NYU Black Renaissance Noire NYU Black Renaissance Noire V. 16.1 | Page 18

PHOTOGRAPH COURTESY OF BILL UNRUH.

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Canadian theoretical
physicist Bill Unruh.

Black holes became a theoretical reality
with the Schwarzschild solution of
general relativity (that describes how
a star and a black hole warps space
and time), and they became a physical
possibility with the understanding of
stellar evolution. In 1958 (around the
same time that Leon Cooper found
his solutions to superconductivity),
one of my heroes in physics, David
Finkelstein, discovered something truly
remarkable to make the black hole
story more interesting yet.

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David Finkelstein is quiet, sage-like,
and beaming with genius as if the
entire cosmos were contained in his
head. So inspiring is he that it’s not
altogether surprising that pioneers
of the two main competing theories of
the unification of quantum mechanics
with gravity, Lee Smolin and Lenny
Susskind, were both mentored by
David. It is interesting that both of
them became pioneers in Loop
Quantum Gravity and String Theory
respectively. David was the chair of
Yeshiva University in New York City
in the 1960’s, when it had humble
beginnings. He suddenly left New
York for Tougaloo College in Jackson,
Mississippi, for three years to
“participate in the fight for human
rights for all, regardless of race” — to
help African Americans attain civil
rights. I became such a fan of David’s
that, in 2014, I hosted a symposium
at Dartmouth to celebrate his lifetime
achievement.

What David wanted to understand
was how a beam of light moved in
the warped space-time around a black
hole. After all, it was the observation
of the bending of light from a distant
star around our sun that confirmed
Einstein’s idea that gravity was, in fact,
the warping of space-time around a
massive object. But, as David found
out, the movement of light around a
black hole was even more bizarre.
By an ingenious reshuffling of the
equations that govern space-time,
David found that there was a spherical
bubble-like region surrounding the
singularity in Schwarzschild’s solution
such that if anything entered this
region, including light itself, it could
never escape. That’s why John Wheeler
coined the term black hole to describe
these things, in fact. If no light could
escape the Schwarzschild region
surrounding the singularity, you’d never
be able to see it. Anything entering this
region would essentially disappear into
blackness. What David had discovered
was a one-way invisible spherical surface,
which he called a horizon. It was a
horizon no one could see beyond, not
completely dissimilar to our visual
horizon into the universe’s past, making
its study all the more intriguing.

When David made his calculations,
black holes were still a subject of
sci-fi novels, a playground for the
imagination, but they were beginning
to be understood, as well. While some
physicists, like Lee Smolin, speculated
that black holes spawned baby
universes at their singularities, we also
learned that black holes can grow in
mass by consuming matter and that
they can radiate, due to quantum
effects near the event horizon of the
black hole. David’s work made the
study of black hole physics concrete.
The event horizon was a definitive,
albeit intangible, mathematical element
to work with, one that might even shed
light on the structure of our universe
and the ancient cosmic horizon.
To better understand how, we need
to look at sound — specifically, how
sound moves in water.
Canadian physicist Bill Unruh found
this brilliant analogy in terms of sound
that captures a great deal of the physics
of black holes. Bill is one of Canada’s
and the world’s most revered theoretical
physicists. I spent a half year at his
home institution, the University of
British Columbia in Vancouver, to
work on my PhD dissertation. Bill is a
big man with a full beard and usually
wears overalls. He has a tendency to
intimidate other physicists and is quick
to pounce on any inaccuracies that
may present themselves, but he was
always kind to me even when I said
dumb things. His mastery of finding
analogies for physics concepts spoke
loudly and clearly to me one day at the
University of British Columbia, when
he found a mistake in the first seminar
I ever gave and proceeded to suggest
a correction. A year later his proposal
worked impeccably.