Timekeeping has reached an unprecedented level of precision with the advent of optical lattice clocks. These clocks, which use laser-cooled atoms trapped in a grid-like structure, are so accurate that they deviate by less than one second over billions of years. But beyond their role as timekeepers, optical lattice clocks are becoming a powerful tool to probe one of the most profound paradoxes in physics: the incompatibility between quantum mechanics and general relativity.1
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Quantum mechanics, which governs the behavior of particles at the smallest scales, treats time as a fixed background. In contrast, Einstein’s general relativity describes time as a dynamic entity, warped by gravity. Reconciling these two theories has been one of the greatest challenges in physics. Optical lattice clocks, with their unprecedented precision, are now allowing scientists to test the limits of these theories at the intersection of the quantum and relativistic realms.1
What Are Optical Lattice Clocks?
Optical lattice clocks (OLCs) are the most precise timekeeping devices ever developed, surpassing traditional atomic clocks in accuracy. They work by trapping neutral atoms, such as strontium or ytterbium, in a laser-generated optical lattice—an array of standing laser waves that form a grid-like structure. This setup isolates the atoms, shielding them from environmental disturbances and enabling highly stable atomic transitions. By using optical frequencies, which are 100,000 times higher than microwave frequencies in conventional atomic clocks, OLCs achieve unprecedented precision.2-3
Unlike cesium-based atomic clocks, which maintain accuracy within a second over millions of years, optical lattice clocks extend this precision to billions of years. This is due to their ability to operate at "magic wavelengths," where the trapping laser affects both energy states of the clock transition equally, eliminating frequency shifts.4
Their fractional frequency uncertainties reach one part in 10¹⁸, making them ideal for applications in fundamental physics, precision navigation, and tests of relativity. As the pinnacle of modern timekeeping, OLCs not only redefine time standards but also offer new opportunities to explore the fundamental nature of the universe.4
The Conflict Between Quantum Mechanics and General Relativity
At the core of modern physics lies an unresolved contradiction between Einstein’s general relativity and quantum mechanics—two pillars of scientific understanding that fundamentally disagree on the nature of time and space.
General relativity describes gravity as the curvature of spacetime caused by mass, with one of its key predictions being time dilation—the effect where time runs slower in stronger gravitational fields. Precise atomic clock experiments have confirmed that a clock at a lower altitude ticks more slowly than one at a higher altitude, a phenomenon crucial for GPS accuracy. In general relativity, time is not absolute; it stretches and contracts based on gravitational forces.5
Quantum mechanics, on the other hand, treats time as a fixed, absolute background—an unchanging stage on which quantum events occur. The Schrödinger equation, which governs quantum behavior, assumes a uniform flow of time, contradicting general relativity’s dynamic and flexible view. Furthermore, quantum entanglement, where two particles instantaneously affect each other across vast distances, appears to conflict with relativity’s rule that nothing can exceed the speed of light.6
Bridging these two theories into a unified quantum theory of gravity remains one of the greatest challenges in physics. General relativity presents a smooth, continuous spacetime, whereas quantum mechanics suggests a discrete, probabilistic nature at microscopic scales. Understanding time at atomic levels, using ultra-precise optical lattice clocks, may provide new insights into this profound mystery and bring physics closer to resolving this fundamental paradox.5-6
How Lattice Clocks Test Relativity at the Smallest Scales
Lattice Clocks provide an unprecedented ability to test general relativity at microscopic scales. Their extreme precision enables the detection of time dilation over micrometer height differences, offering direct experimental evidence of relativity at the quantum level.7
According to general relativity, time runs slower in stronger gravitational fields, an effect typically observed over vast distances, such as between satellites and Earth. However, OLCs can detect this phenomenon at centimeter and even micrometer scales. With a fractional uncertainty of 10-18, these clocks can measure time dilation for a mere centimeter elevation difference, making them powerful tools for testing Einstein’s equivalence principle.7
