I think these lectures are a good intro to QM. This statement feels very emotional though. Many-worlds just states there is no such thing as wave function collapse. The many worlds are a logical conclusion following. Copenhagen simply refuses to elaborate to when wave-function collapse happens, which to me is not a complete theory.
You could have some critique of many-worlds in relation to statistics, although I think many people have given quite satisfactory explanations.
Many worlds is just bare bones QM.. If any other theory is proposed, it should be falsifiable (like pilot wave theory for example). Doesn't mean it won't be difficult to measure.
That’s a more physically plausible interpretation than ever “tick” of the universe actually creates infinitely more universes because every fundamental particle is quantum and each one is constantly interacting under the wave function every plank second. And the infinite number of possible events that a single particle would experience is multiplied by the joint combination of all particles in the entire universe. It doesn’t seem plausible.
And on the ‘every tick creates more universes’: we are completely fine with superposition in nanoscopic systems. It’s just uneasy to follow that conclusion to the logical end. Just my two cents. I think just because the conclusion seems hard to grasp, the reasoning to get there is completely sound..
Just like Newton’s equations are accurate to describe gravity and motion but only because the full equations Einstein worked simplify to them at small speeds.
So similarly it would make more sense that quantum equations and relativity are two sides of the same coin and there’s some missing mathematical bridge where at small particle sizes it simplifies to quantum and at large particle scales gravity matters and we get Newton and then further relativity.
For example, "Many Worlds, the Born Rule, and Self-Locating Uncertainty" notices that the rule doesn't mean anything, but it is rational to behave like it does. It's not a very satisfying position, I'd say.
Any good scientific theory must create falsifiable hypotheses.
Otherwise it’s just a nice story.
For any behavior inside the world, this is good.
But for the world itself and what harbor's it, asking for falsifiability is like measuring the expansion of space using a ruler that inhabits the same space.
Even science, that works in terms of cause and affect, which implies space and time, cannot be used to describe space and time itself.
Vanilla "scientists" have been trying to do that, but that is a dead end.
Which exactly is what general relativity is centrally about and the reason why general relativity is defined in terms of intrinsic properties of the Einstein manifold describing the space of the universe. Intrinsic properties are, colloquially, indeed properties that can be measured "using a ruler that inhabits the same space".
Gauß' Theorema Egregium [1] is an early predecessor of intrinsic properties of general manifolds, and says that the curvature of a surface can indeed be "be determined entirely by measuring angles, distances and their rates of change on a surface, without reference to the particular manner in which the surface is embedded in the ambient 3-dimensional Euclidean space" ([1]).
[1] https://en.wikipedia.org/w/index.php?title=Theorema_Egregium...
In LIGO, we detect the far away distortions propagating through our rulers and the differential effect of those waves.
But Michelson-Morley failed because the ruler distorted synchronously with the signal we tried to measure — since we mistakenly believed EM and matter were different fields.
We do that, routinely: that’s how we do cosmology.
Basic Concepts Of Quantum Mechanics by L. V. Tarasov
https://archive.org/details/tarasov-basic-concepts-of-quantu...
The dialogues of the author engages with various personas and the reader that are interspersed as Prelude and Interludes are really enlightening.
For example:
Classical Physicist: Can one seriously speak about an object without having an idea of what it looks like? Isn’t it strange that we study, for example, the behaviour of an electron in a crystal while we don’ t even know what an electron actually is?
Author: I don’t agree that we don’t even know what an electron is. I have just indicated a number of precisely determined characteristics and properties of an electron. More detailed properties of microparticles in general and electrons in particular were considered in the preceding sections of the book (and will be considered in the following sections). In fact, we know quite a lot about the electron and know, in particular, about its behaviour in a crystal. This is evidenced by the large number of semiconducting devices fabricated and used by us in practice. As you will see, the absence of a “physically intuitive” model of the electron has in no way turned out to be a serious obstacle. We may even go a step further and state that an understanding of natural phenomena in which Planck’s constant plays an important role is possible only through a significant rejection of a graphic description. By the way, this idea was given by Heisenberg, in whose works much attention was given to questions of the use of physically intuitive methods.
