When we read about celestial objects and events, it is sometimes difficult to relate what scientists say happens “out there” in space to life “down here” on Earth. Exotic entities like dark matter do not seem to have any direct impact on terrestrial affairs. But in her new book, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe (Ecco, 2015), particle physicist Lisa Randall proposes a connection between this dark matter, the mysterious material thought to make up around 27 percent of our universe, and the impact that spelled the dinosaurs’ demise about 66 million years ago. Although scientists have largely assumed that dark matter interacts with other matter only via the gravitational force and resides in spherical “halos” around galaxies, Randall suggests that a fraction of dark matter not only interacts gravitationally but also experiences a force analogous to electromagnetism, which she dubs “dark light.” Through its interactions with “dark light” this weird subset of dark matter could form an invisible disk that overlaps with the visible disk of spiral arms in our Milky Way galaxy. This dark disk might have interrupted the orbit of a comet on the outer fringes of the solar system, sending it on a collision course with Earth.
To explore the plausibility of this scenario, Randall’s book traverses a range of scientific fields including particle physics, planetary science and cosmology. Guided by her theory about the dinosaurs’ destruction, Randall explores the underappreciated connections between the tiniest particles right under our noses to the vast structures that rule the universe. In the edited transcript below, she talks to Scientific American about her ideas.
How does your theory about a special subset of dark matter that interacts through “dark light” compare with other models of dark matter interactions?
People have debated whether dark matter has any nongravitational interactions at all. But [my colleagues and I] thought, maybe just a fraction of dark matter does. Just the way ordinary matter is only 15 percent of all the matter in the universe, maybe there’s a fraction of dark matter—even 5 percent of the matter in the universe—that has its own interactions. It’s not the usual dark matter that forms this spherical halo [around the galaxy], it’s a new type of dark matter. So you still have the ordinary halo but in addition you have this dark matter disk.
Invoking the idea of a special kind of dark matter to explain the dinosaurs’ demise might, at first glance, seem to violate Occam’s razor [the philosophic rule that states explanations should be the simplest, involving as few assumptions as possible]. How do you reconcile your theory with that philosophy?
There are a couple answers to that: One is that, in the case of dark matter you could ask which is simpler: Is it simpler to say that dark matter is like our matter, in that it’s [composed of] different particles with different forces [such as “dark light”] or is it simpler to say that dark matter is just one thing with no [nongravitational] interactions? It’s not obvious to me which is the more reasonable option. And the other answer is that the world’s complicated, so Occam’s razor isn’t always the best way to go about things.
What observations or evidence could astrophysicists eventually gather that would prove whether or not the dark matter disk you describe actually exists?
There are actually quite a number of tests that I talk about in the book, but probably the most direct is actually looking for the disk. What I mean by that is looking for its gravitational effects on other stars in the Milky Way—or in another galaxy, for that matter. That’s actually what we’re working on right now, and there’ll be even better data coming out from the Gaia satellite.
Is it ever frustrating to study something like dark matter, which seems so shrouded mystery and is literally impossible for you to see?
Part of what we try to do [as scientists] is figure out ways to “see” it. I mean, our eyes are just one way to see things. Just because we don’t see dark matter directly in our daily lives doesn’t mean it doesn’t exist, doesn’t mean it’s any less real, doesn’t even mean it’s inaccessible. One of my goals in this book to is to explain that dark matter isn’t that exotic a concept. It sounds very exotic but it really is just stuff that doesn’t interact with light. So part of it was just trying to get people into a frame of mind where it doesn’t seem like an exotic thing to have something that we don’t see that can exist.
As a particle physicist, what was it like researching and writing a book that explores so many different disciplines?
I loved learning about these new fields—what’s known and what’s not known, what the methods are. I think I have a much richer view of the universe, of the galaxy, of the solar system. I particularly enjoyed learning in a little more detail about our immediate environment. As a particle physicist, I’m working on very abstract things—and it’s fun bringing those things home in terms of what they mean for our actual environment. I really love the science but I also love telling the story and putting it all together, figuring out what it all means.
What do you hope readers will take away from reading this book?
I think we all live richer lives when we appreciate the big history that underlies where we are today. My book is about an idea my collaborators and I had and it's about dark matter, but it's also about the many connections and processes that happened in the cosmos, the galaxy and the solar system to get us to where we are today.