What is time?

Physicists Propose Radical New Approach to Understanding Time's Origin

Constructor Theory Challenges Fundamental Assumptions About Temporal Reality in Universe

A groundbreaking paper suggests time emerges from impossible transformations rather than being fundamental to physics

 August 16, 2025

In a bold challenge to one of physics' most enduring puzzles, quantum physicist David Deutsch of the University of Oxford and his colleague Chiara Marletto have published a provocative new theory that attempts to explain how time itself emerges in a universe that may be fundamentally timeless. Their work, published in a preprint on arXiv in May 2025, represents the latest development in their ambitious "constructor theory" framework—an approach that seeks to rewrite the laws of physics based on what transformations are possible or impossible rather than on the traditional language of forces and particles.

The paper, titled "Constructor theory of time," asserts that "the laws of physics are expressible as specifications of which transformations of physical systems can or cannot be brought about with unbounded accuracy by devices capable of operating in a cycle ('constructors')" and that such specifications cannot inherently refer to time. This creates what the authors call a fundamental problem: how can laws that don't reference time still give meaning to duration and dynamics?

The Time Problem in Modern Physics

Time has undergone a profound conceptual evolution in physics, creating increasingly complex puzzles about its fundamental nature. To understand why time poses such difficulties today, it's essential to trace how our understanding has transformed over centuries.

From Universal to Relative Time

Classical Newtonian physics, which dominated scientific thinking for over two centuries, treated time as absolute and universal. In Newton's framework, time was the stage upon which all physical processes unfolded—a uniform, unchanging backdrop that flowed equally for all observers throughout the universe. Galileo had similarly assumed time was universal, flowing the same way everywhere. This made intuitive sense: a second was a second whether you were on Earth or moving through space, and the laws of motion could be expressed as precise mathematical relationships describing how position, velocity, and acceleration changed with this universal time parameter.

Einstein's special relativity in 1905 shattered this comfortable assumption. Time, Einstein showed, was not universal but relative—different observers moving at different speeds would measure different elapsed times for the same events. A clock on a speeding train would literally run slower compared to one at rest. This wasn't just a measurement effect; time itself was relative, woven together with space into a four-dimensional spacetime fabric.

General relativity, published in 1915, made the situation even more complex. Einstein revealed that gravity itself was the curvature of spacetime, meaning time could be stretched and compressed by massive objects. Near a black hole, time runs slower than in empty space. Suddenly, there wasn't just one time—there were infinitely many possible ways to slice up spacetime into moments, like infinitely many ways to put a coordinate grid on a map.

The Quantum Gravity Crisis

The real crisis emerged when physicists tried to unify Einstein's curved spacetime with quantum mechanics. In general relativity, time is a coordinate that can be chosen in countless ways—there's no preferred universal time. But quantum mechanics seems to require a definite notion of time to describe how quantum states evolve according to the Schrödinger equation.

This creates a profound paradox: if space and time themselves have quantum properties—as they must in a theory of quantum gravity—then we could have multiple quantum states of time existing simultaneously. "If you don't know what it means to have several different times at once, don't worry. No one knows. That's the problem," explains the current theoretical challenge.

Even more troubling, many approaches to quantum gravity suggest that time doesn't exist at all at the most fundamental level. Instead, what we experience as time's passage might be an "emergent" phenomenon—something that appears only when we look at the universe from a particular perspective, like how the smooth flow of water emerges from the chaotic motion of individual molecules.

The Paradox of Periodic Timekeeping

Adding another layer of complexity is the fundamental way we actually measure time: through periodic phenomena. From the earliest civilizations tracking rotating planets and lunar cycles to modern atomic clocks based on oscillating cesium atoms, all timekeeping relies on repetitive processes. The rotation of Earth gives us days, its orbit around the Sun gives us years, and the vibrations of quartz crystals or atomic transitions give us precise seconds.

