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JANUARY/FEBRUARY 2020. DISCOVER 59i
You never forget your first, they say, but it appeared for a long time as if the
universe had forgotten its first molecule.
Known as a helium hydride ion (HeH+), this conglomeration of Big Bang
leftovers is just a helium atom and a hydrogen nucleus, aka a proton. Scientists
had expected to find it throughout the cosmos, but for decades they couldn’t spot
it anywhere. (Researchers managed to create some in 1925, so they knew at least
it could exist.)
Finally, in Nature this April, an international team of astronomers described how
they used the flying observatory SOFIA to detect HeH+ molecules within a gas
cloud known as planetary nebula NGC 7027, about 2,900 light-years from Earth.
“The chemistry of the universe began with this ion,” the study’s authors wrote.
“The unambiguous detection reported here brings a decades-long search to a
happy ending at last.”Our Universe’s
Forgotten First
Molecule
BY BILL ANDREWS29
Sensitivity is special. Other ways
to gauge complexity of Boolean func-
tions exist, but they’re all known to be
related to each other mathematically.
Sensitivity, though, has always been
an outlier. The sensitivity conjecture,
basically, describes how this measure
could fit into the mathematical family.
It made sense t hat it shou ld be i ncluded ,
but actually proving how it belonged was
a trickier matter.
Huang says the problem’s deceptive
simplicity first piqued his interest in- “Every time I decided to pick it
up again, I would spend three or four
days and go nowhere,” he says. “That’s
my approach to a lot of problems.” He
thinks he spent hundreds of hours on it
over the years.
Huang’s breakthrough came last June
on a warm night in Madrid, where he was
holed up in a hotel room with a noisy
air conditioner. He should have been
finishing a tortuous grant application,
but instead, he found himself think-
ing again about sensitivity. Like other
mathematicians before him, he thought
the most natural way to work with the
binary inputs of Boolean functions was
to treat them as coordinate points, the
corners of an imaginary kind of high-
dimensional cube. Twenty-seven years
ago, a mathematician and a computer
scientist showed that if you take a set of
at least half of these points and could find
connections between them, you could
then prove the sensitivity conjecture.
And that’s what Huang did: He used
tools from the field of linear algebra to
prove that 1992 statement. Afterward, he
wrote up his work, posted it online, and
experts marveled equally at his proof ’s
unequivocal argument and its compact,
elegant structure.
Huang isn’t surprised it took a math-
ematician to crack the computer science
problem. “Theoretical computer science
is abstract mathematics,” he says, and
this problem shows the connection.
“Computer scientists often need problems
to have applications, but for us math-
ematicians, we care about elegance, and
whether the problem can be stated nicely.”