Science-classroom icon Bill Nye recently sat down with leading cosmologist and critical theorist Lisa Randall to discuss her new book, Dark Matter and the Dinosaurs. Full of science jokes and big questions about the origins of our universe, their conversation at 92nd Street Y offers insights into some of the most complex questions scientists are tackling today. (This conversation has been edited and condensed for clarity.)
Bil Nye: I claim there is a question that everyone has asked, and if you meet somebody who says he or she has never asked this question, they’re lying to you: Where did we all come from? You have written three books about it.
When I was young, people presumed that the universe was going to expand and expand indefinitely, never quite reaching an end. Then there were other people that ran around saying, “It’s going to collapse again!” But in my lifetime people found that it’s accelerating, right?
Lisa Randall: People solved these equations for the cosmos, for the universe, and it looked like the solution led to an expanding universe. That was kind of shocking. 100 years ago no one had seen evidence the universe was expanding at all.
It’s very interesting, though, because equations not only tell you that it’s expanding, but they connect the expansion rate to what is actually in the universe. That could be ordinary matter, or it could be dark matter — as far as gravity goes, those are pretty much the same. The equations lead to the expansion to slow down. But there is this other term, “dark energy,” that actually leads to an acceleration of the expansion.
Bill: How was it detected, with supernovae?
Lisa: The first detection did have to do with supernovae because supernovae have a couple of advantages. One is that they are really bright so you can look at the galaxy in which they sit, and you can see how quickly they are receding, and you can see that they were giving less light than what you would expect, given where they were. Basically, there had to be something that was going on. They had to be further away. That’s dark energy.
I don’t like talking about dark energy as much as dark matter because, yeah, we found it, we know it’s there, but theoretically we haven’t really made much progress in understanding its source. There is no reason it shouldn’t be there. The big question is, why this particular amount? We don’t really have the answer to that. Dark matter, on the other hand, that’s actual matter. It acts like ordinary matter with respect to gravity.
“I like to think of dark matter as the unsung hero.”
Bill: You don’t like the word “dark matter,” though?
Lisa: If I had my druthers, I would call it transparent matter. You can see my jacket, right? It’s dark, and it’s dark because it absorbs light, so dark stuff interacts with light. Dark matter is defined by the property that it is matter that light passes through. Billions of dark matter particles are going through every second, and we don’t know about it.
Bill: You think they are particles?
Lisa: Yeah, that is an assumption. But I do think they are particles, and I think they’re particles that aren’t made out of ordinary atoms. They’re a different kind of particle.
Bill: Is it reasonable then that these particles are slashing through us, and sloshing around in this room, and we haven’t come up with a way to detect them? Or is it that they’re concentrated, or specifically not concentrated, the way the sun is concentrated matter? Is this stuff inherently unconcentrated? Does it repel itself?
Lisa: That’s a big question. I like to think of dark matter as the unsung hero. We see the world through a lens of our eyes, so we focus on ordinary matter, but dark matter not only is out there, there is five times as much of it as there is ordinary matter.
Bill: So five-sixths of everything is transparent?
Lisa: Of matter — not everything. But yes, to us.
Bill: Then how would we find it? Are you saying you have found it, seen evidence of it?
Lisa: I, personally, have not seen much of it, but there is evidence of dark matter, and the evidence is gravitational. Because even though an individual dark matter particle has very little effect and interacts with gravity, if you have a lot of dark matter, like the amount that we have in our galaxy, it has a huge gravitational effect. Why is that important? It tells you, for example, how stars move. Stars in our galaxy respond to the gravitational potential set up by the matter in the galaxy.
Bill: In other words, there is something drawing together that is transparent, but is apparent when you measure the motion of these galaxies?
Lisa: That’s right. Or measure the motion of stars in a galaxy, or galaxies in a galaxy cluster. If there was no dark matter and stuff was going back quickly, it would fly away because there wouldn’t be enough gravity that pulled it back. So looking at the velocities of stars, which is what Vera Rubin and Kent Ford did, as you know, in the 1970s, they realized there wasn’t enough visible matter around to account for it.
It’s very funny, the way that we think about all these other forms of matter. We know our matter is really complex. We know there are different types of particles. There are quarks, there are electrons. Particles interact differently. Yet in the case of dark matter, we assume it’s all the same, all the dark matter is either all interacting or all weakly interacting or all non-interacting.
Bill: All the transparons are the same.
