The famous EPR paradox paper [1] that Einstein co-authored remains one of my favourite "wrong" papers of all time.
This paper points out some of the most basic, paradigm shifting implications of entanglement and concludes, not that reality is truly this strange, but that the strangeness of these implications indicate the theory is incomplete. To be completely fair to Einstein, even if he had come up with Bell's inequalities at the time of writing this paper, they still would not have been possible to test with the technology of the day.
There is absolutely nothing to scoff at here. It may be wrong in it's conclusion, but the way in which it is wrong changed the course of physics. This is an example of how genius lies in asking the right questions even if you get the wrong answers. The EPR paradox remains a stunningly brilliant paper that should be closely ready by anyone in quantum physics.
Einstein was wrong, but in the most illuminating way possible!
I wouldn't say Einstein was wrong. It is a sad story that he never got to see the work of Nobel laureate Julian Schwinger on Quantum Field Theory, or the contributions of other Nobel laureates like Frank Wilczek. Switching from a particle-centric theory (like Quantum Mechanics) to a field-centric theory makes all the QM paradoxes disappear, and problems like locality, the double-slit experiment, etc., become trivial. What we have been calling particles are instead oscillators in fields. The Schrödinger equation is a wave function, but Quantum Mechanics has been using it to represent a probability distribution instead of an actual wave. Why is that? Because of the focus on particles. If everything has to be a particle, then of course we have to use a wave function as a probability distribution. But why force that view? We know how to describe fields since the days of Faraday and Maxwell, yet after Copenhagen all we want to do is to force wave-describing partial differential equations into a probabilistic model full of paradoxes.
What most physicists refer to as "particle" is very different from what lay people understand by that term. If you ask physicists at the LHC about particles, they will explain what I've already mentioned, because what I'm saying is far from being revolutionary.
You can count individual quanta of any kind (photons, electrons, etc.), and you can measure their quantum collapse. But that does not mean they are localized "particles" the way Dirac liked to think about them.
Of course you can do that, if we couldn't detect state collapses then they wouldn't be a staple of quantum mechanics. If you measure a rotating wave function over and over then it wont rotate since it will collapse into the same state again, while if you let it be it can rotate into another state giving you another measurement result.
This works since rotations aren't linear, small rotations are quadratic and hence will almost always result in the original state. You can also use this technique to rotate a state by making many measurements slowly changing the axis, so each measurement results in a small rotation.
Edit: But you are right that we can't see the history of state collapses, but they are definitely required for our current theories to work as you get the wrong experimental results without them in the theory.
>> Of course you can do that, if we couldn't detect state collapses then they wouldn't be a staple of quantum mechanics.
No. If you shoot pairs of entangled particles in opposite directions, someone receiving one stream of particles can take or not take measurements thereby collapsing or not collapsing the wave function of the particles going in the other direction. If you could tell the difference between a particle with a collapsed wave function and one without, this could be used for FLT communication. Bottom line is we can't tell if a wave function is "collapsed" or not. It's not a real event.
> if you let it be it can rotate into another state giving you another measurement result.
But then following your prior reasoning, that's just another collapse. So if the only way to measure is to collapse then pmkahler is right: there is no way to discern a collapsed wave function from a non collapsed one.
> But then following your prior reasoning, that's just another collapse.
But it isn't random, if we know how fast it rotates then the second time we measure it we can get a close to exact result.
The most famous experiment for this is the double slit experiment. Normally when you fire particles through you will get an interference pattern on the other side since the particles passes through like a wave. Measuring it in one will collapse the wavefunction and therefore destroying the inference pattern, so now the particles mostly just travels straight and creates a distribution of hits as if it passed through just a single slit.
Edit: Can look at this picture from wikipedia showing the difference between single and double slit, just measuring at one of the slits will even cause particles passing through the other slit to go much straighter.
(Disclaimer: I'm not a quantum physicist and the following is not the mainstream opinion)
Yet, it's just twisting fields together.
The right picture to have is fermions being something like knots on a rope, in a portion of space either you have a knot or you don't. But the knot can be more or less tight, and can be moving and have various shape.
