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Explain the double slit experiment to me. The single photon somehow calculates the interference pattern with photons that don't exist, in our world at least.

Explain the double slit experiment to me. The single photon somehow calculates the interference pattern with photons that don't exist, in our world at least. If there's not parallel universes, how does a single atom calculate that interference pattern? Where do those calculations occur? All other theories say it happens by magic and give all sorts of complicated non-real obfuscations to explain it.

raverbashing 9y ago
Bohmian mechanics seems like a better explanation than "there are infinite universes out there"
- mr_overalls 9y ago
  "We don't understand the deeper mechanisms yet" is also a valid explanation - although one that's sure to be unpopular with scientists. "Entia non sunt multiplicanda praeter necessitatem" and all that.
- ridewinter 9y ago
  I'm sure you could confuse the heck out of me trying to explain it. Deutsch would say it's just a clever obfuscation of what's really happening. And his many-worlds explanation is so incredibly simple that my small mind uses Occam's razor to figure out what to believe.
  - raverbashing 9y ago
    As simple as using God to explain how the universe came to be It's not about a simple explanation, it's about explaining using simple things. A wave is simpler than "infinite worlds"
    - akvadrako 9y ago
      The wavefunction is exactly the same as "infinite worlds". The essential aspect of the Everett / many-worlds view is that the wavefunction is the complete description of reality. The "worlds" are just approximate ways of slicing up the wavefunction which correspond to our sense of classical reality. They are clearly (and uncontroversially) identifiable if you run a simulation.
      - raverbashing 9y ago
        Ah that's a better explanation
    - yorwba 9y ago
      The "worlds" are just points in the space that supports the wave. You can use any other name, if you prefer, but there seem to be an infinite number of them.
      - millstone 9y ago
        All quantum mechanics has notions of "points in space that support the wavefunction," but these are different from the "worlds" in MWI.
        yorwba 9y ago
        Seems like I have been misunderstanding MWI all the time. How are they different?
- jerf 9y ago
  Yes, I find the whole "Explain this in any other way!" sort of a bizarre demand. You've got your choice of 5 or 6 decent isomorphic explanations. Preference for "many worlds" seems to boil down to "it annoys my common sense less than some of the others", which is a very personal concern rather than a scientific one. Personally, I actually think retrocausality, once you twist your brain around it, is another perfectly sensible option that doesn't involve multiplying the universe into a near-infinity of inaccessible, unobservable parallels. The solution in my mind is that retrocausality actually isn't; what it means instead is that what "the present" is is even less like what you thought it was, and is even more of a partial order than relativity already told us it was. The answer to the "double-slit" experiment, to my mind, is that there simply isn't a "before" and "after" and some sort of "retrocausal self-interference" of the wave... the whole thing is simply one atomic event that doesn't have a before and an after, and your imposition of a temporal order on the events is much simplified from the reality. The mystery lies in your attempt to impose a global temporal order, which is appropriate enough at scale, onto a single local event where it doesn't apply in the way you expect. The results of the double-slit experiment itself is not that mysterious, merely counterintuitive, once you release the small scale from your imposed large-scale idea of how time flows. It means that "the present" is actually quite complicated in a quantum universe with as many qubits as ours has, but, just as QM may be counterintuitive but eventually looks classical at a large enough scale, time may in fact be massively complicated but at scale it still works as we expect.
  - ridewinter 9y ago
    It's more of a recognition that many-worlds actually has a physical basis to it, while the others talk about an obfuscated non-physical construction.
