Hacker News Reader

Comment by dang | Hacker News Reader

[stub for offtopicness]

If this is the definition of offtopic then 90% of HN comments are offtopic. https://en.wikipedia.org/wiki/Sturgeon%27s_law at work!

Hard to respond to this unless you say specifically why! Which comment do you think should not have been treated as offtopic?

In the context of a submission about “A century of QM” that says things like

“To celebrate 100 years of quantum mechanics, the CERN Courier looks back at the impact of this theory and examines how it keeps delivering new puzzles, experimental ideas and technologies.”

the following comment may be completely misguided but I didn’t find it offtopic (maybe offreality):

“This is probably the slowest branch of the sciences, not able to get out of labs even after a century.”

But now I see that the author says that he meant quantum computing so the comment makes even less sense and is indeed slightly offtopic.

There are degrees of offtopicness, of course. In this case I moved the subthread because it was a generic tangent that had little to do with the specific article.

Good on-topic subthreads usually show some sign of contact with the body of the article—not just the title, and certainly not the most generic phrase (in this case "quantum mechanics") that can be abstracted from the title.

But one can always argue particular cases either way and I agree that the counter-argument was stronger for that one.

This is probably the slowest branch of the sciences, not able to get out of labs even after a century. The fundamental problem appears to be that we are trying to control probabilistic nature using concrete real world things. I suspect this is not allowed, at least at an industrial scale. At some point humanity might need to stick what they need instead of what they can.

What do you mean? We have been profiting massively from the fruits of QM for the last five decades easily. Transistors, LEDs, Lasers, MRT imaging, solar panels, CCD cameras, etc. have arguably changed the world and would not have been possible without QM. It came out of the lab a long time ago.

Not to mention the entire science of chemistry and everything made from it.

Quantum mechanics started with the description of electron orbitals around an atom; how they work is the foundation of chemistry.

I think GP was thinking of quantum computers, maybe?

That’s engineering not science

Engineering is how science gets out of the lab...

I think you just wrote that 'off the cuff' but it's really a brilliant quote that succinctly clarifies science vs engineering.

How else does science get 'out of the lab'?

>not able to get out of labs even after a century

It got out of labs in a quite spectacular way in the summer of 1945, eighty years ago.

Thought experiment: did we really need quantum mechanics to build an atom bomb? Couldn't we have built one with a model of the atom based on classical particles (with protons leading to a chain reaction)? Is either the quantized or the uncertainty aspect of QM necessary for this?

The very idea of matter and energy being quantized into particles and photons is the starting point of “quantum” mechanics.

>Planck discovers the quantum nature of energy in 1900

https://www.pbs.org/wgbh/aso/databank/entries/dp00qu.html

It wasn’t developed into a mechanics until the 1920’s.

Nucleon orbitals rely a little on Pauli exclusion principle, which you need to add as an ad hoc hypothesis every time in classical physics.

Most of the science going into the Manhattan Project was experimental measurements and phenomenological models, not fundamental physics at the QM level. There were no usable quantum-mechanical models of nuclear physics at that point.

No; they didn't really need it.

It’s just a coincidence that they employed so many experts in quantum mechanics to do those nothing-to-do-with-quantum-mechanics experimental measurements.

Coincidence. If you wanted to do something with elementary particles, you couldn't possibly ignore quantum physics.

>did we really need quantum mechanics to build an atom bomb?

Yes. Nuclear reactions require understanding and modeling of the strong force, you can't understand or even see what protons and neutrons are without understanding the strong force. The mixture of positively charged and neutral particles being stuck together with enormous force which essentially does not exist at all outside of the nucleus of an atom. (there is more than three pounds of force between every pair of protons inside every nucleus with the strong force counteracting the electrostatic force)

You couldn't design a bomb without being able to model the strong force and you couldn't get to that point of investigating the atom without coming up with QM.

You couldn't get the idea of isotopes and enriching U-235 to U-238 or transmuting uranium to plutonium without understanding QM.

Or the circumstances that would lead someone to blindly creating a controlled nuclear reaction without coming up with QM in the process would be pretty absurd.

The idea for the bomb came from the understanding of the strong force. Step one: notice that there's a crazy powerful force keeping positively charged particles stuck together in the nucleus. Step two: the eureka moment of realizing you can "release" that force by causing a chain reaction of fission in heavy elements.

