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kith

Science Advisor

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## Main Question or Discussion Point

Work in quantum foundations is partly considered important because of the hope that the way we think about QM may point to a road to quantum gravity. Lucien Hardy, who is well-known in quantum foundations for his reformulation of QM in terms of five "reasonable" axioms, is one of the people who try to make this really tangible.

A couple of weeks ago, he put a new preprint with the title The Construction Interpretation: Conceptual Roads to Quantum Gravity on the arXiv.

This sounds ambitious, and it is. The main theme of the paper is to learn the right lessons from the conceptual development of GR which combined Newtonian gravity with special relativistic field theory and apply them in order to discover QG.

He notes that GR as the solution to the problems with Newtonian gravity (action at a distance) looked nothing like the efforts of Newton and his successors to solve these problems. And he argues that even if they did manage to solve the problem from the inside, they would have only gotten Newton-Cartan theory which isn't of much physical interest. So instead of trying to solve the problem of QG from within the paradigm of either QM or GR, he argues for a more radical starting point. (String theory, for example, operates under the quantum paradigm and attacks the problem by making gravity quantum (please correct me if I'm wrong))

For people who are interested in the interpretation of QM, Hardy has a rather sobering message:

Thoughts? It looks like a very rough sketch to me. But I can neither judge how sensible this research program is, nor how far it already got. In QM, he does cite quite a bit of research of which I wasn't aware.

A couple of weeks ago, he put a new preprint with the title The Construction Interpretation: Conceptual Roads to Quantum Gravity on the arXiv.

This sounds ambitious, and it is. The main theme of the paper is to learn the right lessons from the conceptual development of GR which combined Newtonian gravity with special relativistic field theory and apply them in order to discover QG.

He notes that GR as the solution to the problems with Newtonian gravity (action at a distance) looked nothing like the efforts of Newton and his successors to solve these problems. And he argues that even if they did manage to solve the problem from the inside, they would have only gotten Newton-Cartan theory which isn't of much physical interest. So instead of trying to solve the problem of QG from within the paradigm of either QM or GR, he argues for a more radical starting point. (String theory, for example, operates under the quantum paradigm and attacks the problem by making gravity quantum (please correct me if I'm wrong))

For people who are interested in the interpretation of QM, Hardy has a rather sobering message:

His suggestion is to chop the process of the discovery of GR into 7+3 distinct conceptual steps and perform analogous steps in order to find QG. His steps involve a quite instrumentalist and unusual way of looking at things which I can't digest easily. But I have only superficial knowledge of QFT and GR, so I can't follow the details anyway.Lucien Hardy (https://arxiv.org/abs/quant-ph/0101012 - p.7) said:If Quantum Gravity requires a radical departure from existing theories, the most interpretations of Quantum Theory can aspire to is to be the correct limiting version of Quantum Gravity just as the Newton-Cartan formalism is the correct limit of General Relativity. [...] Few physicists wish to dedicate themselves to merely providing amusement to future historians. The point of The Construction Interpretation is to take the noble instincts that lead us to attempt to understand Quantum Theory in conceptual terms and re-purpose them to the problem of constructing a theory of Quantum Gravity.

Thoughts? It looks like a very rough sketch to me. But I can neither judge how sensible this research program is, nor how far it already got. In QM, he does cite quite a bit of research of which I wasn't aware.

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