[Paleopsych] NS: Why time keeps going forwards

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Why time keeps going forwards
http://www.newscientist.com/article.ns?id=mg18825210.100&print=true

      * 13 October 2005
      * Amanda Gefter

    YOU wake up one morning and head into your kitchen, where you get the
    distinct feeling that something strange is going on. A swirl of milk
    separates itself from your coffee, which seems to be growing hotter by
    the minute. Scrambled eggs are unscrambling and leaping out of the pan
    back into their cracked shells, which proceed to reassemble. And the
    warm sunlight that had flooded the room seems to be headed straight
    for the window. Apparently, you conclude, time is flowing in reverse.

    You can deduce this because it is obvious that time has an arrow,
    which, this morning aside, always points in the same direction. We
    take the unchanging arrow of time for granted. Yet there is nothing in
    the laws of physics as we know them that says it can't point the other
    way. So the riddle is: where does time's arrow come from?

    Our perception of the direction of time is linked to the fact that the
    world's entropy, or disorder, tends to increase. When you pour milk
    into your coffee, the concoction, at first, is highly ordered, with
    all the milk molecules entering the coffee in a neat stream. But as
    time passes, the milk loses its organisation and mixes randomly with
    the coffee. Keep watching and you will see it become thoroughly mixed,
    but you won't see the milk suddenly regroup. Strange as it may seem,
    it's not that such a scenario is impossible. It's just incredibly
    unlikely.

    That's because there are vastly more ways for the molecules to arrange
    themselves in a random, spread out, high-entropy fashion than in the
    tight formation in which they began. It's a matter of probability: as
    the molecules perpetually rearrange, they almost always find
    themselves in high-entropy arrangements. Of course, if they start off
    in a high-entropy arrangement, we won't notice any change. But if
    entropy is low at the start, it's bound to increase.

    Therein lies the origin of the arrow of time as we perceive it. It has
    two essential ingredients. The first is a low-entropy beginning, like
    the milk starting out in an ordered arrangement. The second is mixing:
    the constant rearrangement of the milk and coffee molecules. Mixing is
    necessary for the system to evolve and rearrange from a low-entropy to
    a higher-entropy state.

    And exactly the same must be true on much grander scales. The
    cosmological arrow of time - the process that started with the big
    bang - requires the universe to have started off with low entropy, and
    the contents of the cosmos to have mixed ever since.

First evidence

    So can we find these ingredients for time's arrow in our universe?
    Cosmologists already have evidence for the first one. They see that
    the universe had a low-entropy beginning by looking at the arrangement
    of the photons in the cosmic microwave background radiation that
    provides a snapshot of the universe near the beginning of time.

    The CMB photons are uniformly spread out, with variations in density
    and temperature detectable at a mere 1 part in 100,000. If the spread
    of the CMB photons is uniform, we can assume that the other contents
    of the nascent universe - such as the atoms - were also spread
    uniformly at that time.

    At first glance, that seems like the very definition of a disordered,
    high-entropy state, but it's not. The universe is governed by gravity,
    which always clumps things together, so a spread-out state is
    incredibly unlikely. Although no one knows exactly why, it seems the
    universe was born in a low-entropy state.

    So what provides the second ingredient? What mixes and rearranges the
    contents of the universe? According to Vahe Gurzadyan, a physicist at
    the Yerevan Physics Institute in Armenia and La Sapienza University in
    Rome, the answer is the shape of space itself.

    In 1992, Gurzadyan and his student Armen Kocharyan were looking at
    what a universe with "negative curvature" would do to the CMB.
    Negative curvature - the exact opposite of the curvature of a sphere -
    means that every point in space would be curved both up and down, like
    the mid-point of a saddle or a Pringle chip. Physicists have long
    considered this to be a possible geometry for the universe.

    The temperature of the CMB varies slightly from point to point in the
    sky, and maps of this variation reveal a multitude of hot and cold
    spots. These maps have enabled cosmologists to infer many things about
    the universe: its age and composition, for example. In their
    theoretical work, Gurzadyan and Kocharyan found that negative
    curvature would stretch the CMB spots into ellipses. That's because
    the CMB photons we observe today have been travelling through the
    universe for nearly 14 billion years. If that journey took them
    through negatively curved space, each little patch of light would
    appear as if it has been through a distorting lens. Five years later,
    Gurzadyan was looking at data from NASA's COBE satellite, one of the
    first to map the CMB, and saw exactly what he and Kocharyan had
    predicted: all the spots appeared elongated (Astronomy and
    Astrophysics, vol 321, p 19).

