- NewScientist.com Physics & Math Article
- © Copyright Reed Business Information Ltd.
11 November 2006
- [[Stuart Clark]]
Gravity has a secret side. As well as the brute force that holds us to the ground, large masses should also exert a subtle swirling influence when they rotate, a force called gravitomagnetism. It's so faint that a NASA spacecraft called Gravity Probe B has been orbiting the Earth for over two years to accrue enough evidence to have a chance of confirming this force.
Yet in a lab in Austria, Martin Tajmar and his team have already succeeded in detecting a faint signal that seems to be due to this elusive component of gravity. A reason for celebration? Not quite. Puzzlingly, the force they seem to have generated is vastly more powerful than anyone else expected.
Despite its name, gravitomagnetism has nothing to do with magnetic fields as we think of them. According to Einstein's general theory of relativity, a rotating mass such as a planet should twist the fabric of space-time, and any object nearby should be dragged around by the vortex. It is really just another case of matter telling space-time how to curve and space-time telling matter how to move. Just as a stationary mass creates a "dip" in space-time that we perceive as gravity, a rotating mass creates a twist in space-time.
This gravitomagnetism is a feeble phenomenon: an object orbiting close to the Earth should be shifted just a few nanometres per year. In contrast, the gravitomagnetic force Tajmar's team have seen is trillions of times stronger, which is why they are treating the results so cautiously. What's more, their force is only generated by a spinning superconductor, not any other kind of matter. "We cannot find a mechanism to explain this in either general relativity or quantum mechanics," says Clovis de Matos, who works at the European Space Agency in Paris and helped establish the theory behind the experiment.
Their startling measurement might point towards a new quantum theory of gravity. It might even herald a futuristic technology that could be used to pull, push or levitate any object, regardless of its composition, electrical charge or shape. With so much at stake, it's no wonder Tajmar and his collaborators are treading carefully. "We tried everything we could think of to make this reading go away," says Tajmar. And yet after three years and more than 250 experimental runs at the Austrian Research Centers facility in Seibersdorf, near Vienna, the gravitomagnetic signal remains.
By sheer coincidence, their experiment was originally designed to investigate an old mystery about the innards of Gravity Probe B. NASA launched this spacecraft on 20 April 2004 to directly measure the effects of this long-sought-after component of gravity. The spacecraft finished collecting data in August 2005, and the science team is on course to announce its results in April next year.
Gravitomagnetic fields affect spinning objects more strongly than non-spinning objects, so Gravity Probe B's detector is based around four gyroscopes. The Earth's gravitomagnetism should tilt them by 11 millionths of a degree per year. To register this minuscule shift, the gyroscopes must run as smoothly as possible, and each one contains a rotating quartz sphere so perfectly crafted that if it were blown up to the size of the Earth, the tallest mountain would be less than 3 metres high. At the size of the gyroscopes, about the diameter of a ping-pong ball, that's an accuracy of just 40 atoms' thickness.
Having made something so flawless, the Gravity Probe B team realised that they had painted themselves into a corner. "How do you measure a spinning, perfectly uniform sphere that has no marks on it?" asks the spacecraft's principal investigator, Francis Everitt of Stanford University in California.
The trick was to coat each quartz sphere in a layer of niobium. When cooled to the point where it superconducts, the niobium generates a magnetic field as it spins, whose axis is exactly the same as the sphere's axis of rotation. The team then adapted sensitive magnetometers called SQUIDs (superconducting quantum interference devices) to measure the axis of this field and so track the motion of the sphere. The result is a gyroscope 30 million times more accurate than any previously constructed.
In the mid-1980s, Blas Cabrera, also of Stanford University, saw Everitt's work on these gyroscopes and realised that they offered a way to test the theory of superconductors proposed in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer, known as BCS theory. It says that when the temperature of the material falls below the critical temperature for superconductivity, pairs of electrons overcome their normal repulsion and join into bound systems known as Cooper pairs. Cabrera realised that since the gyroscope's magnetic field is due to the motion of electrons inside the superconductor, the field could reveal whether those electrons were indeed pairing up.
Janet Tate, now of the University of Oregon in Eugene, ran the experiments. She took one of the gyroscopes and spun it at different speeds to measure the resulting field produced by an accelerating superconductor. That's when the trouble started. Tate found that the magnetic field she measured was stronger than BCS theory predicted.
