December 2019, ScientificAmerican.com 55although the actual process was apparently more
complex), it would have happened when the dust
reached the density of air at the top of Mount Everest.
(Air on top of Everest does not form a black hole, be-
cause there is not enough of it; one would require an
accumulated 6.5 billion solar masses.) Some drastic
and superluminal new physical process would need
to take over in such a low-density regime to instantly
convert the collapsing cloud into a massive remnant
instead of allowing a black hole to form.
A related idea is that something could cause black
holes to change into massive remnants containing
the original information after they form but long be -
fore they evaporate. But once again, this story re -
quires nonlocal transfer of information from the in -
ter ior of the initial black hole to the final remnant.Despite their problems, physicists have explored
versions of both these scenarios. For example, in
2003 Samir Mathur put forward a proposal based on
string theory, which posits that fundamental parti-
cles are tiny strings. His idea is that a black hole
transforms into a “fuzzball,” a kind of massive rem-
nant, or that a fuzzball forms instead of a black hole
in the first place. Thanks to the complicated physics
of string theory and its allowance for more than the
traditional four dimensions of spacetime, fuzzballs
might have a complex higher-dimensional geome-
try; instead of the sharp traditional boundary of a
black hole at the event horizon, a fuzzball would
have a fuzzier and larger boundary where one en-
counters strings and higher-dimensional geometry.
Alternatively a more recent version of a remnant
scenario is the proposal that instead of a black hole
with an event horizon, a massive remnant forms with
a surface “firewall” of high-energy particles where
the horizon would be. This firewall would in cin er -
ate anything that encountered it, turning it into
pure energy that added to the firewall. Both the fire-
wall and the fuzzball, though, share the problem
of needing locality violation, and the resulting
objects would have other properties that are very
hard to explain.MODIFYING LOCALITY
a common thread in massive-remnant proposals is
that saving quantum mechanics appears to require
violation of the locality principle. But doing so care-
lessly is expected to be as disastrous as modifying
quantum me cha nics and, in fact, typically leads to
another paradox. Specifically, the laws of relativity
say that if you send a faster-than-light signal in emp-
ty, flat space, observers traveling past you at a high-
enough speed will see the signal going backward in
time. The paradox arises be cause this superluminal
signaling then allows you to send a message into
your past, for example, asking someone to kill your
grandmother before your mother is born.
Even though this kind of answer appears to con-
tradict fundamental physical principles, it is worth
a closer look. Modifying locality seems crazy, but we
have not found an alternative that does not. The
severe nature of the black hole crisis strongly sug-
gests a resolution via some subtle violation of the
locality principle, one that does not produce such
paradoxes. Put differently, quantum mechanics im -
plies information is never destroyed, so information
that falls into a black hole must ultimately escape,
possibly through some new, subtle “delocalization”
of information that might become clear when we
can finally find a way to unify quantum mechanics
and gravity—one of the most profound problems of
present-day physics. In fact, we have other reasons
to think such a subtlety could be present. The very
idea of localized information—that it can exist in
one place and not in another—is more delicate in© 2019 Scientific American © 2019 Scientific American