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Benchtop cosmos: how to see quantum gravity in the lab
I
N THE control centre on the Hanford Nuclear Reservation in Washington state, banks of plasma screens await a signal that might never come. Hope springs from two concrete tubes that stretch out at right angles from the control centre and extend 4 kilometres towards the horizon. Inside them, laser beams ping relentlessly back and forth. The site is one of two that make up the Laser Interferometer Gravitational-Wave Observatory, LIGO, the largest experiment so far for spying the ripples in space-time known as gravitational waves. Off the coast of west Africa, perched on the highest point of the Canary Islands, a gammaray telescope called MAGIC – the name stands for the Major Atmospheric Gamma-ray Imaging Cherenkov telescope – scans the heavens for bursts of high-energy photons from far corners of the universe. Every now and again it catches a fleeting glimpse of something. Seconds, perhaps, of activity are followed by silence again. Back in the US, meanwhile, teams work flat out on plans for a $650 million space probe called the Joint Dark Energy Mission. It is just the latest and most ambitious bid to study how the universe is expanding and tell us what the vast bulk of the cosmos is made of. These are just three of many experiments
that could deliver breakthroughs in our understanding of nature’s most enigmatic force, gravity. If so, they will do it in the traditional way of big physics, with large collaborations and hefty bank balances. But that might not be the only way. If ideas being explored by a good few physicists are right, quantum gravity and dark energy could all be laid bare on the bench-top by the strange dances of atoms cooled to within a nudge of absolute zero. Familiar yet unfathomable, gravity is a perennial tease. Its quiet muscularity binds stars and galaxies together, steadies Earth on its trek around the sun, and keeps our feet firmly on the ground. According to our current theory of gravity, Einstein’s general relativity, all this is down to massive objects warping space and time and so making things slide towards them. There are a few wrinkles to this explanation: for example, the failure of instruments such as LIGO to spot gravitational waves despite general relativity indicating that accelerating cosmic bodies should be producing them. Yet overall the theory seems solid. No experiment has delivered a result in disagreement with general relativity’s predictions. But still many physicists are unsatisfied. For
Gravity’s big picture might be writ small in the lab, says Matthew Chalmers
Desktop cosmos 38 | NewScientist | 19 June 2010
them it is profoundly unsettling that general relativity is not a quantum theory, unlike the theories that describe the other three forces of nature. Add to that the observation that the cosmos seems to be expanding ever faster – something hard to explain if gravity does indeed dominate the universe – and it is clear why we feel we have much still to learn.
Splitting the atom The lab-based tricks that could allow us to do this without spending a fortune are variants on a technique called atom interferometry. That itself is a newer model of an old and powerful weapon in the armoury of physicists known as optical interferometry. This involves splitting a beam of light, sending it down two different paths and recombining it using mirrors. Any delay to the light travelling down one or other path caused by an outside disturbance will be revealed in a characteristic “interference pattern” created where the two beams recombine. This optical method is used by LIGO and other state-of-the-art interferometers in Germany, Italy and Japan to look for stretches and contractions in space on scales as small as 10-18 metres caused by passing gravitational waves. But the approach poses challenges: the mirrors used to guide the light are susceptible to confounding mechanical vibrations, and because light travels so fast you need an interferometer with very long arms for a noticeable delay to accumulate. Cold atoms mooch around at a more sedate pace, and thanks to the weird wave-particle duality that lies at the heart of quantum physics, they can be made to interfere too. In an atom interferometer, atoms are “split” to go down two paths at once and recombined, producing clear interference signals with the smallest of disturbances. That makes for superlative sensors of rotation and acceleration that have already been used to test the predictions of general relativity with unparalleled accuracy in the lab (see “Atom nav”, page 40). The impressive sensitivity of atom interferometers could also be used to pick up gravitational waves at frequencies lower than existing optical interferometers can detect. Such low-frequency waves are predicted to be radiated by the pas de deux of pairs of orbiting black holes or neutron stars, and perhaps by the cosmic ructions in the first fraction of a second after the big bang. In practice, though, there’s a big problem: although the waves associated with atoms >
