Tomáš Müller for Quanta Magazine |

In 1985, when Carl Sagan was writing the novel

*Contact*, he needed to quickly transport his protagonist Dr. Ellie Arroway from Earth to the star Vega. He had her enter a black hole and exit light-years away, but he didn’t know if this made any sense. The Cornell University astrophysicist and television star consulted his friend Kip Thorne, a black hole expert at the California Institute of Technology (who won a Nobel Prize earlier this month).

Thorne knew that Arroway couldn’t get to Vega via a
black hole, which is thought to trap and destroy
anything that falls in. But it occurred to him that she might make use of
another kind of hole consistent with Albert Einstein’s general theory of
relativity: a tunnel or “wormhole” connecting distant locations in space-time.

While the simplest theoretical wormholes immediately
collapse and disappear before anything can get through, Thorne wondered whether
it might be possible for an “infinitely advanced” sci-fi civilization to
stabilize a wormhole long enough for something or someone to traverse it. He figured
out that such a civilization could in fact line the throat of a wormhole with
“exotic material” that counteracts its tendency to collapse.

The material would
possess negative energy, which would deflect radiation and repulse space-time
apart from itself. Sagan used the trick in

The flurry of findings
started last year with a paper that reported the first traversable wormhole that
doesn’t require the insertion of exotic material to stay open. Instead,
according to Ping Gao and Daniel
Jafferis of Harvard University and Aron Wall of
Stanford University, the repulsive negative energy in the wormhole’s throat can
be generated from the outside by a special quantum connection between the pair
of black holes that form the wormhole’s two mouths. When the black holes are
connected in the right way, something tossed into one will shimmy along the
wormhole and, following certain events in the outside universe, exit the second.

*Contact*, attributing the invention of the exotic material to an earlier, lost civilization to avoid getting into particulars. Meanwhile, those particulars enthralled Thorne, his students and many other physicists, who spent years exploring traversable wormholes and their theoretical implications. They discovered that these wormholes can serve as time machines, invoking time-travel paradoxes — evidence that exotic material is forbidden in nature.
Now, decades later, a new species of traversable
wormhole has emerged, free of exotic material and full of potential for helping
physicists resolve a baffling paradox about black holes. This paradox is the
very problem that plagued the early draft of

*Contact*and led Thorne to contemplate traversable wormholes in the first place; namely, that things that fall into black holes seem to vanish without a trace.
This total
erasure of information breaks the rules of quantum mechanics, and it so puzzles
experts that in recent years, some have argued that black hole interiors don’t
really exist — that space and time strangely end at their horizons.

Remarkably, Gao, Jafferis and Wall noticed that
their scenario is mathematically equivalent to a process called quantum
teleportation, which is key to quantum cryptography and can be demonstrated in
laboratory experiments.

John
Preskill, a black hole and quantum gravity expert at Caltech, says the new
traversable wormhole comes as a surprise, with implications for the black hole
information paradox and black hole interiors. “What I really like,” he said,
“is that an observer can enter the black hole and then escape to tell about
what she saw.” This suggests that black hole interiors really exist, he
explained, and that what goes in must come out.

The paradox has loomed
since 1974, when the British physicist Stephen Hawking determined that black
holes evaporate — slowly giving off heat in the form of particles now known as
“Hawking radiation.” Hawking calculated that this heat is completely random; it
contains no information about the black hole’s contents. As the black hole
blinks out of existence, so does the universe’s record of everything that went
inside.

Lucy Reading-Ikkanda/Quanta Magazine |

The new wormhole work began
in 2013, when Jafferis attended an intriguing talk at the Strings conference in
South Korea. The speaker, Juan Maldacena, a professor of physics at the Institute for
Advanced Study in Princeton, New Jersey, had recently concluded, based on
various hints and arguments, that “ER
= EPR.” That is, wormholes between distant points in space-time, the
simplest of which are called Einstein-Rosen or “ER” bridges, are equivalent
(albeit in some ill-defined way) to entangled quantum particles, also known as
Einstein-Podolsky-Rosen or “EPR” pairs. The ER = EPR conjecture, posed by Maldacena
and Leonard Susskind of Stanford, was an attempt to solve the modern
incarnation of the infamous black hole information paradox by tying space-time
geometry, governed by general relativity, to the instantaneous quantum
connections between far-apart particles that Einstein called “spooky action at
a distance.”

