[Paleopsych] New Scientist: The first evidence for string theory?

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The first evidence for string theory?
18 December 2004

    IF YOU consider them separately, these two observations are hardly
    going to set the scientific world on fire. But together they add up to
    a spectacular possibility. In a tiny region of sky, astronomers have
    seen a dozen galaxies that appear as a curious sequence of double
    images. They have also observed a quasar whose brightness oscillates
    in an unexpected way. What could cause these odd phenomena? The only
    explanation that covers both is pretty mind-bending: "superstrings" of
    pure energy that can stretch millions of light years across the
    universe. Is this the first experimental evidence for string theory?

    The theory is our best hope of understanding how the universe works at
    its most fundamental level. It suggests that the basic constituents of
    matter are impossibly narrow threads of concentrated energy. The
    various different ways these superstrings can vibrate correspond to
    different fundamental particles, such as the up-quark and the
    muon-neutrino. The idea is well on the way to becoming a "theory of
    everything", uniting the laws of physics to explain how all matter and
    energy behave.

Visible strings

    One of the strangest features of string theory is that it requires
    many more dimensions than we can see: the only way the vibration modes
    of the superstrings can be sufficiently diverse to create all
    particles is if the superstrings vibrate in a space-time of 10
    dimensions. Of course, we appear to live in a universe with only four
    dimensions - three of space and one of time - so string theorists have
    postulated that the extra dimensions are "rolled up" much smaller than
    the dimensions of an atom. However, until now no one had seen evidence
    to support string theory, and many scientists dismiss its ideas as
    untestable conjectures. But are they about to be proved wrong?

    The answer lies with the big bang that kicked our universe into
    existence. String theory suggests that our universe may be a
    three-dimensional island or "brane" moving through 10-dimensional
    space, and that the big bang might have been caused by a collision
    between two such branes (New Scientist, 16 March 2002, p 26). This
    kind of collision would release a tremendous amount of energy, which
    would create a plethora of different kinds of stringy object. One type
    is the fundamental superstrings. Another is strange objects called
    Dirichlet or "D" branes that exist within each brane and as
    connections between branes, but intersect with only one dimension of
    our universe. As a result, they look to us like one-dimensional

    But these are not necessarily the tiny strings we associate with
    fundamental particles: they can be of all sizes right up to
    astronomical dimensions. "Contrary to what we used to think,
    fundamental strings need not be ultra-tiny," says Tom Kibble of
    Imperial College London. And the bigger strings can be big enough to
    leave a visible mark on our universe. That's because a string distorts
    the space around it in a unique way. We are used to objects with mass
    or energy distorting the space around them, rather like a person's
    weight distorting the flat surface of a trampoline. This distortion of
    space is the origin of every object's gravitational attraction.
    However, a string is somewhat different from a normal object. All its
    energy is held on a one-dimensional line, not spread through space,
    and this concentrated energy distorts the space around it into a
    conical shape, with the string as its axis.
    "Superstrings are well on the way to becoming a 'theory of
    everything', uniting the laws of physics to explain how all matter and
    energy behave"

    If there were a string between us and a distant galaxy, it would
    distort the light of the galaxy so that it could take two possible
    routes to the Earth. The result would be two identical images of the
    galaxy only a few arc-seconds apart in the sky (an arc-second is
    roughly the angle a small coin would make when seen from 2 kilometres
    away). And this is exactly what an Italian-Russian group claims to
    have found last year. The team, led by Mikhail Sazhin of Capodimonte
    Astronomical Observatory in Naples and the Sternberg Astronomical
    Institute in Moscow, christened the image pair Capodimonte-Sternberg
    Lens Candidate 1, or CSL-1. It consists of two apparently identical
    elliptical galaxies roughly 10 billion light years from Earth and a
    mere 2 arc-seconds apart (Monthly Notices of the Royal Astronomical
    Society, vol 343, p 353).

