What do you think are the challenges for young researchers these days — and how do they compare to when you were a graduate student?

It is always difficult to compare generations. When I started graduate school in the 1970s, it was right at the time when lasers were becoming a useful tool for atomic physics. That was an incredibly exciting time — almost everything you did would be something new. Although the laser was invented in the 1960s, it was only by about the 1970s that it was possible to buy a laser that you could tune to an atomic resonance, shine at atoms in a gas and excite them. This changed experimental physics in a very deep way.

Lasers are used to trap and cool atoms to fractions of a degree above absolute zero, so that all atoms are in the same quantum energy state in a Bose–Einstein condensate. Credit: PASCAL GOETGHELUCK/SCIENCE PHOTO LIBRARY

If I look at physics as a whole, I am not sure that I can easily tell what gives people an advantage today, but I can see some opportunities. One of the things that happened during this year's Lindau meeting was the announcement from CERN of the discovery of the Higgs boson. For many people, it completes the standard model of particle physics, and there are two ways in which you can look at this. One is to say that it closes a chapter of physics; I think that is a very disappointing way of thinking of the discovery. A better way is to realize that it tells you where to go next. If you want to do something else, then you should look outside the standard model. There are lots of things that are not apparently included: dark matter and dark energy, for example (see 'Out of the darkness', page S2). These are areas where we can focus our attention.

We live in an incredibly exciting time for physics. We don't know what 96% of the Universe is made of — what can be more exciting than that? We have 4%, which is matter that we know about; 23% is dark matter, which we cannot yet identify; and the remaining 73% we know even less about. When I was a graduate student we didn't know what we didn't know about the Universe. Now we know what we don't know — and that is really a good place to be.

Did you ever suspect how far the field of cold atoms would go?

When I first started working on cold atoms in 1978, the motivation was to make a better atomic clock, which we achieved. It's amazing that all this work on cooling is useful for the thing we first imagined it would be. But there are so many other things — our inability to see where a field is going is an almost universal fact of the way we do research. When I was a postdoc I was studying atomic beams of sodium using tunable lasers and separately the Bose–Einstein condensation (atoms cooled to a very low temperature all sharing the lowest energy state) of atomic hydrogen. After finishing my postdoc, I first started to use lasers to slow atomic beams, so that the slow atoms could be used for atomic clocks. It was not in my mind that this was going to be a route to Bose–Einstein condensation. In fact, it turned out to be the route to a Bose–Einstein condensate (whereas the initial approach we took turned out not to be a particularly good way to make one). Finally, when we did have Bose–Einstein condensates, I don't think any of us imagined where this work would lead: into a whole new field of quantum degenerate gases (ultracold gases that exhibit quantum-mechanical behaviour).

There is an important lesson here: you must not imagine that you can see where a field is going, or think that you are smart enough to know where we should devote our resources if we want to accomplish a particular goal. Almost always, things turn out to be so much richer than you imagine. This is a lesson for everyone: for people who are going into research, and for people who are funding research. You just don't know.

Can we ever predict the future when it comes to science?

There are some subfields of science that become mature, full of things that have become well understood. But very often this is a prelude to finding out that there was something you were not paying attention to. All of a sudden you don't understand things you thought you understood. Atomic physics was like that in the 1950s — it was kind of boring. People thought that we understood quantum mechanics and hence everything about atoms. And then the laser came along, and we started doing experiments that nobody had even imagined. We go into the lab and learn new things every day.

What do you think is the most beautiful experiment in atomic physics today?

Let me choose two things. One is the idea of artificial fields. To take a neutral atom and make it behave as if it is a charged particle in a magnetic field — this is amazing. Second is the ability to image individual atoms and distinguish them from their neighbours. You can make a two-dimensional array of atoms and see where every one of those atoms is and how they have arranged themselves. These experiments open up opportunities for doing things that we could not even imagine a few years before.

boxed-text If you were a graduate student now, would you choose a different career?

I have seen a tremendous amount of excitement during my time, in particular the discovery of sub-Doppler cooling and of optical lattices. However, if you were to ask me what was the most exciting time in my whole career, I would say right now! There are so many incredible things going on: artificial fields and seeing individual atoms; recreating phenomena that occur in solids; 'atomtronics' — making circuits with atoms. This is why atomic physics and cold-atom physics is an exciting place to be.

At the same time, there is amazing work being done in other areas of science that would also interest me. For example, in genomics there are so many things that we still don't know. We don't know how most of our characteristics depend on the genome or on non-genomic things. And for a scientist, being somewhere where there is a great deal of ignorance is the best place to be.

Is it possible to be both a successful researcher and a good teacher and mentor?

Teaching and research are linked in an important way and are mutually beneficial. Unless somebody is actively engaged in research it is very difficult to teach at the level required to produce the next generation of researchers. People active in research are thinking about things in a cutting-edge way. But this works both ways — teaching helps you do research. That's because when you teach you are forced to re-examine the subject matter that you thought you knew well. You have to understand it in a way that is deeper than you ever understood it before. This gives you a much more mature understanding and raises questions in your own mind that will be important for your research. When you teach, students ask questions that you don't have answers for, and those questions make you think about new directions. That's why nobody who does research should ever be divorced from the mentoring of students. Those of us who supervise students and postdocs learn as much — if not more — from them as they learn from us.