Every fundamental physical law works equally well backward - yet we only experience time flowing forward. Why can't we remember tomorrow? The origin of time's arrow is one of physics' unsolved mysteries.
George Grantham Bain Collection, Library of Congress Prints and Photographs Division Washington, D.C., Public domain, via Wikimedia Commons
The Puzzle of Past and Future
At the microscopic level, the laws of physics are almost entirely time-symmetric. A video of two particles colliding looks just as plausible when played backward. Gravity, electromagnetism, and the rules of quantum mechanics work equally well in either temporal direction. Yet, our macroscopic world shows a clear and irreversible flow of time. A shattered glass does not reassemble itself, and cream mixed into coffee never separates back out. This discrepancy is explained by the Second Law of Thermodynamics, which states that the total entropy, a measure of disorder or randomness of an isolated system can increase over time.
This concept was mathematically grounded in the late 19th century by Austrian physicist Ludwig Boltzmann. He proposed that what we perceive as the one-way flow of time is a statistical phenomenon. There are vastly more ways for particles to be arranged in a disordered, high-entropy state than in an ordered, low-entropy one. A system, therefore, almost always evolves toward disorder simply because it is the most probable outcome. The Second Law is not an absolute rule for any single particle, but an overwhelming statistical certainty for systems containing vast numbers of them.
An Exceptionally Orderly Beginning
The Second Law explains the direction of time's arrow, but it raises a deeper question: why was the universe so highly ordered in the first place? the source of the arrow of time seems to trace back to the initial conditions of the cosmos. The Big Bang, approximately 13.8 billion years ago, produced a universe that was incredibly hot and dense, yet in a state of low entropy. This highly ordered initial state, known to cosmologists as the "Past Hypothesis," is the reason why entropy has been increasing ever since, driving everything from the formation of galaxies to the processes of life.
The term "arrow of time" was coined by British astrophysicist Arthur Eddington in his 1927 Gifford Lectures in Edinburgh, later published in his book "The Nature of the Physical World." Standing on Blackford Hill at the Royal Observatory, Edinburgh—a historic and active center for cosmological research—one can look at the sky and contemplate this deep mystery. Scientists at the observatory's Institute for Astronomy and the UK Astronomy Technology Centre continue to study the universe's origins, seeking to understand the very conditions that set time on its irreversible course. Why the universe began in this special, low-entropy state remains one of the unsolved problems in physics.
💡Fun Facts
The equation defining entropy, S = k log W, is engraved on Ludwig Boltzmann's tombstone in Vienna.
Arthur Eddington, who coined the phrase "arrow of time," delivered the lectures that formed the basis of his famous book at the University of Edinburgh in 1927.
The "Past Hypothesis" is the formal name for the postulate that the universe began in a state of very low entropy, which is not explained by other physical laws but is assumed as an initial condition.
Some physicists theorize that if the universe were to collapse in a "Big Crunch," entropy would continue to increase, meaning time would not run backward even if the cosmos were shrinking.
The Royal Observatory Edinburgh Visitor Centre is generally open for special events and pre-booked tours; check the official website for public opening times and event calendars.
Admission
General admission to the Visitor Centre is often free, though specific tours, lectures, and stargazing events may have a fee.
Accessibility
The main Visitor Centre is wheelchair accessible. However, the historic Victorian telescope dome is only accessible via a spiral staircase.
