can be copied (classical) and information that cannot (quantum), grounding the no-cloning theorem of quantum mechanics in deeper principles
  • Knowledge as a special kind of information — Marletto and Deutsch define knowledge as information that, once instantiated, tends to cause its own preservation and replication — a bridge toward explaining biological and cultural evolution

Thermodynamics Recast

  • Irreversibility from counterfactuals — the second law of thermodynamics is re-expressed as: certain macroscopic transformations are possible in one direction but impossible in reverse, without needing statistical-mechanical assumptions about initial conditions
  • Work and heat distinguished rigorously — constructor theory provides exact definitions of work (a transformation that can be reversed) and heat (one that cannot), resolving long-standing ambiguities in classical thermodynamics

Implications Beyond Physics

  • Biology and evolution — living organisms are approximate constructors: they perform reliable transformations (metabolism, reproduction) and can be understood as entities that embody counterfactual knowledge about their environment
  • Computation and AI — by clarifying what computations are physically possible and what are not, constructor theory may sharpen the boundaries of computability and inform the design of future quantum computers

Personal Reflection

[To be added]

  • The Information - Gleick traces information theory historically; Marletto redefines it from first principles
  • Transformer - Lane’s thermodynamic account of life connects to Marletto’s counterfactual reformulation of irreversibility
  • Life on the Edge - Both push physics into biology — quantum mechanics vs constructor theory

Parent: Books

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