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Lisa: Right. We said, suppose that weren’t true. Suppose it was more like our matter, and there was some small fraction that actually did have an interaction. It could be that it interacts with our matter, or it could be it just interacts with itself. Remember, dark matter obviously doesn’t see our light — that’s what we’ve been talking about — but maybe our matter doesn’t see dark matter’s light. Dark matter has its own light that we do not see.
Bill: Its own energy.
Lisa: No, it’s really light. It exchanges its own dark light, dark photons. It’s its own type of light that we don’t see.
Bill: I was with you until you said, “These are the photons that we can’t see…”
Lisa: Our eyes don’t interact with them, but dark matter does. What does that mean? It means that dark matter does carry a charge but not the charge that our matter carries. It’s a different kind of charge. It’s a charge that only the dark matter sees.
If that’s too confusing, think about the fact that some small fraction of the dark matter could be different than most of the dark matter, and that small fraction could interact differently. It could behave differently. It could cluster differently; it could form structure differently. In particular, it could actually form a disk like the Milky Way disk, even a thinner, narrow disk, in the mid plane of the Milky Way. It could be that, in addition to the dark matter that surrounds us, in this enormous spherical halo that we do know about, there is also a dark matter disk.
Bill: I have a 45 rpm record inside an upside-down Frisbee?
Lisa: Yeah, okay. Although more like a 33 rpm record because it really is as big as the Frisbee.
Bill: These things form a disk because there are discontinuities. Matter is not pulled together evenly from every direction.
Lisa: Actually it is, but then what happens is some of the matter has a means of radiating. We have a Milky Way disk because ordinary matter can lose energy. It can lose energy because it can radiate. Because it can radiate, it cools down, and when it cools down, it doesn’t move around as much, and it ends up like … the gas ends up in this disk. If dark matter, at least a small amount of dark matter, really radiates, then it too can form a disk. It can be this thin narrow disk in the mid plane of the Milky Way. That would be incredibly interesting because it means that the gravity from the galaxy would be different, which could be reflected in the motion of the stars, and it could be reflected in what happened to our star, the sun. The sun goes around the galaxy.
Bill: How long does that take?
Lisa: It takes around 240 million years. It bobs up and down through the plane of the Milky Way as it does so. That is a much shorter time period.
Bill: Does it wobble, nutate?
Lisa: Yeah, it oscillates up and down a little bit as it goes around. What we propose is that every time it goes through the dark matter disk, about every 30 million years, there is extra tidal gravitational force. The other thing to know is that, in our solar system, there is something called the Oort cloud, which is really far away, in some sense at the edge of the solar system. It’s 50,000 times further away than the Earth is from the sun. Because it’s so far away, the stuff there is very weakly gravitationally bound to the sun.
Bill: It’s still bound to the sun even at that crazy distance, right?
Lisa: It is, but it would be much more susceptible to some other perturbation, so if there is some other force that acts on it, then it won’t be bound — in principle. Our proposal is that every time it goes through the disk, stuff gets kicked out. Comets, in particular, can get kicked out of the Oort cloud, and one of them could be the one that’s on the cover of my book.
“I think that we lose track of the importance of basic research, what it does for people being curious about the world.”
Bill: If ancient dinosaurs were knocked out 66 million years ago…
Lisa: Along with two-thirds of the species on the planet. Not just dinosaurs — it was a mass extinction.
Bill: Yeah, it was a big day.
Lisa: It was a big day.
Bill: It’s like control alt+delete.
Let me ask you this, thinking out-louding. If we are rotating or nutating or whatever we’re doing, as the sun is passing through this transparent disk, aren’t we right around one of the nodes? Two times 30 is 66?
Lisa: Yeah, actually. Fortunately we’re on the right side of history because about 2 million years ago, we passed through, so we probably won’t be passing through for another 30 million years.
Bill: There are still 100,000 Earth orbit-crossing asteroids, I’m assuming?
Lisa: Oh, there is a lot of asteroids. It depends on what size you want to talk about. If you want to talk about ones that are bigger than a kilometer, there are only about 1,000, and they are not all Earth-crossing. There are near-Earth asteroids, and if you go down to smaller sizes, we have to distinguish what are the things that could hit the earth and what’s out there. Of course, we have a limited knowledge, but we can see that obviously there is a lot more small stuff, a lot less big stuff. Big stuffs hits a lot less frequently. Small stuff hits all the time. There are tons of asteroids.
Bill: 100,000 tons a night.
Lisa: It’s a lot of stuff that comes in, meteoric material.
