The entangled time of quantum gravity

Theories of quantum mechanics and gravitation are considered incompatible, despite the efforts of many physicists in the last fifty years. However, recently, an international team of researchers led by physicists from the University of Vienna, the Austrian Academy of Sciences, the University of Queensland (AUS) and the Stevens Institute of Technology ( United States) combined the key elements of both theories. describing the flow of time and discovering that the temporal order between events can present real quantum features.

According to general relativity, the presence of a massive object slows down the passage of time. This means that a clock placed near a massive object will work more slowly than the same one, farther away.

However, the rules of quantum theory make it possible to prepare any object in a state of superposition. An overlay state of two locations is different from placing an object in one or the other location randomly: it is in another way for an object of 39, exist, authorized by the laws of quantum physics.

One of the open questions in physics is: what happens when an object powerful enough to influence the flow of time is placed in a state of quantum superposition?

This is a controversial subject: some physicists claim that such scenarios are fundamentally impossible – a new mechanism must prevent superposition from forming at the base – while others develop whole theories based on the assumption that this is possible.

"We started by addressing a question: what would a clock measure if it were influenced by a massive object in a state of quantum superposition?" says Magdalena Zych of the University of Queensland.

Scientists expected to face the obstacles making the scenario impossible, but surprisingly, they were able to describe exactly what was happening using classical physics.

They thus discovered that, when a massive object is placed in a quantum superposition in the vicinity of a set of clocks, their temporal order can become truly quantum, defying any conventional description.

Caslav Brukner, co-author of the University of Vienna and the Austrian Academy of Sciences, added that the regime in which a quantum order of time might exist was far removed from our everyday experience, "but the conclusion the most important of our work is that this order is possible, and that it results in new physical effects ".

To illustrate what is happening, imagine a pair of spaceships in formation for a mission. They are asked to shoot each other at a specific time and immediately start their engines to avoid their attacks. If one of the ships pulls too early, it will destroy the other, which establishes an indisputable chronological order between shooting events. If a powerful agent could place a massive enough object, say a planet, closer to a ship, it would slow down the countdown. As a result, the farthest vessel will pull too early for the first to escape.

The laws of quantum physics and gravitation predict that by manipulating a state of quantum superposition of the planet, ships can end up in an overlay of one or the other. between them being destroyed. Such a state of superposition, involving two systems, is called entangled. The new work shows that the temporal order between events can present a superposition and an entanglement, truly quantum features of particular importance for testing quantum theory versus alternatives. The result can now be used as a theoretical testing ground for quantum gravity frameworks, and thus help advance in the formulation of the correct theory of quantum gravity.

The study will also be relevant for future quantum technologies. Quantum computers that exploit the quantum order of operations can beat devices that operate using only fixed sequences. Practical implementations of the quantum temporal order do not require extreme conditions, such as superimposed planets, and can be simulated without using gravity. The discovery of the quantum properties of time can lead to better quantum devices in the next era of quantum computers.