[HN Gopher] Did the Particle Go Through the Two Slits, or Did th...
___________________________________________________________________
Did the Particle Go Through the Two Slits, or Did the Wave
Function?
Author : Tomte
Score : 109 points
Date : 2025-03-13 14:49 UTC (3 days ago)
(HTM) web link (profmattstrassler.com)
(TXT) w3m dump (profmattstrassler.com)
| SiempreViernes wrote:
| 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_ wrote:
| For a single particle they are easy to confuse. A wave function
| ps(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
| ps(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 wrote:
| That's true, but it's also true of the classical probability
| distribution p(t,x1,x2).
| tsimionescu wrote:
| 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 wrote:
| 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 wrote:
| 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".
| aap_ wrote:
| I morally agree, but not quite: think of the wave function
| as not more than a bookkeeping device. It does get the job
| done but be careful to ascribe it too high an ontological
| status! The path integral formulation seems a lot more
| natural to me and it does not need a wave function, instead
| you can derive it and treat it as a bookkeeping device. The
| way I think about it is that it's an attempt to
| deterministically model non-deterministic behavior: you
| "pretend" that the system _is_ deterministic by keeping
| track of all the possible ways it could have evolved in
| time. sure enough, once you make a measurement this
| probability distribution "collapses" and you find out what
| is actually the case.
| criddell wrote:
| 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 wrote:
| 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 wrote:
| 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 ps(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 wrote:
| 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" ?
| Willingham wrote:
| 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 wrote:
| 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
| ryoshu wrote:
| Live experiments have been done and they get really weird:
| https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
| marcosdumay wrote:
| 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 wrote:
| 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
| chowells wrote:
| > It is as if every photon that went through the slit is
| somehow aware of all other photons that did so too
|
| Why isn't it just that there's a probability density
| function that describes the aggregate outcomes of a large
| number of samples from a random process? Why is "memory"
| involved?
| apt-apt-apt-apt wrote:
| I think because instead of two clusters like you'd expect
| from random BBs being shot, you get multiple bands like
| you'd see with interfered waves. Even when shot one at a
| time.
| Etheryte wrote:
| The experiment came first, all the rest of it came after to try
| and figure out why the results are as weird as they are.
| out_of_protocol wrote:
| Double slit experiment did happen and totally reproducible even
| then photons/electrons are sent by one at a time.
|
| "two photons come out" part makes no sense though. On a target
| side, there's always single hit after single photon/electron,
| but distribution of theses hits as if said electron got through
| both slits and interfered with itself
|
| P.S. the funny thing is - this works on any small thingy,
| measured up to 2000 atoms-big, as if it's the property of the
| universe itself
| bad_haircut72 wrote:
| I would love to try this experiment with something basketball
| sized out in space. Like we build an enormous basketball
| detector behind a double slit inside an unobservable black
| box. If thr basketball started acting like a wave I would be
| sooo freaked out
| whatshisface wrote:
| The largest double-slit projectile I know of is C-60, a
| soccer-ball shaped molecule of sixty atoms.
|
| https://iopscience.iop.org/article/10.1088/2058-7058/12/11/
| 4
| randunel wrote:
| That website's captcha is horrible!
| heylook wrote:
| Or amazing.
| nextts wrote:
| I got cats with sunglasses
| out_of_protocol wrote:
| Wiki, citation 19
|
| > The largest entities for which the double-slit
| experiment has been performed were molecules that each
| comprised 2000 atoms (whose total mass was 25,000 atomic
| mass units).[19]
|
| https://en.m.wikipedia.org/wiki/Double-slit_experiment
| rusk wrote:
| If the slits are big enough, isn't that just gravity?
| idontwantthis wrote:
| I don't know what it's called but I read about a proposed
| experiment to do it in space with macrosized glass beads.
| namaria wrote:
| The experiment working on clusters of atoms is news to me and
| I loved getting to know about that. But the thing that really
| breaks my mind is the experiment that proves that the
| behavior depends on the _possibility_ of getting information
| from which slit the particle went through. So we can rule out
| the act of measurement itself interfering with the behavior
| of the particle.
|
| They did it by splitting a beam of particles into a pair of
| entangled particles and then setting up a way to measure the
| polarity of one of them _after_ the point in time where it
| even hits the final screen. If you measure the polarity then,
| after the other stream of particles from the beam had already
| had time to make the pattern, the pattern will be two
| clusters. If you don 't, it goes back to an interference
| pattern.
