In the previous chapters, we have followed the arrow of time from its thermodynamic origins to quantum considerations and cosmological scales. We have seen how irreversibility can emerge statistically in both classical and quantum systems, how early low-entropy conditions shape the grand evolution of the universe, and how advanced debates in quantum gravity may or may not fundamentally revise our understanding of temporal asymmetry. Yet, even with all these scientific perspectives, there remains a striking everyday experience: humans perceive time flowing from the past to the future, forming memories of what has already happened but not of what is yet to come. We infer causality in events, attributing present circumstances to prior influences rather than future ones. This psychological dimension—the sense that we remember the past but anticipate the future—seems intimately tied to the same arrow that organizes physical processes.
In this chapter, we explore two major themes related to the "psychological" or "perceptual" arrow of time. Section 9.1 examines how our faculties of memory and the notion of causality align with the forward progression of time, emphasizing the entropic underpinnings behind these phenomena. Section 9.2 addresses the role of observation and measurement, asking whether our subjective experience of time's direction stems from some fundamental property of the universe or from how we, as observers, engage with and track our environment. Throughout, we aim to connect these introspective and philosophical viewpoints with the more concrete physical arguments laid down in earlier chapters, showing that the arrow of time unites deep theoretical questions with the immediacy of lived experience.
9.1 Memory and CausalityWhy We Remember the Past and Not the Future
Suppose you watch a short video snippet: a shattered wineglass spontaneously reassembles itself on the floor and flies up onto the table. You instantly recognize this reversal as absurd in ordinary life. Similarly, you recall dropping the glass but never the "pre-memory" of it coming back together tomorrow. In everyday terms, you know the event in the past, not the future. Indeed, from purely classical equations of motion, there is no law forbidding that reversed scenario. Yet, from a thermodynamic perspective, such a reassembly is so improbable that we never witness it (Price 2004; Carroll 2010).
On a psychological level, the arrow of time is strongly manifest in how we form memories of events that occur, forging mental records that become the basis for knowledge about "what happened." Meanwhile, the future remains open, undetermined, and unknown in experience. This dichotomy—memory of the past but no memory of the future—tracks the broader principle that the universe transitions from low-entropy states to higher-entropy states. Our records (neural or otherwise) are, in effect, correlations formed between a system (our brain) and the environment in a moment that has already happened (Lebowitz 2008).
To break this down, consider the following:
Memory formation implies that a certain physical state inside our brain changes to reflect an event in the external world.This correlation-building process demands energy, typically increasing overall entropy by rearranging molecules in the brain and dissipating heat (Landauer 1961; Carroll 2010).Reversing this correlation would require erasing memory from the brain and precisely recollecting the energy used to record it—a feat that parallels unscrambling an egg.
Thus, the arrow of time that emerges in thermodynamic arguments resurfaces in how memories become imprinted in neural structures. This link resonates with the discussion in previous chapters: irreversibility is not just a phenomenon of steam engines or cosmic expansions, but also of mental processes constrained by statistical improbabilities (Halliwell 1994; Price 2004).
The Physical Basis of Memory
Though memory might feel intangible, it is firmly grounded in physical processes. Neurons encode events through changes in synaptic strengths, protein expressions, and structural rearrangements in neural networks—collectively known as "synaptic plasticity." Each new memory represents a reconfiguration of matter, subject to the second law (Kandel et al. 2014 in Carroll 2010). The direction of these reconfigurations is not spontaneously reversible. Erasing a memory requires additional physical processes, typically incurring further entropy production as the system and environment reorganize.
In a more general sense, memory can be viewed as the creation of correlations between one subsystem (the observer's brain or any measuring device) and another subsystem (the environment or the phenomenon observed). As with quantum measurement arguments (Chapter Six), once these correlations are established and spread among countless microscopic degrees of freedom, reconstituting the original uncorrelated state is practically impossible (Zurek 2003 in Halliwell 1994). Physicist Huw Price (2004) underscores how this correlation-building is central to forming records and thus to the psychological arrow of time.
Causality: The Forward Linking of Events
Tied closely to memory is the concept of causality. We typically view the cause as preceding the effect. If you drop a glass, you cause it to smash, not the other way around. Mechanistically, one might try to define causality purely in terms of physical laws that are time-symmetric. Yet everyday experience insists that causal chains run forward in time.