If anyone is interested in self-studying quantum physics, QFT for the Gifted Amateur [1] is very well regarded.
It is much more shut up and calculate.
But I do believe that is atleast in part Feynmans point. The way we have explored QM for the past... 120 years (? Wow is it really that old!?) is you don't need to understand it, here are the rules and tools to carry out extraordinary calculation.
I believe the likes of Sean Caroll are trying to bring understanding back to physics and meet philosophy.
It doesn't. It's model. All models are wrong (including quantum mechanics and attention) but some are useful. You understand the domain of the model, the problems it can solve, the math behind the models and use the model to make predictions. If you are competent at linear algebra and functional analysis, you can do quantum mechanics.
If you make the error of believing that quantum mechanics is ontological god's honest truth of nature, you will end up with blunders like using it to explain consciousness like Prof Penrose. His math was impeccable, but he was making a category error.
> All models are wrong
That famous quote could use attribution: https://en.wikipedia.org/wiki/All_models_are_wrong
> If you make the error of believing that quantum mechanics is ontological god’s honest truth of nature
This is why science & experiment exists. The author holds no such belief, and the paper makes that very clear. That is what he actually meant by “I Don’t Understand Quantum Physics”. Jump to the very last section, “Is Quantum Theory Correct and Complete?” He’s pointing out the confusing and contradictory nature of the model so far, and saying that Quantum is incomplete and needs work. He is not making any category error.
It could, but doesn't need it.
That models are not direct truth is just logical. If there were such a thing that were indistinguishable from the direct truth, it would be the direct truth, not some other thing called a model.
This is why I've never understood the "revalatory" nature of Godel's incompleteness. Of course any axiomatic system must not contain all truths. Axiomatic systems are models and models can't contain all truth. If an axiomatic system contained all truth it would just be all truth rather than some other thing called an axiomatic system.
The only case in which Godel's argument surprises is the case in which one has a fundamental misunderstanding of what a model can and cannot be, which I suppose must have been more common in his time, but I was really confused when my highschool teacher went to great lengths to explain that a model cannot contain the whole truth because...of course it can't?
If obvious, can you prove it without reading Gödel’s proof or anyone else’s?
The important (2nd) part of Gödel’s proof is not that a model can’t contain all truth, it’s that a model cannot prove itself true, which is a far stronger statement than the intuition you describe. Of course, belief that your own intuition is independent of history is also highly questionable; you were raised and educated in an environment in which people were already aware of Gödel. It’s like saying the number zero is obvious (which it is now) despite the fact that it took thousands of years before it was used formally as a number. There was a time when it wasn’t obvious to the smartest people alive.
Note that formal axiomatic systems aren’t physical systems; what Gödel was talking about and what George Box was talking about are related but somewhat different concepts. It is obvious in physics that a parabolic ballistic trajectory model doesn’t account for air turbulence. It’s not at all obvious that all of math itself is guaranteed to produce contradictions and cannot prove itself, especially if you’re allowed to change math or use any system you want.
I also think you can be a realist without having to make the jump to ontological irreducibility. You can say that our latest theory is a better approximation than the previous one and the previous one was an approximation as well.
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".
> 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]!
[0]: https://www.scientificamerican.com/article/this-simple-exper...
[1]: Haag's Theorem, Aizenman–Fröhlich Triviality Theorem, etc.
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.
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.
... 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.
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.
Penrose has fallen into the same logical fallacy that many older, retired theoretical physicists have (notable einstein?).
Sure, but what if you don’t understand the model?
Quantum mechanics is a framework for how theories are built, not a specific theory, and it's really all or nothing. There's not much between the quantum and classical ways of viewing the world.
