Time Compression and the Perception of Aging:
How the Brain’s Clock, Aging, and the Earth’s Spin Shape Our Sense of Time – and What is it
By Stephen Pendergast (for Scientific American–style, with assistance of ChatGPT)
When you were a child, summer seemed endless. Now, years flash by like the pages of a rapidly flipped calendar. We tend to chalk this up to nostalgia or busyness, but the deeper story is biological. The brain does not see time — it builds it, using networks of neurons, hormones, and rhythms that evolved to keep us aligned with a turning planet. As we age, those mechanisms change, altering not just how fast life feels, but how our very cells sense the passing day.
Unlike vision or hearing, time has no single organ or receptor. Instead, it is constructed by multiple systems, each tuned to a different tempo. For split-second judgments — catching a ball, playing a note — the cerebellum and basal ganglia act as neural stopwatches, relying on the timing precision of dopamine-driven circuits. For durations of seconds or minutes, prefrontal networks estimate intervals by keeping events in working memory. And for the larger cycles that govern sleep, body temperature, and hormones, the master conductor is a cluster of neurons in the hypothalamus called the suprachiasmatic nucleus, or SCN.
The SCN’s role is less like a ticking clock than a coordinated symphony of oscillators. Each cell keeps its own rhythm through an elegant molecular loop: genes named CLOCK, BMAL1, PER, and CRY activate and inhibit one another in a feedback cycle lasting a little more than 24 hours. The SCN synchronizes these cellular pulses and uses light entering through the eyes to keep them locked to the solar day. Every morning, sunlight resets the system, ensuring that the inner day matches the outer one.
When the Clock Is Cut Off
In the 1960s and ’70s, German chronobiologists Jürgen Aschoff and Rütger Wever asked what would happen if that daily cue disappeared. They built underground bunkers with no windows, no clocks, and no sense of day or night, and had volunteers live there for weeks at a time. The results were startling: deprived of external signals, participants’ sleep–wake cycles drifted later each day, eventually stabilizing at about 25 hours. The same phenomenon has been observed in laboratory isolation and in astronauts orbiting Earth.
In other words, the human biological day runs longer than the planet’s 24-hour rotation. Our internal clock must be adjusted each morning by sunlight, or we will slowly fall out of phase with the world.
Echoes of an Ancient Earth?
That finding has long inspired a romantic hypothesis: perhaps our 25-hour rhythm is a biological relic from an era when the Earth itself turned more slowly. Geophysical data show that hundreds of millions of years ago, days were indeed shorter — about 21 hours in the Cambrian, 23 by the time of the dinosaurs. Tidal friction with the Moon is still lengthening the day by roughly two milliseconds per century. Could our circadian system have been set during a slower rotation?
Probably not, say modern chronobiologists. The circadian clock is far older than any single species, appearing even in bacteria that predate multicellular life. Across organisms, these clocks are continuously tuned to contemporary environmental cycles, not ancient ones. The small mismatch between the human clock and the 24-hour day is not a fossil but a feature — the result of biochemical delays in gene transcription and protein degradation that make the loop slightly imperfect. Sunlight acts as a daily correction.
Still, the coincidence lingers in the imagination: a faint resonance between biology and cosmology, a reminder that life itself is built on planetary rhythm.
Aging and the Acceleration of Life
But while our circadian machinery keeps time with the planet, our experience of time depends on the brain’s ability to process change. This is where aging alters perception.
As Alvin Toffler observed in his 1970 classic Future Shock, modernity’s accelerating pace compresses human experience — too much novelty, too little time to adapt. Yet biological time compression moves in the opposite direction. With age, internal processes slow, neural oscillations lose amplitude, and dopamine levels decline. The older brain produces fewer discrete “temporal markers,” those micro-events by which we measure duration. The result is that fewer internal changes occur per unit of external time — and time seems to speed up.
Experiments confirm this: older adults tend to underestimate intervals and perceive days as shorter. The effect isn’t merely psychological. As melatonin secretion wanes and the SCN’s response to light weakens, the body’s circadian rhythms flatten. Sleep becomes fragmented, temperature cycles dull, and the internal clock loses precision. Subjective and biological time thus converge — both become blurred.
How the Brain Measures the Flow
Neuroscientists now think the brain tracks time much as it tracks motion: by detecting change rather than counting fixed intervals. Neural activity itself provides the raw material. Each pulse of perception, each shift in attention, adds to an internal timeline. When life is full of novelty — travel, learning, discovery — the brain records dense sequences of change, and time feels expansive. When days repeat, the intervals between meaningful events lengthen, and weeks vanish into routine.
This interplay of external rhythm, internal chemistry, and experiential novelty explains why time seems to accelerate in later life. The environment offers fewer new stimuli; the body generates fewer signals; and memory consolidates events less distinctly. In the end, our perception of time’s speed is a measure of how richly we are living it.
The Elastic Present
From the slow feedback loops of clock genes to the fading response of aging neurons, time in the human brain is both biological and experiential. The same rhythms that once synchronized our ancestors to the rising sun now contend with electric light and 24-hour culture, while the planet’s own spin continues its imperceptible deceleration.
