David: Can you please tell us just what the fine-tuning argument and problem has been in physics.

MG: Right, so fine-tuning: imagine the old radios that you had to tune in to the station, and if you didn’t quite get it right, you wouldn’t hear the music. So fine-tuning is somewhat like that. Imagine that the way we understand nature is through this alphabet: the alphabet of constants – the constants that physicists like to call the Constants of Nature. So you have the electron mass, the electron charge, the proton mass, the proton charge. There are quite a few of those, and the point is this: those constants have been measured through the ages to have certain values which are very specific, and had they been otherwise, meaning if those values were different, nature would have worked in a very different way than it does. And if nature worked in a very different way than it does, we wouldn’t be here.

So the fine tuning essentially means that the Constants of Nature have the value that they do have and because of that we are possible. So the question becomes, why those values?

MG: If they were different, we wouldn’t be here.

Ard: So why would we not be here? Would there be no life?

MG: If you tweak just a little bit of the proton mass, stars would not be possible. If you tweak the proton mass, if you tweak the neutron mass just a tiny bit – less than a percent – stars would behave differently. They would burn much faster. They would not produce the heavier elements – like calcium, iron – that we need.

Ard: So what would happen if the stars were different?

MG: Without fine-tuning, the stars would not behave the way they do. They would not produce the chemical elements that they do, and life as we know it, which depends on a whole set of different chemical elements, would not be possible.

So when people jokingly say, ‘We’re all stardust’, it’s not a joke. It is actually beautiful. It’s true: we definitely are stardust, and, in fact, all the chemical elements that we have in our bodies – the calcium in your bones, the iron in your blood – they belonged to stars, billions of years ago before the solar system existed.

David: So what have been the responses to that problem over the last 30 years?

MG: So one of the responses is, who cares? You know, it just happened to be that way. It’s an accident, and the fact that it’s an accident doesn’t make it special at all. So that’s one possibility: it’s all random, and because of that there is no reason to try to explain it, which is quite appealing, actually, in many ways.

The other one is to say there is a reason for this: everything has a reason, and the goal of science is to explain why things are the way they are. Hence, the fact that the electron has this mass etc. must have a causal explanation. And so what would that be? Then it becomes not so much a fine-tuning, but a search for an explanation for the fine-tuning.

David: Right.

MG: There have been a few out there, but the most popular one nowadays comes from something called String Theory, which is a very bold attempt to understand nature in a completely different way than we usually understand, which is that instead of things being made of little bits called elementary particles, they are actually made of vibrating wiggly things called strings. And the same way that when you play a violin or a guitar you pluck a string, and if you change where your finger is, you’re going to get a different sound, a different frequency or vibration, those strings can vibrate in different ways, and depending how they vibrate, they actually emulate the properties of all the different particles of nature. So it’s a very cool idea, and it’s an idea that, in principle, could bring together all different forces of nature: so the big, grand unifying theory.

The problem is, on these string theories, that you would hope, originally, when they were proposed in the, early 80s – they were called Super Strings – they would say, ‘We’re going to solve these equations, and the solution is going to be the universe as we know it!’ You know, everything is in there. And, people tried and tried and tried.

Ard: Did you try?

MG: I tried, yes. And one of the problems with these theories is that you have ten dimensions: nine spatial and one time. And then you look around, you’re like, ‘Wait a second, I only see three.’ You know, is it north, south, east, west, up, down? Right, three? Where are the other six?

David: So the other dimensions are here, they’re just curled up really small and we can’t see them?

MG: That’s right. They’re really so small that you cannot see them.

David: So they’re all around us?

MG: Imagine each point of space has a little six-dimensional blob, or sphere, associated with it, and that’s what it is. And it’s not so crazy because if you look at this. [Holds up a stick] This is a stick. If you look at it from very, very far away, it’s going to look like a line, and a line is a one-dimensional thing. You can only go this way or that way. But you look closer and you realise it’s not really a line because you can also go around. So this is more like a cylinder. But from far away it looks like it has one dimension, because this circular dimension around it is too tiny compared to the length of the stick.

