*How a new black hole paradox has set the physics world ablaze.*

Alice and Bob Meet the Wall of Fire

An
illustration of a galaxy with a supermassive black hole shooting out jets of
radio waves.

Illustration by NASA/JPL-Caltech

Alice and Bob, beloved characters
of various thought experiments in quantum mechanics, are at a crossroads. The
adventurous, rather reckless Alice jumps into a very large black hole, leaving
a presumably forlorn Bob outside the event horizon — a black hole’s point of no
return, beyond which nothing, not even light, can escape.

Conventionally, physicists have
assumed that if the black hole is large enough, Alice won’t notice anything
unusual as she crosses the horizon. In this scenario, colorfully dubbed “No
Drama,” the gravitational forces won’t become extreme until she approaches a
point inside the black hole called the singularity. There, the gravitational
pull will be so much stronger on her feet than on her head that Alice will be
“spaghettified.”

Now a new hypothesis is giving
poor Alice even more drama than she bargained for. If this alternative is
correct, as the unsuspecting Alice crosses the event horizon, she will
encounter a massive wall of fire that will incinerate her on the spot. As
unfair as this seems for Alice, the scenario would also mean that at least one
of three cherished notions in theoretical physics must be wrong.

When Alice’s fiery fate was
proposed this summer, it set off heated debates among physicists, many of whom
were highly skeptical. “My initial reaction was, ‘You’ve got to be kidding,’”
admitted Raphael Bousso, a physicist at the University of California, Berkeley.
He thought a forceful counterargument would quickly emerge and put the matter
to rest. Instead, after a flurry of papers debating the subject, he and his
colleagues realized that this had the makings of a mighty fine paradox.

**The ‘Menu From Hell’**

Paradoxes in physics have a way
of clarifying key issues. At the heart of this particular puzzle lies a
conflict between three fundamental postulates beloved by many physicists. The
first, based on the equivalence principle of general relativity, leads to the
No Drama scenario: Because Alice is in free fall as she crosses the horizon,
and there is no difference between free fall and inertial motion, she shouldn’t
feel extreme effects of gravity. The second postulate is unitarity, the
assumption, in keeping with a fundamental tenet of quantum mechanics, that
information that falls into a black hole is not irretrievably lost. Lastly,
there is what might be best described as “normality,” namely, that physics
works as expected far away from a black hole even if it breaks down at some
point within the black hole — either at the singularity or at the event
horizon.

Together, these concepts make up
what Bousso ruefully calls “the menu from hell.” To resolve the paradox, one of
the three must be sacrificed, and nobody can agree on which one should get the
ax.

Physicists don’t lightly abandon
time-honored postulates. That’s why so many find the notion of a wall of fire
downright noxious. “It is odious,” John Preskill of the California Institute of
Technology declared earlier this month at an informal workshop organized by
Stanford University’s Leonard Susskind. For two days, 50 or so physicists
engaged in a spirited brainstorming session, tossing out all manner of crazy
ideas to try to resolve the paradox, punctuated by the rapid-fire *tap-tap-tap* of
equations being scrawled on a blackboard. But despite the collective angst,
even the firewall’s fiercest detractors have yet to find a satisfactory
solution to the conundrum.

Joseph
Polchinski, a string theorist at the University of California, Santa Barbara,
is the “P” in the “AMPS” team that presented a new hypothesis about black hole
firewalls.

According to Joseph Polchinski, a
string theorist at the University of California, Santa Barbara, the simplest
solution is that the equivalence principle breaks down at the event horizon,
thereby giving rise to a firewall. Polchinski is a co-author of the paper that started it all, along with Ahmed
Almheiri, Donald Marolf and James Sully — a group often referred to as “AMPS.”
Even Polchinski thinks the idea is a little crazy. It’s a testament to the
knottiness of the problem that a firewall is the least radical potential solution.
If there is an error in the
firewall argument, the mistake is not obvious. That’s the hallmark of a good
scientific paradox. And it comes at a time when theorists are hungry for a new
challenge: The Large Hadron Collider has failed to turn up any data hinting at exotic
physics beyond the Standard Model. “In the absence of data, theorists thrive on
paradox,” Polchinski quipped.
If AMPS is wrong, according
to Susskind, it is wrong in a really interesting way that will push
physics forward, hopefully toward a robust theory of quantum gravity. Black
holes are interesting to physicists, after all, because both general relativity
and quantum mechanics can apply, unlike in the rest of the universe, where
objects are governed by quantum mechanics at the subatomic scale and by general
relativity on the macroscale. The two “rule books” work well enough in their
respective regimes, but physicists would love to combine them to shed light on
anomalies like black holes and, by extension, the origins of the universe.
**An Entangled Paradox**

