IT LOOKS like a pile of rubbish. As I inspect the equipment on the floor of his office, Raymond Chiao laughs. âIâm not a very good experimentalist,â he says. We are looking at two paint cans, some bits of wood and a lot of polystyrene cups. This experimental detritus, languishing in a battered cardboard box, is the disassembled components of a âgravity radioâ.
If it werenât for his impressive track record in experimental physics, Iâd be tempted to believe Chiaoâs assessment of his skills. But this is the physicist who demonstrated that photons can tunnel through materials at speeds greater than the speed of light, a remarkable experimental feat that has had wide-ranging implications. So how come his latest attempt to create cutting-edge science has us laughing?
Perhaps it is the contrast between the experimentâs grand aims and its shabby appearance. Chiao, a professor at the University of California, Berkeley, was trying to construct a desktop device that could detect gravitational waves, ripples in the fabric of space-time whose existence is predicted by Einsteinâs general theory of relativity.
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Gravitational waves form whenever a mass accelerates â waving your fist in the air creates them, though on an unbelievably tiny scale. Even the gravitational waves from a cataclysmic event such as the collision of two black holes would be almost insignificant by the time they reached Earth. Three giant multimillion-dollar detectors in the US called LIGO, two in Hanford, Washington, and one in Livingston, Louisiana, are using sophisticated lasers to look for gravitational waves â and they havenât seen a thing so far. It is hard to see how Chiaoâs contraption, made on a shoestring budget out of his own pocket with parts ordered from high-school lab suppliers, could do better.
And thatâs not to mention Chiaoâs other goal: generating measurable gravitational waves. Every physicist will accept that a moving mass can generate ripples in space-time. But almost none will accept that you could ever create detectable gravitational waves in the laboratory. Especially, as Chiao is claiming, by using an electromagnetic field.
However, this strange experiment will shortly have a peer-reviewed paper behind it, to be published early next year, as part of a volume of research papers presented at a symposium held at Princeton in March last year. At the meeting, which celebrated the 90th birthday of Princeton University physicist John Wheeler, Chiao left most of the physics establishment reeling. Chiao told the assembly he had long been exploring the intersection of general relativity and quantum mechanics, keen to find experiments where they might be united. And he had now found one that might have enormous consequences.
It wasnât the first he had found: in 1982, Chiao published a paper suggesting that superfluids might be used to detect gravitational radiation (Physical Review B, vol 25, p 1655). At the time, the experiment was beyond anyoneâs capabilities. But at the Wheeler symposium Chiao announced to the assembled physicists that he was now planning to build a machine that should efficiently convert gravitational waves into electromagnetic waves â and vice versa. Its main working parts would be a small piece of superconductor and a microwave antenna, held within a closed metallic container. He had done all the relevant calculations and reckoned the whole thing would be no bigger than a small TV. It should be finished and working within a few months, he predicted.
Reputation on the line
It is hard to know whether the assembly was more astonished by the idea that this might be possible, or by Chiaoâs lack of concern for his own reputation. âIt would be sensational, if it were true,â Freeman Dyson, a theorist at the Institute for Advanced Study in Princeton, tactfully told The New York Times.
Paul Davies, a physicist at Macquarie University in Sydney, was also at the Wheeler symposium. Chiaoâs idea was âimaginativeâ, he told me shortly after the meeting. He couldnât see any flaw, but then he admitted this was not his area of expertise. Davies was willing to divulge a physicistâs hunch, though. âItâs too good to be true.â
A couple of days later, when I finally got through to Chiao on the telephone, he endorsed Daviesâ assessment. âUntil I do the experiment Iâm not going to believe it â itâs too good to be true,â he said. His scepticism wasnât based on any fundamental theoretical problem, however, just â like Davies â a hunch. He couldnât find any mistake in his calculations, and neither could anyone else.
Indeed, Chiao even suggests that his claim shouldnât be so shocking. âThis is new to most people, but itâs old in the sense that Michael Faraday started work on this.â In 1850, Faraday published âExperimental Researches in Electricityâ, a compendium of papers that detailed some experiments on gravity and electromagnetic effects. âHe tried to see, in an extension of his experiments in induction, if he could find any coupling mechanism between gravity and electromagnetism.â
Itâs not difficult to see why. An electron has both mass and charge. Though they are both tiny, when the electron reacts to gravity by falling towards Earth, its moving electrical charge creates a magnetic field. Conversely, accelerate an electron using an electromagnetic field, and youâre also moving the tiny gravitational field associated with its mass.
Faraday never got very far in quantifying the link between gravity and electromagnetism, though, and neither did University of London physicist Patrick Blackett, who carried on Faradayâs work in the 1950s. But then, in 1961, Robert Forward, a researcher at the Hughes aircraft company in Malibu, published a paper detailing âMaxwell equation-like general relativistic effectsâ, including âgravitomagnetismâ.
Five years later Bryce DeWitt of the University of North Carolina in Chapel Hill explored the theoretical side of gravitomagnetism in superconductors. His calculations showed that gravitational effects should affect a superconductorâs properties (Physical Review Letters, vol 16, p 1092). And this paper formed the starting point for Chiaoâs extraordinary claims.
