Philosopher Sam Baron (Melbourne) discusses. As always, I’m interested to hear what the philosophers of physics make of a bold claim like this.
Let me recommend Eleanor Knox’s essay on IAI a few months ago for what I think is a much more…
2 responses to “Spacetime does not exist”
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There are many places in the essay where I would get off the argumentative train, but here’s perhaps the first. I don’t think we need to have a notion of spacetime in place to get a notion of emergence off the ground.
Baron claims that emergence requires “arrangement” and “causal interaction,” both of which presuppose spacetime. But in quantum mechanics, the relevant space of possibilities one is working in is Hilbert space, and a Hilbert space can be perfectly well thought of as being composed of smaller Hilbert spaces — via the mathematical operation of a tensor product, which doesn’t necessarily require a notion of spatiotemporal arrangement. And in many Hilbert spaces, one can define (again using the machinery of quantum theories without necessarily appealing to spacetime) collective degrees of freedom whose dynamics are robust and autonomous, and hence deserve the title ‘emergent’ if anything does.
This happens all the time outside of the realm of quantum gravity. For instance, to make quantum computers error-tolerant, one defines what are called ‘logical qubits’ which are constituted from physical qubits. These physical qubits don’t have to be spatiotemporally distinct (they could, e.g., be energetically distinct, or distinct via angular momentum) and, what is much more important to the argument here: the relation that the logical qubit bears to the physical qubits need not be, and typically isn’t, one of spatiotemporal arrangement: rather, it is one of encoding in a subspace of the larger Hilbert space.
Unsurprisingly, then, the ideas of quantum error-correction have been taken up in quantum gravity as well, and there are proposals for how spacetime might emerge as a kind of quantum error-correcting code from more fundamental degrees of freedom. My aim here isn’t to defend those proposals but merely to point out that it’s false, both outside and within quantum gravity, that emergence must be thought of as something necessarily to do with spatiotemporality.
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Let me recommend Eleanor Knox’s essay on IAI a few months ago for what I think is a much more defensible take than Baron’s on how to think about spacetime in the context of quantum gravity.
https://iai.tv/articles/spacetime-is-an-idea-not-a-reality-auid-3281
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