As the anomaly didn't affect the performance of the gyroscopes, finding the cause wasn't essential to the workings of the NASA spacecraft. So, after an initial flurry of interest by physicists, the problem was quietly dropped. "The measurement has remained unexplained for the last 20 years," says Tate.
Enter de Matos and Tajmar. Intrigued by this puzzle, they began to dig around the theory of superconductors looking for clues. They found one in a 1997 paper by John Argyris and Corneliu Ciubotariu of the University of Stuttgart in Germany. Argyris proposed that the hypothesised gravity particle, the graviton, might have mass, rather than being massless as traditional theories of quantum gravity had assumed.
Argyris's idea piqued de Matos and Tajmar's interest because of the parallel with the normally massless photon, which inside a superconductor develops a mass when the temperature drops below the critical temperature and the substance becomes superconducting. Tajmar and de Matos wondered what would happen if the gravitons inside a superconductor behaved like photons and gained mass as well.
Their calculations showed that the more massive the graviton becomes in a superconductor, the stronger the gravitomagnetic field becomes when the material's rotation speeds up. In turn, that should increase the magnetic field by altering the movement of the Cooper pairs. Could that explain Tate's measurement? To fit her findings, de Matos and Tajmar found they had to set the graviton mass to be 10-54 kilograms (Physica C, vol 432, p 167). By comparison, an electron's mass is about 10-30 kilograms. Although that makes the graviton sound like a lightweight, it would give superconductors a gravitomagnetic force 17 orders of magnitude greater than that produced by normal matter.
At that level, they realised, it should be possible to measure the field in a laboratory. So they designed an experiment to test the idea, and built it with funding from the US air force and the European Space Agency. Last year Tajmar's team began to look for evidence of their extraordinary prediction - not really expecting to find it. They set a ring of superconducting niobium spinning, and positioned accelerometers around the ring. Any gravitomagnetic field produced by the spinning superconductor should tug on these sensors.
Initially, they ran tests at room temperature, where niobium is not superconducting, and saw no anomalous readings. That was expected, consistent with the immeasurably tiny field predicted by general relativity. Then as they dropped the temperature, Cooper pairs formed in the niobium and it lost its electrical resistance. Suddenly the accelerometers produced a signal. It was exactly as they hoped: as soon as the niobium became superconducting, the instruments appeared to feel a strong gravitomagnetic field pulling on them (www.arxiv.org/abs/gr-qc/0603033).
It seemed too good to be true. Tajmar's team knew how heretical such a large gravitomagnetic field would seem to other physicists (see "The attraction of gravity"). So they began running their experiment time and time again, looking for any hint of instrumental problems that might be fooling them. Next, they swapped the niobium for other superconducting materials, making predictions about the gravitomagnetic field they expected from each. They included extra sensors to improve the accuracy of their results and added two laser gyroscopes to their set-up to best measure the twist (www.arxiv.org/abs/gr-qc/0610015). Every time, the experiment gave them the right answers.
After 250 runs, they began to believe that perhaps the signals were real after all. It seemed they had found a way to generate a large gravitomagnetic field unanticipated by Einstein or anyone else. They have submitted a paper to the journal Physica C and have been attending conferences to talk about their work - and met a sceptical response.
James Overduin, a theorist from Stanford University is doubtful about the claims. He points to the remarkable strength of the supposed gravitomagnetic field. "Seventeen orders of magnitude is not to be sniffed at." At that strength, says Overduin, we would expect to see gravitomagnetic effects throughout the cosmos. To make the graviton massive would limit the distance it can travel, and since all astronomical observations suggest that gravity travels the entire breadth of the universe, there is a big conflict to resolve.
De Matos counters that the gravitons only gain mass and enhance the gravitomagnetic effect inside superconductors, which in the universe would only occur in certain highly compressed dead stars called neutron stars. "Some models suggest that neutron stars have a superconducting layer inside them. This would lead to enhanced gravitomagnetism, but at the moment the observational effects are not clear because no one has yet done the calculation," he says.
More fundamentally, Overduin points out that introducing massive gravitons into physics could cause more problems than it solves. "A massive graviton would mean that you had to rewrite the entire standard model of particle physics," he says.
