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In the bottom of a Sardinian mine,
in the middle of the Mediterranean,
physicists are building an elaborate experiment
to find the weight of…
absolutely nothing.
Now, that might sound strange,
but they’re trying to get to the bottom
of what’s unofficially called
the Worst Prediction In The History Of Physics.
See, our universe is not only expanding,
that expansion is also accelerating.
And scientists aren’t really sure why.
Because right now, theory and observation don’t quite match up.
And when I say “quite,”
I mean one theory has predicted an expansion that’s 10 to the 120th power too intense.
So scientists need to weigh the vacuum of space
to find out why our universe hasn’t ripped itself apart yet, and which, if any,
of our theories are to blame for the colossal mismatch.
[Intro music]
When Einstein first published his equations of general relativity in 1915,
he inadvertently predicted that the universe
was contracting under the effects of gravity.
But at the time, physicists were pretty sure that it wasn’t.
Including Einstein himself.
So a couple of years later,
he tweaked his equations
by adding a fudge factor to get the result he wanted.
It included a constant that, as long as it wasn’t equal to zero,
balanced everything out and made the cosmos stand still.
And because scientists are great at naming things,
it became known as the cosmological constant.
This was all well and good until 1929,
when Edwin Hubble noticed that galaxies
were moving away from us all across the sky.
The only reasonable explanation for
this was that the universe was expanding
like a loaf of bread in an oven.
Physics no longer needed a fudge factor
to maintain a static universe,
and Einstein famously referred to it as his “biggest blunder.”
But decades after his death,
we learned he wasn’t as wrong as he thought he was.
In the late 1990s, scientists found that all those galaxies
were not only moving away from us,
most were doing it faster than they should.
The universe wasn’t just expanding,
but that expansion was accelerating.
Physicists called the source of this phenomenon
Dark Energy.
And based on how fast distant galaxies were flying away,
they calculated that dark energy must make up
about 68 percent of all the stuff in the universe.
But despite being so abundant,
it’s still not clear what dark energy is.
That’s where the cosmological constant comes back into our story.
And it’s also where we get into the baffling physics of nothing.
In the equations for general relativity,
the cosmological constant
is represented by the Greek letter lambda.
And it tells us there’s an inherent energy to space itself.
Each cubic meter of space
has the exact same amount of this repulsive energy.
And it doesn’t dilute as the universe expands.
So the more space you have,
the more repulsion you have,
explaining how as the universe gets bigger and bigger,
the expansion of the universe gets faster and faster.
While Einstein didn’t know exactly
how each bit of seemingly empty space
could come equipped with an innate amount of energy
there was a new field of physics that could step in.
Yep, that’s right, we’re going quantum.
Quantum mechanics describes the physics of the subatomic,
both the particles you’re familiar with,
like protons, electrons, and even massless photons …
but it also governs the behavior of so-called virtual particles.
Virtual particles may or may not physically exist,
depending on the physicist you ask.
But at the very least,
they exist inside physics equations
so that we can accurately describe our observations.
And all types of particles,
whether they’re quote-unquote real or virtual,
abide by the principle of uncertainty.
The Heisenberg uncertainty principle
tells us that there’s a limit to how much
we can know about the quantum world.
Like, you may have heard that we can never know both
exactly where a particle is
and precisely where it’s going at the exact same time.
But the same rule applies
to measurements of time and energy, too.
The more precisely we pinpoint a moment in time,
the less sure we can be about
how much energy is present in that moment.
So at any given time and place in the universe,
there’s a statistical possibility that there will be a little bit of energy
that could just as easily disappear in the next moment.
And this phenomenon has a name.
These are known as vacuum fluctuations.
And because this energy is carried by virtual particles,
these fluctuations are often described
as virtual particles instantaneously
popping into and out of existence all over the universe.
Which means even a vacuum,
which is the very definition of nothing,
isn’t actually empty.
It’s filled with an astounding number
of short-lived virtual particles
and the energy they carry, giving rise
to what’s known as vacuum energy.
So we know there’s this thing called dark energy out there
that is causing the universe’s expansion to speed up over time.
We also know that vacuum energy is out there.
And if it’s the same
as the cosmological constant that Einstein imagined,
it’s capable of causing the universe to expand.
So does that mean that dark energy
is literally just vacuum energy?
that would be the simplest solution,
as well as a solution that combines
two of the major paradigms in physics today.
General relativity covers the physics
of the extremely large,
and quantum mechanics
covers the extremely small.
Unfortunately, there’s a pretty big hitch.
Because Einstein taught us a few other important things about reality.
First, his famous equation
E equals M C squared
tells us that energy and mass are equivalent.
That means that any amount of vacuum energy
has an equivalent amount of mass.
And second,
one of the fundamental tenets of general relativity
is that mass distorts the fabric of spacetime.
So if vacuum energy really exists,
it has to leave a mark on the structure of our universe.
An accelerated expansion is one such mark,
but the amount of acceleration
we can actually observe and measure
doesn’t come anywhere close
to what quantum models predict.
It falls short by as many as 120 orders of magnitude.
That’s a one followed by one hundred twenty zeroes.
And even in more sophisticated modeling,
that number of zeros is only cut in, like, half.
This extreme discrepancy
was actually a problem first noted back in the 1920s
by the physicist Wolfgang Pauli,
when the science of vacuum energy was still in its infancy.
He calculated that vacuum energy,
if it existed, it would have already made the universe expand so much that,
given the age of the universe,
light wouldn’t have had enough time to traverse the distance
between the Earth and the Moon.
From humanity’s perspective,
we would be stranded in an observable universe
without any other stars,
without galaxies without other planets,
without the Sun, without the moon.
