The earlier thread on The fatalism of entropy. The dynamism of spontaneous order ended up in a discussion about the claims of the Many World Interpretation to explain the observations made in quantum physics experiments. Put simply, the claim is that each time a choice is made, a new universe springs into existence. This is meant quite literally. Brian Scurfield argued this Many World Interpretation is both real, and required to explain free will -- to which I and others objected (you can visit the thread to see that debate).
Brian wished to open a thread on which to argue directly his claims for the Many World Interpretation. This is that thread. The following is Brian's opening salvo.
The Many Worlds Interpretation is the claim that Quantum Mechanics is a true description of reality and that this description applies at all scales, from the microscopic to the macroscopic. It is the claim that not only atoms are subject to quantum phenomena but also observers such as you and me. Taken seriously, the Many Worlds Interpretation implies that physical reality is a vastly bigger thing than we perceive and that reality is partitioned into entities that to a very good approximation are classical universes identical to ours. In the Many Worlds Interpretation, other universes exist and affect each other and we can observe the results of this experimentally - for example when we carry out interference experiments.
The entire ensemble of universes in the Many Worlds Interpretation is contained in an entity called the 'multiverse.' Each universe in this multiverse obeys exactly the same laws of physics and there is an infinity of universes. Universes differ in how events turn out. When two previously identical universes become different because events turn out differently we say the universes have differentiated. The multiplicity of each identical universe in the multiverse is infinite. We can speak of the measure of each universe. Basically it is the relative proportion of one set of identical universes with respect to another set of identical universes. Measure is an important concept in the Many Worlds Interpretation - it means that things do not turn out equally. It means that our choices in the multiverse are important because they govern the future measure of ourselves. And it means that we need not be concerned about the possibility of giant Sperm Whales appearing in orbit around the Earth.
The original version of the Many Worlds Interpretation was put forward by Hugh Everett III in 1957. In its original conception, universes did not differentiate, they split. This creates a number of problems with measure and also in reconciling the MWI with existing physical law. For these reasons the idea of splitting was dropped and replaced with the idea of differentiation by freedomist physicist David Deutsch. It was Deutsch's interest in the Many World Interpretation that led him to discover the almost thaumaturgical possibility of a universal quantum computer, laying the foundation for the modern field of quantum information theory. Universal quantum computers are probably just years away from realisation.
Peter has asked "why the Many Worlds Interpretation is a meaningful physical explanation and not just a useful concept or method". My outline above hints at some reasons. In the equations of Quantum Mechanics we have a tool that describes and predicts our observations in precise detail. We are entitled, therefore, to regard these equations as telling us something about the nature of reality for where does that predictive power come from if the equations do not reflect truths about reality? When the mathematics of quantum mechanics tell us that a quantum computer can perform prodigious feats of calculation in real time by differentiating into, say, 10^1000 versions of itself, then what are we to conclude? That those 10^1000 versions are just mathematical conveniences? How are we to explain the calculation when the entire observable universe contains just 10^80 atoms?
But we don't need complicated mathematics to tell us multiple universes exist. We can infer it from a purely non-mathematical, physical argument, as David Deutsch does in Chapters 2 and 9 of his book The Fabric of Reality when he considers various single-photon interference experiments (see also this video).
Another reason to regard the Many Worlds Interpretation as an explanation is that we have begun to elucidate in detail the structure of the multiverse and how it is determined by information flow (the multiverse is rather more than just an ensemble of universes!). If the multiverse were not real, it seems inconceivable that we could do this. Furthermore, the Many World Interpretation can be used to derive the Born probability rule of Quantum Mechanics. If correct, then even this in itself indicates that the Many World Interpretation has deep explanatory power for previously the Born rule had to be assumed.
