Theoretical physicists describe a new kind of quantum temporal order

One of the fundamental principles of quantum mechanics is quantum superposition, in which a particle exists simultaneously in two or more states. In one paper published in the journal Nature CommunicationUniversity of Queensland physicist Magdalena Zych and her colleagues show that particles aren’t the only objects that can exist in a state of superposition – time too.

Zych et al provide the first direct analysis of quantum causal relationships arising from a spatial superposition of a massive object; they show how the temporal order between temporal events can overlap or even overlap. Image credit: University of Queensland.

“The sequence of events can become quantum mechanical,” said co-author Dr. Igor Pikovski, a physicist at the Stevens Institute of Technology.

“We looked at the quantum temporal order where there is no distinction between one event causing the other or vice versa.”

The team’s work is among the first to reveal the quantum properties of time, according to which the flow of time does not observe a straight forward arrow, but one where cause and effect can coexist at the both forward and backward.

In the coming era of quantum computers, the work is particularly promising: quantum computers that exploit the quantum order of runtime operations could beat devices that operate using only fixed sequences.

To show this scenario, physicists merged two seemingly contradictory theories – quantum mechanics and general relativity – to conduct a Gedankenexperimenta way of using the imagination to investigate the nature of things.

“The discovery arose from an experiment we designed to bring together elements of the two great – but contradictory – physical theories developed over the last century,” explained Dr Zych.

“Our proposal aimed to discover: what happens when an object massive enough to influence the flow of time is placed in a quantum state?”

“Einstein’s theory described how the presence of a massive object slowed down time,” she added.

“Imagine two starships, called upon to fire on each other at a specific moment while dodging each other’s attack.”

“If one shoots too soon, it will destroy the other.”

“In Einstein’s theory, a powerful enemy could use the principles of general relativity by placing a massive object – like a planet – closer to a ship to slow the passage of time.”

“Due to the time lag, the ship furthest from the massive object will fire sooner, destroying the other.”

“The second theory, from quantum mechanics, says that any object can be in a state of superposition. This means it can be found in different states – think Schrödinger’s cat.

“Using quantum mechanical theory, if the enemy puts the planet in a state of quantum superposition, then the weather should also be disrupted,” Dr Zych said.

“There would be a new way for the order of events to unfold, with neither event being first or second – but in a true quantum state of being both first and second.”

“Although a ‘superposition of planets’ is never possible, the technology has enabled a simulation of how time works in the quantum world – without using gravity,” said co-author Dr Fabio Costa, from the ‘University of Queensland.

“Even if the experiment can never be done, the study is relevant for future technologies.”

“We are currently working on quantum computers that, very simply, could actually jump in time to perform their operations much more efficiently than devices operating in a fixed sequence in time, as we know it in our ‘normal’ world.”


Madeleine Zych et al. 2019. Bell’s Theorem for Temporal Order. Nature Communication 10, item number: 3772; doi: 10.1038/s41467-019-11579-x

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