All those things would still be there
but we wouldn’t be able to see them
because the light would have to travel so far to get to us
At the time, Pauli’s thought experiment wasn’t taken too seriously,
but once vacuum energy came back into the spotlight
as a dark energy candidate,
the discrepancy became a big issue.
And it’s a clear signal
that our laws of physics aren’t quite right,
in some unknown way.
Now, though, scientists are hoping
that an upcoming experiment
at the bottom of an Italian mine will shed some light.
It’s called the Archimedes Experiment,
and to fix the worst prediction in the history of physics,
it needs to measure the weight of Nothing…
…or, more accurately,
the virtual particles that fill the vacuum of space.
Right now, the laws of physics assume
that virtual particles interact with gravity
the same way as “real” matter does,
giving them a certain,
predictable weight.
But like, maybe that’s not true.
After all, scientists used to assume
the universe wasn’t expanding,
and look what happened there.
While it’s not possible to trap
or directly measure virtual particles,
we can see the effects they have on certain objects.
And the Archimedes experiment
is going to try and measure one of these:
the Casimir effect.
It’s super subtle, but here’s what happens.
If you hang two plates made out of a special material
inside a vacuum chamber,
the plates can act kind of like mirrors
and reflect the virtual particles
that randomly appear between them.
But we’re still investigating a quantum world, here.
And on quantum scales,
particles don’t really act like solid little balls.
They also act like waves,
with peaks and troughs spaced according
to how much energy they have.
The more energetic the virtual particle,
the more bunched up its wave structure, and vice versa.
Normally the virtual particles
that spontaneously appear in a vacuum
can have pretty much any energy, and therefore any wavelength.
But between the plates,
the wave structures will bounce back and forth
and interfere with one-another.
And when the troughs of one wave overlap with the peaks of another,
they cancel each other out.
And a canceled out wave basically means
a canceled out virtual particle.*
So this interference means
there will be fewer virtual particles
between the plates than there are outside of them.
And that difference pushes the plates inwards by a tiny bit.
Demonstrating this effect
helped to prove the existence of vacuum energy back in 1997,
but how can it help us weigh the vacuum?
Well, that’s where this new experiment gets its name.
Because Archimedes…
that polymath from ancient Greece…
had something to say about weight
in relation to the density of particles,
and how well things float. Have you ever tried
holding an inflatable pool toy under the water?
Well, in order for it to exist
in that particular patch of space,
it has to displace a volume of water
that’s equal to its volume.
And that volume of water has a weight to it.
According to Archimedes’s principle of buoyancy,
the weight of this displaced water
is equal in strength to the buoyant force
that’s trying to push the toy towards the surface.
Since the toy is filled with air,
it has way fewer particles packed inside it.
So the buoyant force is greater than the toy’s weight.
Which means it floats. And if you had the right equipment,
you could measure how much holding that toy
under the water
would make you float a little higher than you would without it.
That’s kind of what this Archimedes Experiment
is going to do with the Casimir Effect.
Inside the vacuum chamber,
the team will hang two disks,
each made from a different material,
on either side of a balance beam.
Then, they’ll swap between
warming the disks up and cooling them down
by a few degrees Celsius.
Because they’re not made of the same stuff,
the disks won’t respond the same way.
Only one will switch between
being an electrical insulator and a conductor.
And while that disk is a conductor,
the Casimir Effect should activate inside it.
That means there should be fewer virtual particles
on that side of the balance beam,
and it should tilt upwards by a tiny amount.
As the disk switches between conductor and insulator,
and the Casimir Effect turns on and off, the beam should seesaw back and forth.
And That periodic movement
is what the Archimedes team will be looking for.
that’s a lot of “shoulds”.
And if the beam doesn’t appear to move at all,
it could mean a few things.
For one, the experiment might not work.
The team isn’t 100% sure that the Casimir effect
works on the stuff they made their flip-flopping disk out of.
But assuming that it does,
the experiment might not be sensitive enough.
In terms of scale,
it’s like trying to weigh the DNA inside a cell,
so the slightest amount of jiggling
would bury a signal caused by the Casimir Effect.
That’s why Archimedes is going to be housed
deep undergrounåd inside an abandoned Sardinian mine.
The island is one of the tectonically quietest places in Europe.
But on top of that, molecules and atoms jiggle,
and the warmer they are,
the more they move.
So the whole setup is also going to be cooled
to -180 degrees Celsius.
Hopefully that will be enough to catch the Casimir effect in action.
And then, if the team still can’t observe any movement,
it might mean that virtual particles don’t weigh anything.
The Archimedes experiment
is still under construction,
and it’ll be a few years before we can expect to see any results.
Certainly, weighing nothing
is going to be a big challenge for experimental physics,
but the implications of the results could be huge.
If virtual particles end up not weighing
what we think they should,
it means they don’t interact with gravity like normal matter does.
Which again, is what scientists currently assume is what’s happening.
And it means that all the virtual particles populating the universe…
all the vacuum energy out there…
doesn’t bend spacetime
as much as our laws of physics predict they should.
Which is exactly what we need to happen
if all that dark energy out there is just vacuum energy.
No more, no less.
That said,
such a revelation would also open up the question of
why virtual particles behave so strangely.
And Archimedes definitely won’t be able to answer that on its own.
On the other hand,
the experiment might produce a weight that matches our predictions,
leaving the worst prediction in the history of physics solidly intact.
General relativity and quantum mechanics
will continue to not play nice together.
And one or both of these theories might need tweaking.
Or scientists could throw something else into the mix.
For example, one physicist has suggested that spacetime is actually a kind of
foam with a constantly changing structure, at least on a tiny scale.
Whatever ends up being true,
it’ll be a radical update to our understanding of physics.
But hey, it’s the entire universe we’re trying to explore, here.
If we need to find the weight of nothing in order to do it,
I say go right ahead.
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