Oxford, contemporary
Chiara Marletto
A theoretical physicist who works with Deutsch on constructor theory, the proposal that the laws of physics are at bottom statements about which transformations can be brought about and which cannot. Within it, knowledge gets an exact physical definition for the first time.
The view
Physics is usually written in one form. You specify the state of a system at some moment, apply the laws of motion, and get its state at any other moment. Everything is cast as what will happen, given what did.
Constructor theory, proposed by Deutsch in 2012 and developed since by Marletto, tries another form. Its basic statements are about tasks: transformations of physical systems from one specified condition to another. A task is possible if the laws of physics allow something to carry it out, to any desired accuracy, and keep the ability to do so again. That something is called a constructor. A catalyst is one, and so are a heat engine, a factory robot and a living cell. A task is impossible if the laws forbid any such thing.
Several deep principles of physics already have this form. Energy cannot be created. Nothing can be sent faster than light. No engine can turn heat wholly into work. Marletto's book The Science of Can and Can't (2021) argues that such statements about what could or could not be made to happen, which she calls counterfactuals, are the proper foundation, and that the laws of motion follow from them.
The argument
The new form allows physics to speak exactly about things that the old form can only describe loosely. Information is the first. In a paper of 2015, Deutsch and Marletto define an information medium as a physical system with states that can be copied and permuted in every way. Nothing in the definition refers to minds or meanings. They then show that quantum information is what you get when a system has states that cannot all be copied.
Knowledge comes next. Some information has a special property: once it is embodied in a suitable environment, it causes itself to stay embodied. A gene does this by building an organism that copies it. A useful idea does it by being worth remembering, teaching and writing down.
Knowledge is information that can act as a constructor: it can bring about transformations and keep the ability to do so, and that is why it lasts. After Marletto, The Science of Can and Can't
Marletto applied this to biology in her constructor theory of life (2015). The laws of physics contain no design for a bacterium or an eye. She argues that accurate self-reproduction is nonetheless possible under such laws, provided the recipe is held in digital form and copied separately from the body it builds. That is the logic of genes that biology in fact found.
The fourth button in the figure shows what follows for knowledge in general. Ore does not become an aircraft by itself, and no law of physics forbids it. Whether it happens depends on one thing: whether the knowledge of how to do it exists. In this picture knowledge is the factor that decides which of the physically possible transformations actually take place.
Where she meets Deutsch
Throughout. Marletto is Deutsch's collaborator and co-author, and her work is an extension of his. The definition of knowledge on the Deutsch page is the one given here. Constructor theory supplies what Popper's account lacked, a statement in physical terms of what objective knowledge is, and it gives precise form to Deutsch's principle that every transformation is either forbidden by the laws of nature or achievable with the right knowledge.
Where they part
There is no disagreement to report, so this section records what is still open. Constructor theory is itself a conjecture. It is a research programme and not yet an established part of physics, and most physicists have not adopted it. Its standing will depend on whether it yields new laws or new testable predictions that the usual formulation cannot. Marletto has worked on that directly. With Vlatko Vedral she proposed an experiment in which two masses become entangled through gravity alone, which would show that gravity cannot be a purely classical field.
The account of knowledge also leaves a gap, and neither author hides it. It says what knowledge is and what it does. It does not yet say how a mind creates it. That question is where section 6 of the Deutsch page takes over.
| Marletto | Deutsch | |
|---|---|---|
| Where does knowledge come from? | Conjecture and criticism in minds; variation and selection in genes. | Conjecture: guesses, improved by criticism and experiment. |
| What does justification do? | Nothing. The same answer as Deutsch. | Nothing. No theory is ever justified, and none needs to be. |
| Can knowledge grow without limit? | Yes. Whatever the laws of physics do not forbid, knowledge can bring about. | Yes. Problems are inevitable, and problems are soluble. |
| What is knowledge, physically? | Resilient information: it can act as a constructor and so remain embodied. | Information that, once embodied, tends to keep itself in existence. |
Further reading
- Chiara Marletto, The Science of Can and Can't: A Physicist's Journey Through the Land of Counterfactuals (Allen Lane, 2021).
- David Deutsch, "Constructor theory", Synthese 190 (2013).
- David Deutsch and Chiara Marletto, "Constructor theory of information", Proceedings of the Royal Society A 471 (2015).
- Chiara Marletto, "Constructor theory of life", Journal of the Royal Society Interface 12 (2015).
- constructortheory.org: the research group's own site, with papers and introductions.