22.08.2019 | Quantum physics

Quantum gravity’s tangled time

The theories of quantum mechanics and gravity are notorious for being incompatible, despite the efforts of scores of physicists over the past fifty years. However, an international team of researchers now discovered that the temporal order between events can exhibit genuine quantum features.

© Magdalena Zych

According to general relativity, the presence of a massive object slows down the flow of time. This means that a clock placed close to a massive object will run slower as compared to an identical one that is further away. However, the rules of quantum theory allow for any object to be prepared in a superposition state. A superposition state of two locations is different to placing an object in one or the other location randomly – it is another way for an object to exist, allowed by the laws of quantum physics.

In their new study, scientists from the Austrian Academy of Sciences, University of Vienna, the University of Queensland (AUS) and the Stevens Institute of Technology (USA) have researched the question what happens when an object massive enough to influence the flow of time is placed in a quantum superposition state?

New discovery

They were expecting to face the roadblocks making the scenario impossible, but surprisingly, using standard textbook physics they were able to exactly describe what happens. They so discovered that when a massive object is placed in a quantum superposition in the vicinity of a set of clocks, their time order can become genuinely quantum, defying any classical description.

Caslav Brukner, coauthor from the Institute for Quantum Optics and Quantum Information  (IQOQI Vienna) of the Austrian Academy of Sciences, states that the regime where quantum time order could arise is quite remote from our everyday experience. “The most important insight from our work is that quantum time order is at all possible, and that it results in new physical effects.”

Thought experiment with starships

To illustrate what happens, imagine a pair of starships training for a mission. They are asked to fire at each other at a specified time, and immediately start their engines in order to dodge each other’s attack. If either of the ships fires too early, it will destroy the other, and this establishes an unmistakable time order between the firing events. If a powerful agent could place a sufficiently massive object, say a planet, closer to one ship it would slow down its counting of time. As a result, the ship farther away from the mass will fire too early for the first one to escape.

The laws of quantum physics and gravity predict that by manipulating a quantum superposition state of the planet, the ships can end up in a superposition of either of them being destroyed. Such a superposition state, involving two systems, is called entangled.

The new study shows that the temporal order among events can exhibit superposition and entanglement – genuinely quantum features of particular importance for testing quantum theory against alternatives. The result can now be used as a theoretical testing ground for frameworks for quantum gravity, and thus help to move forward in formulating the correct theory of quantum gravity.



"Bell’s theorem for temporal order"
M.Zych, F.Costa, I.Pikovski, and Č. Brukner, Nature Communications. Volume 10, Article number: 3772 (2019)