Drillingsraum: Analyzing the Cosmic Microwave Background is not the only way to have a look into the past of the universe: Many scientists pin their hopes on the detection of gravitational waves. Let us assume that we will find them some day. What can we learn at best from these signals, and how much time and effort will it take to interpret them properly?

Prof. Dr. Simon White: Well, there are several different ways to do this. I spoke a bit earlier about the polarisation of the microwave background. Gravitational waves actually influence the pattern of this polarisation, so the great hope for the polarisation experiments is, that they will actually detect these influenced polarisation pattern. This would basically tell us something about processes that happened a tiny fraction of a second after the big bang. Well, that of course is still an indirect detection of the gravitational waves, we would just see their influence on the material which is reflected in its polarisation pattern. People are also trying to detect gravitational waves directly, they are doing this by setting up various kinds of experiments which detect the very small distortions of space time caused by the gravitational waves. One of them, the GEO-Experiment, is located in northern Germany for example. I was told that the main noise at the moment comes from the waves breaking on the beach of the ostsee. (laughs)

Drillingsraum: Or from the gardener, walking over the experiment.

Prof. Dr. Simon White: (laughs) Well, they are looking for really small fluctuations, so they have to isolate their experimental apparatus from all oscillations in the outside world. There are larger interferometers working in America, which are also looking for these signals. The sensitivity of these experiments is being increased rapidly as people understand the laser-technology that is needed at higher and higher levels, it has to be extremely precise. They are now reaching levels where it will start to become a problem if they don't see gravitational waves. But these signals will not come from the early universe, they will come from merging black holes. So, the first

from the early universe are much more difficult to detect, and it is unclear when and how this will happen. If they could be detected in the same way as we detect the microwave background, which is heat radiation from the early universe, then they essentially will come directly from the initial singularity itself, because gravitational waves don't interact significantly with anything else after this very first stage. It then would be a direct image of effects within the inflation epoch itself.

Drillingsraum: Why don't we make a little bet: In which year do you think gravitational waves will be detected for the first time? I then will say before or after. As a wager I would suggest a coffee from the coffee-machine upstairs.

Prof. Dr. Simon White: Well, of course I could say they already have been detected, because the Nobel Prize was given for this (laughs). But you can still count this as an indirect effect, since we just saw an energy loss.

Drillingsraum: Of two neutron stars.

Prof. Dr. Simon White: Right. But ok, lets say: They will be detected directly by these detectors in … I think ... five years.

Drillingsraum: All right. So that would be in 2016.

Prof. Dr. Simon White: Yes.

Drillingsraum: Ok, so I say before. Is a coffee ok as a wager?

Prof. Dr. Simon White: That's fine. Young people are always more optimistic (laughs). I remember the 1980's when they were still saying the detection was five years away (laughs). It's a little like nuclear fusion.

Drillingsraum: In the 1980's Stephen Hawking used the path integral formulation to find an access to quantum gravity. To accomplish this effort, he had to assume a closed spacetime without borders and edges, a little bit like the surface of this Klein Bottle here, which is the second present for you.

the structure of the universe involved a technical working with relativity, he rigorously carried through specific kinds of geometries. We think that the geometry he initially used is not relevant to the geometry of the universe, it is not consistent with special relativity for example. So, this was just done for some technical reasons. I think it depends a little on which point you are discussing. The evaporation of black holes for example, which maybe is what you referring to, is the fact that black holes may not be truly black but actually can emit thermal radiation. That for example didn't depend on these particular assumptions, this was Hawkings intuition about how you could combine quantum mechanics and general relativity. Other people have explored these possible connections in various ways. I think it is true to say that these techniques are still ad hoc. The current attempt to put gravity and quantum mechanics together is string theory. There the problem is: They have a theory which they think may describe both, but they cannot take the limits to check.

Drillingsraum: You would need an accelerator as big as the solar system.

Prof. Dr. Simon White: No, no, that wouldn't be enough. Much larger. Another thing is: The mathematics in the string theory is very beautiful. This theory seems to have the potential to describe both quantum mechanics and gravity in a unified way, but it needs a larger dimensional space whose symmetries will no longer hold. You have to understand how these symmetries will be broken and see whether the broken limits will correspond to the physics that we know, and so far it is not possible to demonstrate this in a rigorous way. People have found ways to describe how this might happen, which looks encouraging, but I think it is still not clear whether string theory is a description that is relative to our universe or not.

Drillingsraum: Let us talk a little bit more about this topic. As you described, physicists are looking for a theory of quantum gravity to describe both the processes in the microcosm as well as those in the large scales of the universe. However, it is impossible for us to observe the whole universe. Furthermore, we are limited in many

Prof. Dr. Simon White: I hope not. What would you do afterwards? (laughs) I think it is very unlikely. But what does it mean to say it is just an interpretation? Interpretation is understanding. For example, let us assume we had an enormous computer like the one in The Hitchhikers Guide To The Galaxy and we thought we understood the laws of physics.

Drillingsraum: Did you read it?

Prof. Dr. Simon White: Of course. So we thought we understood all the laws of physics, program them together with the initial conditions into the computer, let the computer run and in the end something comes out which looks like our universe. I think we still wouldn't have understood anything about how the galaxies formed. We wouldn't be further forward in understanding the formation of the galaxies, because effectively we are just looking at them. What we would learn is that the present universe is consistent with the initial conditions. That would be a gain in physical understanding, which would be important. But it still doesn't tell us how the galaxies form and we still wouldn't understand it any better.