that would levitate from grocer’s scales? Are there negative mass boxes of chocolate-covered cherries that would actually remove poundage with each serving? After enough bites, could we float like Mary Poppins? Despite numerous experiments, scientists have yet to detect particles with negative mass. Even positrons, the oppositely charged antimatter counterparts to electrons, have positive mass. Experiments at the Stanford Linear Accelerator have confirmed that positrons indeed fall down, not up.

Curiously, the laws of gravitational physics—whether expressed in Newtonian or general relativistic form—don’t explicitly rule out the existence of negative mass. Therefore, following the dictum (attributed to physicist Murray Gell-Mann) “Whatever isn’t forbidden is compulsory,” surely it must lie somewhere. British astronomer Hermann Bondi once speculated that every positive mass particle could possess a negative mass companion, just as magnetic north poles must waltz with south poles. Then where are these sub-weightless creatures hiding? Could they be huddled in some remote corner of space—banished to the universe’s Siberia through sheer gravitational repulsion? Or could they reside closer to Earth, albeit in some dim attic of possibilities we have yet to explore?

As it turns out, you wouldn’t need all that much exotic matter to prop open a wormhole. In 2003, Visser and two colleagues calculated that the spatial vacuum—the fuzzy realm of fluctuating quantum fields where uncertainty reigns supreme—could well provide such material. As modern quantum theory has shown, no vacuum is truly empty. The Heisenberg uncertainty principle, a key element of quantum physics, permits particles to materialize from sheer nothingness, as long as they remain only for brief intervals. Conceivably, through this process, tiny amounts of negative mass could randomly emerge from the void. Normally, these bits of flotsam and jetsam would return to the great emptiness, but perhaps they could somehow be captured first. If just a smattering could be netted, Visser’s team showed that it would suffice to keep a