The fabric of reality

I just finished reading ‘The fabric of reality’ by David Deutsch. It is a very thought provoking and impressive book. The book argues that the (current) reductionist approach to understanding modern physics and the universe is, although useful to a certain extent, unable to explain certain higher level emergent phenomena. The book ties together the four ideas of computational theory, quantum theory, the theory of evolution and the theory of knowledge in a rather persuasive way. Delightful insights tie the four main concepts together, giving an exciting glimpse of what could be a very aesthetic theory of everything.

I must admit that I found the chapter with the crypto-inductivist conversation a little difficult to follow, probably due to my lack of knowledge of the field of epistomology (no pun intended). However I very much enjoyed the chapters on ‘Time: the first quantum concept’ and ‘Time travel’.

Deutsch provides a very accessible and convincing overview of the many-worlds interpretation of quantum mechanics This has perhaps somewhat converted my own views on the subject; I never much liked the many worlds approach before reading this book. However when you take into account the role of time and lose the idea of worlds ‘splitting’ – (they don’t ‘split’, they just exist) then it makes a lot of sense.

The book also discusses ‘Omega point theory’ – a theory of events leading up to the big crunch, which was interesting as I hadn’t heard of the theory before.

It was fairly predictable that I’d like this book, as it discusses the intersection of quantum physics and the theory of computation, of which I am rather interested. Throw in the role of intelligence and knowledge in such a unified theory and you’ve pretty much got me hooked 🙂

The book was first published in 1997 (I don’t read books in any particular date order) so it’s certainly not new – hence I’ll definitely be doing some ‘further reading’ in ths area. I’m looking forward to the next book, which I think is scheduled for release in 2009.

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7 thoughts on “The fabric of reality

  1. rrtucci says:

    You say:
    However when you take into account the role of time and lose the idea of worlds ’splitting’ – (they don’t ’split’, they just exist) then it makes a lot of sense.

    There are only two possibiities for each of these worlds: (1) it always exists, or (2) it is created at some point in time. I think the word splitting refers to (2). Are you saying that (1) is the case? (It’s also possible that the worlds exist only in our imagination. That they have no physical reality)

  2. physicsandcake says:

    I think I’m saying that it makes more sense from the point of view (1) although I’ still not entirely convinced of either. (1) definitely makes more sense than (2). TBH I don’t often consider the interpretation of QM as it makes my brain hurt. I think I have a lot more foundations stuff to understand before I really decide which ‘camp’ I’m in.

  3. (1) is how Everett originally formulated the interpretation. The Schrödinger equation is time-independent. Our subjective experience of splitting is due to our embedding within a subset of the Hilbert space, and the ‘branching’ is a subjective consequence of entangled subsystems in each possible outcome.

  4. quantummoxie says:

    Well, there is a time-independent version of the Schr̦dinger equation, but the full version is time-dependent. As for the whole many-worlds thing, despite using branching spacetimes (i.e. non-Hausdorff topologies) in a recent paper, I have another argument (unpublished but available here) that seems to indicate just how unlikely such a scenario would be. In fact if the ideas of permutation symmetry that I discuss there could be tested it might be possible to at least disprove MWI (the result of such an experiment would either disprove the theory or offer no further clarification Рit could not actually prove it).

  5. Correct, though to clarify, the fundamental equation in which Everett framed his relative states fomulation is the Wheeler-DeWitt equation – an analogue of the original time-independent Schrödinger equation – which does not contain time as a variable at all. The account of continuous ‘splitting’ at each decision is a popular misconception. See also the Hartle-Hawking state, which is calculated from the Feynman path integral approach.

    A test to disprove MWI would be interesting – Everett did regard the theory as falsifiable, though in the sense that any test that falsifies conventional quantum theory would also falsify MWI.

    Deutsch has proposed a test to verify the Everett interpretation in Quantum mechanics near closed timelike lines (1991) as well as in The Ghost in the Atom (1986), wherein he proposes that an intelligent quantum computer could test for validity of the theory using the quantum eraser experiment.

    Zurek has used entanglement to derive the Born rule under the relative states formulation, and Saunders and Wallace presented an independent derivation of the Born rule from a decision-theoretic probability basis at the Everett@50 conference (Perimeter Institute), published initially at quant-ph and expanded upon in the Br J Philos Sci, also widely covered by the press, e.g. New Scientist (2007).

    I’m not uncritically advocating the relative states formulation through these references – rather trying to clarify the popular misconception of universes splitting and reproducing every time a decision is made. This is a widely-propagated mistake, and one that leads to all sorts of misplaced debates using Ockham’s razor. A parallel misconception is often employed in use of Ockham’s razor, which is a philosophical principle, not a scientific law – and is further based upon parsimony of assumptions, not parsimony of entities. If the latter were the case, we’d still be speaking from the geocentric perspective, rather than postulating a plethora of ‘other worlds’ out there in the universe.

  6. Amitabh says:

    Commenting when people who are neck deep in Physics are discussing Physics is always a hazard. What if I am considered a fool?

    Ok! At least by the end of this whole exercise – when someone responds to my response – I would be (just that bit) wiser fool. So here goes …

    *

    Correct me if I am wrong, but isn’t the MWI something similar to the Monte Carlo simulation results? The alternate (histories) worlds are merely possibilities. The one that occurred is real for us. Philosophically we could say that the other histories have happened, but this can neither be proved or disproved.

    *

    At the most, as it seems to me, MWI is a tool to arrive at a conclusion. As long as the conclusion is valid, the tool will be used. Unless someone knocks of the basis of the tool itself.

    No?

  7. physicsandcake says:

    My impression (which may well be wrong) of MWI is that all the alternatives are just as real as each other. I’m not sure about ‘the one that occurred is real for us’ – as if you include backwards time travel the Grandfather paradox etc. can be automatically avoided when you subsequently follow a different worldline.

    For me these aspect of QM get a bit too philosphical too quickly… I’m afraid I shy slightly into the ‘shut up and calculate’ camp, but that’s probably due to a lack of confidence whilst debating, and a lack of knowledge of all the different interpretations! This probably isn’t helped by the fact that when I perform QM experiments in the lab I just have to get them done and get the data, rather than contemplating the measurement problem too deeply…

    …sometimes I joke that I have enough ‘measurement problems’ of my own 😉

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