Thermodynamics and statistical mechanics explain this by referencing how systems evolve from states of lower entropy to states of higher entropy (Boltzmann 1872 in Price 2004). The cause occurs before the effect because the environment plus system transitions from a prior microstate to a subsequent macrostate, each step dissipating available free energy in some manner. Memory formation tracks these changes, hence conferring the sense of temporal ordering. Rewinding that chain of events requires an improbably precise alignment of microstates, reversing the direction of entropy flow—a scenario that reality does not manifest.
Researchers in the philosophy of science, like Reichenbach or Salmon, have examined how correlations can be interpreted as signs of causal processes (Reichenbach 1956 in Price 2004). In the thermodynamic arrow viewpoint, processes that yield a stable record or imprint on the environment can be recognized as events that happened in the past, while those that have not occurred yet leave no such imprint (Carroll 2010). As with memory, causality's forward direction is less about fundamental laws being time-asymmetric than about boundary conditions and entropic constraints that make reversed sequences vanishingly unlikely.
Bullet Points: Memory and CausalityMemory Formation: A physical correlation-building process that dissipates energy and increases entropy.No Memory of Future: Arises because the future is not physically correlated with our brain states in the same sense as the past.Causality Forward: Events produce effects in a direction consistent with entropy growth, reinforcing the everyday notion that cause precedes effect.Statistical Underpinnings: The improbability of reversing these correlation-building steps cements the psychological arrow of time.
So the psychological arrow—manifest in memory and causality—echoes the thermodynamic arrow. The direction in which we recall events lines up with how the universe moves from less probable to more probable configurations. This sets the stage for a deeper question: does our perception of time's arrow simply reflect how we measure and track phenomena, or is it embedded in the fundamental structure of reality?
9.2 The Role of ObservationFundamental or Observer-Dependent?
As we advanced through earlier chapters, we encountered the idea that the arrow of time emerges from boundary conditions, improbabilities, or decoherence. Yet, from a day-to-day vantage point, it can feel as if our observation or measurement creates the arrow of time, especially in quantum contexts. If we never measure a system in reverse, does that mean the arrow is merely an artifact of observation?
There are two major lines of thought:
Observer-Independent Realism: The arrow of time is truly out there in the universe, shaped by low-entropy initial conditions and the unstoppable progress of the second law (Penrose 2004; Carroll 2010). Observers—be they humans, machines, or alien intelligences—simply track that arrow. If no conscious observer were present, thermodynamics would still push processes forward. Observer-Dependent Emergence: The arrow of time might be intimately tied to how observers gather and process information, forging "records" of the past. On this view, irreversibility is fundamentally about measurement, correlation, and the development of knowledge. Without an observer, the notion of "past events being remembered" becomes moot (Price 2004; Zeh 1970 in Halliwell 1994).
In a sense, both views can be reconciled. The environment itself (in classical or quantum form) can serve as a "measuring apparatus," embedding irreversible records in the distribution of photons or molecules even if no conscious observer is around. But from a human perspective, the reason we experience a forward flow of time is that we are effectively part of the environment's correlation-building process, forging mental or written records that are no more reversible than any other entropic phenomenon.
Analogies and Illustrations
Imagine you walk into a messy room. The disordered state of scattered papers and clothes "records" the fact that over time, no one has tidied up. Even if you were not present to witness the mess accumulating, the room's condition is an environmental record. Observing it now, you glean information about processes that must have occurred in the room's recent past. The arrow of time emerges from how these records are formed and left behind by irreversible processes. That recording would exist whether or not you were there to see it. Yet, to experience the arrow of time personally, you rely on your sensory apparatus to gather these environmental cues and integrate them into memory.
Similarly, at the quantum scale, decoherence ensures that many degrees of freedom in the environment pick up entanglements with a measured system, effectively "observing" it. The environment becomes an unwitting observer or measurement device, locking in one of the possible outcomes and rendering superpositions into classical alternatives (Zurek 2003 in Halliwell 1994). This irreversible environment imprint is conceptually similar to how a photographic plate records the passage of a photon in an old-school camera. Once recorded, there is no simple erasure of that event. The second law's arrow points forward because reassembling all those photons and molecules into a pristine unobserved state is incredibly unlikely.
Observation, Free Will, and Temporal Experience
Discussions of the psychological arrow of time often branch into debates about free will and consciousness, especially around whether the apparent openness of the future is an illusion. If one regards classical or quantum laws as fully deterministic (with or without hidden variables), one might argue that the future is as determined as the past, but we simply lack knowledge of future microstates (Price 2004). Our sense of "choosing" the future might then reflect incomplete information and the entropic processes that define how we form expectations.