jon krause
19 June 2010 | NewScientist | 39
”One end of the apparatus might fit in the lab – but it would have to be a lab with a 1-kilometre mineshaft beneath it”
move agreeably slowly, they are also flouncily delicate beasts and cannot travel more than a metre or so before losing the quantum character that allows them to interfere. That might well place a limit on the sensitivity that can realistically be achieved with atom interferometry. “It’s a sexy idea, but atoms at the moment are many orders of magnitude away from the optical systems,” says Craig Hogan of the Fermi National Accelerator Laboratory in Batavia, Illinois. All is not lost, however. In 2008, Savas Dimopoulos and his colleagues at Stanford University, California, proposed that a hybrid atom-optical interferometer could combine the advantages of both schemes. They suggest using pulsed laser beams from a single source to impart kicks to cold atoms in two interferometers separated by a large distance. These kicks will usually occur with clockwork precision, producing the same interference pattern in each interferometer. Should a passing gravitational wave disturb the beam on its way to one of the interferometers, however, that interference pattern will change (Physical Review D, vol 78, p 122002). It is a clever idea, ridding the apparatus
Atom nav Applying atom interferometry to measuring the effects of gravity is not new. In 1991, the year the technique took off, Mark Kasevich and Steven Chu of Stanford University in California cooled sodium atoms to near absolute zero, gave them an upward kick and compared the rate at which they fell back on two paths. That allowed their team to determine the acceleration due to gravity to within 3 parts in a million, a degree of precision that has since been improved 1000-fold. We now know that the rates at which, say, a glass paperweight and a caesium atom are pulled to earth are the same to within 7 parts in a billion (Nature, vol 400, p 849). In a paper published in February this year, Chu – now the US energy secretary – and
40 | NewScientist | 19 June 2010
his colleagues Holger Müller and Achim Peters show with the same precision that time flows faster for an object as the gravitational field it experiences weakens. This effect, predicted by general relativity and known as the gravitational red shift, is tiny: during the 0.16 seconds it takes for atoms to fly through two interferometer arms, one positioned a millimetre above the other, the atoms in the upper arm “age” more than those in the lower by 10-20 seconds. That is still enough of a difference to cause a measurable shift in the interference pattern (Nature, vol 463, p 926). It might sound esoteric, but that has practical implications. Precise measurements of Earth’s gravitational gradient are a handy tool for oil
prospectors and geologists, and since GPS satellites move in different orbits high above the Earth, an improvement of our knowledge of the gravitational red shift translates directly to sharper navigation on the ground. Indeed, so good are atom interferometers at sensing motion that a California-based company called AOSense is supplying them to the US Department of Defense for use in navigational devices. But Müller points out that while a fully fledged interferometer is a relatively small piece of kit by the standards of big physics, it will still be a challenge to incorporate into a helicopter or submarine. “It usually doesn’t fit through the lab door and needs a team of PhD level physicists to build and keep alive,” he says.
of the vibration-prone rigid mirrors that are a weak point of existing optical interferometers. But the resulting device is not much smaller than existing gravitational wave detectors: to allow enough space for a gravitational wave to stir things things appreciably, the two atom interferometers must be connected by a vertical laser arm about 1 kilometre long. While one end of the apparatus might fit in the lab, it would have to be a lab with a deep mineshaft beneath it.
