Viewers of Quantum Events are also subject to uncertainty

It’s better. What if B is also anchored to a quantum object that is in a superposition of two places? So the quantum state of A is now smeared out in two different ways, depending on the possible locations of B. Because the determination of the quantum state of B determines the state of A, A and B are now intertwined.

Renato Renner, a physicist at the Swiss Federal Institute of Technology in Zurich, thinks that a careful consideration of quantum reference frames will resolve the paradoxes in our understanding of the quantum world.

Credit: Giulia Marthaler

In the example above, two quintessential properties of quantum systems – superposition and entanglement – turn out to depend on the frame of reference. “The main message is that many of the properties that we think are very important, and in an absolute way, are relational” or relative, he said. Anne-Catherine de la Hamettea co-author of the recent paper.

Even the order of events succumbs to the rigors of quantum reference frames. For example, from a frame of reference, we can observe the click of a detector that happens at a certain time. But from a different frame of reference, the click could end up in an overlap of happening before and after some other event. Whether you observe the click as occurring at a particular time or as being in a superposition of different orders of events depends on the choice of frame of reference.

Stepping Stone to Gravity

The researchers hope to use these changing quantum perspectives to make sense of the puzzling nature of gravity. Einstein’s general relativity, which is a classical theory of gravity, says that gravity is the deformation of the fabric of space-time by a massive object. But how will space-time be deformed if the object itself is in a superposition of two places? “It’s very difficult to answer with the usual quantum physics and gravity,” he said Victoria Cablea researcher in Brukner’s group and a co-author of the new paper.

Change to a frame of reference whose origin is in an overlay, however, and the massive object can end up in a defined location. Now it becomes possible to calculate its gravitational field. “By finding a convenient quantum frame of reference, we can take a problem that we cannot solve (and make) a problem for which we can only use standard known physics,” Kabel said.

Such changes in perspective should be useful for analysis future experiments which aims to place extremely small masses in superpositions. For example, physicists Chiara Marletto and Vlatko Vedral of the University of Oxford have proposed by placing two masses each in a superposition of two places and then studying how this affects their gravitational fields. The growing attempts to formally describe the quantum reference marks could help to make sense of these investigations of the interaction between gravity and quantum theory – an essential step for a theory of quantum gravity.