Is Time Real or Illusory? A Deep Dive with Jim Al-Khalili on Manifest Time, Physical Time and the Arrow of Time

Overview

In a conversation hosted at Greenwich, Jim Al-Khalili explores the enduring question of time. He draws a distinction between manifest time, our subjective experience of time, and physical time, as treated by physics, and outlines the core tensions that arise when these pictures are brought together.

Key insights

• Time as a concept has both experiential and physical facets, which can diverge yet potentially be reconciled.
• The arrow of time is closely tied to thermodynamics and entropy, but its full origin remains debated among physicists.
• Modern physics suggests a block universe or eternalism where all times are real, challenging the immediacy of the present moment.
• Achieving a theory of quantum gravity that also respects thermodynamics could illuminate the true nature of time.

Introduction and Core Question

The interview opens with a provocative question about the reality of time. Jim Al-Khalili explains that many physicists regard time as an illusion, yet he argues that time itself is real, albeit in a way that resists a simple flowing narrative. The discussion is anchored by the idea that understanding time requires integrating three perspectives: time as a dimension (as in relativity), time as a label or parameter in physical laws, and time as an arrow linked to irreversible processes in thermodynamics.

Manifest Time vs Physical Time

Two distinct notions of time are introduced. Manifest time is the subjective, psychological experience of time flowing and the sense of a moving present. Physical time is the time variable that appears in equations of physics, such as Newtonian mechanics, Schrödinger equation, and cosmological dynamics. The conversation emphasizes that there is no evidence for a literal flow of time in the fundamental laws; instead, time is a parameter used to describe change.

The Flow of Time and the Arrow of Time

The speakers distinguish the everyday sense of a river-like flow from the arrow of time, which in physics is associated with irreversibility. They discuss how entropy and thermodynamics appear to provide a direction to time, but this is not the full story since the underlying dynamical laws are time-reversal invariant. Decoherence and environmental interactions play a crucial role in creating practical irreversibility observed in the macroscopic world.

Pillars of Time in Physics

The dialogue highlights three pillars: relativity (time as a dimension of spacetime), quantum mechanics (time as a label in evolution equations), and thermodynamics (the arrow of time via entropy). The host argues that any complete theory of time must reconcile all three, not just pair two of them. This triad motivates ongoing work in quantum gravity and quantum thermodynamics.

Time, Now, and Presentism vs Eternalism

Addressing philosophical positions, the guests discuss presentism (only the present is real) versus eternalism (all times are real). The majority of physicists lean toward eternalism due to relativity, where past, present, and future are interwoven in spacetime, and the present has no universal ontological privilege. The conversation acknowledges this clashes with human experience but emphasizes coherence with the best existing theory of spacetime.

Past Hypothesis and the Beginning of Time

Entropy-based explanations of the arrow of time lead to debates about the universe’s beginning. The past hypothesis suggests the universe started in a very low-entropy state, with entropy increasing thereafter. They also consider quantum formulations of the past, including entanglement-based initial states and the possibility of mirror universes as speculative tools to address symmetry in time.

Emergence of Time and the Role of Decoherence

The discussion turns to whether time could be emergent, perhaps from more fundamental quantum properties such as entanglement, or if time remains baked into reality at the deepest level. A key point is that decoherence provides a robust, irreversible mechanism that contributes to the observed arrow of time, potentially offering a more solid ground than thermodynamic entropy alone.

Time Travel and the Frontier of Time Theory

Time travel is treated as a possibility within the framework of general relativity, with future travel achievable through time dilation near massive objects or near light-speed travel. However, backward time travel raises paradoxes, which are sometimes addressed by speculative ideas such as parallel universes. The interview maintains that while time travel remains unlikely, it cannot be completely ruled out due to potential future breakthroughs in physics.

Looking Ahead

The speakers close by acknowledging that a complete reconciliation of manifest time with physical time remains a grand challenge. They stress humility in the face of unsolved problems and express optimism that deeper theories in quantum gravity and thermodynamics will eventually illuminate the true nature of time, including how subjective time might be integrated into the fundamental laws of physics.