Quantum probe Atom interferometry truly within the confines of the lab might come into its own in another context: the search for evidence of quantum gravity. Seekers of quantumgravitational truth suffer from a big problem: any observable effects are expected to surface only at lengths smaller than 10-35 metres, a scale known as the Planck length. vA fundamental relation in physics says that the tinier the scale you want to observe, the more energetic a probe you need. To observe phenomena at the Planck scale particles have to be a million billion times as energetic as the protons accelerated by the world’s most powerful particle smasher, CERN’s Large Hadron Collider. There might be a workaround. Most theories of quantum gravity predict that below the Planck scale, space is not a smooth, continuous fabric but a pixelated foam or a mash-up of microscopic extra dimensions. This might well have detectable effects. In the late 1990s, for example, theorists realised that photons of different energies emitted at the same time by distant cosmic explosions might arrive on Earth at different times, even though they all travel at the speed of light. In 2005, the MAGIC telescope saw evidence of just such an effect in a burst of gamma rays from a galaxy 500 million light years away (New Scientist, 15 August 2009, p 26). That remains an isolated observation, however, and the interpretation of such measurements is fraught with assumptions about the workings of extreme and distant astrophysical objects. Then late last year Giovanni Amelino-Camelia of the Sapienza University of Rome in Italy and his colleagues suggested that there might be a smarter way to go about things: cold-atom experiments could provide a way to tune into quantum space without looking beyond Earth. If a cold atom is struck by a laser with a specific frequency, it can absorb a photon and recoil slightly. Meanwhile, a second laser can
The idea is highly speculative, and testing it would require the accuracy of atom interferometry to be improved considerably in order to distinguish the shift from spurious effects, says Guglielmo Tino at the University of Florence, Italy. “But it addresses a very interesting possibility,” he says.
jon krause
Repulsive energy
be tuned to make the atom emit a photon. The researchers showed that in a quantum spacetime the amount of energy needed to drive this process would be slightly different from that needed in a smooth, classical one, and this would show up as an interference pattern when the two lasers are combined (Physical Review Letters, vol 103, p 171302). David Mattingly, a physicist at the University of California, Davis, says this could present a whole new line of attack on quantum gravity. The problem with astrophysical observations, he says, is that the theories that explain them only apply “to one or two particle species and to a certain high-energy sector”. Indeed, Amelino-Camelia reckons that overlooking low-energy phenomena in favour of dramatic cosmic processes could turn out to be the decisive factor in 80 years of failures in quantum gravity research. His is not a lone voice, either. In March this year, Charles Wang of the University of
Aberdeen, UK, and colleagues sketched out an atom interferometer experiment that they claim could reveal traces of quantum gravity – using not the interference patterns of a normal atom, but those of a quantum superatom. Their proposal is to load an interferometer with around 100 million rubidium atoms and then cool them close to absolute zero. At such temperatures, the atoms all crowd into the same low-energy quantum state, becoming an exotic gloop called a Bose-Einstein condensate (arxiv.org/ abs/1002.2962). This quantum monster brings into focus effects that are difficult to observe in a lighter object. Wang and colleagues suggest that just as random quantum fluctuations of the electromagnetic field are known to change the energy levels of a hydrogen atom – an effect called the Lamb shift – so the foaming fluctuations of space-time will produce tiny energy shifts in a Bose-Einstein superatom.
The ambition of that experiment, though, is nothing compared with a proposal made in January this year by Holger Müller of the University of California, Berkeley, and Martin Perl of the SLAC National Accelerator Laboratory in Stanford, California. Müller is an atom-interferometry guru, while Perl is a hard-headed experimentalist who won a Nobel prize in 1995 for his part in the discovery of the tau lepton, a heavier cousin of the electron. Perl thinks that atom interferometers could be the right tools to unearth the nature of dark energy, the source of the mysterious repulsive force causing the expansion of the universe to speed up. We know little about dark energy, and while further insights may come from large telescopes and space probes such as the Joint Dark Energy Mission, Perl thinks we might learn more by looking for tiny differences in the free fall of atoms in two interferometers positioned side by side, 1 metre apart (arxiv. org/abs/1001.4061). Not everyone is a convinced. “New technology often opens the door to new effects,” says Robert Caldwell of Dartmouth College in Hanover, New Hampshire. “But until there is a good theory that explains how one can detect dark energy in the lab, the best way to learn about it will continue to be through cosmological observations.” With big physics continually pushing the boundaries, it remains to be seen whether cold-atom experiments will live up to their new star billing. Within the next decade, for example, a trio of spacecraft collectively known as the Laser Interferometer Space Antenna, or LISA, might blast off, bound for a point 50 million kilometres from Earth. There, the craft will form themselves into a triangular array 5 million kilometres across to continue the hunt for gravitational waves. At a cost of well over a billion dollars, though, LISA won’t come cheap. If the money runs out on such grand schemes, we may be grateful for solutions closer to home. n Matthew Chalmers is a freelance writer based in Bristol, UK 19 June 2010 | NewScientist | 41
