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Did the Particle Go Through the Two Slits, or Did the Wave Function?

205 points by Tomte 2y ago

SiempreViernes 2y ago
I'm not sure very many people will actually be helped by reading the linked discussion, which appears both too technical to be clear for newcomers to Quantum mechanics while also not providing any interesting detail for the more experienced reader. This seems to be entire argument: > But the wave function is a wave in the space of possibilities, and not in physical space. Which is fair enough as an initial claim, but it doesn't really get motivated further, or at least not before I got bored reading and started skimming.
- aap_ 2y ago
  For a single particle they are easy to confuse. A wave function ψ(t,x) for a single particle gives a probability amplitude to find the particle at coordinate x at time t. In this case one can imagine an amplitude at each point in space and time, like a field. This interpretation however completely breaks down once you introduce a second particle: the wave function ψ(t,x1,x2) gives a probability amplitude to find particle 1 at x1 and particle 2 at x2 at time t. This no longer admits an interpretation of assigning some value to locations in space. Intuitively one might think you get one amplitude for each particle at some location but that's not how QM works, so we shouldn't think of the wave function as living in physical space.
  - whatshisface 2y ago
    That's true, but it's also true of the classical probability distribution p(t,x1,x2).
    - tsimionescu 2y ago
      Yes, which is exactly the point. The main difference is that the wave function has a complex value with norm <= 1, while a probability distribution function has a real value <= 1.
  - SiempreViernes 2y ago
    But if you aren't trying to map the wave function to physical space somehow you are essentially saying that the central construct of your theory has no direct relation to the actual physical processes happening "underneath". This reduces to a kind of "shut up and calculate" attitude, so it seems poor starting point from which to write an interpretation text.
    - tsimionescu 2y ago
      Space is a part of the wavefunction, as the article explains clearly. The wave function describes where the particles can be in physical space. And, the wave function has the same shape as the wave equations for traditional mechanical waves, like a sound wave or a sea wave. However, if a classical three-dimensional wave equation describes how matter osciallates in three-dimensional physical space, a quantum wavefunction doesn't do that. Quantum particles don't oscillate in physical space like that. A three-dimensional wavefunction might describe three particles' positions along a one-dimensional line, and it's oscillations are oscillations of probability, not position. The particles don't move, say, up and down. Their probability to be here or there on that 1-d line waxes and wanes. This is what the article is trying to explain: the basic mathematics of quantum mechanics, the definition of the wavefunction. The value of a wavefunction for the position of three particles is not a position in space at a moment in time. It is a (complex) probability for the position of every particle at that moment. This only seems confusing when looking at wavefunctions that describe positions. But wavefunctions often have many more observables, such as spin or polarization. A wavefunctions for two electrons moving around on a plane will not be a two-dimensional wave. It will be a wave in a six-dimensional space, whose axis may be "particle 1 has spin up/down, particle 2 has spin up/down, particle 1 position along x axis, particle 2 position along x axis, particle 1 position along y axis, particle two position along y axis".
      - SiempreViernes 2y ago
        I'm honestly confused; it's fine to say the wave function lives in some high dimensional phase space and that it's not actually describing some vibration of spacetime. But I don't recall ever imagining the wave function being a vibration of spacetime, is that really something people think? If I were to express some sort of wave-function-in-spacetime theory, I'd invoke lots of classical fields filling space and have those wiggle. In any case, the whole bit about the proper two-particle wave function living in a higher dimensional space is somewhat spoiled by the fact that you can factorise it into normal 3-space pieces (so long as you don't have your particles interacting), it doesn't seem such an alien space to me.
- criddell 2y ago
  Considering a particle is an excitation of a quantum field, the space of possibilities could be seen as the only space there is. At least that’s what I think (but don’t know for sure) that the mathematical universe hypothesis people posit.
- bryan0 2y ago
  I had the same reaction. If you make it to the end he concludes with: > The wave function’s pattern can travel across regions of possibility space that are associated with the slits. Which to me conflicts with his emphatic “no” at the beginning of the article because this implies you can define some mapping between the physical and probability space. And of course you can because if you couldn’t the theory would not be physically predictive.
  - tsimionescu 2y ago
    His point from the beginning is this: the particle described by the wavefunction can't be said to move through both slits at once, because ψ(t, x, y) has a single value for a particular x and y at a particular time. The particle has non-0 probability for both x, y1, t and for x, y2, t, of course - but that just means the particle has non-0 probability to pass through either slit. And as for saying that the wave moves through both slits, that also doesn't make sense, by the very definition of the wave function - it's a wave in probability space, not in space, so it just doesn't move through space.
    - bryan0 2y ago
      I’m with you on point 1, (I think this is also obvious from experiment because you will never measure a particle at both slits). for point 2 it seems you can define a mapping from the physical space to probability space. Saying that the wave doesn’t “move through” space might be technically correct but also seems like semantics on the definition of the phrase “move through” ?
      - tsimionescu 2y ago
        Of course it is to some extent semantics. But the important point is that the wavefunction is not something like a sound wave, or even something like a classical EM wave. Those are all waves defined over 3-dimensional physical space. In the original QM model, light is not a wave in the classical electrical theory sense. Light is made up entirely of photons, which are particles just like electrons or billiard balls, and they are described by a wavefunction. That wavefunction gives them various probabilities of being in various states at a certain time, and those probabilities can increase or decrease when more particles come into the mix. The states can represent position, momentum, charge, spin, energy levels, etc.
Willingham 2y ago
I’ve always wondered, has there ever been a definitive experiment where one photon hits a slit and on the other side two photons come out, but then when you add a photon observer, it immediately only comes out on one side? Or has the proof always been mathematical rather than a live experiment? Edit: Thank you all for the responses, it has been very educational. It appears I was misunderstanding the most important aspect of the double slit experiment. A photon is a wave function when unobserved, it literally goes through both slits and creates an interference pattern like how waves in water would. However, when observed at the slit, or at the detector screen, the wave function collapses and only one photon(billiard like particle) will be detected.
- judofyr 2y ago
  Are you referring to the double-slit experiment? If so, yes: It has always been an experiment. The experiment came before any theory explaining the behavior AFAIK. https://en.m.wikipedia.org/wiki/Double-slit_experiment https://en.m.wikipedia.org/wiki/Double-slit_experiment
- ryoshu 2y ago
  Live experiments have been done and they get really weird: https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
- marcosdumay 2y ago
  What does "one photon hits a slit and on the other side two photons come out" mean? There is no photon multiplication happening on the double slit.
  - galaxyLogic 2y ago
    One photon hits the slit and one photon comes out. It is only if you repeat the experiment many times that you start to see a strange wawe-like pattern in where the photons hit. It is as if every photon that went through the slit is somehow aware of all other photons that did so too so each photon can choose the (random) position where it hits on the wall behind the slit such that together they look like as if a WAWE went through the slit. That is (one reason) why they call it "Quantum Weirdness". God is playing dice with us
falcor84 2y ago
I'm a bit confused by the argument posed here: > Figure 4: The wrong wave function! Even though it appears as though this wave function shows two particles, one trailing the other, similar to Fig. 3, it instead shows a single particle with definite speed but a superposition of two different locations (i.e. here OR there.) I understand that if treat the act of adding two particles' wave functions as creating a new wave function for one particle, then we have this problem, essentially by definition. But it got me thinking - would it not make sense to treat the result as an expected value, such that we could then measure how many particles are likely to be to the right of the door at each point in time?
- nathan_compton 2y ago
