快猫短视频

ON YOUR kitchen table are the following implements: a chainsaw, a wooden
mallet and a pair of boxing gloves. Your mission, should you choose to accept
it, is to use one of these tools to split an atom.

It is, of course, a ridiculous assignment, but it would sound like child鈥檚
play to researchers studying quantum gravity. They believe that the very fabric
of space-time is a seething foam of wormholes and tiny black holes a hundred
billion billion times smaller than a proton. But the experimental tools
available to test this idea are absurdly clumsy: the best particle accelerators
can barely examine scales a million billion times larger.

鈥淢any people have said it鈥檚 going to be impossible to test quantum gravity,
so there鈥檚 no use even thinking about it,鈥� says John Ellis, a theorist at CERN,
the Geneva-based European centre for particle physics. But, he says, it鈥檚 too
important to ignore. Quantum gravity is needed to describe the first instants of
creation, when quantum fluctuations ruled the Universe, and it could even lead
us to a full understanding of how our Universe works鈥攖he elusive Theory of
Everything that will tie all the forces of nature together. 鈥淭his is the grand
theoretical challenge the 20th century has left physics to solve in the 21st
century,鈥� says Ellis. 鈥淓ven if it looks hopeless you should nevertheless think
about it.鈥�

Astonishingly, it doesn鈥檛 look hopeless any more. Since the beginning of this
year, physicists have proposed a handful of foam-probing experiments that could
shed light on quantum gravity. Against all the odds, they can now embark on a
journey down to the lowest level of reality, where quantum mechanics and gravity
meet.

Quantum mechanics describes how particles interact with each other to
generate all but one of the forces in nature. So most physicists believe it must
work for gravity, too. But how? The best description of gravity we have is
Einstein鈥檚 theory of general relativity, which says that what we feel as gravity
is actually the effect of curved space-time. General relativity works
beautifully for gravitational forces in the Universe, successfully predicting
the existence of such outlandish objects as black holes.

But problems are looming, Ellis says. 鈥淲e know there are inconsistencies in
these theories. It鈥檚 just a question of when the inconsistencies are going to
show up in the data.鈥� The best solution would be to find the underlying theory
from which relativity and quantum mechanics can be inferred.

There鈥檚 no telling what insights such a theory would yield. Physicists
struggling to marry Einstein with quantum mechanics have already made one
startling discovery. In 1971, Russian physicist Yakov Zel鈥檇ovich guessed that
black holes aren鈥檛 truly black, but instead combine with quantum-mechanical
fluctuations to emit photons and other particles. Stephen Hawking proved the
idea three years later, and these emissions are now called Hawking
radiation.

All fledgling theories of quantum gravity also make a more general and even
weirder prediction: the structure of space and time is very different from the
gentle curves predicted by general relativity. The American physicist John
Wheeler realised in the 1950s that if you look at things on a scale of about
10^-35 metres, quantum fluctuations become powerful enough to play tricks
with the geometry of the Universe. Space and time break down into 鈥渇uzziness鈥� or
鈥渇oaminess鈥�. A spaceship that size could find itself negotiating virtual black
holes, or getting sucked into one wormhole after another and tossed back and
forth in time and space.

If you think this idea of a space-time foam sounds horribly vague, you鈥檙e in
good company. So do the researchers. 鈥淚t鈥檚 a very vague thing,鈥� says Chris
Isham, a theoretician at Imperial College, London. 鈥淕eneral relativity is about
space-time, and quantum theory tends to involve quantum fluctuations in things.
Therefore, if you talk about quantum gravity, there might be some sort of
fluctuation in something to do with space-time. It鈥檚 that sort of level of
补谤驳耻尘别苍迟.鈥�

In the race to create a more substantial theory of quantum gravity, there are
two main contenders. Abhay Ashtekar of Pennsylvania State University contends
that space and time aren鈥檛 fundamental properties of the Universe. Instead, they
are supposed to emerge from a purely mathematical theory
(鈥淏eyond space and time鈥�, 快猫短视频, 17 May 1997, p 38).
But impressive as the mathematical framework is, no one is sure how to pull physical realities, like
space, time and gravity, from it.