Bill: Have you ever put a sheet out at night? You come out, and there will be dust, and you can get a magnet and pick up cosmic dust if you’re inclined. You’re a cosmologist, right?
Lisa: I am a critical theorist and a cosmologist, yes.
Bill: In the book, you mention the relation between cosmology and cosmetology, which I thought was charming.
Lisa: Actually, I have been confused for that when I say cosmologist.
Bill: Like when somebody says astrology when they mean astronomy?
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Lisa: Yeah, which is a little embarrassing, but I thought it was kind of interesting. Why do they sound so much alike? You look it up and it turns out they’re both talking about something that is attractive and has an underlying order; the universe as a whole is like a face. It could be something that you think of as very orderly and having this nice structure to it, which I thought was quite lovely.
Bill: You feel that way, right?
Lisa: That the universe has order? On a good day.
Bill: What does it have on bad days?
Lisa: The Republican debates.
Bill: You’re an academic, so your work is funded by an academic institution. Or is it funded another way?
Lisa: Part of it is NSF [National Science Foundation] funding, and some of it comes from Harvard.
Bill: Do you have a concern that funding is going to get cut?
Lisa: Oh, it’s clear that the amount of money going into basic science, at least the kind of science that I do, is going down. It’s really very worrisome.
Bill: What do you feel we should do about it?
Lisa: In some indirect way, it’s one of the reasons I wanted to write this book. We have pretty short-term thinking. Even when we do fund science, we often think of it as applied science. I see medical research as applied. There is a lot of people that will say medical research is basic research, and in some sense, it is, but if you think about it, genetic research isn’t the same as the discovery of DNA. No one was thinking about curing cancer when they were trying to identify the structure of DNA. They were doing research, and many years later, that is what’s leading to all sorts of developments. I think that we lose track of the importance of basic research, what it does for people being curious about the world.
The iPad is great, but, let’s face it, it’s just a bigger iPod. It’s not a basic revolutionary discovery. We want to have basic discoveries like quantum mechanics that led to the electronics revolution. Those are the kinds of things we are in danger of losing.
Bill: There have been, let’s say, five major mass extinctions. The ancient dinosaurs was catastrophic.
Lisa: The last one that we know of.
Bill: You would say we’re in a sixth mass extinction.
Lisa: I do think it’s likely. I can’t say for sure, but if you look at the rate of extinction of, for example, large mammals in the last 5 million years, it’s 16 times the rate that it was ordinarily.
We look at mammals because those are the ones we can measure. It’s very hard to do the measurement. That’s why no one can say for sure. We don’t know how many species exist. We don’t know for sure how many are dying out, but it is clear that we are changing habitats in a way that’s incompatible with many of the species of life on Earth. It’s hard to imagine that is not going to have enormous consequences.
Bill: Can we see evidence of the extinction event that killed the dinosaurs?
Lisa: I have a whole chapter about this. When I started writing the book, I had heard that we think a big object, a meteorite, killed the dinosaurs, but it was presented as a somewhat controversial theory.
It’s amazing, the strength of the evidence, because not only can we go and see the layer of the extinction, we can see the fossils underneath, the fossils above, the structure of the rocks, we can see lots of evidence that there was an impact.
This is a great story in itself too: the crater was found. There was no guarantee that crater would be found. There was a proposal, there was a crater, and not only was it found, but it was the right size. From the amount of iridium, you could figure out how big that crater should be.
Because we actually found this crater, we can do an incredibly accurate timing measurement to see that these things happened within 20,0000 years of each other, 66 million years ago. It is very hard to believe that that was a coincidence, given the amount of destruction that happened, and there is lots of evidence. Once this theory was proposed, there were many different geological formations, so for me it was a lot of fun to study all this geology and see all the different ways that we could actually identify that happened.
Bill: Do you feel we do enough to avoid getting hit with another asteroid?
Lisa: Honestly, as a scientist, I am very happy for us to study what asteroids are out there. In terms of the actual dangers to the planet, yes, it would be bad if we get hit.
Bill: If everything that you know were destroyed in an instant.
Lisa: That is not going to happen for millions of years. The ones that are going to hit are going to be relatively small. Yes, they can do some damage if they do hit, but we’re talking hundreds and thousands of years. We’re not talking tomorrow. I think there are many dangers that are more pressing. So no, we’re not doing everything we can, but I don’t think it’s the most pressing problem on the planet.
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Bill: Are we doing everything we should do?
