For decades now, black holes have been the rock stars of popular astrophysics, both fact and fiction. Physicists rely on them to explain all sorts of mysterious astrophenomena, and black holes have been essential plot devices in various films, from Star Trek (2009) to Galaxy Quest (1999) to (obviously) The Black Hole (1979).
But black holes are not the only famous members of the hole family. Their distorted spacetime cousins, wormholes, are also offspring of Einstein’s general theory of relativity. Black holes are regions of extreme gravity, crushing anything they swallow into subatomic smithereens. Wormholes are “bridges” connecting one region of spacetime to another. Black holes are the spacetime analog of bottomless pits; wormholes would be intergalactic (or interuniverse) superhighways. If they exist, wormholes could be the key to establishing theories that the universe occupied by humans is just one cosmic bubble in a vast multiverse. Wormholes could connect different times and places like Twitter connects tweeters.
The possibility of such spacetime tunnels was noticed shortly after Einstein published his general theory in 1916. Einstein and Nathan Rosen worked out the math for wormholes more thoroughly in a 1935 paper, so they are known technically as Einstein-Rosen bridges.
Unlike black holes, inhabitants of galaxies everywhere, wormholes are common only in fiction. You can see them in action in movies — Green Lantern (2011), Bill & Ted’s Excellent Adventure (1989), Carl Sagan’s Contact (1997) — but real space has not yet yielded any sightings of wormholes.
“Although just as good a prediction of Einstein’s theory as black holes, they have so far eluded detection,” Farook Rahaman of Jadavpur University in Kolkata, India, and collaborators write.
But maybe that’s because nobody has properly looked yet.
For decades, Einstein and Rosen’s paper had discouraged would-be wormhole travelers by pointing out that as soon as anybody entered a wormhole, it would collapse. Wormhole interest was revived in the 1980s, though, when Carl Sagan went fishing for a way that characters in his novel Contact could travel quickly through space. He asked the physicist Kip Thorne for advice and Thorne figured out a way to keep wormholes propped open. All you need is some exotic form of matter possessing negative energy.
In fact, it is possible to construct devices creating negative energy (by exploiting a trick called the Casimir effect), but only on tiny scales. Whether nature provides negative-energy material to prop open big wormholes in space remains a matter for Saganesque speculation.
Lately, though, some physicists have suggested that wormholes may actually populate the cosmos but just haven’t been properly identified. Rahaman and colleagues, for instance, say that evidence for invisible “dark matter” in the outer regions of galaxies could actually be evidence for wormholes.
“Seen from the Earth, we would not be able to distinguish the gravitational nature of a wormhole from that of a compact mass in the galaxy,” they write in a paper posted online at arXiv.org.
Their calculations show that the distribution of mass in the outer regions of most galaxies and the rate at which galaxies spin are both consistent with the presence of wormholes big enough to travel through — complete with the negative energy conditions needed to keep the wormholes open.
Still other wormholes may have been detected but misidentified as black holes (which is understandable, as black holes may form one end of a wormhole’s throat). Current conventional wisdom maintains that every ordinary galaxy harbors a huge black hole at its core, ranging in mass from thousands to billions of times as heavy as the sun. Plenty of evidence supports that conclusion, but physicist Cosimo Bambi points out that the case is not airtight. In fact, based on current observations, traversable wormholes are “indistinguishable from the black holes of general relativity” in galactic cores, he writes in a recent paper posted at arXiv.org.
Bambi notes that both black holes and wormholes would cast “shadows” — regions of darkness surrounded by a bright background — but a wormhole shadow would be smaller than a black hole’s, with more brightness surrounding it.
Telescope projects now being considered ought to be able to measure the shadow size precisely enough to determine whether it belongs to a black hole or wormhole, Bambi writes. He says the radio source Sagittarius A*, marking the spot of the black hole supposedly residing in the center of the Milky Way galaxy, should be a prime target for such an investigation.
“It should be relatively easy to check if SgrA* is actually a wormhole,” he writes.
If such an observation did confirm the existence of a big-time wormhole, physicists would be ecstatic. Travel agents would probably have to wait a few millennia before booking trips to distant galaxies, but physicists could begin imagining trips not just to distant galaxies, but also to other universes. And filmmakers obsessed with wormholes could start making documentaries.