But it just turns out that there's a limit in which the quantum world looks approximately classical.
what about gravity? If we don't know the correct account of that, how can we know those are exactly correct?
...unless I have understood it otherwise, but quantum mechanics is still an approximation of things we simply are unable to measure individually - mostly because of speed and have to resort to guessing that approximation of that limited time period, which is still too long. It would be the same as trying to guess position of a ship during voyage across the ocean and taking the smallest measurable time period as 100 years, in which the ship has been already sunk out of existence, while smaller time period would have given us better picture and quantum mechanics that we are able to use has exactly that issue - we are unable to measure and record particles at smaller time intervals. At best we can see trajectories of particles long gone.
Also, this book is not agreeing with your statement, to me which sounds like Religion(because you have no explanation at all!) and should not be part of Science.
"The number of different wavelengths of electromagnetic radiation, N(λ)dλ, with wavelengths in a small interval between λ and λ + dλ, (dλ means a small interval in wavelength) can be shown to be proportional to 1/λ⁴."
doesn't make sense. It should not talk about a number of wavelengths, but the number of allowed standing-wave modes (number of eigenmodes that a mathematical solution to the electric field in the relevant situation has) in the small wavelength interval.
Perhaps we've got it backwards and quantum entanglement is what defines "locality" rather than mysteriously existing in the face of it.
Superdeterminism and retrocausality both allow for local hidden variables - it’s just that they don’t satisfy Bell’s assumptions.
Superdeterminism implies that things are fixed in advance in such a way that the things we "choose" to measure always looks as if there are no local hidden variables (e.g. Bell's inequality) even though there are.
My theory: The reason we perceive collapse as irreversible is because the computational complexity of reversing the equations is very high. Especially as the state converges towards an eigenvalue of the measurement operator, it becomes more difficult to reverse. This is similar to how a hash function is reversible in principle but not practice. My two cents.
Many worlds and such are just pseudo religious nonsense.
I don't have much to add on the article itself except that it's almost certainly worth reading.
- Richard Feynman
>The next reason that you might think you do not understand what I am telling you is, while I am describing to you how Nature works, you won't understand why Nature works that way. But you see, nobody understands that. I can't explain why Nature behaves in this peculiar way.
Feynman's own QED is a part of QFT, but this is no longer quantum mechanics. Special relativity and quantum physics are reconciled since then. Now, we only need to reconcile GR and QFT, but this leap was so far beyond our attempts to scale it. Yet, QED/QFT/SM can be "understood" as there are fewer obvious paradoxes that plagued QM.
>>>Physics steeped enough in quantum theory do understand QM enough to have meaningful discussions free (at times) of rigorous math.
That sounds a lot like gate keeping and has no actual backing - to understand QM you still need to understand Physics, as mechanics of both of them are explaining each other - the scale is the only difference.
Most of the times the formal version with equations agree with the hand waving, but sometimes you have nasty surprises and must learn to hand wave better.
That depends on how you define nature, world and universe. For me nature is defined by things that can be perceived by human senses.
Blokhinstev's ensemble interpretation, which I like, is unattractive to many.
Quantum Bayesianism (QBism): We need to eliminate the interpretational conundrums that have beset quantum theory. A quantum state should not be thought to offer a description or representation of physical reality – instead it represents the degrees of belief an agent has about the possible outcomes of measurements.
Situation: There are 13 major interpretations of quantum mechanics.
1) Disregard many worlds as a purely remaining mathematical outcome of bounded probability rather than discrete reality. (No 1 thing exists just 1ish things but ultimately that doesn’t mean there are many versions of any give 1)
2) just accept you can’t find the “stuff” of reality. You simply measure increasingly small probability domains which interact with other probability domains
3) time is the sequential working out of probability of anything existing and other things that might exist co defining themselves.
4) space is just the interaction of these probabilities working themselves out into a stable projection.
