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The argument in favor of the Everettian multiverse (“where else could the computation have happened, if it wasn’t being farmed out to parallel universes?”) seem

The argument in favor of the Everettian multiverse (“where else could the computation have happened, if it wasn’t being farmed out to parallel universes?”) seems illogical to me. Aren't these parallel universes running the same computation at the same time, and thus also "farming out" part of their computations to us? If so, it's a zero-sum game, so how could there be an overall performance gain for all the universes?

Filligree 2y ago
That's not really how MWI works. There isn't some fixed number of universes; actually, there aren't really distinct universes (or timelines) at all. Attempting to count them is about like trying to measure the length of a coastline; they blend together once you zoom far enough in. Running the quantum computer causes 'new timelines' to be created. Though so would ordinary atoms just sitting there; the tricky thing about quantum computers is making it so the split is temporary. So the quantum computer gets split into multiple versions of itself, does some computation in each, and merges the results. This isn't map-reduce; there's a strictly limited set of ways in which you can do the merger, all of which are weird from a classical perspective. You can argue for MWI based on this, because the computations that got merged still had to happen somewhere. It's incompatible with Copenhagen, more so the bigger and longer-lasting the computation gets. It's not, strictly speaking, incompatible with pilot wave theory; but pilot wave theory is MWI plus an additional declaration that "Here, you see this timeline here? That's the real one, all the others are fake. Yes, all the computation needed to instantiate them still happens, but they lack the attribute of being real." Though that makes PWT incompatible with computationalism, and hence with concepts such as mind-uploading. Which is a bullet you can choose to bite, of course...
- munchler 2y ago
  Thank you. This makes sense.
- monero-xmr 2y ago
  I’d prefer to say we just go back in time the moment it collapses. How can you disagree with me? No proof of anything and we can all contrive untestable solutions. No wait, it’s actually God who exists in the wave collapse, and His divine intervention does the computation.
  - astrange 2y ago
    > I’d prefer to say we just go back in time the moment it collapses. How can you disagree with me? No proof of anything and we can all contrive untestable solutions. That's close to retrocausality which is a serious theory/interpretation of quantum mechanics.
    - monero-xmr 2y ago
      I'd like to see some serious attempts to prove any of these theories. If not, we can just wildly imagine all sorts of possibilities.
- fidotron 2y ago
  The easy comparison for devs is quantum computing is like git branching. The big question being the PR approval process.
  - whimsicalism 2y ago
    no that's not a good comparison for MWI, it's more continuous
    - fidotron 2y ago
      That sounds like you having a narrow understanding of git - there is no way it is not continuous.
      - whimsicalism 2y ago
        git branches are not continuous
        fidotron 2y ago
        You mean as opposed to discrete? (And not in the usual sense of continuous) There is nothing stopping you putting probability distributions in git and branching them.
        whimsicalism 2y ago
        i think we are just not connecting in what we are saying, nbd
        dartos 2y ago
        Git operates on commits (branches are really nicknames for specific commits with some extra features), which are discrete steps. That’s what’s stopping you from doing anything continuous in git.
  - amelius 2y ago
    Maybe our consciousness also branches, and all branches are approved.
- adastra22 2y ago
  I don't think this is accurate. There is no classical computation happening here, and there is no reason you have to have "room" for the classical computation to happen. That seems to be assuming that the universe is a classical substrate and a classical algorithm has to be instantiated somewhere. Quantum computing isn't classical computing. It isn't a Turing machine. It is a fundamentally different kind of information processing device, making use of non-classical physical phenomena. I'm not a defender of Copenhagen, but the wave collapse interpretation has no difficulty explaining quantum computation. A quantum computer creates extremely large, complexly entangled wave functions that upon collapse result in states that can be interpreted as solutions to problems that were encoded in the sequence of operations that setup the entangled wave function. The Everett interpretation is easier to think about in my opinion, and I prefer thinking in terms of MWI when I try to make sense of these results. But it is not necessary. Computer science is the study of universal Turing machines and their application. But Turing machines are only “universal” in the sense that they can represent any statement of mathematical logic, and that can (we believe) be used to simulate anything we can dream up. But there are intrinsic performance limitations of Turing machines, studied by algorithmic theory, which are artifacts of the Turing machine itself, not physical limitations of the universe we live in. That searching an unordered list with a serial processor takes O(n) time, for example. Grover showed that there are non-Turing machine quantum processes that could be used to perform the same computation in O(sqrt(n)) time. That doesn't mean we need to go looking for "where did that computation actually happen". That doesn't even make sense.
  - slowmovintarget 2y ago
    According to this ( https://arxiv.org/pdf/2302.10778 https://arxiv.org/pdf/2302.10778 ), there are no branches, just decoherence (minus the "collapse").
    - adastra22 2y ago