When the fields are not coupled, i.e. when particles are far away, the only stable solutions, have a discrete quantity. These quantities are the conserved quantities that are preserved by the field evolution. Typically they are the quadratic values that the symplectic integrator conserve locally.
When particles get closer, they can exchange continuously some of the quantities between their fields, but as in a game of musical chair, as soon as the particles get away from each other, they must have taken a seat and settled in one of their discrete values.
QFT or quantum physics is like keeping track of the counts of the number of knots on the ropes and model the probabilities of how these values evolve upon collisions. But if you keep track of the rope shape (aka fields phases), you can more precisely predict where the knots are.
The catch-22 is that the rope shape is not observable, (in a similar fashion as you can't observe the seed of a random number generator), so you can't make better prediction using the rope model than you could with quantum mechanic.
But the "answer" to this catch-22, is that even though with rope mechanics you can't compute the probabilities any faster than QM would (as marginalization isn't fast), you can simulate in same compute complexity as classical system a universe that behaves according (convergence in law) to the probabilities of QM.
The case for particles is quantization. We've never seen half a photon or half an electron. This dates back to the ultraviolet catastrophe. If it was all about waves that would be easy; we reluctantly acknowledge particles because Nature has forced us to.
Quanta is fundamental to Quantum Field Theory so it can't be the deviding factor. I would say we are biased to think in terms of particles because our brains and senses have evolved to perceive macro objects as having a precise location and definite boundaries, thus we have a tendency to project that macro structure onto everything we want to describe.
But they still aren't waves. Waves can be split, quantum particlewaves can't. This is a fundamental difference and makes them neither waves nor particles.
It is equally wrong to view them as classical particles as it is to view them as classical waves. Classical particles with a probability wave is more accurate than both of those, but still not fully accurate. However I don't think there is any better likeness than that, to explain better you'd need to teach the math and equations behind quantum mechanics which usually takes years.
Can this probabilities field simply be the magnetic field? An electron has magnetic field that travels at the speed of light, while the electron is crawling behind, and by the time it enters one of the slits, its magnetic field has already formed the interference pattern that will guide the electron further.
Not all waves can be split, e.g. Solitons which are named because of their particle like nature and can be observed in optics, water chains of oscillators and more.
I think that is the crux of the issue, we have waves with a discrete energy, we can call them particles but they are very different from the traditional image that people have of what a particle is.
You can easily split solitons in water, just cut it in the middle and both sides will continue to live on as the water displacement field is still there.
On the other hand, try grab a part of an electron cloud around an atom and you will either get the entire electron or you will grab nothing. No matter what you do you can't separate one part of the cloud from the other, they are always connected. Trying to grab the electron will either remove all of the wave parts outside, or remove all of the wave part you try to grab. There is no classical system that behaves like this.
But quantum field theory doesn't replace probability distributions on particles with fields; it replaces them with probability distributions on fields.
“Switching from a particle-centric theory
(like Quantum Mechanics) to a field-centric theory
makes all the QM paradoxes disappear, and problems
like locality, the double-slit experiment, etc., become
trivial.“
I feel the same way. Would you know of any references that described the actual experiments seemingly revealing the paradoxes from quantum field theory perspective? Would appreciate it if you could share the references. Thanks!
I take it you're a MWI fan then? Isn't the answer for why we "force wave-describing partial differential equations into a probabilistic model" because in our reality, when we look at an electron we observe something that looks like a particle and not a wave?
I'm not a fan of the Many-Worlds Interpretation :-)
As for the electron, it is an oscillator described by a wave function, quantized, without locality. Here is an image of the wave function interpreted as a probability density:
The Quantum Mechanics interpretation is that the electron is a particle in an indeterminate location and the plot describes the probability of where the electron can be located. The Quantum Field Theory interpretation is that what we see is a field in an excited state, quantized. By looking at those plots, we can see a quantized field vibrating. If we send it through a double slit, it will behave like a wave. If instead we think about it as a single, indivisible particle, then we need to explain how it passes through two different slits at the same time. Thinking about it as a quantized oscillator disolves the paradox.