Just because at a quantum scale everything becomes potential doesn’t mean all potential has been manifested.
It baffles me that so many serious scientists don’t understand this basic tenet.
It's useful, not metaphysically real. (Hot take btw) This is called Instrumentalism. The belief that it's real is called Scientific Realism. However over and over again, science can't cover edge cases, and we find better fitting models. The Copernician revolution is a good example of this dichotomy.
2. Academia basically forces you to believe the doctrine or you fail the class. From philosophy to physics to sociology. If you read the philosopher survey, you will find trendy things being quite high. It's a religion in itself.
Bonus 3: Philosophical Pragmatism and Instrumentalism generally solves this, but is intellectually a dead end. There is a reason analytic philosophy continues despite gaping holes in linguistics (late Wittgenstein) that ruin it. But at the same time, best fits might be missing a new paradigm. I wish I could tell people to ask AI to explain Hegel to understand, but I think people need to read it themselves.
Tl;dr: quantum mechanics is the best we have. It's definitely wrong, but it's the most useful we have.
Yes, but if you can make a reliable experiment that disagree with the doctrine, it will change, even if everyone hates the idea. In particular https://en.wikipedia.org/wiki/Wu_experiment
If you read late Wittgenstein or Karl Popper, you will find that science is a useful simplification. Rorty is boring and not useful, but he kills any sort of Realism.
The 3 gigantic holes:
1. It's all language. (Wittgenstein)
2. The edges of systems do not connect with each other.
3. There are exceptions to everything.
I know that might seem a bit generic, but these are the major flaws in the philosophy of science.
I was a true believer too.
If you ever want to learn a ton about science, read Popper. He tortures himself, but he's also scientific meta.
I guess if/when we discover a theory of everything then we could claim "this is real/reality", but what would that even mean other than modelling reality/outcomes with 100% fidelity?
This is known as a "hidden variable theory". People will tell you that a local (things can only affect their immediate surroundings) version of the theory is impossible because it would be a violation of Bell's inequality, but there is a loophole that requires accepting the idea of "superdeterminism". This is the idea that where what measurement a scientist chooses to measure is predetermined and correlated with what the particle being measured is going to do.
So if a particle lands on a screen according to distribution p when going through a double slit, but with distribution p' when one of the slits is measured at, conventional QM says that the measurement "collapses the wave function". But a superdeterministic local hidden variable theory would imply that the choice of whether to measure at the slit or not was already set in stone at the same time that the particle "decided" which path to take to land on the screen, more or less.
Quantum physics makes predictions that suggest this isn't so. You can design experiments (e.g. A Bell inequality test) in which quantum physics predicts correlations that go beyond what would be possible with some hidden local information attached to the entangled particles that we just don't know about. Experiments don't prove theories to be correct. That's not how science works. However, experimental tests of Bell inequalities have pretty conclusively shown that local hidden variable theories are inadequate to explain what's going on.
Yes, it appears what Einstein called "spooky action at a distance" seems to propagate close to instantly. It's important to appreciate that you can't transmit information this way though. We can only make sense of the quantum correlations after communicating other information at classical speeds. Sometimes you see quantum entanglement devices allowing FTL communication in sci-fi, but that's pure horse hockey.
That all being said, I don't understand quantum physics either.
It sounds all superior and mysterious to quote Feynman here, but the fact is many people absolutely do understand quantum mechanics.
It isn't about being superior, mysterious, or clever. The following "nobody understands" type of quips and thought experiments are different ways of saying our intuitions don't help us understand it :
- "God does not play dice with the universe.", Albert Einstein
- "Shut up and calculate", N. David Mermin
- "Schrödinger's cat" being both alive and dead
- various interpretations/analogies explaining quantum physics leading to absurdities or contradictions
Yes, humanity has harnessed quantum mechanics with mathematic frameworks and repeatable experiments to make useful things... but that's a different layer of "understanding" than the deeper levels in the above quotes.
