If an astronaut were suddenly adrift in the void of interstellar space, they would have to propel their bodies to safety, kicking their legs and swinging their legs to a sanctuary in the vacuum.
Unfortunately for them, physics is not so forgiving, leaving them to float without hope for eternity. If the universe were curved enough, their fluttering wouldn’t be so pointless.
Centuries before he left Earth, Isaac Newton succinctly explained why things move. Whether it’s the expulsion of a gas, a push against a solid, or a fin hitting a liquid, the speed of motion is conserved by the sum of the elements involved, resulting in a reaction that propels the object forward.
Take the air surrounding a bird’s wing, or the water around a fish’s tail, and the effort of each wing will push in the same way as it pulls in the other, and the poor animal will flap feebly with no net motion toward the target.
At the beginning of the 21st century physicists think a space for this rule. If the 3D space in which this motion occurs is curved, changes in the object’s shape or position will not necessarily follow the usual rules for how velocity changes, meaning it will not need a thruster.
The very geometry of warped spacetime can mean that an object’s deformation—a right-whack, a snap, or a flutter—may see a subtle net change in its position after all.
On the one hand, the idea that the curvature of spacetime affects motion is as obvious as watching a rock drop to the ground. Einstein covered this in his work over a century ago general relativity.
But showing how the rolling hills and valleys of distorted space can affect an object’s ability to propel itself is a whole other game.
To observe it in action without going to the nearest space black holeA team of researchers from Cornell University, Cornell University, University of Michigan and University of Notre Dame built a model of curved space in the laboratory.
Their mechanical version of spherical space consists of a set of masses driven by motors along arcing intersections of tracks. Attached to the rotating arm, the entire unit is positioned so that gravity and friction are minimized.
- The “space” swimmer moves along the path of the rotating boom arm. (Georgia Tech)
Although the masses are not cut off from the physics that govern our somewhat flat Universe, the system was balanced so that a bend in the rails would have the same effect as a significantly curved space. Or so the team predicted.
As the robot moves, the mixture of gravity, friction, and curvature is transformed into a motion with unique properties best explained by the geometry of the space.
“To study motion in curved space, we allowed our shape-shifting object to move in a simple curved space, a sphere” he says Georgia Tech physicist Zeb Rocklin.
“We found that the predicted effect, so counterintuitive, was actually rejected by some physicists: as the robot changed its shape, it moved around the sphere in a way that could not be attributed to interactions with its environment.”
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However small the impact, the theoretical use of these experimental results may help improve the deployment of technology where the curvature of the Universe is important. Even in gentle descents like the Earth’s own gravity, understanding how it moves can change ultra-precise positioning in the long run.
Of course, physicists went the zero-fuel route.impossible engines‘before. Small hypothetical forces in experiments have a way of coming and going, and there is no end to debate about the validity of the theories behind them.
Further studies with more precise mechanisms may reveal more understanding of the complex effects of drifting over the sharp edges of the Universe.
For now, we can only hope that the gentle slope of space surrounding our poor astronaut is enough to see him reach a safe haven before he runs out of oxygen.
This study was published PNAS.