  It isn't by definition, presuming the relationship between quantum mechanics and reality. You can have a _two particle_ state and a _one particle state_ with non-trivial probability of being in two places. They are distinct things. The key idea here (and really, in Quantum Mechanics generally) is that superpositions are important things in the theory. This is the statement that if you have a wave function for one situation and another wave function for another than the sum of the two is also, necessarily, a valid wave function for a physically realizable system. This is different from a classical probability. Suppose we simply don't know whether the baseball was fired from HERE or from THERE. In a classical situation, we can carry forward our understanding of the situation in time by simply calculating what the classical particles would do independently. In quantum mechanics the mechanics are of the wave function itself, not of the things we measure. We cannot get the right answer by imagining first that we measure the particle in one location and calculate forward and then by imagining we measure the particle in another and calculating forward and then adding the results. It isn't how the theory works. We must time evolve the wave function to predict the statistical behavior of measurement in the future.
gitfan86 2y ago
Particles don't exist. We just perceive waves with high decoherence rates as particles. Things we call objects effectively have a 100% decoherence rate. Things we call waves like light have low decoherence rates. But underneath it is all quantum mechanics.
- polishdude20 2y ago
  What's a decoherence rate?
  - whatshisface 2y ago
    Decoherence is the process that makes it impractically difficult for an experiment to be designed that makes your observations the two interfering possibilities in some kind of double-slit experiment. Interpreting this in the many-particle case is more difficult, but the basic idea is that due to single-particle uncertainty, you can't have a definite number of particles indexed by momentum and a definite number of particles indexed by position at the same time. If I had 100 particles that were definitely at x=0, in terms of momentum they'd be spread out over the range of possibilities unpredictably.
    - moi2388 2y ago
      There is a difference between them actually having these momentum, and your knowledge about these attributes. The Heisenberg uncertainty principle is not about particles. It’s about statistics and our knowledge about something.
      - tsimionescu 2y ago
        Not exactly. The Heisenburg uncertainty principle doesn't apply to knowledge (actual observations), it applies to observables (things that could affect interactions in principle). That is, Heisenburg uncertainty is not merely a limit on how fine our measurement instruments could get, or even how much information about an interaction we could conceive of and store. It's a limit on how strongly those properties can affect an interaction at all. That is, the future direction and momentum of an interaction between two particles can't depend very strongly on both the position where the interaction happened, and on the momentum the particles had before the interaction. If the interaction is a direct collision, so the position is heavily constrained, then the momentum the particles had before the collision will not really matter a lot for what happens after they collide. If you were to "put yourself in the shoes of" one of the particles, you could say that, because it "knows" where the other particle is at the time of the collision with high precision, it can't "know" the momentum the other particle had with any precision, so it's future movement can't depend strongly on that. But this stretches the definition of "knowledge" far beyond the normal understanding of the word.
waveletsyo 2y ago
I’m tired of metaphorical discussions about this experiment. Stop with the “wave particle duality”. Stop with the “until it’s measured”. Explain the experimental setup in grosse detail. What do you mean by “a particle is emitted?”. What do you mean by “a particle is measured?”. Even within the bounds of self described “double slit experiment”s there are numerous variations on how it is designed, constructed, and conducted. Stop explaining the abstract notion of the experiment through a lens of your preconceived interpretation. Show me data. Show me numerical analysis.
- whatshisface 2y ago
  Sure, here you go. https://iopscience.iop.org/article/10.1088/1367-2630/15/3/033018 https://iopscience.iop.org/article/10.1088/1367-2630/15/3/03...
baq 2y ago
Isn't it one of the 'does it matter if you didn't interact with it?' questions, and keep in mind 'observation' at quantum scales is to a good approximation synonymous to 'interaction'.
- eptcyka 2y ago
  Approximation? One can only measure by interacting, there is no other way.
mplewis9z 2y ago
Yes.
mrdlm 2y ago
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punnerud 2y ago
Rather than viewing wave functions as abstract mathematical objects in possibility space, we might understand them as describing the probabilistic nature of fundamental spinning energy entities whose rotational states generate wave-like behavior in measurement outcomes. Strassler's "possibilities" could be reinterpreted as different rotational configurations of spinning energy, with interference patterns emerging not from physical objects passing through slits but from how these spinning states evolve when constrained by sequential measurements.
aeblyve 2y ago
Two words, Bohmian Mechanics Three words, pilot wave theory To quote Cockshott, the Copenhagen Interpretation is an idealist recapitulation of Russian Machism/Bishop Berkley. The statement "nothing /is/ until it is observed" is not necessarily a Weird Quantum formulation but just a solipsistic attitude applicable towards all scientific observation in general. https://en.wikipedia.org/wiki/Pilot_wave_theory https://en.wikipedia.org/wiki/Pilot_wave_theory
- nathan_compton 2y ago
  If one tries to formulate QFT theory with Bohmian Mechanics the results are less than satisfying. Regular Quantum Mechanics in a Bohmian mode is, in addition to failing to be invariant, also pretty paltry if pressed to really serve, primarily in that it appears to be the case (for both theories) that one has quite a lot of freedom with respect to what precisely lives with the particle and what lives with the pilot wave function. In another sense Bohmian mechanics just kicks the can down the road - we may decide to associate the specific thing we observe with a particle situated on the pilot wave, but in fact, as far as the theory goes, the particle can live at any point in the pilot wave it wishes and nothing about the dynamics of the pilot wave changes at all. Thus we simply place the non-determinism in the past rather than in the present. Furthermore, Bohmian mechanics seems to break Newton's First Law, since the pilot particle, as hinted above, is influenced by the pilot wave but not vice versa. The appeal of Bohmian mechanics is obvious, but superficial. It does not dispense with the can of worms, just opens it from the other side, in my opinion.
  - jostylr 2y ago
    Bohmian mechanics is based on the idea that we perceive stuff to be in a certain position in a single reality because there is a correspondence to stuff being actually there. That's nice. If the particles are surfing a wave and not impacting it, so be it. It is also rather nice to think of the particles as just being points in space with nothing else associated with them; an electron is just an electron because the portion of the wave function that is relevant and guiding it is the electron portion; see a paper from 2004 entitled "Are all particles identical?" [1] (I am a coauthor on that). If one thinks about it, we only know about particles through their motion so having things like mass and charge linked to the object guiding the particle seems perfectly reasonable. Points are not only not labelled by numbers (particle 1, 2, etc) but also not labelled by mass and charge. The nondeterminism of not knowing the initial conditions is fine; the point was to have a theory with well-defined objects that give some plausible story and connection to our experiences, such as stuff existing and being somewhere. The fact that non-relativistic Bohmian mechanics happens to be deterministic is just happenstance for many of its supporters. In some QFT versions, the dynamics of creation is not deterministic and there is no reason for that to be a problem. But it is well-specified without having to invoke some special magic action called "observation". As for QFT, the biggest problem for Bohmian mecahnics is the need to have an actually well-defined evolution of the wave function. The idea of particles being created and annihilated is not particularly hard. And, in fact, recent work has shown that if one takes that seriously and respects probability leaking from n particle space to n+1 and n-1, then at least some of the divergence problems go away. See [2] 1: https://arxiv.org/abs/quant-ph/0405039 https://arxiv.org/abs/quant-ph/0405039 2: https://arxiv.org/abs/1809.10235 https://arxiv.org/abs/1809.10235
zombiwoof 2y ago
We are assuming the “particle” is just a particle and somehow not attached to a larger wave of other unknown (smaller) things We are looking at a birds body calling it a particle but it has wings we don’t see which effect the direction the particle flies
- astrange 2y ago
  This is a classic "midbrow dismissal" where you assume experts haven't thought of the first thing you thought of.
HarHarVeryFunny 2y ago
It's always seemed to me that these types of question only exist because we're considering a choice between two imperfect models. If we had a better model of what a "particle" really is, then there would be no dualing models nor paradox. Do we really have to choose between wave and particle? What does the "particle" model bring to the table that a localized (wavelength-sized) wave/vibration could not?
- gus_massa 2y ago
  We have a better model, but it's an ecuation so horrible that nobody want to solve it. Luckly, sometimes the exact solution can be very accuately aproximated with a wave ecuation. Luckly, sometimes the exact solution can be very accuately aproximated with a particle ecuation. (Sometimes, the exact solution can be aproximated saying that the lowest energy state is an eigenvector of the Schoedinger equation. Is that a wave? It's not localized, but not very wavy.) But neither are the exact solution, just aproximations that solve tpgether 99% of the experiment. It's difficult to explain, because to explaing the detials you need like two years of algebra and calculus and then like another 2 years of physics, and now you get a degree in physics. It's possible to solve the difficult ecuation only in very simple cases like electron-electron colissions, if you allow some cheating and a tiny error. For more complicated systems like electron-muon there are some problems. And for more complicated systems, you get more technical problems and more aproximations.