      A paper proposing a radically different formulation of quantum mechanics, with only a half dozen non-author-citations, so heavy with jargon that it's hard to figure out what the author is claiming. Sorry, my crank meter is dialed to 11 right now and I don't think this is worth spending further time on.
- athrowaway3z 2y ago
  [flagged]
- 3form 2y ago
  Why does the computation have to "happen somewhere"? I understand the desire to push the boundary of the classical reasoning, but I don't think it's something to be taken as an axiom, and I haven't heard about any proof either.
- feoren 2y ago
  There's no reason to think the computation is "happening" anywhere but our universe, or even "happening" at all in any real sense. Light follows a geodesic. Atomic reactions tend to minimize energy. Rivers follow the most efficient path to the sea. Where does any of that computation "happen"? Why is it any stranger that an electron "knows how" to follow a gradient in an electric field than that a quantum computer "knows how" to perform a random circuit sampling?
deleted 2y ago
[deleted]
adastra22 2y ago
I’m not sure the sibling comment is correct, so I’ll take another stab at it. First, I also think the “this favors the Everett interpretation” argument is incoherent, but for different reasons. I won’t be defending that view. But I can try to answer your question about the nature of computation and “where” that computation is taking place. A quantum computer is essentially a random number generator, but with a non-uniform distribution that can be programmatically controlled. Let’s start with the random part: a qubit can be either a 1 or a 0. When first initialized, when you then read it you will get either a zero or a one with 50% probability. (I am oversimplifying to the point of error! This is technically incorrect, but please excuse me for the purpose of a simplified explanation that makes sense to computer scientists rather than quantum physicists.) In the Copenhagen interpretation the wave function randomly collapsed in a way that can be measured as a 0 or as a 1 with 50% probability for either outcome. In the Everett interpretation there are two universes (really, two disjoint sets of universes): one where you see a 0, and the other where you see a 1, and your likelihood of ending up in either is 50%. For example, with three qubits you can read a random value between 0 to 7, where each qubit provides a 0 or 1 of a classical 3-bit binary value. If you're making a D&D simulator, you can take three of these registers (9 qubits total) and read them to get three values between 0 to 7, then increment each and add together to get the simulated value of three 8-sided dice, for the attack value of some game weapon. But so far we're assuming even and independent odds for a 0 or 1 in each qubit. A quantum gate operation lets you modify that probability distribution in interesting ways. Specifically you can take multiple qubits and entangle their state such that the eventual outcome (still random) is constrained to a non-uniform probability distribution. (The details don't matter, but it has to do with the fact that you can add wave amplitude, which is complex-valued and therefore can constructively or destructively interfere.) So instead of reading three separate 1d8 registers and adding them together, we can use quantum gate operations to perform the addition: entangling the state of a new 4 qubit register (large enough to hold the sum of two 3 qubit registers) such that it represents the sum of two separate, randomly chosen with independent probability values between 0..7. The combined sum will be a random value between 0 and 14, but with the value 7 being most likely, 6 or 8 next most likely, etc. (You can see the probabilities here: https://anydice.com https://anydice.com) We then add in the third 3-bit register, getting a 5-qubit result. For good measure, we then add a constant 3 value using quantum gate operations so the result will be what we expect for adding 3 dice rolls, which start counting at 1 rather than 0. Now we have a great random number generator for D&D games. It's a 5-bit register that when you read it will give you a value between 3 and 24, with exactly the probabilities you would expect of three rolls of a fair 8-sided die. You can interpret what this means in Copenhagen vs Everett, but there isn't a way in which it makes material difference. I will say that the Everett interpretation is easier to think about: you setup the entangled operations such that there simply isn't a universe in which the register reads 31, or some value not representative of a 3d8 dice roll, and if you count the number of possible universes under the born rules and holding the rest of the multiverse constant, the counts match the 3d8 dice probability distribution. So you don't know which universe you will end up in when you finally read the register and, under the many worlds interpretation, split off into one of many possibilities. But you know it will be a good 3d8 dice roll for your D&D game. Now imagine you have two 1,024 qubit registers (each with random probability for each bit), and you run a full set of shift-adders to multiply them together into a 2,048 qubit register. If you read the result, it'll be a random non-prime number. Why? Because it's necessarily the product of two 1,024 composites. What's interesting is that you can kinda do the reverse, as Peter Shor figured out how to do in the 90's: start with a 2,048 bit register, run the shift-adder in reverse (really, the method of continued fractions, but I'm going for an intuition pump here), then read the two 1,024 registers. What you will get is two values that when multiplied together will result in the input, the factors. If you put "60" in the input, you will maybe get the outputs (15, 4), or (5, 12), or (2, 30), or (1, 60). It's random which will pop out, but what you get WILL be a factorization of 60. Now put your bank's SSL public key in the register, and read the factors out. The factors are the private key. I don't think any talk of multiverses is needed to understand this. It's simply mucking around with the probability distribution of a random number generator in clever enough ways that the random number generated, whatever it is, will be A solution (perhaps one of many) to your problem. You do this by destructively interfering the register with wave functions that represent non-solutions, and keep doing this until only solutions are left as possible values. Then you read your random number register and know the result is drawn from the solution set.