    The observation was exciting but inconclusive because COBE did not
    provide sufficiently fine resolution to measure the shape of the spots
    precisely. Perhaps, Gurzadyan and Kocharyan reasoned, this apparent
    elongation was just an illusion created by the low-quality images. But
    when vastly more detailed CMB maps arrived from NASA's Wilkinson
    Microwave Anisotropy Probe (WMAP) in 2003, Gurzadyan and colleagues
    ran the data through their programs, removing all irrelevant
    distortion effects - and there it was (Modern Physics Letters A, vol
    20, p 813). "All the spots have the same constant elongation,
    independent of temperature and the size of the spots," Gurzadyan says.

    Because spots of all sizes are distorted in exactly the same manner,
    this effect can't be due to something that happened at the time the
    radiation was created. Some of the spots are so big that their
    extremities were already out of causal contact at the time of their
    creation: light from one side could never reach the other. Just as
    there is no way for us to communicate with a region that has slipped
    beyond our causal horizon (New Scientist, 20 October 2001, p 36),
    there is no way a distortion effect at that point in time could have
    produced the symmetry of the ellipse. So it must have happened some
    time later, during the photons' journey through the universe.

    And if that's the case, Gurzadyan says, we have all the ingredients we
    need for the arrow of time. The universe starts out in an unlikely,
    low-entropy arrangement, with all of its contents almost perfectly
    spread out. But as particles travel through the universe, their paths
    follow the curves of space. In a negatively curved space, any two
    particles that start off next to one another quickly diverge, which
    means all the particles dramatically rearrange: the geometry of space
    mixes the cosmos.

    Since most particle arrangements correspond to high entropy, the
    negative curvature inevitably guides matter into higher-entropy
    states. In the case of the universe, that means states with
    gravitational clumping: as entropy increases, things like stars and
    galaxies form and with them heavy elements and, eventually, us.

    Evidence of this process is encoded in the CMB. The elliptical shape
    of the CMB spots reveals that the photons' paths diverged in precisely
    the way Gurzadyan expected for a negatively curved universe. If
    spatial geometry mixed the photons, then it also mixed everything
    else. And low-entropy beginnings plus mixing equals the arrow of time.

    Although Gurzadyan has published his ideas and his data in various
    places, the work remains controversial: the traditional view is that
    the universe is flat, not negatively curved. The usual interpretation
    of the WMAP results, which comes not from looking at the shape of the
    temperature spots but instead from what's called the power spectrum,
    is that the universe is flat. And most cosmologists believe this
    flatness supports the cherished theory of inflation, the idea that the
    universe underwent a fleeting moment of faster-than-light expansion
    shortly after its birth.

    The trouble with that objection is that a different aspect of WMAP's
    findings goes against inflation's predictions. When astronomers plot
    the power spectrum of the data, they see a big problem - hints of
    which had also been seen with COBE. The power spectrum compares the
    amount of temperature variation at different scales in the sky. When
    close regions of the sky are being compared, the temperature
    variations of the CMB fit with the predictions of inflation. But on
    very large angular scales the variation conflicts with inflation's
    prediction. The anomaly, for which there is no accepted explanation,
    suggests that there is something strange going on in the large-scale
    geometry of the cosmos, perhaps because it is not flat. "This anomaly
    is very curious," says Roger Penrose, a mathematical physicist at the
    University of Oxford. "It seems to be out of kilter with the inflation
    model, and it could be due to negative curvature."

    Gurzadyan regards the elongation of the hot and cold spots as powerful
    evidence that the universe is negatively curved, and Penrose agrees.
    Negative curvature would distort the CMB far more than a flat universe
    could, Penrose explains, squashing the light in one direction and
    stretching it in another. "If the geometry of space is negative, then
    you expect the ellipses to stretch much more than they would in
    positively curved or flat space," he says. "And this is exactly what
    Gurzadyan sees."