Since superconductivity is a quantum phenomenon, Chiao thought that DeWittâs exploration of the effects of a gravitational field on a superconductor might prove fruitful. He was rewarded almost immediately.
When Chiao took DeWittâs equation, and expanded out all its terms, something odd popped out. Chiao credits his graduate student, Daniel Solli, with spotting it: a term in the equation where the effect of an electromagnetic field is directly linked to the effect of any gravitational field in the superconductor.
That means there might indeed exist a direct coupling between electromagnetic and gravitational radiation in a superconductor. Within the material, each packet of electromagnetic radiation, or photon, could be converted into a packet of gravitational radiation. This coupling is linear and reciprocal: the superconductor can convert power from one form of radiation into the other with equal efficiency. As Chiao examined the equations, he reasoned that the efficiency could in principle be 100 per cent.
âIt came as a total surprise,â he says. âI was working away on this and it fell out of my calculations. Weâre not talking about tiny effects, weâre talking about potentially really huge effects.â
So huge, in fact, that at the Wheeler symposium Chiao talked about detecting the particles of gravity: gravitons â something no one has ever managed to do. He spoke about a piece of laboratory equipment so sensitive that it could detect the primordial gravitational waves that were wrenched into existence at the beginning of time.
LIGO would never be sensitive enough to spot these tiny waves. Indeed, seeing these gravitational waves has been such a remote possibility that no one has even worked out exactly what they should look like. Some of the younger cosmologists at the symposium couldnât wait to get started. âThey were very, very excited and said âplease donât do the experiment until we do the calculationâ,â Chiao says. âIt is a very exciting possibility.â
But while the younger folks were getting excited, the older researchers at the symposium â including Chiao himself, he is at pains to point out â were all rather sceptical. And sometimes, it seems, experience pays. It took Chiao a few months to build the experiment with the help of Walter Fitelson of the Space Sciences Laboratory of Berkeley. The superconducting antenna, housed in a paint can to protect the equipment from the effects of any stray electromagnetic fields, was set up to beam gravity waves to the antenna in the other can. They saw nothing.
Quantum lifelines
Initially, Chiao thought the problem might lie in the type of superconductor he was using. For experimental ease, Chiaoâs gravity detector used a ceramic âYBCOâ superconductor, which only needs cooling to liquid nitrogen temperature (77 kelvin), to become superconducting. The YBCO disks were held inside a polystyrene cup that contained liquid nitrogen. The whole experimental assembly, which included a microwave antenna a couple of centimetres across, was housed in the closed paint can to stop electromagnetic fields leaking out or in.
But there are subtle differences between the superconductivity of âhigh- temperatureâ ceramic superconductors and that of âlow-temperatureâ superconductors, such as niobium. Chiao thought that if he could get access to a helium cryostat to perform similar experiments with low temperature superconductors, he might see the effects he was looking for. But here he hit an unexpected problem: no one with the equipment he needed wanted to work with him. People were already turning their backs on what looked uncomfortably close to crank science. Chiao also had his paper on the gravitational antenna idea rejected from a peer-reviewed journal â without an explanation. He was beginning to find the whole experience rather wearisome.
And then, with Chiao already at a low ebb, a physicist refereeing his paper for the Wheeler symposium proceedings spotted the elusive flaw in Chiaoâs thinking. âDeWittâs work was correct, but has a hidden assumption,â Chiao says. The coupling between electromagnetism and gravity DeWitt had spotted dealt with just one component of the gravitational field, called the Lense-Thirring field.
Chiao had failed to appreciate that the strength of this field decays away extremely quickly with distance. Once you get far from the source, there is no detectable Lense-Thirring field, even for a huge mass. It would be like standing in New York and hoping to detect the short-range forces holding a helium nucleus together in London.
Just as the weak force doesnât affect anything beyond the atomâs nucleus, the Lense-Thirring field doesnât travel â there was no hope of detecting this kind of gravitational radiation via superconductors. It looked as if Chiao, like everyone who had been down this path before him, was doomed to fail.
And then he found a lifeline; in fact he found two. Firstly, with the help of Berkeley colleague Achilles Speliotopoulos, an expert on general relativity, Chiao found a component of gravitational radiation, the âWeyl tensorâ, that does propagate over large distances. Chiao and Speliotopoulos have now re-analysed the interaction of matter with gravitational radiation, and had their ideas accepted for publication in Physical Review D.
The other lifeline came when Chiao realised that there was another possible direct link between relativity and quantum mechanics: the electronâs quantum mechanical spin. This provides a kind of handle by which space-time ripples can grab hold of the electron and twist it around. This effect is analogous to a well-documented â but very strange â phenomenon in physics, called the âgeometric phaseâ, also known as âBerryâs phaseâ, after its discoverer, Michael Berry of the University of Bristol.
