52 Scientific American, December 2019
H
umankind caught its first glimpse of a black hole
on April 10, 2019. The Event Horizon Telescope
(EHT) team, which uses an Earth-spanning net-
work of radio observatories acting in concert,
shared images it had captured of an apparent
black hole with 6.5 billion times the mass of our
sun in the center of the nearby M87 gal axy. This
was a breathtaking achievement—our first views of one of the most mysterious
objects in the universe, long predicted but never directly “seen.” Even more
exciting, the images, and the observations that should follow, are beginning
to provide new clues about one of the deepest puzzles in physics.This enigma is the “paradox” of what happens to
information in a black hole. By investigating this ques-
tion, physicists have discovered that the mere exis-
tence of black holes is inconsistent with the quantum-
mechanical laws that so far describe everything else in
our universe. Resolving this inconsistency may require
a conceptual revolution as profound as the overthrow
of classical physics by quantum mechanics.
Theorists have explored many ideas, but there has
been little direct evidence to help resolve this prob-
lem. The first image of a black hole, however, begins
to offer actual data to inform our theories. Future
EHT observations—especially those that can show
how black holes evolve over time—and recent detec-
tions of colliding black holes by gravitational-wave
observatories could provide important new insights
and help to usher in a whole new era of physics.THE INFORMATION PROBLEM
though deeply mysterious, black holes seem to be
ubiquitous in the cosmos. The EHT observations and
the gravitational-wave measurements are just the lat-
est and most robust evidence that black holes, despite
sounding fantastical, do indeed appear to be real—and
remarkably common. Yet their very existence threat-
ens the present foundations of physics. The basic prin-
ciples of quantum mechanics are thought to govern all
the other laws of nature, but when they are applied to
black holes they lead to a contradiction, exposing a
flaw in the current form of these laws.
The problem arises from one of the simplest ques-
tions we can ask about black holes: What happens to
stuff that falls into them? We need a little refinement
here to fully explain. First, according to our present
quantum-mechanical laws, matter and energy can
shift between different forms: particles can, for exam-
ple, change into different kinds of particles. But theone thing that is sacred and never de stroyed is quan-
tum information. If we know the complete quantum
description of a system, we should always be able to
exactly determine its earlier or later quantum de -
scrip tion with no loss of information. So a more pre-
cise question is, What happens to quantum informa-
tion that falls into a black hole?
Our understanding of black holes comes from
Albert Einstein’s general theory of relativity, which de -
scribes gravity as arising from the curvature of space
and time; a common visualization of this idea is a
heavy ball deforming the surface of a trampoline. This
warping of spacetime causes the trajectories of mas-
sive bodies and light to bend, and we call that gravity.
If mass is sufficiently concentrated in a small-enough
vicinity, the nearby spacetime deformation is so strong
that light itself cannot escape a region inside what we
call the event horizon: we have a black hole. And if
nothing can travel faster than light—including infor-
mation—everything must get stuck inside this bound-
ary. Black holes become cosmic sinkholes trapping
information along with light and matter.
But the story becomes stranger. What may be Ste-
phen Hawking’s greatest discovery is his 1974 predic-
tion that black holes evaporate. This finding also led
to the startling idea that black holes destroy quantum
information. According to quantum mechanics, pairs
of “virtual particles” pop into existence all the time,
everywhere. Typically such a pair, consisting of a par-
ticle and its antimatter counterpart, quickly annihi-
lates, but if it forms near the horizon of a black hole,
one particle might pop up inside this boundary and
the other outside. The outside particle can escape, car-
rying away energy. The law of energy conservation
tells us that the black hole has thus lost energy, so the
emission of such particles causes the black hole to
shrink over time until it completely disappears. TheSteven B. Giddings
is a quantum physicist
at the University of
California, Santa Barbara,
who focuses on high-
energy theory, quantum
aspects of gravity and
quantum black holes.
IN BRIEF
According to quantum
mechanics, information
can never be destroyed.
But when combined
with general relativity,
quantum rules say
that black holes
destroy information.
Scientists have proposed
modifications to the clas-
sical picture of black
holes that could solve the
paradox, but they lack
evidence to test them.
That is changing with
the new Event Horizon
Telescope, which recent-
ly took the first picture
of a black hole, as well as
with gravitational-wave
measurements of black
holes colliding.
© 2019 Scientific American