It may seem to violate basic laws of motion, but a physicist has figured out a way to propel a spacecraft without expelling hot gas or anything else in the opposite direction.
At first glance, the idea of propulsion without propellants seems self-contradictory. Newton's third law of motion, which states that every action has to have an equal and opposite reaction, implies that something must compensate for the spacecraft's propulsion. Rockets do this by spewing gas, driving the machines forward; solar sails, by bouncing particles of light back in the direction opposite to the spacecraft's acceleration.
But as Jack Wisdom, a physicist at the Massachusetts Institute of Technology (MIT), explains in his paper, published online today by Science, there's another way to move about in space. Although it took some complicated math to prove it, the central idea is analogous to something you can do in an office swivel chair. By thrusting your arms in and out and moving them about, you can slowly turn the chair around. You can't make the chair pick up some spin, but you can change your orientation.
According to Wisdom, the same sort of idea can work when a body is sitting on a curved surface, like the four-dimensional surface of spacetime. You (or any other extended object) can stretch your limbs, move them, and retract them again, over and over and over, and slowly move forward. "It really is a swimminglike motion," says Wisdom. As in the swivel-chair experiment, where you can change orientation but not spin, the spacetime swimmer can change its position but not its overall velocity. Before and after each "stroke," the swimmer will be moving at the same speed.
"The whole thing is fascinating, actually," says Frank Wilczek, a physicist at MIT. "I don't know if it will really help us with any fundamental problem in physics, but it can pose questions that are almost philosophical--simple-seeming questions that have profound answers." For instance, he says, Wisdom's work raises questions about what it means to be "moving" against the pure fabric of spacetime with no stars and galaxies to help you judge your motion--a question philosophers of science have been debating for more than a century.