    - jerf 9y ago
      "Many worlds has a physical basis to it" has no physics-meaning that I can discern. In order to have one, it would have to deviate from the other isomorphisms and cease to be an isomorphism, and thus somehow be testable, which it isn't. (My previous post is word salad to some extent too, since I'm jamming it into an HN post. But the core insight there is basically, let the math be the math and stop imposing your globally-based ideas on the local world.) It would have a "physical basis" to it if you could visit the alternatives somehow, in which case they could be demonstrated to physically exist and all doubt would be removed. But so far that is not on offer.
- akvadrako 9y ago
  Bohmian mechanics is just many worlds in denial, for people with prior ontological beliefs they can't give up. The entire wave-function is there in the theory, containing all the "worlds" and it's responsible for 100% of the dynamics. Additionally, Bohmians add a single "world particle" indicating which world we are in, and it's guided by the wavefunction. They claim only that particle is "real", but since it has no effect on the state of the universe, I would call it the only part of the theory which isn't real.
TheOtherHobbes 9y ago
There are no photons. There are only photon-like events, with associated probabilities and scattering processes. It's a lot easier to assume that reality is the visible output of an heroically counter-intuitive but relatively tractable probabilistic calculation process, than it is to assume there are infinite universes with poorly defined boundary conditions between them and even more poorly defined properties, and that by some magical process some, or maybe all, of those universes conspire together in certain situations only to create quantum effects. The Shor's Algorithm argument is irrelevant. There's a huge leap between saying saying "Qubits can handle superpositions and we don't know why yet" and saying "The only possible explanation is that qubits exist in multiple universes at the same time. Because they just have to. Obviously." The problem is simple - you're inventing entire multiple universes out of nothing just to explain the behaviour of a single qubit. That may indeed be how reality works. but it's a very strong claim and it needs some correspondingly irrefutable evidence to support it.
- ridewinter 9y ago
  That's exactly what I'm saying! All other explanations say something confusing and obfuscating like "there are no photons". Many-worlds keeps it simple and says "there are photons, and the reason they behave like that is because they're interacting with parallel universes". You can't explain why they behave like that, so you say that the photon isn't really real, and have a complicated explanation of it.
  - BoiledCabbage 9y ago
    Clearly this is all subjective, but Pilot Wave theory (De Broglie & Bohm) is the simplest least "crazy" of all the explanations. Horribly simplified: Space isn't empty, there is an "ether". Matter moving through the ether interacts with it and produces waves just like a very light boat in a lake - these crests and troughs exactly line up with the magical waveform. All of the quantum behavior we see is explained by the waves in this "ether". Ie the waveform is no longer this crazy math thing its a real thing. And just like "air" was considered empty until we discovered a vacuum, space itself is considered empty until we fully define the "ether". It matches up with all of the results of Copenhagen, but was dismissed because physicists like "locality" and it gives up locality. But guess what? The surface of a lake is non-local. Your boat is affected by boats far away via waves, the effect just diminishes over distance. And the double slit experiment is so straightforward now that it makes Copenhagen and many-worlds look kinda silly.
mikeash 9y ago
The atom is a wave, which can interfere with itself.
- ridewinter 9y ago
  I think Deutsch would say there isn't actually a wave-particle duality. That's just an obfuscation that doesn't have any reality to it. Many-worlds has a much simpler explanation - there is simply a particle that interacts with particles in parallel universes that produce a wave-like interference effect.
  - mikeash 9y ago
