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 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.

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. 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.

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.”

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. 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.

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.

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. 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.

During
an interview this month in his tranquil office at the IAS, Maldacena, a
reserved Argentinian-American with a track record of influential insights,
described his radical musings. 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.”

Say
Alice throws qubit q into black hole A. She then measures a particle of its
Hawking radiation, a, and transmits the result of the measurement through the
external universe to Bob, who can use this knowledge to operate on b, a Hawking
particle coming out of black hole B. 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.

The
quantum-teleportation format precludes using these traversable wormholes as
time machines. Anything that goes through the wormhole has to wait for Alice’s
message to travel to Bob in the outside universe before it can exit Bob’s black
hole, so the wormhole doesn’t offer any superluminal boost that could be
exploited for time travel. 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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