Physicists Have Been Trying to Remove Time From the Universe

For centuries, time has seemed like the one thing nobody could argue with. Clocks move forward, people grow older, stars form and disappear, and every event appears to happen somewhere between a past and a future. But when physicists tried to write an equation capable of describing the entire universe, they ran into a bizarre problem: there was nowhere to put the clock. The deeper they pushed the mathematics, the less obvious it became that time belonged in the most fundamental description of reality at all.

The strange story began with a two-hour airport layover in 1965, when physicists John Wheeler and Bryce DeWitt began working on an equation intended to describe the universe as a whole. What emerged became one of the most unsettling ideas in modern physics: a description of the universe in which time appears to disappear completely. More than six decades later, physicists are still debating what that means, and whether time is a fundamental ingredient of reality or something that emerges from the way different parts of the universe interact.

The Airport Layover That Changed Physics

Wheeler had a two-hour gap between flights at Raleigh-Durham airport in North Carolina. Rather than spending the layover alone, he called Bryce DeWitt, who was then at the University of North Carolina in Chapel Hill, and asked him to come and keep him company. The meeting sounds almost casual in retrospect, but the problem they started working on was anything but ordinary. Wheeler wanted a mathematical description capable of applying quantum mechanics and gravity to the universe itself, rather than to a small system sitting inside it.

The pair spent the layover working through the mathematics and eventually produced the foundations of what became known as the Wheeler-DeWitt equation. Wheeler initially wanted to call it the Einstein-Schrödinger equation, reflecting the attempt to bring together ideas associated with Einstein’s theory of relativity and Schrödinger’s quantum mechanics. DeWitt, however, would later spend much of his career trying to distance himself from the equation, which became famous for creating a problem far stranger than either physicist may have expected.

The problem appeared when the equation was applied to the universe as a complete system. In ordinary quantum mechanics, scientists can describe a system changing relative to an external clock. The clock provides a reference point, allowing researchers to say that one event happened before another. But the universe, by definition, contains everything that exists. There is no obvious external laboratory and no outside clock against which the entire universe can be measured.

That left physicists with an uncomfortable result. When the equation was written down in its most fundamental form, the familiar time variable disappeared. The mathematics appeared to describe a universe that did not evolve from one moment to another in the conventional sense. DeWitt reportedly called it “that damn equation,” and the fact that one of its own creators remained deeply uncomfortable with it only added to the mystery surrounding the idea.

The Universe Has Nowhere To Hang A Clock

The problem becomes easier to understand by imagining an ordinary experiment. If scientists want to measure how quickly a particle moves, they need some way to compare its position at different moments. A clock provides that reference. The same basic idea appears throughout physics: something changes, and time provides the parameter used to describe that change. The trouble begins when the object being studied is no longer a particle, planet or laboratory, but everything that exists.

There is nowhere outside the universe to place the reference clock. A scientist can stand outside a laboratory, but nobody can stand outside the universe and watch it evolve from an external vantage point. Once physicists attempted to formulate a quantum description of the entire universe, the role normally played by an outside clock became impossible to define. Time did not simply become difficult to measure. It disappeared from the central equation.

The Wheeler-DeWitt equation was formally published in 1967, and its implications have continued to trouble physicists. Instead of producing a familiar picture in which the universe changes continuously, the equation can be interpreted as describing the complete state of the universe without an obvious parameter representing its passage through time. Everything is there, but there is no universal ticking mechanism telling the system how to move from one state to another.

The idea remains controversial because the equation is not an experimentally confirmed description of the universe. Theoretical physicist Carlo Rovelli wrote in 2015 that the Wheeler-DeWitt equation was “ill-defined, never empirically tested” but had nevertheless exerted a powerful influence on fundamental physics. That distinction is important. The mathematics raises a serious problem about the nature of time, but it does not establish that clocks, aging or change are somehow imaginary.

