Am I correct that decoherence isn’t really an interpretation of quantum mechanics, but an empirically verified statistical consequence of the standard model?
Most of the interpretations aren’t “pure” in the sense of being completely independent of empirical constraints. Decoherence is one that posits a physical process (unlike many worlds and quantum Bayesianism) that doesn’t require new laws of physics (unlike objective collapse theories, including gravity mediated). It requires a collective effect from the environment, which is why I say it’s like thermodynamics.
However, it isn’t as well-understood as thermodynamics. I’ve read some papers in which the authors describe a simple, special-case example of an environment and show that a quantum system does get pushed toward basis states like 0.001% spin-up and 99.999% spin-down. In fact, part of the problem is, “What’s special about the basis states we see in the lab, like position, momentum, energy, spin up/down, field value, particle number, etc., rather than any other linear combination of them?” I remember reading about simple models in which an “isolated” particle, i.e. one surrounded by low-energy photons spontaneously coming out of the vacuum, is pushed toward its energy basis (think of a hydrogen atom, defined by its energy levels) and a colliding particle involved in one high-energy collision is pushed toward its position basis (a wave that becomes more particle-like on impact).
You’re reminding me that I found decoherence more compelling than the rest. But what’s missing, unless there’s been a breakthrough I don’t know about, is generalization. These were very simplified special cases. Also, there’s still a philosophical choice to be made because this world-view is taking the collapse seriously as an objective physical process, but not a discrete one that leaves us in exact basis states. It’s more like a phase transition that isn’t infinitely sharp.
I doubt that anything specific to the Standard Model (high-energy generality, like electroweak symmetry breaking) has anything to do with it, since low-energy electrons also need to decohere, but you probably meant, “standard physics, no new laws,” right? Decoherence is that: no new laws (unless there are yet more models of decoherence that I don’t know about).
Got it. So in a ways it’s more like a mathematical conjecture than a philosophical theory. We posit a statistical result, we have some toy examples which provide us with some intuition for it, but right now we’re not able to prove the general case. We hope to do so in the future, and people are actively working on doing so.
Also isn’t many worlds a straightforward interpretation of decoherence? Decoherence says that regions of large complex superpositions stop interfering with each other, and hence such regions will act classically, many worlds just says that the regions you’re not in presumably still exist? Or are there some extra hoops there?
There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible opposites, but are treated as interchangeable in Yudkowsky’s writings. Decoherent branches are large, stable, non interacting and irreversible...everything that would be intuitively expected of a “world”. But there is no empirical evidence for them (in the plural) , nor are they obviously supported by the core mathematics of quantum mechanics, the Schrödinger equation.Coherent superpositions are small scale , down to single particles, observer dependent, reversible, and continue to interact (strictly speaking , interfere) after “splitting”. the last point is particularly problematical. because if large scale coherent superposition exist , that would create naked eye evidence at macrocsopic scale:, e.g. ghostly traces of a world where the Nazis won.
We have evidence of small scale coherent superposition, since a number of observed quantum.effects depend on it, and we have evidence of decoherence, since complex superposition are difficult to maintain. What we don’t have evidence of is decoherence into multiple branches. From the theoretical perspective, decoherence is a complex , entropy like process which occurs when a complex system interacts with its environment. Decoherence isn’t simple. But without decoherence, MW doesn’t match observation. So there is no theory of MW that is both simple and empirically adequate, contra Yudkowsky and Deutsch.
Decoherence says that regions of large complex superpositions stop interfering with each other
It says that the “off diagonal” terms vanish, but that would tend to.generate a single predominant outcome (except, perhaps, where the environment is highly symmetrical).
I have heard of versions of many-worlds that are supposed to be testable, and you’re probably referring to one of them. The one that I’m most familiar with (“classic many-worlds”?) is much more of a pure interpretation, though: in that version, there is no collapse and the apparent collapse is a matter of perspective. A component of the wavefunction that I perceive as me sees the electron in the spin-down state, but in the big superposition, there’s another component like me but seeing the spin-up state. I can’t communicate with the other me (or “mes,” plural) because we’re just components of a big vector—we don’t interact.
On the other hand, classic decoherence posits that the wavefunction really does collapse, just not to 100% pure states. Although there’s technically a superposition of electrons and a superposition of mes, it’s heavily dominated by one component. Thus, the two interpretations, classic many-worlds and classic decoherence, are different interpretations.
