Suggesting that dark matter is actually gravitational leakage from MWI ‘parallel universes’: You tell me.
How much do you know about the relevant physics? If you’re not an expert, and you’re just saying whatever comes to mind, it’s pseudo-science. You need a lot of evidence to narrow down the set of hypotheses enough to find the correct one. Not just anybody knows enough to do that.
My impression is that MW vs. other theories are not a disagreement about the math, but about how the math should be interpreted.
Doesn’t that actually put the question out into philosophy? (specifically ontology in this case, I think)...because, without really knowing much about the physics, it seems to me that the whole disagreement stems from not defining the word “reality” rigorously enough.
Essentially, I suspect that we should be able to make a simplified model that doesn’t involve extremely complex math, which showcases the disagreement inherent in MW / copenhagen / etc so that non-physicists can think about the problem.
...And I might be completely wrong about that, because I don’t know physics...I’m drawing my conclusions mostly from the pattern of disagreement that I see in discussions involving this issue. It’s the sort of pattern that arises from a philosophical dispute rather than an empirical one.
My impression is that MW vs. other theories are not a disagreement about the math, but about how the math should be interpreted.
Just sticking to the quantum interpretations discussed in the sequences, Everettian/MW quantum mechanics has 1 fewer postulate then Copenhagen quantum. The question as to whether they are empirically identical is still open- no one has derived the Born probabilities in many worlds. Until someone does, no one really knows what many worlds predicts. Most pop-science sweeps this under the rug- including the sequences.
The inability to derive the Born probabilities is THE key area where many worlds fails, and its a technical problem with the physics/math.
Born didn’t pull the Born rule out of nowhere. He derived it from the supposition that you’d want to treat the wavefunction probabilistically.
Under MWI, the notion that you’d want to treat the wavefunction probabilistically is of course still motivated by experiment, but within-theory, it is specifically enabled by decoherence. It is something you find, much like, say, the atomic orbitals, or ferromagnetism… even though we made the theory with finding that in mind, we didn’t need to put it in specially once we’d defined the system.
Born started by considering scattering, and discovered you got ridiculous answers if you thought of the wavefunction as charge-density. He solved this conundrum by treating the wavefunction probabilistically, but its not DERIVED, its a postulate grafted on.
In MWI, you don’t have the measurement postulate (which is the whole point). Decoherence gets you from off-diagonal to diagonal densities, but thats it. It won’t tell you how to interpret a diagonal density matrix.
There is a reason that Many World’s supporters spend a lot of time trying to derive Born (Everett’s original paper, Deustch and Wallace, etc)- they realize its an important open problem. Its also why that (and the preferred basis problem) are the most commonly cited reasons to oppose many worlds (see for instance the review papers Adrian Kent has written.
The general consensus even among many worlds proponents (see Deutsch and Wallace for instance) is that decoherence solves the preferred basis problem, but it doesn’t solve the probability issues.
Paragraph 1: Well, yes. We seem to agree that he derived it from the requirement to treat it probabilistically.
The rest: Decoherence sets you up to the point where all you need to do is say is ‘These things act exactly like probability. Since we see probability experimentally, let’s say they’re the same thing.’ Once you’ve seen it in action, it would take willful ignorance not to draw the connection.
That is a postulate, but it’s the sort of postulate that you want to have in a theory—the kind where the theory points to it and says ‘Hey! There is a connection you can draw to real life right here.’
Decoherence sets you up to the point where all you need to do is say is ‘These things act exactly like probability. Since we see probability experimentally, let’s say they’re the same thing.’ Once you’ve seen it in action, it would take willful ignorance not to draw the connection.
Sure, but that is NOT the many worlds claim. The many worlds claim is that we can remove the measurement postulate all together. Maybe this is the case, but it has not been proved.
You seem to agree on that point, and are arguing that we can rescue the spirit of many worlds by putting in a slightly weaker measurement postulate. That is clearly true, but it leads to technical issues: what exactly do we take for our postulate?
There are lots of ways to do this, for instance- consistent histories gives us a consistency operator, and we have a probability over histories (more or less, there are obviously technical details being smoothed over here, see Bob Griffith’s book for more on this). “many minds” puts a probability over mind-sets, The ensemble interpretation tells us that our density matrix is JUST a probability distribution and that quantum mechanics only applies to ensembles of systems,etc (Ballentine is famous for championing this view). All of the interpretations that have an explicit postulate for measurement tend to go by a different name then ‘many worlds’.
And then the philosophical argument- no matter how you add your measurement postulate, your new many-worlds interpretation will have the same number of postulates as consistent histories or ensemble interpretations (which also have no collapse), BUT it also adds an uncountable number of (fundamentally) unmeasurable worlds- shouldn’t occam throw it out?
p1: Many Worlds says that the only dynamical rule is Schrodinger’s equation acting on a real wavefunction (not numerically ‘real’). That’s the end of it. If you hold that, you end up with many worlds whether you like it (or realize it) or not. People may feel unsatisfied with that and add more explanation, but when it comes down to it, that is what MWI is.
