December 2019, ScientificAmerican.com 57half times the diameter of the event horizon. And for
LIGO, part of the gravitational-wave signal that we
detect is likewise produced from the region where
the colliding objects reach similarly small separa-
tions. Although study of these signals is still in early
phases, the EHT and LIGO have revealed very dark
and very compact objects that produce signals just
like those predicted for unmodified black holes.
Still, it is important to investigate these signals
more closely. Sufficiently careful analysis might in
fact uncover more clues about the quantum physics
of black holes. Even if no new effects are observed,
we then have information that constrains possible
descriptions of their quantum behavior.
Sufficiently large-diameter remnants are now
ruled out, but what about remnant scenarios that
modify the black hole description only very near the
horizon? Although a complete discussion would re -
quire a fuller theory of these remnants—such as fuzz-
balls or firewalls—we have some initial indicators.
Specifically, if these objects had radii barely larger
than the radius of the corresponding black hole hori-
zon, then it is likely that neither EHT nor LIGO obser-
vations would be able to reveal such a structure
because very little light or gravitational radiation
escapes from the region very near the horizon.
One possible exception is the possibility of grav-
itational “echoes.” As first suggested in 2016 by
Vitor Cardoso of the University of Lisbon, Edgardo
Franzin of the University of Barcelona and Paolo
Pani of Rome University, if two such remnants com-
bine to form a final remnant that has similar proper-
ties, gravitational waves can reflect off the merged
remnant’s surface and might be observed. Whereas
most near-horizon scenarios are hard to rule out
through observation, however, it is difficult to ex -
plain how such structures could be stable, instead of
collapsing under their own weight to form black
holes. Of course, this is a general problem for all
massive-remnant scenarios, but it becomes even
more challenging in the presence of the extreme
forces in such a collision.
Prospects are better for testing some of the scenar-
ios where new interactions behave like subtle modifi-
cations of spacetime geometry but extend well outside
the horizon. For example, in the strong nonviolent sce-
nario, the rippling of a black hole’s quantum halo can
distort light passing near the black hole. If this scenar-
io is correct, the shimmering could cause distortions
of the EHT’s images that change with time.
In my work with EHT scientist Dimitrios Psaltis,
we found these changes could happen over roughly an
hour for the black hole in the center of our galaxy.
Because the EHT combines multihour observations
into an average, such effects may be hard to see. But
the relevant fluctuation time for the black hole in M87,
which is more than 1,000 times larger, is more like
tens of days. This work suggests we should look for
these distortions by using longer-duration EHT obser-
vations than the project’s initial seven-day span. If the
experiment found such distortions, they would be a
spectacular clue to the quantum physics of black
holes. If they do not appear, that will begin to point to
the subtler weak quantum scenario or to something
even more exotic.
The weak nonviolent scenario is harder to test be -
cause of the relative smallness of the expected changes
to the geometry. Yet preliminary investigation shows
that this scenario can alter how gravitational waves
are absorbed or reflected, possibly yielding an observ-
able modification to gravitational-wave signals.
If either scenario is correct, we will learn more
not only about what quantum black holes are but
also about the deeper laws of nature. Right now we
do not fully understand how to think about informa-
tion localization when gravitational fields are pres-
ent. Quantum physics suggests that spacetime itself
is not a fundamental part of physics but instead aris-
es only as an approximation of a more basic mathe-
matical structure. Evidence for quantum black hole
effects could help make this concept more concrete.
To learn more, it is important to extend and im -
prove both EHT and gravitational-wave measure-
ments. For the EHT, it would be useful to have signif-
icantly longer-duration observations, as well as imag-
es of other targets such as our galaxy’s central black
hole, both of which are anticipated. For gravitational
waves, more observations with in creased sensitivity
would be helpful and will be assisted when additional
detectors come on line in Japan and India, adding to
the existing facilities in the U.S. and Europe. Further-
more, a strong complementary theoretical effort is
needed to refine scenarios, to better clarify their ori-
gins and explanations, and to assess more thoroughly
the question of how significantly they can affect EHT
or gravitational-wave signals.
Whatever the resolution to the crisis, black holes
contain crucial clues to the basic quantum physics of
gravity, as well as to the very nature of space and time.
Just as with the atom and quantum mechanics, a bet-
ter understanding of black holes is likely to help guide
the next conceptual revolution in physics. EHT and
gravitational-wave observations have the potential to
provide us with key information, either by ruling out
quantum black hole scenarios or by discovering new
phenomena associated with them.MORE TO EXPLORE
Particle Creation by Black Holes. Stephen W. Hawking in Communications in Mathematical
Physics, Vol. 43, No. 3, pages 199–220; August 1975.
Jerusalem Lectures on Black Holes and Quantum Information. Daniel Harlow in
Reviews of Modern Physics, Vol. 88, Article No. 015002; February 2016.
Black Holes in the Quantum Universe. Steven B. Giddings in Philosophical Transactions of
the Royal Society A, Vol. 37 7, Article No. 20190029; November 2019.
FROM OUR ARCHIVES
Burning Rings of Fire. Joseph Polchinski; April 2015.
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