This creates a profound conceptual challenge. Physics began with the goal of predicting periodicities—when eclipses would occur, where planets would be, how pendulums would swing. Yet to make these predictions, we need to already have a notion of time to describe the periods. We define a second as a certain number of oscillations of a cesium atom, but this assumes we already know what it means for the atom to oscillate "regularly" through time.

The circularity becomes even more apparent when we consider that all our fundamental equations of motion—from Newton's laws to Schrödinger's equation—describe how systems evolve by relating their state at one time to their state at slightly later times. But to verify these equations, we must measure time intervals using... other physical systems governed by the same types of equations. We're essentially using clocks to study the physics of clocks.

This raises deep questions: How do we know that atomic oscillations are truly regular? What if our atomic clocks are gradually slowing down or speeding up in unison? Without an independent, external timekeeper, we would have no way to detect such changes. We assume the laws of physics are consistent and that identical atomic transitions always take the same amount of time, but this assumption is built into our very definition of what constitutes equal time intervals.

Complicating matters further is the question of time's direction. Most fundamental laws of physics are time-reversible—Newton's equations, Maxwell's electromagnetic equations, and even Einstein's relativity work equally well whether time runs forward or backward. Yet we clearly experience time as having a definite direction: eggs break but don't spontaneously reassemble, heat flows from hot to cold but not the reverse, and we remember the past but not the future.

This directional aspect of time appears to emerge from thermodynamics and the second law, which states that entropy—roughly, the measure of disorder in a system—always increases over time in isolated systems. When a drop of dye dissolves in water, entropy increases and the process becomes effectively irreversible. The dye molecules will never spontaneously clump back together, even though nothing in quantum mechanics explicitly forbids such an event.

This thermodynamic arrow creates another layer of puzzle for any fundamental theory of time. If time itself is emergent from more basic, timeless laws, how does the directional, irreversible character of thermodynamics arise? Why does entropy increase in one temporal direction rather than the other? Some physicists argue that the arrow of time is purely statistical—a consequence of the universe starting in an extremely low-entropy state after the Big Bang. Others suggest that time's arrow might be more fundamental, requiring explanation at the deepest level of physical law.

The Bootstrap Problem

This leads to what physicists call the "bootstrap" problem: if the fundamental laws of physics don't involve time, how do we construct or derive the time that we clearly experience—including its directional character? Most approaches try to "bootstrap time from matter"—using the relative changes in matter configurations to construct something resembling a clock, then extracting time from that. But this creates conceptual difficulties about what it even means for matter to "change" if there's no pre-existing time for it to change within, and how irreversible thermodynamic processes can emerge from reversible fundamental laws.

The challenge becomes even more acute when physicists attempt to unify quantum mechanics with general relativity into a theory of quantum gravity. Many researchers consider this problem to be one of the greatest unsolved puzzles in physics, with some questioning whether time exists at all at the most fundamental level.

"Many physicists believe now that time fundamentally doesn't exist at all," notes physicist Sabine Hossenfelder, summarizing the current mainstream view that time emerges from more fundamental, timeless processes.

Constructor Theory's Revolutionary Approach

Deutsch and Marletto's constructor theory takes what they call an "audaciously" different approach to fundamental physics. Rather than starting with differential equations that describe how systems evolve over time, constructor theory begins with statements about what transformations are possible or impossible in the universe.

The theory "expresses physical laws exclusively in terms of which physical transformations, or tasks, are possible versus which are impossible, and why" and represents what the authors describe as a complete departure from reductionist physics.

In their new paper, Deutsch and Marletto propose that time can be "constructed" through what they call the "null task"—essentially the shortest possible task, which involves a constructor turning on and immediately turning off. By grouping these null tasks together and measuring changes in any system relative to these minimal operations, they argue that a meaningful measure of time emerges.

This approach attempts to sidestep the traditional reliance on periodic phenomena for timekeeping. Rather than using oscillating systems like atomic clocks, constructor theory proposes measuring time relative to the most basic possible operations. However, this raises the question of whether their "null tasks" can truly avoid the circularity inherent in all timekeeping methods.