The issues are complicated and
subtle — if they were simple, there would be no paradox — but a large part of
the AMPS argument hinges on the notion of monogamous quantum entanglement: You
can only have one kind of entanglement at a time. AMPS argues that two
different kinds of entanglement are needed in order for all three postulates on
the “menu from hell” to be true. Since the rules of quantum mechanics don’t
allow you to have both entanglements, one of the three postulates must be
sacrificed.

Entanglement — which Albert
Einstein ridiculed as “spooky action at a distance” — is a well-known feature
of quantum mechanics (in the thought experiment, Alice and Bob represent an
entangled particle pair). When subatomic particles collide, they can become
invisibly connected, though they may be physically separated. Even at a distance,
they are inextricably interlinked and act like a single object. So knowledge
about one partner can instantly reveal knowledge about the other. The catch is
that you can only have one entanglement at a time.

Under classical physics, as Preskill explained on Caltech’s
Quantum Frontiersblog, Alice and Bob can both have copies of the
same newspaper, which gives them access to the same information. Sharing this
bond of sorts makes them “strongly correlated.” A third person, “Carrie,” can
also buy a copy of that newspaper, which gives her equal access to the
information it contains, thereby forging a correlation with Bob without weakening
his correlation with Alice. In fact, any number of people can buy a copy of
that same newspaper and become strongly correlated with one another.

With
quantum correlations, Bob can be highly entangled with Alice or with Carrie,
but not both.

Illustration courtesy of John
Preskill

But with quantum correlations,
that is not the case. For Bob and Alice to be maximally entangled, their
respective newspapers must have the same orientation, whether right side up,
upside down or sideways. So long as the orientation is the same, Alice and Bob
will have access to the same information. “Because there is just one way to
read a classical newspaper and lots of ways to read a quantum newspaper, the
quantum correlations are stronger than the classical ones,” Preskill said. That
makes it impossible for Bob to become as strongly entangled with Carrie as he
is with Alice without sacrificing some of his entanglement with Alice.

This is problematic because there
is more than one kind of entanglement associated with a black hole, and under
the AMPS hypothesis, the two come into conflict. There is an entanglement
between Alice, the in-falling observer, and Bob, the outside observer, which is
needed to preserve No Drama. But there is also a second entanglement that
emerged from another famous paradox in physics, one related to the question of
whether information is lost in a black hole. In the 1970s, Stephen Hawking
realized that black holes aren’t completely black. While nothing might seem
amiss to Alice as she crosses the event horizon, from Bob’s perspective, the
horizon would appear to be glowing like a lump of coal — a phenomenon now known
as Hawking radiation.

The
entanglement of particles in the No Drama scenario: Bob, outside the event
horizon (dotted lines), is entangled with Alice just inside the event horizon,
at point (b). Over time Alice (b’) drifts toward the singularity (squiggly
line) while Bob (b”) remains outside the black hole.

Illustration courtesy of Joseph
Polchinski

This radiation results from
virtual particle pairs popping out of the quantum vacuum near a black hole.
Normally they would collide and annihilate into energy, but sometimes one of
the pair is sucked into the black hole while the other escapes to the outside
world. The mass of the black hole, which must decrease slightly to counter this
effect and ensure that energy is still conserved, gradually winks out of
existence. How fast it evaporates depends on the black hole’s size: The bigger
it is, the more slowly it evaporates.

Hawking assumed that once the
radiation evaporated altogether, any information about the black hole’s
contents contained in that radiation would be lost. “Not only does God play
dice, but he sometimes confuses us by throwing them where they can’t be seen,”
he famously declared. He and the Caltech physicist Kip Thorne even made a bet
with a dubious Preskill in the 1990s about about whether or not information is
lost in a black hole. Preskill insisted that information must be conserved;
Hawking and Thorne believed that information would be lost. Physicists
eventually realized that it is possible to preserve the information at a cost:
As the black hole evaporates, the Hawking radiation must become increasingly
entangled with the area outside the event horizon. So when Bob observes that
radiation, he can extract the information.

