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What is dark matter?

Dark matter is stuff like the matter we know. Matter is defined by how it interacts with gravity. Matter is stuff that attracts other matter and clumps. Basically, it is stuff attracted by any form of energy which clumps together to form structures like galaxies. The difference between dark matter and the matter we are familiar with is that the latter interacts via the Standard Model forces including electromagnetism, such as light. So, ordinary matter interacts with light, but as far as we know, dark matter does not, which means it is made up of other stuff, it is not made of these things you and I are made of. It is made up of completely different matter. So, essentially, it is matter, in a sense that it interacts gravitationally like matter, but it is not the familiar matter that’s made up of atoms, for example.

You propose a new kind of dark matter different from WIMP (weakly interacting massive particles). What is it like?

Basically, we don’t know what dark matter is. People have assumed for a long time that it has something to do with the Standard model or maybe even a particular extension of the standard model. In fact, in those cases which you call WIMPs there is an interaction, which is very small, with ordinary matter. We ask the question: ”suppose there is no such interaction, so you could never find it through the traditional searches”. What could dark matter be, that we might still be able to observe?

So our proposal is that maybe dark matter interacts with itself. It doesn’t mean that we know this is correct. Or maybe even some of the dark matter, not all of it, interacts with itself. We ask the question: if it is not interacting directly with us but only with itself, would there be detectable signs in the structure of galaxies, in the structure of matter, which can tell us that those interactions exist, so that we would know about them?

You were saying that it is still unclear what dark matter is. Who will be the first to shed light on the nature of dark matter, GAIA (observatory spacecraft) or LHC (Large Hadron Collider: the most powerful particle collider)?

It depends what dark matter is. At this point, LHC has looked for what you call WIMPs, things that do have Standard model interaction. So at this point it is not looking very likely that LHC will find it because they will not reach much higher energy. So, I would say GAIA is more likely because it is already finding out that the structure of dark matter isn’t exactly what we expected, for example, there are strings of dark matter. Some people would claim that GAIA might even rule out the dark disk but it’s too soon to say, at least to rule it out with the parameters we suggest. It’s too soon to say because it is not clear that the stars they study are in equilibrium.

We don’t know yet. I would say that GAIA will teach us a lot more about dark matter in the immediate future.

In your last book “Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe” you suggest a link between dark matter and the dinosaur extinction. Which is it?

It’s speculative. The idea is that maybe some of the dark matter interacts, it collapses to form a dark matter disk the same way we have the Milky Way plane, which is formed by ordinary matter.

The reason that happens is because matter radiates and collapses. Maybe dark matter does the same thing. And if that’s the case, maybe the solar system experiences the effects of dark matter every thirty or thirty five million years as it passes through the Milky Way plane (the galaxy). So the solar system orbits around the galaxy, but it bobs up and down as it does that (like a horse on a carousel).

So our suggestion is maybe every thirty or thirty five million years, every time the solar system bobs up and down, it’s more likely that gravity would trigger comets strikes, particularly from the distant objects in the “Oort cloud”, which are thousands of times farther away from the Sun than the Earth and therefore very weakly gravitationally bound. And it turns out that, 66 million years ago a very big object hit the Earth triggering the dinosaur extinction, extinguishing most of life on Earth. We suggest maybe this was triggered by the solar system going through the dark matter plane.

You have written about many fascinating subjects in physics. Which phenomena have captivated you the most?

I think everything captivates you at the time you are working on it if you are excited about an idea. When I present it to the public, I present the ideas that I know can capture them and it is very exciting. However, sometimes when I am working on a very detailed problem and I get to find the solution, it is still very exciting.

Sometimes science and art/humanities seem to be independent worlds, but you have built many bridges between them. Should science and arts be thought of as one? Should they be integrated?

I don’t think that they should be integrated because I think specialization is important. Maybe if someone is very talented in one subject doesn’t have to be necessarily very talented in another … But I do think that it is important for scientists and artists to talk to each other and also for general people not to think of them as entirely different domains. They are all part of culture.

I just spoke at Kosmopolis (literature festival) last night and one of the things I really liked about the events that the CCCB does is that they treat all of these things as part of culture. So I’ve had fantastic audiences that were there for the literature festival but they were also interested in science. So it’s all part of the cultural world: the advancement of knowledge and how we understand the world around us, how we understand each other. So I think that is important. I don’t think that it should be thought of as one thing, but I do think that is nice to build bridges and also nice to just learn about things other than what you do every day.

And speaking of how we understand the world around and how communicate with each other, what do you think about the use of analogies and metaphors to explain concepts in physics? Should science communicators be aware of how to use them?

It’s a good question. When I first started writing my book I thought: “These are so silly, I don’t know that they always help…” So I was very dismissive. But then, when I started writing I realized that if you have the right metaphor, it could be very helpful. It is nice to trigger different ways of thinking, but what is really important and when I would present things to smart friends who would read things, you have to make sure your metaphors are consistent. Sometimes, when you are not a writer, you get carried away and you mix metaphors and the logical conclusions don’t follow. So if you could have a metaphor that you can really follow through, then that is a lot of fun. Also as a writer, I have to say it is one of the times you get to be creative in your way of thinking, so it is actually quite fun. So often I use metaphors that have their independent messages other than just explaining the science.

What do you think about the role of women in the “dimension” of physics or science in general? Has it been hard for you being the first tenured female theoretical physicist at Harvard?

Of course women can do science, there is no reason for them not to be there and yes, it has been hard for me and actually, I am not always very happy with the way women are treated at Harvard.