Recent experiments have demonstrated OLCs’ ability to observe time dilation over atomic-scale separations. In 2016, a study comparing two OLCs at RIKEN and the University of Tokyo (15m height difference) confirmed gravitational redshift with a measured fractional frequency difference of (−1652.9±5.9) ×10-18—matching GR’s predictions.8 In 2020, an even more precise experiment at Tokyo Skytree placed two OLCs 450m apart, detecting a frequency shift of Δν≈21.18 Hz, further verifying gravitational time dilation.9
By measuring time dilation at such small scales, OLCs provide direct experimental validation of relativity at the quantum level. These findings reinforce the validity of general relativity at microscopic distances while setting constraints on potential violations of local position invariance, offering crucial insights into the relationship between quantum mechanics and gravity.7
Bridging the Gap Between the Theories
Experimental data from OLCs is reshaping our understanding of time in the quantum realm and challenging the long-standing separation between general relativity and quantum mechanics. OLCs, capable of detecting time dilation at micrometer scales, reveal that relativistic effects extend into atomic and quantum domains, forcing physicists to reconsider how time operates at these fundamental levels.7
This challenges the idea that general relativity and quantum mechanics function independently. Findings from OLCs suggest that spacetime, as described by general relativity, may have a deeper influence on quantum systems than previously thought. This opens the door for quantum gravity models, such as string theory, which proposes extra dimensions, and loop quantum gravity, which suggests that spacetime itself is discrete. OLCs could provide experimental constraints on these theories by detecting subtle deviations from relativity at the smallest scales.7
Ongoing research with OLCs is refining our understanding of spacetime, testing the equivalence principle with extreme precision, and probing potential quantum gravity effects. Future advancements, such as placing OLCs in space, could enable even more precise measurements, offering new insights into the relationship between gravity and quantum mechanics.7 These breakthroughs may eventually help bridge the gap between these two fundamental yet conflicting theories.
Future Prospects and Implications
Advancements in OLCs are set to redefine timekeeping and revolutionize fundamental physics. Their precision could lead to a redefinition of the second based on optical frequencies, providing a more stable time standard.7
Beyond timekeeping, OLCs enable groundbreaking tests of the equivalence principle, quantum gravity, and dark matter. They also promise centimeter-level GPS accuracy, benefiting navigation and precision agriculture.
As precision improves, OLCs may help unify general relativity and quantum mechanics, deepening our understanding of spacetime. Ultimately, OLCs are more than just timekeepers—they are powerful tools for unraveling the fabric of reality, offering new insights into the deepest mysteries of physics.
References and Further Readings
- Colombo, S.; Pedrozo-Peñafiel, E.; Adiyatullin, A. F.; Li, Z.; Mendez, E.; Shu, C.; Vuletić, V., Time-Reversal-Based Quantum Metrology with Many-Body Entangled States. Nature Physics 2022, 18, 925-930.
- Derevianko, A.; Gibble, K.; Hollberg, L.; Newbury, N. R.; Oates, C.; Safronova, M. S.; Sinclair, L. C.; Yu, N., Fundamental Physics with a State-of-the-Art Optical Clock in Space. Quantum Science and Technology 2022, 7, 044002.
- Derevianko, A.; Katori, H., Colloquium: Physics of Optical Lattice Clocks. Reviews of Modern Physics 2011, 83, 331-347.
- Grebing, C.; Al-Masoudi, A.; Dörscher, S.; Häfner, S.; Gerginov, V.; Weyers, S.; Lipphardt, B.; Riehle, F.; Sterr, U.; Lisdat, C., Realization of a Timescale with an Accurate Optical Lattice Clock. Optica 2016, 3, 563-569.
- Allori, V., What Is It Like to Be a Relativistic Grw Theory? Or: Quantum Mechanics and Relativity, Still in Conflict after All These Years. Foundations of Physics 2022, 52, 79.
- Riedel, T., Relational Quantum Mechanics, Quantum Relativism, and the Iteration of Relativity. Studies in history and philosophy of science 2024, 104, 109-118.
- Shinkai, H.; Takamoto, M.; Katori, H., Transportable Optical Lattice Clocks and General Relativity. arXiv preprint arXiv:2502.06104 2025.
- Takano, T.; Takamoto, M.; Ushijima, I.; Ohmae, N.; Akatsuka, T.; Yamaguchi, A.; Kuroishi, Y.; Munekane, H.; Miyahara, B.; Katori, H., Geopotential Measurements with Synchronously Linked Optical Lattice Clocks. Nature Photonics 2016, 10, 662-666.
- Takamoto, M.; Ushijima, I.; Ohmae, N.; Yahagi, T.; Kokado, K.; Shinkai, H.; Katori, H., Test of General Relativity by a Pair of Transportable Optical Lattice Clocks. Nature photonics 2020, 14, 411-415.
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