54 Scientific American, December 2019EHT COLLABORATION AND EUROPEAN SOUTHERN OBSERVATORYsuggested that a mistake in the original analysis
implies information never fully enters a black hole but
instead leaves a kind of imprint in the form of what
they called “soft hair” outside it. Closer examination
seems to be closing this loophole, however, and most
experts do not believe this can be the answer. In short,
more radical steps appear to be needed.
An obvious idea is that there is some unknown
physics that prevents true black holes from existing
at all. The conventional picture of black hole forma-
tion says that when very large stars burn out and die,
their mass collapses under the force of gravity into a
black hole. But what if they never reach that stage
and actually transform into objects with “better” be -
hav ior? In fact, we know that when lower-mass stars
such as our sun burn out and collapse, they do notform black holes and instead form dense remnants—
for example, white dwarfs or neutron stars. Perhaps
some unknown laws of physics also prevent larger
stars from forming black holes and instead lead
them to become a kind of “massive remnant”—some-
thing more like a neutron star than a black hole.
The problem with this suggestion is that we can-
not explain what would stabilize such objects—no
known physics should prevent their continued col-
lapse under gravity, and any imagined physics that
did would apparently require superluminal signaling
from one side of the collapsing matter to the other. In
fact, conventional large black holes can form from
very low-density matter. To illustrate, if the 6.5-bil-
lion-solar-mass black hole in M87 arose from the col-
lapse of a dust cloud (which is theoretically possible,GRAVITY BENDS
light around an
apparent black
hole at the center
of the M87 galaxy
in this seminal
image from the
Event Horizon
Telescope.
© 2019 Scientific American