klabb3 2y ago
> “where else could the computation have happened, if it wasn’t being farmed out to parallel universes?” Right. I’m definitely a layman here but as a CS generalist this rings out as narrow-minded to me, biased towards the way we designed physically and then mathematically formalized “computation”. Not that I have anything against multiverses but.. if I had to choose between “Turing-style computation happened, but needed parallel universes to do so” and “what happened is something counter-intuitive and insufficiently understood about the universe we inhabit” I would bet on the latter.
- adastra22 2y ago
  I'm an actual physicist here. Not in quantum computers, but that was my advisor's field and I've kept paying attention in the years since. Your intuition here is correct, I believe. Classical computer science deals with information processing limits of devices constructed using the classical physics laws of Newton and Maxwell. Quantum physics introduces new elements that permit new kinds of information processing machines, which have no obligation to be constrained by classical law.
  - ayewo 2y ago
    > Quantum physics introduces new elements that permit new kinds of information processing machines, which have no obligation to be constrained by classical law. Question then becomes, how do you build such non-constrained machines? Also, how do you confirm that such machines—or even small scale prototypes—are not constrained by classic laws of physics?
    - rlupi 2y ago
      > Question then becomes, how do you build such non-constrained machines? Also, how do you confirm that such machines—or even small scale prototypes—are not constrained by classic laws of physics? You mean, like... transistors? ;-) https://en.wikipedia.org/wiki/History_of_the_transistor https://en.wikipedia.org/wiki/History_of_the_transistor (search for quantum)
    - adastra22 2y ago
qnleigh 2y ago
But to emphasize one quote from the blog: "the new experiment doesn’t add anything new to this old debate" about multiverse interpretations vs. something else. An equally viable and perhaps conceptually simpler interpretation of the experiment is that the qubits are temporarily put in a 'superposition' of many different bitstrings, some operations are performed, and then measurement collapses this superposition back into a single definite bitstring. No multiverse required.
- superq 2y ago
  > No multiverse required. True, but no exciting plot twists either. :(
  - galaxyLogic 2y ago
    Multiple Universes is a "Deus ex Machina" applied by Hollywood a lot these days.
fizx 2y ago
The Everettian multiverse is a bit like seeing some bubbles and foam in the surf and concluding that there are "many oceans." Yes, there are many surfaces of water, but calling each bubble an ocean is a bit disingenuous. Like the bubble analogy, new Everettian universes are local, small, and the total amount of probability density (water in this analogy) is conserved.
Strilanc 2y ago
The main issue with this line of argument is that you don't see people who like other interpretations claiming quantum computers won't work. Saying quantum computers imply many worlds is the classic mistake of wanting to show A=>B and arguing B=>A instead of ~B=>~A. If quantum computers were inconsistent with collapse interpretations, you'd expect people who think collapse interpretations are correct to be confidently predicting quantum computers will never work. But they don't; they just think the state inside the computer isn't collapsing. I'm often tempted to argue quantum computers at least clearly favor ontic interpretations (ones where quantum states are real). Because good luck computing things by subjectively having a computer instead of actually having a computer. But, as far as I know, you don't see quantum-bayesianism-ists having any qualms with quantum computers. I think because if you're already biting the bullet of interpreting diagonally polarized light as subjective, and Bell tests as subjective, then interpreting a computation as subjective isn't fundamentally different. It's just more in your face about it.
- adastra22 2y ago
  Quantum computers do defeat hidden-variable models, which would have to hide the computation somewhere. But yeah, Bell and EPR ruled those out years ago anyway.
  - Strilanc 2y ago
    I don't see people who believe hidden variable models, like pilot wave, claiming quantum computers won't work. So I don't think quantum computers disprove hidden variable models. I do agree quantum computers disprove local hidden variable models. Because they can run Bell tests, and local hidden variable models can't violate the Bell inequalities.
    - adastra22 2y ago
      Last time I checked pilot wave theory was not developed enough to permit modeling of quantum computers, or at least no one had done the work yet to apply the theory to that use case. It is very undeveloped.
    - 2y ago
S_A_P 2y ago
I am also curious if Mark Everett, Hugh’s son, has an account here.
- munchler 2y ago
  EELS!
- Mistletoe 2y ago
  In many worlds, he certainly does.
blueprint 2y ago
It’s not necessary for there to be parallel universes when all possible paths are already taken within this universe.
- kokanee 2y ago
  Parallel universes enter the conversation because we (non physicists) can't identify any scenario explainable with classical physics that allowed those paths to be taken in the allotted time and space.
