# Q&A Part 1: String theory, cosmology and gravity for beginners

#### For many people, Theoretical Physics is an abstract concept. How would you describe it to a layman, who is not scientifically minded?

Well, we try to understand how nature works and how all the matter we see in the universe works including the forces in between. We do this in Theoretical Physics by thinking about the equations that describe matter, space and time. We then try to find laws that we can use to make predictions, and test these with experiments. In this way, we make progress in our understanding of nature.

#### String theory is quite a ‘hip’, modern concept in physics. Can you explain what string theory involves?

Quantum mechanics is needed to able to describe what is going on with particles at the smallest scales, and we know quantum theory works very well. We can also look at things involving gravity, space and time, and General Relativity. It has been a long-standing puzzle to combine quantum mechanics and General Relativity into one framework. String theory does this in quite an elegant way. It has a very precise mathematical structure that you can build on. It then allows you to do calculations that bring these two aspects – quantum mechanics and General Relativity – together.

#### You’re famous for developing this new theory, or idea, on gravity in which you say that gravity is an illusion. Can you explain what you mean by that?

Well, of course gravity is not an illusion in the sense that we know that things fall. Most people, certainly in physics, think we can describe gravity perfectly adequately using Einstein’s General Relativity. But it now seems that we can also start from a microscopic formulation where there is no gravity to begin with, but you can derive it. This is called ‘emergence’.

We have other phenomena in Physics like this. Take a concept like ‘temperature’, for instance. We experience it every day. We can feel temperature. But, if you really think about the microscopic molecules, there’s no notion of temperature there. It’s something that has to do with the property of all molecules together; it’s like the average energy per molecule.

Gravity is similar. It’s really something that only appears when you put many things together at a microscopic scale and then you suddenly see that certain equations arise.

#### What’s the practical importance of having this new manner of considering gravity?

As scientists, we first want to understand nature and our universe. In doing so, we have observed things that are deeply puzzling, such as phenomena related to dark matter. We see things happening that we don’t understand. There must be more matter out there that we don’t see. There’s also something called ‘dark energy’. And then there’s the whole puzzle of the beginning of the universe. We now have what is called the ‘Big Bang’ theory.

I think the ideas I have will shed a totally new light on and resolve some of these puzzles.

#### Can you explain the concept of ‘dark matter’ and ‘dark energy’ and why they’re important in relation to gravity?

We think we understand gravity in most situations, but when we look at galaxies and, on much larger scales, at galaxy clusters, we see things happening that we don’t understand using our familiar equations, like Newton’s equation of gravity or even Einstein’s gravity. So we have to assume there’s this mysterious form of matter, which we call dark matter, which we cannot see. Now dark energy is even weirder, in the sense that we don’t even know what it consists of. It’s something we can put in our equations to make things work, but there’s really a big puzzle to be solved in terms of why it’s there and what it’s made of. At present, we have not really found the right equations to describe it.

There’s clearly progress to be made in terms of finding a better theory of gravity, and understanding what’s happening in our universe.