On the night of 30 June 2005, the sky high above La
Palma in Spain's Canary Islands crackled with streaks of blue light
too faint for humans to see. But on the Roque de los Muchachos, the
highest point of the island, a powerful Magic eye was waiting and
watching.
Magic - the Major
Atmospheric Gamma-ray Imaging Cherenkov Telescope - scans the
sky each night for high-energy photons from the distant cosmos.
Most nights, nothing remarkable comes. But every now and again, a
brief flash of energetic light bears witness to the violent
convulsions of a faraway galaxy.
What Magic saw on that balmy June night came like a bolt from
the blue. That is because something truly astounding may have been
encoded in that fleeting Atlantic glow: evidence that
the fabric of space-time is not silky smooth as Einstein and
many others have presumed, but rough, turbulent and fundamentally
grainy stuff.
It is an audacious claim that, if verified, would put us
squarely on the road to a quantum theory of gravity and on towards
the long-elusive theory of everything. If it were based on a single
chunk of Magic data, it might easily be dismissed as a midsummer
night's dream. But it is not. Since that first sighting, other
telescopes have started to see similar patterns. Is this a
physics revolution through
the barrel of a telescope?
Such incendiary thoughts were far away from
Robert Wagner's mind
when the Magic data filtered through to the Max Planck Institute of
Physics in Munich, Germany, the morning after. He and his fellow
collaborators were enjoying a barbecue. Not for long. "We put our
beers aside and started downloading the full data set," says
Wagner.
It was easy to pinpoint the source of the data blip - a
20-minute burst of hugely energetic
gamma rays from a galaxy some 500 million light years away known as
Markarian 501. Its occasional tempestuous outbursts had already
made it familiar to gamma-ray telescopes worldwide.
This burst was different. As Wagner and his colleagues analysed
the data in the weeks and months that followed, an odd pattern
emerged.
Lower-energy photons
from Markarian 501 had outpaced their higher-energy
counterparts, arriving up to 4 minutes earlier
(Physics Letters B, vol
668, p 253).
This should not happen. If an object is 500 million light years
away, light from it always takes 500 million years to get to us, no
more, no less. Whatever their energy, photons always travel at the
same speed, the implacable cosmic speed limit: the speed of
light.
Perhaps the anomaly has a mundane explanation. We do not really
understand the processes within objects such as Markarian 501 that
accelerate particles to phenomenal energies and catapult them
towards us. They are thought ultimately to have something to do
with the convulsions of supermassive black holes at the objects'
hearts. It could be that these mechanisms naturally spew out
low-energy particles before high-energy ones.
Or they might not. "The more fascinating explanation would be
that this delay is not intrinsic to the source, but that it happens
along the way from the source to us," says Wagner.
Quantum signature
What piqued the interest of Wagner and his colleagues was that
the Magic observations were showing just the sort of effect that
quite a few models of quantum gravity predict. Physicists have been
on the lookout for experimental signposts to the right theory for
the best part of a century (see "Quantum gravity: why we
care").
Physicists have been on the lookout for signposts to the right
theory of quantum gravity for the best part of a century
"All approaches to quantum gravity, in their own very different
ways, agree that empty space is not so empty after all," says
theorist Giovanni
Amelino-Camelia of Sapienza University of Rome in Italy. Many
models based on string theory suggest that space-time is a foamy
froth of particles, and even microscopic black holes, that spark up
out of nothing and disappear again with equal abandon. The
alternative approach favoured by Amelino-Camelia, loop quantum
gravity, posits that space-time comes in indivisible chunks of
about 10-35 metres, a size known as the
Planck length.
Last year, it was suggested that the signature of just such a
quantum space-time had popped up in unexplained noise plaguing a
gravitational-wave detector in northern Germany (New Scientist, 17
January 2009, p 24). But that interpretation is far from a done
deal, and most experts agree that a more substantive sighting could
only come from observing the possible interactions of space-time
with particles passing through it.