When a spinning particle travels around a loop in a particular way, its spin gains a small but significant amount of phase, or an extra amount of rotation, on completion of the loop â as if it has had the chance to rotate slightly more than should be allowed, due to the geometry of the path it has travelled through space. Essentially, the geometry of the loop affects the particleâs rotation. This additional geometric phase has been measured in many different systems, from turbulent fluids to interference effects between spinning particles such as electrons.
Because gravitational waves scrunch and stretch space-time, their passage has exactly the same effect on an electron: electrons hit by a gravity wave gain some extra phase in their spin because the wave can grab hold of the spin and crank it around periodically. The resulting âgeometricâ phase shift changes the properties of the material in which the particle sits. If the electron is part of a conductor, for instance, the shift might manifest itself as a tiny induced current in the conductor. And so, Chiao realised, in the right material, a gravitational wave could produce a measurable effect.
But what is the right material? Chiao believes it certainly has to be a macroscopic quantum system; quantum mechanics is the breakthrough that Faraday lacked, Chiao says. âFaraday didnât have any inkling of quantum mechanics, but it plays an essential role here,â he says.
Thatâs because the effects of gravitational waves are best measured through a âquantum fluidâ. Superconductors are one example of a quantum fluid: when cooled sufficiently, the electrons in a superconductor move into a strange quantum state â a Bose Einstein condensate â where they behave as if they were one gigantic quantum object.
The other important property of a quantum fluid is that it takes a large amount of energy to move it into any of its higher energy states. The result of these two properties is that quantum fluids display a kind of rigidity. A gravitational wave hitting part of the fluid will affect all of it at once, but the energy of the wave wonât be enough to move the quantum fluid out of its ground state. That energy will have to go somewhere, though. âThe system has no choice but to convert the energy into something else,â Chiao says. That something else, he says, is electromagnetic radiation.
It works the other way, too, Chiao believes. âIf gravity can affect the spin, why canât the spin affect gravity?â In other words, Chiao believes an antenna generating the right kind of electromagnetic field should produce waves in a gravitational field. So it should be possible, in a manner reminiscent of Hertzâs experiments over a century ago, to build a âgravity radioâ that can convert electromagnetic waves to gravitational waves and relay them to a microwave detector, where they could be converted back into electromagnetic waves. Thereâs no accepted way to work out what the conversion efficiency will be, however. âWeâre dealing with really unexplored territory. Experiment is the best way to decide,â he says.
And, Chiao says, the disassembled gravity transmitter and receiver in the cardboard box have provided the first measure of an upper limit on the conversion efficiency. Although he didnât design the apparatus with his new theoretical ideas in mind, he has worked out what its failure to register any gravitational waves means. It means that, for YBCO superconductor, the conversion efficiency between gravitational and electromagnetic radiation is less than 16 parts in a million.
On the face of it, thatâs not good news. It is a lot less than the 100 per cent efficiency he was talking about a year or so ago. Is this the best a gravity radio can manage? Are we doomed never to see the link between quantum and gravitational effects?
Not necessarily. There are other quantum fluids besides superconductors, and Chiao believes they might couple to gravity even more strongly. One possibility, he says, is the âquantum Hall fluidâ. This is found in an extremely thin sheet of metal or semiconductor subjected to a large magnetic field and cooled until the electrons it contains fall into their lowest energy state. The properties of the quantum fluid of electrons cause the resistance of the material to change in discrete steps as the magnetic field is ramped up or down. Whatever the material, the size of those steps is determined by two just constants of physics: Planckâs constant and the electron charge.
This quantum fluid has a much larger intrinsic quantum spin than the quantum fluid consisting of an YBCO superconductor. And so it might reveal a gravitational wave as a tiny but measurable electromagnetic field.
But letâs face it, thatâs a great big âmightâ. Itâs an intriguing, alluring possibility, but is it still too good to be true? It may be a while before we find out. Chiao doesnât believe he is the man to make this machine work. Quantum Hall fluids are beyond his experimental capabilities, he says.
And the question of generating measurable gravitational waves is even more open. Many gravity researchers have told Chiao that, while they accept a that gravitational wave affects a spinning particle, they think the âquantum back actionâ of the spin on the gravity field will be too weak to measure. But Chiao insists they are wrong. âIf you believe in quantum mechanics and look at the coupling, itâs reciprocal,â he says. Nobody says a gravitational field will have a negligible effect on a spinning electron, so why should it be any different the other way round? His papers, he hopes, will trigger experiments that might shed more light on the matter. âIâm throwing it out to the community now to have it explored in detail,â he says.
Chiao is aware that some of his colleagues will say he has embarked on a wild goose chase, refuse to follow, and counsel others to steer clear. But this experiment is worth doing, he insists. âIt raises some really fundamental issues: there are tensions between general relativity and quantum mechanics that really should be looked at â and looked at experimentally.â And is he worried about the effect that the past 18 months have had on his reputation? No; the issues involved are too important to ignore, he believes. âI donât regret it at all,â he says. âIâm really happy I had this opportunity to open the discussion.â
So there you have it: what Faraday started, no one has yet managed to finish. More than a century on from Faradayâs original efforts, Chiao is sure of only one thing about the link between electromagnetism and gravity. âThere is something there,â he says. âThere is something there.â