Does gravity need a quantum theory?
Your research pursues the question as to whether gravity follows the laws of the quantum world. Can you briefly outline the main thrust of your work?
Markus Aspelmeyer: In the quantum world, objects can behave as if they were in two places at once. Called quantum superposition, this phenomenon completely contradicts our everyday experience, but it can be demonstrated in experiments. But we also know that every mass is a source of gravity. Albert Einstein showed that mass curves spacetime and that this curvature is responsible for gravitational effects.
This said, the following question still remains unanswered: what happens to the gravitational field of an object that is in a state of quantum superposition? While we assume that spacetime itself must also be in a state of superposition, we have no experimental evidence for this yet. Hence, we want to find out whether there are gravitational experiments that cannot be explained by Einstein’s theory of relativity, but instead require a quantum theory of gravity.
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.
Markus Aspelmeyer has received the FWF Wittgenstein Award 2026. The physicist and his team in Vienna are the only group in the world conducting both gravitational and quantum experiments with small masses.
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 commercialization. After all, there are many attractive opportunities for this in Austria.
About the research projects
Markus Aspelmeyer's research group is seeking to obtain experimental evidence that supports the hypothesis that gravity, too, requires a quantum mechanical description. The objective is to make solid-state quantum systems so massive that they generate measurable gravitational fields of their own. The researchers intend to use quantum objects of this kind in order to investigate whether the phenomenon of quantum superposition also applies to gravitational fields. If they do, this would provide proof that spacetime itself can be brought to quantum superposition.
The Wittgenstein Prize 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 Prize 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 17 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.
About the researcher
Markus Aspelmeyer is the Scientific and Executive Director of the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences (ÖAW) and a professor in the Department of Physics at the University of Vienna. His research focuses on macroscopic quantum physics, gravitational quantum physics, and the quantum-optical control of solid-state objects. He has been awarded several grants from the European Research Council (ERC). The Austrian Science Fund FWF has supported Aspelmeyer’s work to date with both a START Prize and a series of project grants.
The Wittgenstein Award
The FWF Wittgenstein Award is Austria's most highly endowed research award and is granted to outstanding researchers from all disciplines. The award, endowed with €2 million, supports the researchers’ work and guarantees them independence and flexibility in implementing their projects, giving them the opportunity to advance their research activities at the highest international level.