|
| That one really cemented the notion in my head that this is
| just how the Universe is and not some local weirdness with
| particles and measurements.
| canadiantim wrote:
| Hadn't hear that, that one is wild
| smolder wrote:
| That version is called the quantum eraser experiment. It
| really is brain bending.
| wruza wrote:
| I think Sabine explained this social effect few years ago.
| I know she's a little controversial, but the key thing in
| the video (as opposed to all other videos about DSE on the
| internet) was that you don't get "two clusters" actually.
| They are both statistical parts of a single
| [non-]interference pattern. "||" is a lie. I'm not in a
| physics rebel camp and don't prefer Sabine either, but
| after that I sort of lost trust in the interpretations that
| can't even get the resulting picture right. I even suspect
| that showing dumbed results amplifies "wow" effect and
| monetizes better.
|
| https://www.youtube.com/watch?v=RQv5CVELG3U
|
| This is the video if you're interested. Again, I'm no
| physicist and don't know if explanations are legit or
| statistically correct. But that little || trick that all
| other popsci videos play on you, that's a true concern.
| shagie wrote:
| The double slit experiment has been replicated even with fairly
| hefty molecules.
|
| https://www.nature.com/articles/s41567-019-0663-9
|
| > Here, we report interference of a molecular library of
| functionalized oligoporphyrins with masses beyond 25,000 Da and
| consisting of up to 2,000 atoms, by far the heaviest objects
| shown to exhibit matter-wave interference to date.
|
| It would be awkward to say that the 2000 atom molecule comes
| out of both sides... but it does, until you look.
|
| The double slit experiment is not a duplication cheat of
| reality... it's weirder than that.
| somat wrote:
| I think the problem is in insisting on referring to the photon
| as a particle.
|
| In fact the photon may not actually exist. and I have questions
| as to what "single photon experiments" are actually measuring.
| let me explain.
|
| The EM field is not quantized, or at least not quantized at the
| level of a photon, what we call a photon is the interaction of
| the EM field with matter, or more precisely with the electron
| shell of matter. it is the sound of the wave breaking on the
| shore, not the wave.
|
| Now none of this actually matters as the only method we have of
| interacting with the EM field is through matter(electrons
| really) so we can only measure it in photon sized increments.
| jayd16 wrote:
| Just for sake of argument, when looking at it from this
| angle, EM particles could exist and we lack the ability to
| emit a single one? But then why would these "single photon"
| double slit problems not split the particle bunch further?
| somat wrote:
| I honestly don't know, that is my question as well.
|
| However note that we can only perturb the em field in
| photon sized energy levels, and we can only pick up
| disturbances of the em field in photon sized bunches as
| well. Not sure what this implies for how em field energy is
| accumulated on electrons in order for us to detect it.
| jabl wrote:
| Well, the EM field CAN be quantified. Just look up any
| textbook on quantum field theory. And the quanta of the EM
| field is called the photon.
|
| But, to "solve" the wave /particle conundrum, I like to think
| of it as fields all the way down. A "particle" is then a
| localized and quantisized interaction of said field with
| another field.
|
| If you think of particles as small billiard balls flying
| through space on some ballistic trajectory, you'll soon run
| into all kinds of trouble and the mental model breaks down.
| kgwgk wrote:
| > I think the problem is in insisting on referring to the
| photon as a particle.
|
| Or in insisting on referring to the electron as a particle.
|
| "We begin by throwing an ultra-microscopic object -- perhaps
| a photon, or an electron, or a neutrino"
| wasabi991011 wrote:
| > The EM field is not quantized, or at least not quantized at
| the level of a photon, what we call a photon is the
| interaction of the EM field with matter, or more precisely
| with the electron shell of matter.
|
| I don't agree with this. You can absolutely consider a
| classical (non-quantized) EM field interacting with quantized
| matter. This semi-classical model can describe the
| photoelectric effect, but it cannot describe other
| experimental observations such as sub-poissonian photo-
| detections / photon anti-bunching.
| financetechbro wrote:
| If you want to learn more of about current theory and
| experiments I suggest you pick up "Waves in an impossible sea"
| by Matt Strassler
| analog31 wrote:
| Perhaps the closest thing would be some nuclear decays that
| spit out two gamma rays of equal energy in opposite directions.