Cat鈥檚 cradle

The other idea is based on superstrings: minuscule loops or strings about
10^-35 metres long, floating through space-time. Matter arises from
specific kinds of vibration in these strings, just as notes are the result of
certain vibrations of a violin string. There are a huge number of variants of
the strings idea, but researchers believe that they are merely different
versions of a single, all-encompassing structure called M-theory (鈥淚nto the
eleventh dimension鈥�, 快猫短视频, 18 January 1997, p 32). This is
physicists鈥� favourite Theory of Everything, with the potential to unite all the
forces of nature and explain the properties of every subatomic particle. But it
is still in its infancy, and so far has little to say about how quantum gravity
manifests itself in the Universe.

Giovanni Amelino-Camelia of the University of Neuch芒tel in Switzerland
decided not to wait around for the theorists to agree on what exactly is going
on. Earlier this year, he published some calculations in Naturewhich
imply that quantum gravity is accessible to experiments after all. If space-time
is a frothing mess, he reasoned, the distance between two objects should always
have some random fluctuations as the bubbles constantly form and burst. And by
measuring the amounts of fluctuation, we might be able to rule out some of the
theories鈥攐r even discover some real quantum foam.

So rather than the usual tool of fundamental physics鈥攁 superpowerful
particle accelerator鈥攚hat he needed was a good tape measure. The
California Institute of Technology has just such a device. Their interferometer
splits a laser beam in two, and bounces the resulting beams off two mirrors,
each 40 metres away but in different directions
(see Diagram). The
reflected beams are then recombined, producing an interference pattern that
reveals tiny changes in the paths they took to reach the mirrors. If the path
lengths fluctuate, the interference pattern will fluctuate too鈥攊t will be
鈥渘辞颈蝉测鈥�.

Amelino-Camelia compared the noise levels in the Caltech interferometer with
the noise that quantum gravity theories predict. So far, he reckons this
experiment has seen off at least one approach to quantum gravity. Theories based
on 鈥渄eformed Poincar茅 symmetry鈥� say that quantum mechanics distorts
certain symmetries of space-time鈥攊ts immunity to rotation, inversion and
other similar changes. But it turns out that that would produce bigger random
fluctuations than the Caltech system鈥檚 noise limit, so Amelino-Camelia politely
suggests that this approach is almost certainly wrong. This is no mean feat, as
the fluctuations he鈥檚 talking about are equivalent to a change of 1 metre in the
diameter of the Universe.

That still leaves superstrings and the Ashtekar approach undamaged. But
finally, quantum gravity theories are tethered on an experimental leash, and
there are other plans in the making to help pin down this fuzzy foaminess. Last
year, working with Amelino-Camelia and researchers from the University of
Athens, Houston Advanced Research Center and Texas A&M University, Ellis
suggested using gamma-ray bursts. These flashes of high-energy photons arrive at
Earth from the other side of the cosmos, and if they have travelled through a
space-time that is fuzzy, says Ellis, they should have become distorted. Roughly
speaking, the shorter wavelength photons in the burst should arrive at Earth
later than their long wavelength companions, because they fall down the
microscopic holes in space-time more easily. Using today鈥檚 gamma-ray detectors,
it should be possible to see this effect. Unfortunately, the researchers are
still working out exactly what a quantum gravity signature would look like.

