Causal Asymmetry in a Quantum World

ABSTRACT

How can we observe an asymmetry in the temporal order of events when physics at the quantum level is time-symmetric? The source of time’s barbed arrow is a longstanding puzzle in foundational science. Causal asymmetry offers a provocative perspective. It asks how Occam’s razor— the principle of assuming no more causes of natural things than are both true and sufficient to explain their causally – such that the model makes statistically correct future predictions based only on information from the past – what is the minimum past information we must store? Are we forced to store more data if we model events in one particular temporal order over the other?

Take a glass shattering upon impacting the floor. In one temporal direction, the future distribution of shards depends only on the glass’s current position, velocity and orientation. In the opposite, we may need to track relevant information regarding each glass shard to infer the glass’s prior trajectory. Does this require more or less information? For stochastic process, this potential divergence is quantified in the theory of computational mechanics. It is not only generally non-zero, but can also be unbounded. This phenomenon implies a simulator operating in the ‘less natural’ temporal direction is penalized with potentially unbounded memory overhead, and is cited as a candidate source of time’s barbed arrow[1]. These studies assumed models are implemented using classical physics. Could the observed causal asymmetry have been a consequence of this classicality constraint?

In this presentation, we answer this question in the affirmative, by directly constructing a process where there is a classical arrow of time, but at the quantum level this arrow vanishes [2]. Our work suggests that causal irreversibility could be an artefact of forcing classical causal explanations in a fundamentally quantum world.

[1] James Crutchfield, Christopher Ellison, and John Mahoney, Phys. Rev. Lett. 103, 094101
[2] Jayne Thompson, Andrew Garner, John Mahoney, James Crutchfield, Vlatko Vedral, and Mile Gu
Phys. Rev. X 8, 031013
 

BIOGRAPHY

Dr Jayne Thompson is a research fellow at the centre for Quantum Technologies (CQT) located at the National University of Singapore. She obtained her PhD in Theoretical Particle Physics from the University of Melbourne in 2012, before shifting to work on topics in quantum information at CQT. She has made significant contributions to fields of quantum illumination and quantum foundations. Her current interests include quantum algorithms, causal modeling, and quantum resource theories.