4 July 2020 | New Scientist | 41R
IGHT now, roughly half a proton’s
worth of dark matter is passing
through your thumb. That points
to the really exasperating thing about this
mysterious stuff. Astronomical observations
tell us that the gravity of something huge
and unseen is tugging on stars and galaxies,
making them whirl faster than they
otherwise would. They suggest there is an
awful lot of whatever it is – a whopping five
times more than ordinary matter. And yes,
we can even calculate how much of it should
be passing right through us here and now.
Yet open the box, and there’s nothing
there. A no-show of dark matter in
experiments designed to detect it on Earth
increasingly has physicists questioning every
notion they have about it. Now, a group of
them is proposing we cut to the chase by
focusing on the one thing we can say for
certain about dark matter: that it interacts
through gravity. We can use that, they say, to
detect its ghostly gravitational influence as it
passes through us and everything else.
Well, possibly. Gravity is such a ridiculously
weak force that detecting dark matter’s
touch directly would involve connecting
together as many as a head-spinning billion
mini-sensors, each one more sensitive to tiny
disturbances than the huge set-ups used to
spot gravitational waves. Then again, that
worked, so why not this? “It’s new, it’s exciting,it’s coming out of left field,” says Rafael Lang,
a particle physicist at Purdue University in
West Lafayette, Indiana. “We all agree that
it’s a bit crazy, but I think everybody will give
you a different idea of how crazy it is.”
It might seem ridiculous to say gravity
is a weak force, given the calamitous effects
it has if, say, you step out of an aeroplane in
mid-flight without a parachute. That’s true
enough – but equally, you buck the gravity
of an entire planet with ease every time
you pick up your phone. To put a physicist’s
measure on that, the gravitational attraction
between two protons is roughly 40 orders
of magnitude, or 10 thousand billion billion
billion billion times, weaker than their
electrostatic repulsion.
Gravity’s comparative feebleness is one
of its many mysteries. Another is why it is the
only one of the four fundamental forces of
nature that can’t be described using quantum
mechanics. Physicists have searched for
decades for an overarching framework that
might combine quantum theory with Albert
Einstein’s space and time-warping general
theory of relativity, which describes gravity,
but they have had little success.
This was the ballpark that theoretical
physicist Daniel Carney at the University of
Maryland and his colleagues were playing on
in 2017. They were considering how to design
PA an experiment to search for the graviton, >
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Features
Seeing
the dark
Could a billion-sensor array detect dark
matter all around us – through its gravity?
Adam Mann weighs up an audacious proposal