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I'm a quantum physicist (well, we don't say that, I'm an atomic physicist, but for anyone not aware it is 99% quantum mechanics we do all day), and the MWI isn'

I'm a quantum physicist (well, we don't say that, I'm an atomic physicist, but for anyone not aware it is 99% quantum mechanics we do all day), and the MWI isn't universally accepted, but it's not universally rejected either. Plenty of important physicists interpret quantum mechanics that way, and I do too (I am not important though). That's not to say I'm confident it's correct, just that it's the most sensible way to understand the theory as we know it so far. It might not completely solve the measurement problem, in that the purported derivations of the Born rule are hotly contested, but neither does any other interpretation - it gets further than most. I don't see anyone else except pilot wave theory (which violates locality) deriving the Born rule either. "measurement is not special, and observers are just quantum systems like any other" seems like it ought to be the default assumption to make, in the absence of any evidence to the contrary, and honestly I think the reputation of MWI as a bit crazy comes about for purely historical reasons. Namely, Bohr didn't like it and he held a lot of sway. Physicists are human, and parallel universes is pretty strange, so most people reject the idea without thinking about it in much detail. Most physicists' opinions shouldn't count for much anyway, because the measurement problem simply doesn't come up in our day to day work, so most haven't thought about it much. Furthermore, plenty of physicists don't actually grasp the fundamentals of their own field - they specialise a lot and can use specialised theories to get what they need done without understanding in detail where it came from. So I really think the decoherence people, as cranky as their website looks and as oddly as they write, ought to be the ones whose opinions count on the matter. Edit: seriously, check out their website, straight outta the nineties: http://decoherence.de http://decoherence.de

Xcelerate 10y ago
> Furthermore, plenty of physicists don't actually grasp the fundamentals of their own field I once met a professor at a quantum chemistry conference who argued with me that I could not converge on the exact eigenvalues of a helium atom (assuming a simplified Hamiltonian with a few Coulombic terms). He stated the oft repeated mantra that "the Schrödinger equation can't be solved for any element other than hydrogen", so I wrote a program that brute force diagonalized the Hamiltonian to demonstrate otherwise. He was confusing the notion of a closed-form solution (an analytical expression that gives the solution in terms of specified elementary functions) with that of a numerically exact solution (one that converges on the exact solution but can't be expressed in terms of specified elementary functions). Another professor who was teaching a course on statistical mechanics once said that the single particle wave function is more fundamental than the multi-particle wave function. Nevermind the fact that his research involved density functional theory, which famously fails on those cases where the molecular wave function can't be well approximated using a product of single particle wave functions.
- codethief 10y ago
  > Another professor […] once said that the single particle wave function is more fundamental than the multi-particle wave function. Nevermind […] his research […] where the molecular wave function can't be well approximated using a product of single particle wave functions Depends on what you mean by "multi-particle wave function". The way it is usually understood (I think), it includes all possible tensor products of single-particle wave functions. Then it should be possible, shouldn't it?
  - monktastic1 10y ago
    Tensor products only describe the separable (i.e., unentangled) states.
    - tomonl 10y ago
      As I understand it, that is only true when dealing with density matrices. For example, |0>⊗|0> + |1>⊗|1> is entangled and has tensor products. I think that Xcelerate is correct in saying that all combinations of the basis vectors form the basis of the multi-particle Hilbert space, as a single-particle wavefunction is just a vector/ket.
      - monktastic1 10y ago
        Sure, the tensor product space has a basis that is formed by the tensor products of all pairs of basis elements. But this is different from saying that any particular vector in the space is a tensor product of elements from the individual spaces. But I'm possibly just misunderstanding what you're saying.
    - deleted 10y ago
      [deleted]
    - codethief 10y ago
      Fair enough, I should have been more precise: By "all possible tensor products" I actually meant all elements in the tensor product space, including all linear combinations of tensor products of single-particle states.
M_Grey 10y ago
I think that MWI is pretty much like any ontology, in that you find a lot more interest in it when you're not dealing with physicists. At the end of the day, it doesn't change the math, it doesn't change much of anything, and it probably isn't testable. For me, that has all of the hallmarks of an uninteresting topic.
codethief 10y ago
> Furthermore, plenty of physicists don't actually grasp the fundamentals of their own field - they specialise a lot and can use specialised theories to get what they need done without understanding in detail where it came from. This. Sometimes physics does feel like a religion. The fundamentals (including basic assumptions and "proofs" that are complete bs in the mathematical sense) are getting reiterated time and again till people actually start believing in them. From what I've experienced myself it basically works like this: In the beginning, you come across some claim or fundamental assumption you can't follow and make a mental note to follow up on it and sit down to fully understand it. Then you get sidetracked and next time you come across said claim, you're like "Damn, I wanted to look that up!" and here you are, making a mental note again… Now somewhere between the fifth and tenth time this happens, you've forgotten about your mental note completely and have just accepted the claim as fact. It's basically the brain tricking you into thinking the claim is correct, the more often you read about it. It's like habitual behavior you don't actually process consciously anymore.
- doubleunplussed 10y ago
  I completely agree, and I would say that's probably the historical origin of the Copenhagen interpretation. It was clearly initially a note-to-self that something wasn't right and required figuring out, and somewhere along the line that confusion became an axiom that later generations just accepted, not getting the message that it was unfinished. Physicists aren't dumb, but they just haven't looked into it more because it's surprisingly not relevant to the majority of work. So they're just repeating the confusion of past generations as if it's fact. But, particularly in my field, we're getting seriously good at manipulating more and more exotic quantum states. I'm working on things involving 'weak' measurement, where you really need to know what counts as a measurement or not, and how much - you can do a partial measurement and only partly (appear to) collapse a wavefunction. These types of experiments are becoming more are more common, and it's happened a few times now that I've heard people saying "hang on, what counts as a measurement in this context?" and then having to read a decoherence textbook in order to do their atomic physics. With these sorts of things as well as quantum computers, decoherence and the measurement problem is intruding into a larger proportion of people's work, which means repeating the mantra of the previous generation isn't going to cut it anymore, people are going to actually read decoherence textbooks and decide for themselves.
Razengan 10y ago
I'm a layman and I don't really understand much of this, but I'm really intrigued: What's the general consensus in your field on the existence of more than 3 spatial dimensions? Seems to me, that there are a few phenomena that appear to be more or less random. Could they be perturbations caused by activity in dimensions we cannot perceive?
- adrianratnapala 10y ago
  For first question: I'd say pop-science does a surprisingly good job of conveying the consensus on extra dimensions -- they are a totally reasonable possibility, but their effects are to be felt in realms far beyond the realms that experimental physicists can study well. But if they exist, they probably have important (but unknown) effects. And your proposal that they explain randomness is a good example of that. But to me it doesn't feel right. Quantum randomness somehow doesn't seem to be of that kind. Perturbations from unknown dimensions might well look like randomness, much link an RNG in a computer. But that is still masquerading as a classical random process -- it would not show the weird correlations found in QM.
  - Razengan 10y ago
    I must clarify that by "activity" I don't mean directed actions (i.e. not talking about ghosts or gods) but fundamental elements — the waves and particles and fields — possessing properties and movement in more than 3 dimensions (not including time). Like something poking through or pulling on a fabric from the other side, if such a property has, say, wavelike motion in a 4th spatial dimension, it may produce regular effects that manifest as fundamental features of our 3D space, or seem random to us if a bunch of things are bumping and deterministically interacting with each other in extra dimensions. Has this ever been considered in explaining things like [0] and [1]? Might have something to do with "dark" matter/energy too; i.e. stuff is there, just occupying other dimensions. [0] https://en.wikipedia.org/wiki/Pair_production https://en.wikipedia.org/wiki/Pair_production [1] https://en.wikipedia.org/wiki/Quantum_foam https://en.wikipedia.org/wiki/Quantum_foam
monktastic1 10y ago
Thanks for weighing in. Decoherence is great at explaining the loss of interference. It may even address the "preferred basis" problem (though there seems to be good reason to believe that it might not). But ultimately what I want to know is why I experience just one outcome, and decoherence doesn't really help here. MWI partially resolves that ("you don't just experience one; there are many of you!"), but not in a way that satisfies me.

