Exploring the Frontiers of Science: A Conversation with Leo Piilonen

I’ve interviewed hundreds of scientists and engineers. Typically there are no surprises. The scientists are happy to share what they know, more or less by rote. Every once in a while, you meet someone who has the ability to explain complex ideas in a language the rest of us can comprehend, and isn’t afraid to explore the boundaries beyond what we think we know. Enter Leo Piilonen, head of the Physics Department at Virginia Tech, and the physics teacher I wish I’d had in school. His open attitude and sense of humor catalyzed a line of questioning that surprised both of us. With Leo as a guide, I was reminded of one of Einstein’s precepts: that even the most complex ideas in science can be explained simply. But an interview is also a clue to, perhaps even a road map of, how someone thinks. The journey, the line of thought, can be as compelling as the subject itself. With Leo, it was a mind-boggling tour from the outer reaches of the universe to what the Next Big Thing might be in materials science. Along the way, there were unexpected connections and Aha! moments, the hallmarks of exploring any frontier.

—Jim Metzner

Leo Piilonen

Leo Piilonen

Jim Metzner: What are some of the compelling ideas appearing in science—from your perspective?
Leo Piilonen: From the perspective of particle physics, the most interesting things are our understanding the mass of particles, and understanding the overall content of the universe.

JM: Why is this important?
LP: It’s important because it helps us understand how the universe is put to­gether. We know that the universe is put together in a certain way, but we don’t know why it’s put together in that way.

So, one of the interesting things is the mass of the content of the universe, and that’s where the Higgs boson comes in. The other is the overall content of the universe. Normal matter makes up about five percent of the content of the universe, and then there are these very mysterious things that have only come to light—to our perception in the past decade—the so-called dark matter and, even more mysterious, dark energy.

The Standard Model, revealing the Higgs field’s central role in elementary particles

The Standard Model,
revealing the Higgs
field’s central role in
elementary particles

JM: Let’s talk about the Higgs boson.
LP: The Higgs boson was a particle that was discovered in 2012. The Higgs boson is the face of the Higgs field that gives rise to the mass of all the other normal matter that we have in the universe—the electron, the proton, the neutron, and so on.

JM: Can you say what this particle is and how it does what we think it does?
LP: This particle is the manifestation of a field that permeates all of space. The particle itself—the Higgs boson—doesn’t give rise to mass. The Higgs field that it’s a manifestation of is sort of like molasses throughout all of space. It fills space, and it impedes the passage of all particles through this “molasses”—this Higgs field. The heavier a particle is, that’s a measure of how much it’s impeded in its motions through the molasses. And so that’s a sign that it interacts more often than a lighter particle. So, for instance, a proton moving through space moves more slowly than an electron would of the same energy because the proton’s bouncing and colliding with this Higgs field more often.

JM: And there are gazillions of Higgs bosons in this room right now?
LP: They’re coming and going throughout this room, but the boson itself is hard to capture. The field itself is everywhere in the room, of course, and it’s hard to put your hand on the field. The only way that you sense the presence of the field is as stuff moves through it and it interacts with it and is, in some sense, slowed down by it.

JM: What led to the discovery of this?
LP: The reason is that there was a mystery as to why any particle—electrons or protons and so on—would have any mass at all. By all rights, all particles should have no mass at all, but something was causing them to behave like they carried mass. And that something was, according to Peter Higgs and his contemporaries, this so-called Higgs field, which was interacting with electrons and protons in a way that would slow them down and, therefore, cause them to appear like they had mass.

JM: Appear like they had mass?
LP: Right. Mathematically, it’s indistinguishable from having mass.

Higgs event created by collision of two protons at CERN, Switzerland

Higgs event created by collision of two protons at CERN, Switzerland

JM: This wallet, this table—they have mass. By golly, I can feel it. Why do I need a Higgs boson theory to prove to me that this has mass?
LP: Right. It has mass, but why does it have mass? It shouldn’t have any mass. That’s the idea. We can observe, of course, that everything has mass—your wallet, the table, me—and the puzzling thing for physicists is that this shouldn’t be. Everything should be like the photon—light. We know that the light is massless. All the photons streaming towards us from a light bulb carry no mass at all.

We would expect, based on principles of symmetry and so on, that everything should behave the same as light. Every­thing should all be massless, but for some reason, some things have mass and some things don’t. So Peter Higgs and his colleagues came up with this mech­anism—the so-called Higgs field—that would ascribe mass to things and ascribe a different value of mass to an electron than it would to a proton.

JM: Let’s switch gears here. What’s the current thinking about the content of the universe?
LP: We’ve been led to the belief that the universe is made up of ordinary matter in the form of electrons, protons, neutrons. That’s what makes up our tables and ourselves and our planet, the Earth, the sun. Everything that we knew about the universe seemed to be made up of this stuff called matter. And then, about a decade or so ago, cosmologists looking out into the far reaches of the universe discovered that there was much more to the universe than this ordinary matter.

