Quantum mechanics finds its application in particular at the scale of atoms and molecules. Nonetheless, quantum effects are studied in ever growing systems of ever growing mass. Gravitational effects, which have so far been neglected, could soon become relevant. This opens the chance for an experimental evaluation of our concepts of space, time, and matter, and can lead the way to the yet unknown theory uniting general relativity and quantum mechanics. An unambiguous and well-established theoretical understanding of the gravitational interaction of quantum matter, however, is still lacking.
Decoherence in quantum systems in the gravitational field
For a quantum particle in the Earth’s gravitational field, Newtonian gravity has already been experimentally tested, however, corrections due to general relativity have not. Gravitational decoherence effects are one of the most promising ways to look for experimental guidance. A systematic evaluation of those effects can provide insight into which of them are the result of classical fluctuations of spacetime and which have their roots in a gravitational field with quantum properties.
The gravitational field of quantum matter
Our knowledge is even less solid when it comes to the question of how quantum matter sources the gravitational field. Take the famous double slit experiment for example, and con- duct it with a heavy quantum particle. Is the resulting gravitational field itself a superposition of two fields—as one would expect in analogy to electrodynamics—or is the geometrical concept of classical space-time valid even at small scales, as sometimes suggested? To date, neither of these possibilities has been ruled out, by observation nor by theoretical reasoning. The two different approaches can, however, lead to different predictions for laboratory experiments that might be feasible in the foreseeable future.