| I'm struggling to remember which isotope does this.
| GeoAtreides wrote:
| RE: your edit
|
| You still do not understand what is happening, please READ the
| article, it shows that the wave function doesn't go through
| anything and the it certainly doesn't create the interference
| pattern.
| DanielVZ wrote:
| I really don't understand the topic much but this veritasium
| video is quite eye opening and goes into further depth than any
| layman explanation I've ever seen in the topic:
| https://youtu.be/qJZ1Ez28C-A?si=6gSQYcJPpaSIt1x1
| falcor84 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| What's a decoherence rate?
| whatshisface wrote:
| 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.
| gitfan86 wrote:
| How frequently a wave would go through just 1 of the slits.
| If you threw a baseball at a wall with two baseball sized
| slits it would basically always go through just one of the
| slits. You would never see an interference pattern.
|
| This is because a baseball is interacting with other matter
| on the way to the slit. A photon on the other hand might not
| interact with any matter and it stays as a wave and you can
| see an interference pattern on the other side.
| waveletsyo wrote:
| 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 wrote:
| Sure, here you go.
|
| https://iopscience.iop.org/article/10.1088/1367-2630/15/3/03...
| baq wrote:
| 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 wrote:
| Approximation?
|
| One can only measure by interacting, there is no other way.
| mplewis9z wrote:
| Yes.
| punnerud wrote:
| 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 wrote:
| 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
| nathan_compton wrote:
| 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.
| drpossum wrote:
| If only all these people who spend their lives and jobs
| studying this in rigor and have all heard of Bohmian Mechanics
| WOULD ONLY JUST LISTEN TO YOU.
| aeblyve wrote:
| I don't think it's quite as quack of me to present as you
| might think: it has a somewhat fringe but not unsuccessful
| body of recent study.
|
| https://pubmed.ncbi.nlm.nih.gov/26989784/
|
| I tend to think (as some others do) that it's also a much
| better way to reason about quantum computation. Should a
| factorization of a large semiprime number by Shor's algorithm
| be attributed to the semi-mystical power of The Observer
| collapsing the wave function (which is who by the way, the
| sensor, or the person reading that sensor?), or are we
| instead exploiting realism to do the work?
| zombiwoof wrote:
| 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
| HarHarVeryFunny wrote:
| 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 wrote:
| 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 wrote:
| What is the name of the better model which you are talking
| about?
| kadoban wrote:
| I'm also not sure, but I'm thinking some variation of
| Quantum Field Theory.
| gus_massa wrote:
| Yes, in general
| https://en.wikipedia.org/wiki/Quantum_field_theory but
| also the list of details in
| https://en.wikipedia.org/wiki/Standard_Model
| tim-kt wrote:
| The photoelectric effect [0] can be explained if light behaves
| as discrete particles, but not when it's a wave since a higher
| amplitude does not imply a higher energy transfer.
|
| [0] https://en.m.wikipedia.org/wiki/Photoelectric_effect
| rusk wrote:
| If I emit a bass signal at a low amplitude, but then emit it
| at a higher amplitude, I can see the effect on a glass of
| water on the table. What's happening here if amplitude does
| not carry power?
|
| My understanding is that theoretically energy transfer is a
| function of wavelength.
| tim-kt wrote:
| Sorry, my last sentence wasn't formulated well. Yes, a wave
| with higher amplitude (or one could say "intensity") has a
| higher energy. The photoelectric effect happens when you
| shine light with "enough" energy on some material such that
| the atoms of the material are ionized, i. e. electrons are
| freed. You need a minimal energy for this and if you use
| dim light with a low frequency, you will not see the
| effect. Now, if you increase the frequency of the light,
| you can measure electrons. If, instead, you make the light
| brighter, that is, increase the amplitude of the wave (if
| it were a wave), you _don 't_ see electrons. So at least in
| this experiment, light does not function as a wave.
| HarHarVeryFunny wrote:
| But once you've increased the light frequency (i.e.
| photon energy) above the required threshold, THEN making
| the light brighter (more photons) will increase the
| number of electrons emitted.