  - tim-kt 2y ago
    What is the name of the better model which you are talking about?
    - kadoban 2y ago
      I'm also not sure, but I'm thinking some variation of Quantum Field Theory.
      - gus_massa 2y ago
        Yes, in general https://en.wikipedia.org/wiki/Quantum_field_theory https://en.wikipedia.org/wiki/Quantum_field_theory but also the list of details in https://en.wikipedia.org/wiki/Standard_Model https://en.wikipedia.org/wiki/Standard_Model
kazinator 2y ago
Simpleton's view: Particles are just standing waves, so to speak. They are not just an amorphous clay-like lump of matter. They are made of smaller things and those things are churning around. That in-place churning becomes a wave when the particles move at speeds that approach a significant fraction of c.
simpaticoder 2y ago
Observation is more important than model; if we take the model too seriously, we can be led astray. It's much like extending a metaphor too far. We observe double-slit diffraction and model it with the wave-function. This doesn't preclude other models, and some of those models will be more intuitive than others. The model we use may only give us a slice of insight. We can model a roll of the dice with a function with 6 strong peaks and consider the state of the dice in superposition. The fact that the model is a continuous real function is an artifact of the model, a weakness not a strength. We are modeling a system who's concrete state is unknown between measurements (the dice is fundamentally "blurred"), and we keep expecting more from the model than it wants to give. Programmers may have better models, actually. The world is a tree where the structure of a node births a certain number of discrete children at a certain probability, one to be determined "real" by some event (measurement), but it says little about "reality". The work of the scientist is to enumerate the children and their probabilities for ever more complex parent nodes. The foundations of quantum mechanics may be advanced by new experiments, but not, I think, by staring at the models hoping for inspiration.
- nimish 2y ago
  Finally! Too much of physics is obsessed with the map and not the territory. This is how you get the tortured reasoning that views measurement and observation as somehow different. Even einstein struggled.
  - deleted 2y ago
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  - GeekyBear 2y ago
    Doesn't the difference between measurement and observation stem from an extension of the double slit experiment discussed in thus artucle? It you place a detector on one of the two slits in the prior experiment, (so that you measure which slit each individual photon goes through) the interference pattern disappears. If you leave the detector in place, but don't record the data that was measured, the interference pattern is back.
bookofjoe 2y ago
"Nobody really understands quantum mechanics." — Richard Feynman
FollowingTheDao 2y ago
There are no particles, only waves. I do not know how long it will take people to accept this because I think it effects their very psyche, realizing that there is no mass outside of our observations. The wave went through the slits, not the "wave function". There is no "quantum" because there is nothing to measure so there is no quantum physics. The fact that we are quantifying things is the problem. When we look at everything as a whole which is effected by waves we will find the solution.
- oneshtein 2y ago
  But waves are just group dynamics of particles, so there is no waves, just particles.
  - FollowingTheDao 2y ago
    Do particles change back into waves? No.
    - oneshtein 2y ago
      Quantum physics says: yes.
    - soulofmischief 2y ago
      Yes, you can reintroduce another beam splitter at the end and "lose" the path information, and provided you aren't measuring anywhere along the path, you get wave interference at the end even if you split the beam along two paths in the beginning. Look up the quantum eraser experiment.
  - lmm 2y ago
    > But waves are just group dynamics of particles No. Group movement of particles is one medium in which waves can occur, but the concept is more fundamental and general. The waves described in the article are not in particles.
- rapht 2y ago
  Probably it's because I'm not a quantum physicist, but the argument boiling down to "the wavefunction is an object of a probability space not of physical space" seems to make the whole article moot. Can the "wavefunction" be anything else than a _representation_ of the particule(/wave)?... but then who could ever think that a representation would actually travel in space?
oneshtein 2y ago
Vibration of charged particle creates EM waves. Particle goes trough one slit. Wave goes trough two slits.
jas39 2y ago
One thing I've thought about is whether observations in the present can influence past events. I'm thinking it must be so, though probably only on a microscopic level.
- whatshisface 2y ago
  The chain isn't this: Choice of how to measure -> History it is, Choice of how to measure + physical system -> Observations -> Interpretation of observations -> History The choice of what and how to measure will influence the history you conclude, but that is true of actual "Caesar and Napoleon" history too, and in that case it's definitely not that past events are being changed, instead it is your knowledge of them. A really interesting principle is that any philosophical question that can be phrased without referring to ideas that only exist in quantum mechanics can usually be answered without referring to them.
stared 2y ago
I always feel that we are inclined to ask this question because we want to treat the wavefunction as if it were a probability distribution. While they share some properties, fundamentally they are not the same thing. In typical probability, we deal with an ensemble of fixed states, or at least phenomena that can be simulated as such. In quantum physics, the wavefunction is fundamental. The question "what was the exact path?" is meaningless. In particular, if we take the approach of Feynman path integrals, we find that particles take many paths - including circular paths through each slit - before arriving somewhere else where they interact (i.e., become entangled) with, say, an electron in the screen. Sure, we may consider different experiments (e.g., quantum erasers, see https://lab.quantumflytrap.com/lab/quantum-eraser https://lab.quantumflytrap.com/lab/quantum-eraser), but analogies with deterministic particles are whimsical - sometimes they work, sometimes not.
- bop24 2y ago
  So I only have a B.S. in physics but my impression is that the weird parts of quantum mechanics are fundamentally a measurement problem. At the quantum level, we are very very limited in what we can use to measure properties of a quantum system - which is why we resort to probabilities. Wave functions are just a mathematical representation of a physical property that are (only?) ever operated on using quantum operators which result in a statistical distribution. Because they are so closely tied to probabilities I struggle with interpretations that try to say that perhaps these wave functions are something physical and based in reality (i.e. they are in superposition so particles must take on every possible state at once). An analogy I use is its like when we talk about sample sizes of a population of people, what is an ‘average person’? An average person is not something physical we can pick out, it exists in abstract. I’m curious if anyone with more experience in QM can shed light on how sound my thinking is here.
  - stared 2y ago
    > Wave functions are just a mathematical representation of a physical property that are (only?) ever operated on using quantum operators which result in a statistical distribution. It is not correct— at least not unless you subscribe to the Copenhagen interpretation. Yet, while this interpretation is a simple heuristic for interaction with big systems (e.g., a photon hits a CCD array), none of the quantum physicists I know treat it seriously (for that matter, I have a PhD in quantum optics theory). I mean, at some certain level, everything is "just a mathematical representation" - in the spirit of "all models are wrong but some are useful". But the wavefunction is more fundamental than measurement. The other can be thought of as a particle entangling with a system so large that, for statistical reasons, it becomes irreversible - because of chaos, not fundamental rules. For some materials, I recommend materials on decoherence by WH Zurek, e.g. https://arxiv.org/pdf/quant-ph/0105127 https://arxiv.org/pdf/quant-ph/0105127. Some other references (here a shameless self ad) in https://www.spiedigitallibrary.org/journals/optical-engineering/volume-61/issue-08/081808/Visualizing-quantum-mechanics-in-an-interactive-simulation--Virtual-Lab/10.1117/1.OE.61.8.081808.full https://www.spiedigitallibrary.org/journals/optical-engineer... - mostly in the introduction and, speaking about interpretations, section 3.7. EDIT: or actually even simpler toy model of measurement, look at the Schrodinger cat in this one: https://arxiv.org/abs/2312.07840 https://arxiv.org/abs/2312.07840
Ericson2314 2y ago
There was a few lines on this, but I wish it clearer that everything it said is also true classically about particles for which we are uncertain. IMO so much writing about quantum mechanics gets harder to follow by trying to jump classic -> quantum, and certain -> probabilistic at the same time. If one does the latter switch first, it cuts out the noise of easier-to-understand things to get to the second.
fracus 2y ago
I'm enjoying this tutorial so far. Every sentence was carefully considered which I think is important for my level of understanding of quantum mechanics. I'm reading very slowly and carefully. It was really helpful to define the wave function as not existing in the physical world such as a water wave but exists as description in probability space.
GoblinSlayer 2y ago
Ugh, that's bad, the post is very handwavy.
deleted 2y ago
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TriangleEdge 2y ago
Not a physicist - how is it possible to only send 1 photon at a time? How is this ensured?
- analog31 2y ago
