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Quantum theory based on real numbers can be experimentally falsified

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Re: Quantum theory based on real numbers can be experimentally falsified

#41
post #3

Can we go further and ditch the reals, relying instead on rational numbers or even IEEE floats? After all, the computers that we use for predicting empirical results all run on integers.

Can you? Most likely. Should you? You’ll need to reprove more than a handful of theorems, and for what? What advantage does using rationals instead of reals get you? You might enjoy taking courses in real & complex analysis, the general purpose of which is to impart upon the receiver an understanding of why we’ve constructed those particular number systems and how despite the names they both describe things which are…

I enjoyed your comment and agree with all your well put points except one: on the rationals tie with reality.

Having implemented exact real computation to better understand reals, I think of a real number as a kind of machine that generates infinite streams. Operations on them instantiate new machines which query their real operands, computating until there's sufficient information to emit a next term of the stream. When the next term needs an infinite amount of information to decide what to spit out next, it results in an "unproductive" infinite loop.

Rational numbers are interesting, more realistic, because they always terminate. In the real world, measurement tolerances and physical limits means at some point having to extract a rational. When we work with reals we are really only working with rational approximations or symbols with associated properties and relations.

Reals are a powerful and elegant tool to rigorously reason about mathematical spaces and operations on algebraic objects but trying to work with them in reality in their exact form is a fun and visceral lesson on the nature of undecidability. It's hard to go two steps without tripping over a non-terminating loop (such as any operation that starts with an irrational and results in a rational or equality testing in general).

This is an observation on our interface with reality and not on its true nature, which may or may not admit reals (although my non-serious guess is that black holes form whenever you try to do something that requires a proper real number).

Re: Quantum theory based on real numbers can be experimentally falsified

#42
post #30

Note that it is trivial to split the real and imaginary parts into two separate real-numbers and write quantum mechanics that way with only real numbers. Instead of i you get a 90 degree rotation matrix, instead of individual numbers you get a 2-element vector, etc. Lacking "numbers" with the right arithmetic properties for other things in quantum mechanics, we indeed use matrices and vectors for other stuff all the…

This was my first thought on seeing the title. Complex numbers are just vectors with special behavior for some operations, right? I haven't read through the paper, but this statement from the abstract confuses me: > Here we investigate whether complex numbers are actually needed in the quantum formalism. We show this to be case by proving that real and complex Hilbert-space formulations of quantum theory make differe…

> just vectors with special behavior for some operations

> basically just a structure containing two real numbers and some modified behavior

I think the issue might be that you're brushing away what is central to their utility and interestingness? Yes, you can take the view that it's a 2D euclidean vector space but it's not just. It's a 2D commutative algebra over the reals, an algebraically closed field and its algebraic properties and the addition of a notion of a rotation operation to our concept of number is what's of central importance.

Re: Quantum theory based on real numbers can be experimentally falsified

#43
post #11
post #7

Earlier quoted context omitted.

You can describe it to within measurement error without any irrational numbers. And with fewer decimal places than you'd probably imagine. See https://www.jpl.nasa.gov/edu/news/2016/3/16/how-many-decimal... for more.

That often works, but not always, some systems generate sequences of operations which can be symbolically simplified, or equivalently could be exactly computed using reals, but which if computed using any finite precision will fail. A simple example would be solving for the position of a planet in orbit under simple Newtonian gravity, a sufficient number of revolutions latter. For any finite precision the number of o…

I believe that has to do with discretization of a math operation (derivative, differential equations) that's inherently continuous. There are algorithms (symplectic integrators? Verlet?) that are perfectly capable of computing orbits without adding any drift.

Re: Quantum theory based on real numbers can be experimentally falsified

#45
post #30

Note that it is trivial to split the real and imaginary parts into two separate real-numbers and write quantum mechanics that way with only real numbers. Instead of i you get a 90 degree rotation matrix, instead of individual numbers you get a 2-element vector, etc. Lacking "numbers" with the right arithmetic properties for other things in quantum mechanics, we indeed use matrices and vectors for other stuff all the…

This was my first thought on seeing the title. Complex numbers are just vectors with special behavior for some operations, right? I haven't read through the paper, but this statement from the abstract confuses me: > Here we investigate whether complex numbers are actually needed in the quantum formalism. We show this to be case by proving that real and complex Hilbert-space formulations of quantum theory make differe…

> Complex numbers are just vectors with special behavior for some operations, right?

Decades since I took Complex Analysis, but:

Not if you care about poles, zeros, residuals, cuts, conformal mappings, multiple layers of some sort overlaying the same point on the complex plane.

It goes way beyond "declaring two variables" vs. "declaring a struct containing two variables." Not just a representation issue.

