What does “reality” mean, in this context?

Is the universe "actually made of" a flexible substance that moves according to the equations of GR? In one sense, it doesn't matter. It acts like it does, and so well that we can make very precise predictions about what will happen.

Suppose a naysayer in 2010 said "Well, GR is a nice mathematical formalism, but mathematical formalism isn't reality. It's preposterous to think space is actually made of this mysterious flexible substance. You'll never see gravitational waves. It's a fiction of the math."

The naysayer has conflated the claim "GR is ontologically true" with the claim "GR makes accurate predictions." The first is irrelevant, and may be freely denied without casting meaningful doubt on the second, which has been well-tested for a century. It would be a great surprise to conduct an experiment and learn that GR mispredicted the result.

QM predicts QC. To doubt QC is to doubt that QM accurately predicts experiments we can conduct. In this case, once again, a century of experiments cuts the other direction. The failure of QC would be the surprise, not its success.

A very simple argument is that there's strong reasons to believe that energy is required to represent all information in the physical universe. You can't have "states" without mass/energy storing that state somewhere.

2^n is clearly super-linear in 'n', so as you scale to many particles, the equations suggest that you'd need a ludicrously huge state space, which requires a matching amount of energy to store. Clearly, this is not what happens, increasing the mass/energy of a system 10x doesn't result in 2^10 = 1024x as much mass/energy. You get 10x, plus or minus a correction for binding energy, GR, or whatever.

Quantum Computing is firmly based on pretending that this isn't how it is, that somehow you can squeeze 2^n bits of information out of a system with 'n' parts to it.

The ever increasing difficulties with noise, etc... indicate that no, there's no free lunch here, no matter how long we stand in the queue with an empty tray.

You simply do not need to believe this. The universe doesn't need to be "stored" somewhere.

> Quantum Computing is firmly based on pretending that this isn't how it is, that somehow you can squeeze 2^n bits of information out of a system with 'n' parts to it.

Quantum computing does not believe this. It is a theorem that you can only get n bits out of n qubits, and quantum computing speedups do not rely otherwise.

Noise is hard, but error correction is a mathematically sound response.

Information that can have any effect on anything physical must have an energy associated with it, because that's basically the definition of energy: the property of physical systems that can cause change over time. No energy, no change in state. These are practically axioms.

Information that has zero energy can have only zero effect on observables in (our) universe.

> It is a theorem that you can only get n bits out of n qubits, and quantum computing speedups do not rely otherwise.

I'm pretty sure you've misunderstood something somewhere, because the 2^n states represented by n qubits is mentioned in practically all QC materials.

The argument is metaphysical, it doesn't pose any concrete questions about the validity of QM, which would go like "does QM predicts reality correctly". All the physics is built upon a nice mathematical formalism of real numbers, and it doesn't make physics invalid, it can be used to build planes and computers, despite those pesky real numbers, that are unreal.

Intellectual responsibility means that you filled your metaphysics with some substance. I don't really know, how to do it for quantum mechanics, but if I wanted to, I might start with the inability of QM to explain gravity, I'd dig this topic to the point when I would be able to propose a specific way how research on quantum gravity could overturn QM and make it wrong. Or at least I'd try to make an argument that QM predictions about QC might become false.

But I cannot make such arguments, because I don't know QM, and I'm not going to dive into it, because I know better ways to spend my time, so I'll keep my mouth shut and will not voice vague statements about QM being not reality.

In any case from metaphysical standpoint, I'm sure that physics is not reality, physics is a mathematical description of reality. It doesn't matter if this description is incomplete or even wrong if it works for our case. Like Newton's gravity still works, while being proved wrong. We just need to bear in mind the limits of applicability. So I see the argument Scott Aaronson discusses as a very general truth "a map is not territory" which is used in an incorrect way. The correct (intellectually responsible) way is to point to the limits of applicability and to build an argument that they can bite.

> Asking because it's basically what I think[2]

I had followed your link after I wrote the first part, and I was delighted to see that you targeted the limits of applicability. Your argument is different from just saying "mathematical formalism is not reality".

I cannot verify your thoughts, because I don't know QM or how QC are supposed to work, but you are talking about the limits of applicability: "a theoretically-capable Quantum Computer will be testing the predictions of Quantum Mechanics to a degree of precision hundreds of orders of magnitude greater than any physics experiment to date". It doesn't seem to me as intellectual irresponsibility, it is not intellectual irresponsibility if you really know what you a talking about and can defend your statement when talking with PhD in QM.

> but I promise not to argue with any explanation given here.

Feel free to argue, I have nothing against it. :)

BTW, do you don't take QC seriously because of that? Do you expect QM (as a theory) to fail as a result or R&D work that is done on QC? I agree that there is a probability of QM failing, but my uneducated guess that the probability is low enough (maybe 0.1?) to take QC seriously.

> BTW, do you don't take QC seriously because of that?

Not as a threat to real-world cryptography, but I do take it seriously as a scientific endeavor.

> Do you expect QM (as a theory) to fail as a result

Yes, similar to how Newtonian Mechanics fails at high precision or in extreme circumstances.

Instead, what Aaronson refers to is probably more like people saying "planets moving in epicycles are a nice mathematical formalism, but mathematical formalism is not reality" — you know, the kind of thing you say when you want to doubt, but not take full intellectual responsibility for your doubts...

Reminds me of this way in which laser interference patterns can be used to find primes. But, like many analog computing situations, it requires a degree of sensitivity to signal vs noise that is essentially impossible.