The concepts of time and causality form the very fabric of our understanding of reality. They are woven into every aspect of how we interpret events, predict outcomes, and relate cause to effect. But as we have ventured deeper into the nature of quantum mechanics, these seemingly unbreakable structures have revealed surprising flexibility. What if the notions of causality are not as fixed or absolute as we have assumed? Instead, what if causality is indefinite at its most fundamental level?
According to this idea, which we will call a Quantum Causal Emergence (QCE) model, the strict causal sequences of the classical world emerge from quantum processes where causal order is not fixed, but indefinite—a superposition of causal sequences.
The Mystery of Causality in Quantum Mechanics
In classical physics, causality has been seen as an immutable chain of events: causes precede effects, and events unfold along a strict timeline. But quantum mechanics has long presented challenges to this picture. Quantum phenomena defy our intuition about time and causality in ways that classical physics cannot account for. Entangled particles, for example, exhibit correlations that seem to ignore the boundaries of space and time. And in quantum superposition, a particle exists in multiple states at once, challenging the notion that an event must unfold in a single, definitive sequence.
A QCE model takes these quantum behaviors a step further by proposing that the very concept of causality is subject to superposition at the quantum level. In the QCE perspective, causality is not fixed or fundamental in the way we’ve assumed. Instead, it can be indefinite—quantum events can exist in superpositions of different causal orders, where event A causes event B, and simultaneously, event B causes event A. This notion of indefinite causal order provides a more flexible, probabilistic structure underlying what we experience as a linear, ordered reality.
Indefinite Causal Order
Imagine a quantum system that can transition between states A and B. In the classical view, either A causes B, or B causes A. But in the quantum realm, both sequences can coexist in a superposition. This concept is illustrated by the quantum switch, where two operations (A and B) are applied in a superposition of possible orders. In this state, the system does not "decide" on a single causal sequence until we observe it. The quantum switch demonstrates that causal order, like other quantum properties, can exist in a superposition of possibilities.
A QCE model proposes that indefinite causal order is not a rare or special state but rather the norm in the quantum world. Classical, definite causality—the ordered sequence of cause and effect we experience in daily life—is an emergent property, arising from quantum interactions that lose their indefiniteness when observed or when interacting with a larger environment. In this way, indefinite causal order can be thought of as the "native" state of causality in the universe, with the appearance of definite causal sequences emerging as a kind of large-scale approximation.
Mechanisms of Causal Emergence
Three known processes could feasibly explain a transformation from the indefinite causal orders of quantum systems into the well-defined causal chains of classical physics:
Decoherence: In the quantum world, systems are often in superposition until they interact with an environment, which disrupts the quantum coherence that maintains superpositions. As a system decoheres, it effectively “chooses” a specific causal order, transitioning from indefinite to definite. Decoherence acts like a filter, stripping away the multiple possible causal orders and collapsing them into a single, observable sequence.
Coarse-Graining: As we observe quantum systems at larger scales, finer details of their structure are averaged out. This coarse-graining process eliminates the quantum ambiguity, allowing only the most stable, large-scale causal structures to survive. In effect, coarse-graining serves to reduce the complex superpositions of causal orders into simpler, more deterministic structures that can be perceived at the macroscopic scale.
Renormalization Group Flow: In the QCE model, causal structure varies with scale, becoming more defined as we move from microscopic to macroscopic observations. At small scales, quantum systems can exhibit indefinite causal order, but as we scale up, this indefinite order “flows” into stable, classical causal structures. This flow process is mathematically analogous to the renormalization group flow in quantum field theory, which describes how systems evolve toward simplicity and stability at larger scales.
These processes would enable definite causal structures to emerge from an underlying substrate of indefinite causal order. Classical causality—the ordered chain of events we take for granted—is thus not a fundamental feature of the universe but an emergent property of quantum systems interacting with their environment.
Does Indefinite Causal Order Makes Sense?
Indefinite causal order seems to provide a more accurate picture of the quantum world. It also offers a cohesive framework that addresses longstanding problems in physics. For instance, it provides a fresh perspective on the measurement problem. In classical terms, measurement is the act of observing a system to determine its state. But in the quantum realm, measurement is transformative, collapsing a superposition into a single state. The QCE model suggests that measurement also “collapses” causal order: it forces an indefinite, quantum state of causality into a single, classical sequence. Thus, measurement can be seen as the act that fixes causality itself, supporting the proposal that causality, like quantum state, is not predetermined but emergent.
Additionally, indefinite causal order provides new insights into some of the most vexing puzzles in modern physics, including the black hole information paradox. Traditionally, causal structure and information preservation are tightly linked in physics; causality ensures that information about a system’s state is conserved. But if causality is not fundamental, then the strict rules of information preservation may also be flexible, perhaps allowing information to escape through indefinite causal paths. The implications are profound, potentially reconciling quantum mechanics with general relativity and offering new ways to think about the fabric of spacetime itself.
Free Will and Consciousness
Causal indeterminacy also raises fascinating questions about free will and consciousness. Many prominent minds of today adhere to the idea that free will does not exist - that our future is determined. But in a universe where causality is flexible and emergent, we might envision a form of agency that is not strictly bound by these assumed, predetermined causal chains. If causality itself is probabilistic, then perhaps choices influence outcomes within this framework without being strictly determined by prior states.
Consciousness might play a role in interacting with, or even selecting from, an indefinite ordering of causal events at the quantum level. In certain interpretations of quantum mechanics, the conscious observer is essential to the collapse of the wave function. The QCE model extends this idea, suggesting that consciousness might influence the emergence of causal structure itself. We could speculate that consciousness has the capacity to “collapse” not just quantum states but also the indefinite causal orders in which they exist, thus selecting a particular causal sequence from among a superposition of possibilities.
This model encourages us to rethink the very nature of reality and free agency. The arrow of time, the chain of cause and effect—these are constructs that arise out of a deeper quantum structure that is flexible, probabilistic, and, in a sense, unbound. Time and causality become approximations, tools forged from quantum possibilities that only emerge as stable, observable features on a macroscopic scale.
I contend this is a reason for optimism. In this view, the fabric of reality is much richer and more intricate than our daily experience suggests. We are free to act instead of being acted upon, quite literally shaping the world around us. It is a universe not of fixed sequences and linear progressions but of dynamic, interacting possibilities, with causality emerging like a shadow from an underlying quantum flux.