| HarHarVeryFunny wrote:
| OK - a bit like the fairground game of trying to knock
| coconuts off a stand by throwing a wooden ball at them. It
| doesn't matter how many balls you are throwing per minute
| (total energy being delivered) if the energy of each ball
| doesn't cross the threshold to knock the coconut off.
|
| OTOH, the energy of a photon is such an abstract concept (not
| like the kinetic energy of a ball) that I'm not sure it
| really helps explain it.
| wasabi991011 wrote:
| You can explain the photoelectric effect with classical light
| (i.e. as EM waves) as long as you properly quantize the
| atomic energy levels. This is often called s semi-classical
| model.
|
| However, photo-detections with sub-poissonian statistics
| cannot be explained under this semi-classical model, but it
| can be explained with properly quantized EM field (i.e. with
| photons).
|
| For reference, see Mandel and Wolf's Quantum Optics textbook.
| GoblinSlayer wrote:
| Particles aren't necessary for it, any quantization is
| sufficient.
| fallingknife wrote:
| I also don't understand this. AFAIK "particle" in this context
| means quantized unit rather than contiguous solid object. And I
| see no reason why a quantized unit of a wave can't propagate
| through two slits simultaneously. But my level of understanding
| here is YouTube level so if you know more please correct me.
| ithkuil wrote:
| I trust the physics works out.
|
| The problem in these discussions is how to build an intuition
| about the underlying physical model.
|
| I fail to have an intuition of how can a quantized unit of
| wave propagate through both slits.
|
| I know that the equations say that the probability of finding
| the particle at a given location is given by the amplitude
| squared of the wave function (Born rule).
|
| The image that a "quantized unit of wave propagates through
| two slits simultaneously" doesn't help me build any further
| intuition.
|
| Do the two parts going through the two different paths carry
| half the unit? Clearly that's not the case otherwise they
| wouldn't be quanta anymore. So does it mean that the entire
| wavefront is "one unit" no matter how spread out? But in that
| case, "one unit" of what?
| aqme28 wrote:
| We do have a better model, but the mathematical model doesn't
| analogize well to the real-life concepts we're used to.
| IshKebab wrote:
| Precisely. The question "is it a particle or a wave" is wrong.
| It's neither. It's a particle-wave. Something that behaves
| _like_ a classical wave or particle depending on the situation,
| but it doesn 't switch between them or anything like that. It's
| not a "particle that has interference" or a "wave with a
| location".
|
| Classic labelling issue.
| jfengel wrote:
| Being pedantic about the language, there is only one model, and
| effectively every physicist agrees on it.
|
| What they differ about is the _interpretation_ of that model.
| The equations are the same, but differ in what the variables
| refer to in the real world. It 's really a matter of solving
| the equation for X vs Y, saying which one is independent and
| which is dependent.
|
| The purpose is to take the fact that none of the variables
| correspond directly to anything we have any experience with.
| The best we can hope for is to isolate part of it and say "this
| much is like this thing we understand, but there's an
| additional thing that we'll treat as a correction".
|
| We can try to take the whole thing seriously, and just call it
| "a quantum thingy" which is not like anything else. This is
| sometimes called "shut up and calculate", but even that makes
| assumptions about what things are feasible to calculate and
| which are hard. That skews your understanding even if you're
| trying to let it speak for itself.
| kromem wrote:
| In video games that have procedural generation, there's often a
| seed function that predicts a continuous geometry.
|
| But in order to track state changes from free agents, when you
| get close to that geometry the engine converts it to discrete
| units.
|
| This duality of continuous foundation becoming discrete units
| around the point of observation/interaction is not the result
| of dueling models, but a unified system.
|
| I sometimes wonder if we'd struggle with interpreting QM the
| same way if there wasn't a paradigm blindness with the
| interpretations all predating the advances in models in
| information systems.
| canjobear wrote:
| > What does the "particle" model bring to the table that a
| localized (wavelength-sized) wave/vibration could not?
|
| A lot of the article is about this. Start with the section "The
| Wave Function of Two Particles and a Single Door". The wave
| packet view can't explain why you don't for example see a
| "particle" (that is, a dot on a detector) show up
| simultaneously having gone through two different doors. You
| have to think about it in terms of a wave in the space of
| possible joint particle positions.
| kazinator wrote:
| 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 wrote:
| 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 wrote:
| 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.