This violates a principle
called “unitarity,” the backbone of quantum theory, which holds that as
particles interact, information about them is never lost, only scrambled, so
that if you reversed the arrow of time in the universe’s quantum evolution,
you’d see things unscramble into an exact re-creation of the past.

Almost everyone believes in
unitarity, which means information must escape black holes — but how? In the
last five years, some theorists, most notably Joseph
Polchinski of the University of California, Santa Barbara, have argued
that black holes are empty shells with no interiors at all — that Ellie
Arroway, upon hitting a black hole’s event horizon, would fizzle
on a “firewall” and radiate out again.

Many theorists believe in
black hole interiors (and gentler transitions across their horizons), but in
order to understand them, they must discover the fate of information that falls
inside. This is critical to building a working quantum theory of
gravity, the long-sought union of the quantum and space-time descriptions
of nature that comes into sharpest relief in black hole interiors, where
extreme gravity acts on a quantum scale. The quantum gravity
connection is what drew Maldacena, and later Jafferis, to the ER = EPR idea,
and to wormholes.

When Maldacena read Gao,
Jafferis and Wall’s paper, “I viewed it as a really nice idea, one of these
ideas that after someone tells you, it’s obvious,” he said. Maldacena and two
collaborators, Douglas Stanford and Zhenbin
Yang, immediately began exploring the new wormhole’s ramifications for the
black hole information paradox; their paper appeared
in April.

The implied relationship
between tunnels in space-time and quantum entanglement posed by ER = EPR
resonated with a popular recent belief that space is essentially stitched
into existence by quantum entanglement. It seemed that wormholes had a role
to play in stitching together space-time and in letting black hole information
worm its way out of black holes — but how might this work? When Jafferis heard
Maldacena talk about his cryptic equation and the evidence for it, he was aware
that a standard ER wormhole is unstable and non-traversable. But he wondered
what Maldacena’s duality would mean for a traversable wormhole like the ones
Thorne and others played around with decades ago. Three years after the South
Korea talk, Jafferis and his collaborators Gao and Wall presented their answer.
The work extends the ER = EPR idea by equating, not a standard wormhole and a
pair of entangled particles, but a traversable wormhole and quantum
teleportation: a protocol discovered in 1993 that allows a quantum system to
disappear and reappear unscathed somewhere else.

Susskind and Ying
Zhao of Stanford followed this with a paper about wormhole
teleportation in July. The wormhole “gives an interesting geometric picture for
how teleportation happens,” Maldacena said. “The message actually goes through
the wormhole.”

In their paper, “Diving Into
Traversable Wormholes,” published in Fortschritte der Physik, Maldacena,
Stanford and Yang consider a wormhole of the new kind that connects two black
holes: a parent black hole and a daughter one formed from half of the Hawking
radiation given off by the parent as it evaporates. The two systems are as
entangled as they can be. Here, the fate of the older black hole’s information
is clear: It worms its way out of the daughter black hole.

On the right side of a
chalk-dusty blackboard, Maldacena drew a faint picture of two black holes
connected by the new traversable wormhole. On the left, he sketched a quantum
teleportation experiment, performed by the famous fictional experimenters Alice
and Bob, who are in possession of entangled quantum particles a and b,
respectively. Say Alice wants to teleport a qubit q to Bob. She prepares a
combined state of q and a, measures that combined state (reducing it to a pair
of classical bits, 1 or 0), and sends the result of this measurement to Bob. He
can then use this as a key for operating on b in a way that re-creates the
state q. Voila, a unit of quantum information has teleported from one place to
the other.

Maldacena turned to the
right side of the blackboard. “You can do operations with a pair of black holes
that are morally equivalent to what I discussed [about quantum teleportation].
And in that picture, this message really goes through the wormhole.”

Bob’s operation reconstructs
q, which appears to pop out of B, a perfect match for the particle that fell
into A. This is why some physicists are excited: Gao, Jafferis and Wall’s
wormhole allows information to be recovered from black holes. In their paper,
they set up their wormhole in a negatively curved space-time geometry that
often serves as a useful, if unrealistic, playground for quantum gravity
theorists. However, their wormhole idea seems to extend to the real world as
long as two black holes are coupled in the right way: “They have to be causally
connected and then the nature of the interaction that we took is the simplest
thing you can imagine,” Jafferis explained. If you allow the Hawking radiation from
one of the black holes to fall into the other, the two black holes become
entangled, and the quantum information that falls into one can exit the other.