  - blueprint 2y ago
    Have a look at e.g. the path integral. It's a formulation for calculating the result of the mechanism behind probabilistic mechanics. I like to use the word "possible" instead of "probable", when explaining it, sometimes. When a car is hurtling down a road, it cannot really diverge from its "classical" path too much because it is already highly localized and entangled. There's a single car we're talking about. But a single isolated particle is not even so localized as to be able to be called a particle, but more so a wave of possibility of measuring in a certain state, such as a location. (I'm still a student though, so I do not yet fully understand how particle location and space-patch entanglement connect.) But, anyway, as I grasp it so far... nature already makes it possible to look like one can "take multiple paths" when one is coherent, i.e. isolated from entanglement with environment during that path, i.e. where the "which path" is not already decided via e.g. measurement. For example, the Lamb shift is the consequence of the fact that the universe really doesn't know which paths of particle creation and annihilation, if any, the force carriers between the electron and the proton took. So, it is able to take all of them. Why should it rather be our expectation rather than how nature says it can work? It seems like a mental contortion is needed - but nature really does operate based on possibility because it's the consequence of the more fundamental fact that there being no one / nothing in the entire universe, including the particle itself, that "knows" what state it's in or going to be in, during a certain span of the system's evolution. At the moment there is such a record keeper, the influence of "possibility" diminishes.
AnotherGoodName 2y ago
You just need to devise a system where the vast majority of universes give the right answer for this to work. Start by at least having a universe for each possibility so that every code path is computed. Then add a mechanism to create many more universes when a correct result is hit. So each wrong result has 1 universe and the correct result alone has 2^300 a universes. Run this. You now end up with the correct result 99.99999% of the time. I’m not arguing on the above take or not fwiw, it’s just that it easy to see how this could happen in many worlds. Effectively error correction just becomes a mechanism to create more universes for correct answers than incorrect answers and it all works. In fact it’s very reasonable to think of quantum error correction this way (it really is a mechanism to favour correct answers being observed and in many worlds that means it creates more correct answer universes).
- skirmish 2y ago
  > add a mechanism to create many more universes when a correct result is hit Simple count has not much to do with the outcome observed, IMU each branch in a split is not equally likely. If you finely sub-divide a low-probability branch into 2^300 sub-branches, so what. Instead, you need to merge multiple high probability branches back into a single outcome that is observed "99.99999% of the time".
  - AnotherGoodName 2y ago
    In many worlds the probability observed can absolutely be thought of as the ratio of worlds with that outcome vs another. Any specific numbers here are just as an example.
- knome 2y ago
  This feels like taking quantum bogo sort seriously.
lmm 2y ago
> Aren't these parallel universes running the same computation at the same time, and thus also "farming out" part of their computations to us? If so, it's a zero-sum game, so how could there be an overall performance gain for all the universes? The result is the same in all universes. There's a kind of scatter-gather/map-reduce feel - each universe computes part of the problem, then you add up all their results to get the final one (in all universes). The argument works for me, although I already believed the conclusion so I'm biased.
laserbeam 2y ago
I think the "parallel universe" analogies are misleading at best. I actually think most "narrative" explanations of quantum physics are as well. For a long time I used to imagine quantum physics with the same language we talk about superheroes and time travel and consciousness and whatever. But then I actually saw some of the math (very basic Hisenberg uncertainty equations) and good comparisons to classical wave mechanics and wave interference. Everything made more sense. As a layman with some very rudimentary understanding of the math, I would ignore ALL talk about "farming out" and "parallel universrs" and similar concepts. I think thise ideas are charged with too many interesting but fantastical elements which you probably don't find in actual quantum mechanics.
- tbrownaw 2y ago
  > I think the "parallel universe" analogies are misleading at best. I actually think most "narrative" explanations of quantum physics are as well. This is inherent to the problem of attempting to describe something that fundamentally does not work like the macroscopic world, using vocabulary rooted in that macroscopic world.
- atq2119 2y ago
  As a layperson with a math background who has dabbled in quantum computing, I agree. It would help to call the "Everettian" interpretation by the original title given by Everett: the theory of the universal wave function. Ignore the techno-babble about the multiverse. The way I understand Everett is simply as taking the math literally and seriously to its logical conclusion.
  - smolder 2y ago
    I think the minimum amount of understanding needed to grasp MWI is understanding the double slit experiment. The interference pattern is real, thus the waves are real, but we only experience one measurable outcome where a photon lands in a specific location. The probabilistic part is not which outcome happens but which outcome your reality follows.
    - 2y ago
ssener2001 2y ago
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Filligree 2y ago

