The River That May Not Flow

Augustine confessed that he knew what time was — until someone asked him. Twenty-six centuries of philosophy have not improved much on this admission. We experience time with a vividness unmatched by any other feature of reality. The present moment is here, blazingly actual, in a way that the past and future are not. And yet, when we try to say what time is — not what clocks measure, not what calendars mark, but the thing itself — language buckles.

Physics, which is supposed to be our most precise account of reality, offers surprisingly little help. The equations of fundamental physics are, with one exception, time-symmetric. They work equally well run forward or backward. Newton's laws, Maxwell's equations, Schrodinger's equation, Einstein's field equations — all are indifferent to the direction of time. If you filmed the motion of a single particle obeying any of these laws and played the film in reverse, the reversed motion would also obey the laws. There is nothing in the fundamental equations that distinguishes "past" from "future."

The one exception is the second law of thermodynamics: entropy tends to increase. But the second law is statistical, not fundamental. It describes the probable behavior of large collections of particles, not the necessary behavior of any individual particle. The arrow of time — the felt directionality of experience, the fact that we remember the past and not the future — appears to be a macroscopic phenomenon emergent from microscopic symmetry. Time's arrow is, in some sense, an artifact of our scale.

We do not perceive time. We perceive change. And we call the asymmetry of change "time." But what if the asymmetry is in us, not in the world?

The Block Universe

The most radical challenge to our intuitive understanding of time comes from special relativity. Einstein showed that simultaneity is relative — two events that are simultaneous in one reference frame are not simultaneous in another. This means there is no absolute "now" that stretches across the universe. My present moment and yours, if we are moving relative to each other, slice the four-dimensional spacetime manifold at different angles.

The Minkowski metric makes this precise. In special relativity, the spacetime interval between two events is:

This metric treats time ($t$) and space ($x, y, z$) as components of a single four-dimensional geometry. The minus sign in front of $c^2 dt^2$ is what distinguishes time from space — it gives spacetime a Lorentzian signature rather than a Euclidean one — but time is nonetheless woven into the same geometric fabric as space. There is no privileged "now." There is only the manifold, extending in four dimensions, with all events — past, present, and future — equally real.

This is the block universe. The metaphor is spatial: imagine all of spacetime laid out like a four-dimensional block of ice. Your birth is at one end. Your death is at the other. Every moment of your life exists simultaneously (if that word even makes sense in this context) within the block. The passage of time is an illusion — or rather, it is a feature of consciousness embedded within the block, not a feature of the block itself.

Each position has consequences for how we understand human experience, moral responsibility, and — if one is theologically inclined — eternity.

Entropy and the Felt Arrow

Even if fundamental physics is time-symmetric, our experience is emphatically not. We age. Eggs break but do not unbreak. We remember yesterday but not tomorrow. This asymmetry requires explanation, and the standard explanation involves entropy.

The Boltzmann entropy of a macrostate is:

where $\Omega$ is the number of microstates compatible with the macrostate and $k_B$ is Boltzmann's constant. The second law says that $S$ tends to increase — systems tend to evolve toward macrostates with more compatible microstates. This is overwhelmingly probable, not logically necessary. The probability of a significant entropy decrease in a macroscopic system is:

where $N$ is the number of particles involved. For any macroscopic system ($N \sim 10^{23}$), this probability is so small that it has never been observed and never will be.

But here is the puzzle: why was entropy low in the past? If the second law merely says that entropy is overwhelmingly likely to increase from any given moment, it should apply equally to the past and the future. Starting from the present, entropy should be expected to increase in both temporal directions. The fact that it was lower in the past — that the universe began in an extraordinarily low-entropy state — is not explained by the second law. It is an additional fact about the universe, sometimes called the Past Hypothesis.

The arrow of time is not written into the laws of physics. It is written into the initial conditions. Someone, or something, set the universe going in an extraordinarily improbable state — and the rest is thermodynamics.

Roger Penrose has estimated the probability of the universe's initial state, in terms of phase-space volume, at roughly $1$ in $10^{10^{123}}$. This number is so large that writing it out would require more digits than there are particles in the observable universe. The low-entropy beginning is, in some frameworks, the deepest unexplained fact in all of physics.

Time and Mind

Carlo Rovelli, in The Order of Time, argues that time, as we experience it, is not a feature of fundamental reality but an artifact of our particular way of interacting with the world. We are macroscopic systems, sensitive to entropy but blind to individual microstates. We form memories (which requires entropy increase). We predict the future (which requires statistical regularities). Our entire experience of temporal flow is generated by our thermodynamic situation — by the fact that we are open systems far from equilibrium, maintaining our structure by exporting entropy to the environment.

If Rovelli is right, then asking "What is time?" is like asking "What is temperature?" — the answer is not a fundamental property of reality but a statistical description useful for beings at our scale. Temperature is the average kinetic energy of molecules. Time, in some analogous sense, might be the felt experience of entropy increase by systems complex enough to have experiences at all.

This has a strange consequence for theology. If time is mind-dependent — if it is a feature of how conscious beings process information rather than a feature of the universe itself — then the theological concept of eternity takes on a different character. Eternity is traditionally understood as either endless time (an infinite duration) or timelessness (existence outside of time altogether). If time is itself a construct of finite minds embedded in an entropic universe, then eternity-as-timelessness becomes not a mystical paradox but almost a truism: from the perspective of the block universe, everything is timeless. Your life, my life, the entire history of the cosmos — all of it simply is, laid out in four-dimensional geometry, neither beginning nor ending in any absolute sense.

But this is not the eternity that the mystics describe. The mystics speak of a fullness of presence — an experience of the present moment so total that past and future dissolve. This is not the block universe's cold eternalism. It is something warmer: an experiential state in which the mind stops projecting forward and backward and simply inhabits the now. If time is indeed mind-dependent, then this state — which contemplatives across traditions report achieving — might be the closest a finite being can come to perceiving reality as it is, rather than as our entropy-processing brains construct it.

The clock ticks. The universe does not care. But we do — we care enormously, desperately, about the passage of hours and years, about the shortness of life and the uncertainty of what follows. Perhaps this caring is itself the arrow of time. Not a feature of physics, but a feature of love — the love of finite beings for a world they know they must leave. Time is what it feels like to be a creature that knows it will die, living in a universe that does not know anything at all.