Pioneering research into quantum computing reveals time moves forward and backward, upending much of what we know about the nature of time.

A team of quantum physicists at the National University of Singapore has found that time behaves differently in quantum computing models. 

They say what is true at the quantum level is true for the rest of the universe as well.

What Is Causal Asymmetry?

It is pretty obvious to everyone that time always moves forward, never backward. It makes sense as 7:00 rolls into 7:01, Monday into Tuesday, and middle age into senior years and finally to death.

This understanding of time also applies to classical computers, which store information as either one of two states, 0 or 1. It is far less challenging for computers to figure out what will happen to the system in the future than to predict what happened in the past based on the information it has in the present.

This is called causal asymmetry, the idea that a system needs fewer resources to move forward than backward. In the real world, it is widely known as cause and effect. One thing happens now that results in another thing in the future. What happens in the future cannot affect what happened in the past.

Information theorists have long assumed that causal asymmetry is a fundamental property of the universe. In 1927, astrophysicist Arthur Eddington proposed that causal asymmetry is the reason why time moves forward, not backward.

Think of the universe is a massive cosmic computer. It requires far less information to move forward in time than backward. The cause happens before the effect, not the reverse.

No Causal Asymmetry In Quantum Computers

In a study published in Physical Review X, the team of quantum scientists challenge the prevailing theory of causal asymmetry. The new paper explains how the linear movement of time into the future could be a relic of the bygone era of classical computing.

Using theoretical quantum computers, the researchers found that causal asymmetry does not exist in quantum models. They say that what applies to a quantum computer also applies to massive objects in the universe.

Quantum physics is the study of very small particles exhibiting strange behaviors that are not obvious on the bigger level. What is true for quantum particles is also true for massive objects, even when our limited tools keep us from actually perceiving these behaviors.

This means that, if causal asymmetry does not exist at the quantum level, it also does not exist for the universe.

How Quantum Computers Work

Unlike classical computers, quantum computers store information in subatomic particles that can exist in more than one state at a time. Both quantum and classical computers can easily predict what happens in systems that are either very orderly (such as a pendulum) or very random (such as a gas-filled chamber).

The researchers looked at systems with just the right amount of disorder and randomness, such as weather systems. They found that quantum computers studying these systems were able to go forward and backward without using up more memory than going in a single direction.

"While classically, it might be impossible for the process to go in one of the directions [through time], our results show that 'quantum mechanically,' the process can go in either direction using very little memory," says Jayne Thompson, complexity theorist and quantum physicist at NUS.

The researchers point out that their discovery does not mean there is zero causal asymmetry at every point in the universe. Thompson says it is possible that a quantum model exists that demonstrates causal asymmetry.

The team hopes to find out if they can devise models where this phenomenon exists.

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