In the early days of research on black holes, before they even had that
name, physicists did not yet know if these bizarre objects existed in the
real world. They might have been a quirk of the complicated math used in the
then still young general theory of relativity, which describes gravity. Over
the years, though, evidence has accumulated that black holes are very real
and even exist right here in our galaxy.
Today another strange prediction from general relativity—wormholes, those
fantastical sounding tunnels to the other side of the universe—hang in the
same sort of balance. Are they real? And if they are out there in our
cosmos, could humans hope to use them for getting around? After their
prediction in 1935, research seemed to point toward no—wormholes appeared
unlikely to be an element of reality. But new work offers hints of how they
could arise, and the process may be easier than physicists have long
thought.
The
original idea of a wormhole
came from physicists Albert Einstein and Nathan Rosen. They studied the
strange equations that we now know describe that unescapable pocket of space
we call a black hole and asked what they really represented. Einstein and
Rosen discovered that, theoretically at least, a black hole’s surface might
work as a bridge that connected to a second patch of space. The journey
might be as if you went down the drain of your bathtub, and instead of
getting stuck in the pipes, you came out into another tub just like the
first.
Subsequent work expanded this idea but turned up two persistent challenges
that prevent the formation of easily spotted, humanly usable wormholes:
fragility and tininess. First, it turns out that in general relativity, the
gravitational attraction of any normal matter passing through a wormhole
acts to pull the tunnel shut. Making a stable wormhole requires some kind of
extra, atypical ingredient that acts to keep the hole open, which
researchers call “exotic” matter.
Second, the kinds of wormhole-creating processes that scientists had studied
rely on effects that could prevent a macroscopic traveler from entering. The
challenge is that the process that creates the wormhole and the exotic
matter that stabilizes it cannot stray too far from familiar physics.
“Exotic” does not mean physicists can dream up any sort of stuff that gets
the job done on paper. But so far, familiar physics has delivered only
microscopic wormholes. A bigger wormhole seems to require a process or type
of matter that is both unusual and believable. “That’s the delicacy,” says
Brianna Grado-White, a physicist and wormhole researcher at Brandeis
University.
A breakthrough occurred in late 2017, when physicists Ping Gao and Daniel
Jafferis, both then at Harvard University, and Aron Wall, then at the
Institute for Advanced Study in Princeton, N.J.,
discovered a way to prop open wormholes
with quantum entanglement—a kind of long-distance connection between quantum
entities. The peculiar nature of entanglement allows it to provide the
exotic ingredient needed for wormhole stability. And because entanglement is
a standard feature of quantum physics, it is relatively easy to create.
“It’s really a beautiful theoretical idea,” says Nabil Iqbal, a physicist at
Durham University in England, who was not involved in the research. Though
the method helps to stabilize wormholes, it can still deliver only
microscopic ones. But this new approach has inspired a stream of work that
uses the entanglement trick with different sorts of matter in the hopes of
bigger, longer-lasting holes.
One easy-to-picture idea comes from a preprint study by Iqbal and his Durham
University colleague Simon Ross. The two tried to see if they could make the
Gao-Jafferis-Wall method produce a large wormhole. “We thought it would be
interesting, from a sci-fi point of view, to push the limits and see whether
this thing could exist,” Iqbal says.
Their work showed
how special disturbances within the magnetic fields surrounding a black hole
could, in theory, generate stable wormholes. Unfortunately, the effect still
only forms microscopic wormholes, and Iqbal says it is highly unlikely the
situation would occur in reality.
Iqbal and Ross’s work highlights the delicate part of wormhole construction:
finding a realistic process that does not require something added from way
beyond the bounds of familiar physics. Physicist Juan Maldacena of the
Institute for Advanced Study, who had suggested connections between
wormholes and entanglement back in 2013, and his collaborator Alexey
Milekhin of Princeton University
have found a method
that could produce large holes. The catch in their approach is that the
mysterious dark matter that fills our universe must behave in a particular
way, and we may not live in a universe anything like this. “We have a
limited toolbox,” Grado-White says. “To get something to look the way we
need it, there’s only so many things we can do with that toolbox.”
The boom in wormhole research continues. So far, nothing like a
made-to-order human-sized wormhole machine looks likely, but the results do
show progress. “We’re learning that we can, in fact, build wormholes that
stay open using simple quantum effects,” Grado-White says. “For a very long
time, we didn’t think these things were possible to build—it turns out that
we can.”
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