One of the fundamental principles of quantum mechanics is quantum superposition, in which a particle exists in two or more states simultaneously. In a paper published in the journal Nature Communications, University of Queensland physicist Magdalena Zych and colleagues show that particles are not the only objects that can exist in a state of superposition — so can time itself.
“The sequence of events can become quantum mechanical,” said co-author Dr. Igor Pikovski, a physicist at Stevens Institute of Technology.
“We looked at 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, whereby the flow of time doesn’t observe a straight arrow forward, but one where cause and effect can co-exist both in the forward and backward direction.
In the upcoming era of quantum computers, the work holds particular promise: quantum computers that exploit the quantum order of performing operations might beat devices that operate using only fixed sequences.
To show this scenario, the physicists merged two seemingly conflicting theories — quantum mechanics and general relativity — to conduct a Gedankenexperiment, a 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 big — but contradictory — physics theories developed in the past century,” Dr. Zych explained.
“Our proposal sought 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 time,” she added.
“Imagine two space ships, asked to fire at each other at a specified time while dodging the other’s attack.”
“If either fires too early, 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 one ship to slow the passing of time.”
“Because of the time lag, the ship furthest away from the massive object will fire earlier, destroying the other.”
“The second theory, of quantum mechanics, says any object can be in a state of superposition. This means it can be found in different states — think Schrodinger’s cat.”
“Using the theory of quantum mechanics, if the enemy put the planet into a state of quantum superposition, then time also should be disrupted,” Dr. Zych said.
“There would be a new way for the order of events to unfold, with neither of the events being first or second — but in a genuine quantum state of being both first and second.”
“Although ‘a superposition of planets’ may never be possible, technology allowed 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 towards quantum computers that — very simply speaking — could effectively jump through time to perform their operations much more efficiently than devices operating in fixed sequence in time, as we know it in our ‘normal’ world.”
Magdalena Zych et al. 2019. Bell’s theorem for temporal order. Nature Communications 10, article number: 3772; doi: 10.1038/s41467-019-11579-x