N THE control centre on the Hanford Nuclear Reservation in Washington state, banks of plasma screens await a signal that might never come. Hope springs from two concrete tubes that stretch out at right angles from the control centre and extend 4 kilometres towards the horizon. Inside them, laser beams ping relentlessly back and forth. The site is one of two that make up the Laser Interferometer Gravitational-Wave Observatory, LIGO, the largest experiment so far for spying the ripples in space-time known as gravitational waves. Off the coast of west Africa, perched on the highest point of the Canary Islands, a gammaray telescope called MAGIC – the name stands for the Major Atmospheric Gamma-ray Imaging Cherenkov telescope – scans the heavens for bursts of high-energy photons from far corners of the universe. Every now and again it catches a fleeting glimpse of something. Seconds, perhaps, of activity are followed by silence again. Back in the US, meanwhile, teams work flat out on plans for a $650 million space probe called the Joint Dark Energy Mission. It is just the latest and most ambitious bid to study how the universe is expanding and tell us what the vast bulk of the cosmos is made of. These are just three of many experiments
that could deliver breakthroughs in our understanding of nature’s most enigmatic force, gravity. If so, they will do it in the traditional way of big physics, with large collaborations and hefty bank balances. But that might not be the only way. If ideas being explored by a good few physicists are right, quantum gravity and dark energy could all be laid bare on the bench-top by the strange dances of atoms cooled to within a nudge of absolute zero. Familiar yet unfathomable, gravity is a perennial tease. Its quiet muscularity binds stars and galaxies together, steadies Earth on its trek around the sun, and keeps our feet firmly on the ground. According to our current theory of gravity, Einstein’s general relativity, all this is down to massive objects warping space and time and so making things slide towards them. There are a few wrinkles to this explanation: for example, the failure of instruments such as LIGO to spot gravitational waves despite general relativity indicating that accelerating cosmic bodies should be producing them. Yet overall the theory seems solid. No experiment has delivered a result in disagreement with general relativity’s predictions. But still many physicists are unsatisfied. For
Gravity’s big picture might be writ small in the lab, says Matthew Chalmers
Desktop cosmos 38 | NewScientist | 19 June 2010
them it is profoundly unsettling that general relativity is not a quantum theory, unlike the theories that describe the other three forces of nature. Add to that the observation that the cosmos seems to be expanding ever faster – something hard to explain if gravity does indeed dominate the universe – and it is clear why we feel we have much still to learn.
Splitting the atom The lab-based tricks that could allow us to do this without spending a fortune are variants on a technique called atom interferometry. That itself is a newer model of an old and powerful weapon in the armoury of physicists known as optical interferometry. This involves splitting a beam of light, sending it down two different paths and recombining it using mirrors. Any delay to the light travelling down one or other path caused by an outside disturbance will be revealed in a characteristic “interference pattern” created where the two beams recombine. This optical method is used by LIGO and other state-of-the-art interferometers in Germany, Italy and Japan to look for stretches and contractions in space on scales as small as 10-18 metres caused by passing gravitational waves. But the approach poses challenges: the mirrors used to guide the light are susceptible to confounding mechanical vibrations, and because light travels so fast you need an interferometer with very long arms for a noticeable delay to accumulate. Cold atoms mooch around at a more sedate pace, and thanks to the weird wave-particle duality that lies at the heart of quantum physics, they can be made to interfere too. In an atom interferometer, atoms are “split” to go down two paths at once and recombined, producing clear interference signals with the smallest of disturbances. That makes for superlative sensors of rotation and acceleration that have already been used to test the predictions of general relativity with unparalleled accuracy in the lab (see “Atom nav”, page 40). The impressive sensitivity of atom interferometers could also be used to pick up gravitational waves at frequencies lower than existing optical interferometers can detect. Such low-frequency waves are predicted to be radiated by the pas de deux of pairs of orbiting black holes or neutron stars, and perhaps by the cosmic ructions in the first fraction of a second after the big bang. In practice, though, there’s a big problem: although the waves associated with atoms >
jon krause
19 June 2010 | NewScientist | 39
”One end of the apparatus might fit in the lab – but it would have to be a lab with a 1-kilometre mineshaft beneath it”