It seems traversable
wormholes might be permitted in nature as long as they offer no speed
advantage. “Traversable wormholes are like getting a bank loan,” Gao, Jafferis
and Wall wrote in their paper: “You can only get one if you are rich enough not
to need it.”

While traversable wormholes
won’t revolutionize space travel, according to Preskill the new wormhole
discovery provides “a promising resolution” to the black hole firewall question
by suggesting that there is no firewall at black hole horizons. Preskill said
the discovery rescues “what we call ‘black hole complementarity,’ which means
that the interior and exterior of the black hole are not really two different
systems but rather two very different, complementary ways of looking at the same
system.” If complementarity holds, as is widely assumed, then in passing across
a black hole horizon from one realm to the other, Contact’s Ellie Arroway
wouldn’t notice anything strange. This seems more likely if, under certain
conditions, she could even slide all the way through a Gao-Jafferis-Wall
wormhole.

The wormhole also safeguards
unitarity — the principle that information is never lost — at least for the
entangled black holes being studied. Whatever falls into one black hole
eventually exits the other as Hawking radiation, Preskill said, which “can be
thought of as in some sense a very scrambled copy of the black hole interior.”

Taking the findings to their
logical conclusion, Preskill thinks it ought to be possible (at least for an
infinitely advanced civilization) to influence the interior of one of these
black holes by manipulating its radiation. This “sounds crazy,” he wrote in an
email, but it “might make sense if we can think of the radiation, which is
entangled with the black hole — EPR — as being connected to the black hole
interior by wormholes — ER. Then tickling the radiation can send a message
which can be read from inside the black hole!” He added, “We still have a ways
to go, though, before we can flesh out this picture in more detail.”

Indeed, obstacles remain in
the quest to generalize the new wormhole findings to a statement about the fate
of all quantum information, or the meaning of ER = EPR.

In Maldacena and Susskind’s
paper proposing ER = EPR, they included a sketch that’s become known as the
“octopus”: a black hole with tentacle-like wormholes leading to distant Hawking
particles that have evaporated out of it. The authors explained that the sketch
illustrates “the entanglement pattern between the black hole and the Hawking
radiation. We expect that this entanglement leads to the interior geometry of
the black hole.”

But according to Matt Visser,
a mathematician and general-relativity expert at Victoria University of
Wellington in New Zealand who has studied wormholes since the 1990s, the most
literal reading of the octopus picture doesn’t work. The throats of wormholes
formed from single Hawking particles would be so thin that qubits could never
fit through. “A traversable wormhole throat is ‘transparent’ only to wave
packets with size smaller than the throat radius,” Visser explained. “Big wave
packets will simply bounce off any small wormhole throat without crossing to
the other side.”

Stanford, who co-wrote the
recent paper with Maldacena and Yang, acknowledged that this is a problem with
the simplest interpretation of the ER = EPR idea, in which each particle of
Hawking radiation has its own tentacle-like wormhole. However, a more
speculative interpretation of ER = EPR that he and others have in mind does not
suffer from this failing. “The idea is that in order to recover the information
from the Hawking radiation using this traversable wormhole,” Stanford said, one
has to “gather the Hawking radiation together and act on it in a complicated
way.” This complicated collective measurement reveals information about the particles
that fell in; it has the effect, he said, of “creating a large, traversable
wormhole out of the small and unhelpful octopus tentacles.

The information would then
propagate through this large wormhole.” Maldacena added that, simply put, the
theory of quantum gravity might have a new, generalized notion of geometry for
which ER equals EPR. “We think quantum gravity should obey this principle,” he
said. “We view it more as a guide to the theory.”

In his 1994 popular science book, Black Holes and Time Warps,
Kip Thorne celebrated the style of reasoning involved in wormhole research. “No
type of thought experiment pushes the laws of physics harder than the type
triggered by Carl Sagan’s phone call to me,” he wrote; “thought experiments
that ask, ‘What things do the laws of physics permit an infinitely advanced
civilization to do, and what things do the laws forbid?’”

Via QuantaMagazine

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