Question: What do particle physicists do on a day-to-day basis?
Lisa Randall: So, particle physicists, we’re trying to understand the substructure of matter; what’s really out there, what’s at the core of matter, how it interacts, what are the fundamental forces. And as you go to deeper and deeper scales you learn different things. So the sort of two-pronged approach, we have some very theoretical work that goes on and some very experimentally oriented work. We’re trying to see what we can find out at the collider that will turn on again next week, we hope. The collider that will be running for the next couple of years, the Large Hadron Collider, that will collide together protons of high energy and tell us what’s out there. We also try to combine it together with theoretical ideas we’ve been working on. Some of them generated from more high energy theoretical ideas beyond what we can see with experiment and we’re trying to put those together.
On a day-to-day basis, we could be thinking about many different things. We could be thinking about what the experiments will do. We could be thinking about the theoretical idea of how they fit together, doing the calculations to see what the consequences of some ideas, which we call models, would be. There might be some speculations or hypotheses for what’s out there and we try to derive what the consequences would be and test whether those ideas work, test these ideas that might synthesize the various phenomena that we’ve observed, but we don’t yet understand.
Question: What insights do you hope the LHC will yield?
Lisa Randall: So, the LHC stands for Large Hadron Collider. It’s large; it’s 27 kilometers in circumference. It’s a big tunnel where two beams of protons will circulate around it and then collide at various points within the tunnel. When it collides, it will collide together at high energy, about seven times the energy of the highest energy collider we have yet had on earth. And when it does that, the idea is for the mass of the particles converts into energy.
The energy that’s created in the collision can then convert into new particles. Those particles can have heavier mass than anything that’s been created before, E=MC2, so having bigger energy means that we can create particles that have bigger mass. It also will have what’s called high luminosity, which is to say, there’s a high collision rate. So the hope is that by having these new parameters, this high rate of collision, these high energies, we’ll be able to study matter in ways we haven’t been able to study it before.
In the process, we want to understand the answers to questions like; where do the masses of fundamental particle come from? That might sound like an odd question since mass seems like an intrinsic property to matter, but it turns out that in the simplest versions of the theories you’ve ****, particles don’t have mass. And we know that it’s actually only possible for particles to have mass because of something like a phase transition that happened **** universe, something called the Higgs Mechanism, named after the physicist, Peter Higgs, who thought of it. And the idea is that particles at high energy might seem not to have mass, but at low energies they do. And the question is; how did that come about? If it’s a simple Higgs Mechanism implementation, we’ll spy something called the Higgs Boson. It’s a particle that tells us that this is really what happens and really puts it all together.
The standard model works to an incredibly high degree of precision. We believe that it’s right. The standard model of particle physics tells us what are the basic elements of matter? What are the forces through which they interact? It’s been tested to a high degree of precision. But it doesn’t answer questions like; where do the particle masses come from, the elementary particle masses come from? And the other question it doesn’t answer is; why are those masses about the scale that they are? Why aren’t those masses much bigger? That has to do with something what we call the hierarchy problem of particle physics, which is connected to the question of why gravity is so weak. And those are the types of questions we hope to answer at the LHC.
If we’re really lucky, we might also get some insight into dark matter. The matter that doesn’t emit light that we know is out there in the universe. So, there’s a number of questions that we hope the LHC might help us answer.
Question: What is dark matter?
Lisa Randall: As far as dark matter goes, we could be lucky in the sense that it seems that if you look at just the amount of dark matter that’s out there in the universe, we can figure out what is called the relic density is given a sort of type of interaction. That is to say, how much stuff should be left over now that the universe is big and cool. And it turns out that if particles had mass about the scale it’s going to be probed at the LHC, the Large Hadron Collider, that it turns out to have about the right relic density. Now, this might be just a coincidence, or it might be something deep and fundamental telling us that the Large Hadron Collider might actually be able to produce the dark matter particle. It might not produce it directly, but it could produce something that decays into the dark matter particles. So there is some hope that by probing this energy, that the LHC will study, we can learn something about dark matter.
Question: What is dark energy?