  Literally dim the light source until the probability of two showing up within the time scale of the measurement is low enough. This is not impractical. For instance there are many kinds of detectors that can be set up to discriminate single photons or particles.
sunami-ai 2y ago
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nine_k 2y ago
According to modern QFT, there are no particles except as an approximation. There are no fields except as mathematical formalisms. There's no locality. There is instead some kind of interaction of graph nodes, representing quantum interactions, via "entanglement" and "decoherence". In this model, there are no "split particle" paradoxes, because there are no entities that resemble the behavior of macroscopic bodies, with our intuitions about them. Imagine a Fortran program, with some neat index-based FOR loops, and some per-element computations on a bunch of big arrays. When you look at its compiled form, you notice that the neat loops are now something weird, produced by automatic vectorization. If you try to find out how it runs, you notice that the CPU not only has several cores that run parts of the loop in parallel, but the very instructions in one core run out of order, while still preserving the data dependency invariants. "But did the computation of X(I) run before or after the computation of X(I+1)?!", you ask in desperation. You cannot tell. It depends. The result is correct though, your program has no bugs and computes what it should. It's counter-intuitive, but the underlying hardware reality is counter-intuitive. It's not illogical or paradoxical though.
- soup10 2y ago
  just because QFT follows an internal logic, doesn't mean the jump from macro physics to quantum physics itself is logical. In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics.
  - perlgeek 2y ago
    The Universe is fundamentally quantum in nature; if anything, we'd need a model that explains why classical physics works so well most of the time.
    - soup10 2y ago
      i started writing a response about how the human brain is designed to operate in an environment where classical physics is the norm, so we need to bridge the deviations from that if we are to really understand the world. But I don't know how much that's really true if you consider neural biology and I won't claim to know where quantum stops and classical begins as it relates to brain function.
est 2y ago
I have a dumb question since school days, how to you build two "slits" as small as particle size? I assume there's tons of interacting forces when you zoom in to that scale. The "slits" are made of particles themselves afterall. Is it really similar to the "slits" we see in daily life or something different going on here?
- moi2388 2y ago
  The slits are not made of particles, they are slits, so the absence of particles. You could make a double slit experiment by shining a laser on a single strand of hair, this would create two wide slits on either side. Or draw black sharpie on some glass, and scratching two openings on it with a needle. Since effects are clearer when it matches wavelength, you can also buy pre-made ones if you don’t feel like making it yourself
- throwawayffffas 2y ago
  The slits do not have to be particle sized. You just cut a couple on a sheet of metal.
dschuetz 2y ago
This again. The take is that possibility space does not have the same constraints as physical space. But I cannot recall that that was a real problem, but at least that's an opener for a discussion. The question was and still is what influences individual particles to form an interference pattern over time. One interpretation is that something changes, or interferes with, the probability wave function of the particle's 'virtual' trajectory. The empirical evidence for this statistical anomaly is the pattern on the screen. Well of course, the double slit causes the interference pattern, but it's still not clear why. Best working guesstimate so far is that it behaves like a bullet in physical space and like a wave in possibility space at the same time. This guesstimate was enough to become the foundation of modern quantum physics, and so much more. But is it the whole story? Do we want to know? I think the double slit experiment is just flawed, and it's a miracle that someone was able to derive a working model from its results.
torbjornbp 2y ago
Made me think of this exchange with Heinz von Foerster about theory, particles and reality: https://www.youtube.com/watch?v=ev7e9sfWIJo https://www.youtube.com/watch?v=ev7e9sfWIJo
akomtu 2y ago
Has anyone tried this with non-magnetic particles? Something that's not a photon, not an electron and not a C60 electron-wrapped molecule?
- lmm 2y ago
  It's been done with neutrons if that's what you're asking.
  - akomtu 2y ago
    Yeah, that's right.
ashoeafoot 2y ago
The experiment result becomes much more clearer once you add more slits. Until you have infinite slits. The possibility of a lightwave travells on all roads possible, but most chancel out by likelihood . The action goes all the way.
- zweizlitz 2y ago
  Is there a way to filter out all the people who watched the Veritasium video last week but didn’t read this article? Because i would like to understand infinite slits from wave probability space described in the article. Although the article author says that’s coming next week or so.
beyondCritics 2y ago
https://m.youtube.com/watch?v=qJZ1Ez28C-A https://m.youtube.com/watch?v=qJZ1Ez28C-A I had learned about the double slit experiment in school, but in my mind it was something of a theoretical construct. This veritasium video demonstrates, that quantum waves are very real and tangible. This is how physics should be.
lmm 2y ago
The wave went through the two slits, for any normal everyday definition of "go through". Yes, you can say "the wave function is just a function that assigns an amplitude for the particle's presence at every (x, y, z, t) co-ordinate, it doesn't go anywhere". But that's no more valid than saying that a regular water wave is "just a function that assigns a height to the water at every (x, y, t) co-ordinate, it doesn't go anywhere". There is a pattern to the wavefunction, where the amplitude at (x+delta, y, z, t+delta) is closely related to the amplitude at (x, y, z, t). (Specifically, it's that amplitude rotated by delta times the mass of the particle). Or, unless you're being wilfully obtuse, the wave packet moves from x to x+delta in time t to t+delta, rotating as it goes as quantum mechanical waves do. You can, if you really want, insist in Zeno's paradox fashion that nothing ever goes anywhere, that things just exist at given places and times, and in a certain sense that's true. But there's nothing QM-specific about that, and it's misleading to complicate a discussion of QM by claiming so. If we allow that things can move through space, and waves can move through space, then the wave moves through the two slits in the normal sense of all those concepts. I wish people would stop going out of their way to make QM sound confusing/weird/"spooky". Most of it is just normal wave behaviour for which you can observe exactly the same thing with everyday classical waves.
- scotty79 2y ago
  My personal interpretation is that elementary particles physically are those waves and never anything else. Those waves interact with each other with probabilistic events exchanging some energy and momentum and reshaping each other. They can get narrowed down if they exchange energy/momentum or get spread apart, for example through interactions on the edge of an object. What's completely virtual for me is the idea of pointlike particles occupying some specific location and having some specific momentum. Almost everything we know contradicts this idea, and yet we cling to it.
  - adornKey 2y ago
    I think Craig Bohren wrote in one of his books, that to get anything calculated and done the waves are all you need. Particles are nice for some kind of visualization, but they don't really help getting things done. I liked that. Instead of particles I like to view the interactions like the forming of a lightning in a thunderstorm. The energy-field builds up, And at some point of contact the energy is being released in a single lightning strike. What I still wonder is, if the interaction really depletes the energy-field instantly in a single point, or if there is more going on (on different timescales - maybe with speeds not related to the speed of light).
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pantulis 2y ago
Strassler is usually more technical than your run-of-the-mill popsci writer. But his book "Waves in an impossible sea" is much more accessible.
rob_c 2y ago
One of the best ways of thinking about it is that it's "a quanta propagating with these properties". Trying to collapse a quanta to either a particle or a wave loses some of the behaviour of the thing you're taking about and is where some of the confusions come from trying to take one viewpoint to it's wrong conclusion
maunke 2y ago
If you want to play with quantum wave packets, I built a quantum Web-based Simulation tool for non-relativistic quantum mechanics in 2016: https://quantum-simulation.de https://quantum-simulation.de The split-operator method for the numerical solution of the time-dependent Schrödinger equation is used to simulate the propagation of a Gaussian wave packet in arbitrarily adjustable potentials.
gejose 2y ago
Somewhat related recent Veritasium video - https://www.youtube.com/watch?v=qJZ1Ez28C-A https://www.youtube.com/watch?v=qJZ1Ez28C-A
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lisper 2y ago
Here is my take on explaining this, written seven years ago: https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz... https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-deux.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz... https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-drei.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz...
_ce5e 2y ago
If you consider the photon (particle), a single photon does not go through both slits at once. It takes the path of least action. It's only when the experiment is repeated that the wave-like behavior emerges. The wave function is simply a probability approximation of the least action required for any set of start and end position. So the question in the title doesn't make much sense.

