Lisa Randall on Knocking on Heaven’s Door
Harvard physicist Lisa Randall talks about particles colliding, high energy, and the recent revelations that might upend Einstein’s theory.
Lisa Randall, theoretical particle physicist at Harvard, explained the strange physics of string theory and tiny extra dimensions in her first book, Warped Passages. She met The Daily Beast for a cappuccino at L.A. Burdicks to discuss how scientific progress happens, the largest particle accelerator ever made, the origin of mass, the future of physics, and her most recent book Knocking on Heaven’s Door.
One of the central motifs of Randall’s book is the sometimes messy process of scientific discovery. The world looks fuzzy at the far boundaries of scientific knowledge. On the ordinary scale of everyday life we have enormous amounts of data that confirm our understanding of physics. But on unfamiliar scales, like the extremely small or the very high energy, we bump against the limits of experience. This doesn’t mean that we should distrust physics—far from it, as modern physics has made and verified some astoundingly accurate predictions—but it introduces both scientific uncertainty and the potential for incredible new discoveries.
The Large Hadron Collider (LHC) in Switzerland, the most complicated machine every built and the most powerful particle accelerator in the world, now places us on this boundary of knowledge. In her book, Randall manages to transform these experiments at distant and unfamiliar scales into crucial acts in a cosmic drama.
There has been some buzz recently from CERN—which houses the Large Hadron Collider—about an experiment that seemed to find faster-than-light neutrinos, something I thought was impossible. What do physicists think about this?
Most people suspect the measured speed of these neutrinos is wrong— due to an experimental error or an incorrect interpretation of results. Nonetheless it is not impossible that we will eventually measure such a violation of one of the underlying assumptions of the theory called special relativity.
It could be that the underlying symmetry that is assumed is somehow violated. One way this might happen is with an extra dimension, and a warped extra dimension of the sort I’ve thought about in particular. It doesn’t necessarily occur but it is possible.
This is challenging science in action. We want to test our fundamental assumptions as precisely as possible. Even if they work in regimes we’ve already measured, a violation could be a sign of a deeper underlying theory. Theories build on each other. This is one of the key elements of science that I discuss in my book.
What happens inside the LHC?
First of all you have to know, what’s happening is protons are colliding together. At these energies, it’s actually not the protons themselves that collide together en masse, but ingredients inside the protons. It’s quarks, which are inside, or gluons, which are particles associated with the strong force that holds it together. So these particles collide and they then turn into pure energy. E=mc-squared—Einstein’s’ famous equation. And that energy can then turn into new particles. It can turn into what we call standard model particles, and that describes everything we’ve seen so far, in fact. Or it could be some new particles. So what the experiments try to do is report what came out of the collision.
One of the particles the LHC is looking for is something called the Higgs boson, right? What is this?
In some sense, there are two main goals at the LHC. One is the Higgs boson, which we’ll go into in a minute. And the other, we’re not just looking for the Higgs boson, there’s another very exciting idea of why particles have the mass they do, which could involve these very exotic ideas about space-time symmetries or dimensions of space or very exotic things, and we want to learn both. What makes the Higgs so interesting is that we really think it should show up. These other ideas, might be just beyond the energy reach of the LHC, or they might be there. But the Higgs really should be there.
So let me tell you about the Higgs. The Higgs has to do with how particles acquire their mass. That might sound like an odd concept, particles and mass. I should say how fundamental elementary particles acquire their mass. Shouldn’t particles just have mass? But it turns out, according to actually the symmetries in the standard model, according to the fundamental principles involved in the field there underlying it, unless there’s some extra mechanism, they won’t, it would just be an inconsistent theory. So you need this new mechanism. This mechanism is called the Higgs mechanism. And it involves the idea that particles are part of their mass in some sense by scattering against charges spread throughout the universe. It’s called a Higgs field, not a Higgs particle, a Higgs field. It’s a different type of charge that in some sense exists throughout the universe. And particles acquire their mass in some sense by scattering off that charge. So basically, particles that scatter more are heavier. Now if the Higgs mechanism is right, and I think most particle physicists think it is right, there should be direct experimental evidence. You might say the fact that particles have mass is in some sense evidence. What we really want to know is, what is the model underlying this Higgs mechanism? What is it that gives you this charge spread throughout the universe?