Physicists Mean Several Different Things By Time

Part of the confusion comes from the fact that physicists use one word to describe several different ideas. There is time as a quantity in equations, where it acts as a parameter allowing scientists to describe change. There is time as a dimension within spacetime, alongside the three familiar dimensions of space. Then there is the arrow of time, the direction that makes the past different from the future and gives everyday events their apparent one-way flow.

Those concepts do not necessarily have to rise or fall together. A theory could remove one version of time while leaving another intact. This is particularly important when physicists discuss the arrow of time, because the direction we experience may be connected to thermodynamics rather than being a basic ingredient of the universe’s fundamental equations. Entropy generally increases, and that increase gives physical processes a preferred direction.

Think about a glass falling from a table and shattering across the floor. The equations describing microscopic physics can work in ways that do not obviously care whether the process is played forward or backward, yet everyday experience gives the event a clear direction. Nobody expects the broken pieces to leap from the floor and reconstruct the glass. The difference between those two descriptions is closely tied to entropy, which makes the passage from an ordered state to a more disordered state statistically favoured.

That leaves physicists with a strange possibility. The sensation that time is flowing may not require time to exist as a basic substance. It could instead arise from physical relationships, entropy and the way observers experience changes within a larger system. As physicist Jim Al-Khalili put it in 2025, “the closer we look, the more bizarre time becomes.”

Your Head Really Does Run Slightly Faster Than Your Feet

Einstein’s theory of relativity had already destroyed the idea that the universe possesses one universal clock shared by everyone. According to general relativity, gravity affects the rate at which clocks run. The difference becomes larger as the gravitational field changes, meaning two clocks positioned at different heights can record slightly different amounts of elapsed time.

In 2010, researchers at the National Institute of Standards and Technology demonstrated this effect using two extremely precise aluminium ion clocks. One clock was positioned about 33 centimetres above the other, roughly the height of a laptop screen. The higher clock ran slightly faster than the lower one, demonstrating that the difference was not merely a theoretical prediction buried inside Einstein’s equations.

The difference was extraordinarily small, but it was measurable. Over a 79-year lifetime, the difference between the two locations would amount to roughly 90 billionths of a second. That is far too small for a person to notice, yet the experiment makes the underlying point clear: there is no single universal rate of time that applies identically to every location.

Even the idea of “now” becomes complicated when viewed through relativity. Your head and your feet are technically experiencing slightly different rates of elapsed time while you sit or stand. Your everyday experience remains completely normal because the difference is tiny, but the physics underneath that ordinary experience is already very different from the universal clock imagined in classical physics.

Two Theories Describe Reality, But They Don’t Fully Agree

Modern physics has another problem sitting underneath the mystery of time. Quantum mechanics provides an extraordinarily successful description of the microscopic world, while general relativity describes gravity, spacetime and the large-scale structure of the universe. Both theories have survived enormous numbers of experimental tests within their respective domains, yet combining them into one complete theory has proved remarkably difficult.

As philosopher of science Sam Baron has noted, “Both theories work extremely well in their own right, but the two are thought to conflict with one another.” The conflict becomes especially important when physicists examine extreme situations, such as the earliest moments of the universe or the interior of black holes, where quantum effects and strong gravity are expected to become impossible to separate.

One proposed route toward a solution is loop quantum gravity. Rather than treating space and time as perfectly smooth, the theory explores the possibility that spacetime has a deeper structure made from extremely small discrete elements. At the smallest scales, the smooth world humans experience could therefore be an approximation of something much stranger underneath.

The search for a theory that combines quantum mechanics with gravity has produced several competing approaches. Some of them make time look emergent, while others raise questions about whether space itself is fundamental. The Wheeler-DeWitt equation sits directly inside this broader problem, which explains why an equation written down during an airport layover continues to attract attention decades later.

The Strange Experiment That Made Time Look Emergent

A possible way around the problem appeared in 1983, when physicists Don Page and William Wootters proposed an idea called “Evolution without evolution.” Their proposal accepted the strange feature of the Wheeler-DeWitt picture rather than trying to remove it. The universe as a complete system could remain static, they suggested, while observers inside that universe could still experience change.

