[Paleopsych] Economist: Cosmology: A braney theory

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Cosmology: A braney theory

    Oct 6th 2005

    An explanation for the anthropic principle comes a little closer

    DID God have a choice? Or, to put the matter less theologically, does
    the universe have to be the way that it is? The answer to this
    question, posed by Einstein among others, remains elusive. But it is
    important, not least because a universe with laws only slightly
    different from those actually observed would be one in which life--and
    therefore human life--could never have come into existence.

    That observation, known as the anthropic principle, disturbs many
    physicists because they cannot see any fundamental reason why things
    could not be different. In particular, they cannot see why space has
    to have three dimensions. But a paper due to be published this month
    in Physical Review Letters by Andreas Karch of the University of
    Washington and Lisa Randall of Harvard University suggests that the
    laws of physics may, indeed, be biased towards three-dimensions.
    Curiously, though, they have a similar bias towards seven-dimensions.

    The idea that there may be more dimensions than the familiar ones of
    length, breadth and height (and also, to be strictly accurate, the
    fourth dimension of time) is a consequence of attempts to solve an old
    problem in physics. Ever since Einstein developed his theories of
    space, time and gravity, physicists have sought a "theory of
    everything" that would unite those theories with quantum
    mechanics--the part of physics that describes electromagnetism and the
    forces that hold atomic nuclei together. Such a theory would, it is
    hoped, describe how the universe developed from the Big Bang. It would
    explain why there appears to be more matter than anti-matter. It would
    even indicate the nature of the dark energy and dark matter that lurk
    at the edge of perception.

    To date, the best candidates for a theory of everything are various
    versions of a branch of mathematics called string theory.
    Unfortunately for common sense, these theories require the universe to
    have ten or even 11 dimensions rather than the familiar three of space
    and one of time. To get round this anomaly, some physicists propose
    that the familiar dimensions are "unfurled", while the other six or
    seven are rolled up so tightly that they cannot be seen, even with the
    most powerful instruments available. For an everyday analogy, think of
    a thread of cotton. This appears one-dimensional for most purposes.
    Only under a magnifying glass are the other two dimensions

    A second interpretation of multidimensionality, however, is that the
    extra dimensions are not always rolled up, but that even when they are
    not humans cannot readily observe them because they are not free to
    move in them. In this version, the space inhabited by humans is a
    three-dimensional "surface" embedded in a higher dimensional
    landscape. The particles of which people are composed, and the
    non-gravitational forces acting on them, are strictly confined to this
    surface--called a brane (short for membrane)--and, as such, have no
    direct knowledge of the higher dimensional space around them. Only
    gravity is free to pervade all parts of the universe, which is one of
    the reasons why it obeys a different set of rules from the other

    It is this second interpretation that is invoked by Dr Karch and Dr
    Randall. They assume that, initially, the universe was filled with
    equal numbers of branes and anti-branes (the antimatter equivalent of
    a brane). These branes and anti-branes could take any number of up to
    ten different dimensions. Dr Karch and Dr Randall then demonstrated,
    mathematically, that a universe filled with equal numbers of branes
    and anti-branes will naturally come to be dominated by 3-branes and
    7-branes because these are the least likely to run into their
    anti-brane counterparts and thus be annihilated.

    This result is interesting for two reasons. It is the first piece of
    work to show that branes alone can explain the existence of hidden
    dimensions. They do not have to be rolled up to be inaccessible. It is
    also the first to suggest an underlying preference in the laws of
    physics for certain sorts of universe, and thus perhaps provide a
    solution to the anthropic principle. Yet it is not a total solution.
    Other realities, whether three- or seven-dimensional, could be hidden
    elsewhere in the landscape. And life in seven-dimensional space would
    look very different from life on Earth--if, indeed, it existed at all.
    That is because the force of gravity would diminish far more quickly
    with distance than it does in this world. As a result,
    seven-dimensional space could not have planets in stable orbits around
    stars. Like dark matter and dark energy, therefore, the anthropic
    principle is still grinning from the sidelines, taunting physicists to
    explain it.

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