Decay and transformation

Ellis has helped to develop yet another plan for unveiling quantum gravity,
one first suggested in 1995. The delicate physics of neutral kaons, subatomic
particles that exist for less than a millionth of a second, could be affected by
quantum fluctuations in space-time. Kaons and their antiparticles (antikaons)
decay and transform into each other, but they do it at very slightly different
rates. Ellis believes that quantum gravity may affect鈥攊n a very small
way鈥攖hese decay and transformation rates. As with the gamma-ray bursts,
predicting the effect precisely is still beyond the theorists, but it might be
possible to isolate it in future particle accelerator experiments

While we wait for these experiments to mature, a new generation of
interferometers could eliminate a few more theories. These interferometers are
designed to search for another peculiar gravitational phenomenon: gravity waves.
Although gravity waves have nothing to do with quantum gravity directly, they
could still have a big impact on its theory-makers. When massive objects such as
stars move very suddenly, general relativity says that they should send
space-time ripples out across the Universe. Astrophysicists hope to see these
gravity waves emitted by supernova explosions, or by black holes orbiting one
another or even colliding.

The biggest new gravity-wave detector, the Laser Interferometer
Gravitational-Wave Observatory (LIGO), is being built at Hanford in Washington
State, and Livingston, Louisiana (two versions are needed to rule out the
effects of seismic waves). As in the Caltech interferometer, laser light from a
single source is split and sent down two perpendicular arms, and reflected by
mirrors suspended at the end of each. But LIGO鈥檚 arms are 4 kilometres long, and
two more mirrors at the junction of the arms send the light back along the same
path so the beams can bounce back and forth many times before recombining. A
gravitational wave passing though this apparatus would change the lengths of the
two arms by different amounts, and so change the interference pattern caused
when the two light beams recombine.

When it is fully operational by 2002, LIGO will be the world鈥檚 largest
precision optical instrument. The device is so sensitive that, despite its
massive scale, it should detect movements in the mirrors as small as
10^-18 metres, or a thousandth of the diameter of a proton. VIRGO, a
slightly smaller European interferometer, will have about the same
sensitivity.

Amelino-Camelia says LIGO鈥檚 noise levels will set new limits on quantum
gravity. Mark Coles, head of the LIGO Livingston observatory, is unsure. 鈥淲e
don鈥檛 have any operational experience as yet, so all the predictions of noise
performance are simply extrapolations from the Caltech interferometer.鈥�

But even if that is true, there is a grander scheme to look forward to. LISA,
the Laser Interferometer Space Antenna project, will consist of six spacecraft
arranged in pairs at the corners of an equilateral triangle orbiting the
Sun鈥攁n interferometer stretching over millions of kilometres. LISA is due
for completion in 2015.

In the meantime, atom interferometry could provide yet another avenue for
quantum gravity research. Ian Percival, a theoretical physicist at London
University鈥檚 Queen Mary and Westfield College, believes that atom
interferometers, which replace laser light with a beam of atoms, should be able
to detect fluctuations in the time element of the foam.

It鈥檚 not just space that is beaten to a froth: time is also stretched and
squashed, fluctuating by around 10^-44 seconds as the bubbles appear and
disappear. Small, but possibly detectable, Percival says. According to quantum
mechanics, atoms have a wave-like nature, so a single atom can be split into two
separate waves and sent along two different paths. When the two atomic waves
recombine, any difference in their 鈥渋nternal clocks鈥� due to the effects of
quantum gravity should destroy the atomic wave interference pattern.

Steven Chu of Stanford University and Mark Kasevich of Yale University have
managed to separate atomic wave packets by 1 centimetre before recombining them.
They saw an interference pattern. According to Percival, that could be
interpreted in two ways. Either space-time fluctuations don鈥檛 exist鈥攊n
which case quantum gravity theories are in real trouble鈥攐r both paths
experienced the same fluctuations. He favours the latter: the fluctuations could
be 鈥渃orrelated鈥� over these distances, he says. They might even spread from one
place to another. As yet, however, no one really knows.

Few people believe that a satisfactory theory of quantum gravity is just
around the corner. 鈥淚t may be that the actual theory is so different from
anything we know about that we are hundreds of years away from it,鈥� Ellis says.
But now experiments are now becoming possible, things are looking up. Eventually
we should narrow in on one true description of the fabric of the Universe. The
apple, one might say, has fallen from the tree.