Yeah. It shouldn't be that surprising that when you arbitrarily assume that part of the wavefunction magically disappears sometimes, you get weird results like non-locality.
It's always amusing to see the kind of excuses that pop up when you try to explain entanglement or interference in the mindset of wavefunction collapse.
I'm still not entirely sure why so many people consider collapse to be simpler. Is it too daunting to think of the world as an amplitude distribution that propagates in a way that we're not intuitively used to? Too hard to think of ourselves as part of quantum mechanics, because that thing I see there on the measuring device must be the reality, damn it, and what the hell do you mean I've just entangled myself and the device along with the system?
Or maybe just tradition and accepting the "scripture" coming from the established authority. How could we best test that?
The many world's interpretation is based on the assumption that both the observed system and the observer is modeled by a wavefunction. In that case observation is entanglement, which seems weird to us, but which makes perfect logical sense.
Other interpretations of quantum mechanics start with the assumption that the world is fundamentally like our naive experience leads us to think of it as. Therefore when we observe something, we couldn't really be thrown into a superposition of states, each of which observed a different thing and which cannot meaningfully interact due to quantum mechanical principles. If we can't be thrown into such a superposition, then there must be some sort of underlying reality, or quantum collapse, or other weirdness that is not explained by QM.
All of the weirdness of interpretations other than many worlds can be understood as the conflict between how they would like to understand the world (there is a reality that happened), and how quantum mechanics described things.
All of the weirdness of the many worlds interpretation can be understood as, "We can't perceive the process of quantum superposition, so it seems really, really weird to us."
Lets say you have some experiment which gives you a 1 or a 0, and you have two entangled particles which will give the same result depsite the result being random and the measurements being taken outside each other's lightcones. In Collapse QM there was some instantanious communication making that possible.
In Many Worlds you'd find that for each measurement the wavefunction split into two branches, one corresponding to 1 and one to 0, in each location. From the sites of both measurements the decoherence spreads outwards as particle interactions happen, and when they finally touch the two 1 segments and the two 0 segments merge, and there is one area split along 1/0 in the wave function instead of two.
That's all horribly simplified, but should get the idea across.
Can you explain how MWI is local? In the period of time before the observer is 'entangled' into the same system as the observed particles, the different possibilities can interfere with each other. If this can happen even if the entangled particles are separated by a lot of space, surely that is the same as nonlocality.
Maybe my confusion is that I'm not sure a which point the universe splits. Suppose I entangle two particles, then separate them by a large amount, my universe hasn't split yet, because before I make the measurement, I haven't become entangled with the state of the particles, but when I do, surely the spreading entanglement of myself with the possibilities centers at both entangled particles (i.e. it is nonlocal)?
> Maybe my confusion is that I'm not sure a which point the universe splits.
The answer to that is that the universe never really splits. It remains one big very complicated universe, with very very very messy superpositions of state in it. But different components of these superpositions lose "coherence", which gives the appearance to us of wave collapse, even though there actually is no wave collapse.
The splitting is a quantitative rather than a qualitative process. You can observe quantum effects with 100 atom molecules the same way as with single particles, it's just they require far finer observations to detect. If no collapse occurs, the the time entangled complexes reach visible scales there's no way you could ever detect the quantum interference.
It isn't that the universe has split, it's that one section of the universe's wavefunction has split, or more exactly become a bimodal distribution. At both locations there are bunches of the wavefunction corresponding to both possible observations - but both locations are entangled since the two particles that touched this off were. That means that when you observe the two experiments you don't "split" into four, but only into two since they are already entangled and you only get entangled with this complex once. The information of "What's entangled with what" spread at strictly sub-light speeds, and the information of "Did it pass through the polarizing filter or not" also traveled at sub-light speeds.
Yes, but this complex is growing the whole time, as it entangles with other parts of the universe around it in both locations. That means that I become entangled with it in one location, and simultaneously become entangled with some other part of the universe that is not remotely local to me.
That's why I find the description of it as 'local' somewhat weird, but I accept that I'm probably using the term incorrectly.
Two particles are entangled and separated by a vast distance.
At some point, I conduct a measurement on the first particle, causing me to split into two different versions of myself that have measured two different states. Now, the effect of that measurement spreads outwards from the site of the measurement, so I may affect other things based on the measurement, etc.
My problem is that if a measurement is done on the other particle, long before any message from me could possibly have got there, then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement.
I imagine this as a wave of universe splitting spreading out from the site of my measurement and the entangled particle despite the separation.
