The High-Pressure Physics of Creating Metallic Hydrogen
Hydrogen, the most abundant element in the universe, is found as a gas in our atmosphere. In 1935, Eugene Wigner and Hillard Bell Huntington put out a theory that at high enough pressures, hydrogen could be converted into a metal. Since that time, scientists have chased the idea of creating metallic hydrogen. In a recent study appearing in Science, researchers reported that they used two diamonds to compress the element to pressures high enough that would produce metallic hydrogen. Physicist Isaac Silvera, one of the authors on that study, tells us how metallic hydrogen could be used to make superconductors and high-powered rocket propellant.
Isaac Silvera is a professor of physics at Harvard University in Cambridge, Massachusetts.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Hydrogen is the most abundant element in the universe. If you picture the periodic table– got that in your mind? You know it’s in the upper left corner, and it fits in that stair-step of nonmetals. But 80 years ago, a pair of researchers theorized that you could turn hydrogen, the gas, into a solid metal.
And since then, scientists have been trying all sorts of ways of creating metallic hydrogen. They’ve zapped it, fired lasers at it. They have squeezed it. And now– drum roll, please– a group claims to have created shiny metallic hydrogen by squishing it really, really hard between two diamonds. Their study was published last month in the journal Science.
So why are scientists so interested in turning hydrogen into a metal? What could you use it for? And can we really create it? Did they really create it? Isaac Silvera is an author on that study in Science. He’s also a professor of physics at Harvard University. Welcome to Science Friday.
ISAAC SILVERA: Thank you, Ira. Pleasure to be here.
IRA FLATOW: You’re very welcome. As a physicist, why, for so many years, are people interested in metallic hydrogen? What’s it for? What do you do with it? What can it tell us about physics that we don’t understand yet?
ISAAC SILVERA: Well, first of all, hydrogen is the most fundamental, simplest atom in the periodic table, as you mentioned. It’s got a single proton and an electron. And if you take these and put them together, they actually form a bond and form a molecule. But the hydrogen atom itself was studied over a century ago by Niels Bohr, and that brought in the ideas of quantum mechanics.
So it’s very, very simple and very fundamental. The molecule itself is a very fundamental, simple molecule with two protons and two electrons, and the electrons are between the protons and hold the protons together to form this very small molecule. If you now take that gas of hydrogen and cool it down to just below 14 degrees absolute– so absolute temperature scale is a scale that zero, you can’t get lower than zero, and I’m talking about Kelvin.
IRA FLATOW: Right, right.
ISAAC SILVERA: On the centigrade scale–
IRA FLATOW: So you cool it really down, what happens to it?
ISAAC SILVERA: It becomes a solid. First it’s a liquid, then it’s a solid. And then you take that, and you start compressing it. So when you start out at very low pressure, the two atoms in a molecule are very close together, and the molecules themselves are very, very far apart. They’re about six times farther apart than the atoms are in the molecule. And as you press it together, the molecules get closer and closer.
And finally they get so close that a proton in one molecule can’t decide, should I stay bonded to this other one, or should I bond with my neighbor? And they solve some kind of an equation. You know, they can do the calculation.
IRA FLATOW: Well, can you press it hard enough to make it– from what you’ve said, you’ve pressed it together hard enough between these two diamond anvils, sort of speak, that you’ve actually created metallic form?
ISAAC SILVERA: Yeah, we’ve pressed it to just under 5 million atmospheres of pressure, 5 million bars. A bar and an atmosphere is about the same. But what happens is when you get them really close, the molecules dissociate, and you form an atomic solid. And now the electrons are free to run through the system. They no longer hold the molecules together. And it’s a metal.
IRA FLATOW: Does it–
ISAAC SILVERA: And that’s what we’ve done.
IRA FLATOW: Does it have all the properties of a metal, what you’ve done?
ISAAC SILVERA: Well, so far it has. What we have measured to show that it’s a metal is you measure the reflection. It’s not enough to measure reflection of one wavelength or one color of light, but we measured from the green, blue, red, and into the infrared. And we find exactly the behavior that you would expect for a metal. And this is–
IRA FLATOW: Well, a metal is supposed to be a good conductor too. Have you run electricity through it to see that it’s a good conductor, those sorts of things?
ISAAC SILVERA: That’s right. The most rigorous test to show that a material is a metal is just to show that its electrical conductivity remains finite in the limit that the temperature goes to zero. And we’ve studied this at low temperature, at liquid nitrogen and liquid helium temperatures. That’s 80 degrees and about 5 degrees in temperature.
IRA FLATOW: Now there are some critics that point out that they don’t think you have proved it enough. And I understand you’re willing to send this out for a further testing. Is that right?
ISAAC SILVERA: Well, we’re not going to send it out. We’re going to take it.
IRA FLATOW: I wouldn’t trust it either.
ISAAC SILVERA: We’re going to do the measurement. We’re going to do the measurements ourselves. But it really is. It’s sufficient to show reflection. Reflection comes from high-frequency electromagnetic waves. And light is an electromagnetic wave. And that makes the electrons respond. They move back and forth, and they reflect the light.
So we’re actually measuring the very high-frequency electrical conductivity. But what you really would like to do is measure the DC conductivity, where you put a voltage on it and measure a current going through it.