If the state of theory and/or experiment developed further and these asymptotic projections were shown to exist in generality, that would bolster decoherence but not eliminate many-worlds: you could still imagine a 0.0001% me being coupled with the spin-up electron. It would, however, undermine the motivation. If, instead, these asymptotic projections were shown to not exist, then that would undermine decoherence to the point of refutation (i.e. only die-hards would keep looking for variants of the theory that aren’t ruled out). So classic decoherence is more falsifiable than classic many-worlds. That’s what I mean by saying that many-worlds is more purely philosophical.
But I was careful to say “classic” everywhere. With this being such an active area of research, I’m sure there are versions of these theories that don’t fit the description above. (They’re all “more than one theory.”) As I said, I’ve heard of variants of many-worlds that are less purely philosophical, and that must be what you’re referring to, @TAG.
I have heard of versions of many-worlds that are supposed to be testable
The’re are versions that are falsified, for all practical purposes, because they fail to.predict broadly classical observations—sharp valued real numbers, without pesky complex numbers or superpositions. I mean mainly the original Everett theory of 1957. There have been various attempts to patch the problems—preferred basis, Decoherence , anthropics, etc, -- so there are various non falsified theories.
The one that I’m most familiar with (“classic many-worlds”?) is much more of a pure interpretation, though: in that version, there is no collapse and the apparent collapse is a matter of perspective. A component of the wavefunction that I perceive as me sees the electron in the spin-down state, but in the big superposition, there’s another component like me but seeing the spin-up state. I can’t communicate with the other me (or “mes,” plural) because we’re just components of a big vector—we don’t interact.
Merely saying that everything is a component of a big vector doesn’t show that observers dont go into superposition with themselves, because the same description applies to anything which is in superposition..it’s a very broad claim.
What you call classic MWI is what I the have-your-cake-and-eat-it … assuming nothing except that collapse doesn’t occur, you conclude that observers make classical observations for not particular reason...you doing even nominate Decoherence or preferred basis as the mechanism that gets rid of the unwanted stuff.
On the other hand, classic decoherence posits that the wavefunction really does collapse, just not to 100% pure states. Although there’s technically a superposition of electrons and a superposition of mes, it’s heavily dominated by one component. Thus, the two interpretations, classic many-worlds and classic decoherence, are different interpretations.
OK. I would call that single world decoherence. Many worlders appeal to Decoherence as well.
So classic decoherence is more falsifiable than classic many-worlds.
If classic MW means Everetts RSI, it’s already false.
Am I correct that decoherence isn’t really an interpretation of quantum mechanics, but an empirically verified statistical consequence of the standard model?
Most of the interpretations aren’t “pure” in the sense of being completely independent of empirical constraints. Decoherence is one that posits a physical process (unlike many worlds and quantum Bayesianism) that doesn’t require new laws of physics (unlike objective collapse theories, including gravity mediated). It requires a collective effect from the environment, which is why I say it’s like thermodynamics.
However, it isn’t as well-understood as thermodynamics. I’ve read some papers in which the authors describe a simple, special-case example of an environment and show that a quantum system does get pushed toward basis states like 0.001% spin-up and 99.999% spin-down. In fact, part of the problem is, “What’s special about the basis states we see in the lab, like position, momentum, energy, spin up/down, field value, particle number, etc., rather than any other linear combination of them?” I remember reading about simple models in which an “isolated” particle, i.e. one surrounded by low-energy photons spontaneously coming out of the vacuum, is pushed toward its energy basis (think of a hydrogen atom, defined by its energy levels) and a colliding particle involved in one high-energy collision is pushed toward its position basis (a wave that becomes more particle-like on impact).
You’re reminding me that I found decoherence more compelling than the rest. But what’s missing, unless there’s been a breakthrough I don’t know about, is generalization. These were very simplified special cases. Also, there’s still a philosophical choice to be made because this world-view is taking the collapse seriously as an objective physical process, but not a discrete one that leaves us in exact basis states. It’s more like a phase transition that isn’t infinitely sharp.
I doubt that anything specific to the Standard Model (high-energy generality, like electroweak symmetry breaking) has anything to do with it, since low-energy electrons also need to decohere, but you probably meant, “standard physics, no new laws,” right? Decoherence is that: no new laws (unless there are yet more models of decoherence that I don’t know about).
