Lisa Randall, physicist at Harvard University, explains where particle physics falls short, and a little thing called a brane.
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The Standard Model for Particle Physics 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 want to understand 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 t