The idea is exactly the same. Every point in space has this six-dimensional sphere hidden in it, and it’s just so tiny, we don’t see it. And so the question is, why are they so tiny?

So, back to strings. Strings, to exist, have to vibrate in this nine-dimensional space, and the point is those extra six dimensions, they can be folded up in many different ways – just like if you get a balloon you can twist it, you can make holes in it. These are different topologies. So the six-dimensional extra space has different topologies. When people start to calculate how many of these could be around, instead of ‘the universe’ coming out, they came up with a ridiculously huge number, which is a ten with 500 zeros on top: so, one with 500 zeros afterwards.

David: Isn’t that many more particles than we have in the universe?

MG: Oh, pfft! Yeah, many, many, many, many more particles.

David: That’s a big number.

MG: It is a ridiculous number, which means, now what? So the goal, the dream, of finding ‘the universe’ became, ‘What do we do with all this stuff?’ That’s where the multiverse came up. So each solution, each folding of this extra-dimensional space, is, potentially, a different kind of universe.

David: So we’re going to have 10 to the power 500 different universes to explain this one?

MG: Yes, exactly.

David: But instead of having the difficulty of explaining how one universe comes into being, now we’ve got to explain how 10 to the 500 universes came into being.

MG: Right.

David: And since we can’t explain this one, it seems to me we’ve just made the problem a whole lot worse, not better!

Ard: Yeah, but I think the argument would be that now you’ve got a mathematical theory, at least, a beautiful theory which explains that.

David: I don't know. It doesn’t seem that beautiful to me. We used to have the one universe we needed to explain, now we’ve got loads.

MG: Exactly. It’s like the universe now becomes a data point in a vast manifold of possible points, and are you really explaining something with that? Did you gain any knowledge from this?

Ard: And there’s a question maybe about can you do experiments on…?

MG: That, to me, is the fundamental question. Physics is supposed to be an empirically validated science. You come up with some hypothesis, doesn’t matter how crazy it is, but it has to be empirically tested. You’ve got to make an experiment, an observation, and say, ‘Yeah it’s okay’, or ‘It’s not okay’. In practice, it’s not so black and white. There are many, many subtleties to this argument, but at the end of the day you need to be able to prove your idea, otherwise it’s not physics, it’s something else.

Ard: Prove it by experiments.

MG: You prove it by experiments, and that’s why there is the rift, right? Because… It’s something else because it’s a different way of doing science. Because what you’re trying to do now is you’re bringing up an idea that is based on a-posteriori reasoning, which is, ‘We’re here’. We start from that, and usually the explanation is, ‘How did we get here?’ You go from beginning to end. Now, it’s starting from the end and you want to create an argument based on our existence. And the point is, is that good enough as an explanatory tool, or are we just throwing in the towel and pretending we are smart?

Ard: So wouldn’t you also say that this method of trying to find out about other universes is like the method Dirac used to predict antimatter? He used the tools of mathematics: he tried to make something consistent and out of it pops antimatter. You do it again and you get the Higgs boson. You do it again, and out pops other universes. Isn’t that…?

MG: Yeah. That would be beautiful if I could go and do an experiment to see the multiverse the same way I see the Higgs or the positron, but I can’t. So it…

Ard: That’s where it’s different?

MG: Yeah, so the mathematics is compelling. But being compelling doesn’t mean it’s right. And that’s very important.

Ard: Do you think that because, in the past, that compelling mathematics has turned out to be true, that physicists feel that it must be true about the multiverse?

MG: Well, you have to be careful. It has been true a few times, sometimes, not always.

Ard: Okay.

MG: And of course, when it is true, it’s so mind-bogglingly spectacular that you go, ‘Whoa! There is something going on here.’ But you can’t make that into a rule.