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on a standard laptop with no special equipment.
Chesler calls it “superb computing, rather than
supercomputing.”
THE INNER TRUTH
So what does this work — much of it described in a
Physical Review D paper — reveal about the innards
of a Kerr black hole? Like the non-rotating variety, a
Kerr black hole has a central singularity and an invis-
ible event horizon — a spherical surface of no return
from which matter and energy can never escape. But a
spinning black hole, unlike its stationary counterpart,
has an “inner horizon,” too — a second surface of no
return, also spherical and invisible, lying somewhere
between the event horizon and central singularity. The
faster the black hole spins, the closer the inner horizon
is to the event horizon.
To visualize the inner horizon, consider this
thought experiment: Imagine you could enter a spin-
ning black hole and cross the event horizon while
holding a flashlight that’s pointed outward. That light
would eventually freeze along, or within, a spherical
surface. That’s the inner horizon, Chesler explains: the
point where the black hole’s gravity is strong enough
to keep light from leaving, but not strong enough to
drag it all the way into the center. The trapped light
becomes so energized that it turns into a firewall of
radiation, incinerating anything that passes through it.
The inner horizon is important for another rea-
son, the BHI team concluded. If you were reckless or
unlucky enough to get pulled into a Kerr black hole,
“that’s where you’d die,” says Chesler. He compares
crossing the inner horizon to “closing your eyes and
driving into a brick wall.” Only in this case, it’s a wall
of extremely curved space-time, tremendous gravi-
tational forces and extreme radiation. For an intrepid
traveler, the inner horizon represents the end of the
line — a place you’d be instantly ripped apart and
flash-fried, with your ashes dispatched at nearly the
speed of light toward the central singularity. “It will
not end happily,” Chesler notes. But it will end quickly.
And what of the hypothetical wormholes and other
exotica that some researchers have speculated about?
Wormholes, Chesler explains, “would be sealed off by
the unboundedly large curvature of space-time inside
the black hole.” It’s as if our universe were a giant, elon-
gated balloon that is twisted until a knot, which is a
kind of singularity, forms in the middle. “If you lived
on one side of the knot, you couldn’t get through to the
other side to reach the other universe,” Chesler says.
In fact, there’d be no way of knowing whether another
universe, or anything else, exists beyond that point.
It effectively marks the end of geometry itself. Time
travel would be ruled out, too, in this increasingly
mundane picture of a black hole’s interior.
While that may disappoint sci-fi fans, the firewall
the BHI trio discovered is a dramatic fixture in its own
right — one that could deliver a slam-bang ending
to many a tale of cosmic exploration. Plus, it finally
provides an answer to the familiar question: What
happens if you fall into a black hole?
D
Steve Nadis, a contributing editor to Discover and
Astronomy, plays handball and volleyball in Cambridge,
Massachusetts, where he lives with his wife, two daughters
and an unruly dog. R
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OUT THERE
A spinning black
hole actually has two
types of horizons,
and it’s against the
inner that hapless
travelers would meet
their end.
The inner
horizon
represents
the end of
the line
— a place
you’d be
instantly
ripped
apart and
flash-fried.
Outer event horizon
Inner horizon
Black hole