Between gravity and the quantum world

It is often at the boundaries where things get exciting: They offer space for exchange, tension – but also conflict. And while we are used to boundaries in the outside world, it is surprising that the world of physics is also divided into two parts.

On the one hand, there is the sphere of quantum theory, which has gone from one success to the next since its development a good 120 years ago: From the most volatile particles to the glow of stars, physicists use quantum physics to describe an impressive range of phenomena.

Seemingly separate from this world, there is also the realm of gravity, which forces planets, gas nebulae and galaxies into their orbits. Both theories are an integral part of modern physics. There is only one problem: Gravity and quantum physics do not go well together.

Looking for the link

This fact rankles physicists like Philip Walther. "We are actually looking for a theory that describes everything," says the Professor of Quantum Physics at the University of Vienna. "That is why it is interesting to see how the two fundamentally different theories interact.”

But that is not so easy, because gravity and quantum physics do usually not get in each other's way: At large masses and high energies, quantum effects are negligible, just as, conversely, weak gravity usually plays no role in the microcosm.

Only recently, and thanks to state-of-the-art experiments, have researchers analysed the effects at the boundaries of the two hemispheres of physics. There are two ways to see the effects of gravity on quantum systems: "Either we achieve extremely high precision, or we work with larger masses," says Walther.

Photon test object

Walther and his team are pursuing the first approach: "We are working with massless particles, and therefore directly within the scope of Albert Einstein's general theory of relativity," says the physicist. Massless – this is where photons, the particles of light, come into play. As Einstein's formulae show, photons have no (resting) mass.

In fact, light is the most prominent inhabitant of the border area between gravity and quantum physics: Although individual photons are quantum objects, they experience the influence of gravity when they whizz around clusters of galaxies on curved paths in space, for example.

For Walther, an expert in quantum states of light, photons are therefore the ideal object of research – especially as their interaction with gravity can only be correctly described using the general theory of relativity and not Newton's law of gravity, which is already obsolete today.

Together with gravitational physicist Piotr Chruściel from the University of Vienna, Walther will investigate exactly how gravity and photons interact as part of the international GRAVITES project, which also involves research groups from the Massachusetts Institute of Technology (MIT) and the University of Munich.

Precise measurements

At the heart of the project, which was recently awarded an ERC Synergy Grant worth almost nine million euros, will be a fibre optic interferometer that will measure the influence of gravity on entangled photons over a total length of 50 to 100 kilometres.

This is a set-up in which an entangled pair of photons has two paths of equal length at its disposal, which reunite at the end. The trick is that one of the paths will lie above the other. As the force of gravity becomes weaker with increasing distance from the Earth, the paths through the interferometer are exposed to different gravitational influences.

This difference in gravity should be noticeable in a change in the entanglement state of the photons. But: "This prediction has not yet been tested," says Walther. According to the physicist, the planned interferometer could be used to experimentally investigate the influence of gravity on quantum entanglement – one of the key characteristics of quantum physics – for the first time and possibly detect deviations. "That would be particularly exciting, of course."

Highly challenging research

The precision required for this is a huge technical challenge: The interferometer must be shielded against thermal fluctuations, vibrations and other interference and the length of the arms must be actively stabilised. Following the remodelling of the Faculty of Physics, the experiment will even have its own room, which will be decoupled from the environment as far as possible.

This will create the most sensitive experiment of its kind on a laboratory scale. However, the ability to precisely manipulate light quanta – and potential technical spin-offs – are only a pleasant side effect for the experts. Their focus is on measuring this mysterious border area between gravity and quantum physics.