That's not really how MWI works. There isn't some fixed number of universes; actually, there aren't really distinct universes (or timelines) at all. Attempting to count them is about like trying to measure the length of a coastline; they blend together once you zoom far enough in. Running the quantum computer causes 'new timelines' to be created. Though so would ordinary atoms just sitting there; the tricky thing about quantum computers is making it so the split is temporary. So the quantum computer gets split into multiple versions of itself, does some computation in each, and merges the results. This isn't map-reduce; there's a strictly limited set of ways in which you can do the merger, all of which are weird from a classical perspective. You can argue for MWI based on this, because the computations that got merged still had to happen somewhere. It's incompatible with Copenhagen, more so the bigger and longer-lasting the computation gets. It's not, strictly speaking, incompatible with pilot wave theory; but pilot wave theory is MWI plus an additional declaration that "Here, you see this timeline here? That's the real one, all the others are fake. Yes, all the computation needed to instantiate them still happens, but they lack the attribute of being real." Though that makes PWT incompatible with computationalism, and hence with concepts such as mind-uploading. Which is a bullet you can choose to bite, of course...

munchler 2y ago
Thank you. This makes sense.
monero-xmr 2y ago
I’d prefer to say we just go back in time the moment it collapses. How can you disagree with me? No proof of anything and we can all contrive untestable solutions. No wait, it’s actually God who exists in the wave collapse, and His divine intervention does the computation.
- astrange 2y ago
  > I’d prefer to say we just go back in time the moment it collapses. How can you disagree with me? No proof of anything and we can all contrive untestable solutions. That's close to retrocausality which is a serious theory/interpretation of quantum mechanics.
  - monero-xmr 2y ago
    I'd like to see some serious attempts to prove any of these theories. If not, we can just wildly imagine all sorts of possibilities.
fidotron 2y ago
The easy comparison for devs is quantum computing is like git branching. The big question being the PR approval process.
- whimsicalism 2y ago
  no that's not a good comparison for MWI, it's more continuous
  - fidotron 2y ago
    That sounds like you having a narrow understanding of git - there is no way it is not continuous.
    - whimsicalism 2y ago
      git branches are not continuous
      - fidotron 2y ago
        You mean as opposed to discrete? (And not in the usual sense of continuous) There is nothing stopping you putting probability distributions in git and branching them.
        whimsicalism 2y ago
        i think we are just not connecting in what we are saying, nbd
        dartos 2y ago
        Git operates on commits (branches are really nicknames for specific commits with some extra features), which are discrete steps. That’s what’s stopping you from doing anything continuous in git.
- amelius 2y ago
  Maybe our consciousness also branches, and all branches are approved.
adastra22 2y ago
I don't think this is accurate. There is no classical computation happening here, and there is no reason you have to have "room" for the classical computation to happen. That seems to be assuming that the universe is a classical substrate and a classical algorithm has to be instantiated somewhere. Quantum computing isn't classical computing. It isn't a Turing machine. It is a fundamentally different kind of information processing device, making use of non-classical physical phenomena. I'm not a defender of Copenhagen, but the wave collapse interpretation has no difficulty explaining quantum computation. A quantum computer creates extremely large, complexly entangled wave functions that upon collapse result in states that can be interpreted as solutions to problems that were encoded in the sequence of operations that setup the entangled wave function. The Everett interpretation is easier to think about in my opinion, and I prefer thinking in terms of MWI when I try to make sense of these results. But it is not necessary. Computer science is the study of universal Turing machines and their application. But Turing machines are only “universal” in the sense that they can represent any statement of mathematical logic, and that can (we believe) be used to simulate anything we can dream up. But there are intrinsic performance limitations of Turing machines, studied by algorithmic theory, which are artifacts of the Turing machine itself, not physical limitations of the universe we live in. That searching an unordered list with a serial processor takes O(n) time, for example. Grover showed that there are non-Turing machine quantum processes that could be used to perform the same computation in O(sqrt(n)) time. That doesn't mean we need to go looking for "where did that computation actually happen". That doesn't even make sense.
- slowmovintarget 2y ago
  According to this ( https://arxiv.org/pdf/2302.10778 https://arxiv.org/pdf/2302.10778 ), there are no branches, just decoherence (minus the "collapse").
  - adastra22 2y ago
    A paper proposing a radically different formulation of quantum mechanics, with only a half dozen non-author-citations, so heavy with jargon that it's hard to figure out what the author is claiming. Sorry, my crank meter is dialed to 11 right now and I don't think this is worth spending further time on.
athrowaway3z 2y ago
[flagged]
3form 2y ago
Why does the computation have to "happen somewhere"? I understand the desire to push the boundary of the classical reasoning, but I don't think it's something to be taken as an axiom, and I haven't heard about any proof either.
feoren 2y ago
There's no reason to think the computation is "happening" anywhere but our universe, or even "happening" at all in any real sense. Light follows a geodesic. Atomic reactions tend to minimize energy. Rivers follow the most efficient path to the sea. Where does any of that computation "happen"? Why is it any stranger that an electron "knows how" to follow a gradient in an electric field than that a quantum computer "knows how" to perform a random circuit sampling?