    Seeing two identical galaxies is nothing new: it also arises from a
    phenomenon known as gravitational lensing (New Scientist, 13 November,
    p 42). This occurs when light from a distant galaxy passes close to
    another galaxy on its way to Earth. The mass of the intervening galaxy
    distorts the path of the light, producing multiple images of the
    distant galaxy. But gravitational lensing tends to manifest itself as
    an odd number of images that differ in brightness, often greatly. In
    the case of CSL-1, no intervening galaxy or cluster of galaxies is
    visible, there are just two images, and they are of equal brightness.
    So gravitational lensing doesn't seem to offer an explanation. "It
    looks like the signature of a string to me," says Kibble.

    Tanmay Vachaspati of Case Western Reserve University in Ohio is
    similarly optimistic. When he first noticed Sazhin's paper, he and his
    student Dragan Huterer tried to come up with reasons why a string
    could not be responsible. One of the first things that occurred to
    them was gravitational lensing, and they soon realised this hypothesis
    could easily be tested. The way galaxies are randomly distributed
    throughout the universe means that if you look at a patch of sky,
    gravitational lensing should be a rare phenomenon. If there's a string
    around, however, double images will be a lot more common. "A string
    should create other double images of galaxies in the neighbourhood -
    far more than would be expected by random chance," he says. "A simple
    follow-up observation should be enough to resolve the issue."

    Sazhin and his colleagues have now made just such an observation. In a
    "field" 16 arc-minutes square centred on CSL-1, they found 11 other
    double images. Between nine and 200 would be expected for a string,
    they say, but just two would be expected by chance from the
    gravitational lensing of intervening galaxies. "This already sounds
    very exciting," Vachaspati says.

Good vibrations

    It's particularly exciting because CSL-1 is not the only observational
    evidence for a string: there is also the curious case of the double
    quasar known as Q0957+561A,B, the first confirmed case of a
    gravitationally lensed object, observed by the Jodrell Bank telescope
    in the UK, in 1979. The two images are formed by the gravity of a
    galaxy that bends the light of the quasar so that it follows two
    distinct paths to Earth. The paths are different lengths and so the
    light takes a different time to travel along each one. As a result,
    outbursts in one image are mimicked by identical outbursts in the
    other image 417 days later.

    This year, a team from the US and Ukraine, led by Rudolph Schild of
    the Harvard-Smithsonian Center for Astrophysics, noticed some peculiar
    anomalies. Four times between September 1994 and July 1995, the two
    images of Q0957+561A,B brightened and faded by about 4 per cent, but
    without any time delay. Each oscillation in brightness lasted about
    100 days, and they were not repeated.

    The only way such a synchronous change in brightness could occur would
    be if the cause was not the quasar itself but rather an object between
    the quasar and the Earth. Schild and his colleagues claim that the
    idea that best fits the bill is an oscillating loop of string. These
    oscillations would occasionally cause the string to encroach on the
    two light paths from the quasar, altering the images we see. The
    string also appears to be moving across our line of sight at about 70
    per cent of the speed of light - which is why it affected the quasar
    for only a limited time.

    To oscillate once every 100 days or so, the loop has to be very small
    in astronomical terms - roughly 1011 kilometres. It also has to
    subtend an angle at the Earth substantially smaller than the
    separation of the images or it would create a spiky variation in the
    quasar's brightness rather than the smooth, periodic variation
    observed. The combination of these two conditions implies that the
    string is shockingly close to us - in our own galaxy, within about
    10,000 light years of the sun.
    "We are left with the conclusion that we are very lucky to have a
    string on our doorstep"

    So is it pure coincidence that a stringy relic of the big bang has
    ended up in our neighbourhood, or are these things scattered liberally
    throughout the universe? Strings would also emit gravitational waves
    and these should distort the space between them and us, introducing
    fluctuations in the time light takes to reach us and therefore in the
    observed timings of pulses. Though Kibble points out that there are a
    number of uncertainties in this calculation, the fact we do not see
    such an effect suggests a limit on how many strings there are between
    us and known pulsars. So we are left with the conclusion that we are
    very lucky to have a string on our doorstep.

    Scientifically speaking, that's not a very satisfying conclusion.
    Indeed, the whole question of string observation is still riddled with
    uncertainty, and many researchers are wary of rushing to conclusions
    about Sazhin's observations. "I think it is too early to get excited,"
    says Edmund Copeland of the University of Sussex in the UK. "There may
    be other possible explanations. Until the unique string aspects are
    confirmed, I think we should remain a little cautious."