Lisa: We’re not doing everything we should do for anything.
Bill: Since dark matter doesn’t interact with other matter, could there be dark life?
Lisa: There absolutely could, and I talk about that, but I remind you that life is an incredible thing, in the sense that there is life, and then there is complex life. We should probably distinguish those.
Life requires an amazing amount of coincidences, and we don’t know how life came to this planet, but it could actually be connected to some of these asteroids and comets hitting. I talk about the various possibilities, but there is an amazing number of coincidences that have to happen. Yes, in principle, there could be dark life, but life is never likely.
Bill: What would you tell the youngest of scientists to encourage a love of science? I would tell you, do it. What’s more exciting than science?
Lisa: That’s a great answer. There are two ways that the joy of science is presented. One is the awe and wonder of the universe. The other is getting your hands dirty, or, in my case, doing theoretical stuff, doing math where you see things fit together, and the joy of discovery and the joy of things fitting together.
Bill: You dropped a phrase which is one of my favorites. Next time you’re on Mars, I hope you go up to the rovers like Mark Watney, the character in The Martian. In very small letters, on the edge of the test pattern for the cameras, it says, “To those who visit here, we wish a safe journey and the joy of discovery.” That to me, that’s what this thing is all about.
I would also say to young scientists, be sure you get good at algebra. I’m not joking. They have done very compelling studies. Algebra is the single most reliable indicator of whether or not you will pursue a career in science. It’s not clear that it’s cause and effect, but the correlation is indisputable in people who pursue careers in science and, more importantly, engineering.
Lisa: Algebra relies on abstract reasoning in a way that addition does not.
Bill: Thinking abstractly about numbers helps you think abstractly about all sorts of things. My proposal is we should start having letters represent numbers sooner, like in fourth grade.
Here’s another question. Since the universe is expanding…
Lisa: What is it expanding into? It’s not expanding into anything. We always think of something as expanding because that’s what we see, but the universe is everything, so the analogy that I use, and many other people do too, is a balloon. Imagine the balloon is the entire universe. I call it the balloonaverse. You have a balloon, and suppose you drew some points on that balloon, and you blew up the balloon. Forget that it’s in a room. Imagine that the balloon is the entire universe. Those points are going away from each other. They are growing apart because we are blowing up the balloon. In the same way, the universe is getting bigger. Space itself is expanding. It’s not expanding into anything. It’s space itself expanding.
Bill: We have the big bang, and there are perturbations that lead to structure that cause enough gravity to make things coagulate, accrete. Are those perturbations analogous to ripples in a river? And why would there be any ripple? Is it the nature of nature that there is always a little roughness?
“We are one way that the universe knows itself, which fills me with reverence every day.”
Lisa: The universe looks like it started off completely smooth. It’s homogeneous, the same everywhere, and isotropic, the same in every direction.
For structure to form, you need small deviations because if everything is smooth and the same everywhere, nothing is going to happen. You need small deviations, so where did those deviations come from?
The proposal is the very same thing that flattened out the universe caused them. The exponential phase of expansion in the early universe, cosmological inflation, tended with what are called quantum perturbations that led to these deviations. I am not going to explain quantum mechanics to you, but it’s not so hard to understand why this is. The idea is that, in different regions of the sky, inflation ended at a slightly different time because of random effects.
Bill: Like you can’t know where an electron is or how fast it’s going. It has a statistical property. There is a quantum fuzziness to this …
Lisa: Right. In this case, it’s a quantum fuzziness exactly where inflation ended. It’s a tiny, tiny effect, but there could be these tiny ripples that are imprinted in the sky at the end of inflation.
Bill: Did they detect those in Antarctica?
Lisa: No, those have not been detected, but what has been detected is a lot of evidence that is consistent with the theory of having these fluctuations that it expanded.
Bill: God, it’s so cool. Because it’s fundamental. It’s where did we all come from.
Lisa: It is, it’s true. And the Kobi satellite, in the ’80s, measured that cosmic microwave radiation background very accurately. It’s an understated discovery because that really is the source of all the structure in the universe. These tiny, tiny perturbations is the source of everything we see today.
Bill: We are made of the same stuff, the dust of exploding stars and other dust. We are one way that the universe knows itself, which fills me with reverence every day.
This conversation was originally recorded at 92nd Street Y — the New York cultural center that convenes influencers and innovators who inspire a world of ideas. From the arts to business to politics to science, it’s where tomorrow’s most important issues are revealed, and today’s most intriguing conversations begin.