Modifying a variable that is read by another thread needs special care in all multithreaded languages I know. If you don't use a synchronization primitive to tell the other thread "hey, this thing here may have changed, read it again from memory"; the other thread is free to use its old value for as long as it whishes.
It's completely counterintuitive, but that's how it works experimentally. For example:
Measure the spin of electrons in direction X (that's done with a weirdly shaped magnet). You'll get either +1 or -1 in a conveniently chosen unit system. That's already weird, you'll never measure +0.5 for example. Keep only the +1 electrons. You can measure them again and again, you'll always get +1 in the X direction.
Now measure those electrons in the Y direction, you'll also get either +1 or -1.
But now, if you measure in the X direction again, you'll get +1 or -1 with 50% probability; although before measuring in the Y direction, these electrons always had spin +1 in the X direction.
Oh I get that experiments prove out. What has always been unintuitive to me to the point of seeming wrong is that said spin is random and lives in a probability cloud that is probabilistic only until we observe it.
The math works and the experiments work, but it feels like a broken clock situation. It happens to correlate well, but the understanding and models are missing something in the actual underlying physics that, if understood, would throw the whole randomness part out the window.
> [the] spin is random and lives in a probability cloud that is probabilistic only until we observe it.
My mental model of it is kinda reversed. The maths assign a specific, deterministic state to each electron between each measurement. However, the "left" state is equal to "(up + down) / sqrt(2)". It then follows that when we measure an electron that is in the "left" state along the vertical direction, we get 50% up and 50% down: the probabilistic behavior happens when measuring, which "forces" an electron that was in the "left" state to become either "up" or "down".
I agree though that it would be less frustrating if there was some property of the electrons that would allow to predict in which state it'll collapse.
Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".
Models are simplifications/approximations made on purpose, as in "I know this is wrong on some level, but it's close enough for my use case", or "this makes the maths solvable".
As someone with a degree in physics and math, I find the line you're trying to draw rather blurry.
> Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".
Is it?
If by "mathematical exactness" you merely mean we have somehow been able to produce some numbers that later on we measured in experiments, then yes. (Though we haven't been able to produce such numbers in all cases, see e.g. [0].)
I my book, though, mathematical exactness means your model is mathematically consistent and is applied with the necessary mathematical rigor.
Quantum mechanics and quantum field theory, as they are commonly taught in universities, are everything but. They are a bunch of concepts and recipes that, albeit successful, are held together loosely with duct tape and lots of hand waving to make you ignore the logical gaps & jumps and the fact that many of the "mathematical" objects physicists refer to don't even exist[1]!
I’m not sure most physicists would make that specific distinction between "theory" and "model". We know QM is wrong or at the very least incomplete as long as there’s no theory of quantum gravity. Every theory, like every model, has a domain of applicability and gives wrong or even nonsensical answers when applied outside its domain.
What a theory is, is more like a model (or maybe meta-model) with a particularly broad scope or high explanatory value, often a mathematical framework that gives a common foundation and theoretical justification for multiple experimentally-derived models.
This is really a misunderstanding of what quantum mechanics is. It's not a particular theory. It's a category of theories that fit into a certain framework.
Quantum electrodynamics is a quantum theory. Quantum chromodynamics is a quantum theory. String theory is a quantum theory. I'm sure there is a correct quantum theory of gravity - we just don't know what it is yet.
This is an important distinction. Quantum mechanics as a framework is the correct description of reality as far as we can measure (excluding some loopholes like superdeterminism). This is as close to the foundation or "god" or whatever you want to call it as it gets. If you look at specific quantum model descriptions (like the Schrödinger equation), we know they are not complete. Even relativistically correct Dirac equation is still not the full picture, because we don't fully understand how gravity fits into this on short length scales. But the current principles like Bell's theorem exist independently with all their implications on our reality and they will not go away, no matter what else we discover at the bottom.
>>>Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".
... which "we think" is functioning that way not to mention that theory itself might be wrong and be replaced as most of the supporters of quantum theory doesn't seem to look on it as something that explain mechanics of it, but rather how magic of "unexplainable" quantum mechanics works.
Eh I would disagree on such a sharp distinction. Both quantum mechanics and general relativity are theories, and we know that both of them are not accurate descriptions at all scales.
Heck your descriptions of a model is also what we would call an effective theory (i.e. a description valid for a specific range of parameters/scale).
I don't think there's a commonly agreed difference between model and theory and I sure have heard many physicists use them interchangeably when it suits them (and I'm guilty of it myself). Perhaps theory feels 'wider', as in quantum theory encompasses models for many quantum systems, but that's as far as I'd go.
> [...] when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical, they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together.
Some countries solve that by just banning mail-in voting and moving paper ballots.
In France for example, voting happens on a specific day with transparent ballot boxes, with the counting happening in the polling station (you may stay for the whole day in your polling station if you want to oversee the whole process). Yes this is problematic for voters in foreign countries, because they need to go to the embassy to vote, but the voting protocols chose transparency over practicality.
IIRC, deployed military get a polling station in their military base. A voter may also designate someone in the same polling station as theirs to vote in their place.
By adding up the uncertainties of all intermediate measurements they made.
As a parallel, for a regular clock, you'd measure and compute the uncertainties of:
- The distance between the axis of rotation and the center of mass of the pendulum,
- The local acceleration of gravity.
And deduce the pendulum period and its uncertainty by calculus.
In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).
That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.
In places where it's been living for hundreds of years, that's a legitimate concern.
In Europe though, where aedes aegypti didn't exist 20 years ago, it's probably the other way around: what did the introduction of this new species damage? (apart from making the summer unbearable outside if any of your neighbour isn't very strict with standing water elimination)
I never used it in practice, so maybe the support isn't that good, but I was under the impression that it was possible to pass custom allocators basically everywhere in the STL. See for example the definition of a vector here:
haha yeah especially this summer! thanks for the feedback. yeah, the region/country aggregation can be a bit off. i'm still tuning the best parameter for each preset.
haha european summers are the best though. long sun hours festivals everywhere. from my travels, i learned the cold countries (well i guess not spain) have the best summer.
Modifying a variable that is read by another thread needs special care in all multithreaded languages I know. If you don't use a synchronization primitive to tell the other thread "hey, this thing here may have changed, read it again from memory"; the other thread is free to use its old value for as long as it whishes.
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