Now, perhaps this is a nontechnical use of the word nonlocal, but I'd describe that as a nonlocal phenomenon.
> My problem is that if a measurement is done on the other particle, long before any message from me could possibly have got there, then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement.
Imagine that your friend Joe writes the same word on two pieces of paper, puts them in envelopes and hands one envelope to you and mails the other envelope to Paris. At some point in time, you open up your envelope and read the word "popcorn". At some other point in time, Pierre open up his envelope and reads the same word.
The word that Pierre reads is not "dependent on your measurement". Nothing nonlocal occurred.
Sure, I used the word 'theory' incorrectly, I should have said 'explanation'.
As I understand it, hidden variables are fine if you're happy to throw away locality, which is exactly what Bohm interpretation does and exactly what we're talking about here.
Since your analogy is purportedly showing me how MWI is local, showing me that something that is significantly different (i.e. is a hidden variable explanation) is also local doesn't help a lot.
then that measurement and the effects it has on its local environment and the effects they have are all also dependent on my measurement.
I think that's your trouble there. Those are totally independent of your measurement, the waveform splits at the measurement of the other particle the exact same way, regardless of what happened to you locally.
except I can never be in a universe where it split at my measurement site in one way and it split at the other measurement site the other way. That means that I am entangled with a measurement that happens nonlocally to me and everything that it interacts with.
You're not entangled with the measurement, you're entangled with the entire wave despite the inconceivable number of measurements that happen within it. The same way my friends are still my friends despite us being in different places. I won't know instantly if they die or something, but I react differently when I hear about what happens to them than I do with ordinary people. If one of them gets into a car accident outside my light-cone and I'm more sad when I hear about it than about the car accident of a stranger, that doesn't mean that I have a non-local interaction with the car accident.
Correct if I'm wrong, but I think that Many Worlds Interpetation does not explain a thing.
It's like religion - one can think it's possible, other - impossible. But it can't be proved or disproved
I'm pretty sure that, in theory, all the major interpretations of QM are experimentally distinguishable from each other. The feasibility of ever being able to perform some of these experiments, however, is rather questionable.
Two exceptions to this are (1) that the Many Worlds Interpretation is experimentally indistinguishable from the Bohm Interpretation, unless you want to commit suicide many times with a quantum revolver; and (2) the Copenhagen Interpretation doesn't define what a "measurement" is, so scientifically determining whether it's correct or not is impossible.
None of this implies that any of this is "like religion".
Be specific. What experiment do you have in mind that could distinguish between interpretations? Note that if the "experiment" involves dying to see what's on the other side then it doesn't exactly make it seem less like a religion.
Well, all the theories of quantum mechanics that rely on waveform collapse all predict that waveform collapse happens, while those that don't don't. Observing that larger and larger objects still have tinier and tinier but still existent quantum effects is evidence against collapse. And if you can demonstrate quantum effects in people, then you've pretty much ruled out the Copenhagen and von Neumann interpretations. Those experiments are inconceivably difficult with our current level of technology, though.
So you're saying, if Quantum Mechanics turns out to correctly describe larger and larger systems--in other words, unless we discover some new physics which makes QM break at a certain scale--then your pet interpretation will turn out to be true?
No, I'm saying that if Schrödinger's equation describes larger and larger systems that means that Quantum Mechanics is Schrödinger's equation rather than being Schrödinger's equation plus waveform collapse. Maybe you could try reading through the Interpretations of Quantum Mechanics Wikipedia page[1] to get a better sense of the issues being talked about here? Whether or not waveforms collapse at a certain scale is precisely the most important issue of disagreement here, and it actually is subject to experiment in theory. And experiment could probably even distinguish between the various families of waveform collapse too. Not that this will distinguish between all interpretations, but it would at least cut the possible number in half.
Remember, waveform collapse or "some new physics which makes QM break at a certain scale" was actually the assumption of the people who started quantum physics, and the default assumption of popular writing including the article we're discussing. It wasn't until much later that Everett proposed that it might be an unnecessary hypothesis like Maxwell's aether.
You might as well say that the Bohm interpretation is "like religion" then, rather than saying that the MWI is.
If your assertion is that we might never know which is true, the Many Worlds Interpretation or the Bohm Interpretation, then you might be right. But, if it comes down to these two, most scientists are going to go with the Many Worlds Interpretation by Occam's razor.
There are many other theories that we reject by Occam's razor that we can't disprove, and yet we don't typically consider these rejections to be a matter of "religion".
