[ intro ]
One of the big quests of modern science is the search for dark matter.
It’s a kind of particle that fills the universe,
outweighing regular matter more than five to one.
Except… we don’t really have any clue what it is or how it works.
We know it’s there because astronomers can directly see
and measure things like its gravitational pull.
But that’s basically where our knowledge ends.
And a big part of that is because
we’ve never detected even a single dark matter particle up close.
Now, though, physicists have proposed a way we might do it.
The proposal appeared last week in the journal Physical Review D,
and if you want to give it a go, all you need are a few millimeter-sized pendulums or metal
Well, actually, more than a few—
you’re gonna need about a billion of them.
What makes the search for dark matter so challenging is that
dark matter doesn’t work like the ordinary matter we’re used to.
Like ordinary matter, dark matter has a gravitational pull.
But unlike the regular stuff, it doesn’t interact with light
or any other aspect of electromagnetism.
And that’s a huge pain,
because instruments like telescopes, microscopes,
and mass spectrometers all rely on electromagnetism to work.
So there’s no good, conventional way to try to study dark matter.
Instead, scientists have traditionally relied on experiments
with a proposed type of dark matter particle called a WIMP,
or a weakly-interacting massive particle.
WIMPs are too light for us to detect their gravity.
So in these tests,
researchers wait for one to run smack into the nucleus of a regular atom.
That would cause the atom to emit a bit of light that detectors can pick up.
Except... even after decades,
no experiment like this has ever conclusively picked up a WIMP’s signal.
Physicists have long favored these particles
as the source of dark matter because they’re strongly supported by theoretical research.
But that doesn’t mean they definitely exist…
or that they’d interact with regular atoms in this way.
And if they don’t, none of these experiments will be able to find them.
But this new paper suggests another option.
Their proposed test would search for a different category of potential dark matter particles,
ones with a mass equal to around half a grain of salt.
That’s at least a billion, billion times more massive than the WIMPs
we’ve been looking for so far.
And it’s so heavy that each of these particles exerts a detectable amount of gravity.
Which is great, because gravity is the only force we’re certain dark matter interacts
To detect one of these particles, the authors propose two possible techniques.
The first uses a three-dimensional grid of tiny, dangling force sensors.
As a dark matter particle passes through the array,
its gravity would tug a little on every sensor it passed, creating a trail of small disturbances.
Their second idea is to levitate tiny spheres
or beads using a magnetic field or laser.
When the field or laser is turned off,
Earth’s gravity would cause the pieces to fall.
And if a dark matter particle happened to be wandering by,
the force of its gravity would disturb the spheres’ trajectories ever so slightly,
and they’d land in slightly different spots than expected.
Making either approach work, though,
will require something like a billion sensors packed into about a cubic meter.
That’s because ordinary protons and neutrons
could easily whack into a single sensor and trigger it.
In fact, this would probably happen a lot.
But if you had a billion sensors,
it would be more obvious
when an enormous dark matter particle plowed through and triggered a bunch of them sequentially.
Now, this proposal could seem like a longshot,
but it’s not that different from something we’re already doing.
Something that’s been really successful lately:
searching for gravitational waves.
These are ripples in spacetime created by massive, accelerating objects —
like, black holes interacting with each other.
And by the time they reach Earth, they’re tiny, even smaller than protons.
So, to find them, we built huge, ultra-sensitive detectors,
then just sat around and waited for a gravitational wave to swing by.
And in a way, this dark matter detector is a lot like that.
In fact, the force sensors these authors propose using
are basically much-smaller versions of the tools
used by gravitational wave observatories like LIGO!
So we have the knowledge on a large scale,
and the rise of smartphones has also led to a major increase in our ability to manufacture
Virtually all of the 1.5 billion smartphones sold last year had an accelerometer onboard
to detect when you rotate the screen.
And accelerometers are just tiny force sensors.
They’re probably not sensitive enough for detecting dark matter,
but they show that the mass manufacturing is both practical and economical.
So, there’s no doubt that all this would be a huge undertaking.
And so far, nobody has signed up to actually build these designs.
But we have the capabilities.
And given that there’s around five times more dark matter out there than ordinary matter,
this proposal is a really exciting start.
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