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[♪ INTRO]
Even if you feel like you’re standing still on the surface of the Earth,
you never really are.
Our planet is spinning, and it’s orbiting the Sun.
But the entire solar system is also hurtling around the galaxy,
orbiting the supermassive black hole at its center.
And while most of us are thankful that we can’t feel these dizzying motions,
some scientists are hoping to use our galactic flight
to help detect some of the most elusive particles of all: dark matter.
Interstellar space is far from empty.
In fact, it has some particles that are potent, but invisible, like dark matter.
And although we haven’t seen it,
Earth might be experiencing some of it as headwinds.
You see, as a solar system, we’re whirling around the center of the galaxy
at a whopping 220 kilometers per second.
Adding all these motions together, the Earth is moving fastest
through the galaxy during June, ​when the planet’s orbital motion
aligns with the sun’s motion through the Milky Way.
Whereas in December, the Earth is moving ‘backward’
compared to the Milky Way’s spin, kind of like swimming against the current,
so our overall speed is at its lowest.
Luckily for us, most other things in the Milky Way
are also wheeling around the galaxy at a similar rate,
so we’re not in much danger of barrelling into another star or planet.
But although it might seem like a black void,
we might be plowing through a lot of dark matter.
Now, dark matter is thought to outweigh normal matter by about six to one.
But we’ve never managed to pinpoint the particles responsible for it.
Something powerful but invisible must be exerting a force
on the stars orbiting in galaxies, or else those galaxies would fly apart.
So it’s thought that there’s a shell or halo of dark matter that sits like a cloud
over the galaxy, providing the gravity needed to keep those stars in check.
If that’s correct, then as we orbit the Milky Way,
we are plowing through this dark matter slightly faster in June than in December.
And the Earth is experiencing a dark matter headwind as it travels.
Compared to the motion of the stars and planets, dark matter is pretty much static,
so the strength of the headwind we experience
depends on the speed of Earth at any given time.
Just like when you stick your hand out of a car when you’re driving along.
You feel a wind from all the air molecules your hand is hitting.
The faster the car, the faster the wind feels.
So as the relative speed of our planet changes through the year,
so too does the strength of the dark matter headwind.
Meaning that we should experience more of that headwind during June,
when we go the fastest, and less in December when we’re at our slowest.
And now, an experiment deep in the Italian Apennine mountains
claims to have detected this variable headwind signal,
with exciting implications for the detection of dark matter, too.
The experiment, named DAMA, has been running
at the Gran Sasso National Laboratory in Italy for about 20 years.
They use crystals of sodium iodide doped, or impregnated, with thallium.
These crystals are often used to detect radioactivity in lots of different
scientific fields, like medicine, geophysics, and nuclear physics.
So when any subatomic particle or strong electromagnetic radiation
strikes these crystals, they absorb that radiation and release the excess energy
as a photon, which we see as a flash of light.
The tricky part here is that Earth gets lots of radiation from things like the Sun
and space in general, so researchers had to prevent
all the sources of radioactivity from getting into the crystals
So you can imagine they’re heavily shielded.
The crystals are engulfed in a pure nitrogen atmosphere,
surrounded by thick layers of copper, paraffin, and concrete,
buried under nearly one and a half kilometers of overlying rock.
And even with all these protective layers, the crystal detectors still emit light.
A signal that rises and falls over the year, with more flashes in June,
and fewer seen in December. Kind of on schedule with the projected headwinds.
So if the shields can block out all known particle bombardments,
then the scientists conclude that the variable signal they are detecting
must be coming from something we have not yet seen and accounted for.
And the best candidate for that is the dark matter headwind.
After 20 years of measuring, the team is certain that their signals
are real and not random background noise.
But confirming the detection of dark matter isn’t so straightforward.
There’s a bit of controversy surrounding this result.
Many scientists are skeptical that the annual signal is really coming
from beyond the solar system.
Some suggest it could be something unaccounted for,
that changes with the seasons and not with the headwinds per se.
And other similar experiments have failed to find the annual variation at all.
But a new project on the other side of the world hopes to combine its results
with DAMA, to test the galactic headwind hypothesis once and for all.
It’s called SABRE, and it’s located at the bottom
of an active gold mine in southeastern Australia.
The experiment will use the same setup as DAMA,
and hopes to find the same annual variation that the Italians have detected.
But the timing of that rise and fall will be critical,
given that this other experiment is in the southern hemisphere.
If the detectors peak during the Australian summer, in December,
then it is likely that the annual variation is down to the changing seasons.
But if the detections peak in June, as it does in the Italian experiment,
then it is more likely to be caused by something beyond the Earth,
like the dark matter headwinds.
And if they don’t detect any annual variation at all…
well, it’ll be back to the drawing board!
Solving the mystery of dark matter is never going to be easy,
but these cutting-edge experiments are showing the power of thinking
outside the box, or at least outside the solar system!
Thanks for watching this episode of SciShow Space, if you want to learn more
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[♪ OUTRO]