You might as well say that the Bohm interpretation is "like religion" then, rather than saying that the MWI is.
I might except MWI clearly has more True Believers.
If your assertion is that we might never know which is true
My assertion is that since all interpretations lead to the same exact predictions for every imaginable experiment, it doesn't make sense to say that in any meaningful way one interpretation is true and the others aren't. You might as well be arguing whether the electromagnetic field is weaved by tiny angels out of their hair or not.
The Copenhagen interpretation is the closest to "no interpretation" in that it's the most direct way of translating the math into actual predictions.
Quoting the guy who said you shouldn't multiply entities without necessity, to justify the introduction of infinitely many parallel worlds, is an interesting rhetorical maneuver.
> My assertion is that since all interpretations lead to the same exact predictions for every imaginable experiment
This is not the case. Other than MWI and Bohm, the major interpretations (not including Copenhagen) make different predictions that are, in theory, distinguishable. It's just that it is completely infeasible to perform the experiments at this time. Maybe in 100 years or in 1,000 years, we'll have the technology to perform these experiments.
When the time comes that we can perform these experiments, one of the possible outcomes is that we narrow it down to MWI/Bohm. Once that happens, however, we will have no way to scientifically determine which of the two is the correct interpretation, except to the degree that we trust our intuitions about Ockham's razor. But that is certainly not going to let us know the answer for sure.
> The Copenhagen interpretation is the closest to "no interpretation" in that it's the most direct way of translating the math into actual predictions.
If we ever want to actually do experiments to determine under precisely which situation the probability wave collapses, then we have to do better than the Copenhagen Interpretation, since it doesn't define what a "measurement" is. Without such a definition, there's no way to test whether it is correct or not.
> Quoting the guy who said you shouldn't multiply entities without necessity, to justify the introduction of infinitely many parallel worlds, is an interesting rhetorical maneuver.
It's not a "rhetorical maneuver". I'll just refer you to the Stanford Encyclopedia of Philosophy for more info:
It seems that the majority of the opponents of the MWI reject it because, for them, introducing a very large number of worlds that we do not see is an extreme violation of Ockham's principle: "Entities are not to be multiplied beyond necessity". However, in judging physical theories one could reasonably argue that one should not multiply physical laws beyond necessity either (such a verion of Ockham's Razor has been applied in the past), and in this respect the MWI is the most economical theory. Indeed, it has all the laws of the standard quantum theory, but without the collapse postulate, the most problematic of physical laws. The MWI is also more economic than Bohmian mechanics which has in addition the ontology of the particle trajectories and the laws which give their evolution. Tipler 1986 (p. 208) has presented an effective analogy with the criticism of Copernican theory on the grounds of Ockham's razor.
This is not the case. Other than MWI and Bohm, the major interpretations (not including Copenhagen) make different predictions that are, in theory, distinguishable. It's just that it is completely infeasible to perform the experiments at this time. Maybe in 100 years or in 1,000 years, we'll have the technology to perform these experiments.
So, what exactly are those experiments that allegedly could distinguish between interpretations?
Also, this is a marvel of a sentence: "Other than MWI and Bohm, the major interpretations (not including Copenhagen)"
"Other than Android and Windows Mobile, all major cell phone operating systems (not including iOS)"
The long and short of it is, for purely philosophical reasons you don't like the notion of the state vector collapse. You freely admit that there is no way to experimentally distinguish between your favorite interpretation and the Copenhagen interpretation. You just declare that it's not even a contender, using arguments which have nothing to do even in principle with the outcome of any experiments.
Saying that something is not precisely defined sounds to me totally like grasping at straws. Nothing's ever precisely defined in science, you could criticize any theory including Newton's mechanics by saying that it doesn't define precisely what a measurement is. Which never stopped anyone from measuring things and comparing the values they measured with what the theory predicted.
You quote someone who made an analogy with Copernicus. The Copernican theory simplified the calculations right away, whereas with Quantum Mechanics, the calculations stay exactly the same no matter what story you feel like telling yourself so that you can take the outcome of these calculations and compare them with the real world.
Let me make this clear that I'm not against MWI. I care about MWI exactly as much as about the Copenhagen interpretation (which is not very much.) I am however opposed to pretending that one of the two exactly equivalent ways of saying something is "more true" than another.