205 points by Tomte 2y ago

I'm not sure very many people will actually be helped by reading the linked discussion, which appears both too technical to be clear for newcomers to Quantum mechanics while also not providing any interesting detail for the more experienced reader. This seems to be entire argument: > But the wave function is a wave in the space of possibilities, and not in physical space. Which is fair enough as an initial claim, but it doesn't really get motivated further, or at least not before I got bored reading and started skimming.

aap_ 2y ago
For a single particle they are easy to confuse. A wave function ψ(t,x) for a single particle gives a probability amplitude to find the particle at coordinate x at time t. In this case one can imagine an amplitude at each point in space and time, like a field. This interpretation however completely breaks down once you introduce a second particle: the wave function ψ(t,x1,x2) gives a probability amplitude to find particle 1 at x1 and particle 2 at x2 at time t. This no longer admits an interpretation of assigning some value to locations in space. Intuitively one might think you get one amplitude for each particle at some location but that's not how QM works, so we shouldn't think of the wave function as living in physical space.
- whatshisface 2y ago
  That's true, but it's also true of the classical probability distribution p(t,x1,x2).
  - tsimionescu 2y ago
    Yes, which is exactly the point. The main difference is that the wave function has a complex value with norm <= 1, while a probability distribution function has a real value <= 1.
- SiempreViernes 2y ago
  But if you aren't trying to map the wave function to physical space somehow you are essentially saying that the central construct of your theory has no direct relation to the actual physical processes happening "underneath". This reduces to a kind of "shut up and calculate" attitude, so it seems poor starting point from which to write an interpretation text.
  - tsimionescu 2y ago
    Space is a part of the wavefunction, as the article explains clearly. The wave function describes where the particles can be in physical space. And, the wave function has the same shape as the wave equations for traditional mechanical waves, like a sound wave or a sea wave. However, if a classical three-dimensional wave equation describes how matter osciallates in three-dimensional physical space, a quantum wavefunction doesn't do that. Quantum particles don't oscillate in physical space like that. A three-dimensional wavefunction might describe three particles' positions along a one-dimensional line, and it's oscillations are oscillations of probability, not position. The particles don't move, say, up and down. Their probability to be here or there on that 1-d line waxes and wanes. This is what the article is trying to explain: the basic mathematics of quantum mechanics, the definition of the wavefunction. The value of a wavefunction for the position of three particles is not a position in space at a moment in time. It is a (complex) probability for the position of every particle at that moment. This only seems confusing when looking at wavefunctions that describe positions. But wavefunctions often have many more observables, such as spin or polarization. A wavefunctions for two electrons moving around on a plane will not be a two-dimensional wave. It will be a wave in a six-dimensional space, whose axis may be "particle 1 has spin up/down, particle 2 has spin up/down, particle 1 position along x axis, particle 2 position along x axis, particle 1 position along y axis, particle two position along y axis".
    - SiempreViernes 2y ago
      I'm honestly confused; it's fine to say the wave function lives in some high dimensional phase space and that it's not actually describing some vibration of spacetime. But I don't recall ever imagining the wave function being a vibration of spacetime, is that really something people think? If I were to express some sort of wave-function-in-spacetime theory, I'd invoke lots of classical fields filling space and have those wiggle. In any case, the whole bit about the proper two-particle wave function living in a higher dimensional space is somewhat spoiled by the fact that you can factorise it into normal 3-space pieces (so long as you don't have your particles interacting), it doesn't seem such an alien space to me.
criddell 2y ago
Considering a particle is an excitation of a quantum field, the space of possibilities could be seen as the only space there is. At least that’s what I think (but don’t know for sure) that the mathematical universe hypothesis people posit.
bryan0 2y ago
I had the same reaction. If you make it to the end he concludes with: > The wave function’s pattern can travel across regions of possibility space that are associated with the slits. Which to me conflicts with his emphatic “no” at the beginning of the article because this implies you can define some mapping between the physical and probability space. And of course you can because if you couldn’t the theory would not be physically predictive.
- tsimionescu 2y ago
  His point from the beginning is this: the particle described by the wavefunction can't be said to move through both slits at once, because ψ(t, x, y) has a single value for a particular x and y at a particular time. The particle has non-0 probability for both x, y1, t and for x, y2, t, of course - but that just means the particle has non-0 probability to pass through either slit. And as for saying that the wave moves through both slits, that also doesn't make sense, by the very definition of the wave function - it's a wave in probability space, not in space, so it just doesn't move through space.
  - bryan0 2y ago
    I’m with you on point 1, (I think this is also obvious from experiment because you will never measure a particle at both slits). for point 2 it seems you can define a mapping from the physical space to probability space. Saying that the wave doesn’t “move through” space might be technically correct but also seems like semantics on the definition of the phrase “move through” ?
    - tsimionescu 2y ago
      Of course it is to some extent semantics. But the important point is that the wavefunction is not something like a sound wave, or even something like a classical EM wave. Those are all waves defined over 3-dimensional physical space. In the original QM model, light is not a wave in the classical electrical theory sense. Light is made up entirely of photons, which are particles just like electrons or billiard balls, and they are described by a wavefunction. That wavefunction gives them various probabilities of being in various states at a certain time, and those probabilities can increase or decrease when more particles come into the mix. The states can represent position, momentum, charge, spin, energy levels, etc.

Willingham 2y ago

I’ve always wondered, has there ever been a definitive experiment where one photon hits a slit and on the other side two photons come out, but then when you add a photon observer, it immediately only comes out on one side? Or has the proof always been mathematical rather than a live experiment? Edit: Thank you all for the responses, it has been very educational. It appears I was misunderstanding the most important aspect of the double slit experiment. A photon is a wave function when unobserved, it literally goes through both slits and creates an interference pattern like how waves in water would. However, when observed at the slit, or at the detector screen, the wave function collapses and only one photon(billiard like particle) will be detected.

judofyr 2y ago
Are you referring to the double-slit experiment? If so, yes: It has always been an experiment. The experiment came before any theory explaining the behavior AFAIK. https://en.m.wikipedia.org/wiki/Double-slit_experiment https://en.m.wikipedia.org/wiki/Double-slit_experiment
ryoshu 2y ago
Live experiments have been done and they get really weird: https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
marcosdumay 2y ago
What does "one photon hits a slit and on the other side two photons come out" mean? There is no photon multiplication happening on the double slit.
- galaxyLogic 2y ago
  One photon hits the slit and one photon comes out. It is only if you repeat the experiment many times that you start to see a strange wawe-like pattern in where the photons hit. It is as if every photon that went through the slit is somehow aware of all other photons that did so too so each photon can choose the (random) position where it hits on the wall behind the slit such that together they look like as if a WAWE went through the slit. That is (one reason) why they call it "Quantum Weirdness". God is playing dice with us

falcor84 2y ago

I'm a bit confused by the argument posed here: > Figure 4: The wrong wave function! Even though it appears as though this wave function shows two particles, one trailing the other, similar to Fig. 3, it instead shows a single particle with definite speed but a superposition of two different locations (i.e. here OR there.) I understand that if treat the act of adding two particles' wave functions as creating a new wave function for one particle, then we have this problem, essentially by definition. But it got me thinking - would it not make sense to treat the result as an expected value, such that we could then measure how many particles are likely to be to the right of the door at each point in time?

nathan_compton 2y ago
It isn't by definition, presuming the relationship between quantum mechanics and reality. You can have a _two particle_ state and a _one particle state_ with non-trivial probability of being in two places. They are distinct things. The key idea here (and really, in Quantum Mechanics generally) is that superpositions are important things in the theory. This is the statement that if you have a wave function for one situation and another wave function for another than the sum of the two is also, necessarily, a valid wave function for a physically realizable system. This is different from a classical probability. Suppose we simply don't know whether the baseball was fired from HERE or from THERE. In a classical situation, we can carry forward our understanding of the situation in time by simply calculating what the classical particles would do independently. In quantum mechanics the mechanics are of the wave function itself, not of the things we measure. We cannot get the right answer by imagining first that we measure the particle in one location and calculate forward and then by imagining we measure the particle in another and calculating forward and then adding the results. It isn't how the theory works. We must time evolve the wave function to predict the statistical behavior of measurement in the future.