Re: Quantum theory based on real numbers can be experimentally falsified

#46
post #30

Note that it is trivial to split the real and imaginary parts into two separate real-numbers and write quantum mechanics that way with only real numbers. Instead of i you get a 90 degree rotation matrix, instead of individual numbers you get a 2-element vector, etc. Lacking "numbers" with the right arithmetic properties for other things in quantum mechanics, we indeed use matrices and vectors for other stuff all the…

This was my first thought on seeing the title. Complex numbers are just vectors with special behavior for some operations, right? I haven't read through the paper, but this statement from the abstract confuses me: > Here we investigate whether complex numbers are actually needed in the quantum formalism. We show this to be case by proving that real and complex Hilbert-space formulations of quantum theory make differe…

"Quantum theory based on real numbers" means a specific thing -- quantum mechanics with real amplitudes (and real anything-else-that-would-follow-from-that). It doesn't mean just any way of representing quantum mechanics with real numbers. Of course you can represent quantum mechanics with real numbers, for the reason you say; but for that very reason, that isn't what anyone means by "quantum theory based on real numbers", because there's not much point in discussing trivial rephrasings like that!

Re: Quantum theory based on real numbers can be experimentally falsified

#47

Note that it is trivial to split the real and imaginary parts into two separate real-numbers and write quantum mechanics that way with only real numbers. Instead of i you get a 90 degree rotation matrix, instead of individual numbers you get a 2-element vector, etc. Lacking "numbers" with the right arithmetic properties for other things in quantum mechanics, we indeed use matrices and vectors for other stuff all the…

The article reads to me as reductionistic in the sense that provided QM could use real numbers only, it would not matter if aerodynamics or whatever at a higher, emergent level required them -- almost suggesting the other levels are not part of physics.

In the same sense I did not see reference to General Relativity in the article -- as another fundamental ground in physics besides QM. Never learned manifolds stuff for GR, so I don't know if complex numbers become naturally essential there.

Re: Quantum theory based on real numbers can be experimentally falsified

#48

Note that it is trivial to split the real and imaginary parts into two separate real-numbers and write quantum mechanics that way with only real numbers. Instead of i you get a 90 degree rotation matrix, instead of individual numbers you get a 2-element vector, etc. Lacking "numbers" with the right arithmetic properties for other things in quantum mechanics, we indeed use matrices and vectors for other stuff all the…

Agreed. So what you need is the 'complex structure' behind rather than just 'complex numbers'. Any form of representations (numbers, matrices, and so on) should correspond to a unique structure. The question why the complex structure emerges in quantum mechanics is more interesting.

Re: Quantum theory based on real numbers can be experimentally falsified

#49
post #30

Note that it is trivial to split the real and imaginary parts into two separate real-numbers and write quantum mechanics that way with only real numbers. Instead of i you get a 90 degree rotation matrix, instead of individual numbers you get a 2-element vector, etc. Lacking "numbers" with the right arithmetic properties for other things in quantum mechanics, we indeed use matrices and vectors for other stuff all the…

This was my first thought on seeing the title. Complex numbers are just vectors with special behavior for some operations, right? I haven't read through the paper, but this statement from the abstract confuses me: > Here we investigate whether complex numbers are actually needed in the quantum formalism. We show this to be case by proving that real and complex Hilbert-space formulations of quantum theory make differe…

So not having yet read through OP I am not terribly surprised that this is true and I can kind of give a quick sketch in terms of a QM game that I want everyone to know, called Betrayal.

The idea is that it's a collaborative game for three people, you are trying to work together to beat the rules of the game. Meanwhile the rules are trying to set you up so that one of the people betrays the other two. In 3 relativistically separated rooms (so they can’t communicate) they go, where they find a screen and buttons labeled 0 and 1. The screen displays a prompt, each teammate presses exactly one of the buttons once before time runs out, then the three numbers pressed get summed together into a number.

25% of the time we run a “control round,” everyone gets a prompt to make the sum of their numbers even, and they win if the sum is even. The easiest way is if everyone hits 0, 0+0+0 is even. But a team can also answer 0+1+1 or so and win. Otherwise we randomly choose one to be the traitor and send them the control prompt, to make the sum even. But we send the other two the prompt to make the sum odd! In this case the team will only win if their joint sum is odd.

Long story short, classical players of this game have a success probability bounded from above by 75%. This is the Bell inequality. But quantum capable players can walk in with a GHZ state,

    |+++> + |–––>,
which only collapses to even sums. If they have to do a control round they will all just measure this in the computational basis.

The more interesting thing, where i really matters, comes during the traitor rounds. Here you want to perform the phase rotation gate in the Hadamard basis,

    |+> → |+>,
    |–> → i |–>,
And any two of them can thereby switch the state to

    |+++> – |–––>,
a state which only has odd configurations. Quantum players can win 100% of the time. Over multiple independent trials you should be able to observe the inequality violations even if quantum coherence were to limit your success probability to 90%.

I suspect that the inequality here is something similar, quantum mechanics but you can only form real-coefficient superpositions, and therefore you cannot take the square root of a unitary transformation just by doing it for half the time, per Schrödinger.

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