Inventors have been trying to build a perpetual motion machine for around a thousand years. Around 1150 the Indian astronomer Bhāskara II described a wheel with hollow spokes part-filled with mercury that he was sure would turn forever. It didn’t. And after him came a long parade of weighted wheels, self-filling flasks and closed-loop water mills, none of which worked. Energy is never created or destroyed, only moved around and converted from one form to another. A machine that kept doing useful work without drawing on a source would have to make energy from nothing, and so it cannot run.
In that answer, impossibility is a conclusion, something the laws hand down once you already have them. FQxI members Chiara Marletto, David Deutsch, Vlatko Vedral suspect it runs the other way as they have outlined in a new preprint on the arXiv. Constructor theory, the program Deutsch proposed at Oxford in 2012 and has built up since with Chiara Marletto, treats impossibilities not as conclusions but as starting points: law-like constraints in their own right, what the authors call “meta-laws,” which the ordinary equations have to obey. Most of physics, they argue, is written to tell you what happens next, and says almost nothing about what could never happen at all, even though some of our surest knowledge is exactly that. You can’t clone an unknown quantum state and you can’t signal faster than light (both examples discussed in their new paper). And you definitely can’t build Bhāskara’s wheel. Marletto wrote a whole book on the subject, aptly titled The Science of Can and Can’t.
For a while, the objection to this was that a physics of the impossible would itself be impossible to test. The new preprint from Marletto, Deutsch and Vlatko Vedral, “Tests of constructor theory,” is the answer to such an objection, pulling together the experiments meant to probe principles that, in their words, “supplement current dynamical laws, leading to new predictions for experimental tests.”
A few have already happened, as we’ve covered before on the podcast. At Italy’s national metrology institute in Turin, Fabrizio Piacentini and Laura Knoll built an optical experiment around a transformation that could be carried out reliably one way but not in reverse, even though the quantum operation behind it is perfectly reversible in time, which is just the sort of irreversibility constructor theory predicts. A more demanding test, a witness for whether gravity carries any non-classical features, has yet to be built.
Further reading:
“Tests of constructor theory,” Marletto, Deutsch & Vedral — the preprint: arXiv
“Reconstructing Physics” — the Turin photon experiment: QSpace
“Testing Time in Constructor Theory” — Piacentini and Knoll on the experiment: QSpace
“Constructor Theory and Irreversibility” — the earlier FQxI Podcast on the idea: QSpace
“Constructing a Theory of Life” — constructor theory and living things: QSpace
“The Universal Constructor” — David Deutsch on the FQxI Podcast: QSpace