Staying within Quantum Mechanics, there are two ways of writing the equations of motion: the Heisenberg picture and the Schroedinger picture. In the former the state vector is constant but the operators are a function of time, in the latter the operators are constant and the state vector evolves with time. The two formulations are equivalent, sometimes it is convenient use one or the other for a specific calculation and often you use a mix of both (so called interaction picture.) Nobody argues that say the Heisenberg picture is "really true" as opposed to the Schroedinger picture. If someone did, that would be inane, even if they invoked Copernicus and Occam (even though the analogy with Copernicus would be maybe better, since the calculations actually are different depending which picture you choose.)
> The long and short of it is, for purely philosophical reasons you don't like the notion of the state vector collapse.
That's not the long and the short of it. I didn't say anything of the sort.
You ramble on apparently oblivious that to the fact that different interpretations of QM can and do make different predictions about what causes wave-function collapse. E.g., GRW makes different predictions from Penrose's interpretation. Why you are oblivious to this fact, I cannot fathom, as I've mentioned this fact several times now.
In theory, someday we'll be able to design experiments that determine which one of these interpretations, if either, is correct.
So, as you should be able to see, this is in no way comparable to different forms for equations that make identical predictions.
As to the measurement problem
http://en.wikipedia.org/wiki/Measurement_problem
Newtonian mechanics has no such measurement problem, so it is you who are grasping at straws. If a QM Interpretation is going to be considered a complete theory, it has to answer certain questions. For instance, if were were to train cats to operate quantum measurement devices and to perform experiments for us, and we consequently determined that measurements performed by cats did not cause wave function collapse, would that confirm or deny the Copenhagen Interpretation?
A. Neither, because the Copenhagen Interpretation doesn't define "measurement", and so we have no idea whether or not measurement by cats counts as "measurement".
You don't like cats, substitute in nanoscale molecular robots instead.
I'm not familiar with GRW, Penrose's theory is obviously more than an interpretation, he postulates new physics which is experimentally testable. The MWI does nothing of the sort to they're not at all similar.
Looks like you are trying to confuse the issue: to deny that QM interpretations are not experimentally testable, you bring into discussion a bunch of things which are not interpretations, but actual scientific theories, and call them "interpretations." You might as well argue that the ancient Greek religion was testable (you could climb Mount Olympus and see whether Zeus was there or not), therefore religions are testable, therefore the existence of God is a scientific fact.
Regardless of wonderful qualities of the GRW theory and the Penrose interpretation, the fact stands that there is no experimental way, even in principle, to distinguish between MWI and Copenhagen. If you teach cats or mice or West Highland Terriers to perform quantum mechanical experiments, they will still not be able differentiate experimentally between MWI and Copenhagen. And you will not be able to determine whether measurements performed by cats cause wave function collapse, because you will have to observe the cat to ask him what he observed.
> I'm not familiar with GRW, Penrose's theory is obviously more than an interpretation
I have no desire to get into a debate on terminology. I've just been using the terminology that was taught to me in an entire semester-long class I took at MIT on QM and its various "interpretations". GRW was called an "interpretation", as was Penrose's. The term "interpretation" is also the term used on the Wikipedia page for "Penrose's Interpretation".
> the fact stands that there is no experimental way, even in principle, to distinguish between MWI and Copenhagen.
See the text starting at "However, in 1985 David Deutsch published three related thought experiments which could test the theory vs the Copenhagen interpretation."
You know what, this is interesting. I would love to be proven wrong.
I searched for that paper but couldn't find it, nor any description of the experiment it proposes. I wrote an email to professor Deutsch asking him to send me a pdf copy. I will get back to you if/when he responds.
I still kind of suspect that the paper will make some assumptions that will effectively mean "if <magic> then we could test the MWI in the following way: ...".
He has a book. I haven't read it, but I'm sure he must discuss this issue in it.
In any case, I'm sure you can come up with a definition of "measurement" that might make the Copenhagen Interpretation experimentally indistinguishable from MWI, but why? MWI is and always will be a simpler theory, and thus preferred by Occam's razor.
The problem with the Copenhagen Interpretation is that it is NOT a scientific theory. It is not a scientific theory because it is not falsifiable. It also builds non-fundamental things like "measurement" right into the fundamental laws of physics, which is absurd. Copenhagen is not falsifiable, because if I were to attempt to falsify it by demonstrating that a "measurement" did not cause the wave function to collapse, you could always assert that I had used the wrong definition of "measurement".
GRW, on the other hand, can be seen as a sub-interpretation of the Copenhagen Interpretation because it rigorously defines the term "measurement". I.e., entanglement of the particles in question with a "large enough" collection of additional particles. "Large enough" here needs some experimental tuning, but some day we may be able to perform these experiments and attempt to falsify GRW. Because we can falsify GRW, it IS a scientific theory.
