New Scientist 19 June 1999


Quantum foam


Is the fabric of the Universe a seething mass of black holes and wormholes? We may soon be able to venture into this maelstrom in search of the theory of everything, reports Michael Brooks


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's 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.

"Many people have said it's going to be impossible to test quantum gravity, so there's no use even thinking about it," says John Ellis, a theorist at CERN, the Geneva-based European centre for particle physics. But, he says, it's 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-the elusive Theory of Everything that will tie all the forces of nature together. "This is the grand theoretical challenge the 20th century has left physics to solve in the 21st century," says Ellis. "Even if it looks hopeless you should nevertheless think about it."

Astonishingly, it doesn't 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's 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. "We know there are inconsistencies in these theories. It's just a question of when the inconsistencies are going to show up in the data." The best solution would be to find the underlying theory from which relativity and quantum mechanics can be inferred.

There's 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'dovich guessed that black holes aren't 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 "fuzziness" or "foaminess". 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're in good company. So do the researchers. "It's a very vague thing," says Chris Isham, a theoretician at Imperial College, London. "General 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's that sort of level of argument."

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't fundamental properties of the Universe. Instead, they are supposed to emerge from a purely mathematical theory ("Beyond space and time", New Scientist, 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's 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 ("Into the eleventh dimension", New Scientist, 18 January 1997, p 32). This is physicists' 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 Neuchâtel 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 Nature which 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-or even discover some real quantum foam.

So rather than the usual tool of fundamental physics-a superpowerful particle accelerator-what 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-it will be "noisy".

Amelino-Camelia compared the [Detecting quantum foam] noise levels in the Caltech Detecting quantum foam 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 "deformed Poincaré symmetry" say that quantum mechanics distorts certain symmetries of space-time-its immunity to rotation, inversion and other similar changes. But it turns out that that would produce bigger random fluctuations than the Caltech system's noise limit, so Amelino-Camelia politely suggests that this approach is almost certainly wrong. This is no mean feat, as the fluctuations he's 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's 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-in a very small way-these 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's 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's 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's noise levels will set new limits on quantum gravity. Mark Coles, head of the LIGO Livingston observatory, is unsure. "We don't have any operational experience as yet, so all the predictions of noise performance are simply extrapolations from the Caltech interferometer."

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-an 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's 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's 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 "internal clocks" 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't exist-in which case quantum gravity theories are in real trouble-or both paths experienced the same fluctuations. He favours the latter: the fluctuations could be "correlated" 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. "It may be that the actual theory is so different from anything we know about that we are hundreds of years away from it," 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.

Further reading:

* Further reading:Gravity-wave interferometers as quantum-gravity detectors by Giovanni Amelino-Camelia, Nature, vol 398, p 216 (1999)

* The Elegant Universe: Superstrings Hidden Dimensions and the Quest for the Ultimate Theory by Brian Greene, Jonathan Cape (1999)

Author: Michael Brooks Lewes, East Sussex

From New Scientist magazine, vol 28 issue 2191, 19/06/1999, page 28

© Copyright New Scientist, RBI ltd 1999