By Peter Weiss
Just as a bicyclist can summon gravity’s help by turning onto a descending street, it might be possible to use the topography of space itself for a propulsion assist, albeit a tiny one, says a relativity theorist.
The scheme starts with Einstein’s general theory of relativity, which says that space-time curves near massive objects (SN: 12/21&28/02, p. 394: Getting Warped). In such a universe, certain idealized machines, such as one made of balls attached to long, adjustable struts, can propel themselves through curved space-time by shifting the relative positions of their parts, suggests Jack Wisdom of the Massachusetts Institute of Technology in the March 21 Science.
By means of mathematical and physical analyses, Wisdom argues that such machines can “swim” through space-time without thrust from an engine or other external forces. In effect, the contortions of such machines and probably more complex bodies would be like the acrobatics of a cat held feet up and then dropped to the ground, Wisdom says. The cat reorients itself and lands on its feet.
Wherever space-time isn’t curved–as is effectively true in the weak gravitational field of Earth–such rotations only produce changes in an object’s orientation. However, in strongly curved space, the same gyrations cause a simultaneous shift in location, Wisdom shows. For example, a contorting cat might actually move sideways even if it weren’t falling. Unfortunately, it would take forever to move anywhere.
“When I started this, I hoped it would be a practical way of moving around,” Wisdom confesses. However, a swimmer would travel only as far as the diameter of a proton after even 100 million strokes, he calculates. Nonetheless, Wisdom’s proposal might point to new ways of testing general relativity.
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