IRA FLATOW: And so it’s still under pressure now, I understand?
ISAAC SILVERA: Yeah.
IRA FLATOW: What if you just open up the tank and let it go? Would it still stay there as a metal?
ISAAC SILVERA: So it’s not a tank. It’s a really small sample. It’s smaller than the cross-section of a hair on your head and about 1 micron– a micron’s about, there are 1,000 microns in a millimeter– about a micron thick, something like that. So it’s a small sample. And it’s been predicted– I don’t know if it’s true, and you have to show these things experimentally.
But it’s been predicted that hydrogen might be metastable. That means that if you get it up to the high pressure, it becomes a metal. And then you release the pressure, it will stay in the form of a metal. It will stay metallic. And if that happens, that would be terrific. That would be really–
IRA FLATOW: Who’s going to unscrew it from there and try it out? Are you going to do that?
ISAAC SILVERA: Myself and my post-doc, I have a terrific post-doc, Ranga Dias, and we’ve worked on this together. And we’ll do it together, and perhaps pray a little and maybe have something to settle our hands, a little drink to settle our hands as we turn it down. But–
IRA FLATOW: You’re confident. So you don’t know what’s going to happen when you open up the vice.
ISAAC SILVERA: No. So, you know, there are other materials that are metastable. And I’ll give you a well-known one, diamond. If you take carbon, graphite, and compress it up to about 50,000 atmospheres and heat it up, the carbon turns into diamond. You take the pressure and the temperature off, and you still have diamond. But if you take that diamond and put it in an oven and heat it up to about 1,500 degrees Kelvin, you’re going to have a carbon ring instead of a diamond ring.
IRA FLATOW: It’s going to go back to carbon.
ISAAC SILVERA: It’ll convert back to the lowest energy states, yes.
IRA FLATOW: I never saw Superman do that with the diamond.
ISAAC SILVERA: Yeah, but that’s what metastability is.
IRA FLATOW: So what is so great about if you could have metallic hydrogen? What are the wonderful things you could do with it?
ISAAC SILVERA: Well, the first thing that is interesting is that it has been predicted to be a high temperature, possibly room temperature or higher, superconductor. A superconductor is a metal that conducts electricity without dissipation. So we can test for that under these very high pressures. It’s a challenging experiment, and what we’ll do is put electrical leads into our sample– and that’s not going to happen this time because you have to do it before you pressurize the sample– and then measure its conductivity.
But imagine that it’s metastable. You take the pressure off, and it stays in the metallic phase. If you could now develop a technique to scale it up– that is make lots of it, and I’ve got ideas on how to do that– then you could make electrical wires out of it. And currently, just an example of an application is that in our electrical power grid, we lose perhaps 15% of the energy to dissipation in the wires.
The wires are copper, but they have resistance. And so you just dissipate, or you’re heating the wires when the electricity goes through it. But if they were superconducting wires, you would have no loss, no dissipation in the system. So that would be a great advent.
You could make superconducting magnets. Superconducting magnets are used, a well-known application is Magnetic Resonance Imaging, MRI. You could make superconducting magnets that don’t have to be cooled. The current ones have to be cooled with liquid helium, which is–
IRA FLATOW: It’s in short supply.
ISAAC SILVERA: We do it all the time, but it’s challenging.
IRA FLATOW: Yeah.
ISAAC SILVERA: You wouldn’t have to have helium, and you could have these magnets any place in the world, places where they didn’t have helium.
IRA FLATOW: So how long before you can scale this up and actually prove, I guess by scaling it up? Can anybody do this? Do you have a special technique that no one else can do?
ISAAC SILVERA: Well, we have gotten the highest pressure that anyone has ever gotten on hydrogen to convert it into a metal. And we used a lot of special processes and techniques that we’ve developed over the years, and it worked. We got to these super high pressures. Other people have been trying it, and I hear complaints and whining, our diamonds always break. Why aren’t yours?
Well, in our paper that’s published, we tell exactly what we’ve done to get these pressures. And people can copy that if they want. And I’ll even help them if they want to come over here to Cambridge, Mass. And we can get them into our clean room and get them access to the facilities that we use to create special diamonds.
IRA FLATOW: OK, well, we’re waiting to see when you actually start scaling this up. Send us a little piece of that liquid– solid hydrogen for us to look at,
ISAAC SILVERA: Where are you located?
IRA FLATOW: We’re in New York. We’re not too far.
ISAAC SILVERA: Oh, you’re in New York.
IRA FLATOW: We’re a little Amtrak ride, yeah. You can bring it right down here.
ISAAC SILVERA: Well, if you wanted to– come on over here, and we’ll show you metallic hydrogen.
IRA FLATOW: I was just in Cambridge–
ISAAC SILVERA: It’s going to be around for–
IRA FLATOW: I was just in Cambridge earlier in the week. If I had known, I would have stopped.
ISAAC SILVERA: Yeah.
IRA FLATOW: I would have stopped by. Of course I was at MIT and Harvard, you know MIT, Harvard. It would have been a little tough together. Thank you, Dr. Silvera. Good luck to you.
ISAAC SILVERA: Thank you very much, Ira.
IRA FLATOW: Isaac Silvera, author of that study in Science, professor of physics at Harvard University.