4 July 2020 | New Scientist | 43lower than the microgram masses the new
detectors would be most sensitive to. The
higher range, known as the Planck scale,
would instead contain objects that are
extremely compact yet incredibly massive,
making them subject to both quantum
mechanics and general relativity. If new
particles and forces existed at this scale,
they might not just solve the dark matter
puzzle – they could provide crucial hints, too,
into how to unify these two theories.
The new detector would be agnostic as to
what exactly this dark matter would be. It
might be a collection of dark particles similar
to visible molecules, a bunch of black hole
remnants or a topological defect in space-
time. Or it could be a curled up cosmic string
as favoured by string theory – a prominent
attempt to combine quantum theory and
general relativity – or hypothetical extra-
large WIMPs created shortly after the big
bang, dubbed WIMPzillas.
Were such a detector to find anything, itwould be big news. “The first thing you do,
which earns you the Nobel prize, is you show
that dark matter is real,” says David Moore,
an experimental particle physicist at Yale
University. Even if a detector found nothing,
it would tell physicists that the mass range
it was looking at is empty of dark matter.
There are still a few hurdles to jump before
that becomes reality, however. For a start,
there’s how you distinguish the tiny shaking
of a bead from other, equivalently sized
gravitational effects – say an elevator
descending in a distant part of the lab
building. Such effects have been known
to confound previous attempts to measure
the fundamental strength of gravity.
There is a workaround. Earth is ploughing
through a thicket of dark matter as it,
and the entire solar system, rotates
around the centre of the Milky Way at some
800,000 kilometres per hour. Carney and his
colleagues showed that if you arranged many
of the new zeptonewton sensors in a regular
three-dimensional grid they would vibrate
in a coordinated manner when a dark matter
nugget passed. In a similar way, LIGO and
its European partner, VIRGO, require signals
from multiple detectors at the same time
to ensure their gravitational-waves are real.
Here’s the snag, however. Given the
calculated local density of dark matter,
an experiment that is a cubic metre in size
would need somewhere between a million
and a billion hypersensitive sensors spaced
between a centimetre and a millimetre apart
to uncover dark matter’s signature within a
year. With a single zeptonewton sensor
currently costing around $1 million, the price
would need to come down considerably for it
to be feasible. Then you would need ways to
skirt less worldly problems, such as the
stringent limits that the fuzzy laws of
quantum mechanics set on measurement
precision at such small scales.
Then again, these are just the sort of
problems that experimental physicists
love to overcome. “This is really the
opening shot in a longer conversation,”
says Gordan Krnjaic, a theorist at Fermi
National Accelerator Laboratory near Batavia,
Illinois, who is part of Carney’s team.
He and around 30 other physicists
gathered at the University of Maryland lastCA
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>“ This simple
idea is a world
away from
the complex
detectors we
rely on now”
LIGO’s concrete tunnels are
home to the most sensitive
detectors so far built