We inhabit several timescales at once — the cosmic, the biological, the psychological — and the friction between them gives rise to the mystery of felt time. Our inner day drifts longer than the outer one; our later years seem shorter than our early ones. Between those scales, consciousness stitches together the illusion of continuity. The physics of time is universal; its perception, profoundly human.
Time in Classical and Relativistic Physics
In Newtonian physics, time was absolute — a universal parameter flowing evenly everywhere, independent of what happens in space. That picture worked for centuries until Einstein’s relativity showed that time is not absolute at all.
Einstein’s contribution
- In Special Relativity (1905), time and space form a unified entity: spacetime.
- Motion, gravity, and energy curve spacetime, and time runs at different rates depending on relative motion and gravitational field.
- Time is still a dimension — the “fourth” — but it is not universal. There is no single “now” for the whole universe.
So in relativity, time exists, but it is woven into geometry, not a cosmic metronome.
The Problem of Time in Quantum Gravity
Things get stranger when we try to reconcile quantum mechanics and general relativity — the two great pillars of modern physics.
- In quantum theory, time is a parameter: the wavefunction evolves in time according to Schrödinger’s equation.
- In general relativity, time is part of the geometry itself, and that geometry is dynamic.
When you try to merge them, the equations don’t agree on what “time” even is.
The Wheeler–DeWitt equation
In the 1960s, physicists John
Wheeler and Bryce DeWitt developed a candidate equation for a
“wavefunction of the universe.”
But their equation — a quantum analog of Einstein’s field equations — has no
time variable at all.
It’s written simply as:
[
\hat{H}\Psi = 0
]
where (\Psi) is the wavefunction of the universe and (\hat{H}) is the
Hamiltonian operator.
There’s no (t). Nothing “evolves.” The universe just is.
This led to what physicists call the problem of time: in quantum gravity, time disappears from the fundamental equations. The appearance of change — of clocks ticking, stars moving, people aging — must then be emergent, not fundamental.
Emergent Time: The Relational View
One way physicists interpret this is
that time is not a basic ingredient of the universe, but a relationship
among events.
Change doesn’t happen in time — time is what we call the ordering of
change.
This idea has roots in Leibniz’s philosophy and has been revived in modern theoretical physics by thinkers like:
- Carlo Rovelli, in The Order of Time (2018)
- Julian Barbour, in The End of Time (1999)
Rovelli’s approach, called relational quantum mechanics, says that what exists are correlations between variables — not an independent time variable. The “flow” of time emerges from how systems interact and exchange information.
Barbour goes further: the universe is a vast collection of possible “nows” — configurations of the cosmos. What we experience as motion and time is just a sequence of these configurations that our consciousness stitches together.
Quantum Theory and the Search for a “Time Quantum”
Planck time
At extremely small scales — the Planck
scale — spacetime may cease to be smooth and become discrete.
The Planck time is about
[
t_P = 5.39 \times 10^{-44}\ \text{seconds}
]
It’s the time it takes light to travel one Planck length, the smallest
meaningful distance in quantum gravity.
Below that scale, the usual concepts of space and time break down. Some approaches, like loop quantum gravity, predict spacetime is built from discrete “chunks” — spin networks — and that time steps might effectively come in Planck-sized units.
So while we don’t have a measured quantum of time, we suspect that the smooth flow of time we experience is a coarse-grained illusion built from much more granular, quantum-level processes.
What Does It Mean for Us?
If time isn’t fundamental, how can we experience its passage so vividly?
The answer may be that our brains are time-making machines — biological systems that integrate change, store memory, and project anticipation. Just as temperature emerges from molecular motion, time may emerge from change and memory.
At the deepest level, physics might describe a static universe — a “block” of spacetime or configuration space — but within that block, structures like us experience ordered sequences of states we interpret as “time’s flow.”
In summary
|
Framework |
How Time Appears |
Key Idea |
|
Newtonian |
Absolute, uniform flow |
Time exists independently of space |
|
Einsteinian |
Dimension of spacetime, relative |
Time bends and dilates with motion and gravity |
|
Quantum mechanics |
External parameter |
Time drives evolution of wavefunctions |
|
Quantum gravity |
Disappears from equations |
Time may be emergent or relational |
|
Speculative (loop gravity, causal sets) |
Possibly quantized |
Fundamental “atoms” of time and space |
So when physicists say “there is no such thing as time,” they don’t mean clocks stop or causality vanishes — they mean that, at the deepest level, the universe might not have an intrinsic temporal variable.
Time could be a property of the way matter and energy change and relate, not a thing that flows on its own.
Suggested Reading
- Aschoff, J., & Wever, R. (1962). “Spontanperiodik des Menschen bei Ausschluß aller Zeitgeber.” Naturwissenschaften, 49, 337–342.
- Czeisler, C. A. et al. (1999). “Stability, precision, and near-24-hour period of the human circadian pacemaker.” Science, 284(5423), 2177–2181.
- Toffler, A. (1970). Future Shock. New York: Random House.
- Roenneberg, T., & Merrow, M. (2005). “Circadian clocks — the fall and rise of physiology.” Nature Reviews Molecular Cell Biology, 6(12), 965–971.
- Duffy, J. F., & Czeisler, C. A. (2002). “Age-related change in the relationship between circadian period, circadian phase, and diurnal preference in humans.” Neuroscience Letters, 318(3), 117–120.
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