Wormholes make for great science fiction because they get us around the speed-of-light conundrum. Taking a shortcut through spacetime, they connect one part of the universe to another, though where and when you would come out if you went in a wormhole would be an interesting experiment, and not one for the faint of heart. But do we have any evidence that wormholes exist, and if they did, what could we look for that might reveal their presence?
Perhaps it’s time to revisit a fascinating 1994 paper called “Natural Wormholes as Gravitational Lenses.” The authors are a compendium of names known to anyone with an interest in the physics of interstellar flight or its depiction in science fiction: John G. Cramer (whose columns in Analog set high standards for science writing); Geoffrey A. Landis (Mars Crossing and innumerable short stories); Gregory Benford (whose bibliography of novels is too long to list); Robert Forward (the leading proponent of interstellar studies) and two other physicists whose work deserves a wider audience: Michael Morris and Matt Visser.
It was Visser (Washington University, St. Louis) who suggested a possible configuration for a wormhole that frames it with ‘struts’ of exotic material, the struts having a negative mass density that could result in an interesting object indeed, what the paper describes as ‘…a flat-space wormhole mouth framed by a single continuous loop of exotic cosmic string.’
Geoffrey Landis calls cosmic strings ‘flaws in geometry,’ but you can also think of them as flaws in the structure of spacetime itself. They’re quite useful in imagining wormholes because to preserve a primordial wormhole formed at the beginning of the universe, you need to wrap it in negative energy, and a negative mass cosmic string could do the trick. There are plenty of conditions here, but Landis put it this way in an interview I did with him back in 2003: “If one of these hypothetical negative mass cosmic strings got wrapped around a hypothetical primordial wormhole, you could have a hypothetical stable primordial wormhole, one that could still exist.”
Detecting such an object becomes a fascinating exercise in itself. We know how to look for the signature of gravitational lensing, as in imagery of a distant galaxy that has been shaped by the gravitational influence of an intervening galaxy. A wormhole should show a negative mass signature that, instead of focusing light, does the opposite. The signature of that ‘defocusing’ is characteristic. Landis again:
“If the wormhole is exactly between you and another star, it would defocus the light, so it’s dim and splays out in all directions. But when the wormhole moves and it’s nearer but not in front of the star, then you would see a spike of light. So if the wormhole moves between you and another star and then moves away, you would see two spikes of light with a dip in the middle.”
As theoretical as all this sounds, it’s actually quite useful. As the authors of the wormhole paper put it: “…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.” It makes sense, then, given the interest among astronomers in observing normal gravitational lensing with positive mass objects, to keep open the possibility of finding such a signature, which would provide our first solid evidence of wormholes.
And it also corresponds with how science works today. Much of the analysis of the voluminous data collected by our instruments is performed by computer software. And the value of a paper like this one, beyond its purely scientific interest, is that it identifies a pattern that such software should be written to recognize among all the other patterns that clue researchers in to interesting findings. It would be absurd to have solid evidence of a wormhole in data that was never analyzed in precisely the right direction.
As for that wormhole itself, an ancient one from the earliest days of the cosmos wouldn’t be of much use from a transportation perspective — you have to get to it first, after all, and it might be millions of light years away. And then there’s that problem of figuring out where it comes out on the other side. But it may be that a Kardashev Type II civilization, able to use all the energies of a star, could create artificial wormholes using negative energy, and if that is the case, the universe may have shortcuts galore through the Einstein barrier.
The paper is John Cramer, Robert L. Forward, Gregory Benford et al., “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124–27, also available on the arXiv site (and thanks to Gregory Benford for forwarding a copy). Be sure to read Robert Forward’s novel Timemaster for his fictional take on negative energy and many other ideas from the outer limits of science. In addition to being a fascinating speculative romp, it’s a rich and funny book.