Could astronauts take a leaf out of H. G. Wells's book The First Men in the Moon, and use spacecraft propelled by antigravity devices? Some see the idea as science fiction, but major space agencies take it seriously.
In 2001, the European Space Agency (ESA) commissioned two scientists to evaluate schemes for gravity control. They have concluded that, even if such control were possible, the benefits for lifting spacecraft out of the Earth's gravitational field would probably not be worth the effort1.
But scientists working on such propulsion schemes dispute the report. "I regard the conclusion, even if correct, as uninteresting and, frankly, irrelevant", says James Woodward of California State University at Fullerton, who has worked for NASA on gravity-control propulsion.
NASA ran a research programme on speculative propulsion methods, called Breakthrough Propulsion Physics, from 1996 until its funding was cut in 2003. The project's founder and former manager, Marc Millis of NASA's Glenn Research Center in Cleveland, Ohio, says that the ESA report corrects some misconceptions in the field of gravity control. But he thinks its scope is too limited to rule out future research in the area.
"The risk of this paper is that the casual reader will more broadly interpret the negative findings to apply to all inquiries into gravitational or inertial manipulation," says Millis.
The report is not meant to kill off all such ideas, says one of its authors, cosmologist Orfeu Bertolami of Lisbon's Technical University in Portugal. "Our recommendation to ESA was to keep a critical eye on them," he says. But, he adds, "this should be a low-intensity activity. Our estimates show that conventional ideas [for propulsion] are much more effective."
Wells's fantasy hinges on the invention of a substance that shields any object placed above it from the Earth's gravity. But can such a material really exist? Antigravity seems to violate the law of conservation of energy, which prohibits perpetual motion. Place a wheel half over such a gravity shield and the shielded segment will rise, causing the wheel to rotate forever without a power source.
What's more, gravity cannot be screened out in the same way as light or sound: Einstein's general theory of relativity explains that gravity results from the way mass distorts space-time itself.
But relativity is not the last word on the subject. "Gravity does not fit into the standard model of particle physics," says Clovis de Matos, technical officer in charge of the ESA study. "And we do not understand the gravitational interaction at the quantum level."
De Matos explains that ESA commissioned the survey of gravity control partly to establish whether a quantum theory of gravity might expose loopholes in our current understanding that space technology could exploit.
Bertolami and his co-author, Martin Tajmar of the space technology company ARC Seibersdorf in Austria, looked at proposals for assisting spacecraft launch by weakening gravity. They were not impressed. "None of the proposals seemed convincing and detailed enough," says Bertolami. "Experimentally and theoretically they do not seem to meet a standard we could qualify as scientific."
All the same, the researchers did feel that some ideas for modifying gravity are worth exploring. For example, as they are reaching the edge of the Solar System, NASA's Pioneer spacecraft are deviating from their expected trajectories. This has led some scientists to suggest that the current theory of gravity is incomplete.
There have also been suggestions that magnetic effects in materials whose behaviour is dominated by quantum effects, such as superconductors, might induce a kind of artificial gravity. NASA scientists have studied claims by Russian physicist Eugene Podkletnov that a spinning superconductor can act as a gravity shield, reducing the weight of an object placed above it by about 2%.
Independent scientists have been unable to reproduce this and similar claims, says Tajmar. He and Bertolami conclude that there are currently no good grounds for taking such effects seriously. All the same, they don't rule out the possibility of gravitational anomalies in quantum materials.
Other options involve the gravitational and inertial masses of objects. Gravitational mass determines the force of gravity experienced by the object; inertial mass determines how much force is needed to set it in motion. General relativity says that the two definitions are identical, but some theories of quantum gravity suggest that they differ.
Tajmar and Bertolami looked at schemes to alter one kind of mass, leaving the other unchanged. They found that reducing the inertial mass has no effect on the amount of fuel needed to launch a spacecraft. And altering the gravitational mass alone, by gravity shielding for example, doesn't help unless the shielding is almost total.