The Physicist Who Asks What Can’t Happen
Most physics is written as a prediction. Here is the state of things now; here is the law; here is what happens next. It is a powerful way to write, and it has one blind spot that took a long time to notice. It has no natural way to talk about a seed.
A seed is not a prediction. It is a small object that can, under the right conditions, build a copy of the thing that built it, and that copy can do the same. Write that down as a set of initial conditions and a law of motion and you get a description of atoms moving. You do not get the fact that matters, which is that a coneflower seed is capable of a task, growing a coneflower, that a pebble the same size is not. The difference between the two is not in where their atoms are. It is in what they can be made to do.
Chiara Marletto is a theoretical physicist at Wolfson College, Oxford. She grew up in Turin, trained at the Polytechnic there, and came to Oxford to work in quantum information, the field that treats information as something physical, with its own laws, rather than as a description laid over physics from outside. With David Deutsch she is the principal author of constructor theory, which she has spent the last decade building into something more than a proposal.
The idea is this. Instead of stating physics as what will happen, state it as what can and cannot be made to happen, and why. A transformation that can be brought about, reliably and repeatedly, by something that survives the process is a possible task. One that cannot, whatever you build to attempt it, is impossible. The thing that brings a possible task about, and is still able to do so afterward, is a constructor. A catalyst is a constructor. So is a factory, an enzyme, a bird that eats seed. The laws of physics, on this view, are the complete list of which tasks are possible and which are not. Everything else follows.
Her first solo paper in the program, in 2015, took this straight to biology. The question was old: are self-reproduction and natural selection actually compatible with a physics that contains no design, or is there something in the living cell that the laws of motion cannot account for? Her answer was that they are compatible, but only if the laws permit one specific kind of thing to exist. That thing is information which, once present in matter, causes itself to be copied, and so remains present while everything around it is transformed. She called it knowledge. The word is deliberate and it carries no mind with it. A seed contains knowledge in exactly this sense. The coneflower does not know anything. The sequence in the seed is the kind of information that survives its own use.
This is why she belongs in a plant publication. The framework was built, in part, to make room for the seed, for the enzyme, for the copying that a living thing does and a crystal does not, without treating any of it as a special case outside physics. For most of its life the theory has been a way of stating things, and its critics have said so: elegant, but where is the experiment? In June, Marletto, Deutsch, and Vlatko Vedral published a review titled, plainly, “Tests of constructor theory.” It lays out what would falsify it. One test asks whether gravity can entangle two small masses, which it can only do if gravity is not a classical field. Another asks whether irreversibility, the arrow that makes a stalk blacken and never unblacken, can be recovered from laws that run equally well backward. The theory now names the results that would prove it wrong. That is the point at which a way of thinking becomes a science.
Marletto’s own line, from her 2021 book, is that the most important things about the world are the ones that never happen. The coneflower in front of the house is a list of what can happen, standing up in a row. Physics has been slow to read it.
• https://www.chiaramarletto.com/about/
• https://en.wikipedia.org/wiki/Chiara_Marletto
• https://arxiv.org/abs/2606.07352
• https://www.penguin.co.uk/books/306121/the-science-of-can-and-cant-by-marletto-chiara/9780141986463