    Nonetheless, most cosmologists are still not ready to abandon the flat
    universe or inflation. Although no one has actually shown or even
    suggested that there is anything wrong with Gurzadyan's elliptical
    spots, they are hesitant to accept its implications. "At the moment, I
    don't feel that we have any compelling evidence against space being
    flat," says Max Tegmark, a cosmologist at the Massachusetts Institute
    of Technology. Princeton University's Lyman Page, a member of the WMAP
    team, is similarly reluctant. "Though I'm a strong believer in
    alternative analyses of data, it is too early to put much stock into
    the interpretation of Gurzadyan's result," Page says.

    Penrose, however, is excited by the result, and says there is much
    more to be gained from the CMB than physicists so far seem to realise.
    "There's vastly more information in the data than people look at
    normally. So far we've seen an infinitesimal amount, and people tend
    to look at the same things that everyone else is looking at. Gurzadyan
    is only using a tiny bit, but it's a different tiny bit. I think the
    analysis has to be taken very seriously."

Elliptical time

    Of course, directly linking the ellipses to the flow of time is even
    more controversial, but we don't have any other satisfactory
    explanation. The flow of time we observe is certainly not compulsory:
    it is perfectly possible for the time-symmetry of relativity, quantum
    theory and our other descriptions of the universe to produce a
    universe where time doesn't flow - or even one where time flows in the
    opposite direction to the one we experience. In 1999 Lawrence Schulman
    of Clarkson University in Potsdam, New York, showed that in principle
    regions of the universe where time flows in the normal direction can
    coexist with regions where it flows backwards (New Scientist, 6
    February 2000, p 26).

    But in our universe a negative curvature would stop this by imposing a
    global condition for the increase of disorder. This may even be what
    allows life to exist in the universe, Gurzadyan suggests: a new kind
    of anthropic principle (see "Life and time").

    Of course, if the saddle-shaped universe provides us with a mechanism
    for the increasing cosmic disorder, it still doesn't explain the
    arrow's ultimate origin: it doesn't explain the first ingredient, why
    the universe began with low-entropy conditions. "Of course you need
    mixing," explains University of Chicago physicist Sean Carroll, "but
    that's the easy part. The hard part is getting the initial entropy to
    be low."

    That remains a mystery, perhaps only to be resolved by the "theory of
    everything" that physicists are avidly searching for. And we do have
    hints that this final theory might address the problem. For example,
    Rafael Sorkin of Syracuse University in New York state has proposed
    "causal set theory", which attempts to unite quantum theory and
    relativity. It supposes that the fabric of the universe grows as
    effects follow causal events - giving a sense of time's flow (New
    Scientist, 4 October 2003, p 36). Although Sorkin and his colleagues
    admit it is not yet a complete theory of quantum gravity, it does at
    least install a one-way arrow of time and a low-entropy beginning.

    Of course, all these attempts to understand the irrepressible passage
    of time assume that time's arrow is a "real" phenomenon to do with the
    physical universe - and that is not entirely certain. Some think it
    might arise from the strange metaphysics of the quantum world; others
    see it as a purely psychological phenomenon, an artefact of our
    consciousness.

    But Gurzadyan is now convinced that the passage of time is a
    cosmological process. The hands on the cosmic clock are driven round
    by the chaotic movements of photons through the negatively curved
    universe, he says. Though that may be a little beyond what most
    cosmologists are willing to accept for now, the idea must be worth
    exploring: the search for answers to the flow of time goes to the
    heart of physics, Penrose believes. "The problem of the arrow of time
    is absolutely fundamental," he says. "It's telling us something very
    deep about the universe."

Life and time

    Vahe Gurzadyan's idea has a startling implication: if the geometry of
    space were different, there would be no "arrow" of time. Could life
    exist in a universe without an arrow? If not, would that help explain
    why the geometry of our universe is as we observe? Gurzadyan has
    dubbed this idea the "curvature anthropic principle".

    The standard anthropic principle says that certain aspects of the
    universe - like the values of physical constants - are the way they
    are because otherwise we wouldn't be here to wonder about them. For
    instance, if the mass of the electron were different, the universe
    would be unable to support human life, so we shouldn't be surprised by
    its value, given our very existence. Some scientists consider this
    common sense, while others see it as a sorry stand-in for a real
    explanation. The curvature anthropic principle applies this logic to
    the shape of space: without this negative curvature, we wouldn't have
    evolved as we did, Gurzadyan suggests.