But what happens if Bob were to
compare his information with Alice’s after she has passed beyond the event
horizon? “That would be disastrous,” Bousso explained, “because Bob, the
outside observer, is seeing the same information in the Hawking radiation, and
if they could talk about it, that would be quantum Xeroxing, which is strictly
forbidden in quantum mechanics.”

Physicists, led by Susskind,
declared that the discrepancy between these two viewpoints of the black hole is
fine so long as it is impossible for Alice and Bob to share their respective
information. This concept, called complementarity, simply holds that there is
no direct contradiction because no single observer can ever be both inside and
outside the event horizon. If Alice crosses the event horizon, sees a star
inside that radius and wants to tell Bob about it, general relativity has ways
of preventing her from doing so.

Susskind’s argument that
information could be recovered without resorting to quantum Xeroxing proved
convincing enough that Hawking conceded his bet with Preskill in 2004,
presenting the latter with a baseball encyclopedia from which, he said,
“information can be retrieved at will.” But perhaps Thorne, who refused to
concede, was right to be stubborn.

The
Hawking radiation is the result of virtual particle pairs popping into
existence near the event horizon, with one partner falling in and the other
escaping. The black hole’s mass decreases as a result and is emitted as
radiation.

Illustration courtesy of Joseph
Polchinski

Bousso thought complementarity
would come to the rescue yet again to resolve the firewall paradox. He soon
realized that it was insufficient. Complementarity is a theoretical concept
developed to address a specific problem, namely, reconciling the two viewpoints
of observers inside and outside the event horizon. But the firewall is just the
tiniest bit outside the event horizon, giving Alice and Bob the same viewpoint,
so complementarity won’t resolve the paradox.

**Toward Quantum Gravity**

If they wish to get rid of the
firewall and preserve No Drama, physicists need to find a new theoretical
insight tailored to this unique situation or concede that perhaps Hawking was
right all along, and information is indeed lost, meaning Preskill might have to
return his encyclopedia. So it was surprising to find Preskill suggesting that
his colleagues at the Stanford workshop at least reconsider the possibility of
information loss. Although we don’t know how to make sense of quantum mechanics
without unitarity, “that doesn’t mean it can’t be done,” he said. “Look in the
mirror and ask yourself: Would I bet my life on unitarity?”

Polchinski argues persuasively
that you need Alice and Bob to be entangled to preserve No Drama, and you need
the Hawking radiation to be entangled with the area outside the event horizon
to conserve quantum information. But you can’t have both. If you sacrifice the
entanglement of the Hawking radiation with the area outside the event horizon,
you lose information. If you sacrifice the entanglement of Alice and Bob, you
get a firewall.

David Kaplan, Petr Stepanek and
MK12 for Quanta Magazine; Music by Steven Gutheinz

**Video:** David Kaplan explores black
hole physics and the problem of quantum gravity in this In Theory video.

That consequence arises from the
fact that entanglement between the area outside the event horizon and the
Hawking radiation must increase as the black hole evaporates. When roughly half
the mass has radiated away, the black hole is maximally entangled and
essentially experiences a mid-life crisis. Preskill explained: “It’s as if the
singularity, which we expected to find deep inside the black hole, has crept
right up to the event horizon when the black hole is old.” And the result of
this collision between the singularity and the event horizon is the dreaded
firewall.

The mental image of a singularity
migrating from deep within a black hole to the event horizon provoked at least
one exasperated outburst during the Stanford workshop, a reaction Bousso finds understandable. “We should be
upset,” he said. “This is a terrible blow to general relativity.”
Yet for all his skepticism about
firewalls, he is thrilled to be part of the debate.
“This is probably the most exciting thing that’s happened to me since I entered
physics,” he said. “It’s certainly the nicest paradox that’s come my way, and
I’m excited to be working on it.”
Alice’s death by firewall seems
destined to join the ranks of classic thought experiments in physics. The more
physicists learn about quantum gravity, the more different it appears to be
from our current picture of how the universe works, forcing them to sacrifice
one cherished belief after another on the altar of scientific progress. Now
they must choose to sacrifice either unitarity or No Drama, or undertake a
radical modification of quantum field theory. Or maybe it’s all just a horrible
mistake. Any way you slice it, physicists are bound to learn something new.