From River to River, Iowa Public Radio:
Gold: the stuff of treasure hunts, fairy tales, alchemy and wedding rings.
Gold is a metal, but, surprisingly, we know little about its origins. How is gold made? How did it get on earth? Why is gold more rare than other metals on Earth? Being the stuff of legends, these seem like questions we should have answered long ago.
It turns out Burlington-local Brian Metzger was recently able to bring some clarity to these long-standing questions.
Metzger studied math and physics at the University of Iowa and went on to receive his Ph.D. from the University of California at Berkeley. He was also a NASA Einstein Fellow and did his post-doctoral work at Princeton University. He is currently a professor of physics at Columbia University in New York City and a Senior Research Scientist at the Flat Iron Institute. In 2020, Metzger received the prestigious Blavatnik Award for his research into the origins of gold and other heavy metals in the universe.
River to River host Ben Kieffer recently spoke to Metzger about his career and how his theories on the origins of gold in the universe came to be.
Kieffer: “Okay. Let's get to the story of this mystery and how you theorized that gold was created in our universe and your prediction that was confirmed not too long ago. Where do we start with this story?”
Metzger: “Well, I think maybe we could start from the fact that we have a pretty good understanding where the lighter elements come from, the elements of life, even up to iron. We know that these are formed in massive stars and expelled into space when they explode, a supernova. But it turns out it's much harder to create elements that are much heavier than iron. What you need to do is you need to take an iron nucleus and you need to bombard it at a very high rate with neutrons. So, but as you may know or maybe don't know, neutrons are unstable. They actually decay in a vacuum away into protons in about 10 minutes.
"And so in order to create the heaviest elements, we need a site in nature where there's a very high abundance of neutrons. And this has led a lot of people to think about these objects called “neutron stars.” So neutron stars are the cores of stars that are left over — big stars, much larger than our sun. At the end of their life, when they explode, they burst these neutron stars. Neutron stars have about the mass of our sun, but confined to a size of about New York City, so 10 kilometers or so. And we think that these neutron stars may be critical to creating the conditions that give rise to these heavy elements and in particular, the collision between two of these neutron stars, which is a very rare event in our universe. The collision of these neutron stars has long been hypothesized to be a site where these heavy elements could be produced. And so the story really starts a long time ago, but it's come to a climax recently, in 2017. There was the first direct detection of these two colliding neutron stars, and we detected it in a very atypical way. Most of the time, we detect things by pointing our telescopes to them. In this case, the two colliding neutron stars were detected through their gravitational wave emission.
"So as these neutron stars get closer and closer together, these are very massive objects. They're strongly distorting the space and time around them, and they can create these ripples that, believe it or not, propagate out from the site where these stars are merging, and we can actually detect them now on Earth. There are these gravitational wave observatories. There are two in the United States, one in Washington state and one in Louisiana. And the name of the experiment is a Laser Interferometer Gravitational-Wave Observatory. And this legal observatory in 2017, for the first time, detected these colliding neutron stars, not with telescopes, but with their gravitational waves. But they immediately told the astronomers of the world, ‘OK. There was a merging neutron star system in that part of the sky right now. You should point your telescopes over there.’ And that's what a large fraction of our community did. There were many of the biggest telescopes in the world pointed towards the direction of this neutron star merger, and what they saw was a fading glow of light that lasted about a week. And it was in a galaxy that was actually fairly close, by astronomical terms, it was 130 million light years away. But essentially, what I did, was make predictions for the appearance of that light, like how bright it was, what color it was. And to connect the signal we saw to the formation of these heavy elements like gold, silver and platinum in the ejecta, the matter that was released into space when these neutron stars collided.”
“So tell me a little bit more about your prediction. You said the color of this, the specific area that you predicted, because I understand the color, even from that vast difference distance can you tell you what elements are being created.”
“Yeah. It turns out that the color of the stellar explosions that create some of the heaviest elements are much redder than ordinary stellar explosions. So one of the hints that this event produced these heavy elements was actually in the very red color, and it has to do with the very complex nature of these heavy elements and the way that they absorb ultraviolet light. So I made predictions about that.