See, there are (at least?) two kinds of postulates. I don’t know of names for them so I’m going to call them types 1 and 2. Type 1 says what the system does. Type 2 is how you map the system onto our perceptions.
Like, in Newtonian mechanics, Newton’s 3rd law is a type 1 postulate. The mapping of the xyz coordinate parameters onto our 3-dimensional space is a type 2 postulate. Alternately, on a world map, the projection used is type 2 (or if you’re using a globe then that fact is also of type 2).
Copenhagen-style objective collapse treats the Born rule as a type 1 postulate. MWI treats the Born rule as a type 2. Anyone else who introduces the Born rule as a type 2 rule somehow and doesn’t add any other dynamical rules, ends up with a flavor of MWI. Their arguments, reasoning, etc. are irrelevant.
P2,3: Those sound like flavors of MWI. If they add dynamical rules then they’re not. If they don’t use the Born rule (unlikely) then they’re not, except that in the many minds interpretation I suppose we could introduce a correction for differences in numbers of minds by way of anthropic reasoning, but that depends how you put the question.
P4: Not all postulates are created equal. Euclid’s 5th is far uglier than the 1st to 4th, for instance. If you take one tine of the fork in a Gödel sentence, that’s going to be waaaay uglier still. And generally speaking, it’s fair to weight type 1 and type 2 differently.
In the globe analogy, the MWI family is the equivalent of using a globe to represent the Earth, while Copenhagen is the equivalent of flattening it by the two-point equidistant projection onto a billion-piece jigsaw puzzle and eating the 99.999% of the pieces that don’t include anything we can see. And with an ontologically real collapse, then that’s what you think the Earth is actually doing (minus your personally eating it) - you’re not even keeping the globe in the back of your mind.
Sure, it’s the same number of postulates, and it ends up describing our experiences as well. Must be just as good!
Your p1: Thats simply not true. Consistent histories, for instance is definitely NOT many worlds, and yet it only has the one dynamical rule. Similarly, Ballentine’s ensemble interpretation has only one world,but only the one dynamical rule (it denies the “reality” of the wavefunction to get there).
Your p2: I’m not sure your two kinds of postulates are distinct categories. Consider the standard quantum postulate: All observables are associated with Hermitian operators. Is this type 1 or 2? It defines what we measure, but its also defining the system. Can you list a type 2 postulate for me that isn’t the measurement postulate?
Your p4: In my mind (and in most literature I’m familiar with) many worlds means specifically Everett’s intepretation. In Everett’s interpretation: you don’t take measurement as EITHER a type 1 or a type 2 postulate, and people like Wallace insist that you can deduce the “type 2” of the theory from the dynamics.
I’d be willing to extend the term “many worlds” to any interpretations that insist on the existence of multiple “worlds”, but to suggest consistent histories or Ballentine’s ensemble are many worlds variants is to weaken the term to the point of meaningless. Neither have any kind of multiple worlds! Consistent histories, for instance, is most often described (by Omnes, for instance) as Copenhagen made more precise.
P penultimate:I haven’t discussed copenhagen in the comments in this chain,or any objective collapse intepretations. This whole paragraph seems off point. Your choices aren’t only between many worlds and Copenghagen (unless you continue with your definition of many worlds as ‘anything not Copenhagen’). There are many other modern interpretations.
I’ve seen variants of MWI that were explicitly MWI, so what you’re calling MWI would be straight Everettian MWI. But really, here, I’m asking: “Does this theory have multiple worlds in it?” I care significantly less what it’s called.
For instance, Consistent Histories looks at things quite differently, but if you ask the critical questions of it, it looks like it has many worlds in it. It primes you to zero in on one of them, but if you’re going to stick with the wavefunction being real then the histories you don’t observe are going to be equally real, just less relevant. On the other hand if you say it’s just a trick for finding the probabilities, well, then it’s just a formalized ontological collapse and not MWI. I don’t see any middle ground or ground off to the side here (aside from throwing your hands in the air and saying you don’t know, which is perfectly legitimate but it isn’t an interpretation).
The associations of the hermitian operators corresponding to observable quantities are very type-2. We should feel about as justified using them as using the Born rule.
The point of mentioning objective collapse in the last 2 paragraphs was as a reference point for the non-equality of type-2 postulates. I know it’s terrible, and you know it’s terrible—that’s the point.
But really, here, I’m asking: “Does this theory have multiple worlds in it?” I care significantly less what it’s called
Right- in consistent histories there is 1 world. When you make a measurement, you get one answer. In ensemble quantum mechanics there is 1 world. Remember- the creators of consistent histories (Hartle, for instance) consider it a formalized and clarified copenhagen variant (though inspired by many worlds). Maybe think about it like Bohmian mechanics- the “world” that the Bohmian particle actually sits in is the ‘real’ one. Similarly, in consistent histories, the answer you get picks out a set of projection operators as “real.”
