What Is Time? The Physics and Philosophy of Our Strangest Dimension

Key takeaway: Time is a dimension we experience through the relentless increase of entropy and the expansion of spacetime, yet its ultimate nature remains one of science’s deepest mysteries. Yet what is time at its core still eludes us.

You are moving through it right now, at the same rate as everyone else on Earth, irresistibly, in only one direction. You remember yesterday but not tomorrow. You age. Things wear out. Causes precede effects.

Time seems obvious. Then you try to define it.

Augustine of Hippo, writing in the 4th century, captured the problem perfectly: “What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.”

Physics has not fully solved Augustine’s puzzle. But it has transformed it. What we’ve learned about time in the last century is stranger than anything pre-scientific imaginations conjured, and some of the deepest questions about time remain wide open.

Newton’s Absolute Time: The Universe‘s Universal Clock

For most of human history, time seemed obvious: a universal, absolute backdrop against which events occur. Isaac Newton codified this intuition in 1687. In Newton’s mechanics, time flows at the same rate everywhere in the universe: an absolute clock ticking uniformly, independent of anything happening in space, unaffected by matter or motion.

In this picture, two observers anywhere in the universe can agree on whether two events happened simultaneously. “Now” is a universal fact, not a perspective.

Newton himself was troubled by this. He distinguished absolute time (“true, mathematical time, of itself, and by its own nature, flows uniformly, without relation to anything external”) from measured time (the hours, minutes, and seconds we track with instruments). But the idea of an absolute, universal time seemed necessary for his mechanics to work.

It held for over 200 years, until a 26-year-old patent clerk rewrote everything.

Einstein’s Revolution: Time Is Relative

In 1905, Albert Einstein’s special theory of relativity demolished Newton’s absolute time. Einstein started from two postulates: the laws of physics are the same for all observers in uniform motion, and the speed of light is constant for all observers regardless of their motion.

The second postulate is what breaks time. If the speed of light is constant, if you can’t catch up to a light beam, then time itself must stretch to accommodate this fact. The consequences are:

Time dilation: Moving clocks run slow. A clock on a fast-moving spacecraft ticks more slowly than an identical clock at rest. This is not an illusion or a mechanical effect of motion, it is a genuine difference in the rate at which time passes. Atomic clocks on airplanes run measurably slower than identical clocks on the ground due to their high speed, though the net real-world effect is complicated by altitude (which makes them run faster due to weaker gravity). GPS satellites must account for both effects, time dilation from motion and gravitational time dilation from altitude, to maintain accuracy.

Loss of simultaneity: Two events that appear simultaneous to one observer may not appear simultaneous to another observer moving relative to the first. There is no universal “now”: simultaneity is relative to your frame of reference. Crucially, relativity preserves causality: no observer ever sees an effect before its cause.

The spacetime interval: Einstein’s mentor Hermann Minkowski reformulated special relativity in terms of four-dimensional spacetime. As explained in detail on What Is Spacetime? Einstein’s Unified View of Space and Time, space and time are not separate: they are aspects of a single four-dimensional fabric. Different observers slice spacetime differently (different “nows”), but they agree on the four-dimensional spacetime interval between events.

General relativity (1915) deepened this further: gravity is the curvature of spacetime caused by mass and energy. A clock deeper in a gravitational well (closer to a massive object) ticks more slowly than a clock higher up. This gravitational time dilation is not metaphorical: it is measurably real. GPS satellites, orbiting above most of Earth’s gravity, tick faster than ground clocks, and this must be corrected for.

Near a black hole, time slows dramatically: at the event horizon, time effectively stops from the perspective of a distant observer. An astronaut falling into a black hole would experience time normally until they crossed the horizon, but an external observer would see them slow to a frozen image, never quite reaching it. For more on these extreme objects, see How Black Holes Form: Stellar Collapse, Mergers, and the Early Universe.

Time in daily life: Next time you use GPS, remember: your device is correcting for the fact that your clock ticks faster than a satellite’s. Time really is relative.

The Block Universe: Is Time an Illusion?

One of the most challenging philosophical implications of special relativity is the block universe view.

In the block universe (also called eternalism), all of spacetime, past, present, and future, exists simultaneously as a four-dimensional structure. The “flow” of time is an illusion created by consciousness moving through this static structure. Past and future events are as real as present ones; they simply exist at different locations in spacetime.

This view follows naturally from the loss of absolute simultaneity. If different observers disagree about which events are simultaneous, if “now” depends on who’s asking, then there can be no objective boundary between past and future. All of spacetime must exist on equal footing.

The block universe raises disturbing questions about free will, the nature of memory, and whether the “present” is special in any objective sense. Most physicists who think about the foundations of physics take some version of the block universe seriously, though it remains philosophically contested.

The Arrow of Time: Why Does It Point One Way? (And the Role of Entropy)

Relativity explains what time is, but it doesn’t explain why time has a direction.

Look at the fundamental laws of physics: Newton’s laws, Maxwell’s electromagnetism, quantum mechanics, general relativity. Almost all of them are time-symmetric: if you reverse all the momenta (run the movie backward), the resulting motion is equally valid under the laws of physics. A movie of two billiard balls colliding is physically valid whether run forward or backward.

Yet the world clearly has a preferred time direction. A coffee cup falls and breaks; broken cups don’t spontaneously reassemble. Smoke disperses; it doesn’t reconcentrate. People age in one direction. Memories are of the past, not the future.

This directionality, the arrow of time, comes from thermodynamics, specifically the second law: entropy (a measure of disorder or the number of available microstates) increases with time in isolated systems. The difference between past and future is ultimately just entropy.