“Maybe I should step back a little bit and explain why we see light at all from the production of these elements. The way you create gold is you take an iron nucleus, which is fairly easy to create in these explosions and you bombard with neutrons. And in this way you create a very heavy element, but it's radioactive, it's not a stable element. So you've created all of this radioactive waste during the early stages of this explosion, and now you're expelling this radioactive waste into space. And, as we know, radioactive waste decays. It actually releases a lot of energy. This is the basis for efficient energy production. And so the energy that was released by the radioactive decay of this newly, freshly formed gold, if you will, was what caused this event to glow. It was the light that we saw, the power source, if you will. So these colliding neutron stars basically create a bunch of radioactive heavy elements and disperse them into space. And I made predictions for how bright that event would be, how long it would last and also the specific colors. So how much optical light it would produce — a red light versus blue light, let's put it that way.”
“This event, this neutron star merger in August of 2017. Of course, to say that you witnessed it or measured it in real-time is a twist of time because this happened 130 million years ago, right? The light reaching us?”
Metzger: “It did. But it plays itself at the right speed. So it happened a long time ago, but we see it unfold at the same pace. But yeah, it's true.
“As I mentioned at the beginning, I'm a theoretical astrophysicist, so I make the models, and I don't go to the big telescopes and take that data myself. But I work with the observational astronomers who do that data and this was actually a worldwide secret LIGO [Laser Interferometer Gravitational-wave Observatory] for various reasons. It's quite secretive about its discoveries, but it was telling the astronomers who had to have this information immediately to be able to point their telescopes there. And so there were astronomers who were taking the data and seeing this kilonova fading, and it was starting to leak to me, and I started to be involved in some of those groups. And so it was really an amazing feeling when you see that this prediction you made in sort-of a vacuum of theory, and thinking about a very exotic event that we've never seen before, and then seeing it unfold, basically in real-time, over the course of a few days, was quite exciting, and people telling you it looks a lot like your predictions. At that point, this was the first time we had directly witnessed the synthesis of these very heaviest elements in the universe.
“So obviously, this neutron star merger did not is not going to put any gold on Earth, it's too far away. But if you think about our galaxy, before the Sun formed, there were many of these mergers that happened in our galaxy, maybe about 10,000 of them. They're very rare, but there was a lot of time before the Sun and the Earth formed in our galaxy. And so one of these mergers that happened, or many of them that happened, before our sun formed, and polluted our galaxy with these elements. And then when the Sun formed condensed out of the gas in our galaxy, it had these elements in it; gold, silver, platinum. They made it onto Earth because they started in one of these merging neutron stars.”
So anyone listening, looking at a gold wedding band, … all of that gold that we have on Earth right now was created through the merger of neutron stars.
“We think most of it or a good chunk of it. I wouldn't say — we have one event, and so it's a little hard to extrapolate that to say, with confidence, to say all of it. But I think this event shows us that these are, that it's highly likely a good fraction of our gold and platinum came from these types of events. So the way I describe it to my wife is: my wedding band, which has some platinum in it, I'm fairly confident that this was, at one point, very close to falling into a black hole, the material. Because after these neutron stars merged, the matter that doesn't get expelled, it doesn't create the gold, ends up going into a black hole.”
“I understand you also met the great theoretical physicist Stephen Hawking before his death in 2018.”
“I did. Yeah, he was launching this initiative called Project Starshot with Yuri Milner, a philanthropist, and he happened to be in New York. And so the president of Columbia hosted him at his house. And so I had a chance to meet him for the first time, and, in addition to these textbooks my mother gave me, I had read "A Brief History Of Time," when I was in middle school. I think many physicists and astrophysicists in my generation were inspired by Hawking. And so even though we're both professional scientists, it was a bit of a fanboy moment, me getting a chance to chat with him and talk to him. And it took him a few minutes to actually respond, to give you a response. But it was great to see him, and we miss him dearly.
“I interview scientists of all types on this program. I love, especially, to talk to astrophysicists, and I like to ask this question. So I'll ask this of you: I am a genie. When I pop out of my bottle, I answer one question for you about your field of astrophysics. What question, unanswered question would you have me answer?”
“Wow, that's a tough one. I'll stay specific to my field. I want to know if these neutron star mergers are the only source of gold in the universe, and are there other events that produce these heavy elements? Are there other ways that we may form black holes that could produce these elements? And can we collect evidence for that over the next few years? Will I know this answer in my lifetime?”
You can hear Metzger's full conversation with Kieffer here.