    He might say that, but so what? It's clear that he favors many-worlds, but that doesn't mean the rest of us have to. My point is that he's attempting to prove many-worlds by assuming that none of the other explanations are true, i.e. he's begging the question.
    - ridewinter 9y ago
      Well the other explanations say that the atom doesn't really exist, it's just an abstraction. And many-worlds says the atom does exist, and this is why you see these quantum effects. That's a meaningful difference.
      - mikeash 9y ago
        I'm pretty sure that's not what the other explanations say at all.
      - BoiledCabbage 9y ago
        Pilot Wave is to Many Worlds, as Many Worlds is to Copenhagen. Pilot wave is the most "concrete". If you're moving away from abstraction, then why stop at Many Worlds?
    - deleted 9y ago
      [deleted]
eximius 9y ago
What? So in other universes the interference pattern is wrong? A photon isn't a computer. It doesn't compute. It acts according to some nature which we equate with computation.
graycat 9y ago
Double slit experiment? It appears that some people, e.g., Feynman, believe that this experiment is profound. I don't have an explanation as good as I would like, but from what I studied in physics class, reading Feynman, etc. here's my explanation. So, shoot a photon, or an electron, neutron, proton, alpha particle, etc., at the double slit barrier. Only one particle, please. Well, now give up on the idea that we shot a particle. F'get about particles because they don't exist and only seem to exist from some interactions. Instead of a particle, what we shoot at the target is a wave or, in quantum mechanics, a wave function. So, the wave function hits the barrier. Nearly all that wave function hits the barrier and disappears. Why? Because physics can't think of anything else it could do. So, what is left are the two parts of the wave function that pass through the two slits. Now that one wave function is in two pieces, but no energy has been lost bumping into the barrier and squeezing through the double slits (amazing thing a wave function) -- but maybe with some barriers some of the wave function is reflected, but let's leave that to the next semester! The two pieces of the wave function that went through the slits continue on to our array of detectors. As wave functions tend to do, especially as they pass through a small hole or slit, they spread out. By the time the two parts of the wave function hit the detectors, they overlap. But, they are just two parts of the same wave function. So, they interfere and create a sine wave pattern at the detectors. Well one of the detectors, at random, gets a detection of the particle. So, lesson: What travels is a wave, not a point like particle. The wave can be split into two parts and come back together, and then it interferes with itself in a way different from having waves from two independent (I hope the physics people do say independent instead of uncorrelated) particles hitting the detectors. When the wave is detected the detection is at a point as if there were a particle. Moreover, there is no more than one detection, as if there were a particle. Still, there are no particles; it's just that when a wave function interacts, say, at a detector, it interacts at a point, just some one point, as if it were a point particle. Amazing. Amazing experiment. Generalize the experiment some and get the Michelson-Morley experiment, even more amazing.
- lisper 9y ago
  > So, the wave function hits the barrier. Nearly all that wave function hits the barrier and disappears. No, it doesn't disappear. The barrier itself is illuminated. Most of the wave function that hits the barrier is absorbed (or reflected) by the barrier. But the light that hits the barrier is uninteresting so it is ignored. (On the other hand, if you make a reflective barrier with a lot of slits, not just two, then you have a diffraction grating and things get interesting again.)
EliRivers 9y ago
When I let go of something, it falls to the floor and hits it at exactly the correct speed. The correct speed accounting for gravity and the air and air currents and all sorts of other things. That's a lot of calculations to make. And it was exactly correct at every instant of the fall as well! Somehow, all these calculations are being made instantly. Where are those calculations occurring?