Side question- do you know a many worlds variant (in the sense of more than one world) that makes explicit what its “type 2” postulate is? The only variant I know of is many minds, which I find sort of abhorrent and disregard out of hand. The reason I insist that “many worlds” is incomplete is that the only formalized version I know is Everettian many worlds (which we both seem to agree IS incomplete).
The associations of the hermitian operators corresponding to observable quantities are very type-2.
But also type 1, because it defines the system (hermitian operators on a Hilbert space). What would you consider the type 2 postulates of Newtonian mechanics? What would you consider the type 2 postulates of GR?
In that case, Consistent Histories is both not WMI and I didn’t say it was, because it doesn’t consider the wavefunction fully real in its own right (there were two criteria, not just one, in that sentence)*. Just as Bohm isn’t, on the same grounds.
Type 1 vs type 2: Normally we don’t even talk about these types—if it were a matter of discussion, we wouldn’t be using these terms! With the observables, using them in the theory is type 1. Associating each one to a part of the world we experience is type 2.
As for the incompleteness of Everett, I hold that you can deduce that the Born Rule is one possible way of finding sapience within wavefunctions. I am not at all sure that you can prove that there aren’t others, so barring such a proof, a postulate is necessary to exclude them—“The way of getting to a perceivable world from this theory is… THIS one, not any others.”
ETA: and in this case Consistent Histories deserves every bit of scorn that Eliezer heaped on Copenhagen in the ‘what does it have to do, kill a puppy’ rant.
To summarize, the universe is a three-dimensional sheet in a four-dimensional universe. It’s predicted that it occasionally splits into two thinner sheets. The probability of being in a sheet is measured to be proportional to the square of the predicted thickness.
The Copenhagen interpretation claims that, once the sheets have separated beyond a certain distance, one of them completely vanishes for no adequately explained reason. It’s not known how far apart they have to be, beyond that it’s further than you can detect a parallel universe.
The Many Worlds Interpretation claims that all the sheets continue to exist. No explanation is given for why you end up in the thicker one at that rate. It’s just assumed that there’s a better reason than that one universe ceases to exist.
It’s just assumed that there’s a better reason than that one universe ceases to exist.
“One universe ceases to exist” doesn’t explain why the universes survive with that probability either.
This was one of the points raised in the QP sequence. You can’t say “MWI doesn’t explain these probabilities” as evidence for Copenhagen, because Copenhagen doesn’t explain them either.
(I don’t actually know the physics, I’m just repeating the teacher’s password.)
I’ve read those sequences—I was more asking about your reasons for believing that one must be a domain expert in QM and be familiar with the empirical evidence on the topic to have an opinion about this issue.
My central point is that the two things you described are empirically identical, and the only difference between them is which parts of the math are defined as “existing”.
The underlying question here is What is Reality and What exists. My intuition is that you don’t really need to be an expert on anything or really understand physics to have an opinion about this largely ontological question. From what I’ve heard of the two arguments so far, I don’t actually see why you need a background in physics, or any empirical knowledge at all, to answer this question once it has been posed.
Unless I haven’t understood the question / something really important was lost in simplification.
The original post wasn’t talking about having an opinion on someone else’s theory. It was making up a new theory. If two experts argue about something, and they explain it well to you, and you form an opinion on this, you will probably be right. There’s still a pretty good chance you’ll be wrong, so you shouldn’t form a strong opinion, but you can get higher than 50%. If you make up your own theory, then you are distinguishing one out of a huge number of possibilities without anyone explaining anything to you. Your theory will probably be nonsense. If it isn’t, it will probably be fundamentally flawed. If it isn’t, it will probably be something that can be readily disproven. If it isn’t, it will almost certainly not be the simplest explanation. You can easily make an accurate opinion in this situation, by assuming that your theory is wrong.
My central point is that the two things you described are empirically identical
They are identical to within measurable error. However, there is a difference. There are ways to detect entangled systems. It just gets exponentially harder as you increase the size of the configuration space by adding particles or letting them move more etc. In principle, no matter how much has to get entangled for a waveform to collapse, you could entangle more than that and check to see if it stays entangled.
and the only difference between them is which parts of the math are defined as “existing”.
No it is not. The math shows how an entangled system of particles evolves. The Copenhagen interpretation does not say that after a certain number of particles get involved it stops being “real”. It says that it collapses, in a manner that presumable could be precisely defined, but since there’s neither theory nor empirical data behind the idea, they can’t actually tell you what the definition would be.
From what I’ve heard of the two arguments so far, I don’t actually see why you need a background in physics, or any empirical knowledge at all, to answer this question once it has been posed.
