We all live with a deep intuition: time flows from the past to the future. A glass falls and shatters, but the pieces never spontaneously reassemble to form the glass on the table. This asymmetry, this unique direction, is what physicists call the arrow of time. Yet, surprisingly, the fundamental laws of physics (Newtonian mechanics, relativity, quantum mechanics) are almost all time-reversal symmetric. The equations work just as well forwards as backwards. So, where does this irreversibility we observe come from? The answer lies at the intersection of several arrows of time, the most powerful of which is thermodynamic.
The most fundamental law explaining the direction of time is the second law of thermodynamics, formulated in the 19th century. It introduces the notion of entropy, a measure of the degree of energy dispersion. The principle states that the entropy of an isolated system can only increase or remain constant: \(\Delta S \ge 0\).
It is this inevitable increase that defines the thermodynamic arrow. When you put an ice cube in your soda, heat diffuses from the drink (hot) to the ice cube (cold) until thermal equilibrium is reached, never the other way around. The universe itself, considered as the ultimate isolated system, has seen its entropy increase since the Big Bang, providing a cosmic direction to time.
N.B. :
The physicist Ludwig Boltzmann (1844-1906) gave a statistical interpretation of entropy. A disordered state (high entropy) is much more probable than an ordered state (low entropy) simply because there are astronomically more ways to achieve it. Time "moves forward" because the universe evolves statistically towards more probable states.
The thermodynamic arrow is intimately linked to another: the cosmological arrow. The universe has been expanding since its initial state of very high density and temperature. This Big Bang state was remarkably ordered (low entropy), although very hot. The expansion created the necessary gradient for the increase in entropy. If the universe were to contract again in a "Big Crunch", would the direction of the arrow of time reverse? The debate is open, but the dominant scenario today is one of accelerated and eternal expansion.
We perceive time passing and remember the past, not the future. This psychological arrow is probably a consequence of the thermodynamic arrow. Our brain, like any biological system, operates by dissipating energy and increasing overall entropy.
In other words, at the finest scale, memorizing information in the brain involves very real material changes: electrical signals propagate, chemical molecules are released between neurons, and neural connections change. Once these transformations have occurred, it is impossible to return exactly to the previous state. The brain therefore retains traces of the past, but cannot obtain information about the future. The future, not having yet produced any correlatable physical states, cannot be extracted from it.
This asymmetry does not come from a mysterious property of time, but from the fact that, like any complex physical system, the brain obeys the laws of thermodynamics, which impose irreversible evolution. Thus, our perception of time passing is rooted in the physical laws that govern our neurons. The process of memory formation is irreversible at the chemical and neurological level.
The radiative arrow is the temporal asymmetry observed in the behavior of light and electromagnetic waves. We constantly receive light emitted by stars years ago, but we never observe the reverse: light from space spontaneously converging towards a source to be "reabsorbed".
This directionality is encoded in the Maxwell's equations. Although they are mathematically time-reversal symmetric, their physical solutions in our universe impose specific boundary conditions: we systematically choose the "outgoing wave" solution (which moves away from the source), and not the "incoming wave" solution. This choice is directly related to the thermodynamic arrow and the expansion of the universe, which establish a global temporal framework. This is why we see the past of stars (their light reaching us), and not their future.
At the microscopic level, the quantum world has its own asymmetries, summarized under the term quantum arrow. The most discussed phenomenon is that of decoherence.
A quantum particle can exist in a superposition of states (such as passing through two slits at the same time). When it interacts irreversibly with its environment (for example, by emitting a photon), this superposition "unravels" to give way to a single classical state. This process seems to go in only one direction: we observe the transition from a pure quantum state to a mixed classical state, rarely the reverse. Decoherence is considered a key mechanism explaining why the macroscopic world appears classical and defined. Its exact relationship with thermodynamic irreversibility and the origin of the arrow of time remains an active research topic in fundamental physics.
A crucial observation is that it is possible to locally reduce entropy. Building a house, assembling a computer, or growing a crystal are actions that create order. However, the second law of thermodynamics is not violated. These processes are always accompanied by an even greater increase in entropy in the immediate environment: heat release, waste production, energy consumption. The overall entropy balance increases inexorably.
Living organisms are the most striking examples of locally ordered systems. A cell, a plant, or a human being maintains a complex structure and low entropy. They only exist as open and out-of-equilibrium systems, constantly fueled by an energy flow (sunlight, food). Life does not contradict the arrow of time; it is a sophisticated and dynamic consequence of it, exploiting the energy gradient to locally delay the march towards disorder.