| GeekyBear wrote:
| 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.
| alserio wrote:
| I'm not a physicist, but that doesn't really sound right.
| Might I ask you a reference or an explanation?
| kevinventullo wrote:
| Do you have a reference for that last paragraph?
| whatshisface wrote:
| That is true for classical probability, but the idea that
| unknown quantities are determining the outcomes in quantum
| mechanics has been disproven in the event of the speed of light
| being a true limit on communication speed. This is known as,
| "Bell's theorem."
| codethief wrote:
| De Broglie and Bohm would like to have a word...
| exe34 wrote:
| And that word would be....?
| codethief wrote:
| De Broglie-Bohm theory is a hidden-variable theory but
| does not allow for FTL communication.
| jfengel wrote:
| The models of quantum mechanics have already withstood
| experiments to a dozen decimal places. You aren't going to find
| departures just by banging around in your garage; you just
| can't generate enough precision.
|
| The only way forward at this point is to start with the model
| and design experiments focusing on some specific element that
| strikes you as promising. Unless you're staring at the model
| you're just guessing, and it's practically impossible that
| you're going to guess right.
| simpaticoder wrote:
| _> You aren't going to find departures just by banging around
| in your garage_
|
| This kind of rhetoric saddens me. Someone says "design an
| experiment" and you jump to the least charitable conclusion.
| That people do this is perhaps understandable, but to do it
| and not get pushback leads to it happening more and more, to
| the detriment of civil conversation.
|
| No, the experiment I had in mind would take place near the
| Schwarzchild radius of a black hole. This would require an
| enormous effort, and (civilizational) luck to defy the
| expectations set by the Drake equation/Fermi paradox. It's
| something to look forward to, even if not in our lifetimes!
| exe34 wrote:
| > No, the experiment I had in mind would take place near
| the Schwarzchild radius of a black hole
|
| I think the GP was thinking of more practical experiments,
| not science fiction.
| jiggawatts wrote:
| One particular model: the electron g-factor.
|
| Now go look up how precise a prediction the same model makes
| for the muon g-factor.
| CooCooCaCha wrote:
| This is one of the reasons I believe science and technology
| as a whole are on an S-curve. This is obviously not a precise
| statement and more of a general observation, but each step on
| the path is a little harder than the last.
|
| Whenever a physics theory gets replaced it becomes even
| harder to make an even better theory. In technology low
| hanging fruit continues to get picked and the next fruit is a
| little higher up. Of course there are lots of fruits and
| sometimes you miss one and a solution turns out to be easier
| than expected but overall every phase of technology is a
| little harder and more expensive.
|
| This actually coincides with science. Technology is finding
| useful configurations of science, and practically speaking
| there are only so many useful configurations for a given
| level of science. So the technology S-curve is built on the
| science S-curve.
| GoblinSlayer wrote:
| A model is supposed to be accurate. When it's inaccurate, you
| should understand where and how it's inaccurate and not just
| become agnostic.
| bookofjoe wrote:
| "Nobody really understands quantum mechanics." -- Richard Feynman
| FollowingTheDao wrote:
| 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 wrote:
| But waves are just group dynamics of particles, so there is no
| waves, just particles.
| FollowingTheDao wrote:
| Do particles change back into waves? No.
| rapht wrote:
| 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 wrote:
| Vibration of charged particle creates EM waves. Particle goes
| trough one slit. Wave goes trough two slits.
| jas39 wrote:
| 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 wrote:
| 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 wrote:
| 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),
| but analogies with deterministic particles are whimsical -
| sometimes they work, sometimes not.
| bop24 wrote:
| 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 wrote:
| > 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. Some
| other references (here a shameless self ad) in
| https://www.spiedigitallibrary.org/journals/optical-
| engineer... - mostly in the introduction and, speaking about
| interpretations, section 3.7.
|
| EDIT: or actually even simpler toy moodel of measurement,
| look at the Schrodinger cat in this one:
| https://arxiv.org/abs/2312.07840
| Ericson2314 wrote:
| 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 wrote:
| 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 wrote:
| Ugh, that's bad, the post is very handwavy.
| oh_my_goodness wrote:
| Quick self-test. Check all that apply.
|
| 1. "I don't understand quantum mechanics."
|
| 2. "People who understand quantum mechanics are missing something
| important about it."
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