According to many string theory models, particles of different
energies should speed up or slow down by different amounts as they
interact with a foamy space-time. A minimum size for space-time
grains, as predicted by loop quantum gravity, could violate the
cherished principle of special relativity known as Lorentz
invariance, which states that the maximum speed of all particles,
regardless of their energy, is the speed of light in a vacuum.
The trouble is that these effects would be observable only with
particles far more energetic than even the beefiest terrestrial
particle accelerators can produce. Even if we could make these
particles, the tiny interactions between them and the fabric of
space-time would not add up to a hill of beans, even over many laps
of the Large Hadron Collider's 27-kilometre-long loop at CERN, near
Geneva, Switzerland.
Summed over hundreds of millions or billions of light years,
such interactions could account for the Magic travel-time anomaly.
It looks like nature might have provided us with particle
accelerators - distant galaxies - whose products could, for the
first time, allow us to test predictions of quantum gravity against
hard experimental evidence.
As yet, we have only seen a handful of gamma-ray bursts of the
energy and intensity needed to see whether the delay effect is a
consistent feature. In July 2006, the High Energy Stereoscopic
System (HESS), an array of gamma-ray telescopes in the desert of
Namibia, saw a high-energy flare erupt from an active galaxy nearly
four times as far away as Markarian 501. The burst contained
marginal evidence for a time-lag of around half a minute for the
most energetic photons, which were considerably less energetic than
those in the flare spotted by Magic. The uncertainties in the data
resulting from the detection process, however, made a definitive
statement impossible (Physical Review Letters, vol 101, p
170402).
It is recent results from
NASA's Fermi Gamma-ray Space
Telescope, launched last year, that provide the most
tantalising glimpse yet of something extraordinary going on out
there. Last September, it spied a burst of gamma rays from a source
nearly 12 billion light years away. According to an analysis by
Amelino-Camelia and Lee Smolin
of the Perimeter Institute for Theoretical Physics in Waterloo,
Canada, the zippiest low-energy photons beat some of the
high-energy stragglers to Earth by anything up to 20 minutes. Two
much closer bursts seem to contain much smaller delays
(www.arxiv.org/abs/0906.3731v2).
The individual observations are pretty consistent with each
other, too, says theorist
John
Ellis at CERN. He and colleagues have taken data from the Magic
and HESS bursts to calibrate a theoretical model inspired by string
theory that assumes the delay effect increases linearly with
distance and photon energy. Using it to estimate the delay that the
highest-energy photon in the Fermi space telescope's September
burst should have experienced, they came up with a figure of 25
seconds, plus or minus 11 seconds. What Fermi had measured for that
particular photon was 16.5 seconds - within the model prediction's
admittedly large margin of error.
The only way to find out conclusively whether the delays are a
consistent signature of a quantised or foam-like space-time, says
Ellis, is to get more data - ideally from sources at many different
distances. "Then we'll be able to see whether we can distinguish
between effects at the source and effects in the propagation," he
says.
Worldwide cover
We also need to observe the same burst with more than one
instrument. Each telescope is sensitive to a different energy
range, owing to its altitude and detector set-up. Combining
different data sets will provide a wider spread of energies from
which to tease out any energy-dependent effect, and also help us
get round a persistent irritant to consistent astronomical
observations: Earth's rotation. Not only does our planet's spin
mean that multitudes of photons from the sun overwhelm any cosmic
source for a large proportion of the day, but it also makes
observing a highly directed beam of gamma rays from one specific
direction tricky, even at night: as you train your telescope on
your target, the Earth moves beneath your feet and eventually the
source slips out of sight.
That means Magic can observe any burst for a maximum of only 6
hours on any given night, assuming it is pointing in the right
direction when a new burst arrives. That period could be doubled by
using it in conjunction with a similar instrument - the
Very Energetic Radiation
Imaging Telescope Array System (VERITAS) - that sits atop Mount
Hopkins in southern Arizona.
