One theory to rule them all
We currently have two very symmetrical theories - relativity and the Standard Model - but if we've managed to compress physics into two theories, then can we go further and squeeze it all into one theory to rule them all? In other words, can we construct a 'Theory of Everything'? Graihagh Jackson spoke to Katherine Freese from Nordita, who first explained what exactly we mean by a 'Theory of Everything'...
Katherine - Well, from a physicist's perspective, a theory of everything is one that would encompass all of the physical laws of nature. In today's universe, we know there are 4 separate forces.
Graihagh - When you say 4 forces, what are you referring to?
Katherine - Well, we have the most important ones of our everyday lives. There's the electromagnetic force which includes electricity and magnetism. Then we also have the strong force. This is actually where holds our nuclei together. Our nuclei have neutrons and protons in there and they don't fly apart because of the strong forces holding them together, so they're really necessary for our existence. We also have a third force - the weak force - which is responsible for many types of radioactivity. And then of course, there's gravity. So today in the universe, we have all these four different separate forces.
Graihagh - And then at the Big Bang, when the universe began, we thought all these forces were one force, one single thing.
Katherine - Well, as we go backwards in time, then we can watch them start to unify. So, we do know that electromagnetism and the weak interactions unified into one single force - electroweak - and then as you go farther back in time then you could also bring in, at least we think we can combine that with the strong interactions into what we call a grand unified force. If we could go even farther back in time, almost all the way back to the Big Bang, that's where gravity would join, if only we knew how to do that. That's what is really missing in our trying to build a theory of everything.
Graihagh - Why is that so tricky? Why can't we seem to unify? Is it just that we can't look that far back in time?
Katherine - Well, that's certainly part of it. On the theory side, there's a real stumbling block that - well, Einstein was already trying to get at it and we've made some progress, but the unification of gravity in general relativity, and then quantum mechanics. Unifying general relativity and quantum mechanics, we don't know how to do that. We do have some ideas if only we can make that work then we have some chance of approaching this theory of everything.
Graihagh - So in some sense, it's joining this quantum world - the tiny world - with a much bigger world - these theories of gravity and Einstein's theories of relativity.
Katherine - Yeah. The quantum world describes very well what's going on in atoms and then in the particles inside the atoms. It works on small scales. And general relativity, we know how to use it on the large scales of clusters of galaxies, galaxies, the earth and things like that. But combining these two has proven to be very, very tricky.
Graihagh - Just stepping back a bit, what has this got to do with symmetry?
Katherine - If we can in fact, unify everything in terms of a single theory then this would be a single symmetry, mathematically speaking, that would describe everything. So, we do know that's what happens when we write down grand unified theories or some of these higher symmetric states of the universe early on. But as the temperature of the universe drops then these symmetries break. It's called spontaneous symmetry breaking. So, what happens is, a single force splits off into the 4 different forces that we have today. So, that's a breaking of that symmetry that we had early on.
Graihagh - Aah ok, I'm with you now. Why do we really want to do this? Why do we need to look back to the Big Bang and before?
Katherine - Well, this is something that I think that everybody wants to know. How did the universe begin? What was the starting point? What is remarkable to me is how well we've done. The Big Bang happened 14 billion years ago and we have a pretty good understanding of everything that went on. We have predictions for things that would have taken place 3 minutes after the Big Bang and they're verified to incredible accuracy. But people, everybody is always asking, "But what happened before that? Let's go farther back in time." If only we had this theory of everything then we hope that will let us address some of these questions of what happened even earlier than that.
Graihagh - When can we see this theory to be announced? Is it likely to be soon?
Katherine - Well, I would say not in the next decade. I guess that's the fun of doing a research.We don't know how long it's going to take. There's a lot of physicists working on a really interesting idea - string theory - which does have the capacity to incorporate both quantum mechanics and general relativity. But the trouble is that it doesn't seem to make predictions that we can test. So, the way string theory works is that the most fundamental objects in the universe would be strings and these strings vibrate. Those vibrations are particles. So, one particular type of vibration would be an electron, a different type of vibration would be a proton. So, it's a beautiful mathematical theory and as yet, it is not making predictions that we can see at the Large Hadron Collider. So, the problem is, it's a mathematically beautiful thing, but we're kind of stuck experimentally.
Graihagh - Does that mean we will have completed the universe?
Katherine - Even if we do have a theory of everything, we still won't be finished. There's always a chance of looking further back in time. The more we know, there's always the more questions that appear. Humans are really creative people and we are explorers. It's fun. We want to go into the unknown. So, there will always be new questions for us to ask.
Graihagh - Why is it called the theory of everything then if it's not going to be describing everything?
Katherine - That's just a physicist's term for the unification of all the forces. Automatically, if you're unifying the forces, you're probably understanding what the universe is made of which right now, we don't. We only know 5 per cent of the total content of the universe - the ordinary atoms. We don't know the dark matter, we don't know the dark energy. So, when physicists use this term 'theory of everything', what they're referring to is the unification of the 4 forces and the understanding of the content of the universe.