Got it. So in a ways it’s more like a mathematical conjecture than a philosophical theory. We posit a statistical result, we have some toy examples which provide us with some intuition for it, but right now we’re not able to prove the general case. We hope to do so in the future, and people are actively working on doing so.
Also isn’t many worlds a straightforward interpretation of decoherence? Decoherence says that regions of large complex superpositions stop interfering with each other, and hence such regions will act classically, many worlds just says that the regions you’re not in presumably still exist? Or are there some extra hoops there?
MWI is more than one theory.
There is an approach to MWI based on coherent superpositions, and a version based on decoherence. These are (for all practical purposes) incompatible opposites, but are treated as interchangeable in Yudkowsky’s writings. Decoherent branches are large, stable, non interacting and irreversible...everything that would be intuitively expected of a “world”. But there is no empirical evidence for them (in the plural) , nor are they obviously supported by the core mathematics of quantum mechanics, the Schrödinger equation.Coherent superpositions are small scale , down to single particles, observer dependent, reversible, and continue to interact (strictly speaking , interfere) after “splitting”. the last point is particularly problematical. because if large scale coherent superposition exist , that would create naked eye evidence at macrocsopic scale:, e.g. ghostly traces of a world where the Nazis won.
We have evidence of small scale coherent superposition, since a number of observed quantum.effects depend on it, and we have evidence of decoherence, since complex superposition are difficult to maintain. What we don’t have evidence of is decoherence into multiple branches. From the theoretical perspective, decoherence is a complex , entropy like process which occurs when a complex system interacts with its environment. Decoherence isn’t simple. But without decoherence, MW doesn’t match observation. So there is no theory of MW that is both simple and empirically adequate, contra Yudkowsky and Deutsch.
It says that the “off diagonal” terms vanish, but that would tend to.generate a single predominant outcome (except, perhaps, where the environment is highly symmetrical).
I have heard of versions of many-worlds that are supposed to be testable, and you’re probably referring to one of them. The one that I’m most familiar with (“classic many-worlds”?) is much more of a pure interpretation, though: in that version, there is no collapse and the apparent collapse is a matter of perspective. A component of the wavefunction that I perceive as me sees the electron in the spin-down state, but in the big superposition, there’s another component like me but seeing the spin-up state. I can’t communicate with the other me (or “mes,” plural) because we’re just components of a big vector—we don’t interact.
On the other hand, classic decoherence posits that the wavefunction really does collapse, just not to 100% pure states. Although there’s technically a superposition of electrons and a superposition of mes, it’s heavily dominated by one component. Thus, the two interpretations, classic many-worlds and classic decoherence, are different interpretations.
If the state of theory and/or experiment developed further and these asymptotic projections were shown to exist in generality, that would bolster decoherence but not eliminate many-worlds: you could still imagine a 0.0001% me being coupled with the spin-up electron. It would, however, undermine the motivation. If, instead, these asymptotic projections were shown to not exist, then that would undermine decoherence to the point of refutation (i.e. only die-hards would keep looking for variants of the theory that aren’t ruled out). So classic decoherence is more falsifiable than classic many-worlds. That’s what I mean by saying that many-worlds is more purely philosophical.
But I was careful to say “classic” everywhere. With this being such an active area of research, I’m sure there are versions of these theories that don’t fit the description above. (They’re all “more than one theory.”) As I said, I’ve heard of variants of many-worlds that are less purely philosophical, and that must be what you’re referring to, @TAG.
The’re are versions that are falsified, for all practical purposes, because they fail to.predict broadly classical observations—sharp valued real numbers, without pesky complex numbers or superpositions. I mean mainly the original Everett theory of 1957. There have been various attempts to patch the problems—preferred basis, Decoherence , anthropics, etc, -- so there are various non falsified theories.
Merely saying that everything is a component of a big vector doesn’t show that observers dont go into superposition with themselves, because the same description applies to anything which is in superposition..it’s a very broad claim.
What you call classic MWI is what I the have-your-cake-and-eat-it … assuming nothing except that collapse doesn’t occur, you conclude that observers make classical observations for not particular reason...you doing even nominate Decoherence or preferred basis as the mechanism that gets rid of the unwanted stuff.
OK. I would call that single world decoherence. Many worlders appeal to Decoherence as well.
If classic MW means Everetts RSI, it’s already false.