What do you mean by belief in MWI? What sort of experiment could settle whether MWI is true or not?
I suspect that a lot of people object to the stuff including copies of humans and other worlds we should care about and hypotheses about consciousness tacitly build on MWI, rather than MWI itself.
First, the links say that MWI needs a linear quantum theory, and lists therefore the linearity among its predictions. However, linearity is a part of the quantum theory and its mathematical formalism, and nothing specific to MWI. Also, weak non-linearity would be explicable using the language of MWI saying that the different worlds interact a little. I don’t see how testing the superposition principle establishes MWI. A very weak evidence at best.
Second, there is a very confused paragraph about quantum gravity, which, apart from linking to itself, states only that MWI requires gravity to be quantised (without supporting argument) and therefore if gravity is successfully quantised, it forms evidence for MWI. However, nobody doubts that gravity has to be quantised somehow, even hardcore Copenhageners.
The most interesting part is that about the reversible measurement done by an artificial intelligence. As I understand it, it supposes that we construct a machine which could perform measurements in reversed direction of time, for which it has to be immune to quantum decoherence. It sounds interesting, but is also suspicious. I see no way how can we get the information into our brains without decoherence. The argument apparently tries to circumvent this objection by postulating an AI, which is reversible and decoherence-immune, but the AI will still face the same problem when trying to tell us the results. In fact, postulating the need of an AI here seems to be only a tool to make the proposed experiment more obscure and difficult to analyse. We will have a “reversible AI”, therefore miraculously we will detect differences between Copenhagen and MWI.
However, at least there is a link to Deutsch’s article which hopefully explains the experiment in greater detail, so I will read it and edit the comment later.
“Many-worlds is often referred to as a theory, rather than just an interpretation, by those who propose that many-worlds can make testable predictions (such as David Deutsch) or is falsifiable (such as Everett) or by those who propose that all the other, non-MW interpretations, are inconsistent, illogical or unscientific in their handling of measurements”
None of the tests in that FAQ look to me like they could distinguish MWI from MWI+worldeater. The closest thing to an experimental test I’ve come up with is the following:
Flip a quantum coin. If heads, copy yourself once, advance both copies enough to observe the result, then kill one of the copies. If tails, do nothing.
In a many-worlds interpretation of QM, from the perspective of the experimenter, the coin will be heads with probability 2⁄3, since there are two observers in that case and only one if the coin was tails. In the single-world case, the coin will be heads with probability 1⁄2. So each time you repeat the experiment, you get 0.4 bits of evidence for or against MWI. Unfortunately, this evidence is also non-transferrable; someone else can’t use your observation as evidence the same way you can. And getting enough evidence for a firm conclusion involves a very high chance of subjective death (though it is guaranteed that exactly one copy will be left behind). And various quantum immortality hypotheses screw up the experiment, too.
So it is testable in principle, but the experiment involved more odious than one would imagine possible.
The math works the same in all interpretations, but some experiments are difficult to understand intuitively without the MWI. I usually give people the example of the Elitzur-Vaidman bomb tester where the easy MWI explanation says “we know the bomb works because it exploded in another world”, but other interpretations must resort to clever intellectual gymnastics.
If all interpretations are equivalent with respect to testable outcomes, what makes the belief in any particular interpretation so important? Ease of intuitive understanding is a dangerous criterion to rely on, and a relative thing too. Some people are more ready to accept mental gymnastic than existence of another worlds.
Well, that depends. Have you actually tried to do the mental gymnastics and explain the linked experiment using the Copenhagen interpretation? I suspect that going through with that may influence your final opinion.
Have you actually tried to do the mental gymnastics and explain the linked experiment [the Elitzur-Vaidman bomb tester] using the Copenhagen interpretation?
Maybe I’m missing something, but how exactly does this experiment challenge the Copenhagen interpretation more than the standard double-slit stuff? Copenhagen treats “measurement” as a fundamental and irreducible process and measurement devices as special components in each experiment—and in this case it simply says that a dud bomb doesn’t represent a measurement device, whereas a functioning one does, so that they interact with the photon wavefunction differently. The former leaves it unchanged, while the latter collapses it to one arm of the interferometer—eiher its own, in which case it explodes, or the other one, in which case it reveals itself as a measurement device just by the act of collapsing.
As far as I understand, this would be similar to the standard variations on the double-slit experiment where one destroys the interference pattern by placing a particle detector at the exit from one of the holes. One could presumably do a similar experiment with a detector that might be faulty, and conclude that an interference-destroying detector works even if it doesn’t flash when several particles are let through (in cases where they all happen to go through the other hole). Unless I’m misunderstanding something, this would be a close equivalent of the bomb test.
The final conclusion in the bomb test is surely more spectacular, but I don’t see how it produces any extra confusion for Copenhageners compared to the most basic QM experiments.
Frankly, I don’t know what you consider an explanation here. I am quite comfortable with the prediction which the theory gives, and accept that as an explanation. So I never needed mental gymnastics here. The experiment is weird, but it doesn’t seem to me less weird by saying that the information about the bomb’s functionality came from its explosion in the other world.
What do you mean by belief in MWI? What sort of experiment could settle whether MWI is true or not?