But why does entropy increase? Because the universe started in an extraordinarily low-entropy state, the Big Bang, and has been increasing in entropy ever since. We live in the early stages of a universe that began far out of equilibrium, and the arrow of time is ultimately a remnant of those initial conditions. For more on the origin event that set this arrow, see The Big Bang Theory Explained: What It Actually Says About the Origin of the Universe.

This explanation, championed by physicist Ludwig Boltzmann in the 19th century and developed further by Sean Carroll and others, reframes the question: not “why does entropy increase?” but “why did the universe start with such low entropy?” That remains deeply puzzling.

One speculative answer involves the multiverse: in a large enough ensemble of universes, some will by chance start in low-entropy states. We necessarily find ourselves in such a universe because high-entropy universes don’t produce the conditions for conscious observers. This is an anthropic argument, one of the most controversial moves in modern cosmology.

Time in Quantum Mechanics

Time plays an unusual role in quantum mechanics. In standard quantum mechanics, time is a parameter, an external background against which quantum systems evolve, rather than a quantum observable like position or momentum. There is no “time operator” in quantum mechanics; you can’t measure time the way you measure position. Think of it this way: in quantum mechanics, particles have a position you can measure, but time isn’t like that: it’s the stage, not a character.

This asymmetry bothers many physicists. Position and momentum are related by the uncertainty principle; energy and time are related by an energy-time uncertainty relation that is superficially similar but fundamentally different in origin.

The problem of time becomes acute in quantum gravity. When you try to combine quantum mechanics with general relativity, as in approaches like loop quantum gravity, the usual quantum mechanical treatment of time breaks down. In the Wheeler-DeWitt equation (a quantum gravity equation), time seemingly disappears from the fundamental description entirely. (This is one approach among several to quantum gravity, not a settled fact.) Time may be an emergent phenomenon rather than a fundamental ingredient, an approximate description that works at large scales but breaks down at the Planck scale.

This is deeply strange. If time is not fundamental, if it emerges from something more basic, what is that something?

Thermodynamic Time, Psychological Time, and Cosmic Time

We experience time in multiple ways that don’t always align with physics.

Thermodynamic time flows in the direction of increasing entropy. This is why causes precede effects, why you remember the past and not the future, and why your body ages. The thermodynamic arrow and the experienced arrow of time are the same.

Psychological time is notoriously plastic. Time flies when you’re having fun; it drags when you’re bored. Extreme stress can make events seem to slow (not because time genuinely dilates, but because the brain records more memories per interval during high arousal). Psychedelic states dramatically distort time perception. Neuroscientists have identified “time cells” in the hippocampus and shown that dopamine levels influence our subjective sense of duration, research pioneered by David Eagleman and others.

Cosmic time is the time measured by the cosmic microwave background and used in cosmological models. It’s the time since the Big Bang, measured in the frame of the universe’s overall matter distribution. At cosmological scales, time has a natural direction: the universe expands, cools, and becomes increasingly structured.

Does Time Have a Beginning (and End)?

General relativity predicts that our universe began 13.8 billion years ago at a singularity, the Big Bang, where the density was infinite and general relativity breaks down. Before the Big Bang, in the classical general relativistic picture, time did not exist. There was no “before” the Big Bang because time itself began then.

This is one of the most mind-bending claims in all of science. Asking what happened before the Big Bang is like asking what is south of the South Pole. The question may be malformed.

Quantum gravity approaches complicate this picture. Stephen Hawking and James Hartle proposed the no-boundary proposal: near the Big Bang, time smoothly transitions into space: there is no sharp beginning, just a smooth rounding-off of spacetime, like the South Pole, where there’s no edge but the concept of “south of here” ceases to make sense.

Whether time has an end is equally uncertain. If the universe expands forever (as current observations suggest), time continues indefinitely. If a Big Rip occurs (where dark energy eventually tears apart all structure), all physical processes stop at a finite future time. In some models, quantum fluctuations could eventually produce a reversal of time’s arrow, an extraordinarily improbable but theoretically possible event in an infinitely old universe.

The Mystery That Remains

Despite centuries of philosophical inquiry and a century of relativity, time remains profoundly mysterious.

We know time is relative, it flows at different rates for different observers. We know it is bound up with space into the fabric of spacetime. We know its direction is set by entropy and the initial conditions of the universe, that the difference between the past and the future is just entropy. We know it may be emergent rather than fundamental.

But why time flows at all, why we experience the passage of time rather than simply existing in a static four-dimensional block, remains one of the deepest unsolved questions at the intersection of physics, neuroscience, and philosophy. It touches on whether time exists without observers, an enduring puzzle that divides presentists (who believe only the present is real) from eternalists (who believe all moments are equally real).

“Time,” as the physicist John Wheeler once said, “is what prevents everything from happening at once.” It’s a good joke. It’s also a reminder of how little we understand about the scaffolding on which everything we know is built.

Sources

• Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der Physik, 17, 891–921. Available via Wiley Online Library
• Carroll, S. (2010). From Eternity to Here: The Quest for the Ultimate Theory of Time. Dutton.
• Barbour, J. (1999). The End of Time: The Next Revolution in Physics. Oxford University Press.
• Penrose, R. (2010). Cycles of Time: An Extraordinary New View of the Universe. Bodley Head.
• Hawking, S. & Hartle, J. (1983). Wave function of the universe. Physical Review D, 28(12), 2960. Available via APS

Further reading: Time on Wikipedia