FANCY a quick dip in space-time? A physicist has shown it is possible to swim just by pushing against the curved fabric of space and time.
Basic laws of physics say that it is impossible to propel yourself forwards through space without ejecting part of your mass backwards. But Jack Wisdom at the Massachusetts Institute of Technology has demonstrated how to achieve the feat, simply by changing shape in the right way. He was inspired by species of bacteria that shape-shift to propel themselves through water.
On the scale of a bacterium, water is as viscous as treacle. This makes swimming difficult because a simple symmetrical stroke gets you nowhere: the recovery stroke pushes you as far back as the first part of the stroke pulled you forward. So these bacteria adopt different geometrical shapes during the first and second parts of the stroke to maximise the forward movement. Swimming robots and moving parts in nanomachines are already engineered in this way.
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Wisdom wondered whether shape-shifting might also allow a craft to push itself forward against space-time. 鈥淚 was hoping this would be a big enough effect to make propulsion devices,鈥� he told 快猫短视频.
He imagined a three-legged craft, poised in space above the Earth. It would swim by repeating a single stroke, more like the opening and closing of an umbrella than front crawl (see Graphic). Starting with all three legs pointing straight outwards, it would then shorten them and fold them down to point towards the Earth. The central body of the craft would react by moving slightly forwards. The craft would then extend its legs and swing them outwards, causing the body to move back again.
You might think this wouldn鈥檛 get the space bug anywhere, but that leaves out the effect of general relativity. According to Einstein鈥檚 theory, the gigantic mass of our planet stretches and deforms the space-time around it, like a bowling ball sinking into a rubber sheet. And this curvature is key to how the bug moves. The distortion means the short-legged closing action creates more movement forwards than the long-legged opening action does backwards. So each completed stroke leaves the bug a little ahead of where it started.
According to Wisdom鈥檚 calculations, to be published in Science, a metre-sized bug would swim just 10-23 metres with each stroke. So even if the craft performed one stroke per second, it would take more than 3000 billion years to move a millimetre. The movement would be faster in parts of space that are more curved, for example near a star or a black hole, but even so the approach doesn鈥檛 look like it will be of any use for space travel. 鈥淚t鈥檚 fun and whimsical physics,鈥� says Frank Wilczek, a theoretical physicist also at MIT who first drew attention to the mathematics of bacterial swimming.
Wisdom also developed his theory to show how a two-legged bug could swim over the surface of a frictionless sphere. And this does allow it to reach a reasonable speed.