What has physics ever done for us? Apart from computers, satellites, planes, communications, sensors, and health... What has physics ever done for us?

The roads?

Well obviously the roads go without saying!

Nature is probabilistic. And we know how to calculate those probabilities, that is one of the core ideas of quantum mechanics. Why is it "not allowed"? By whom? Since you are commenting on HN, you are already are using the very mature applications of QM out of laboratory. The proverbial cat (is it Schröedinger's?) is out of the bag!

Theory is probabilistic, nature isn't.

I don't know dog, that is a pretty bold statement given everything we can presently firmly say about the universe.

Like we can imagine some kind of purely deterministic thing going on but when the rubber meets the road the best ways of working stuff out seem to very strongly imply some fundamental indeterminism. No one likes it, but thats the way it is.

When the rubber meets the road the best ways of working stuff out is to shut up and calculate, you don't figure out anything by assuming unobservable fundamental indeterminism.

No one assume it.

ok, postulate

No one does that either. The nondeterminism in QM is basically just an experimental fact which sort of inveigles itself into the structure of the mathematical framework which makes it hard to ignore when one tries to understand what the theory may mean philosophically.

I've been to phil. physics conferences and I've never encountered anyone who has any kind of strong attachment to nondeterminism. In fact, in general, I think almost every physicist who learns QM has a prejudice against it which never entirely goes away unless they get deep into foundations which forces a more detached perspective.

Not really, "it just is" isn't how facts work. Preferably it should be a quantity.

But it's the opposite, all people with shallow understanding of QM believe in indeterminism, because it's the first thing they learn from the start, and then this belief ossifies. Oh and Heisenberg uncertainty principle tells you that reality is unreal. Conversely foundations are deeply mathematical with basically nothing uncertain or random is sight. In fact it's foundations that have bias for mathematics and determinism. How do you interpret the evolution operator as random?

I don't really get your point - the time evolution operator is deterministic, but it describes the wave function, which itself gives a probability distribution for the outcome of measurements. Typically exegeses of QM are roughly given in two parts: deterministic time evolution of the wave function and random selection of the outcome of measurements. So it really is just right there in the formalism: some stuff is random.

From the point of view of foundations of physics there really is no easy way out of this apparently fundamental randomness. We can be wave-function substantivalists and then we must explain why and how we don't see wave functions but concrete outcomes. We can adopt t'Hooft's cellular automata interpretation but that framework cannot easily support even something as simple as basic interactions in QFT (last I checked). We may list any number of ways of interpreting QM but all of them that I know of only at most banish randomness to the initial conditions, but not totally eliminate it from the ontology.

I'm not saying that reality is fundamentally non-deterministic. But I am saying that most of the ways one squares basic quantum mechanical predictions with basic physical measurements suggest a type of uncertainty that is at least very close to being "fundamental."

Evolution operator shows that state evolves deterministically and doesn't hide randomness anywhere. You use probabilities when you want to explain human experience, because you can't practically compute human's state. But you don't strictly need fundamental randomness for this optimization, Bayesian probabilities work in the same way, whether randomness is fundamental or not is difficult to observe. That said, computational difficulties start in the measurement device, that's where you should do collapse if you don't want exponentially complex calculations.

Outcomes of observation are explained by linearity of evolution operator and decoherence, but then you get human factor: different people have different problems with this explanation, I think it's because unitary motion is unintuitive, Aristotle knew explanation of heliocentrism, but it didn't work, because geocentrism was more intuitive for him. Some apparently suggest that to explain human experience you need to calculate mind from physics, which needs to solve the hard problem of consciousness, which is impossible.

Nature doesn't obey your opinions.

That's an argument against solipsism at best, not much else.

Of course it is. What you are suggesting is classical determinism.

Slapping the "classical" label on it isn't quite correct. If it's not classical physics, then it's nonclassical determinism.

What a take. All of modern technology, materials, solid state, semiconductors, transistors... And uhhh did you forget chemistry itself?

Just came back from work to see all the battering and downvoting. Clam down. I meant Quantum computing using qubits. Apologies for not being specific. Quantum computing itself is also a century old (almost)? Where are we with this?

Jeez - HN is brutal. Even a bot could have understood the context I meant.

Quantum computing is very much not a century old

Reply on news.ycombinator.com