Tajmar agrees that it is no trivial thing to do. He points out that other theorists have proposed that massive gravitons could explain why the expansion of the universe is accelerating. If confirmed, their discovery would fundamentally change the way we think about gravity. It would mean that superconductors generate gravitational effects differently from normal matter, which would in turn be an unambiguous pointer towards some quantum theory of gravity, because until now only an object's total mass has been assumed to determine its gravitational field. If Tajmar and his collaborators are right, the arrangement of particles inside a superconductor also matters.
Such a departure from mainstream theory does not impress Overduin. "A massive graviton gives you huge problems. I wouldn't bet on this work as a breakthrough," he says.
The best hope for Tajmar and de Matos is that another team will reproduce their experiment and confirm the anomalous gravitomagnetic signal. According to Tajmar, several teams have pledged to recreate the experiments to refute or verify the puzzling signals, but he won't reveal the identities of these teams for fear of putting them under undue pressure. "I am very happy with their interest. It shows that others take us seriously and are willing to spend time on this."
The results could be out in a year or so. If they are positive, it puts the technology of science fiction on the horizon. Levitating cars, zero-g playgrounds, tractor beams to pull objects towards you, glassless windows that use repulsive fields to prevent things passing through. Let your imagination run riot: a gravitomagnetic device that works by changing the acceleration and orientation of a superconductor would be the basis for a general-purpose force field.
The suggestion that gravitomagnetism might one day form the basis of some new technology evokes a quick reaction from Everitt: "Absolutely, unquestionably no!" Then, after a pause, he adds, "But I suppose Simon Newcomb was just as certain in 1900 when he said that humans would never build a heavier-than-air flying machine."
- The attraction of gravity
Any talk of superconductors producing weird gravitational effects makes physicists uneasy. A decade ago, Russian scientist Eugene Podkletnov of Tampere University of Technology in Finland claimed that a rotating superconductor would partially shield objects from the Earth's gravitational pull. Before his results were published, the story of the "anti-gravity device" leaked to the press. In the ensuing melee, Podkletnov withdrew the paper and returned to Russia.
Other researchers have also run aground after being drawn by the siren song of superconductors and gravity. In 1989, [Huei Peng]] of the Institute of Applied Mathematics in Beijing and Douglas Torr of the University of Alabama in Huntsville published a paper claiming that gravitational waves in the fabric of space-time should affect superconductors. This could lead to a new kind of laboratory-based gravitational wave detector, they said. Raymond Chiao of the University of California, Merced, has also claimed that such a "gravity radio" is possible.
No one has succeeded in realising these predictions. "The enthusiasm for an antigravity device is so great that sometimes people see what they want to see. You have to exercise a lot of caution," says James Overduin, a theorist from Stanford University in California.
So in claiming that superconductors have a powerful effect on gravity, Martin Tajmar of the Austrian Research Centers near Vienna and Clovis de Matos of the European Space Agency in Paris have entered a scientific minefield. However, they point out that their effect is completely unrelated to all the earlier ideas.
Gravity might already have given scientists a hint of its twisted side by playing with the orbits of two space probes. The LAGEOS I and II satellites were designed and launched in the 1980s by NASA and the Italian Space Agency to map the Earth's gravitational field in detail, simply by orbiting the planet and being closely tracked by laser ranging from the ground.
In 1998, after a painstaking analysis of 11 years of data, the tracking team found that each satellite's highly tilted orbit was moving around in the direction of Earth's rotation by about 2 metres per year.
Almost all of that could be accounted for by undulations in Earth's gravitational field, caused by the uneven distribution of oceans and mountain ranges. However, after calculating that effect and subtracting it from the measured movement, there was a little left over. The extra shift was microscopic, no more than a few nanometres a year, but it agrees to within 10 per cent of what is predicted by general relativity. It seems to be evidence that the rotating bulk of the Earth slowly drags space-time around with it.
The result remain controversial, however, with some scientists questioning whether the shift from the oceans and mountains can really be calculated so precisely.
From issue 2577 of New Scientist magazine, 11 November 2006, page 36-39
- URL: © Copyright Reed Business Information Ltd.
- © Copyright Reed Business Information Ltd.
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