raverbashing 9y ago

Bohmian mechanics seems like a better explanation than "there are infinite universes out there"

mr_overalls 9y ago
"We don't understand the deeper mechanisms yet" is also a valid explanation - although one that's sure to be unpopular with scientists. "Entia non sunt multiplicanda praeter necessitatem" and all that.
ridewinter 9y ago
I'm sure you could confuse the heck out of me trying to explain it. Deutsch would say it's just a clever obfuscation of what's really happening. And his many-worlds explanation is so incredibly simple that my small mind uses Occam's razor to figure out what to believe.
- raverbashing 9y ago
  As simple as using God to explain how the universe came to be It's not about a simple explanation, it's about explaining using simple things. A wave is simpler than "infinite worlds"
  - akvadrako 9y ago
    The wavefunction is exactly the same as "infinite worlds". The essential aspect of the Everett / many-worlds view is that the wavefunction is the complete description of reality. The "worlds" are just approximate ways of slicing up the wavefunction which correspond to our sense of classical reality. They are clearly (and uncontroversially) identifiable if you run a simulation.
    - raverbashing 9y ago
      Ah that's a better explanation
  - yorwba 9y ago
    The "worlds" are just points in the space that supports the wave. You can use any other name, if you prefer, but there seem to be an infinite number of them.
    - millstone 9y ago
      All quantum mechanics has notions of "points in space that support the wavefunction," but these are different from the "worlds" in MWI.
      - yorwba 9y ago
        Seems like I have been misunderstanding MWI all the time. How are they different?
jerf 9y ago
Yes, I find the whole "Explain this in any other way!" sort of a bizarre demand. You've got your choice of 5 or 6 decent isomorphic explanations. Preference for "many worlds" seems to boil down to "it annoys my common sense less than some of the others", which is a very personal concern rather than a scientific one. Personally, I actually think retrocausality, once you twist your brain around it, is another perfectly sensible option that doesn't involve multiplying the universe into a near-infinity of inaccessible, unobservable parallels. The solution in my mind is that retrocausality actually isn't; what it means instead is that what "the present" is is even less like what you thought it was, and is even more of a partial order than relativity already told us it was. The answer to the "double-slit" experiment, to my mind, is that there simply isn't a "before" and "after" and some sort of "retrocausal self-interference" of the wave... the whole thing is simply one atomic event that doesn't have a before and an after, and your imposition of a temporal order on the events is much simplified from the reality. The mystery lies in your attempt to impose a global temporal order, which is appropriate enough at scale, onto a single local event where it doesn't apply in the way you expect. The results of the double-slit experiment itself is not that mysterious, merely counterintuitive, once you release the small scale from your imposed large-scale idea of how time flows. It means that "the present" is actually quite complicated in a quantum universe with as many qubits as ours has, but, just as QM may be counterintuitive but eventually looks classical at a large enough scale, time may in fact be massively complicated but at scale it still works as we expect.
- ridewinter 9y ago
  It's more of a recognition that many-worlds actually has a physical basis to it, while the others talk about an obfuscated non-physical construction.
  - jerf 9y ago
    "Many worlds has a physical basis to it" has no physics-meaning that I can discern. In order to have one, it would have to deviate from the other isomorphisms and cease to be an isomorphism, and thus somehow be testable, which it isn't. (My previous post is word salad to some extent too, since I'm jamming it into an HN post. But the core insight there is basically, let the math be the math and stop imposing your globally-based ideas on the local world.) It would have a "physical basis" to it if you could visit the alternatives somehow, in which case they could be demonstrated to physically exist and all doubt would be removed. But so far that is not on offer.
akvadrako 9y ago
Bohmian mechanics is just many worlds in denial, for people with prior ontological beliefs they can't give up. The entire wave-function is there in the theory, containing all the "worlds" and it's responsible for 100% of the dynamics. Additionally, Bohmians add a single "world particle" indicating which world we are in, and it's guided by the wavefunction. They claim only that particle is "real", but since it has no effect on the state of the universe, I would call it the only part of the theory which isn't real.

TheOtherHobbes 9y ago

There are no photons. There are only photon-like events, with associated probabilities and scattering processes. It's a lot easier to assume that reality is the visible output of an heroically counter-intuitive but relatively tractable probabilistic calculation process, than it is to assume there are infinite universes with poorly defined boundary conditions between them and even more poorly defined properties, and that by some magical process some, or maybe all, of those universes conspire together in certain situations only to create quantum effects. The Shor's Algorithm argument is irrelevant. There's a huge leap between saying saying "Qubits can handle superpositions and we don't know why yet" and saying "The only possible explanation is that qubits exist in multiple universes at the same time. Because they just have to. Obviously." The problem is simple - you're inventing entire multiple universes out of nothing just to explain the behaviour of a single qubit. That may indeed be how reality works. but it's a very strong claim and it needs some correspondingly irrefutable evidence to support it.

ridewinter 9y ago
That's exactly what I'm saying! All other explanations say something confusing and obfuscating like "there are no photons". Many-worlds keeps it simple and says "there are photons, and the reason they behave like that is because they're interacting with parallel universes". You can't explain why they behave like that, so you say that the photon isn't really real, and have a complicated explanation of it.
- BoiledCabbage 9y ago
  Clearly this is all subjective, but Pilot Wave theory (De Broglie & Bohm) is the simplest least "crazy" of all the explanations. Horribly simplified: Space isn't empty, there is an "ether". Matter moving through the ether interacts with it and produces waves just like a very light boat in a lake - these crests and troughs exactly line up with the magical waveform. All of the quantum behavior we see is explained by the waves in this "ether". Ie the waveform is no longer this crazy math thing its a real thing. And just like "air" was considered empty until we discovered a vacuum, space itself is considered empty until we fully define the "ether". It matches up with all of the results of Copenhagen, but was dismissed because physicists like "locality" and it gives up locality. But guess what? The surface of a lake is non-local. Your boat is affected by boats far away via waves, the effect just diminishes over distance. And the double slit experiment is so straightforward now that it makes Copenhagen and many-worlds look kinda silly.

