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Lisa Randall studies theoretical particle physics and cosmology at Harvard University. Her research connects theoretical insights to puzzles in our current understanding of the properties and interactions of matter. She[…]

Randall explains where the standard model for particle physics falls short.

Lisa Randall: Well the standard model of particle physics, it basically . . . it’s a list of particles and how they interact in some sense. So there’s . . . It turns out there are particles called “quirks” which experience the strong nuclear interactions; weak nuclear interactions; and electro magnetism; and gravity, which is negligible, so we’ll ignore it for now. As a force ________ fundamental particles. And there are particles called “leptons” which don’t experience a strong force. And we know about up and down type quirks which sit inside all matter – inside the proton and neutron. But it turns out there are heavier particles – heavier particles that have the same charge as those particles. In fact there’s what we call three generations of particles where they have the same charges . . . set of charges, but they’re heavy enough. And they interact under the forces I just mentioned – electromagnetism, weak nuclear force, strong nuclear force and gravity. And basically that’s the standard model. Where it falls short is in explaining masses. In fact there’s some aspect of the standard model that we expect to be completed very soon. And there’s another more subtle aspects, and let me explain those. First of all it’s important to know that if all the symmetries were . . . that are part of the standard model were there forever, every particle wouldn’t have mass. The fundamental particles wouldn’t have mass. Now we know fundamental particles have mass. The question is how does that happen. And it’s because there’s a small breaking of these symmetries – a small breaking of the symmetries. And it’s associated with the mass scale, so there’s some mass scales in which the nature of the theory changes in some sense. And there’s a particle called the “Higgs particle” associated with that. The Higgs particle is associated with the mechanism through which fundamental particles acquire mass. I know it’s a mouthful. But that’s what happens. And so the . . . One of the things that we wanna understand is is there this Higgs particle? We haven’t found it yet. I mean the theory seems to only make sense if something like a Higgs particle is there, but no one has observed it yet. So one thing is to find the Higgs particle, study its properties. But another thing is to understand where does this mass scale come from? Why is this mass scale so small when compared to other scales that exist in the problem? And also, why are forces of such different strength? Gravity is much, much weaker than the other fundamental forces when acting on elementary particles. And the question is why is that? Why is this force so much weaker? So we know that if we just assume these masses are what they are – if we assume this force is much bigger – the theory works beautifully. Its predictions work. But right now there’s no explanation for the scales in the problem. And furthermore it seems almost inconsistent. If you just follow the rules of quantum mechanics and special relativity, you would expect that all the forces should have comparable strength. And the fact that there’s this enormous discrepancy between gravity and other forces means we have to make a fudge in the theory – what we call fine tuning. And so no one believes that’s what’s there, and so we believe there’s something that completes this theory. And that thing that completes the theory might well be something as exotic as extra dimensions of space. The hierarchy problem is this question of why gravity is so weak compared to the other fundamental forces. That is to say even though gravity seems strong, it’s because you have big, massive objects that act with gravity. If you had two fundamental particles – say you have two electrons separated by some distance – the force of gravity is something like 42, 43 _________ of magnitude smaller than the force of electro magnetism. It’s really, really weak. So the question is why is that? And you can turn that question into a question about mass scales. You can ask the question, “Why is the mass of the Higgs particle that we talked about earlier so much lighter than the energy scale of which gravity would be strong?” After all, although I said gravity is weak when it acts in elementary particles, we know that the strength of gravity becomes bigger as we go to higher and higher masses, bigger and bigger masses. So in principle you could go to a mass scale where gravity was comparable to the other forces. But that mass scale is 16 orders of magnitude bigger than the mass of where this . . . the Higgs particle, for example. And the question is why are these masses as different as we know them to be? So first of all I wanna say that we didn’t just jump and say, “Let’s solve the problem with extra dimensions.” A lot of very smart theorists have been working on this problem since the standard model of physics . . . particle physics was established around the 1970s. And there are some proposed solutions that work if there are only three dimensions of space. One of the most popular among theoretical particle physicists is called “supersymmetry”, for example, in which the particle spectrum is doubled. And we can come back to that if you’re interested. But supersymmetry works fine in some respects, but it actually . . . there’s no really compelling theory once you include all the (15:15) things you have to include to make the theory work. So it really seemed worth asking, “Is this the only possibility?” And since there are theories like string theories and extra dimensions of space, it was natural to ask could there be solutions with extra dimensions. And one of the ways it might be solved if there was an extra dimension of space is that space time can be very curved – or “warped” is the technical term. Einstein taught us that space time can be curved or warped. It doesn’t have to be flat. It doesn’t have to be the same everywhere. There can be gravitational forces which exist through the geometry of space time. And the consequence of that warping could be that essentially gravity is concentrated elsewhere. It’s much stronger in other regions of space than it is where we are. In fact what we found . . . What my collaborator _________ and I found is that you could have gravity really concentrated in one particular location and exponentially decreasing in strength as you go away. So it’s quite natural to have hierarchies in this scenario. It’s natural for gravity to be weak. It’s natural for masses to be different, because space time itself determines the masses, and those can be different. What are branes? So branes are . . . is perhaps the place where this gravity is concentrated. So the word “brane” is . . . First of all it’s B-R-A-N-E for the listening public. And branes are membrane-like objects in higher . . . in extra dimensions of space, for example. So even when there’s an extra dimension, it doesn’t mean that we travel throughout. After all, I just said gravity is weak if we live apart from where gravity is concentrated. So it could be that there are lower dimensional surfaces in higher dimensions. So even if there is a fourth dimension of space, it could be that we and the stuff we’re made of in galaxies only travel in three dimensions. Say electro magnetism maybe only is experienced in our three dimensions. That is to say charged particles can only exist on the brane where we live. But there still could be another dimension where gravity travels. So this idea of a brane is that there’s lower dimensional surfaces in higher dimensional space, and we can actually live on those. Recorded On: 11/2/07