deleted 2y ago

[deleted]

adastra22 2y ago

I’m not sure the sibling comment is correct, so I’ll take another stab at it. First, I also think the “this favors the Everett interpretation” argument is incoherent, but for different reasons. I won’t be defending that view. But I can try to answer your question about the nature of computation and “where” that computation is taking place. A quantum computer is essentially a random number generator, but with a non-uniform distribution that can be programmatically controlled. Let’s start with the random part: a qubit can be either a 1 or a 0. When first initialized, when you then read it you will get either a zero or a one with 50% probability. (I am oversimplifying to the point of error! This is technically incorrect, but please excuse me for the purpose of a simplified explanation that makes sense to computer scientists rather than quantum physicists.) In the Copenhagen interpretation the wave function randomly collapsed in a way that can be measured as a 0 or as a 1 with 50% probability for either outcome. In the Everett interpretation there are two universes (really, two disjoint sets of universes): one where you see a 0, and the other where you see a 1, and your likelihood of ending up in either is 50%. For example, with three qubits you can read a random value between 0 to 7, where each qubit provides a 0 or 1 of a classical 3-bit binary value. If you're making a D&D simulator, you can take three of these registers (9 qubits total) and read them to get three values between 0 to 7, then increment each and add together to get the simulated value of three 8-sided dice, for the attack value of some game weapon. But so far we're assuming even and independent odds for a 0 or 1 in each qubit. A quantum gate operation lets you modify that probability distribution in interesting ways. Specifically you can take multiple qubits and entangle their state such that the eventual outcome (still random) is constrained to a non-uniform probability distribution. (The details don't matter, but it has to do with the fact that you can add wave amplitude, which is complex-valued and therefore can constructively or destructively interfere.) So instead of reading three separate 1d8 registers and adding them together, we can use quantum gate operations to perform the addition: entangling the state of a new 4 qubit register (large enough to hold the sum of two 3 qubit registers) such that it represents the sum of two separate, randomly chosen with independent probability values between 0..7. The combined sum will be a random value between 0 and 14, but with the value 7 being most likely, 6 or 8 next most likely, etc. (You can see the probabilities here: https://anydice.com https://anydice.com) We then add in the third 3-bit register, getting a 5-qubit result. For good measure, we then add a constant 3 value using quantum gate operations so the result will be what we expect for adding 3 dice rolls, which start counting at 1 rather than 0. Now we have a great random number generator for D&D games. It's a 5-bit register that when you read it will give you a value between 3 and 24, with exactly the probabilities you would expect of three rolls of a fair 8-sided die. You can interpret what this means in Copenhagen vs Everett, but there isn't a way in which it makes material difference. I will say that the Everett interpretation is easier to think about: you setup the entangled operations such that there simply isn't a universe in which the register reads 31, or some value not representative of a 3d8 dice roll, and if you count the number of possible universes under the born rules and holding the rest of the multiverse constant, the counts match the 3d8 dice probability distribution. So you don't know which universe you will end up in when you finally read the register and, under the many worlds interpretation, split off into one of many possibilities. But you know it will be a good 3d8 dice roll for your D&D game. Now imagine you have two 1,024 qubit registers (each with random probability for each bit), and you run a full set of shift-adders to multiply them together into a 2,048 qubit register. If you read the result, it'll be a random non-prime number. Why? Because it's necessarily the product of two 1,024 composites. What's interesting is that you can kinda do the reverse, as Peter Shor figured out how to do in the 90's: start with a 2,048 bit register, run the shift-adder in reverse (really, the method of continued fractions, but I'm going for an intuition pump here), then read the two 1,024 registers. What you will get is two values that when multiplied together will result in the input, the factors. If you put "60" in the input, you will maybe get the outputs (15, 4), or (5, 12), or (2, 30), or (1, 60). It's random which will pop out, but what you get WILL be a factorization of 60. Now put your bank's SSL public key in the register, and read the factors out. The factors are the private key. I don't think any talk of multiverses is needed to understand this. It's simply mucking around with the probability distribution of a random number generator in clever enough ways that the random number generated, whatever it is, will be A solution (perhaps one of many) to your problem. You do this by destructively interfering the register with wave functions that represent non-solutions, and keep doing this until only solutions are left as possible values. Then you read your random number register and know the result is drawn from the solution set.