Their idea involved dividing the universe conceptually into two parts: a clock and everything else. The two parts would become quantum mechanically entangled, creating correlations between them. An observer inside the system could then use the clock portion as a reference and describe the rest of the system as evolving, even though an observer describing the complete system would see a static state.

For decades, the proposal remained a theoretical idea. Then researchers in Turin tested the mechanism using two entangled photons. One photon acted as the clock, while the other represented the system whose evolution was being observed. The experiment provided a laboratory demonstration of how different observers could obtain different descriptions of change depending on how they examined the combined quantum system.

The researchers reported that an internal observer correlated with the clock photon could see the other system evolve, while an external observer examining the global properties of both photons could describe the complete system as static. In other words, time could emerge from the relationship between parts of a system rather than existing as an independent stage on which everything happens.

Physicists Still Don’t Know What Has To Go

None of this proves that time does not exist. The Wheeler-DeWitt equation has never been experimentally confirmed as a complete description of the universe, and the Turin experiment involved two photons rather than the cosmos. It demonstrated a mechanism that helps physicists think about emergent time, but it did not place the universe itself inside a laboratory and switch off its clock.

There are also competing ideas about which part of our familiar picture of reality might disappear at the deepest level. Physicist Fotini Markopoulou has argued that space, rather than time, could be the quantity that fails to exist fundamentally. Other work has proposed relationships in which gravity emerges from quantum time, producing a very different picture of how the universe is constructed.

That means the question is still wide open. Physicists do not currently have an accepted answer saying that time is an illusion, nor do they have a confirmed theory showing exactly what replaces it. The Wheeler-DeWitt equation instead exposes a problem that any successful theory of quantum gravity may eventually have to confront.

The possibilities are surprisingly broad. Time could be fundamental but require a new interpretation. It could emerge from quantum relationships between physical systems. Space could turn out to be the deeper quantity that disappears. Or the eventual theory could force physicists to abandon assumptions about reality that currently seem too obvious to question.

The Clock May Be Something You Carry Inside

The most fascinating part of the problem is that removing time from a fundamental equation does not necessarily remove change from the universe. A person can still age, stars can still form, particles can still interact and events can still occur in an apparent sequence. The question is whether those changes require a universal clock or whether they can emerge from relationships between different parts of the universe.

The Page-Wootters proposal offers one possible explanation. An observer does not need to exist outside the universe to measure change. Instead, the observer can use one physical system as a clock and compare it with another. The relationship between the two systems can create an internal experience of time even if the complete description of both systems contains no external time parameter.

That idea turns the ordinary concept of a clock upside down. A clock might not be measuring some universal substance called time. It could simply be a physical system undergoing change that another system uses as a reference. From inside the universe, that relationship can produce the familiar experience of events unfolding one after another.

It also offers a possible explanation for why time feels so unavoidable. Human beings exist inside the universe rather than outside it, so every experience we have is built from relationships between physical processes. We never get to observe the universe from an external viewpoint where its entire history could be displayed at once.

Six Decades Later, The Airport Equation Is Still Causing Problems

Wheeler and DeWitt were trying to solve an extraordinarily difficult problem during a two-hour airport layover. Instead, they helped expose a question that remains central to modern attempts to understand gravity, quantum mechanics and the basic structure of reality. Their equation did not prove that time is fake, but it revealed how difficult it is to include a conventional clock when the system being described is the entire universe.

More than 60 years later, physicists still do not know whether time is fundamental, emergent or part of a deeper structure that current theories cannot yet describe. The experiments with entangled photons have shown how an observer could experience change inside a globally static system, while relativity has already shown that time does not pass identically everywhere.

The clock on the wall still works perfectly well. It tells people when to catch flights, when to go to work and when another day has passed. But at the deepest level of physics, that familiar ticking may be less fundamental than anyone once assumed.