| Type of Arrow | Manifestation | Probable Origin | Irreversible? |
|---|---|---|---|
| Thermodynamic | Increase in entropy, heat diffusion | Initial low-entropy conditions of the Big Bang (statistical law) | Yes, at the macroscopic scale |
| Cosmological | Expansion of the universe | Big Bang and dark energy | Yes, observed |
| Psychological | Perception of past/future, memory | Consequence of the thermodynamic arrow in brain processes | Yes, subjective |
| Radiative | Waves emitted and not absorbed (delay) | Thermodynamics + boundary conditions of the universe | Yes, empirical |
| Quantum (decoherence) | Collapse of state superpositions to a defined classical state | Irreversible interaction with the environment (coupling) | Practically irreversible at the macroscopic scale |
| Biological | Growth, aging, evolution of species | Consequence of the thermodynamic arrow in open systems (energy flow) | Yes, at the organism level |
| Historical/Causal | Cause > effect sequence, impossibility of changing the past | Emerges from the thermodynamic arrow and the structure of spacetime (light cone) | Yes, in our physical framework |
Source: Inspired by the work of Arthur Eddington (1882-1944), Stephen Hawking (1942-2018), and modern developments in cosmology and thermodynamics.
Ultimately, time flows in only one direction because the universe began in an extraordinarily particular and ordered state. It is this initial condition of low entropy at the Big Bang that gave meaning to thermodynamic evolution and, by extension, to all other arrows. The microscopic laws are reversible, but the history of the universe is not. Understanding the arrow of time is therefore understanding the mystery of the initial conditions of cosmology. Why did the universe begin in such an "improbable" state? This question remains one of the frontiers between physics and philosophy.
The Mystery of the Arrow of Time: Why Can't We Go Back? 
The Big Bang: At the Frontiers of the Model 
When Space Bends: The Tiny Slope That Guides the Universe 
Nabataean Astronomy: Masters of the Desert Between Starry Skies and Stone Constructions 
Polynesian Astronomy: The Art of Navigating the Pacific Ocean 
Mesopotamian Astronomy: The Cradle of Celestial Observation 
Andean Astronomy: A Sacred Link Between Sky and Earth 
Ancient Persian Astronomy: Between Babylon and the Islamic Golden Age 
Mayan Astronomy: Celestial Cycles Dictated Religious, Agricultural, and Political Time 
Islamic Astronomy: When Baghdad Illuminated the Sky of Science 
Indian Astronomy: From Sacred Poetry to Scientific Thought 
Ancient Greek Astronomy: The Universe of Philosophers in Search of Cosmic Order 
The Three Cosmic Shapes: A Hidden Geometry of the Universe 
Egyptian Astronomy: Between Sky and Nile, the Secrets of Time 
Babylonian Astronomy: When the Sky Predicted Destiny 
Chinese Imperial Astronomy: A Millennial Scientific Legacy 
Extreme Cosmic Objects: Where Physics Explodes 
Mirror Universe: Coexistence of Two Worlds in a Cosmic Reflection 
The first second of our history 
Time Dilation: Relativistic Mirage or Reality?
Space Through Time: A Constantly Evolving Concept
The Expanding Universe: What Does "Creating Space" Really Mean? 
From Nothingness to the Cosmos: Why Is There Something Rather Than Nothing? 
Glossary of Astronomy and Astrophysics: Key Definitions and Fundamental Concepts 
How can the Universe measure 93 billion light-years? 
How can we say that the Universe has an age? 
First Proof of the Expansion of the Universe 
Space-time slices of the observable Universe 
Dark Ages of the Universe 
Alternative theories to the accelerated expansion of the universe 
The primitive atom of Abbot Georges Lemaître 
Great walls and filaments: the great structures of the Universe 
The Origins of the Universe: A History of Cosmic Representations 
Lyman-alpha Blobs: Gaseous Traces of the First Galaxies 
Gamma-Ray Bursts: The Ultimate Breath of Giant Stars 
Perspective on the Inflation of the Universe 
The Planck Universe: the Image of the Universe Becomes Clearer
The sky is immense with Laniakea 
Abundance of chemical elements in the Universe 
The Symmetries of the Universe: A Journey Between Mathematics and Physical Reality 
The Geometry of Time: Exploring the Fourth Dimension of the Universe 
How to measure distances in the Universe? 
Why ‘nothing’ is impossible: Do nothingness and emptiness exist? 
The Horizon Problem: Understanding the Uniformity of the Cosmos 
What is Dark Matter? The Invisible Force Shaping the Universe 
Metaverse, the next stage of evolution 
Multiverse: An Ocean of Expanding Space-Time Bubbles 
Cosmological Recombination: When the Universe Became Transparent 
The cosmological and physical constants of our Universe 
The Thermodynamics of the Sandpile and the Avalanche Effect 
The engine of the accelerated expansion of the Universe 
The X-Ray Universe: When Space Becomes Transparent 
The oldest galaxies in the universe 
Hubble constant and expansion of the Universe 
Dark Energy: When the Universe Defies Its Own Gravity 
What is the Size of the Universe? Between Cosmological Horizon and Infinity 
Quantum Vacuum and Virtual Particles: The Physical Reality of Nothingness 
Paradox of the dark night 
Journey into the Heart of Paradoxes: The Enigmas That Revolutionized Science 
Cosmic Microwave Background: The Thermal Echo of the Big Bang