Of course you don’t. You’re not a domain expert. It’s hard to see why you need the background knowledge, when you don’t have the background knowledge and you can’t see how it helps.
There are times where the background knowledge doesn’t help much. Like understanding the Born probabilities, for example. If the domain experts consistently tell you that being a domain expert isn’t going to help, then you can accept that being a domain expert probably isn’t going to help. Even in these situations, you shouldn’t form strong opinions. It’s not so much that an amateur understands it as well as an expert as it is that an expert understands it as badly as an amateur. You’re just as likely to be wrong as normal. It’s just that the experts aren’t any better off.
I’m not an expert. My reasoning process, such as it was, was, “MWI is not generally accepted by the scientific community, but is on LW. Are there any existing scientific mysteries which could potentially be solved by examining them from the perspective of assuming MWI is true, that wouldn’t be thought of without that assumption?”. Dark matter was at the top of the list, and gravity ‘leakage’ the simplest way to describe such a possible solution.
IIRC, less than half of physicists believe in the Copenhagen interpretation, and more than half of the rest believe in MWI. At any rate, actual physicists who ascribe to MWI are not rare, and if your conjecture had anything going for it, I’m sure one of them would have thought of it. I’m under the impression that we have good theoretical reasons to believe that such interactions cannot happen. Making up statements that sound plausible given a few other assertions you’ve heard, without first understanding the technical details behind those assertions, is a poor way to generate hypotheses.
The main alternative paradigm to dark matter is modified gravity (above all, MOND, Modified Newtonian Dynamics). Modified gravity theories usually involve a change at the classical level, but it has been suggested that the galactic rotation curves, etc., might be due to some specifically quantum effect. In quantum field theory, a classical force field generated by an object is understood in terms of virtual particles emitted by that object, the quantum details can cause the field to exhibit properties different from the classical approximation, and maybe virtual gravitons do something funny when they are summed on galactic scales.
If you could explain such quantum modifications to gravity on astronomical scales, as a result of interaction between Everett worlds, then you could have something like your MWI theory of dark matter. The real problem with that, is in finding a coherent causal account of inter-world interaction.
ETA By the way, I suspect that your idea of how MWI gravitational leakage would work, uses paradigms from the wrong multiverse “level”. In string theory there is the idea of membranes in hyperspace, each with its own matter attached, and with gravity being the one force that can cross hyperspace. So what we can see in space might be just one brane, and the dark matter could be matter on other nearby branes. I suspect that you’re thinking of the MWI multiverse in similar terms—a stack of worlds, with some gravitational interaction between neighbors.
But in QM, quantum interference involves whole configurations at once. Suppose you have two entangled particles x and y, that are far apart in space. Schematically, their combined wavefunction will be x1 y1 + x2 y2 + x3 y3 + … where the x’s and y’s are different possible wavefronts for the individual particles, and xi yi is a formal product of two such individual wavefunctions. The point of nonlocality is that the combined wavefunction of the two particles is simultaneously susceptible to changes at the x end and to changes at the y end. It’s a nonlocally defined entity whose manifestation as x and y can’t be reproduced by influences that travel through the space between x and y—that’s the lesson of Bell’s theorem.
So the brane-like scenario of locally propagated gravity leaking between worlds that are neighbors in a hyperspace, doesn’t capture the nonlocality of quantum effects. What would be needed, is a type of Bohmian many-worlds theory, in which the nonlocal quantum force of Bohm (which allows Bohm to reproduce QM within a neo-classical ontology of determinism and definite trajectories), comes from some sort of nonlocal inter-world interaction. MWIers really ought to be investigating this sort of model, especially modified versions of it that approach locality.
The fact that MWI isn’t generally accepted doesn’t mean that there a sizable chunk of physicists who do accept it. I’m not even sure that you need to accept it to run the relevant equations.
This is a misconception. Many worlds has one fewer postulate then Copenhagen quantum, so there are operations you can do in Copenhagen that don’t make sense in Everettian quantum (measurement as a projection operator). Most of the physicist I know who object to many worlds object that it doesn’t have enough structure in the postulates to actually get predictions out.
Most interpretations are similar- they remove or introduce other postulates. Consistent histories, for instance, has a consistency operator that you wouldn’t use in another formulation.
This is a misconception. Many worlds has one fewer postulate then Copenhagen quantum, so there are operations you can do in Copenhagen that don’t make sense in Everettian quantum (measurement as a projection operator). Most of the physicist I know who object to many worlds object that it doesn’t have enough structure in the postulates to actually get predictions out.
The complaint is that MWI is not complete enough to give a satisfactory interpretation for the Born rule. Nevertheless, those who support MWI do believe that Born rule is essentially accurate.
Nevertheless, those who support MWI do believe that Born rule is essentially accurate.
Sure, its obvious empirically.