    It is always possible, for example, that the fluctuations in the
    brightness of Q0957+561A,B looked the same entirely by chance. Abraham
    Loeb of the Harvard-Smithsonian Center for Astrophysics still favours
    the possibility that we have just seen a set of identical twin
    galaxies. "CSL-1 is most likely just a pair of galaxies that happened
    to be close together on the sky," he says. "We know of many close
    pairs of galaxies in the local universe, including our own Milky Way
    and Andromeda."

    Such a coincidence would be disappointing, Vachaspati says. "I am
    hoping nature won't have played such a trick on us." What everyone
    needs now is more evidence. To prove that each galaxy pair is a
    lensed, double image of a single galaxy, it will be necessary to
    measure the spectra of both objects and show them to be the same.
    Another angle of attack would be to find more candidates like CSL-1
    and Q0957+561A,B. But the best approach might be to look for
    gravitational waves.

    Strings would produce gravitational waves because they get kinked as
    they meet each other in space. As two straight strings cross, for
    example, they can emerge from the meeting as two V-shaped strings.
    Every time strings cross, they can become more kinked, and to shake
    off a kink they emit a shockwave, cracking like a whip. This shockwave
    travels at almost the speed of light, and should produce an intense
    burst of gravitational waves. As first pointed out by Thibault Damour
    of the Institut des Hautes Études Scientifiques in Paris and Alex
    Vilenkin of Tufts University in Massachusetts, "cusp" signals could be
    spotted by the VIRGO or LIGO gravitational wave detectors. "The
    signals are very distinctive," says Joe Polchinski of the University
    of California at Santa Barbara. "If they exist, they could be picked
    up in the next few years."

    According to Polchinski, if strings are discovered it will take at
    least a decade to measure the signals precisely enough to deduce their
    properties. This, he says, may enable us to pin down their origins. It
    could be another source of disappointment: the observed strings could
    have nothing to do with string theory, but be low-energy versions of
    the cosmic strings that were once thought to have seeded the
    universe's structure (see "Return of the cosmic string").

    Nevertheless, it's an exciting prospect. String theory is big on
    imagination-stirring concepts, such as vibrating threads of energy
    that inhabit a multidimensional reality, and awesome collisions that
    create new universes. It may be that these elusive and fantastical
    strings have finally shown themselves.
    From issue 2478 of New Scientist magazine, 18 December 2004, page 30

Return of the cosmic string

    WE ALREADY know there cannot be an enormous number of giant
    superstrings out there. That's because they share many characteristics
    with "cosmic strings", concentrated threads of energy that physicists
    once believed to be scattered throughout space. In the 1980s,
    cosmologists were greatly interested in such structures: they were
    thought to be defects in space and time, formed by abrupt
    misalignments in the fundamental fields of nature when the universe
    cooled in the aftermath of the big bang, and locked forever in the
    weave of the fundamental fields threading the universe.

    Cosmic strings would be massive, and according to theory their gravity
    was of exactly the right strength to drag in the cooling debris of the
    big bang and seed the great superclusters of galaxies we see in
    today's universe.

    The observations, unfortunately, did not play ball. The gravity of
    cosmic strings should distort the cosmic background radiation - the
    "afterglow" of the big bang fireball - in a particular way, creating
    distinctive features. These were not seen. More seriously, experiments
    such as Boomerang and NASA's Wilkinson Microwave Anisotropy Probe,
    which each made detailed measurements of the radiation's temperature
    differences across different angular scales, saw sharp fluctuations in
    the temperature of the background radiation. The existence of such
    sharp peaks was seen as a natural consequence of the fact that the
    universe had a very particular size, or scale, at the end of an early
    epoch of super-fast "inflation". If cosmic strings - or the
    superstrings created by brane collisions in string theory - had indeed
    seeded structures in today's universe, no such sharply peaked features
    would be created. That's because, according to the theoretical ideas
    behind them, both types of strings are created with all possible

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