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Scientists have had to come up with some extreme ways to generate the extreme pressures needed to simulate the conditions at the cores of planets!

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It’s not just a surprisingly common topic for song lyrics for the last several centuries, scientists have been fascinated by what happens to materials when they’re under pressure.

The hope is that with these experiments, we can model the chemistry of places we’ll never be able to visit, like deep inside the Earth, or Saturn, or the Sun. And with the help of some diamonds, they’re already able to push some things to galactic extremes.

By definition, pressure is the amount of force exerted over a given area. So there are two basic ways to increase that pressure: increase the amount of force you’re using, or decrease the area. As all good Jedi padawans know, generating more force isn’t always simple.

Which means the easiest way to increase pressure is to decrease the area on which the force is acting. That’s why shark teeth have points. One of the most common ways scientists do this is by using a Diamond Anvil Cell, or DAC, so named because two flawless, cut diamonds are used as anvils to squeeze a sample, which exerts a lot of force over a tiny area.

The area doing the pressing is a teeny facet at the bottom of each diamond call the culet. It can be as small as 50 microns, or millionths of a meter across. Whatever scientists want to study is sandwiched between two of these culets, suspended in a bit of fluid.

Then, they push the diamonds together. Really hard. Pressures of standard DACs can get into the hundreds of gigapascals that’s millions of times more than the pressure that you feel standing on the ground, and equivalent to what we think is happening in the Earth’s core.

Where, to be clear, you would not be able to stand. Because the pressure would crush you into molecular mush. There’s also a special class of DAC called double-DAC, which takes each diamond tip and attaches an even smaller, specially-grown hemisphere of diamond — only 10-20 microns across.

Then the super-mini diamond does the pushing, confining the force to an even smaller area. A double-DAC developed in 2015 is capable of generating 1000 gigapascals, or 1 terapascal, of pressure enough to begin to explore the physics of more massive planets. Astronomers think that’s roughly how strong the pressure is in Saturn’s core, for example.

One main reason we use diamonds is that they’re the hardest substance readily available. But it’s also nice that they happen to be transparent to a wide range of electromagnetic wavelengths, because that allows scientists to X-ray the sample while it’s being squashed so they can see what’s happening. And these DACs provide researchers with essentially unlimited time to do that data collection, because they exert static pressure that is, a fixed, continuous pressure on their samples.

But to really get at what’s happening elsewhere in the universe, you need a lot more pressure. And that means turning to dynamic experiments, which allow for much greater pressures, but less time to see what happens. There are dynamic DACs that can quickly vary the amount of pressure delivered over time, but the diamonds themselves can only handle so much pushing before they shatter.

So when scientists want to generate pressures too strong for the diamonds to withstand, they use lasers to vaporize them instead. You know, as you do. The National Ignition Facility in Livermore, California is equipped with almost 200 super-powerful laser beams.

And researchers have used those lasers to pump so much energy into diamonds that they start to vaporize. That creates an incredibly strong wave of pressure known as a ramp compression wave, without producing too much heat. This whole vaporization process takes about ten billionths of a second, which is actually relatively slow and gentle as far as the diamond is concerned.

The increase in pressure is gradual enough that it won’t necessarily destroy the sample, and you can get up to 5 terapascals of pressure this way. That’s on par with the center of Jupiter, or even bigger super planets that exist in distant star systems. Still, 5 terapascals is nothing compared to the kind of pressure a shock wave can generate.

If the researchers are willing to sacrifice their diamonds and maybe even their sample, they can generate twenty times that much pressure using a device that looks a lot like a normal diamond anvil cell, but with a twist. Instead of just pressing things together, you point a bunch of intense laser beams at one of the diamond anvils, heating it up kind of like how focusing light with a magnifying glass can light things on fire. The light heats the diamond so much so quickly in about a trillionth of a second that the outer layer vaporizes instantly.

The rapid expansion of the carbon atoms creates a one-time shock wave that travels into the sample faster than the speed of sound, vaporizing both it and the other diamond anvil it’s pressed against and then I assume everybody in the lab just jumps and down and are like, “yes!!” Why? I don’t know. We did that!

Theoretically, if the sample is already compressed at the time, the pressure on it could be as much as 100 terapascals. That’s a billion times Earth's atmospheric pressure. But if you really need as much pressure as you can muster to get closer to what’s happening inside of stars, for example you’re gonna need to use the shockwave from something a little more destructive.

Or a lot more destructive. Yeah, I’m talking about nukes. Back during the Cold War, underground nuclear explosions were used to study the compression of metals.

The shockwaves those explosions created regularly reached up to pressures of 20 terapascals, and some Soviet scientists reportedly reached a record of 700 terapascals of pressure in 1983. Granted, we try to stay away from the whole setting-off-nuclear-explosions kind of thing these days, so we’re probably not going to replicate those experiments any time soon. Still, all these high pressure experiments can tell us a lot about what’s going on inside planets in our own galaxy by creating some super weird things.

In February 2018, for example, researchers vaporized diamond to create what they think is superionic ice the stuff astronomers predict is in Neptune and Uranus’s cores. That’s ice that’s compressed so much that it becomes a lattice of oxygens surrounded by flowing hydrogens which behaves like a solid and a liquid at the same time. And that’s just the tip of the superionic iceberg.

While physicists have some ideas about will happen at really high pressures, they won’t know for sure if they’re right until engineers find a way to push things to the extreme. So they’re just going to have to keep pushing things, and pushing them real good. Thank you for watching this episode of SciShow!

To learn more about the results of these nifty machines, you might want to watch our episode on matter under extreme pressures. And thank you for watching and thank you for subscribing, and if you haven’t subscribed, then now you feel bad that I thanked you and you’re not a subscriber so you have to do it now. Just, subscribe.

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