Lisa Randall: There are various cosmological measurements, that’s to say, by studying how things have evolved in looking out at what’s out there in the sky, we can determine what type of matter it out there. It turns out that the kind of matter we see is only about 5% of the universe. That is to say, stuff made up of protons and neutrons, the kinds of stuff we’re made of. It seems only to be about 5% of the energy. Then another 25% seems to be something called dark matter, which is matter that’s just like matter in the sense that it clumps together. If we look at galaxies, there’s lots of dark matter. But it doesn’t interact with light in the same way that the matter we know about does, which is why it’s dark and it’s hard to see, where I mean literally mean see with light. So, we know it’s there because the gravitational effect. And there’s various ways we’ve seen those gravitational effects, and there’s various ways we’ve seen those gravitational effects, but we don’t yet know exactly what it is.
In addition to that, more recently we became aware of the fact that most of the energy density in the universe consists of something that’s become known as dark energy. So that is very different than dark matter. Dark energy is just energy that permeates the universe, it’s not associated with any particular type of particle, any type of matter, it’s just out there. So if there was nothing, no particles around, there could still be this kind of energy. And in addition to that, that energy doesn’t clump and it doesn’t dilute as the universe expands. So it’s a very special type of energy that Einstein first thought could be around. It turns out it was there in the amount that Einstein thought, but there seems to be this amount, this residual dark energy that’s left that seems to dominate the energy density of the universe today.
Question: How did the universe evolve to where it is today?
Lisa Randall: We have a pretty good idea of how the universe has evolved to today and remarkably, in the last however many years, we’ve really been able to test the evolution of the universe, the cosmology model we have in much greater detail then we have ever had before. So it’s really made cosmology into a precision science in a way that it hadn’t been earlier on in this century.
We now have something which we call the Big Bang Theory. Now the Big Bang Theory is a little bit funny because it’s called the Big Bang Theory, but the Big Bang is the one thing we don’t know about. What the Big Bang Theory tells us is really how things evolved at late stages. How the universe given some energy density which could consist of matter, radiation, dark energy. Given those energies, how is the universe evolving, what’s happening? So, Big Bang Theory tells us how it evolves and we have really nice experimental confirmation. We’ve seen that the universe is expanding, we’ve seen the Hubble expansion, we’ve see the relic radiation left over from the early time. In the beginning, it was all hot and dense, but as the universe expanded the radiation has cooled down significantly, so now we see this 3 degree microwave background in the sky. And in addition to that, we even have detailed predictions for the abundance of various nuclei, which is very much connected with what happened as the universe cooled.
So there are really good tests of the Big Bang Theory and this theory of expansion. The thing is, it doesn’t tell us about the early stages. And particularly, there was some mysterious features when we looked at this background radiation, and I’m using “we” very loosely, the other people when they looked out at the sky. And it was seen that radiation was very uniform and was homogeneous and isotropic, it looked the same in all different directions in the sky to a very, very high degree of precision; like one out of ten thousand with very small temperature deviations as you looked out at the sky. Which was very surprising because there were pieces of the sky that didn’t even have enough time, according to the standard Big Bang Theory, to talk to each other. They’re so far away that light could not travel between them in enough time. So, some how all these different regions of the sky had to have originated in some common place. And the theory of cosmological inflation combined together with Big Bang Theory seems to be what really works.
So for cosmological inflation, the idea is that early on
there was actually an exponential expansion of the universe driven actually by
something like dark energy except in a much greater energy density, and it
drove this exponential expansion of the universe. Actually, later on in our evolution, we will also have an
exponential expansion, but it’ll be a much slower exponential expansion. But early on there was this very rapid
phase of exponential expansion where the universe kept doubling its size, and
during that time, the universe grew an enormous amount, and then after it
stopped that whatever it was that drove the expansion might have decayed, but
somehow the stuff that becomes our matter was created. So, in some sense, that is really the
beginning that we can trace. The
fact that there was some expansion, some inflationary stage where it
exponentially expanded, then afterward this more conventional Big Bang Theory
So for cosmological inflation, the idea is that early on there was actually an exponential expansion of the universe driven actually by something like dark energy except in a much greater energy density, and it drove this exponential expansion of the universe. Actually, later on in our evolution, we will also have an exponential expansion, but it’ll be a much slower exponential expansion. But early on there was this very rapid phase of exponential expansion where the universe kept doubling its size, and during that time, the universe grew an enormous amount, and then after it stopped that whatever it was that drove the expansion might have decayed, but somehow the stuff that becomes our matter was created. So, in some sense, that is really the beginning that we can trace. The fact that there was some expansion, some inflationary stage where it exponentially expanded, then afterward this more conventional Big Bang Theory takes over.
