Viewers of quantum events are also subject to uncertainty

It gets better. What if B was also anchored to a quantum object located at a superposition of two locations? Then the quantum state of A is now entangled in two different ways, depending on the possible locations of B. Since the determination of the quantum state of B determines the state of A, A and B are now entangled.

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

Photo credit: Giulia Marthaler

In the example above, it turns out that two essential properties of quantum systems – superposition and entanglement – depend on the reference frame. “The main message is that many of the properties that we think of as very important and in some ways absolute are relational” or relative, he said Anne Catherine de la HametteCo-author of the current article.

Even the order of events is subject to the strictures of quantum reference systems. For example, from a frame of reference we could observe the clicking of a detector at a particular time. But from a different frame of reference, the click could result in a superposition of events before and after another event. Whether you observe the click as occurring at a specific time or as a superposition of different sequences of events depends on your choice of frame of reference.

Springboard to gravity

Researchers hope to use these changing quantum perspectives to understand the mysterious nature of gravity. Einstein’s general theory of relativity, a classic theory of gravity, states that gravity is the deformation of the fabric of space-time by a massive object. But how will spacetime distort if the object itself is in a superposition of two locations? “That’s very difficult to answer using standard quantum physics and gravity,” he said Victoria Cablea researcher in Brukner’s group and co-author of the new paper.

However, switching to a reference frame whose origin is an overlay allows the massive object to end up in a specific location. It is now possible to calculate its gravitational field. “By finding a suitable quantum reference system, we can turn a problem we can’t solve into a problem for which we can simply use standard, well-known physics,” Kabel said.

Such changes in perspective should be useful for the analysis future experiments which aim to bring extremely small masses into superpositions. For example the physicists Chiara Marletto And Vlatko Vedral from the University of Oxford suggested Put two masses at a time in a superposition of two locations and then study how this affects their gravitational fields. The emerging attempts to formally describe quantum reference systems could help to understand these studies of the interaction between gravity and quantum theory – an essential step towards a theory of quantum gravity.