Predicting聽the future is easy, if you are a physicist. Break a glass, and you can boldly assert that it will fall into a number of shards, assuming you know the initial conditions. Knowing the past is more difficult 鈥� you need to store much more information to piece a pile of broken glass back together.
This 鈥渃ausal asymmetry鈥� makes it easier to determine cause and effect and thus place events in order. But it doesn鈥檛 exist in the quantum world, say Mile Gu at Nanyang Technological University in Singapore. Quantum physics uses the same amount of information to trace an event backwards in time as forwards. This find could have implications for how we perceive time itself.
Gu and his colleagues found that when a quantum computer models a system that seems聽causally asymmetrical to聽a聽classical computer, the asymmetry can disappear. In聽some cases, the amount of memory you need to store information is the same in both temporal directions, so there is no concept of cause and effect.
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Quantum movie
Gu describes modelling ordinary physics as watching a film. 鈥淚f you watch the movie in the correct direction, it makes much more sense,鈥� he says. But for a quantum movie, both directions make sense.
His team tested this idea with a simple system, a list of ones and zeroes. Imagine the ones and zeroes as tiles on a path running south to north. You are tasked with crossing out each zero that lies immediately north of a one.
If you walk forwards, south to north, this is simple 鈥� if the last tile you stepped on was a one and now you are on a zero, you can cross it out. But if you walk backwards, north to south, you need to know what the next tile south is before you reach it. That is why the classical system requires storing more information moving backwards than forwards.
In the quantum system, as you walk in either direction, you place all of the tiles with zeroes into entangled quantum states, both crossed off and not crossed off. The entanglement stores some of that information, meaning you use less memory, says Gu.
The weird properties of quantum physics means that once information on all the tiles has been gathered in a quantum state, measuring that state produces the correct answer, no matter which direction you travelled, he says.
鈥淭his is a big area in quantum computing 鈥� to see, once we鈥檝e built these things, what they鈥檒l be聽good for,鈥� says Gu. 鈥淪ystems with causal asymmetry are a class聽of problems where these quantum machines can have a聽significant advantage.鈥�
A quantum model will always need less memory than a classical one, so using a quantum computer to model a process in reverse will take less information than using a classical computer to model the process forwards. Sometimes, a quantum computer will need the same amount of memory to run forwards or backwards, eliminating causal asymmetry all together.
That means that, in some cases,聽our perception of cause and聽effect may come from our inability to process information in a quantum way. 鈥淲e perceive this asymmetry because we reason classically,鈥� says Gu.
鈥淚t is possible that if we evolved to聽reason quantum mechanically, we might not perceive time.鈥�
Others are less convinced. 鈥淚 would not draw any philosophical implications from this work,鈥� says聽Howard Wiseman at Griffith University in Australia聽鈥� after all, large objects like us can never perceive the quantum world directly.
Physical Review X