Makes sense -- but if you're saying "the electron travels through both slits at the same time because it is a wave", then why can't we detect that wave simultaneously at both slits?
At that point of measurement/detection we HAVE to start talking about probabilities, not just waves, right?
What does it mean for a wave to quantize? That is not something I (as a mathematician) are familiar with. It feels like something that bears a lot of explanation.
I would hazard a guess that the explanation makes it decently reasonable to call this process 'a particle'.
Certainly, to me it feels like saying 'it is just a wave' doesn't describe it because this quantization is a special thing.
I guess it means that the governing equation has a solution space which is somehow discrete. I would like to know if there's a more precise definition than that!
not a physicist, but afaik MWI doesn't work like that.
iirc, particles are actually excitations in a quantum field. the more particle-y an electron looks - the closer you bound its position - the more waves are needed to constructively/destructively interfere to make a peak there.
it's like a Fourier transform - if you want a perfect square wave you need infinite sine waves. in this analogy that's momentum space expanding out.
also, like, you're not really seeing individual electrons. you're seeing macroscopic phenomena, like your sensor or photomultiplier tube or whatever. you're seeing the interaction, not the particle. understanding that as your lab equipment, retina and brain entering the state space caused by resolving a wave to a spike makes more sense to me than some decoherence mechanism.
One of the big issues that both Einstein and Bohr had was that the experiments available to them at the time were only statistical--actually creating experiments with single quantum states or small quantum states was just not feasible.
For example, one of the arguments against Einstein's view was that Einstein's formulation implied that electrons in an excited atomic orbital would never decay. This, of course, is quite problematic as we know that electrons in atoms always decay.
Except that they don't!
When we put individual excited atoms in modern laser traps, we find that they don't drop from their excited state at the same rate. And the better the isolation, the slower that decay occurs. Whoops.
So, the problem is that these two greats were arguing with each other when a whole lot of the experimental evidence was still quite shaky and sometimes even wrong.
How certain are we at this point, that there are no hidden variables in quantum mechanics?
I'm asking because I read an article a while ago about some advances in higher-dimensional math that turned out to simplify certain quantum mechanical computations [1]. And it got my hopes up that we might eventually see quantum mechanics explained this way - e.g. as a projection/cross-section of higher dimensional phenomena into our 3D/4D reality. That the "weirdness" of QM could be explained by the effects happening in higher dimensional space. But from our perspective those would be hidden variables, so I'm curious whether we've been able to rule those out completely, or only partially?
I just want to correct some comments that say we are certain there are no local variables and that they have been ruled out experimentally.
This is false information and even Bell himself knew it:
"There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will."
Regarding unfalsifiable, from [0] "If engineers ever succeed in making such quantum computers, it seems to me that the CAT is falsified; no classical theory can explain quantum mechanics." By "such quantum computers" he means computers that can run Shor's algorithm. "...but factoring a number with millions of digits into its prime factors will not be possible – unless fundamentally improved classical algorithms turn out to exist."
Or right in the page I linked: "This makes it possible to construct a local hidden-variable theory that reproduces the predictions of quantum mechanics, for which a few toy models have been proposed."
From Wikipedia about superdeterminism:
"By postulating that all systems being measured are correlated with the choices of which measurements to make on them, the assumptions of the theorem are no longer fulfilled"
By assuming superdeterminism you can indeed construct a valid local hidden variable theory. But there is no way you can say superdeterminism is by itself a local theory, and no physicts will call it so. It's as far from "local" theory as possible, it's actually global.
So, what is supposed to be wrong with "the complete absence of free will"?
It reads like some sort of religious objection. If the data leads you there, you go there.
Sabine, particularly, goes there. Apparently there are no actual problems with giving up the illusion of "free will", whatever the hell it was supposed to mean in the first place.
Distaste seems pretty rich coming from people promoting MWI.