gitfan86 2y ago

Particles don't exist. We just perceive waves with high decoherence rates as particles. Things we call objects effectively have a 100% decoherence rate. Things we call waves like light have low decoherence rates. But underneath it is all quantum mechanics.

polishdude20 2y ago
What's a decoherence rate?
- whatshisface 2y ago
  Decoherence is the process that makes it impractically difficult for an experiment to be designed that makes your observations the two interfering possibilities in some kind of double-slit experiment. Interpreting this in the many-particle case is more difficult, but the basic idea is that due to single-particle uncertainty, you can't have a definite number of particles indexed by momentum and a definite number of particles indexed by position at the same time. If I had 100 particles that were definitely at x=0, in terms of momentum they'd be spread out over the range of possibilities unpredictably.
  - moi2388 2y ago
    There is a difference between them actually having these momentum, and your knowledge about these attributes. The Heisenberg uncertainty principle is not about particles. It’s about statistics and our knowledge about something.
    - tsimionescu 2y ago
      Not exactly. The Heisenburg uncertainty principle doesn't apply to knowledge (actual observations), it applies to observables (things that could affect interactions in principle). That is, Heisenburg uncertainty is not merely a limit on how fine our measurement instruments could get, or even how much information about an interaction we could conceive of and store. It's a limit on how strongly those properties can affect an interaction at all. That is, the future direction and momentum of an interaction between two particles can't depend very strongly on both the position where the interaction happened, and on the momentum the particles had before the interaction. If the interaction is a direct collision, so the position is heavily constrained, then the momentum the particles had before the collision will not really matter a lot for what happens after they collide. If you were to "put yourself in the shoes of" one of the particles, you could say that, because it "knows" where the other particle is at the time of the collision with high precision, it can't "know" the momentum the other particle had with any precision, so it's future movement can't depend strongly on that. But this stretches the definition of "knowledge" far beyond the normal understanding of the word.

waveletsyo 2y ago

I’m tired of metaphorical discussions about this experiment. Stop with the “wave particle duality”. Stop with the “until it’s measured”. Explain the experimental setup in grosse detail. What do you mean by “a particle is emitted?”. What do you mean by “a particle is measured?”. Even within the bounds of self described “double slit experiment”s there are numerous variations on how it is designed, constructed, and conducted. Stop explaining the abstract notion of the experiment through a lens of your preconceived interpretation. Show me data. Show me numerical analysis.

whatshisface 2y ago
Sure, here you go. https://iopscience.iop.org/article/10.1088/1367-2630/15/3/033018 https://iopscience.iop.org/article/10.1088/1367-2630/15/3/03...

baq 2y ago

Isn't it one of the 'does it matter if you didn't interact with it?' questions, and keep in mind 'observation' at quantum scales is to a good approximation synonymous to 'interaction'.

eptcyka 2y ago
Approximation? One can only measure by interacting, there is no other way.

mplewis9z 2y ago

Yes.

mrdlm 2y ago

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punnerud 2y ago

Rather than viewing wave functions as abstract mathematical objects in possibility space, we might understand them as describing the probabilistic nature of fundamental spinning energy entities whose rotational states generate wave-like behavior in measurement outcomes. Strassler's "possibilities" could be reinterpreted as different rotational configurations of spinning energy, with interference patterns emerging not from physical objects passing through slits but from how these spinning states evolve when constrained by sequential measurements.

aeblyve 2y ago

Two words, Bohmian Mechanics Three words, pilot wave theory To quote Cockshott, the Copenhagen Interpretation is an idealist recapitulation of Russian Machism/Bishop Berkley. The statement "nothing /is/ until it is observed" is not necessarily a Weird Quantum formulation but just a solipsistic attitude applicable towards all scientific observation in general. https://en.wikipedia.org/wiki/Pilot_wave_theory https://en.wikipedia.org/wiki/Pilot_wave_theory

nathan_compton 2y ago
If one tries to formulate QFT theory with Bohmian Mechanics the results are less than satisfying. Regular Quantum Mechanics in a Bohmian mode is, in addition to failing to be invariant, also pretty paltry if pressed to really serve, primarily in that it appears to be the case (for both theories) that one has quite a lot of freedom with respect to what precisely lives with the particle and what lives with the pilot wave function. In another sense Bohmian mechanics just kicks the can down the road - we may decide to associate the specific thing we observe with a particle situated on the pilot wave, but in fact, as far as the theory goes, the particle can live at any point in the pilot wave it wishes and nothing about the dynamics of the pilot wave changes at all. Thus we simply place the non-determinism in the past rather than in the present. Furthermore, Bohmian mechanics seems to break Newton's First Law, since the pilot particle, as hinted above, is influenced by the pilot wave but not vice versa. The appeal of Bohmian mechanics is obvious, but superficial. It does not dispense with the can of worms, just opens it from the other side, in my opinion.
- jostylr 2y ago
  Bohmian mechanics is based on the idea that we perceive stuff to be in a certain position in a single reality because there is a correspondence to stuff being actually there. That's nice. If the particles are surfing a wave and not impacting it, so be it. It is also rather nice to think of the particles as just being points in space with nothing else associated with them; an electron is just an electron because the portion of the wave function that is relevant and guiding it is the electron portion; see a paper from 2004 entitled "Are all particles identical?" [1] (I am a coauthor on that). If one thinks about it, we only know about particles through their motion so having things like mass and charge linked to the object guiding the particle seems perfectly reasonable. Points are not only not labelled by numbers (particle 1, 2, etc) but also not labelled by mass and charge. The nondeterminism of not knowing the initial conditions is fine; the point was to have a theory with well-defined objects that give some plausible story and connection to our experiences, such as stuff existing and being somewhere. The fact that non-relativistic Bohmian mechanics happens to be deterministic is just happenstance for many of its supporters. In some QFT versions, the dynamics of creation is not deterministic and there is no reason for that to be a problem. But it is well-specified without having to invoke some special magic action called "observation". As for QFT, the biggest problem for Bohmian mecahnics is the need to have an actually well-defined evolution of the wave function. The idea of particles being created and annihilated is not particularly hard. And, in fact, recent work has shown that if one takes that seriously and respects probability leaking from n particle space to n+1 and n-1, then at least some of the divergence problems go away. See [2] 1: https://arxiv.org/abs/quant-ph/0405039 https://arxiv.org/abs/quant-ph/0405039 2: https://arxiv.org/abs/1809.10235 https://arxiv.org/abs/1809.10235

zombiwoof 2y ago

We are assuming the “particle” is just a particle and somehow not attached to a larger wave of other unknown (smaller) things We are looking at a birds body calling it a particle but it has wings we don’t see which effect the direction the particle flies

astrange 2y ago
This is a classic "midbrow dismissal" where you assume experts haven't thought of the first thing you thought of.