klabb3 2y ago

> “where else could the computation have happened, if it wasn’t being farmed out to parallel universes?” Right. I’m definitely a layman here but as a CS generalist this rings out as narrow-minded to me, biased towards the way we designed physically and then mathematically formalized “computation”. Not that I have anything against multiverses but.. if I had to choose between “Turing-style computation happened, but needed parallel universes to do so” and “what happened is something counter-intuitive and insufficiently understood about the universe we inhabit” I would bet on the latter.

adastra22 2y ago
I'm an actual physicist here. Not in quantum computers, but that was my advisor's field and I've kept paying attention in the years since. Your intuition here is correct, I believe. Classical computer science deals with information processing limits of devices constructed using the classical physics laws of Newton and Maxwell. Quantum physics introduces new elements that permit new kinds of information processing machines, which have no obligation to be constrained by classical law.
- ayewo 2y ago
  > Quantum physics introduces new elements that permit new kinds of information processing machines, which have no obligation to be constrained by classical law. Question then becomes, how do you build such non-constrained machines? Also, how do you confirm that such machines—or even small scale prototypes—are not constrained by classic laws of physics?
  - rlupi 2y ago
    > Question then becomes, how do you build such non-constrained machines? Also, how do you confirm that such machines—or even small scale prototypes—are not constrained by classic laws of physics? You mean, like... transistors? ;-) https://en.wikipedia.org/wiki/History_of_the_transistor https://en.wikipedia.org/wiki/History_of_the_transistor (search for quantum)
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qnleigh 2y ago

But to emphasize one quote from the blog: "the new experiment doesn’t add anything new to this old debate" about multiverse interpretations vs. something else. An equally viable and perhaps conceptually simpler interpretation of the experiment is that the qubits are temporarily put in a 'superposition' of many different bitstrings, some operations are performed, and then measurement collapses this superposition back into a single definite bitstring. No multiverse required.

superq 2y ago
> No multiverse required. True, but no exciting plot twists either. :(
- galaxyLogic 2y ago
  Multiple Universes is a "Deus ex Machina" applied by Hollywood a lot these days.

fizx 2y ago

The Everettian multiverse is a bit like seeing some bubbles and foam in the surf and concluding that there are "many oceans." Yes, there are many surfaces of water, but calling each bubble an ocean is a bit disingenuous. Like the bubble analogy, new Everettian universes are local, small, and the total amount of probability density (water in this analogy) is conserved.

Strilanc 2y ago

The main issue with this line of argument is that you don't see people who like other interpretations claiming quantum computers won't work. Saying quantum computers imply many worlds is the classic mistake of wanting to show A=>B and arguing B=>A instead of ~B=>~A. If quantum computers were inconsistent with collapse interpretations, you'd expect people who think collapse interpretations are correct to be confidently predicting quantum computers will never work. But they don't; they just think the state inside the computer isn't collapsing. I'm often tempted to argue quantum computers at least clearly favor ontic interpretations (ones where quantum states are real). Because good luck computing things by subjectively having a computer instead of actually having a computer. But, as far as I know, you don't see quantum-bayesianism-ists having any qualms with quantum computers. I think because if you're already biting the bullet of interpreting diagonally polarized light as subjective, and Bell tests as subjective, then interpreting a computation as subjective isn't fundamentally different. It's just more in your face about it.

adastra22 2y ago
Quantum computers do defeat hidden-variable models, which would have to hide the computation somewhere. But yeah, Bell and EPR ruled those out years ago anyway.
- Strilanc 2y ago
  I don't see people who believe hidden variable models, like pilot wave, claiming quantum computers won't work. So I don't think quantum computers disprove hidden variable models. I do agree quantum computers disprove local hidden variable models. Because they can run Bell tests, and local hidden variable models can't violate the Bell inequalities.
  - adastra22 2y ago
    Last time I checked pilot wave theory was not developed enough to permit modeling of quantum computers, or at least no one had done the work yet to apply the theory to that use case. It is very undeveloped.
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S_A_P 2y ago

I am also curious if Mark Everett, Hugh’s son, has an account here.

munchler 2y ago
EELS!
Mistletoe 2y ago
In many worlds, he certainly does.

blueprint 2y ago

It’s not necessary for there to be parallel universes when all possible paths are already taken within this universe.