The question is- can you do without the measurement postulate and recover the Born rule. If you can’t, then Everett’s interpretation doesn’t work, you still have an ugly measurement postulate grafted on to the unitary theory, and you gain no elegance.
I don’t even know enough about the physics to know which equations you would have to run. However there are a lot of theoretical physics Phds.
I know one who tries to writes her thesis on two dimensional time. It’s my impression that theoretical physicists just try out various different methods of writing formula in the hope of discovery something “beautiful” or something that at least makes testable predictions.
How much do you know about the relevant physics? If you’re not an expert, and you’re just saying whatever comes to mind, it’s pseudo-science. You need a lot of evidence to narrow down the set of hypotheses enough to find the correct one. Not just anybody knows enough to do that.
My impression is that MW vs. other theories are not a disagreement about the math, but about how the math should be interpreted.
Doesn’t that actually put the question out into philosophy? (specifically ontology in this case, I think)...because, without really knowing much about the physics, it seems to me that the whole disagreement stems from not defining the word “reality” rigorously enough.
Essentially, I suspect that we should be able to make a simplified model that doesn’t involve extremely complex math, which showcases the disagreement inherent in MW / copenhagen / etc so that non-physicists can think about the problem.
...And I might be completely wrong about that, because I don’t know physics...I’m drawing my conclusions mostly from the pattern of disagreement that I see in discussions involving this issue. It’s the sort of pattern that arises from a philosophical dispute rather than an empirical one.
Just sticking to the quantum interpretations discussed in the sequences, Everettian/MW quantum mechanics has 1 fewer postulate then Copenhagen quantum. The question as to whether they are empirically identical is still open- no one has derived the Born probabilities in many worlds. Until someone does, no one really knows what many worlds predicts. Most pop-science sweeps this under the rug- including the sequences.
The inability to derive the Born probabilities is THE key area where many worlds fails, and its a technical problem with the physics/math.
Why oh WHY do people keep claiming this?
Born didn’t pull the Born rule out of nowhere. He derived it from the supposition that you’d want to treat the wavefunction probabilistically.
Under MWI, the notion that you’d want to treat the wavefunction probabilistically is of course still motivated by experiment, but within-theory, it is specifically enabled by decoherence. It is something you find, much like, say, the atomic orbitals, or ferromagnetism… even though we made the theory with finding that in mind, we didn’t need to put it in specially once we’d defined the system.
Born started by considering scattering, and discovered you got ridiculous answers if you thought of the wavefunction as charge-density. He solved this conundrum by treating the wavefunction probabilistically, but its not DERIVED, its a postulate grafted on.
In MWI, you don’t have the measurement postulate (which is the whole point). Decoherence gets you from off-diagonal to diagonal densities, but thats it. It won’t tell you how to interpret a diagonal density matrix.
There is a reason that Many World’s supporters spend a lot of time trying to derive Born (Everett’s original paper, Deustch and Wallace, etc)- they realize its an important open problem. Its also why that (and the preferred basis problem) are the most commonly cited reasons to oppose many worlds (see for instance the review papers Adrian Kent has written.
The general consensus even among many worlds proponents (see Deutsch and Wallace for instance) is that decoherence solves the preferred basis problem, but it doesn’t solve the probability issues.
Paragraph 1: Well, yes. We seem to agree that he derived it from the requirement to treat it probabilistically.
The rest: Decoherence sets you up to the point where all you need to do is say is ‘These things act exactly like probability. Since we see probability experimentally, let’s say they’re the same thing.’ Once you’ve seen it in action, it would take willful ignorance not to draw the connection.
That is a postulate, but it’s the sort of postulate that you want to have in a theory—the kind where the theory points to it and says ‘Hey! There is a connection you can draw to real life right here.’
Sure, but that is NOT the many worlds claim. The many worlds claim is that we can remove the measurement postulate all together. Maybe this is the case, but it has not been proved.
You seem to agree on that point, and are arguing that we can rescue the spirit of many worlds by putting in a slightly weaker measurement postulate. That is clearly true, but it leads to technical issues: what exactly do we take for our postulate?
There are lots of ways to do this, for instance- consistent histories gives us a consistency operator, and we have a probability over histories (more or less, there are obviously technical details being smoothed over here, see Bob Griffith’s book for more on this). “many minds” puts a probability over mind-sets, The ensemble interpretation tells us that our density matrix is JUST a probability distribution and that quantum mechanics only applies to ensembles of systems,etc (Ballentine is famous for championing this view). All of the interpretations that have an explicit postulate for measurement tend to go by a different name then ‘many worlds’.
And then the philosophical argument- no matter how you add your measurement postulate, your new many-worlds interpretation will have the same number of postulates as consistent histories or ensemble interpretations (which also have no collapse), BUT it also adds an uncountable number of (fundamentally) unmeasurable worlds- shouldn’t occam throw it out?
p1: Many Worlds says that the only dynamical rule is Schrodinger’s equation acting on a real wavefunction (not numerically ‘real’). That’s the end of it. If you hold that, you end up with many worlds whether you like it (or realize it) or not. People may feel unsatisfied with that and add more explanation, but when it comes down to it, that is what MWI is.