Chess is superdeterministic as all the moves are known in advance, but free will is still there. The "reality" might be such a superdeterministic playground that simply tells what the possible next moves are, while the players keep a pointer to one "chess" position and advance it one step at a time with their free will.
iirc local hidden variables have been ruled out experimentally. global hidden variables have not, and form the basis of Pilot Wave Theory, but pilot waves have a lot of unresolved issues, and there's no full theory that explains what MWI and Copenhagen do.
I think, ultimately, there are only 3 possible explanations for the paradoxes of the quantum world. 1) superdeterminism (everything including our choices in quantum experiments today were fully determined at the instant of the Big Bang), 2) something "outside" our observable reality acting as a global hidden variable (whether something like the bulk in brane cosmology or whatever is running the simulation in simulation theory) or 3) emergent spacetime (if space and time are emergent phenomena then locality and causation are not fundamental).
Until it's "Bell's Law", I think we should consider quantum mechanics a statistical approximation. Every time QM is questioned people point to Bell's Theorem and challenge anyone to disprove it, when it seems to me that it is QM which is making extraordinary claims that require extraordinary evidence. Every theory of physics has at one point been "the ultimate", and they have all been replaced by better theories as our ability to investigate has expanded.
The only claims QM make is about probabilities of measurements, and you can verify these experimentally, as we did, repeatedly. You are free to interpret these as many universes, pilot waves, or fairies playing tricks on scientists, suit yourself.
But we know for sure if it's fairies (or any other hidden variables) - they have to conspire globally. Cause otherways the experimental results couldn't happen the way they did cause fairies in place X wouldn't know which way to nudge the results to be consistent with the way fairies in place Y nudged them.
No. Local hidden variables can not explain experimental results of QM without implying something many people find distasteful. But no one has demonstrated any objective reason why that distaste has anything to do with the universe or its laws.
So, another way to say it is local hidden variables do explain experimental results of QM if you are willing to accept the implications, none of which contradict observation.
There is this essentially religious notion of "free will" that physical theories have been obliged to preserve, for no objective reason. Hidden variables are inconsistent with experiment only if you demand "free will" be preserved.
You call an interpretation you don't like "nonsense". But calling a thing nonsense is not a way to refute it. Relativity was nonsense, once. Can you devise an experiment that may produce results inconsistent with the interpretation?
That's the same argument every model has made for the history of science, but :shrug:. The value of a model is its predictive capability. Thinking any one to be absolute truth blinds us from progress.
This line of reasoning could probably be applied to any scientific debate between two greats. Both experts are trying to qualitatively describe how the current model coheres or disagrees with their internal models of reality. Since both have very good (but also very different) internal models, you get two accurate impressions of the bigger picture, however vague or incomplete.
It's like the parable of the five blind men who encounter an elephant.
They didn't though! They had access to the same body of work, but their individual biases, tastes, and intuitions would have skewed both which research they paid attention to, and how much credence they gave it.
This is essentially my point. We all have access to the same set of data, but we all look at it a little differently.
Something I never understood: if we take the Big Bang theory at face value, shouldn't we believe everything is entangled together? And thus is there any to think superdeterminism isn't the ultimate explanation?
Doesn't decoherence just mean entanglement with other particles increases as the particle interacts with its environment? I never understood it to mean that the "total entanglement" (loosely speaking) will decrease somehow. Rather, I thought it means that mutual entanglement between N particles starts involving N + 1 particles, N + 2, etc. until it grows large and N just becomes insignificant.
I believe the idea is that lots of 'random' entanglement effectively concentrates the probability distribution, which is what destroys super-positions. Effectively saying things are due to the central limit theorem.
Saying 'everything is entangled' might not be that meaningful if the entanglement is random. If you want to get to super-determinism then it takes some very weird 'knowledge' in the entanglement. Rather than the entaglement being random.
What I have had physics PHD friends explain to me is that the 'wave' evolves completely deterministically. The way they see it, measurement is just 'getting entangled with the wider world, thus concentrating the wave function'.