kokanee 2y ago
Parallel universes enter the conversation because we (non physicists) can't identify any scenario explainable with classical physics that allowed those paths to be taken in the allotted time and space.
- blueprint 2y ago
  Have a look at e.g. the path integral. It's a formulation for calculating the result of the mechanism behind probabilistic mechanics. I like to use the word "possible" instead of "probable", when explaining it, sometimes. When a car is hurtling down a road, it cannot really diverge from its "classical" path too much because it is already highly localized and entangled. There's a single car we're talking about. But a single isolated particle is not even so localized as to be able to be called a particle, but more so a wave of possibility of measuring in a certain state, such as a location. (I'm still a student though, so I do not yet fully understand how particle location and space-patch entanglement connect.) But, anyway, as I grasp it so far... nature already makes it possible to look like one can "take multiple paths" when one is coherent, i.e. isolated from entanglement with environment during that path, i.e. where the "which path" is not already decided via e.g. measurement. For example, the Lamb shift is the consequence of the fact that the universe really doesn't know which paths of particle creation and annihilation, if any, the force carriers between the electron and the proton took. So, it is able to take all of them. Why should it rather be our expectation rather than how nature says it can work? It seems like a mental contortion is needed - but nature really does operate based on possibility because it's the consequence of the more fundamental fact that there being no one / nothing in the entire universe, including the particle itself, that "knows" what state it's in or going to be in, during a certain span of the system's evolution. At the moment there is such a record keeper, the influence of "possibility" diminishes.

AnotherGoodName 2y ago

You just need to devise a system where the vast majority of universes give the right answer for this to work. Start by at least having a universe for each possibility so that every code path is computed. Then add a mechanism to create many more universes when a correct result is hit. So each wrong result has 1 universe and the correct result alone has 2^300 a universes. Run this. You now end up with the correct result 99.99999% of the time. I’m not arguing on the above take or not fwiw, it’s just that it easy to see how this could happen in many worlds. Effectively error correction just becomes a mechanism to create more universes for correct answers than incorrect answers and it all works. In fact it’s very reasonable to think of quantum error correction this way (it really is a mechanism to favour correct answers being observed and in many worlds that means it creates more correct answer universes).

skirmish 2y ago
> add a mechanism to create many more universes when a correct result is hit Simple count has not much to do with the outcome observed, IMU each branch in a split is not equally likely. If you finely sub-divide a low-probability branch into 2^300 sub-branches, so what. Instead, you need to merge multiple high probability branches back into a single outcome that is observed "99.99999% of the time".
- AnotherGoodName 2y ago
  In many worlds the probability observed can absolutely be thought of as the ratio of worlds with that outcome vs another. Any specific numbers here are just as an example.
knome 2y ago
This feels like taking quantum bogo sort seriously.

lmm 2y ago

> Aren't these parallel universes running the same computation at the same time, and thus also "farming out" part of their computations to us? If so, it's a zero-sum game, so how could there be an overall performance gain for all the universes? The result is the same in all universes. There's a kind of scatter-gather/map-reduce feel - each universe computes part of the problem, then you add up all their results to get the final one (in all universes). The argument works for me, although I already believed the conclusion so I'm biased.

laserbeam 2y ago

I think the "parallel universe" analogies are misleading at best. I actually think most "narrative" explanations of quantum physics are as well. For a long time I used to imagine quantum physics with the same language we talk about superheroes and time travel and consciousness and whatever. But then I actually saw some of the math (very basic Hisenberg uncertainty equations) and good comparisons to classical wave mechanics and wave interference. Everything made more sense. As a layman with some very rudimentary understanding of the math, I would ignore ALL talk about "farming out" and "parallel universrs" and similar concepts. I think thise ideas are charged with too many interesting but fantastical elements which you probably don't find in actual quantum mechanics.

tbrownaw 2y ago
> I think the "parallel universe" analogies are misleading at best. I actually think most "narrative" explanations of quantum physics are as well. This is inherent to the problem of attempting to describe something that fundamentally does not work like the macroscopic world, using vocabulary rooted in that macroscopic world.
atq2119 2y ago
As a layperson with a math background who has dabbled in quantum computing, I agree. It would help to call the "Everettian" interpretation by the original title given by Everett: the theory of the universal wave function. Ignore the techno-babble about the multiverse. The way I understand Everett is simply as taking the math literally and seriously to its logical conclusion.
- smolder 2y ago
  I think the minimum amount of understanding needed to grasp MWI is understanding the double slit experiment. The interference pattern is real, thus the waves are real, but we only experience one measurable outcome where a photon lands in a specific location. The probabilistic part is not which outcome happens but which outcome your reality follows.
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ssener2001 2y ago

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