In fact, present thought is that ordinary matter makes up less than five percent of the content of the known universe—that there is five times as much of this mysterious stuff called dark matter, which is seen only by the interactions of galaxies. The way that the stars move around inside galaxies indicates the presence of this invisible stuff we call dark matter, which is causing stars to rotate inside a galaxy and causing galaxies to interact one with the other in a way that would not be if the galaxy contained only ordinary matter.

And then, beyond dark matter, the same observations indicated that there’s even more mysterious stuff out there that we call dark energy. It’s sort of causing the universe to blow up at a faster and faster rate as the universe evolves in time.

Map of the distribution of positrons towards the center of the Milky Way, including the newly discovered antimatter "cloud"

Map of the distribution of positrons towards the center of the Milky Way, including the newly discovered
antimatter “cloud”

JM: Do we know absolutely that dark matter and dark energy exist?
LP: There are good observations. The observations for the existence of dark energy and of dark matter are all based on looking at cosmic microwave background radiation. This is light that’s coming at us from the earliest days of the universe, and that light has been affected by all the stuff that it’s gone through in the intervening billions of years. And as it’s been traveling over those billions of years, the space that it’s traveling through has been stretching.

The universe has been expanding. And through the way that the light has interacted through the expansion of the universe, we have some idea of the presence of this dark energy. And then, as the light is scattered from one galaxy to another as it’s heading towards us, you can see the presence of the dark matter. You can also see the presence of dark matter just in looking at how each star circulates around the center of a galaxy. The stars at the outside of the galaxy should be moving much more slowly than they do if there were only the ordinary matter that gives off light that we used to think was all the universe. The fact that the stars at the outside of a galaxy are moving much faster than we would otherwise expect tells us that there’s a lot more mass in the middle of the galaxy that they’re orbiting. But we don’t see any evidence of this matter through light, so we say it’s “dark” matter.

JM: Is dark matter everywhere—say, in this room?
LP: Well, we think that dark matter is in this room. Dark matter is present wherever there’s a galaxy. Galaxies tend to coalesce where there is dark matter. There’s a dark matter halo surrounding just about every galaxy, and that dark matter, then, pervades everywhere—everywhere within the galaxy, including here in this room.

JM: Does dark matter or dark energy help us understand the dynamics of the universe?
LP: There’s a possibility, certainly, that once we start to see a little bit of the properties of dark matter, we may find that it has just as rich a structure as ordinary matter does. What could we do with that? That’s something for a future engineer to work out, but dark matter, since it can travel great distances, may be a way for us to communicate with distant galaxies.

I still work in another mysterious area of particle physics, and this is in the nature of matter and antimatter. What we’re looking at in my field of research is understanding the difference in behavior between matter and antimatter. Why would antimatter not behave identically with matter? And in most cases it does, but we are finding hints in certain corners of physics where the antimatter version of the stuff that we’re made of behaves just a little bit differently than the matter version. And that extends all the way to the biggest mystery of all, really: What we see as the visible universe, why is it composed almost entirely of matter, and why do we not see any antimatter at all in the visible universe?

Galaxy cluster 1689, with the mass distribution of the dark matter in the gravitational lens overlaid in purple

Galaxy cluster 1689,
with the mass distribution of the dark matter in
the gravitational lens overlaid in purple

JM: Is antimatter the same thing as dark matter?
LP: No, antimatter is ordinary matter, but just the anti- version of it. But there doesn’t seem to be any antimatter in the ordinary universe, although we would expect there to be an equal amount. We believe that in the Big Bang there was an equal amount of matter and antimatter made, so there should be equal amounts still today, even if they’re separated physically, like one galaxy made of matter and another galaxy made of antimatter. But we see no evidence for that at all. And so, the puzzle is, why is there no antimatter? Where did it all go?

JM: And the reason we think that there should be antimatter is?
LP: The Big Bang started with a quantum fluctuation that made equal amounts of matter and antimatter out of nothing. It turned energy into stuff called matter and antimatter. And then, those recombined to make light. And, actually, if you think about it all the way through to its logical conclusion, you would say that all the matter and all the antimatter from the Big Bang would’ve recombined by now and made just light, and the universe would be filled with just light and maybe dark matter and dark energy, but none of the ordinary stuff that we’re made of. There would be no suns. There’d be no planets. There’d be no people. But for our benefit, somehow or other, we ended up with this preponderance of matter over antimatter. One part in ten to the ninth of the matter didn’t find its antimatter equivalent to annihilate with—to recombine with – and that was what was left over to make the visible universe that we see today. So, we’re the one part in a billion that was left over.

