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Retro-: Unravelling the Mysteries of Quantum

Dr Peter Evans RETRO-CAUSALITY: UNRAVELLING THE MYSTERIES OF QUANTUM COSMOLOGY

Despite many years of research aiming to unite with cosmological theories, researchers in fields across and remain in disagreement about a solution. Now, Dr Peter Evans at the University of Queensland sheds new on the debate. He argues that on quantum scales, the idea of cause and effect does not need to follow the one-way passage of , as we understand it. If correct, his theories could dispel some of the most puzzling mysteries of quantum theory – a significant step forward in understanding the true of the .

Quantum Cosmology current idea of , in which time progresses in one direction. For decades, scientists and However, Dr Evans argues that given a philosophers alike have struggled to conceptual framework built from the reconcile our current understanding right background assumptions, his of the nature of the universe with ideas could answer some of the most the theory of quantum mechanics. pressing questions posed by quantum This broad , known as ‘quantum cosmology. To justify these claims, he cosmology’, encompasses a wide variety has drawn together a variety of concepts Contrary to well-established of efforts to unite quantum behaviours, first devised in previous studies, in philosophical theories, Dr Evans which unfold on the very smallest of fields including cosmology, quantum argues that in quantum mechanics, scales, with processes such as the mechanics and philosophy. chains of events do not necessarily universe’s accelerating expansion. need to play out within our limited, Redefining the Cause and Effect Chain one-directional view of the flow of Such a goal is a mammoth undertaking. time. Where conventional physics Despite the that our current Philosophers have incorporated the experiments ultimately rely on this more theories of quantum mechanics and idea of cause and effect into their conventional concept, his work is based cosmology each work well on their arguments for many centuries – many on the possibility of backwards-in-time, own, they still appear to be completely of them using it to justify their beliefs or ‘retro’-causality, where quantum incompatible with each other. So far, no about the origins of the universe. The mechanics allows for the occurrence one has come close to merging them concept describes how an cannot of an event that has been triggered by into a single theory. happen unless it has been triggered another event occurring in its future by another event, which happened in (from our perspective). Dr Peter Evans at the University of its past. Today, the concept of cause Queensland considers a different angle and effect continues to be important Many researchers continue to discredit to the problem: what if events that in quantum mechanics, where it has retro-causality as a flawed idea, happened in the past could have been been given the name ‘causality’. In this since many scientific theories are caused by events in their futures? The context, the idea demands that even on fundamentally based around the idea idea is clearly incompatible with our quantum scales, one event must always that time is one-directional, and cannot have been caused by another.

WWW.SCIENTIA.GLOBAL ‘My research considers the possibility of retro-causal influences playing a role in solving some of the mysteries of quantum theory’

flow backwards. If retro-causality did scales can only be ‘real’ when they are assumptions are made when stating the turn out to be true, they would argue observed; otherwise, they only exist as nature of the universe, there is nothing that many of the fundamental laws of a cloud of potential positions in to say that retro-causality isn’t possible. the universe as we currently understand and time. Many theorists argue that it would need to be completely re- this blurry idea of can’t really Conceptual Framework for Retro- written. However, Dr Evans has set out represent what is actually happening, Causality to prove that retro-causal influences and that it is simply unscientific to say are not only possible, but could that an doesn’t exist simply The first of these unorthodox also provide a variety of meaningful because it isn’t measured. background assumptions is called solutions to the challenges faced by the ‘block universe’ model, which researchers of quantum cosmology. As Dr Evans explains, these types of states that the universe exists as an long-standing problems could be unchanging four-dimensional block. Considering Retro-Causal Influences solved if quantum mechanics allowed Four-dimensional spacetime is a widely the chain of cause and effect, which used concept amongst cosmologists, One of the biggest puzzles currently underlies the behaviour of the universe, describing how the three-dimensional faced by quantum theorists is the to be not restricted to a one-way flow space in which we exist is continually undeniable of ‘action at of time. ‘My research considers the changed by the fourth dimension, a distance’ interactions, where certain possibility of retro-causal influences which we know as time. Recently, events must have been triggered by playing a role in solving some of the some theorists have argued that if others happening simultaneously. As mysteries of quantum theory,’ he says. the universe exists as an unchanging Einstein clearly showed, this shouldn’t ‘Some of these quantum peculiarities, four-dimensional block, time must not be allowed to happen, since the such as apparent action at a distance be some irreversible concept; rather, universe has a fundamental speed limit or the definite reality of the quantum everything that ever happen is – the – which must mean system between , obtain essentially happening simultaneously. that an event can’t happen at the same a natural and intuitive interpretation Within this framework, retro-causal time as the event it has been caused by. when considered as arising as a result of influences may be allowed to occur retro-causal influences at the quantum from our perspective. In addition, quantum theory appears scale.’ Dr Evans that if certain to suggest that objects on quantum perhaps unorthodox background