How does this relate to this Higgs particle?
In the simplest implementation, there should be a particle known as the Higgs boson, the Higgs particle. And based on what we just said, we know a little bit about the interaction, that the Higgs particle interacts the most with the heaviest particles, and there interacts the least with the lightest particles.
There have been some stories in the news very recently about CERN saying the Higgs doesn’t exist in the range they’ve tested, with 95% probability.
If you asked physicists before the LHC story what is the most likely value for the mass of the Higgs particle, based on the evidence of experiments we’ve seen, and based on theories, I’d say a vast majority of them would’ve said that it has a mass that the LHC has not yet tested. So it’s getting close. It might by the end of the year—really have covered the entire range, either ruling it out or finding strong evidence for it. Strong evidence in the sense that it won’t be quite enough that we can say it definitively, in the sense that we just talked about, but we could find strong evidence.
So do you think that there’s a popular misconception about what the Higgs is or what the LHC is doing in its experiments? How do these misconceptions about particle physics come about, and what can particle physicists do about them?
Well, first of all I think misconceptions are easy, it’s almost overstating it. I talk a lot about scale in my book. It’s different from the obvious scales, the scales we see in our daily life. The LHC is giving you enormously high energy, which is equivalent to looking at short distances. So you’re looking at these tiny distance scales. I mean, 10 to the negative 17 centimeters. And we don’t have intuition for that. So either you’re a physicist and you’ve studied it, or someone writes about it or explains it to you or talks about it. And I think the only thing we can do is try to make it comprehensible. And it’s subtle, because the physics itself is subtle.
There are two kinds of people. You could have a zillion categories but one category I think is people who like certainty and people who revel in uncertainty. And people who like to be able to say, ‘This is the answer.’ And people that like to be able to say, ‘That’s a really interesting question; let’s see if we can figure it out.’ The fact is, and again this is something I really try and explain, that although we think about science as something very definitive: we have theories, we make predictions, they’re tested. When you’re actually at the edge, at the frontier of knowledge, knocking on heaven’s door shall we say, when you’re really there: there are ambiguities, there are uncertainties, you really have to test ideas. You start in one direction and it turns out to be wrong or the data doesn’t exactly support what you say but maybe it’s close. It’s evolving. It’s not yet established and I think that’s confusing. It’s actually true with just about everything that’s reported in the newspaper: it’s true with political situations, there’s a lot of messiness. But somehow with science I think people have this expectation, ‘Okay. We want the answer. What is it?’ So a lot of time when science is reported, ‘we found a cure for cancer,’ of course what they often mean is they found a way to change the rate of curing this cancer from 20 percent to 20.3 percent. The numbers aren’t necessarily what people think of but it’s easier to make these very broad, definitive statements. It’s easier to get your head around. But the interesting, exciting thing is more subtle: establishing what really is happening.
What will particle physics be doing 30 years for now?
So this is a really great question. So the question is “what do we need?” You basically have two directions you can go in as far as colliders go: you can go to higher energy, or higher luminosity—that is, just have more collisions.
So the plan as of a few years ago in the back of people's minds was to go to a higher luminosity. That seemed like the least expensive option, but given what we’re seeing now (and again, we haven’t really explored everything), it’s becoming clear that stuff isn’t that light—that is, won’t be visible at these energies. Now, I personally would have said that just based on a lot of the data and information we had already, both theoretical and experimental. From a cost standpoint, you go to higher luminosity.
I have to say, I was extraordinarily excited—I was at CERN at the end of the summer/beginning of September and I talked to a couple of people, including the director who said that they were beginning to think about going to higher energy machine. Just technologically it looks feasible—what you need are stronger magnets you can bend around and have these higher energies.
It doesn’t mean it will happen, but you know, I was feeling like Don Quixote before just because I kept emphasizing how important it could be, especially in the theories that I work on to get to higher energies, because given what we know, I wanted to test those theories, and it will test them. It’s now on the table, which made me so happy from a purely physics perspective.