"Nobody understands what gravity is." ... Wait, if nobody "understands" gravity, how did we send people to the moon?!?
Because those are different usages of the word "understand".
Our monkey minds evolved to throw rocks at birds, and our metaphors are based on concepts like position, velocity, and temperature. We added first-level abstractions like energy and cause-and-effect, then built higher level abstractions on those basic concepts.
QM clearly doesn't work like that. It's probabilistic, possibly non-local, possibly subjective, possibly a holographic projection of some underlying reality that is nothing like this one, and its relationship with causality is slippery.
We have a mathematical UI for it, but that's not at all the same thing as understanding the machinery on the other side of the UI.
Many people of course, Feynman obviously included, understand the theory in the sense of knowing how to correctly apply it to make predictions.
Nobody takes this to mean that classical physics is generally incomprehensible.
Also billions of people understand how to operate mobile phones, but "understanding how they operate" would be rather stretch and even the biggest specialists of QM have understanding only on how to interpret end results in QM - at best and that is basically it.
Quite frankly, I'm confused about these bold statements. I assume there seems to be some knowledge background of what you are saying - it very much reminds me what stupid strawman arguments I had in high school with my schoolmates then as a part of learning process, but rather far from actual wisdom.
We can build black boxes and exploit the behavior of what we see.
But we can’t always go to deeper and deeper layers of understanding.
QM is not a black boxes we built but one could perhaps say it’s a black box that we see.
For QM the main issue is that we are barely able to see results of that black box and have to make guesses even about observable results, because what we see is result of particle long gone and the result has been a mess of paths that it took. And basically we are operating at identifying single particle, while clearly the path that particle took was defined by other particles and "gravity"? from interacting with other particles bumping it around. I'm even less knowledgeable when it comes to QM terminology, but I get impression that most of the commenters here simply are parroting things they have read or being taught without actually trying to understand and taking into account all the unresolved problems in QM, which is why so many of them sounds so confident is my guess. Because knowing limitations of my brain I would be not so confident of anything, only because of massive load of unresolved issues that QM has.
Anyway, I prefer LEDs as an example.
Ancient humans could build boats 5000 years ago and expand throughout the planet, but they didn't know about water being made up of hydrogen+oxygen, or the tides being caused by the moon, etc.
Their calculations served them on the layer they operated on.
Mostly because people around the mediterranean sea didn't have to care, tides being so small there. Yet the Greek had fairly advanced models of tide schedules.
Pretty sure peoples that were experiencing bigger tides and depended on it e.g for fishing figured out the relationship with the moon pretty quickly, whatever esoteric reason they may have plastered upon it.
The math does indeed work, and many experiments and practical uses seem to show that there is something to quantum broadly. But to throw determinism out the window and believe that quantum is fundamentally probabilistic, and that one can never really run scientific experiments because observing state collapses superposition, has always seemed asinine to me.
I could be completely wrong, but it's always seemed obvious to me that the need for probabilities in quantum is a clear indicator that something is missing in the theory. I'm never surprised when a person can't understand quantum as its described today.
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.
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.
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.
Unrelated, but I also wish scientists properly used the term "theory".
He also wrote another similar popular article on Relativity, I DO Understand Relativity - https://news.ycombinator.com/item?id=50220891
With article titles like these, i just had to share it on HN!
Somebody should work on the above two webpages to make them more readable with html+pdf formats. That would make these more accessible to the "interested layman".
The author also wrote a sequel; I DO Understand Relativity - https://news.ycombinator.com/item?id=50220891
Tl;dr spacetime does not exist as a global background where things are "placed", it is not ontic. "Things" are first, and spacetime is derived from interaction statistics of "things". This eliminates nearly all naive paradoxes like double slit etc. Now, these "things" are somewhat complicated and non-intuitive (e.g. there are no individual "particles", they are a limit of discrete distributions), but that's a good start.
Uh oh, some people have no sense of humor =\