See, there are (at least?) two kinds of postulates. I don’t know of names for them so I’m going to call them types 1 and 2. Type 1 says what the system does. Type 2 is how you map the system onto our perceptions.
Like, in Newtonian mechanics, Newton’s 3rd law is a type 1 postulate. The mapping of the xyz coordinate parameters onto our 3-dimensional space is a type 2 postulate. Alternately, on a world map, the projection used is type 2 (or if you’re using a globe then that fact is also of type 2).
Copenhagen-style objective collapse treats the Born rule as a type 1 postulate. MWI treats the Born rule as a type 2. Anyone else who introduces the Born rule as a type 2 rule somehow and doesn’t add any other dynamical rules, ends up with a flavor of MWI. Their arguments, reasoning, etc. are irrelevant.
P2,3: Those sound like flavors of MWI. If they add dynamical rules then they’re not. If they don’t use the Born rule (unlikely) then they’re not, except that in the many minds interpretation I suppose we could introduce a correction for differences in numbers of minds by way of anthropic reasoning, but that depends how you put the question.
P4: Not all postulates are created equal. Euclid’s 5th is far uglier than the 1st to 4th, for instance. If you take one tine of the fork in a Gödel sentence, that’s going to be waaaay uglier still. And generally speaking, it’s fair to weight type 1 and type 2 differently.
In the globe analogy, the MWI family is the equivalent of using a globe to represent the Earth, while Copenhagen is the equivalent of flattening it by the two-point equidistant projection onto a billion-piece jigsaw puzzle and eating the 99.999% of the pieces that don’t include anything we can see. And with an ontologically real collapse, then that’s what you think the Earth is actually doing (minus your personally eating it) - you’re not even keeping the globe in the back of your mind.
Sure, it’s the same number of postulates, and it ends up describing our experiences as well. Must be just as good!
Your p1: Thats simply not true. Consistent histories, for instance is definitely NOT many worlds, and yet it only has the one dynamical rule. Similarly, Ballentine’s ensemble interpretation has only one world,but only the one dynamical rule (it denies the “reality” of the wavefunction to get there).
Your p2: I’m not sure your two kinds of postulates are distinct categories. Consider the standard quantum postulate: All observables are associated with Hermitian operators. Is this type 1 or 2? It defines what we measure, but its also defining the system. Can you list a type 2 postulate for me that isn’t the measurement postulate?
Your p4: In my mind (and in most literature I’m familiar with) many worlds means specifically Everett’s intepretation. In Everett’s interpretation: you don’t take measurement as EITHER a type 1 or a type 2 postulate, and people like Wallace insist that you can deduce the “type 2” of the theory from the dynamics.
I’d be willing to extend the term “many worlds” to any interpretations that insist on the existence of multiple “worlds”, but to suggest consistent histories or Ballentine’s ensemble are many worlds variants is to weaken the term to the point of meaningless. Neither have any kind of multiple worlds! Consistent histories, for instance, is most often described (by Omnes, for instance) as Copenhagen made more precise.
P penultimate:I haven’t discussed copenhagen in the comments in this chain,or any objective collapse intepretations. This whole paragraph seems off point. Your choices aren’t only between many worlds and Copenghagen (unless you continue with your definition of many worlds as ‘anything not Copenhagen’). There are many other modern interpretations.
I’ve seen variants of MWI that were explicitly MWI, so what you’re calling MWI would be straight Everettian MWI. But really, here, I’m asking: “Does this theory have multiple worlds in it?” I care significantly less what it’s called.
For instance, Consistent Histories looks at things quite differently, but if you ask the critical questions of it, it looks like it has many worlds in it. It primes you to zero in on one of them, but if you’re going to stick with the wavefunction being real then the histories you don’t observe are going to be equally real, just less relevant. On the other hand if you say it’s just a trick for finding the probabilities, well, then it’s just a formalized ontological collapse and not MWI. I don’t see any middle ground or ground off to the side here (aside from throwing your hands in the air and saying you don’t know, which is perfectly legitimate but it isn’t an interpretation).
The associations of the hermitian operators corresponding to observable quantities are very type-2. We should feel about as justified using them as using the Born rule.
The point of mentioning objective collapse in the last 2 paragraphs was as a reference point for the non-equality of type-2 postulates. I know it’s terrible, and you know it’s terrible—that’s the point.