Almost everything we interact with has this concentrated wave function. And when you see something without this concentration the process of how it 'decoheres' is chaotic. It just turns out that the chance of concentrating in a spot is proportional to the wave self-conjugation.
I could imagine the idea 'almost everything is engangled' meaning the chaotic nature of this concentration process is essentially perfectly random. Since you'd be interacting with trillions of other waves.
Do I understand correctly that you're claiming it's "at least exponentially unlikely" (so to speak) for a fully-deterministic-since-the-Big-Bang model to result in a superdeterministic world? Or to rephrase, something along the lines of: "a vanishingly small set of evolution trajectories from a fully-deterministic-since-the-Big-Bang would result in a superdeterministic world that would explain ours"?
No, I wasn't talking about superdeterminism at all.
My point is that the evolution of the waveform is deterministic. Waveform collapse is where lots of theories introduce indeterminism. I propose a theory in which waveform collapse is technically deterministic.
My proposition is that the process is deterministic but chaotic. With the simultaneous state of all other particles determining how this collapse happens. Making it deterministic, but truly impossible to predict without a picture perfect representation of the outcome.
If I have entangled particles A and B, and then B interacts with and becomes entangled with particle C, can that interaction not qualify as an "observation" that collapses the wave function of A + B and de-entangles them?
Not as I understand it. I think of it as: A and B were correlated, and now you caused B and C to become correlated. That cannot possibly imply that A and C's correlation dropped to zero.
Or if you prefer to think of balls; if A hits B, and then B hits C, then C's future trajectory is still (forever) affected by what A was doing, as is the trajectory of anything else it hits.
That is classical systems and "classical" probability. In quantum systems particle functions collapses all the time making parts of it go to zero. At least as we understand quantum mechanics today, the math we have doesn't explain experimental results without those collapses.
From what I've read on Wikipedia [1], my guess would be that the answer is probably: "Since it's impossible to falsify, we should only be satisfied with it if science can eliminate literally all other possible falsifiable explanations." In which case, I guess if your goal is solely to "push the boundaries of science", then this is fair enough. But in that case, it seems to me the amount of effort and resources we could ostensibly pour into eliminating falsifiable explanations might well be unlimited, which raises the question of: at precisely what point do you cut your losses and realize you need to move on (to philosophy? idk), so to speak. Science is supposed to be a means to an end, after all, and I thought that end was "understanding the universe"—not "exploring the space of all falsifiable predictions just for the sake of it".
But I'm curious to hear physicists' takes on this! e.g., the implausibility claim in the Wikipedia article would seem like a compelling rebuttal, except that I don't see why I should believe superdeterminism to be an implausible explanation at all. If anything, it sounds like the most plausible explanation we have—either that, or the Big Bang somehow doesn't imply superdeterminism (how is that even possible?), or the Big Bang theory is bogus to begin with.
There have been several comments on particles vs waves, which is more real, possibilities of hidden variables, and similar.
People interested in those questions might like some of the more recent looks at QM that are based on information theory. Zeilinger has an approach based on the idea that various aspects of particles can only have one bit of information.
From this he's able to derive entanglement, the uncertainty principle, the Schrödinger equation, and more.
PBS Space Time did an episode on this just over a month ago [1].
Bohr really didn't offer anything to the debate other than some hand waving philosophical arguments about how you just can't ask anything about what is inside the box until you look.
Einstein, Podolsky and Rosen put the argument into mathematically solid and testable form. Einstein really 'wins' the argument in my mind either way it turns out because his name is attached to the actual experiment you can run to try to settle the question.
He also got the Nobel prize for the photoelectric effect and directly helped to invent quantum mechanics. The idea that there's this Einstein-vs-QM WWE event is sort of false. He'd probably be more or less happy with the way things turned out since quantum observation can't be used for superluminal communication. It turns out QM is nonlocal, but it still has rules.
If Einstein had lived to 143+ and was still around it might be interesting to ask where we'd be, since I don't think he would have accepted the just sort of shrug-and-accept-it-MWI approach to quantum observation. He'd probably be busy with something like twistor theory and trying to turn matter into tiny wormholes in space-time that can stretch and break and produce QM.