mikeash 9y ago

The atom is a wave, which can interfere with itself.

ridewinter 9y ago
I think Deutsch would say there isn't actually a wave-particle duality. That's just an obfuscation that doesn't have any reality to it. Many-worlds has a much simpler explanation - there is simply a particle that interacts with particles in parallel universes that produce a wave-like interference effect.
- mikeash 9y ago
  He might say that, but so what? It's clear that he favors many-worlds, but that doesn't mean the rest of us have to. My point is that he's attempting to prove many-worlds by assuming that none of the other explanations are true, i.e. he's begging the question.
  - ridewinter 9y ago
    Well the other explanations say that the atom doesn't really exist, it's just an abstraction. And many-worlds says the atom does exist, and this is why you see these quantum effects. That's a meaningful difference.
    - mikeash 9y ago
      I'm pretty sure that's not what the other explanations say at all.
    - BoiledCabbage 9y ago
      Pilot Wave is to Many Worlds, as Many Worlds is to Copenhagen. Pilot wave is the most "concrete". If you're moving away from abstraction, then why stop at Many Worlds?
  - deleted 9y ago
    [deleted]

eximius 9y ago

What? So in other universes the interference pattern is wrong? A photon isn't a computer. It doesn't compute. It acts according to some nature which we equate with computation.

graycat 9y ago

Double slit experiment? It appears that some people, e.g., Feynman, believe that this experiment is profound. I don't have an explanation as good as I would like, but from what I studied in physics class, reading Feynman, etc. here's my explanation. So, shoot a photon, or an electron, neutron, proton, alpha particle, etc., at the double slit barrier. Only one particle, please. Well, now give up on the idea that we shot a particle. F'get about particles because they don't exist and only seem to exist from some interactions. Instead of a particle, what we shoot at the target is a wave or, in quantum mechanics, a wave function. So, the wave function hits the barrier. Nearly all that wave function hits the barrier and disappears. Why? Because physics can't think of anything else it could do. So, what is left are the two parts of the wave function that pass through the two slits. Now that one wave function is in two pieces, but no energy has been lost bumping into the barrier and squeezing through the double slits (amazing thing a wave function) -- but maybe with some barriers some of the wave function is reflected, but let's leave that to the next semester! The two pieces of the wave function that went through the slits continue on to our array of detectors. As wave functions tend to do, especially as they pass through a small hole or slit, they spread out. By the time the two parts of the wave function hit the detectors, they overlap. But, they are just two parts of the same wave function. So, they interfere and create a sine wave pattern at the detectors. Well one of the detectors, at random, gets a detection of the particle. So, lesson: What travels is a wave, not a point like particle. The wave can be split into two parts and come back together, and then it interferes with itself in a way different from having waves from two independent (I hope the physics people do say independent instead of uncorrelated) particles hitting the detectors. When the wave is detected the detection is at a point as if there were a particle. Moreover, there is no more than one detection, as if there were a particle. Still, there are no particles; it's just that when a wave function interacts, say, at a detector, it interacts at a point, just some one point, as if it were a point particle. Amazing. Amazing experiment. Generalize the experiment some and get the Michelson-Morley experiment, even more amazing.

lisper 9y ago
> So, the wave function hits the barrier. Nearly all that wave function hits the barrier and disappears. No, it doesn't disappear. The barrier itself is illuminated. Most of the wave function that hits the barrier is absorbed (or reflected) by the barrier. But the light that hits the barrier is uninteresting so it is ignored. (On the other hand, if you make a reflective barrier with a lot of slits, not just two, then you have a diffraction grating and things get interesting again.)

EliRivers 9y ago

When I let go of something, it falls to the floor and hits it at exactly the correct speed. The correct speed accounting for gravity and the air and air currents and all sorts of other things. That's a lot of calculations to make. And it was exactly correct at every instant of the fall as well! Somehow, all these calculations are being made instantly. Where are those calculations occurring?