A further gamma-ray telescope, the Major Atmospheric Cherenkov
Telescope Experiment (MACE), 4500 metres up on the Tibetan plateau
in the remote region of Ladakh, India, will open that observational
window still further. When completed in 2011, MACE will be the
highest-altitude gamma-ray telescope in the world, capable of
observing gamma rays with a wide range of energies. "Then we will
have another observatory 5 to 6 hours in front of Magic," says
Wagner. "That could lead the way to a continuous, 24-hour
observation of certain objects."
What with that and the new high-accuracy data from the Fermi
space telescope, gamma ray telescopes could well uncover quantum
space-time within the next few years. Even so, they still might be
beaten to the line. The definitive answer might come from a very
different source, and a very different quarter of Earth's surface -
the South Pole.
That is because a cubic kilometre of ice under the South Pole
will soon be home to the IceCube Neutrino Observatory, whose
strings of detectors will watch for faint flashes of blue light
emitted when neutrinos from cosmic sources smash into the Antarctic
iceMovie Camera.
Neutrinos are ghostly particles thought to be produced in the
same violent events that produce high-energy gamma rays. As yet, we
have not seen any neutrinos from outside our galaxy, barring some
that burst on us from a supernova in a neighbouring galaxy, the
Large Magellanic Cloud, in 1987Movie Camera. The neutrinos we do
see are lower-energy ones that come from nuclear reactions in the
sun and particle interactions in Earth's atmosphere. IceCube aims
to change that.
And it could see something big. Because the quantum-mechanical
wavelengths associated with neutrinos of the very highest energies
are even smaller than those of high-energy photons, they could be
more susceptible to disruption through interactions with a
space-time that is grainy on very small scales. Francis Halzen of
the University of Wisconsin, Madison, who leads the IceCube
experiment, has calculated together with his colleagues that in one
favoured model of quantum space-time such interactions could
dramatically speed up higher-energy neutrinos (Physical Review D,
vol 72, p 065019). "It's a beautiful signal that could not be
explained by conventional astrophysics," he says.
Humble constructions
That's not the only attractive property of neutrinos when it
comes to testing the idea of a frothy space-time, says Dan Hooper
of Fermilab in Batavia, Illinois. Neutrinos come in three distinct
"flavours", named after the chunkier particles they are associated
with - the electron, the muon and the tau. They tend to morph back
and forth between these different states as they travel, a
phenomenon known as neutrino oscillation. If a distant source is
emitting only electron neutrinos, theory tells us how many should
have changed flavours by the time they reach us.
If neutrinos were interacting with the quantum foam, though,
they would forget their original flavour along the way, leading to
equal numbers of all flavours by the time they arrive here. "That
effect would be hard to explain with normal astrophysics," says
Hooper. He suggests a possible, albeit disputed, source of electron
neutrinos in the Cygnus region of the Milky Way that could be ripe
for investigation (Physics Letters B, vol 609, p 206).
Uncertainties in models of neutrino oscillations make exact
calculations of the expected extent of the flavour-equalising
effect difficult, as Hooper himself points out. And even if we do
strike it lucky and find indisputable signs that either neutrinos
or gamma rays are being affected by the structure of space-time, it
will be a long, hard slog to convert that evidence into a viable
theory of quantum gravity. Amelino-Camelia likens the situation now
to that of a century ago, when anomalous observations - such as the
spectrum of black-body radiation, or the photoelectric effect -
that could not be explained by classical means set physics on the
decades-long path towards a fully fledged quantum theory. It did
not come easy.
And so it will be for quantum gravity. "We have to build,
humbly, very humbly, from what we know," says Amelino-Camelia.
"Construct simple theories, which are very far from being a theory
of everything, but intelligible enough that they can guide us to
the next spark." Whether on Atlantic islands, in the Himalayas,
deep in the Antarctic ice or high above Earth's atmosphere,
watchful eyes are waiting for signs from the universe's quantum
fabric.
Gallery: All eyes on space: The global gamma-ray network