I'm still waiting to see what happens when someone dares to take a shot at describing the physical universe while eliminating 'time' from the equations.
Yeah, it can a useful ruler as a way to map changes; it's as real as a 'cubit', but not the territory. Energies may suffice. Things are constantly changing ... but only, and forever, Now. Without time, entanglement's no surprise.
Weinstein is the director of Thiel capital and holds a PhD. I do not think he is a crackpot. He is butthurt over being victimized in academia for various reasons, and he is not alone in having such experienced. He is using his voice to call them out at the same time Publishing his work.
Regarding the theory. It is is impossible to me to analyze his work, because it is impenetrable. What is a two-tensor, gauge theory, I for one have absolutely no idea. I think this is an Achilles heel of his approach, he needs to convey meaning better. Or maybe that's just how a theory of everything is articulated.
What bugs me somewhat with quantum mechanics is if we give up on localism, how can we rely on any of the science preceding quantum mechanics or even the science behind the experimental equipment, it is very possible I am missing something here
As a person who's been interested in physics but without any depth at all, this was a very interesting article and I'm kind of relieved that it's not settled.
Sadly it's not settled in large part because many physicists have stopped caring about more fundamental interpretations or explanations for QM. "The theory works, the math checks out, so we're done" -- and we're left without a really satisfactory explanation for why or how it works.
I have to go with Einstein on this debate, i.e. suspecting that there’s a deeper phenomena yet to be revealed. The failure to realize a Grand Unified Theory of Everything shows that there are still many unknowns leaving his theory as a possibility. Also, a simple thought experiment reveals that even with the Higgs field it’s nonsensical that there could be the existence of a fundamentally small particle because everything must be made of something smaller.
My takeaway from this article as a nonphysicist was that it’s satire.
I'm with Einstein on this one. The statistical math we use in quantum mechanics is not "real", it's an human invention to approximate something that we don't understand yet. Id est, a mathematical model that approximates, but does not describe the real truth about the physical universe.
do you mind explaining Bell's inequality? i looked up the wikipedia but it is very wordy and has a lot of equations; i need it stripped of Ponderousness as Scott Aaronson did in this piece
While it is not a really hard thing to understand, requiring only high school math, writing it out entirely here is going to be a bit laborious.
I can really recommend this book: "Dance of the Photons: From Einstein to Quantum Teleportation" by Anton Zeilinger. I read it when I was 16 and it's easy to read, but might shake to the foundations your understanding of the universe (as it did with mine). It clearly lays out why the position you take in your post is hard to maintain given some very simple experiments, even though your position looks so intuitive.
Math and equations don't have to reflect reality, but you are missing the point. The problem is with the very real experimental result of QM. Whichever theory or interpretation you come up to explain it, it won't make it any less weird.
> The statistical math we use in quantum mechanics is not "real", it's an human invention to approximate something that we don't understand yet.
What does it mean to understand something?
You could argue understanding is just as much a human invention as math is.
"Reality" (if there is such a thing) is independent of anything the human mind comes up with (whether that's understanding, math, or something else).... or is it?
Doesn’t that apply to all models used by physics, though? All of it is “with what we know right now”. New measurements and new experiments can always modify a constant or an equation.
This paper points out some of the most basic, paradigm shifting implications of entanglement and concludes, not that reality is truly this strange, but that the strangeness of these implications indicate the theory is incomplete. To be completely fair to Einstein, even if he had come up with Bell's inequalities at the time of writing this paper, they still would not have been possible to test with the technology of the day.
There is absolutely nothing to scoff at here. It may be wrong in it's conclusion, but the way in which it is wrong changed the course of physics. This is an example of how genius lies in asking the right questions even if you get the wrong answers. The EPR paradox remains a stunningly brilliant paper that should be closely ready by anyone in quantum physics.
Einstein was wrong, but in the most illuminating way possible!
[1]https://journals.aps.org/pr/pdf/10.1103/PhysRev.47.777