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You can get a $100 60-day credit on a new Linode account at linode.com/scishow. 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. Thanks for watching this SciShow video, which was supported by Linode!

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