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Quantum physics falls apart without imaginary numbers

scientificamerican.com

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Re: Quantum physics falls apart without imaginary numbers

#161

I've always had problems with how complex numbers are taught. The most common explanation is a geometric one, that of the "complex plane", that seems awfully analagous to any old 2D plane. But teachers never seem to explain why you'd have a complex plane in the first place, or when you'd use it instead of a regular plane, and you slowly realize that indeed, nobody's ever using it as a dimensional "plane" at all that'…

i is a number defined to have i² = -1. Everything else follows from that. The complex plane arises naturally as a useful representation of complex numbers, it is not the starting point.

I do not understand at all that stuff about "spiral path between otherwise discontinuous real numbers", but I suspect you may be making things more complicated than they need.

Re: Quantum physics falls apart without imaginary numbers

#162
post #138

Earlier quoted context omitted.

Its simple. Start with your circuit and add one ancila qubit. Then in your set of basis gates (universal for quantum computation), replace every phase shifting operation with a controlled X rotation targeted on that ancila. For example, lets just use the cliffords plus arbitrary phase rotation {X,Y,Z,H,R(theta)}. X,Z and H are all real. Y is real up to an irrelevant global phase (if you really want to implement it an…

I am super confused. Why can't I take this purely classical circuit and run it on a classical computer? Somewhere, there should be some blowup into exponential time?

The state space still grows like 2^n in the number of qubits. Again, all this mapping does is rename some variables from "imaginary number" to "degree of freedom on an imaginary system". But, the computer doesn't care what you call your vars.

Re: Quantum physics falls apart without imaginary numbers

#163

Earlier quoted context omitted.

> if it needs to be solved at all You're right - it's likely a made up issue due to human psych, not anything due to physics, thus it's weird you get hung up on it. > You seem to have missed the whole point (which comes after "yet" at the end of the quote) Ok, so you agree the first part of the quote is sufficiently incorrect? Let's invalidate the next part. Your initial claim was "A classical apparatus is part of th…

> They're pure math, and the agreement with nature Sure, as far as the math is concerned, it appears that there is no need for "classical objects." But physics is not (and has never been) "pure math," which is why Landau and Lifshitz, in particular, keep insisting on the importance of understanding the "physical principles" behind the axioms, whatever they are, and the facts of the theory; and so the "agreement with…

Your claim was "A classical apparatus is part of the QM framework."

I just pointed out it is not, and gave you axioms that define QM, and asked where the classical part is. You ignored, as expected.

>But physics is not (and has never been) "pure math,

This is not clear. I pointed out three pretty solid places that seem to disagree. In fact, a huge chunk of advances over the past 150 years came from where? Pure math. Electron oribital structure - forced by spherical harmonics (we didn't measure the orbits, then write equations - we looked for harmonics, the math game them all, then we experimented and found them all). Spin, forced by the math (relativity + qm had no solutions - to get them forced the representations to have a spin component, forced by pure math, then we found them). Positrons and anti-particles - forced by the math (the equation for electrons had two roots, pure math, at first people ignored the second root, then they decided to look, and found anti-matter). Relativity - forced by the math (the underlying Riemannian geometry is extremely forced - and the "pure math" of it led to a century of predictions, all from the pure math, that later became observed). Noether's theorem underlying all conservation principles - forced by the math (and it's an astounding, very general priciple, devoid of any physical content, but once applied, out pops all sorts of conservation laws, some we had not even observed, all forced by pure math). Quarks, Higgs Boson, nearly all of modern physics - forced by the math, not the other way around.

This list is incredibly extensive, far beyond what is here.

This is why physicists reached the "Unreasonable effectiveness of mathematics" and "It from Bit." More and more of physics is being recast as information theory and computation, since it is becoming more clear that a significant amount of what we call physics and nature is forced by math. Some, like Tegmark as I pointed out, go so far as to claim all math is physics and we just don't see some yet, because our inputs, timeframe, and objervations of multi-verse things are so small. So to claim physics is not math (another of your claims that is too hand wavy and short-sighted to believe) is not demonstrable - it's your belief, one that is slowly but surely giving way to realizing how intertwined they are.

The idea that we simply observe nature than add more terms to model it is long gone. What to look for as pointed to by math, not observation, has led to so many physical discoveries that it is why physicists (and mathematicians, and philosophers) deeply question why is math directing nature, not nature directing math.

Since I provided axioms for QM without your requirement of a "classical apparatus," and you cannot show where in those axioms your "classical apparatus" resides, I guess that part is done now too.

Re: Quantum physics falls apart without imaginary numbers

#164

Earlier quoted context omitted.

yes, specifically doing things without complex numbers breaks if you require composing systems to work like they do in normal quantum mechanics (composing with the tensor product). If you drop that assumption about how things compose then you can use something like the real matrix representation where 1 is a 2x2 identity matrix and i is some anti-symmetric 2x2 matrix.

Help a lapsed mathematician out here... the complex numbers can be represented as skew-symmetric 2x2 real matrices, right? And this is an isomorphism? So if the 2x2 real skew-symmetric matrices are "the same" as the complex numbers, how do we end up having this issue that comes up in quantum physics where you can only use the actual complex numbers and not the representation?

Any explanation I can give in a comment here will be pretty flawed, if you want more detail than my previous comment I think your best bet is to read the papers at the arxiv links I posted above.

Re: Quantum physics falls apart without imaginary numbers

#165

There is much more to the history of complex numbers, and that is also worth a read [0]. In particular, Gauss was very against the term "imaginary numbers" because it implies some mystery around them. I vaguely remember reading that he preferred the term "lateral" numbers, but that may be a mistake. Euler's formula connects them very plainly with rotations in a complex number plane. The intuition I developed with the…

Can complex numbers be seen as just a tuple of length 2, containing two real numbers?

Re: Quantum physics falls apart without imaginary numbers

#166

Earlier quoted context omitted.

> They're pure math, and the agreement with nature Sure, as far as the math is concerned, it appears that there is no need for "classical objects." But physics is not (and has never been) "pure math," which is why Landau and Lifshitz, in particular, keep insisting on the importance of understanding the "physical principles" behind the axioms, whatever they are, and the facts of the theory; and so the "agreement with…

Your claim was "A classical apparatus is part of the QM framework." I just pointed out it is not, and gave you axioms that define QM, and asked where the classical part is. You ignored, as expected. >But physics is not (and has never been) "pure math, This is not clear. I pointed out three pretty solid places that seem to disagree. In fact, a huge chunk of advances over the past 150 years came from where? Pure math.…

> This is not clear.

Of course it is. In physics, experiment (observation) remains the sole criterion of truth, no matter what math may be saying. (And I will continue to stress that observations, experiments, and measurements - all are inevitably "classical" by nature, and so in order to draw connections from those back to the underlying physical theory, the latter must include the notion of a classical measurement device.) Also, math is not physics because a mathematical theory includes a lot of scaffolding - things that do not correspond to anything real, and it's a physicist's job to tell the difference.

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