move agreeably slowly, they are also flouncily delicate beasts and cannot travel more than a metre or so before losing the quantum character that allows them to interfere. That might well place a limit on the sensitivity that can realistically be achieved with atom interferometry. “It’s a sexy idea, but atoms at the moment are many orders of magnitude away from the optical systems,” says Craig Hogan of the Fermi National Accelerator Laboratory in Batavia, Illinois. All is not lost, however. In 2008, Savas Dimopoulos and his colleagues at Stanford University, California, proposed that a hybrid atom-optical interferometer could combine the advantages of both schemes. They suggest using pulsed laser beams from a single source to impart kicks to cold atoms in two interferometers separated by a large distance. These kicks will usually occur with clockwork precision, producing the same interference pattern in each interferometer. Should a passing gravitational wave disturb the beam on its way to one of the interferometers, however, that interference pattern will change (Physical Review D, vol 78, p 122002). It is a clever idea, ridding the apparatus
Atom nav Applying atom interferometry to measuring the effects of gravity is not new. In 1991, the year the technique took off, Mark Kasevich and Steven Chu of Stanford University in California cooled sodium atoms to near absolute zero, gave them an upward kick and compared the rate at which they fell back on two paths. That allowed their team to determine the acceleration due to gravity to within 3 parts in a million, a degree of precision that has since been improved 1000-fold. We now know that the rates at which, say, a glass paperweight and a caesium atom are pulled to earth are the same to within 7 parts in a billion (Nature, vol 400, p 849). In a paper published in February this year, Chu – now the US energy secretary – and
40 | NewScientist | 19 June 2010
his colleagues Holger Müller and Achim Peters show with the same precision that time flows faster for an object as the gravitational field it experiences weakens. This effect, predicted by general relativity and known as the gravitational red shift, is tiny: during the 0.16 seconds it takes for atoms to fly through two interferometer arms, one positioned a millimetre above the other, the atoms in the upper arm “age” more than those in the lower by 10-20 seconds. That is still enough of a difference to cause a measurable shift in the interference pattern (Nature, vol 463, p 926). It might sound esoteric, but that has practical implications. Precise measurements of Earth’s gravitational gradient are a handy tool for oil
prospectors and geologists, and since GPS satellites move in different orbits high above the Earth, an improvement of our knowledge of the gravitational red shift translates directly to sharper navigation on the ground. Indeed, so good are atom interferometers at sensing motion that a California-based company called AOSense is supplying them to the US Department of Defense for use in navigational devices. But Müller points out that while a fully fledged interferometer is a relatively small piece of kit by the standards of big physics, it will still be a challenge to incorporate into a helicopter or submarine. “It usually doesn’t fit through the lab door and needs a team of PhD level physicists to build and keep alive,” he says.
of the vibration-prone rigid mirrors that are a weak point of existing optical interferometers. But the resulting device is not much smaller than existing gravitational wave detectors: to allow enough space for a gravitational wave to stir things things appreciably, the two atom interferometers must be connected by a vertical laser arm about 1 kilometre long. While one end of the apparatus might fit in the lab, it would have to be a lab with a deep mineshaft beneath it.
Quantum probe Atom interferometry truly within the confines of the lab might come into its own in another context: the search for evidence of quantum gravity. Seekers of quantumgravitational truth suffer from a big problem: any observable effects are expected to surface only at lengths smaller than 10-35 metres, a scale known as the Planck length. vA fundamental relation in physics says that the tinier the scale you want to observe, the more energetic a probe you need. To observe phenomena at the Planck scale particles have to be a million billion times as energetic as the protons accelerated by the world’s most powerful particle smasher, CERN’s Large Hadron Collider. There might be a workaround. Most theories of quantum gravity predict that below the Planck scale, space is not a smooth, continuous fabric but a pixelated foam or a mash-up of microscopic extra dimensions. This might well have detectable effects. In the late 1990s, for example, theorists realised that photons of different energies emitted at the same time by distant cosmic explosions might arrive on Earth at different times, even though they all travel at the speed of light. In 2005, the MAGIC telescope saw evidence of just such an effect in a burst of gamma rays from a galaxy 500 million light years away (New Scientist, 15 August 2009, p 26). That remains an isolated observation, however, and the interpretation of such measurements is fraught with assumptions about the workings of extreme and distant astrophysical objects. Then late last year Giovanni Amelino-Camelia of the Sapienza University of Rome in Italy and his colleagues suggested that there might be a smarter way to go about things: cold-atom experiments could provide a way to tune into quantum space without looking beyond Earth. If a cold atom is struck by a laser with a specific frequency, it can absorb a photon and recoil slightly. Meanwhile, a second laser can
The idea is highly speculative, and testing it would require the accuracy of atom interferometry to be improved considerably in order to distinguish the shift from spurious effects, says Guglielmo Tino at the University of Florence, Italy. “But it addresses a very interesting possibility,” he says.
