Scientific American - USA (2019-12)

Illustration by Amanda Montañez December 2019, ScientificAmerican.com 53

problem is that the es cap ing particles, known as

Hawking radiation, carry essentially no in formation

about what went into the black hole. Therefore,

Hawking’s calculations appear to show that quantum

information that falls into a black hole is ultimately

destroyed—contradicting quantum mechanics.

This revelation initiated a deep crisis in physics.

Great advances have followed from previous such

crises. For instance, at the beginning of the 20th cen-

tury, classical physics seemed to predict the inevita-

ble instability of atoms, in obvious contradiction to

the existence of stable matter. That problem played a

key role in the quantum revolution. Classical physics

implied that because orbiting electrons within atoms

are constantly changing direction, they continually

emit light, causing them to lose energy and spiral

into the nucleus. But in 1913 Niels Bohr proposed

that electrons actually travel only within quantized

orbits and cannot spiral in. This radical idea helped

to establish the basis of quantum mechanics, which

fundamentally rewrote the laws of nature. Increas-

ingly it seems that the black hole crisis will similarly

lead to another paradigm shift in physics.

QUANTUM ALTERNATIVES

When haWking first predicted black hole evapora-

tion, he suggested that quantum mechanics must be

wrong and that information destruction is allowed.

Yet physicists soon realized this change would re -

quire a drastic breakdown of the law of energy con-

servation, which would disastrously invalidate our

present description of the universe. Apparently the

resolution must be sought elsewhere.

Another early idea was that black holes do not

completely evaporate but instead stop shrinking at a

tiny size, leaving behind microscopic remnants con-

taining the original information. But, scientists real-

ized, if this were true, basic properties of quantum

physics would predict catastrophic instabilities caus-

ing ordinary matter to explode into such remnants,

also contradicting everyday experience.

Obviously something is deeply wrong. It is tempt-

ing to conclude that the flaw is in Hawking’s original

analysis and that somehow information does escape

a black hole emitting Hawking radiation. The chal-

lenge here is that this scenario would conflict with a

foundational concept of present-day physics, the

principle of locality, which states that information

cannot move from one place to another superlumi-

nally—that is, faster than the speed of light. But

according to our definition of black holes, the only

way to es cape one is to travel faster than light, so if

information does es cape, it must be doing so super-

luminally, in conflict with locality. In the four

decades since Hawking’s discovery, physicists have

tried to find a loophole to this argument that stays

within conventional physics, but none has emerged.

The closest attempt was a 2016 proposal by Hawk-

ing, Malcolm Perry and Andrew Strominger, who

The Information Problem

Black holes were predicted by general relativity, and mounting astro­

physical evidence supports their existence. But in 1974 Stephen Hawking

argued that black holes eventually evaporate. If so, everything that falls

into them is ultimately destroyed, including the information contained in

the matter that fell in. The problem is that quantum mechanics and

energy conservation forbid such destruction of information. In response,

physicists have come up with several suggestions for how to modify our

picture of black holes to make them compatible with quantum physics:

Black hole with an

event horizon; infor-

mation that enters

the black hole is

destroyed when

the black hole

evaporates.

“Classical” black hole

Information is destroyed

Information is not destroyed

HYPOTHESIS DESCRIPTION PROBLEM

Soft hair

Fuzzball

Firewall

Wall of particles

Imprint of information

Event horizon

Quantum halo

Contradicts quan-

tum mechanics and

energy conservation,

which say that infor-

mation cannot

be destroyed.

Most experts do not

regard this picture as

providing a convinc-

ing resolution.

All three of these

scenarios require

modification of the

conventional notion

of locality —that is,

the idea that noth-

ing, including infor-

mation, can travel

faster than light.

Information does not

fully enter the black

hole but instead

leaves an “imprint”

just outside the

event horizon.

A type of massive

remnant in which the

black hole horizon is

replaced by strings

and higher-dimen-

sional geometry.

A type of massive

remnant in which a

“wall” of high-energy

particles replaces the

horizon; there is no

black hole interior.

A quantum black

hole interacts with

its surroundings,

possibly through

small fluctuations

in spacetime, allow-

ing information to

transfer out.

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