The Schrödinger equation remains the central mathematical framework of quantum mechanics, but the measurement problem endures. A developing approach called quantum reference frames treats measuring devices (clocks, detectors, observers) as quantum systems, revealing that entanglement and superposition can be frame dependent. Recent work—including a 2023 study involving Edward Witten—suggests this perspective can simplify thorny problems like black hole entropy. A quickly growing research community is now exploring implications for quantum gravity and foundational puzzles such as Wigner’s friend.
Schrödinger’s Equation at 100: Why Observers Are Back in the Quantum Spotlight
One hundred years after Erwin Schrödinger submitted the equation that became a cornerstone of quantum mechanics, physicists are still confronting its deepest puzzles—especially the role of measurement and the observer.
Why the Equation Still Matters
The Schrödinger equation prescribes how a quantum system’s wave function evolves in time. The wave function encodes the probabilities for a particle’s possible properties—position, momentum, spin—and it is an indispensable predictive tool in laboratories around the world. But the equation’s rules for evolution apply while a system is unobserved. The instant a measurement is made, the wave function appears to "collapse" to a definite outcome, a phenomenon that lies at the heart of the longstanding measurement problem.
Putting the Observer Back Into the Math
A growing research program called quantum reference frames changes how we use the Schrödinger equation: it treats measuring devices—clocks, detectors, even observers—as quantum systems in their own right. That matters because if a clock obeys quantum rules, the time it reports is fundamentally fuzzy, subject to Heisenberg uncertainty.
“We’ve been trying to do physics as though it’s just there,” says Anne‑Catherine de la Hamette of ETH Zurich. “And we forgot to ask, ‘Well, who is actually measuring stuff?’”
When instruments are included in the quantum description, the wave function in the Schrödinger equation is updated to include both the system and its measuring device. This lets researchers ask concrete questions about how different observers—each with their own "quantum clock"—will describe the same physical situation.
Surprising Consequences
Small conceptual changes have yielded striking results. In 2019, researchers showed that entanglement can be frame dependent: a pair of particles that appears entangled from one quantum reference frame may not look entangled from another. The same relativity of description can apply to superposition—what looks like a single superposed state in one frame could appear as a mixture in another.
These ideas also hint at new ways to tackle problems at the intersection of quantum theory and general relativity. For instance, black holes present a major tension: their intense gravity prevents information from escaping, yet they also display thermodynamic properties (temperature and entropy) that arise from quantum considerations. Reconciling these facts has been an enduring challenge.
In a 2023 paper that included contributions from Edward Witten and others, researchers explicitly included an observer equipped with a quantum clock in black hole calculations. To their surprise, certain previously divergent entropies became finite and more tractable. "If you take quantum reference frames into account, you find those infinities are made finite," says Joshua Kirklin of the Perimeter Institute.
A Growing Community And Open Questions
Interest in quantum reference frames has accelerated recently. Last year, researchers gathered in Okinawa for the first conference devoted to the subject, signaling a growing community exploring connections to quantum gravity, thermodynamics, and the foundations of quantum mechanics.
Physicists are also revisiting classic thought experiments—Wigner’s friend and Schrödinger’s cat—through this new lens, hoping to sharpen our understanding of what happens at the moment of observation. Philipp Höhn of the Okinawa Institute of Science and Technology summarizes the aim: to describe quantum systems "from the inside" rather than always from an external, classical viewpoint.
Where This Might Lead
Quantum reference frames are still young as a theoretical approach, but they have already produced testable conceptual predictions and nudged notoriously difficult calculations into more manageable territory. Whether they will provide a full resolution of the measurement problem or be a stepping stone toward quantum gravity remains to be seen. What is clear is that a century after Schrödinger’s breakthrough, the question of who—or what—counts as an observer has returned to center stage.
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