jon krause
Repulsive energy
be tuned to make the atom emit a photon. The researchers showed that in a quantum spacetime the amount of energy needed to drive this process would be slightly different from that needed in a smooth, classical one, and this would show up as an interference pattern when the two lasers are combined (Physical Review Letters, vol 103, p 171302). David Mattingly, a physicist at the University of California, Davis, says this could present a whole new line of attack on quantum gravity. The problem with astrophysical observations, he says, is that the theories that explain them only apply “to one or two particle species and to a certain high-energy sector”. Indeed, Amelino-Camelia reckons that overlooking low-energy phenomena in favour of dramatic cosmic processes could turn out to be the decisive factor in 80 years of failures in quantum gravity research. His is not a lone voice, either. In March this year, Charles Wang of the University of
Aberdeen, UK, and colleagues sketched out an atom interferometer experiment that they claim could reveal traces of quantum gravity – using not the interference patterns of a normal atom, but those of a quantum superatom. Their proposal is to load an interferometer with around 100 million rubidium atoms and then cool them close to absolute zero. At such temperatures, the atoms all crowd into the same low-energy quantum state, becoming an exotic gloop called a Bose-Einstein condensate (arxiv.org/ abs/1002.2962). This quantum monster brings into focus effects that are difficult to observe in a lighter object. Wang and colleagues suggest that just as random quantum fluctuations of the electromagnetic field are known to change the energy levels of a hydrogen atom – an effect called the Lamb shift – so the foaming fluctuations of space-time will produce tiny energy shifts in a Bose-Einstein superatom.
The ambition of that experiment, though, is nothing compared with a proposal made in January this year by Holger Müller of the University of California, Berkeley, and Martin Perl of the SLAC National Accelerator Laboratory in Stanford, California. Müller is an atom-interferometry guru, while Perl is a hard-headed experimentalist who won a Nobel prize in 1995 for his part in the discovery of the tau lepton, a heavier cousin of the electron. Perl thinks that atom interferometers could be the right tools to unearth the nature of dark energy, the source of the mysterious repulsive force causing the expansion of the universe to speed up. We know little about dark energy, and while further insights may come from large telescopes and space probes such as the Joint Dark Energy Mission, Perl thinks we might learn more by looking for tiny differences in the free fall of atoms in two interferometers positioned side by side, 1 metre apart (arxiv. org/abs/1001.4061). Not everyone is a convinced. “New technology often opens the door to new effects,” says Robert Caldwell of Dartmouth College in Hanover, New Hampshire. “But until there is a good theory that explains how one can detect dark energy in the lab, the best way to learn about it will continue to be through cosmological observations.” With big physics continually pushing the boundaries, it remains to be seen whether cold-atom experiments will live up to their new star billing. Within the next decade, for example, a trio of spacecraft collectively known as the Laser Interferometer Space Antenna, or LISA, might blast off, bound for a point 50 million kilometres from Earth. There, the craft will form themselves into a triangular array 5 million kilometres across to continue the hunt for gravitational waves. At a cost of well over a billion dollars, though, LISA won’t come cheap. If the money runs out on such grand schemes, we may be grateful for solutions closer to home. n Matthew Chalmers is a freelance writer based in Bristol, UK 19 June 2010 | NewScientist | 41