Does gravity need a quantum theory?

What would such an experiment be like?

Aspelmeyer: Over the past 20 years, we have developed methods that enable us to manipulate solid-state objects the size of a grain of sand in such a way that they must be described by the laws of quantum physics. These quantum objects consist of many billions of atoms, and in some cases they are even visible to the naked eye. This makes them large enough for us to wonder whether their gravitational field can be measured. On the one hand, we want to make these objects even larger.

On the other hand, we are developing extremely precise measurement systems to detect the gravity of even the tiniest masses. Right now, we are still several orders of magnitude away from bringing these two worlds together. A second task is even more difficult: we must get the quantum objects into superposition and be able to measure the gravitational fields of the two superposition states separately.

To what extent does this bring us closer to a "theory of everything" that unifies the theory of relativity and quantum mechanics?

Aspelmeyer: Einstein himself noted that the general theory of relativity is not sufficient to describe all gravitational phenomena. He assumed that a quantum theory would also require modifications to the theory of gravity, which was new at the time. If we succeed, this assumption would be confirmed. We would then be certain that there must also be a quantum theory of gravity. While we have theoretical explanatory models on paper, we do not know whether they actually describe nature—because it has not been possible yet to test them.

Nobel laureate Anton Zeilinger has revolutionized quantum physics through his breakthroughs in quantum optics. To what extent does your work build on his fundamental research?

Aspelmeyer: Without Zeilinger, I wouldn’t be where I am today. He had the courage to have me join his group, even though I came from a completely different field of research, i.e. solid-state physics. That led me to long years of working on the quantum properties of solid-state objects. At the time, I was a member of a newly founded early-career research group in Vienna, and Zeilinger granted me complete freedom to pursue my ideas. As a next step I switched to gravity – another major thematic shift in my research career.

Experiments in quantum physics often require the development of complex new instruments that are also of interest for further applied research. Could your research also open up commercial applications?

Aspelmeyer: Yes, absolutely. Sensor technology is an obvious field. With our instruments we need to function in areas that were previously unmeasurable – for example, detecting gravity on the smallest scales. We have to develop this type of quantum sensor from scratch. I think it’s possible that there is a market for something like that. If there’s a need and there are people in the group interested in applied research, nothing stands in the way of commercialisation. After all, there are many attractive opportunities for this in Austria.

The Wittgenstein Award gives your research group new freedom. How will this affect your work and your ability to achieve your research goals?

Aspelmeyer: The new funding will definitely be a great boost for our research. We are currently the only group in the world conducting both gravitational and quantum experiments with small masses, and we’re trying to break records in both areas. The Wittgenstein Award gives us the independence and momentum to advance our unique approach. We are doing research at the Champions League level, and this award is an important step toward winning that championship.

What are the next steps?

Aspelmeyer: Our most important resource is our staff. We have a strong international pool of applicants, and we’ll make every effort to bring outstanding researchers from all over the world into the team. The group is already very international, with people from seventeen countries. We also need to carefully consider how to fine-tune our ongoing experiments and where to change course. Someone once said to me that the hardest decision is what experiment to stop, not what one to start. That rings true.

Is the race for the Holy Grail of physics –figuring out the connection between the theory of relativity and the quantum world – your primary motivation, or has this goal evolved organically from your previous research?

Aspelmeyer: Definitely the latter. I've always just focused on what I was interested in. Our motivation is intrinsic. I’ve seen more than once that competition only comes in the wake of success: first, you create a great experiment, which generates attention, and then other research groups start taking an interest in the topic. But it’s a "friendly competition": we learn from one another, keep each other up to date, and advance the field of research together. After all, there’s hardly anything more rewarding than watching a community grow – seeing how a few ideas you originally pursued lead to the development of an entire research community.

Interview by Alois Pumhösel at the scilog - the FWF Science Magazine; Videos: University of Vienna