http://www.newscientist.com/article/dn26267-four-ways-you-can-see-the-multiverse.html#.VCVPb3YcR2E
Adrian Kent tries to refute many-worlds interpretations of quantum mechanics:
http://arxiv.org/pdf/0905.0624v3.pdf
In his paper "One world versus many: the inadequacy of Everettian accounts of evolution,
probability, and scientific confirmation", Adrian Kent aims to show that the many-worlds interpretation of quantum mechanics, first proposed by Hugh Everett, leads to absurdities when one tries to define a "rational" betting strategy for an inhabitant of the multiverse.
Toy multiverses and betting strategies of an inhabitant
In section III of the paper, Kent defines some toy multiverses and discusses possible betting strategies for an inhabitant.In my opinion, the inhabitants of a multiverse are "robots" created by evolution. They do not have "free will". We have to study them like laboratory mice in a labyrinth. It does not make sense to think about their betting strategies from the viewpoint of an outside observer who sees the whole branching universe of the many-worlds interpretation.
One specific question is if it makes sense for an inhabitant to take a maximum risk in betting, because then he maximizes the win in ONE branch of the multiverse. Again, I do not see this as a relevant question at all. Rather, we should study what kind of players thrive in a "typical" branch of the multiverse. Obviously, in most branches, evolution favors players who take only moderate risks. An animal species which would always take huge risks to maximize the return in ONE branch of the universe, would quickly become extinct in most branches of the multiverse.
Probabilities in tossing of a coin
In section IV A, Kent worries about a coin toss experiment in a toy multiverse. Kent describes an experiment where a quantum process prints either 0 or 1 to a tape. The inhabitants of the toy multiverse should determine the relative frequency of 0s and 1s experimentally. Kent is worried that in some branches of the multiverse, the inhabitants will see a wildly biased distribution of the binary numbers.I do not see any problem in this. Even in the one-world case, we can run a similar experiment where we have a big number of participants tossing a coin. If we have 1 000 000 participants in the experiment, then one will probably see a string of 20 tosses that are all heads. What is the problem then? Most of the participants will get roughly 50 % heads and 50 % tails. The ones who get a very biased distribution may deduce from the physics of the coin toss experiment that they have been incredibly "lucky" to get a very biased distribution. There is no philosophical problem in this, whether the experiment happens in one world or in a multiverse.
Multiplication of species in a multiverse
In Appendix C, Kent describes a thought experiment. There are two species, A and B. B takes huge risks and tries to increase its population wildly, while running also a big risk of extinction. Then, there is one branch in the multiverse where B's population grows exponentially. And the sum of populations of B in all the branches quickly dominates the sum of A populations when time increases. If we live in a multiverse, does it mean that it pays to take huge risks for huge returns? If we get a huge population of our species in one branch of the multiverse, do we need to care if our species becomes extinct in almost all other branches?The flaw in the thought experiment is that the resources of the visible universe are not infinite. The population of species B cannot grow exponentially for very long. In the long run, the risk-avoiding species A will dominate the total population of A + B in the multiverse because B becomes extinct in almost all branches.
De Broglie - Bohm interpretation is very close to many-worlds interpretations
Adrian Kent seems to sympathize with de Broglie - Bohm interpretation of quantum mechanics because that interpretation is "one-world".http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#Occam.27s_razor_criticism
But David Deutsch has said: "pilot-wave theories are parallel-universe theories in a state of chronic denial."
Since de Broglie - Bohm interpretation does not involve a collapse of the wave function, and since it evolves the wave function using the exact same equations as many-worlds interpretations, that means that de Broglie - Bohm interpretation is almost equivalent to many-worlds interpretations. The difference is that hidden variables in de Broglie - Bohm pick one branch of the multiverse as "more real" than other branches. De Broglie and Bohm believe that "conscious" observers only exist in this one branch.
But since the other branches may still interfere with the "real" branch of the universe in de Broglie - Bohm interpretation, we may conclude that the other branches also "exist" in some sense. Furthermore, physical phenomena in the other branches are similar to the "real" branch. There are humans living in the other branches, doing physics, and thinking about quantum mechanics. That raises the question: in what sense is the "real" branch more real than the other branches? This question makes de Broglie - Bohm interpretation look awkward.
In many-worlds interpretations, we might define that the branch where I am currently living as an observing "subject", is more "real" than other branches. But it is not mandatory to claim it is more real. Rather, we may say that I as a subject have landed through some random process in one branch, but that one branch is not more "real" than other branches.
I as a living subject was born in Finland. Does that mean that the UK is less "real" than Finland? I do not think so. Similarly, I think all the branches of the multiverse may be as real as the one where I am living.
The problem in one-world interpretations
We saw in the preceding section that de Broglie - Bohm interpretation is essentially a many-worlds interpretation of quantum mechanics though it pretends to be a one-world interpretation.What is the problem in genuine one-world interpretations, like the Copenhagen interpretation where there is a "collapse" of the wave function when a quantum system is measured?
The problem is that there is no natural way to decide what kind of interactions with a quantum system constitute a measurement that collapses the wave function. In the traditional Copenhagen interpretation, a "conscious being" does the measurement and causes the wave function to collapse.
The paradox of the Schrödinger cat raises the question whether a cat is a "conscious observer", or if it is just humans that are "conscious observers". This all looks very ugly, and in recent years experiments in quantum mechanics have shown that almost macroscopic lumps of matter can be in a superposition of quantum states, that is, such a lump of matter cannot be a "conscious observer". Maybe in the future we are able to show that even humans can be in a superposition of quantum states.
True one-world interpretations need some way to prune the branches of the multiverse in a many-worlds interpretation. Such pruning involves a "collapse" of the wave function of a kind. I do not believe such collapse exists, and that is why I have long been a supporter of many-worlds interpretations.
Many-worlds interpretations can explain some miracles in the origin of life
How the first living cell was formed is a mystery. No pathway from simple molecules to a complex cell that contains, say, a billion atoms is known. Many-worlds interpretations can explain such miraculous happenings. However improbable it is that simple organic molecules spontaneously react and form a living cell of one billion atoms, that DOES happen in some branches of a multiverse.Using a similar "anthropic" reasoning we could also explain the origin of intelligent life, if the evolutionary pathway to intelligent beings is extremely improbable.
But evolution in general cannot be explained using an anthropic argument in a many-worlds interpretation, because evolution has created many wonderful creatures that are not necessary in the pathway to intelligent life.