And what’s really remarkable is that there’s sort of an imprint of this state of exponential expansion in the sky. If you look at details at the cosmic microwave background, it’s very much in agreement with the predictions of cosmological prediction. So it really does seem like this happened and it explains some features; explains the flatness of the universe, the fact that the universe doesn’t – we can talk about the geometry of it, even the three-dimensional spatial universe and it seems extremely flat. And again, that’s because if you think about a balloon, if you blow it up, if you blow it up enough even though we know over a large scale, it looks sort of round or spherical or oval, if you look at a small region, it looks very flat. So, if you blow up a balloon enough at any given region, it looks flat, and the same way maybe we blew up the universe enough that it looks large and flat. So, inflation seems to explain many features of the initial conditions of how things started before this Big Bang Theory took over.
Question: What does this research into the universe’s early history tell us about its ultimate fate?
Lisa Randall: Well, what really tells us about the ultimate fate of the universe is what we see today. What we see today seems to be this dark energy that dominates. And as I said earlier, dark energy doesn’t dilute as the universe expands. Radiation dilutes, matter dilutes, so although we know right now we have about 30% matter if we include dark matter and our matter. That matter, as the universe grows, that matter will become more and more dilute. And dark energy is going to just take over at which point we will again have exponential expansion of the universe. At that point, we will cease to have matter creation in the way we do today; star formation in the way we do. Eventually we might have all the matter in black holes, but those black holes will radiate away. So we will ultimately have, if this is right, a large essentially empty universe that will just keep growing.
Question: When do you predict this will happen?
Lisa Randall: This is not going to be the dominant thing that affects any of our lives. This is a projection based on what we observe today of what would happen billions of years from now.
Question: What might the LHC tell us about the existence of extra dimensions of space?
Lisa Randall: The LHC will hopefully find the Higgs Particle; it hopefully will answer the question about the weakness of gravity and why there are different mass scales in the universe. What the answer will be, we don’t know, although we’ve conjectured what it might be. One of the possibilities that I and others have worked on is the idea that there could be extra dimensions of space other than the ones we see the forward, backward, left, right, up, down. There could be other dimensions and they could be connected to explaining some of these features that we can’t otherwise understand. We don’t know that that’s right. Other people conjecture, in fact I’ve worked on it too, the idea that it might be something called supersymmetry, which have doubled the amount of particles in the universe, but again has some chance of explaining why there could be this huge hierarchy of mass scales.
So there could be various possibilities that we conjecture. I don’t think anyone feels 100%
confident, probably there are some people that do, but we really don’t know
what’s there. That’s why we do
experiments. All these theories
have good aspects and bad aspects and so until we actually see how the universe
works, sometimes the universe is far more clever than anything we think
of. So these were all
possibilities, there’s some finer probability for all of them. And we certainly want to make sure that
the machine will test these ideas that it is capable so we’ve made the right –
we tell them what the predictions are, what they should be looking for.
So there could be various possibilities that we conjecture. I don’t think anyone feels 100% confident, probably there are some people that do, but we really don’t know what’s there. That’s why we do experiments. All these theories have good aspects and bad aspects and so until we actually see how the universe works, sometimes the universe is far more clever than anything we think of. So these were all possibilities, there’s some finer probability for all of them. And we certainly want to make sure that the machine will test these ideas that it is capable so we’ve made the right – we tell them what the predictions are, what they should be looking for.
One thing to keep in mind is the experiments at the Large Hadron Collider are very difficult because they are very messy. There’s many billion events per second and you’ve got to narrow that down and be able to look at the small things that the small predictions have a small probability of what could happen and be able to pull those out. So, you have to have a very clear idea of what it is that you are looking for. And that’s why we want to make models and tell them what they should be looking for. What are the unique features that will identify something that’s new and can tell us what’s really going on at these energy scales?
Question: What is supersymmetry?
Lisa Randall: Well the term – to really understand supersymmetry requires you to know something about quantum mechanics. Supersymmetry connects together different kinds of particles known as bosons and fermions. Bosons are particles that like to be in the same place, fermions are particles that don’t like to be in the same place. And the conjecture is, supersymmetry is that for every particle that we know about is a partner that’s the opposite in the sense that if the Higgs boson is a boson there will be a Higgs Zeno, a partner that’s a fermion. If there are Gath Bosons there are Gath Genos, if there are fermions, there are partnered bosons. So it sort of doubles the number of particles which, if you think about it from the point of view of what you’ve seen experimented, is quite dramatic. If supersymmetry is right and explains what we call weak scale phenomena, this questions about masses and about the hierarchy problem, then we’d be able to see a whole school of new particles when the LHC is running at full capacity.
