Physics
A new way to think about why time only flows in one direction has major implications for both the universe’s early period and its eventual demise, says Leah Crane
IN THE beginning, there was no quantum entanglement.
That is the conclusion of a study exploring the so-called entanglement past hypothesis, which is part of a quantum reinvention of our notions about why time only flows in one direction.
When two particles become entangled, each can no longer be thought of as an independent object – their properties are tied together, even if they are physically far apart. However, unless these particles are perfectly isolated from the environment, outside interference will eventually cause their entanglement to break down in a process called decoherence.
This phenomenon inspired an idea called the decoherent, or quantum, arrow of time. This posits that because decoherence is irreversible, it could be the fundamental reason why time flows forwards and never backwards. This is related to the more traditional thermodynamic arrow of time, where the direction of time’s flow is governed by the idea that entropy, or disorder, must always increase – a concept at the core of the second law of thermodynamics.
If you follow the thermodynamic arrow back to the beginning of time and reconstruct the starting state of the universe – known as the thermodynamic past hypothesis – you will conclude it must have been one of extremely low entropy. Jim Al-Khalili at the University of Surrey in the UK and Eddy Keming Chen at the University of California, San Diego, have now performed a similar analysis to define the entanglement past hypothesis.
Their research suggests that there was no quantum entanglement in the earliest moments of the universe. As the cosmos evolved, there was more and more entanglement and, correspondingly, more and more decoherence (arxiv, doi.org/mx7z).
“People have been vaguely aware that you need some kind of past hypothesis to get the decoherent arrow of time, but it hasn’t really been worked out in detail before,” says Emily Adlam at Chapman University in California. “This clarifies what exactly that beginning state of the universe is.”
While we cannot directly observe the beginning state of the universe and it may not seem relevant to the current state of things, it is crucial to our understanding of how things have evolved since then – and even what “since then” really means.
“Once you get beyond that very early universe, you have thermodynamic entropy, you have gravity clumping everything up, so you move away from concerns about quantum entanglement,” says Al-Khalili. “Once you have an arrow, once you have a direction to time,