Most other collapse interpretations that I have heard of can likewise be seen as sub-interpretations of Copenhagen, in that they define "measurement".
One of those sub-interpretations is the Wigner Interpretation, or the "consciousness causes collapse" interpretation. Counter to your previous assertion, we could in theory experimentally determine whether this is true via trained rats: (1) Train a rat to perform measurements, (2) kill the rat before it has a chance to tell you the results, (3) check to see whether wave function has collapsed. We can do this because there are experiments that will tell you if two particles are entangled or not.
Animal consciousness not good enough for "measurement"; it needs to be human? Okay, Nazis could in theory perform this experiment, as could future evil alien overlords.
Now let's go back to MWI: I think that many people have a misconception about MWI, and perhaps this is due to its name. (If we had stuck to the name "Everett Interpretation" perhaps that would have been better.) MWI doesn't really imply multiple worlds. It implies one very complicated superimposed world. It is also the simplest theory, as it does not add the complication of wave function collapse. Furthermore, it is completely consistent with every bit of data that has ever been collected.
Another misconception is that MWI asserts that the "other worlds" that fall out of it are "real". This is not the case. MWI is agnostic on this issue. For instance, Stephen Hawking is in favor of MWI, but he doesn't like the name, because he thinks that asserting that the "other worlds" are "real", rather than just mathematical artifacts of the theory, is not something that we can scientifically know.
Executive summary: MWI is the simplest theory, and is consistent with all data. By Occam's razor, we are required to give this theory preference until we have evidence that contradicts it.
The counter argument to the above is that the ontological cost of all these many worlds (or maybe even the complicated superpositions of state) is too great, and that this somehow violates Occam's razor.
Well, first of all it doesn't, since Occam's razor these days is almost always taken to prefer the SIMPLEST THEORY, regardless of additional philosophical worries like, "It's just creepy to think that there might be so many other worlds."
Furthermore, this objection is based on a misinterpretation of MWI. MWI is completely agnostic about the ontological status of these "other worlds". It's just a mathematical formulation for making scientific predictions. There are many cases in the history of science where "creepy" things fall out of the math, if we were to grant them the status of being ontologically "real", and yet we don't reject the theories because of this. E.g., virtual particles and advanced waves. Sometimes scientists at some point decide that mathematical artifacts of theories are "real". E.g., virtual particles. And at other times, they remain just mathematical artifacts. E.g. (maybe), advanced waves.
Are the other worlds in MWI "real"? You tell me! Science cannot answer that question. This does not imply that MWI isn't the best theory.
OK I got hold of the paper, which is a very nice read, I can email it to you if you want. It's from a talk David Deutsch gave at some conference, I bet he is a very entertaining speaker. My summary won't do it justice, but anyway. The experiments are the following:
Experiment 1:
Measure the current time and call it t1. Note that you are conscious at the time you are observing the value t1. Wait. Check your watch again. It is showing a different time t2 now. You are still conscious and your consciousness is in a different state than at t1. Therefore you've detected experimentally a superposition of distinct states of human consciousness, since there exists a formulation of Quantum Mechanics in which time is a regular operator like any other observable and you've observed two different values of it.
Experiment 2:
Consider a computer which is so well isolated that interference can be observed between it's computational states. The computer is programmed to perform an algorithm which takes one bit of input, and produces one bit of output. The algorithm is very computationally expensive and takes a long time T to complete. We communicate with the computer via two observables, I (for input) and O (for output).
Prepare the initial state as a superposition of both input values 1/sqrt(2) * (|I=0> + |I=1>). After the time T, the computer will be in the state 1/sqrt(2) (|I=0,O=f(0)> + |I=1,O=f(1)>), where f(n) is the output the algorithm produces for the input n. So by measuring I and O at this point we will either learn the value f(0) or f(1), but not both. But say we are really interested in f(0) XOR f(1). Classically, it's impossible to calculate it without computing both f(0) and f(1) so it has to take the time 2T. But with our computer, which is in a quantum superposition of states, and with the help of some clever algebra, we can construct another observable R. When we measure R one of the two things happen with equal probabilities: either we get the correct value of f(0) XOR f(1), or we lose any hope of learning it from our system. We know which one happened, i.e., with probability 0.5 we will have the correct value for f(0) XOR f(1) and we will know for sure it is correct.
Since we obtained f(0) XOR f(1) in half the time, clearly there existed two parallel worlds in which two versions of the computer calculated f(0) and f(1).
