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Can Molecules Hang Glide on Gravity?

Proposed effect, to slow the fall of a rapidly vibrating molecule, faces skepticism
skydiver



© ISTOCKPHOTO/DRAZEN VUKELIC
In the movies, action heroes leap from planes sans parachutes or second thoughts, counting on the air to break their falls long enough to grab a soft landing off of parachuted foes. Now a team says that a properly designed molecule could do essentially the same thing by coasting on gravity. Other researchers, however, may need more convincing before they fall for the idea.

In Einstein's theory of general relativity, planets and stars bend, drag and otherwise contort the taffylike fabric of spacetime. In 2003 physicist Jack Wisdom of the Massachusetts Institute of Technology proposed that a tripod-shaped object could push off from this taffy, slowly swimming jellyfish-style through empty space by varying the lengths of its legs and the angle between them.

Motivated by the discovery, physicists Eduardo Guéron of the Federal University of ABC and Ricardo Mosna of the University of Campinas, both in Brazil, say they have stumbled onto something else lurking in Einstein's equations.

A vibrating, dumbbell-like pair of masses—for example, a molecule—could push off not from empty space but from a gravitational field such as that of Earth, they report in the April issue of the journal Physical Review D.

Compared with Wisdom's effect, "we only need one parameter that oscillates," Guéron says, namely, the distance between the two masses. He says that with each vibration, the dumbbell catches on the spacetime fabric and pushes itself up a tiny bit.

This may sound suspiciously like antigravity, but Guéron says the proposed gliding relies completely on general relativity.

The idea falls in the realm of gravitational tides, or variations in the strength of gravity at different distances, says physicist Clifford Will, an expert on general relativity experiments at Washington University in St. Louis. "You get some interesting tidal effects," he says, but the gliding concept still "looks a little fishy."

The trick, according to the team, is making sure that the oscillation is asymmetric, like an inchworm's crawl, with one motion taking longer than the other. Guéron says that a real object would have to vibrate one billion times per second to slow its fall by 1 percent. That will not help action heroes, but a molecule could do it if given the right asymmetry, he adds.

Will says the paper is intriguing enough that someone will check it in full detail, causing it to fall flat or coast safely.

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