JM: What if there were no Big Bang? Maybe there have been many bangs, so that there isn’t antimatter, and we’re barking up the wrong tree?
LP: Certainly, there are other explanations for the beginning of the universe that don’t start with a Big Bang. We may be going through cycles, and we’re just partway through one of the cycles of many cycles that sees the universe expand and contract, expand and contract, and so on. That’s not been ruled out. Nevertheless, we know that antimatter can be formed because we make antimatter every day.

JM: How do we do that?
LP: We can make antimatter just by converting energy—for instance, energy of motion, kinetic energy—into matter. At the same time, we can convert it into antimatter. You can start with, say, a beam of electrons or a beam of protons, and you can speed them up so they have a lot of kinetic energy. And then you slam each of those into a block of tungsten, for instance, and as they pass through that material, their kinetic energy gets turned into mass. E = mc2. And some of that mass is in the form of matter; some of it’s in the form of antimatter.

JM: And that’s observable?
LP: That’s observable and, in fact, we can make enough antimatter and collect it together and make beams of antimatter, and we can have beams of antimatter smash into beams of matter. We can take particles of antimatter, and we can assemble them into an anti-nucleus of helium. And we can surround that anti-nucleus of helium with two anti-electrons to make an anti-atom of anti-helium. This has been done.

JM: And so, what happens if an anti-atom of helium meets a …
LP: Ordinary atom of helium?

JM: Yeah.
LP: They recombine. They join together, and out comes a lot of light.

JM: It’s not an explosion?
LP: Well, if you have only two atoms, you can’t see it as an explosion but it is, essentially, an explosion. It’s an instantaneous conversion of all of that mass energy in the matter and antimatter back into light. And so maybe the next storage battery is to be a container of antimatter. This is sort like in those movies where you can take a container of antimatter, and then, you release it and combine it with matter in a controlled way, and you get energy released. And that’s a tremendously efficient process for releasing energy.

JM: Is somebody working on this?
LP: There are people working on containing antimatter. You can contain it with electric and magnetic fields, just as you can ordinary matter. I don’t think that, at the moment, they’re thinking about making a battery. They’re just thinking about doing simple experiments with small amounts of antimatter, but, eventually it will lead to the point where you can make such copious amounts of this stuff that you can use that, then, for instance, for energy storage. Or if you combine them rapidly, you can make, not just a nuclear bomb, not just a hydrogen bomb, but you can make an antimatter bomb, and that would blow a planet apart.

JM: That’s the question I was afraid to ask.
LP: Yes. You can harness these technologies in good and bad ways.

Electron cloud

Electron cloud

JM: One more question. I’ve heard that there’s empirical evidence that the universe is infinite. Now, an infinite universe is something that’s easy to say and hard to wrap one’s mind around. Is that something that you have heard of?
LP: I’m not sure what measurable consequences there would be to say “the universe is infinite” versus “the universe is finite but huge.” So I’ve not heard too much talk about infinite universes so much as about the shape of the universe and the size of the universe that we can perceive since the time of the Big Bang, if you believe the Big Bang, or the origin of the universe in its current state as we know it. We do know that things can’t reach us, things can’t propagate within the universe, faster than the speed of light. So we’re limited as to how far out into the universe we can see by the speed of light, and we do know that there are plenty of things out there beyond what has reached us so far from the beginning of time, the beginning of the universe. And so we have a kind of a bubble around us—an expanding bubble. It’s always expanding as you go later into time, or earlier into time. It’s a bubble beyond which we can’t perceive anything because not enough time has elapsed for light or signals or any kind of information to have reached us. And yet we certainly believe, and there’s no reason not to believe, that there is something beyond that. It just hasn’t reached us yet. And why do we believe that? Because, from the beginning of the universe until today, that’s been our experience—that as we wait and wait for things to come from farther away and earlier in time, it still looks the same as it did yesterday, and that looks the same as it did the day before, and the day before.

And so, nothing about the nature of the universe that we see, no matter how far back in time we look, no matter how far away the shell is—this expanding shell that we can finally perceive the edge of—we always perceive the same kinds of things. And so, that tells us that we’re in a bubble, and as the bubble expands, it’s still seeing the same kinds of universe structures and so on that we have seen earlier. And so we believe that there is still more of it out there. How much more? We don’t know.

JM: Could that bubble just be the bubble of our awareness? It may have no corres­pondence to what is actually beyond….
LP: It’s the bubble centered on the Earth. There’s an equivalent kind of bubble centered on a different planet far, far away. We each have our own bubble.

JM: Well, it’s been wonderful sharing your bubble today.
LP: Thank you. ♦

From Parabola Volume 39, No. 2, “Embodiment,” Summer 2014. This issue is available to purchase here. If you have enjoyed this piece, consider subscribing.