Xcelerate 10y ago

> Furthermore, plenty of physicists don't actually grasp the fundamentals of their own field I once met a professor at a quantum chemistry conference who argued with me that I could not converge on the exact eigenvalues of a helium atom (assuming a simplified Hamiltonian with a few Coulombic terms). He stated the oft repeated mantra that "the Schrödinger equation can't be solved for any element other than hydrogen", so I wrote a program that brute force diagonalized the Hamiltonian to demonstrate otherwise. He was confusing the notion of a closed-form solution (an analytical expression that gives the solution in terms of specified elementary functions) with that of a numerically exact solution (one that converges on the exact solution but can't be expressed in terms of specified elementary functions). Another professor who was teaching a course on statistical mechanics once said that the single particle wave function is more fundamental than the multi-particle wave function. Nevermind the fact that his research involved density functional theory, which famously fails on those cases where the molecular wave function can't be well approximated using a product of single particle wave functions.

codethief 10y ago
> Another professor […] once said that the single particle wave function is more fundamental than the multi-particle wave function. Nevermind […] his research […] where the molecular wave function can't be well approximated using a product of single particle wave functions Depends on what you mean by "multi-particle wave function". The way it is usually understood (I think), it includes all possible tensor products of single-particle wave functions. Then it should be possible, shouldn't it?
- monktastic1 10y ago
  Tensor products only describe the separable (i.e., unentangled) states.
  - tomonl 10y ago
    As I understand it, that is only true when dealing with density matrices. For example, |0>⊗|0> + |1>⊗|1> is entangled and has tensor products. I think that Xcelerate is correct in saying that all combinations of the basis vectors form the basis of the multi-particle Hilbert space, as a single-particle wavefunction is just a vector/ket.
    - monktastic1 10y ago
      Sure, the tensor product space has a basis that is formed by the tensor products of all pairs of basis elements. But this is different from saying that any particular vector in the space is a tensor product of elements from the individual spaces. But I'm possibly just misunderstanding what you're saying.
  - deleted 10y ago
    [deleted]
  - codethief 10y ago
    Fair enough, I should have been more precise: By "all possible tensor products" I actually meant all elements in the tensor product space, including all linear combinations of tensor products of single-particle states.