Question: What was the greatest “Eureka!” moment in your career?
Lisa Randall: I mean, for any problem you work on there’s some moment where there’s sort of maybe it’s a moment where no one else cares about, where it all fits together and I found some simple way of doing something. There are also the models that people have heard about such as the models about extra dimensions, something else that we did having to do with a different way of communicating supersymmetry breaking. There were a lot of eureka moments in that and making it work out and the fact that we could all tie it together. So, it’s hard for me to pull those out because a lot of the time when you’re working on something there’s that one thing that you get excited about.
Question: Have you ever been completely surprised by the result of an experiment?
Lisa Randall: I think one thing that surprised us, Braman and I, when we were working together was this existence, or the possibility of the existence, of an infinite extra dimension. People had actually thought there were theorems that you couldn’t have an infinite extra dimension, that gravity would just look wrong, that there was no consistent way to do it. And it was really interesting because we came across this by really following the consequence of our equations to the end and there was just no mistaking that it was allowing the possibility of an infinite extra dimension. And it was interesting to go back and see what assumptions have been made that made us think, or made other people think that it was impossible, and just to see how the theory was almost smarter than we were and worked it out. That was kind of fun.
Question: What excites you most about your current research?
Lisa Randall: Right now I’m enjoying thinking about what we’ll see at the Large Hadron Collider. What is it that they could find that they’re not looking for yet? Making sure that it’s what we have and making sure it does as much as we can. Also thinking about what we can see in dark matter experiments. Making sure, again, that we’ve considered all of the possibilities and we’ve looked for how to search and what could be out there and to make sure that we can look for other candidates for dark matter, you know, what can it be? So, right now I’m thinking really because we are in this era where these experiments are turning on, so it’s important to really make sure that we use them for all they’re worth.
Question: Apart from science, what’s an area of intellectual inquiry that fascinates you?
Lisa Randall: Well, right now, I’m interested in art and music. I wrote a libretto for a small, what we call a projective opera, which was kind of a different kind of way to communicate ideas to the public. It’s not a linear story where we’re explaining things, but just to communicate why we care about exploration discovery. For me it was just an interesting experience to see what it’s like to do something that is connected with performance. It was performed at the Pompidou Center; it was performed at the Opera House in Barcelona, and that was a very exciting and interesting thing for me.
I think right now it’s also a little bit interesting to follow what’s going on with the economy, what’s going on with various political things and by fun -- kind of disturbing sometimes, but I think there are a lot of interesting questions that are lurking there that are fun to think about.
Question: How can scientists reach out to the public more effectively?
Lisa Randall: I think, I’ve obviously made the choice to speak to the public at some level and it’s in part because I enjoy writing and I enjoy being able to think creatively in different ways. I don’t think that any one particular scientist is obliged to do this, to speak to people. But I do think that as a community, if we are expecting resources, it’s very difficult material and I think it’s important that the information is out there.
When I wrote my book “Warped Passages,” I really had in mind an interested audience. I think it’s very easy to try to pander to a large audience – not easy, but – or to write technical things for your audience, but there’s sort of this in-between category of people where they’re really smart and really interested, but they just don’t know the physics yet, and I think it’s important that if they wanted to know what we’re doing and why it’s important and what the full implications are, then that information is out there in a way that they can access. And I just think, generally, it’s important to be able to communicate among different fields. We live in an era where science is important to the decisions we make. Is what happens at the LHC going to determine our policy toward it? Not necessarily, but being able to think like a scientist on some level we’ll be able to think about predictions and risk and probabilities. There’s some basic science training that might help people, but there’s also just curiosity about the world which everyone has and understanding why we care about these questions, what it is that we’re looking for and why it’s going to tell us these deep fundamental properties of the universe at both a small scale and the large scale. Scales that we can directly access just by looking, that we need technology for and saying what is it that we’re after? And I think those are – a lot of people want to know that, and it’s important that they have access to that information.
Recorded on February 17, 2010
Interviewed by Austin Allen