A popular criticism of the Many Worlds Interpretation is that we cannot see or communicate with other universes. We can only infer the existence of other universes from things seen in this universe. The reason for this is that interference occurs when universes that had become different become identical again. The more different the two universes have become the more unusual it is for them to come together again - although this is happening all the time. Controlling interference between two universes requires that we control all the particles that have different states in the two universes and for practical purpose this means that we can control interference only in universes that are very nearly identical. For you to communicate with your doppelgänger would require controlling an astronomical number of particles, including all those in your brain. This is not even a remote possibility. That we infer the existence of other universes indirectly should be no more controversial than that we infer the existence of, say, neutrinos indirectly. We can't be absolutely sure that neutrinos really exist, but the concept provides the best explanation for things we do observe.
Another popular criticism of the Many Worlds Interpretation is that it cannot be distinguished from the philosophically lame Copenhagen Interpretation, which is another popular scientific explanation for the observations of quantum experiments. But the criticism may not stand. David Deutsch proposed the first test to distinguish the Many World Interpretation from the Copenhagen Interpretation in 1977 (see Quantum Concepts of Space and Time, Oxford: The Clarendon Press, pp. 204-214) and there have been a number of other proposals put forward since then. Unfortunately these are not yet technically feasible (and some may be conceptually wrong), but we should not be surprised that the Many World Interpretation makes different predictions to its rivals for it has different assumptions.
I'll quickly summarise some other reasons why you should regard the Many Worlds Interpretation as an explanation. In the Many World Interpretation, a particle is a particle; it is not both localized at a point and spread out over the whole universe. Explanations that invoke the latter idea have only ever caused confusion! The Many Worlds Interpretation does not require "spooky action at a distance" or that, in Einstein's words, "God plays dice." Quantum mechanics turns out to be, after all, local and deterministic, as all good explanations are. In the Many World Interpretation, reality exists independently of consciousness and although we are differentiating into countless versions all the time we can reconcile this with identity and free will. Finally - as implied by the other points - the Many Worlds Interpretation requires no collapse of the wave function (the wave function doesn't even seem to collapse in the Many World Interpretation) so this postulate can be dropped from quantum mechanics and Ockham's Razor slices in favour of the Many Worlds Interpretation.
The Many Worlds Interpretation is not without possible non-trivial problems. But these problems are not obvious objections stemming from incredulity to the whole idea of multiple universes, they are technical problems (and some [pdf] may already have been resolved). The Many World Interpretation has matured [pdf] in the last two decades and many critics have not caught up. We need to think what it would mean if the Many Worlds Interpretation were true, for so far it is our only tenable explanation of the strange quantum reality that we observe.
Feel free to either comment below, or to send me your own substantive five-hundred word response for publication here later in the week.
UPDATE: As Falufulu Fisi suggested, I have re-posted Mark Sadgrove's substantive comment below as number two in this series. You can find it here:
Quantum Physics Debate, 2: The Many Worlds Interpretation rebutted - Mark Sadgrove, at Not PC
LINKS: The fatalism of entropy. The dynamism of spontaneous order - Not PC [thread]
Many Worlds Interpretetation - Wikipedia
Advances in reserch - Hitachi Global
Hugh Everett bio - MIT
David Deutsch - Wikipedia
Quantum computer - Wikipedia
Quantum computers: march of the Qbits - New Scientist
Shor's algorithm - Wikipedia
'The Fabric of Reality,' by David Deutsch - Qubit.Org [book review]
Lecture 2: Interference - David Deutsch video lectures [video lecture]
The structure of the multiverse - David Deutsch
Quantum theory of probability and decisions - David Deutsch
The Quantum Aristotle - Peter Cresswell, SOLO
Copenhagen Interpretation - Wikipedia
An extension of 'Popper's Experiment' can test interpretation of quantum mechanics - R.Plaga
The wave function "does not seem to collapse" - 'Fabric of Reality' message board
The basis problem in Many World Theories - Henry P. Stapp [21-page PDF]
There is no basis ambiguity in Everett quantum mechanics - Mark A. Rubin
100 years of the Quantum - Tegmark & Wheeler [9-page PDF]
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