"Your first idea might be to say we'll use constructors that perform recurring tasks because this is similar to an oscillation of sorts and that's what one normally does to construct clocks," the framework acknowledges. "But this doesn't work because one can't know whether the recurrent task actually happens in the same time periods. It could just be getting slower and slower like Windows updates."

"They identify a shortest task and that is doing nothing," explains the framework. "They take all these possible null tasks and group them together and then they measure the changes in any system relative to these null tasks. This means one has now introduced a measure of change which is time in some sense."

Crucially, constructor theory also addresses the thermodynamic arrow of time through its emphasis on impossibility. The theory explicitly incorporates irreversible processes—transformations that can occur in one direction but not the reverse—as fundamental features rather than statistical accidents. When thermodynamics shows that a dye clumping back together after dissolving is "effectively impossible," constructor theory treats this impossibility as a basic law of physics, not merely an unlikely event. This provides a potential foundation for time's arrow that doesn't rely solely on statistical arguments about low-entropy initial conditions.

Scientific Community's Mixed Reception

The reception to constructor theory has been notably polarized within the physics community. Supporters praise its conceptual boldness and potential to break free from traditional frameworks that may be limiting progress in fundamental physics. As physicist Chiara Marletto notes, "Declaring something impossible leads to more things being possible. Bizarre as it may seem, it is commonplace in quantum physics".

However, critics point to apparent circularity in the reasoning. "The argument seems somewhat circular to me because how can a task run if you don't have time already," observes one physicist reviewing the work. This criticism highlights a fundamental challenge: explaining how temporal processes can operate in a framework that claims to derive time from more basic principles.

Broader Context in Quantum Gravity Research

The Deutsch-Marletto proposal arrives at a time of intense activity in quantum gravity research. Recent developments have begun to draw unexpected connections between gravity, black holes, quantum information, and condensed matter systems, with researchers at institutions like MIT exploring how quantum entanglement, quantum error correction, and computational complexity play a fundamental role in the emergence of spacetime geometry.

Current experiments aim to determine if differences in space-time might alter atomic and subatomic behaviors, while physicists have considered many possible signatures of quantum gravity, such as gravitationally induced quantum entanglement of masses and fluctuating quantum spacetime in gravitational interferometry.

The field is also seeing practical developments, with researchers creating executable Python libraries that translate constructor theory's formalism into testable computational models, allowing for direct exploration of the theory's predictions.

Implications for Future Physics

If constructor theory proves viable, it could fundamentally reshape how physicists approach the deepest questions about reality. The theory suggests that all fundamental properties of nature are contained in the types of constructors that our universe can bring about, representing a complete inversion of traditional reductionist approaches.

Other emerging approaches to quantum gravity, such as emergence theory, also focus on information as fundamental, viewing "all of reality as made of information" and suggesting that consciousness and meaning play crucial roles in actualizing physical reality.

The debate over constructor theory reflects broader questions about the nature of physical laws and the role of time in fundamental physics. Recent work on the emergence of time in quantum gravity suggests that time should be considered "an emergent, non-fundamental notion" with multiple forms of emergence potentially at play.

Looking Ahead

While constructor theory remains highly speculative, its proponents argue that physics needs exactly this kind of radical departure from conventional approaches. The theory's emphasis on impossibility and counterfactual reasoning offers a novel way to think about physical laws that may prove essential for progress on the hardest problems in physics.

Upcoming conferences on quantum gravity in 2025, including events at Penn State University and the Perimeter Institute, will provide forums for further debate about constructor theory and other approaches to understanding time's emergence.

As the field continues to grapple with the fundamental nature of time and space, the Deutsch-Marletto proposal stands as a reminder that the deepest questions in physics may require not just new mathematics, but entirely new ways of thinking about the nature of reality itself.

Whether constructor theory represents a breakthrough or a dead end remains to be seen. What's certain is that it exemplifies the kind of bold theoretical thinking that has historically driven physics forward—even when, as with time itself, the destination remains mysterious.

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