56 Scientific American, December 2019theories that include gravity than in those that do
not, because gravitational fields extend to infinity,
complicating the concept of localization.
If information does escape black holes, it might
not require a change as obvious and abrupt as the
formation of a massive remnant, whether fuzzball,
firewall or another variant. The growing evidence
for black holes suggests there are objects in the uni-
verse that look and act a lot like classical black holes,
without large departures from Einstein’s predictions.
Is Einstein’s general relativity so drastically wrong in
its description of black holes, or might there be some
more innocuous, currently unknown effects that
delocalize information and allow it to leak from
black holes, avoiding such a dramatic failure of the
entire spacetime picture?In my recent theoretical work, I have found two
versions of such effects. In one, the geometry of
spacetime near a black hole is altered, making it
bend and ripple in a way that depends on the infor-
mation in the black hole—but gently, so that it does
not, for example, de stroy an astronaut falling through
the region where the horizon would ordinarily be
found. In this “strong, nonviolent” scenario, such
shimmering of spacetime can transfer the informa-
tion out. Interestingly, I have also found that there is
a subtler, intrinsically quantum way for information
to escape the black hole. In this “weak, nonviolent”
scenario, even tiny quantum fluctuations of the
spacetime geometry near the black hole can transfer
information to particles emanating from the hole.
The fact that the information transfer is still large
enough to save quantum mechanics is related to the
huge amount of possible information a black hole
can contain. In either picture, a black hole effective-
ly has a “quantum halo” surrounding it, where inter-
actions pass information back to its surroundings.
Notably, these scenarios, despite appearing to re -
quire superluminal travel of information, do not
necessarily produce a grandmother paradox. The
information signaling here is tied to the existence of
the black hole, which has a spacetime geometry that
is different from that of flat space, so that the earlier
argument about communicating with the past no
longer holds. These possibilities are tantalizing from
an other perspective: the locality principle is also
what prohibits our own faster-than-light travel; the
quantum me cha nics of black holes seems to be tell-
ing us there is something wrong with the present
formulation of this principle.REWRITING THE LAWS OF PHYSICS
so far such a quantum-halo scenario has not been
predicted by a more complete theory of physics that
reconciles quantum mechanics with gravity, but it is
strongly indicated by the need to resolve the problem
and by assumptions based on what we see. If such a
scenario is correct, it probably represents an approx-
imate description of a deeper reality. Our very no-
tions of space and time, which underlie the rest of
science, appear to require significant revision. The
present work to understand black holes may be akin to
the first attempts to model the physics of the atom by
Bohr and others. Those early atomic descriptions were
also approximate and only later led to the profound
theoretical structure of quantum mechanics. Although
modifying locality seems crazy, we might find solace by
noting that the laws of quantum mechan-
ics also seemed very crazy to the classical
physicists grappling with their discovery.
Given the immense challenge in sort-
ing out the story of quantum black holes
and the more complete theory describing
them, physicists are eager for experimental
and observational evidence to help guide
us. The exciting recent advances have giv-
en humankind two direct observational windows on
black hole be havior. In addition to the EHT’s images
of black holes, the Laser Interferometer Gravitational-
Wave Observatory (LIGO) and its companion facilities
have begun to detect gravitational waves from colli-
sions between ap parent black holes. These waves car-
ry valuable information with them about the proper-
ties and behavior of the objects that created them.
From a naive viewpoint, it seems preposterous
that the EHT or LIGO could detect any departure
from Einstein’s description of black holes. Tradition-
ally his theory has been expected to need modifica-
tion only when spacetime curvatures be come ex -
tremely large, near the center of a black hole; in con-
trast, curvatures are very weak near the horizon of a
large black hole. But the information crisis I have
described suggests otherwise. A large part of the the-
oretical community has now reached the consensus
that some changes to the current laws of physics are
needed to describe phenomena not just deep inside a
black hole but all the way out past the horizon. We
appear to have crossed the Rubicon. For the case of
the black hole in M87, the distance at which we
expect to find deviations from classical predictions is
several times the size of our solar system.
Already LIGO and the EHT have ruled out wilder
possibilities that could be considered in an attempt
to give a logically consistent description of black
holes. Specifically, if black holes were replaced by
massive remnants more than about twice the diame-
ter of the supposed black hole, we would have seen
signs in the data from both experiments. In the case
of the EHT, much of the light that produced the now
famous image comes from a region around one and aOur very notions of space and time,
which underlie the rest of science,
appear to require significant revision.
© 2019 Scientific American