The Physics of Time Travel

If Einstein and Rosen are the architects of the space-time shortcut, then Kip Thorne of Caltech is its structural engineer. Starting from the rough sketch that Einstein and Rosen left behind, Thorne created an algorithm that describes in strict mathematical terms the physics of a working time machine. Of course, actually building Thorne's time portal would require a technological prowess that is at least many centuries away. But his work proves that time travel is possible-at least in theory.



Thorne's problem was finding a way to hold open the wormhole's channel, or throat, long enough for an explorer to pass through. Ordinary matter won't do: No matter how strong it is, any scaffolding made of matter cannot brace against the crush of space-time. Thorne needed a substance that could counteract the squeeze of a black hole. Thorne needed antigravity.



Instead of contracting the space around it, as ordinary matter does, antigravity-or negative energy, as it is sometimes called-pushes it apart. In theory, antigravity would be placed inside a wormhole's throat, opening it wide enough for an astronaut, or possibly even a spaceship, to pass through.
Antigravity does the trick; the problem is finding it. Einstein first postulated the existence of antigravity on cosmic scales in 1915, a conjecture proven correct eight decades later. But Einstein's antigravity is wispy and dilute, a spoonful of sugar dissolved in the Pacific Ocean. Opening a wormhole requires a regular torrent of antigravity.




The best current candidate for creating concentrated antigravity is called the Casimir effect. Because of the quirks of quantum mechanics, two flat metal plates held a hair's width apart generate a small amount of negative energy. That energy, multiplied many times over, could in principle be used to create a traversable wormhole. The widening, meanwhile, would dilute the strength of nearby gravity, preventing the traveler from being torn apart.