Impossible or Fundamentally Impossible

Here is Scott Aaronson's main argument for quantum computing:

I notice that you never responded about a fault-tolerant QC running Shor’s algorithm. Do you believe that that’s fundamentally possible, or not? If not, what physical principle is going to come in and prevent it? Will you agree that the discovery of that principle would be a revolution in physics?

He draws a distinction between what is impossible, and what is fundamentally impossible. I am not sure the distinction makes any sense.

In other words, QC might be impossible, but it is not fundamentally impossible unless some law of physics forbids it. We have not found that law of physics. The Extended Church-Turing Thesis forbids it, but he says it is not fundamental and it "is still on thin ice."

A response:

From an engineering point of view, there are often unforeseen limitations emerging from complex interactions of different domains such as physics of materials, chemistry, thermodynamics, mechanics, economics, etc. Those different knowledge fields are themselves at a much higher conceptual level compared to the underlying basic physics they all share, so their own laws/heuristics only hold in specific domains with specific assumptions, and all those various models (often highly non linear) just don’t overlap.

There’s nothing in the basic laws of physics explicitly saying that you can’t build a stable stack of quarters from here all the way up to the edge of space.

But do you believe it can be done? Given an existing stack of quarters, it’s trivial to just add one more quarter to it, and then by recursion assume the stack can be arbitrarily high. But that’s not how system scalability works in practice: at some point, what works for 100 quarters won’t work for 1000 quarters, because new problems are introduced: e.g. the wind will screw things up, and if you build your stack inside a tube with a vacuum, you’re now facing another totally different engineering challenge (create a 100 mile-high tube that can contain a vacuum). And, even without air, you’d have to deal with the effects of tides, plate tectonics, strength limitations in alloys, etc.

There’s also no specific law of physics telling us whether building room temperature super-conductors is impossible.

Same about building a stealth bomber that can travel faster than mach 5 at sea level.

It also goes the other way: a hundred years ago, it would have seem impossible (given the technology of the day, but given pretty much the same laws of physics) to build a gravitational wave detector that could measure changes in distance around 1/10,000th of the diameter of a proton, between two mirrors separated by 4km.

So, for the vast majority of hard engineering problems (and building a QC *is* a hard engineering problem), the fact that there’s no clear black and white basic principle saying it’s impossible isn’t really helping much at all. It wouldn’t be the first time we set up to build something, and then it never happens because various requirements just can’t be met within the same system (often it’s quietly killed because money runs out and people move on to other things because some new engineering progress makes an entirely different problem more exciting to work on).

So maybe QC is like that gravitational wave detector. It seemed impossible for a long time, until some huge technological advances were made.

In the same thread, Aaronson ridicules the idea that Einstein might have thought that ER=EPR. He did not even believe in either wormholes or quantum mechanics. It is not clear today if anyone really believes this wormhole entanglement nonsense. Somehow Einstein did inspire a lot of bogus physics thinking.

Sabine Hossenfelder has a new video on Quantum Uncertainty Simply Explained. She correctly describe the uncertainty principle as an essential part of quantum mechanics, but also explains that all waves obey an uncertainty principle. The uncertainty principle is a way of saying that electrons have wave-like behavior.

Update: Quanta has an article by Robbert Dijkgraaf saying There Are No Laws of Physics. There’s Only the Landscape. That is because he is a string theorist who studies mathematical abstractions that have nothing to do with reality.