(It is noted that some members of the audience objected that Experiment 2 is conceptually no different from the two-slit interference experiment. The author allows that this may be so in a sense, since indeed, the two-slit experiment alone should be enough to make it obvious that the Everett's interpretation is right, however Experiment 2 makes it even more obvious.)
Experiment 3 (simplified version):
Consider a system consisting of a spin 1/2 particle and a quantum computer running a simulation of human consciousness. Prepare the spin in the state |→> = 1/sqrt(2) (|↑>+|↓>), i.e., measuring the spin along the x axis will always show it's pointing to the right, which means that measuring the spin along the z axis may give up or down with equal probabilities. Have the conscious being in the computer measure the spin along the z axis and communicate to the outside world the fact that he/she observed one of the values 'up' or 'down' (without saying which one). Then undo all the transitions the combined system underwent, i.e., revert it to the original state (which is in principle possible for a system consisting of a quantum computer and a microscopic system). Then measure the spin of the particle along the x axis. If it shows 'right' every time (we need to repeat the whole procedure many times), then the Everett interpretation must be true, since otherwise the fact that a conscious being observed 'up' or 'down' would have caused the particle to collapse to a state in which 'left' and 'right' are equally likely.
(The original formulation of Experiment 3 was more complicated, with three spins not one and some more clever algebra, the purpose of which if I understand correctly is to prove that it's possible to communicate to the outside world the fact that a measurement along z axis was taken, without losing the ability to revert the system to the original state.)
So, there. I'll let you form your own judgement.
Counter to your previous assertion, we could in theory experimentally determine whether this is true via trained rats: (1) Train a rat to perform measurements, (2) kill the rat before it has a chance to tell you the results, (3) check to see whether wave function has collapsed.
It doesn't work that way, because even if the rat doesn't cause the wave function to collapse, it interacts with the system and causes a transition, from the original state to a state which is a superposition of states corresponding to various values the rat might have gotten from the measurement, each of these states individually looking exactly as if the rat collapsed the wave function, with the coefficients such that the probabilities for each value come out right. And killing the rat afterwards does not undo it. So you will observe that the wave function has collapsed. Same if you use a mechanical detector in place of a rat.
Yes, I would like copy of the paper. Please send it to doug at alum dot mit dot edu. Thanks!
> And killing the rat afterwards does not undo it.
Yes, sorry; it's been a long long time since I've thought about this sort of stuff in any detail. This is what I should have written:
Train a rat to perform measurements on an observable with two possible outcomes and have it press lever A for outcome A and lever B for outcome B. Put the rat into a sealed box to perform the measurement. A dial on the outside of box will read either A or B once the rat has performed the experiment and recorded the result.
You can now come up with a complex observable on the whole system, i.e., the original observable being measured by the rat, plus the rat, and the box, that will give two different results on different occasions if the rat did not collapse the wave, but will always give you the same result if the rat did collapse the wave.
The problem with this complex observable is that for it to work, you must consider every molecule in the rat, every molecule of air it interacts with, etc., etc., etc. Miss a single molecule and the results are randomized.
Are we ever going to be up to this task? Not any time soon! But it could be child's play for the aforementioned evil alien overlords.
One complication, I can imagine, is that for this to work, you'd need to have a perfect model of the rat's biology and cognition in order for you to come up with the right observable. In the face of not yet being sure how collapse works, this might be very difficult. But then again, I'm sure that evil alien overlords are up the task.
David Albert talks about doing these sorts of experiments on p. 88 of Quantum Mechanics and Experience, and it was this that I was thinking of. Only Albert's examples don't have a trained rat, but rather other, simpler measuring equipment for which we are trying to determine whether or not it causes collapse.
As for Deutsch three experiments, it looks like Experiment 3 has two huge advantages over my trained rat system: (1) Since it's all contained inside a quantum computer, it seems a lot more feasible without the help of aliens. (2) If I understand correctly, the reversal stages means that you end up with a very simple observable, rather than the unfeasibly complex observable that you would need for my trained rat system.
As for Experiment 1 and Experiment 2, I don't understand #1 at the moment. And #2 seems to me so obvious as to go without saying. But it doesn't seem to prove anything that we didn't already know. Of course an uncollapsed wave can compute more than a collapsed wave!
I want to reiterate here that MWI is a much simpler theory than Bohm's, even if the consequences of MWI might seem more complicated. At times, there have been scientists who want to apply Occam's razor to the consequences, but most scientists these days would apply Occam's razor to the theory itself.