I suspect that a lot of people object to the stuff including copies of humans and other worlds we should care about and hypotheses about consciousness tacitly build on MWI, rather than MWI itself.
From THE EVERETT FAQ:
“Is many-worlds (just) an interpretation?”
http://www.hedweb.com/manworld.htm#interpretation
“What unique predictions does many-worlds make?”
http://www.hedweb.com/manworld.htm#unique
“Could we detect other Everett-worlds?”
http://www.hedweb.com/manworld.htm#detect
I’m (yet) not convinced.
First, the links say that MWI needs a linear quantum theory, and lists therefore the linearity among its predictions. However, linearity is a part of the quantum theory and its mathematical formalism, and nothing specific to MWI. Also, weak non-linearity would be explicable using the language of MWI saying that the different worlds interact a little. I don’t see how testing the superposition principle establishes MWI. A very weak evidence at best.
Second, there is a very confused paragraph about quantum gravity, which, apart from linking to itself, states only that MWI requires gravity to be quantised (without supporting argument) and therefore if gravity is successfully quantised, it forms evidence for MWI. However, nobody doubts that gravity has to be quantised somehow, even hardcore Copenhageners.
The most interesting part is that about the reversible measurement done by an artificial intelligence. As I understand it, it supposes that we construct a machine which could perform measurements in reversed direction of time, for which it has to be immune to quantum decoherence. It sounds interesting, but is also suspicious. I see no way how can we get the information into our brains without decoherence. The argument apparently tries to circumvent this objection by postulating an AI, which is reversible and decoherence-immune, but the AI will still face the same problem when trying to tell us the results. In fact, postulating the need of an AI here seems to be only a tool to make the proposed experiment more obscure and difficult to analyse. We will have a “reversible AI”, therefore miraculously we will detect differences between Copenhagen and MWI.
However, at least there is a link to Deutsch’s article which hopefully explains the experiment in greater detail, so I will read it and edit the comment later.
“Many-worlds is often referred to as a theory, rather than just an interpretation, by those who propose that many-worlds can make testable predictions (such as David Deutsch) or is falsifiable (such as Everett) or by those who propose that all the other, non-MW interpretations, are inconsistent, illogical or unscientific in their handling of measurements”
http://en.wikipedia.org/wiki/Many-worlds_interpretation
None of the tests in that FAQ look to me like they could distinguish MWI from MWI+worldeater. The closest thing to an experimental test I’ve come up with is the following:
Flip a quantum coin. If heads, copy yourself once, advance both copies enough to observe the result, then kill one of the copies. If tails, do nothing.
In a many-worlds interpretation of QM, from the perspective of the experimenter, the coin will be heads with probability 2⁄3, since there are two observers in that case and only one if the coin was tails. In the single-world case, the coin will be heads with probability 1⁄2. So each time you repeat the experiment, you get 0.4 bits of evidence for or against MWI. Unfortunately, this evidence is also non-transferrable; someone else can’t use your observation as evidence the same way you can. And getting enough evidence for a firm conclusion involves a very high chance of subjective death (though it is guaranteed that exactly one copy will be left behind). And various quantum immortality hypotheses screw up the experiment, too.
So it is testable in principle, but the experiment involved more odious than one would imagine possible.
The math works the same in all interpretations, but some experiments are difficult to understand intuitively without the MWI. I usually give people the example of the Elitzur-Vaidman bomb tester where the easy MWI explanation says “we know the bomb works because it exploded in another world”, but other interpretations must resort to clever intellectual gymnastics.
If all interpretations are equivalent with respect to testable outcomes, what makes the belief in any particular interpretation so important? Ease of intuitive understanding is a dangerous criterion to rely on, and a relative thing too. Some people are more ready to accept mental gymnastic than existence of another worlds.
Well, that depends. Have you actually tried to do the mental gymnastics and explain the linked experiment using the Copenhagen interpretation? I suspect that going through with that may influence your final opinion.
cousin_it:
Maybe I’m missing something, but how exactly does this experiment challenge the Copenhagen interpretation more than the standard double-slit stuff? Copenhagen treats “measurement” as a fundamental and irreducible process and measurement devices as special components in each experiment—and in this case it simply says that a dud bomb doesn’t represent a measurement device, whereas a functioning one does, so that they interact with the photon wavefunction differently. The former leaves it unchanged, while the latter collapses it to one arm of the interferometer—eiher its own, in which case it explodes, or the other one, in which case it reveals itself as a measurement device just by the act of collapsing.
As far as I understand, this would be similar to the standard variations on the double-slit experiment where one destroys the interference pattern by placing a particle detector at the exit from one of the holes. One could presumably do a similar experiment with a detector that might be faulty, and conclude that an interference-destroying detector works even if it doesn’t flash when several particles are let through (in cases where they all happen to go through the other hole). Unless I’m misunderstanding something, this would be a close equivalent of the bomb test.
The final conclusion in the bomb test is surely more spectacular, but I don’t see how it produces any extra confusion for Copenhageners compared to the most basic QM experiments.
Frankly, I don’t know what you consider an explanation here. I am quite comfortable with the prediction which the theory gives, and accept that as an explanation. So I never needed mental gymnastics here. The experiment is weird, but it doesn’t seem to me less weird by saying that the information about the bomb’s functionality came from its explosion in the other world.
Fair enough.