Physicists are rethinking time itself. Long treated as a basic part of the universe, time may instead be an illusion\u2014a side effect of quantum entanglement. A new study challenges the traditional idea that time flows independently. Instead, it suggests that time only appears when quantum systems interact in certain ways.<\/p>\n
Published in Physical Review A<\/a>, the study revisits a lesser-known theory from 1983 called the Page and Wootters (PaW) mechanism. This approach treats time not as a fixed backdrop but as something that arises from the properties of quantum systems. The theory turns time into a quantum observable rather than a built-in dimension.<\/p>\n Physics and Time<\/p>\n One major problem in physics is how differently time behaves in two major theories. In quantum mechanics, time acts like an outside ruler\u2014it stays constant and doesn\u2019t exist within the system being studied. Instead, it\u2019s used to track change from the outside. This setup creates a paradox: how can time control change if it doesn\u2019t actually exist inside the quantum system?<\/p>\n The evolving system phase-space with the admitted orbits when cm= 0\u2200m and \u03ba = 3\/4, r = 2\/3 for M = 20 (on the left) or M =50 (on the right). (CREDIT: Physical Review A) <\/p>\n On the other hand, Einstein\u2019s theory of general relativity<\/a> describes time as flexible. Time stretches and bends depending on speed and gravity. For example, time runs slower near a black hole. This means time is not fixed but flows differently depending on where and how it\u2019s observed. The conflict between this view and quantum mechanics has long puzzled scientists, who call it the \u201cproblem of time.\u201d<\/p>\n Alessandro Coppo from the National Research Council of Italy<\/a> puts it plainly: \u201cIt seems there is a serious inconsistency in quantum theory. This is what we call the problem of time.\u201d This mismatch stands in the way of creating a single theory that explains all of physics\u2014from the smallest particles to the entire cosmos.<\/p>\n Quantum Time Emergence Framework<\/p>\n To address this, the researchers took a fresh look at the PaW mechanism. According to this theory, time doesn\u2019t exist on its own. It arises through entanglement between two quantum systems: one acts as a clock, the other as the system being measured. Without this relationship, time disappears completely, and the universe becomes motionless\u2014a frozen snapshot without any flow.<\/p>\n Related Stories<\/p>\n To test the theory, they used a model with two entangled but separate quantum systems<\/a>. One system\u2014a vibrating harmonic oscillator\u2014represented the evolving world. The other\u2014a group of tiny magnetic spins\u2014acted as the clock. The oscillator changed over time, not because of some outside force, but due to its entanglement with the clock. This matched the predictions of the Schr\u00f6dinger equation, which describes how quantum systems evolve.<\/p>\n This new view marks a huge change in how we think about time. It suggests that time isn\u2019t something we move through, like a road. Instead, it may be a product of quantum relationships. In a universe without entanglement, time may not exist at all.<\/p>\n Bridging the Quantum-Classical Divide<\/p>\n The researchers took their study further, applying the PaW mechanism to larger, macroscopic systems. By assuming that either the magnet clock<\/a> or the harmonic oscillator was a classical object, they found that their equations naturally simplified into those used in classical physics. This suggests that the classical flow of time emerges from entanglement, even at larger scales.<\/p>\n Graphical representation of the marginal probability distribution related to |\u03b2|2 w.r.t the space-time coordinates and b) its section at any constant time for the example with \u03ba = 3\/4 introduced in the main text. (CREDIT: Physical Review A) <\/p>\n \u201cWe strongly believe that the correct and logical direction is to start from quantum physics and understand how to reach classical physics, not the other way around,\u201d Coppo said.<\/p>\n This finding supports the idea that classical physics emerges from quantum mechanics<\/a> rather than existing as a separate framework. It also suggests that time itself may not be fundamental but instead a byproduct of quantum correlations.<\/p>\n Implications and Future Challenges<\/p>\n While this perspective is mathematically consistent, physicists remain cautious. Some researchers argue that, although the PaW mechanism offers an intriguing explanation, it lacks direct experimental validation.<\/p>\n \u201cYes, it is mathematically consistent to think of universal time as the entanglement between quantum fields<\/a> and quantum states of 3D space,\u201d said Vlatko Vedral, a professor of quantum information science at the University of Oxford. \u201cHowever, no one knows if anything new or fruitful will come out of this picture\u2014such as modifications to quantum physics and general relativity, and corresponding experimental tests.\u201d<\/p>\n The contour plot of the marginal probability distribution related to |\ud835\udefd|2 with respect to the evolving system phase-space coordinates. (CREDIT: Physical Review A) <\/p>\n Experimental evidence remains the ultimate test of this theory. To truly determine whether time emerges from entanglement, physicists will need to design experiments capable of measuring these effects in real-world systems.<\/p>\n Despite the uncertainties, some believe this approach could be a step toward a deeper understanding of time. Adam Frank, a theoretical physicist at the University of Rochester<\/a>, suggests that time might only make sense when viewed from within a system, rather than as a detached, objective phenomenon. \u201cMaybe the only way to understand time is not from some God\u2019s-eye perspective, but from the inside, from a perspective of asking what is it about life that manifests such an appearance of the world,\u201d he said.<\/p>\n If the concept of time as an emergent property proves correct, it could lead to breakthroughs in quantum gravity and even reshape our understanding of reality. However, until experiments confirm these ideas, time\u2019s true nature remains one of physics\u2019 greatest mysteries.<\/p>\n More About the Page and Wootters (PaW) Mechanism<\/p>\n The Page and Wootters (PaW) mechanism, proposed by Don Page and William Wootters in 1983, offers an intriguing solution to the long-standing “problem of time” in quantum mechanics<\/a> and quantum gravity. Traditionally, quantum mechanics describes time as an external parameter, while general relativity treats it as a dynamic, relational quantity embedded within spacetime itself. PaW reconciles these views by suggesting that time emerges entirely from entanglement and correlations between quantum systems, rather than existing as an absolute or independent background entity.<\/p>\n The energy-time phase-space with the admitted orbits when cm= 0 \u2200m and \u03ba = 3\/4, r = 2\/3 for M = 20 (on the left) and M = 50 (on the right). (CREDIT: Physical Review A) <\/p>\n In the PaW approach, the entire universe is described by a timeless quantum state\u2014often called the “wavefunction of the universe”\u2014that is static and unchanging at the global level. Within this timeless framework, what we perceive as time arises solely through internal correlations between subsystems. One subsystem acts as a quantum “clock,” with other systems evolving in relation to its states. Time, therefore, does not flow externally but emerges naturally from conditional probabilities linking different quantum states to specific “clock” readings.<\/p>\n This mechanism elegantly addresses fundamental conceptual challenges. By treating time as relational, PaW aligns closely with the relativistic notion of time as observer-dependent. It also resonates with quantum entanglement<\/a> theory, as correlations between states underpin both entanglement and the emergence of time itself. However, PaW requires the presence of an appropriate quantum subsystem to act as a clock, raising practical questions about what constitutes a suitable quantum clock system and how accurately such a system can track emergent dynamics.<\/p>\n Despite its theoretical elegance, PaW faces ongoing scrutiny and debate within the physics community. Critics point out the difficulty in defining universally applicable quantum clocks and express concern about PaW’s ability to describe the classical experience of time in realistic physical situations. <\/p>\n