WWW.SCIENTIA.GLOBAL The assumption is named the ‘interventionalist account By extending this notion to quantum causality, Dr Evans claims of causation’, which describes how certain events can be that some of the mysterious behaviours of quantum systems caused by other events, but only if the variable responsible for would begin to look like a symptom of the agent-relative nature the first event was changed in a certain way. This is in contrast of causality, and not as some ad hoc injection of the role of the to the conventional idea that all events must have been agent in quantum mechanics, as one finds in the triggered by past events to which they are linked, no matter the problem. nature of the variable change that gave rise to the first event. If connections between events really are more exclusive, retro- ‘Causality can be understood as an agent-relative notion, causal influences could in fact become a necessary property of thereby highlighting the fact that the agent is able to play a the nature of the universe. significant part in the interpretation of quantum theory, without the postulation of any extra agent-centric phenomena,’ Dr To adopt each of these ideas, Dr Evans argues that we would Evans describes. ‘That is, we don’t need to specify any special need to let go of our current view of the past as something new rules that agents interacting with quantum systems we can know everything about. Instead, he says, we should need to obey; it’s the fundamental causal rules that agents adopt a more flexible idea of time. If these background necessarily obey that bring about the peculiar quantum assumptions are made, he believes that retro-causality can behaviour.’ be seen as a feasible solution to many of the problems posed by quantum cosmology. ‘The notion of retro-causality cannot If this idea is correct, the task of researchers studying the nature be ruled out on analytic grounds, and so comprises a viable of causality would become far easier. Ultimately, it could break conceptual framework for attempting to understand quantum down the mystery of processes including action at a distance, phenomena,’ he explains. and the reality of quantum objects when they are not being observed. Agent-Relative Causality A New Take on Quantum Cosmology Having established this groundwork, Dr Evans then aimed to describe the conditions in which retro-causal events would Quantum cosmology has now become one of the most hotly occur. He has concluded that in order for his ideas to work, a debated fields among both physicists and philosophers, with highly unorthodox piece of philosophy is needed: we would very little agreement being reached amongst researchers need to view causality as an ‘agent-relative’ idea. over the very nature of our reality. In this stagnant academic environment, the approach taken by Dr Evans could prove In this context, an ‘agent’ is a causal actor in the world, which to move the discussion along from its current deadlock we certainly take humans to be. We would probably take certain – dispelling the idea that quantum are not advanced robots, and perhaps even intelligent extra-terrestrial consistent with our current understanding of causality. lifeforms, to be agents also. Viewing causality as an ‘agent- relative’ notion is counterpoised to the idea that causality is an Ultimately, this could allow for important new insights into objective feature of the world – out there independently of the the nature of the universe, bringing researchers one crucial causal players in the world, and existing even if there were no step closer to reconciling quantum mechanics with the most humans. Agent-relative causality implies that there is some key fundamental theories of cosmology. ingredient to the way that agents interact with the world that is integral to what we think causality is fundamentally.

WWW.SCIENTIA.GLOBAL Meet the researcher Dr Peter Evans School of Historical and Philosophical University of Queensland St Lucia Queensland Australia

Dr Peter Evans was awarded his PhD from the University of FUNDING Sydney in 2011, for a thesis entitled A Study of Time in . After graduating, Dr Evans joined the University of Australian Research Council (ARC) Queensland in 2014, where he now works as a Discovery Early Career Research Fellow of the Australian Research FURTHER READING Council (ARC), pursuing the long-term task of ‘a philosophical exploration of simulating and controlling the quantum world’. PW Evans, Quantum Causal Models, Faithfulness, and His main research interests include philosophy of physics Retrocausality, The British Journal for the Philosophy of and science, , and . He has Science, 2018, 69, 745–774. already achieved several prestigious awards to pursue his work, PW Evans, S Gryb, KP Thébault, Ψ-epistemic quantum including the ARC Discovery Early Career Researcher Award, cosmology?, Studies in History and Part which he received in 2017. B: Studies in History and Philosophy of Modern Physics, 2016, 56, 1–12. CONTACT PW Evans, Retrocausality at no extra cost, Synthese, 2015, 192, E: [email protected] 1139–1155. W: https://researchers.uq.edu.au/researcher/10586

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