Alternatively, some philosophers and cognitive scientists highlight that human agency—our ability to intervene in the environment—feels fundamentally forward-directed. We do not choose events in the past. Instead, we plan and act in ways that change future outcomes (Albert 2000 in Carroll 2010). While the underlying physics might not forbid backward causation in a strictly formal sense, the second law, boundary conditions, and the correlation structure of the environment combine to make forward-directed agency the only feasible mode of existence.
Regardless of whether one endorses a strong or weak version of free will, the key point is that observation and the creation of records take place in a forward temporal direction, consistent with how the environment's microstates evolve. This consistency cements our day-to-day sense that "time flows," and we cannot alter what has already occurred.
Bullet Points: Observation and the Arrow of TimeObserver-Independent: Physical processes produce records of the past, whether or not a conscious being sees them.Observer-Dependent: Humans experience time's arrow via the formation of memory and knowledge, giving a subjective lens to an objective phenomenon.Environmental Recording: The universe "observes" itself through scattering, decoherence, and entanglement, locking in irreversible events.Agency and Free Will: Our forward sense of agency aligns with the forward arrow of time, as future states remain less determined in practice than the recorded past.
By integrating these perspectives, we see that our psychological arrow of time does not exist in isolation from physical laws; rather, it emerges naturally when conscious observers—like us—become part of an inherently irreversible universe shaped by initial boundary conditions and entropic constraints.
9.3 Synthesis and Connections to the Larger Narrative
This chapter explored how memory, causality, and observation fold into the arrow of time story, tying it back to the thermodynamic and cosmological contexts developed previously.
Memory is a Physical Record
We gleaned that memory formation is essentially about embedding correlations in material substrates (neurons, data storage devices, environment couplings), increasing global entropy to do so. This process unites everyday psychology with the general principle that irreversibility arises from improbable microstate reversals. Causality Tracks Irreversibility
Just as thermodynamic processes cannot spontaneously revert in everyday life, we do not see effects preceding causes. Causality in physics might be time-symmetric in local laws, but in practice, the second law imposes a unidirectional flow of events at macroscopic scales, shaping how we interpret the world. Observation and the Universe
The quantum mechanical notion that measurement irreversibly picks out classical outcomes extends seamlessly to the question of how we track time. Observers, conscious or otherwise, are part of the environment's correlation-building. This perspective dovetails with earlier chapters on decoherence, the role of boundary conditions, and the universal impetus toward higher entropy. Human Perspective
Ultimately, the psychological arrow of time—our experience of "before" and "after," of "remembering" but not "pre-membering"—mirrors the universal arrow that emerges from low-entropy boundary conditions. It is neither an artifact nor an illusion in a trivial sense, but a reflection of the same entropic logic that shapes glass shards and galaxy formation.
By grounding memory, causality, and observation in physical processes, we reaffirm that the arrow of time is not merely a quirk of human consciousness. Instead, it is deeply woven into the fabric of an evolving universe that, for reasons explored in Chapter Eight, began in a very special, low-entropy state. Our conscious perceptions, neural pathways, and observational stances all feed into that cosmic narrative, making the psychological arrow a localized expression of thermodynamic truth.
Concluding Thoughts
Reflecting on the interplay between physics and human experience, one realizes that the arrow of time is far more than a brute physical law. It organizes how we perceive events, plan for the future, and interpret causality. Yet, the story we have followed through the book demonstrates that this arrow is neither a fundamental preference for future over past in the microscopic laws nor an arbitrary invention of the mind. Instead, it arises from the synergy of improbable initial conditions, statistical mechanics, and environmental interactions that embed records of what has happened and, in so doing, forbid easy or natural rewinding.
As we move to close our exploration, it becomes clear that bridging the gap between fundamental symmetries (Newtonian, relativistic, or quantum) and the richly asymmetrical experience of time is one of modern science's most profound achievements. By linking the psychological arrow (memory and observation) to the cosmic arrow (expanding universe, gravitational clumping), to the thermodynamic arrow (entropy growth), and to the quantum measurement arrow (decoherence), we see multiple vantage points on the same underlying phenomenon. None of these vantage points on time's arrow stands alone; each reaffirms that irreversibility is a consistent, emergent reality within a cosmos that started out in a state so special and so low in entropy that it all but guaranteed a dynamic forward progression ever since.