Right- in consistent histories there is 1 world. When you make a measurement, you get one answer. In ensemble quantum mechanics there is 1 world. Remember- the creators of consistent histories (Hartle, for instance) consider it a formalized and clarified copenhagen variant (though inspired by many worlds). Maybe think about it like Bohmian mechanics- the “world” that the Bohmian particle actually sits in is the ‘real’ one. Similarly, in consistent histories, the answer you get picks out a set of projection operators as “real.”
Side question- do you know a many worlds variant (in the sense of more than one world) that makes explicit what its “type 2” postulate is? The only variant I know of is many minds, which I find sort of abhorrent and disregard out of hand. The reason I insist that “many worlds” is incomplete is that the only formalized version I know is Everettian many worlds (which we both seem to agree IS incomplete).
But also type 1, because it defines the system (hermitian operators on a Hilbert space). What would you consider the type 2 postulates of Newtonian mechanics? What would you consider the type 2 postulates of GR?
In that case, Consistent Histories is both not WMI and I didn’t say it was, because it doesn’t consider the wavefunction fully real in its own right (there were two criteria, not just one, in that sentence)*. Just as Bohm isn’t, on the same grounds.
Type 1 vs type 2: Normally we don’t even talk about these types—if it were a matter of discussion, we wouldn’t be using these terms! With the observables, using them in the theory is type 1. Associating each one to a part of the world we experience is type 2.
As for the incompleteness of Everett, I hold that you can deduce that the Born Rule is one possible way of finding sapience within wavefunctions. I am not at all sure that you can prove that there aren’t others, so barring such a proof, a postulate is necessary to exclude them—“The way of getting to a perceivable world from this theory is… THIS one, not any others.”
ETA: and in this case Consistent Histories deserves every bit of scorn that Eliezer heaped on Copenhagen in the ‘what does it have to do, kill a puppy’ rant.
Eliezer came up with a good analogue in Where Physics Meets Experience and Where Experience Confuses Physicists.
To summarize, the universe is a three-dimensional sheet in a four-dimensional universe. It’s predicted that it occasionally splits into two thinner sheets. The probability of being in a sheet is measured to be proportional to the square of the predicted thickness.
The Copenhagen interpretation claims that, once the sheets have separated beyond a certain distance, one of them completely vanishes for no adequately explained reason. It’s not known how far apart they have to be, beyond that it’s further than you can detect a parallel universe.
The Many Worlds Interpretation claims that all the sheets continue to exist. No explanation is given for why you end up in the thicker one at that rate. It’s just assumed that there’s a better reason than that one universe ceases to exist.
“One universe ceases to exist” doesn’t explain why the universes survive with that probability either.
This was one of the points raised in the QP sequence. You can’t say “MWI doesn’t explain these probabilities” as evidence for Copenhagen, because Copenhagen doesn’t explain them either.
(I don’t actually know the physics, I’m just repeating the teacher’s password.)
I’ve read those sequences—I was more asking about your reasons for believing that one must be a domain expert in QM and be familiar with the empirical evidence on the topic to have an opinion about this issue.
My central point is that the two things you described are empirically identical, and the only difference between them is which parts of the math are defined as “existing”.
The underlying question here is What is Reality and What exists. My intuition is that you don’t really need to be an expert on anything or really understand physics to have an opinion about this largely ontological question. From what I’ve heard of the two arguments so far, I don’t actually see why you need a background in physics, or any empirical knowledge at all, to answer this question once it has been posed.
Unless I haven’t understood the question / something really important was lost in simplification.
The original post wasn’t talking about having an opinion on someone else’s theory. It was making up a new theory. If two experts argue about something, and they explain it well to you, and you form an opinion on this, you will probably be right. There’s still a pretty good chance you’ll be wrong, so you shouldn’t form a strong opinion, but you can get higher than 50%. If you make up your own theory, then you are distinguishing one out of a huge number of possibilities without anyone explaining anything to you. Your theory will probably be nonsense. If it isn’t, it will probably be fundamentally flawed. If it isn’t, it will probably be something that can be readily disproven. If it isn’t, it will almost certainly not be the simplest explanation. You can easily make an accurate opinion in this situation, by assuming that your theory is wrong.
They are identical to within measurable error. However, there is a difference. There are ways to detect entangled systems. It just gets exponentially harder as you increase the size of the configuration space by adding particles or letting them move more etc. In principle, no matter how much has to get entangled for a waveform to collapse, you could entangle more than that and check to see if it stays entangled.
No it is not. The math shows how an entangled system of particles evolves. The Copenhagen interpretation does not say that after a certain number of particles get involved it stops being “real”. It says that it collapses, in a manner that presumable could be precisely defined, but since there’s neither theory nor empirical data behind the idea, they can’t actually tell you what the definition would be.
Of course you don’t. You’re not a domain expert. It’s hard to see why you need the background knowledge, when you don’t have the background knowledge and you can’t see how it helps.
