In an article published in Communications Physics, researchers from the Université libre de Bruxelles and the Institute for Quantum Optics and Quantum Information in Vienna present a new framework for describing physics relative to quantum reference frames, unveiling the importance of previously unrecognized “extra particles.”

In any experiment, specifying a physical quantity of interest always relies on a reference frame. For example, identifying the time at which an event happens only makes sense relative to a clock. Similarly, the position of a particle is usually defined relative to other particles. Reference frames are typically treated as classical systems, that is, they are assumed to have definite values when measured relative to other reference frames.

However, as far as we know, every system is ultimately quantum. As such, it can, in principle, exist in indefinite states called quantum superpositions. What does the physical world look like when described from the perspective of a reference frame that can be in a quantum superposition? Can we define consistent rules for changing between different perspectives?