HarHarVeryFunny 2y ago

It's always seemed to me that these types of question only exist because we're considering a choice between two imperfect models. If we had a better model of what a "particle" really is, then there would be no dualing models nor paradox. Do we really have to choose between wave and particle? What does the "particle" model bring to the table that a localized (wavelength-sized) wave/vibration could not?

gus_massa 2y ago
We have a better model, but it's an ecuation so horrible that nobody want to solve it. Luckly, sometimes the exact solution can be very accuately aproximated with a wave ecuation. Luckly, sometimes the exact solution can be very accuately aproximated with a particle ecuation. (Sometimes, the exact solution can be aproximated saying that the lowest energy state is an eigenvector of the Schoedinger equation. Is that a wave? It's not localized, but not very wavy.) But neither are the exact solution, just aproximations that solve tpgether 99% of the experiment. It's difficult to explain, because to explaing the detials you need like two years of algebra and calculus and then like another 2 years of physics, and now you get a degree in physics. It's possible to solve the difficult ecuation only in very simple cases like electron-electron colissions, if you allow some cheating and a tiny error. For more complicated systems like electron-muon there are some problems. And for more complicated systems, you get more technical problems and more aproximations.
- tim-kt 2y ago
  What is the name of the better model which you are talking about?
  - kadoban 2y ago
    I'm also not sure, but I'm thinking some variation of Quantum Field Theory.
    - gus_massa 2y ago
      Yes, in general https://en.wikipedia.org/wiki/Quantum_field_theory https://en.wikipedia.org/wiki/Quantum_field_theory but also the list of details in https://en.wikipedia.org/wiki/Standard_Model https://en.wikipedia.org/wiki/Standard_Model

kazinator 2y ago

Simpleton's view: Particles are just standing waves, so to speak. They are not just an amorphous clay-like lump of matter. They are made of smaller things and those things are churning around. That in-place churning becomes a wave when the particles move at speeds that approach a significant fraction of c.

simpaticoder 2y ago

Observation is more important than model; if we take the model too seriously, we can be led astray. It's much like extending a metaphor too far. We observe double-slit diffraction and model it with the wave-function. This doesn't preclude other models, and some of those models will be more intuitive than others. The model we use may only give us a slice of insight. We can model a roll of the dice with a function with 6 strong peaks and consider the state of the dice in superposition. The fact that the model is a continuous real function is an artifact of the model, a weakness not a strength. We are modeling a system who's concrete state is unknown between measurements (the dice is fundamentally "blurred"), and we keep expecting more from the model than it wants to give. Programmers may have better models, actually. The world is a tree where the structure of a node births a certain number of discrete children at a certain probability, one to be determined "real" by some event (measurement), but it says little about "reality". The work of the scientist is to enumerate the children and their probabilities for ever more complex parent nodes. The foundations of quantum mechanics may be advanced by new experiments, but not, I think, by staring at the models hoping for inspiration.

nimish 2y ago
Finally! Too much of physics is obsessed with the map and not the territory. This is how you get the tortured reasoning that views measurement and observation as somehow different. Even einstein struggled.
- deleted 2y ago
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- GeekyBear 2y ago
  Doesn't the difference between measurement and observation stem from an extension of the double slit experiment discussed in thus artucle? It you place a detector on one of the two slits in the prior experiment, (so that you measure which slit each individual photon goes through) the interference pattern disappears. If you leave the detector in place, but don't record the data that was measured, the interference pattern is back.

bookofjoe 2y ago

"Nobody really understands quantum mechanics." — Richard Feynman

FollowingTheDao 2y ago

There are no particles, only waves. I do not know how long it will take people to accept this because I think it effects their very psyche, realizing that there is no mass outside of our observations. The wave went through the slits, not the "wave function". There is no "quantum" because there is nothing to measure so there is no quantum physics. The fact that we are quantifying things is the problem. When we look at everything as a whole which is effected by waves we will find the solution.

oneshtein 2y ago
But waves are just group dynamics of particles, so there is no waves, just particles.
- FollowingTheDao 2y ago
  Do particles change back into waves? No.
  - oneshtein 2y ago
    Quantum physics says: yes.
  - soulofmischief 2y ago
    Yes, you can reintroduce another beam splitter at the end and "lose" the path information, and provided you aren't measuring anywhere along the path, you get wave interference at the end even if you split the beam along two paths in the beginning. Look up the quantum eraser experiment.
- lmm 2y ago
  > But waves are just group dynamics of particles No. Group movement of particles is one medium in which waves can occur, but the concept is more fundamental and general. The waves described in the article are not in particles.
rapht 2y ago
Probably it's because I'm not a quantum physicist, but the argument boiling down to "the wavefunction is an object of a probability space not of physical space" seems to make the whole article moot. Can the "wavefunction" be anything else than a _representation_ of the particule(/wave)?... but then who could ever think that a representation would actually travel in space?

oneshtein 2y ago

Vibration of charged particle creates EM waves. Particle goes trough one slit. Wave goes trough two slits.

jas39 2y ago

One thing I've thought about is whether observations in the present can influence past events. I'm thinking it must be so, though probably only on a microscopic level.

whatshisface 2y ago
The chain isn't this: Choice of how to measure -> History it is, Choice of how to measure + physical system -> Observations -> Interpretation of observations -> History The choice of what and how to measure will influence the history you conclude, but that is true of actual "Caesar and Napoleon" history too, and in that case it's definitely not that past events are being changed, instead it is your knowledge of them. A really interesting principle is that any philosophical question that can be phrased without referring to ideas that only exist in quantum mechanics can usually be answered without referring to them.

stared 2y ago

I always feel that we are inclined to ask this question because we want to treat the wavefunction as if it were a probability distribution. While they share some properties, fundamentally they are not the same thing. In typical probability, we deal with an ensemble of fixed states, or at least phenomena that can be simulated as such. In quantum physics, the wavefunction is fundamental. The question "what was the exact path?" is meaningless. In particular, if we take the approach of Feynman path integrals, we find that particles take many paths - including circular paths through each slit - before arriving somewhere else where they interact (i.e., become entangled) with, say, an electron in the screen. Sure, we may consider different experiments (e.g., quantum erasers, see https://lab.quantumflytrap.com/lab/quantum-eraser https://lab.quantumflytrap.com/lab/quantum-eraser), but analogies with deterministic particles are whimsical - sometimes they work, sometimes not.

bop24 2y ago
So I only have a B.S. in physics but my impression is that the weird parts of quantum mechanics are fundamentally a measurement problem. At the quantum level, we are very very limited in what we can use to measure properties of a quantum system - which is why we resort to probabilities. Wave functions are just a mathematical representation of a physical property that are (only?) ever operated on using quantum operators which result in a statistical distribution. Because they are so closely tied to probabilities I struggle with interpretations that try to say that perhaps these wave functions are something physical and based in reality (i.e. they are in superposition so particles must take on every possible state at once). An analogy I use is its like when we talk about sample sizes of a population of people, what is an ‘average person’? An average person is not something physical we can pick out, it exists in abstract. I’m curious if anyone with more experience in QM can shed light on how sound my thinking is here.
- stared 2y ago
  > Wave functions are just a mathematical representation of a physical property that are (only?) ever operated on using quantum operators which result in a statistical distribution. It is not correct— at least not unless you subscribe to the Copenhagen interpretation. Yet, while this interpretation is a simple heuristic for interaction with big systems (e.g., a photon hits a CCD array), none of the quantum physicists I know treat it seriously (for that matter, I have a PhD in quantum optics theory). I mean, at some certain level, everything is "just a mathematical representation" - in the spirit of "all models are wrong but some are useful". But the wavefunction is more fundamental than measurement. The other can be thought of as a particle entangling with a system so large that, for statistical reasons, it becomes irreversible - because of chaos, not fundamental rules. For some materials, I recommend materials on decoherence by WH Zurek, e.g. https://arxiv.org/pdf/quant-ph/0105127 https://arxiv.org/pdf/quant-ph/0105127. Some other references (here a shameless self ad) in https://www.spiedigitallibrary.org/journals/optical-engineering/volume-61/issue-08/081808/Visualizing-quantum-mechanics-in-an-interactive-simulation--Virtual-Lab/10.1117/1.OE.61.8.081808.full https://www.spiedigitallibrary.org/journals/optical-engineer... - mostly in the introduction and, speaking about interpretations, section 3.7. EDIT: or actually even simpler toy model of measurement, look at the Schrodinger cat in this one: https://arxiv.org/abs/2312.07840 https://arxiv.org/abs/2312.07840

Ericson2314 2y ago

There was a few lines on this, but I wish it clearer that everything it said is also true classically about particles for which we are uncertain. IMO so much writing about quantum mechanics gets harder to follow by trying to jump classic -> quantum, and certain -> probabilistic at the same time. If one does the latter switch first, it cuts out the noise of easier-to-understand things to get to the second.

fracus 2y ago

I'm enjoying this tutorial so far. Every sentence was carefully considered which I think is important for my level of understanding of quantum mechanics. I'm reading very slowly and carefully. It was really helpful to define the wave function as not existing in the physical world such as a water wave but exists as description in probability space.

GoblinSlayer 2y ago

Ugh, that's bad, the post is very handwavy.