There are times where the background knowledge doesn’t help much. Like understanding the Born probabilities, for example. If the domain experts consistently tell you that being a domain expert isn’t going to help, then you can accept that being a domain expert probably isn’t going to help. Even in these situations, you shouldn’t form strong opinions. It’s not so much that an amateur understands it as well as an expert as it is that an expert understands it as badly as an amateur. You’re just as likely to be wrong as normal. It’s just that the experts aren’t any better off.
I’m not an expert. My reasoning process, such as it was, was, “MWI is not generally accepted by the scientific community, but is on LW. Are there any existing scientific mysteries which could potentially be solved by examining them from the perspective of assuming MWI is true, that wouldn’t be thought of without that assumption?”. Dark matter was at the top of the list, and gravity ‘leakage’ the simplest way to describe such a possible solution.
IIRC, less than half of physicists believe in the Copenhagen interpretation, and more than half of the rest believe in MWI. At any rate, actual physicists who ascribe to MWI are not rare, and if your conjecture had anything going for it, I’m sure one of them would have thought of it. I’m under the impression that we have good theoretical reasons to believe that such interactions cannot happen. Making up statements that sound plausible given a few other assertions you’ve heard, without first understanding the technical details behind those assertions, is a poor way to generate hypotheses.
The main alternative paradigm to dark matter is modified gravity (above all, MOND, Modified Newtonian Dynamics). Modified gravity theories usually involve a change at the classical level, but it has been suggested that the galactic rotation curves, etc., might be due to some specifically quantum effect. In quantum field theory, a classical force field generated by an object is understood in terms of virtual particles emitted by that object, the quantum details can cause the field to exhibit properties different from the classical approximation, and maybe virtual gravitons do something funny when they are summed on galactic scales.
If you could explain such quantum modifications to gravity on astronomical scales, as a result of interaction between Everett worlds, then you could have something like your MWI theory of dark matter. The real problem with that, is in finding a coherent causal account of inter-world interaction.
ETA By the way, I suspect that your idea of how MWI gravitational leakage would work, uses paradigms from the wrong multiverse “level”. In string theory there is the idea of membranes in hyperspace, each with its own matter attached, and with gravity being the one force that can cross hyperspace. So what we can see in space might be just one brane, and the dark matter could be matter on other nearby branes. I suspect that you’re thinking of the MWI multiverse in similar terms—a stack of worlds, with some gravitational interaction between neighbors.
But in QM, quantum interference involves whole configurations at once. Suppose you have two entangled particles x and y, that are far apart in space. Schematically, their combined wavefunction will be x1 y1 + x2 y2 + x3 y3 + … where the x’s and y’s are different possible wavefronts for the individual particles, and xi yi is a formal product of two such individual wavefunctions. The point of nonlocality is that the combined wavefunction of the two particles is simultaneously susceptible to changes at the x end and to changes at the y end. It’s a nonlocally defined entity whose manifestation as x and y can’t be reproduced by influences that travel through the space between x and y—that’s the lesson of Bell’s theorem.
So the brane-like scenario of locally propagated gravity leaking between worlds that are neighbors in a hyperspace, doesn’t capture the nonlocality of quantum effects. What would be needed, is a type of Bohmian many-worlds theory, in which the nonlocal quantum force of Bohm (which allows Bohm to reproduce QM within a neo-classical ontology of determinism and definite trajectories), comes from some sort of nonlocal inter-world interaction. MWIers really ought to be investigating this sort of model, especially modified versions of it that approach locality.
The fact that MWI isn’t generally accepted doesn’t mean that there a sizable chunk of physicists who do accept it. I’m not even sure that you need to accept it to run the relevant equations.
Do you know anyone who would be willing to run through said equations?
MWI doesn’t have different equations than “mainstream” quantum mechanics. It’s just an epistemological interpretation.
This is a misconception. Many worlds has one fewer postulate then Copenhagen quantum, so there are operations you can do in Copenhagen that don’t make sense in Everettian quantum (measurement as a projection operator). Most of the physicist I know who object to many worlds object that it doesn’t have enough structure in the postulates to actually get predictions out.
Most interpretations are similar- they remove or introduce other postulates. Consistent histories, for instance, has a consistency operator that you wouldn’t use in another formulation.
The complaint is that MWI is not complete enough to give a satisfactory interpretation for the Born rule. Nevertheless, those who support MWI do believe that Born rule is essentially accurate.
Sure, its obvious empirically.
The question is- can you do without the measurement postulate and recover the Born rule. If you can’t, then Everett’s interpretation doesn’t work, you still have an ugly measurement postulate grafted on to the unitary theory, and you gain no elegance.
I don’t even know enough about the physics to know which equations you would have to run. However there are a lot of theoretical physics Phds.
I know one who tries to writes her thesis on two dimensional time. It’s my impression that theoretical physicists just try out various different methods of writing formula in the hope of discovery something “beautiful” or something that at least makes testable predictions.
MWI has no equations distinct from the standard approach.