“Constructor theory puts counterfactuals at the very foundation of physics”
Particle Fever (2013)
From Quanta Magazine:
At the heart of constructor theory is the feeling that there’s something missing in our usual approach to physics.
The standard laws of physics — such as quantum theory, general relativity, even Newton’s laws — are formulated in terms of trajectories of objects and what happens to them given some initial conditions. But there are some phenomena in nature that you can’t quite capture in terms of trajectories — phenomena like the physics of life or the physics of information. To capture those, you need counterfactuals.
The word “counterfactual” is used in various ways, but I mean a specific thing: A counterfactual is a statement about which transformations are possible and which are impossible in a physical system. A transformation is possible when you have a “constructor” that can perform a task and then retain the capacity to perform it again. In biology, we call that a catalyst, but more generally we can call it a constructor.
In the current approach to physics, some laws already have this counterfactual structure — the conservation of energy, for example, is the statement that it is impossible to have a perpetual motion machine.
So the constructor is the perpetual motion machine, and the counterfactual states that this transformation of usable energy to usable energy is not possible?
Yes. Counterfactuals do appear in existing laws, but these laws are regarded as second class. They are not incorporated wholeheartedly. Constructor theory puts counterfactuals at the very foundation of physics, so that the most fundamental laws can be formulated in these terms.
How would this work in practice?
For example, consider quantum gravity. Some people say: “Why do we even need to quantize gravity given that we don’t even have experimental evidence for it? We could have a classical theory of gravity and a quantum theory of everything else.” Well, constructor theory provides us with a robust and general theoretical foundation for an experimental test that would prove that gravity must be quantum.