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TriangleEdge 2y ago

Not a physicist - how is it possible to only send 1 photon at a time? How is this ensured?

analog31 2y ago
Literally dim the light source until the probability of two showing up within the time scale of the measurement is low enough. This is not impractical. For instance there are many kinds of detectors that can be set up to discriminate single photons or particles.

sunami-ai 2y ago

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nine_k 2y ago

According to modern QFT, there are no particles except as an approximation. There are no fields except as mathematical formalisms. There's no locality. There is instead some kind of interaction of graph nodes, representing quantum interactions, via "entanglement" and "decoherence". In this model, there are no "split particle" paradoxes, because there are no entities that resemble the behavior of macroscopic bodies, with our intuitions about them. Imagine a Fortran program, with some neat index-based FOR loops, and some per-element computations on a bunch of big arrays. When you look at its compiled form, you notice that the neat loops are now something weird, produced by automatic vectorization. If you try to find out how it runs, you notice that the CPU not only has several cores that run parts of the loop in parallel, but the very instructions in one core run out of order, while still preserving the data dependency invariants. "But did the computation of X(I) run before or after the computation of X(I+1)?!", you ask in desperation. You cannot tell. It depends. The result is correct though, your program has no bugs and computes what it should. It's counter-intuitive, but the underlying hardware reality is counter-intuitive. It's not illogical or paradoxical though.

soup10 2y ago
just because QFT follows an internal logic, doesn't mean the jump from macro physics to quantum physics itself is logical. In my opinion we still don't have a logical explanation for why the model changes so dramatically from classical to quantum physics.
- perlgeek 2y ago
  The Universe is fundamentally quantum in nature; if anything, we'd need a model that explains why classical physics works so well most of the time.
  - soup10 2y ago
    i started writing a response about how the human brain is designed to operate in an environment where classical physics is the norm, so we need to bridge the deviations from that if we are to really understand the world. But I don't know how much that's really true if you consider neural biology and I won't claim to know where quantum stops and classical begins as it relates to brain function.

est 2y ago

I have a dumb question since school days, how to you build two "slits" as small as particle size? I assume there's tons of interacting forces when you zoom in to that scale. The "slits" are made of particles themselves afterall. Is it really similar to the "slits" we see in daily life or something different going on here?

moi2388 2y ago
The slits are not made of particles, they are slits, so the absence of particles. You could make a double slit experiment by shining a laser on a single strand of hair, this would create two wide slits on either side. Or draw black sharpie on some glass, and scratching two openings on it with a needle. Since effects are clearer when it matches wavelength, you can also buy pre-made ones if you don’t feel like making it yourself
throwawayffffas 2y ago
The slits do not have to be particle sized. You just cut a couple on a sheet of metal.

dschuetz 2y ago

This again. The take is that possibility space does not have the same constraints as physical space. But I cannot recall that that was a real problem, but at least that's an opener for a discussion. The question was and still is what influences individual particles to form an interference pattern over time. One interpretation is that something changes, or interferes with, the probability wave function of the particle's 'virtual' trajectory. The empirical evidence for this statistical anomaly is the pattern on the screen. Well of course, the double slit causes the interference pattern, but it's still not clear why. Best working guesstimate so far is that it behaves like a bullet in physical space and like a wave in possibility space at the same time. This guesstimate was enough to become the foundation of modern quantum physics, and so much more. But is it the whole story? Do we want to know? I think the double slit experiment is just flawed, and it's a miracle that someone was able to derive a working model from its results.

torbjornbp 2y ago

Made me think of this exchange with Heinz von Foerster about theory, particles and reality: https://www.youtube.com/watch?v=ev7e9sfWIJo https://www.youtube.com/watch?v=ev7e9sfWIJo

akomtu 2y ago

Has anyone tried this with non-magnetic particles? Something that's not a photon, not an electron and not a C60 electron-wrapped molecule?

lmm 2y ago
It's been done with neutrons if that's what you're asking.
- akomtu 2y ago
  Yeah, that's right.

ashoeafoot 2y ago

The experiment result becomes much more clearer once you add more slits. Until you have infinite slits. The possibility of a lightwave travells on all roads possible, but most chancel out by likelihood . The action goes all the way.

zweizlitz 2y ago
Is there a way to filter out all the people who watched the Veritasium video last week but didn’t read this article? Because i would like to understand infinite slits from wave probability space described in the article. Although the article author says that’s coming next week or so.

beyondCritics 2y ago

https://m.youtube.com/watch?v=qJZ1Ez28C-A https://m.youtube.com/watch?v=qJZ1Ez28C-A I had learned about the double slit experiment in school, but in my mind it was something of a theoretical construct. This veritasium video demonstrates, that quantum waves are very real and tangible. This is how physics should be.

lmm 2y ago

The wave went through the two slits, for any normal everyday definition of "go through". Yes, you can say "the wave function is just a function that assigns an amplitude for the particle's presence at every (x, y, z, t) co-ordinate, it doesn't go anywhere". But that's no more valid than saying that a regular water wave is "just a function that assigns a height to the water at every (x, y, t) co-ordinate, it doesn't go anywhere". There is a pattern to the wavefunction, where the amplitude at (x+delta, y, z, t+delta) is closely related to the amplitude at (x, y, z, t). (Specifically, it's that amplitude rotated by delta times the mass of the particle). Or, unless you're being wilfully obtuse, the wave packet moves from x to x+delta in time t to t+delta, rotating as it goes as quantum mechanical waves do. You can, if you really want, insist in Zeno's paradox fashion that nothing ever goes anywhere, that things just exist at given places and times, and in a certain sense that's true. But there's nothing QM-specific about that, and it's misleading to complicate a discussion of QM by claiming so. If we allow that things can move through space, and waves can move through space, then the wave moves through the two slits in the normal sense of all those concepts. I wish people would stop going out of their way to make QM sound confusing/weird/"spooky". Most of it is just normal wave behaviour for which you can observe exactly the same thing with everyday classical waves.

scotty79 2y ago
My personal interpretation is that elementary particles physically are those waves and never anything else. Those waves interact with each other with probabilistic events exchanging some energy and momentum and reshaping each other. They can get narrowed down if they exchange energy/momentum or get spread apart, for example through interactions on the edge of an object. What's completely virtual for me is the idea of pointlike particles occupying some specific location and having some specific momentum. Almost everything we know contradicts this idea, and yet we cling to it.
- adornKey 2y ago
  I think Craig Bohren wrote in one of his books, that to get anything calculated and done the waves are all you need. Particles are nice for some kind of visualization, but they don't really help getting things done. I liked that. Instead of particles I like to view the interactions like the forming of a lightning in a thunderstorm. The energy-field builds up, And at some point of contact the energy is being released in a single lightning strike. What I still wonder is, if the interaction really depletes the energy-field instantly in a single point, or if there is more going on (on different timescales - maybe with speeds not related to the speed of light).

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pantulis 2y ago

Strassler is usually more technical than your run-of-the-mill popsci writer. But his book "Waves in an impossible sea" is much more accessible.

rob_c 2y ago

One of the best ways of thinking about it is that it's "a quanta propagating with these properties". Trying to collapse a quanta to either a particle or a wave loses some of the behaviour of the thing you're taking about and is where some of the confusions come from trying to take one viewpoint to it's wrong conclusion

maunke 2y ago

If you want to play with quantum wave packets, I built a quantum Web-based Simulation tool for non-relativistic quantum mechanics in 2016: https://quantum-simulation.de https://quantum-simulation.de The split-operator method for the numerical solution of the time-dependent Schrödinger equation is used to simulate the propagation of a Gaussian wave packet in arbitrarily adjustable potentials.

gejose 2y ago

Somewhat related recent Veritasium video - https://www.youtube.com/watch?v=qJZ1Ez28C-A https://www.youtube.com/watch?v=qJZ1Ez28C-A

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lisper 2y ago

Here is my take on explaining this, written seven years ago: https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz... https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-deux.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz... https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-drei.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz...

_ce5e 2y ago

If you consider the photon (particle), a single photon does not go through both slits at once. It takes the path of least action. It's only when the experiment is repeated that the wave-like behavior emerges. The wave function is simply a probability approximation of the least action required for any set of start and end position. So the question in the title doesn't make much sense.