M_Grey 10y ago

I think that MWI is pretty much like any ontology, in that you find a lot more interest in it when you're not dealing with physicists. At the end of the day, it doesn't change the math, it doesn't change much of anything, and it probably isn't testable. For me, that has all of the hallmarks of an uninteresting topic.

codethief 10y ago

> Furthermore, plenty of physicists don't actually grasp the fundamentals of their own field - they specialise a lot and can use specialised theories to get what they need done without understanding in detail where it came from. This. Sometimes physics does feel like a religion. The fundamentals (including basic assumptions and "proofs" that are complete bs in the mathematical sense) are getting reiterated time and again till people actually start believing in them. From what I've experienced myself it basically works like this: In the beginning, you come across some claim or fundamental assumption you can't follow and make a mental note to follow up on it and sit down to fully understand it. Then you get sidetracked and next time you come across said claim, you're like "Damn, I wanted to look that up!" and here you are, making a mental note again… Now somewhere between the fifth and tenth time this happens, you've forgotten about your mental note completely and have just accepted the claim as fact. It's basically the brain tricking you into thinking the claim is correct, the more often you read about it. It's like habitual behavior you don't actually process consciously anymore.

doubleunplussed 10y ago
I completely agree, and I would say that's probably the historical origin of the Copenhagen interpretation. It was clearly initially a note-to-self that something wasn't right and required figuring out, and somewhere along the line that confusion became an axiom that later generations just accepted, not getting the message that it was unfinished. Physicists aren't dumb, but they just haven't looked into it more because it's surprisingly not relevant to the majority of work. So they're just repeating the confusion of past generations as if it's fact. But, particularly in my field, we're getting seriously good at manipulating more and more exotic quantum states. I'm working on things involving 'weak' measurement, where you really need to know what counts as a measurement or not, and how much - you can do a partial measurement and only partly (appear to) collapse a wavefunction. These types of experiments are becoming more are more common, and it's happened a few times now that I've heard people saying "hang on, what counts as a measurement in this context?" and then having to read a decoherence textbook in order to do their atomic physics. With these sorts of things as well as quantum computers, decoherence and the measurement problem is intruding into a larger proportion of people's work, which means repeating the mantra of the previous generation isn't going to cut it anymore, people are going to actually read decoherence textbooks and decide for themselves.

Razengan 10y ago

I'm a layman and I don't really understand much of this, but I'm really intrigued: What's the general consensus in your field on the existence of more than 3 spatial dimensions? Seems to me, that there are a few phenomena that appear to be more or less random. Could they be perturbations caused by activity in dimensions we cannot perceive?

adrianratnapala 10y ago
For first question: I'd say pop-science does a surprisingly good job of conveying the consensus on extra dimensions -- they are a totally reasonable possibility, but their effects are to be felt in realms far beyond the realms that experimental physicists can study well. But if they exist, they probably have important (but unknown) effects. And your proposal that they explain randomness is a good example of that. But to me it doesn't feel right. Quantum randomness somehow doesn't seem to be of that kind. Perturbations from unknown dimensions might well look like randomness, much link an RNG in a computer. But that is still masquerading as a classical random process -- it would not show the weird correlations found in QM.
- Razengan 10y ago
  I must clarify that by "activity" I don't mean directed actions (i.e. not talking about ghosts or gods) but fundamental elements — the waves and particles and fields — possessing properties and movement in more than 3 dimensions (not including time). Like something poking through or pulling on a fabric from the other side, if such a property has, say, wavelike motion in a 4th spatial dimension, it may produce regular effects that manifest as fundamental features of our 3D space, or seem random to us if a bunch of things are bumping and deterministically interacting with each other in extra dimensions. Has this ever been considered in explaining things like [0] and [1]? Might have something to do with "dark" matter/energy too; i.e. stuff is there, just occupying other dimensions. [0] https://en.wikipedia.org/wiki/Pair_production https://en.wikipedia.org/wiki/Pair_production [1] https://en.wikipedia.org/wiki/Quantum_foam https://en.wikipedia.org/wiki/Quantum_foam

monktastic1 10y ago

Thanks for weighing in. Decoherence is great at explaining the loss of interference. It may even address the "preferred basis" problem (though there seems to be good reason to believe that it might not). But ultimately what I want to know is why I experience just one outcome, and decoherence doesn't really help here. MWI partially resolves that ("you don't just experience one; there are many of you!"), but not in a way that satisfies me.