A new twist on Schrödinger’s Cat could finally unite quantum mechanics and Einstein’s relativity

Quantum mechanics allows a system’s state to combine several possibilities. Yet the universe we observe has a definite large-scale geometry. Explaining how those descriptions fit together remains a difficult question, especially when the system under discussion includes everything, leaving no outside observer to make a measurement.

A study published in the Journal of High Energy Physics explores one possible answer. Matteo Carlesso and José Luis Gaona-Reyes, affiliated with the University of Trieste, worked with Lucía Menéndez-Pidal and Mir Faizal to examine spontaneous collapse models in quantum cosmology.

Their proposal changes the mathematical evolution of the quantum state so that it can narrow toward a definite cosmic geometry. It belongs to a research tradition that asks whether quantum mechanics needs an additional physical process to explain definite outcomes. Laboratory cat-like states make the underlying questions especially tangible.

The work does not show that quantum mechanics fails for large objects, or that gravity and quantum theory have finally been unified. Instead, it demonstrates what a particular modification can accomplish within a simplified model. Experiments involving quantum objects remain essential for deciding which proposed changes, if any, nature actually requires.

A superposition is more than uncertainty about an object’s hidden condition. Different components of a quantum state can interfere, producing effects that an ordinary statistical mixture cannot. Experiments routinely exploit this behavior. The unresolved issue is how to connect quantum evolution with the definite outcomes recorded in individual measurements.

Schrödinger’s cat dramatizes that issue by linking a microscopic event to a macroscopic consequence. It does not establish that a real animal remains visibly both alive and dead until a person looks. The thought experiment exposes questions about applying quantum rules consistently to the detector, the cat and their surroundings.

Decoherence explains how interactions distribute quantum information into a system’s environment, suppressing accessible interference. Preserving molecular entanglement therefore requires careful control of those interactions. However, explaining lost interference and explaining one actual outcome are distinct tasks, whose interpretation remains debated.

Cosmology adds another complication: there is no environment outside the entire universe. Parts of the universe can still decohere through interactions with other parts. The authors seek an additional mechanism that selects a definite global geometry. Broader proposals involving quantum networks illustrate other ways researchers investigate the relationship between quantum behavior, time and gravity.

The researchers began with a highly symmetric universe: spatially flat, homogeneous and isotropic. In plain terms, the model has no preferred location or direction at large scales. Matter takes the form of an idealized fluid. These assumptions make the mathematics manageable while leaving out much of the real universe’s complexity.

They then considered a quantum state containing different possible geometries. Here, geometry concerns the universe’s spacetime structure and expansion, rather than planets appearing in multiple places. The question is whether added collapse dynamics can turn that spread of possibilities into a state associated with one definite geometry.

Their starting equation is a form of the Wheeler–DeWitt equation, used in approaches to quantum cosmology. To describe evolution, they choose an internal physical variable as a clock. Research on relational time explores why defining time from within a system is an important issue in its own right.

That choice allows the equation to take a Schrödinger-like form, with a state evolving relative to the selected variable. The model therefore assumes a usable clock rather than deriving time from nothing. Related questions about quantum time remain separate from demonstrating that a collapse process can operate once a clock has been chosen.

Spontaneous collapse models add random, nonlinear terms to ordinary quantum evolution. These terms make collapse part of the physical dynamics, rather than something triggered by a special measuring device. In this paper, the added process acts continuously and randomly. It is not a sequence of collapses occurring at regular intervals.

The team chose a collapse operator proportional to the Hamiltonian, the operator governing the system’s dynamics. This is a modeling choice, not an experimentally established law. Its role is to specify which property the random process makes increasingly definite and how strongly the state responds to the added noise.

Within the model, uncertainty in the selected quantity decreases as evolution proceeds. Individual simulated histories head toward different outcomes, depending on the noise realization. The authors also show that narrowing can be slow. The result concerns an approach toward localization, rather than an instantaneous cosmic event established by observation.

“Our model describes a quantum universe that eventually collapses, becoming effectively classical,” Carlesso explained. Here, collapse means localization of a quantum state, not a universe shrinking under gravity. Other work on collapse dynamics and levitated particles explores different theoretical or experimental aspects of these foundational questions.

The researchers also applied their framework to parametrised unimodular gravity, an alternative formulation of gravitational dynamics. In that description, the quantity selected by collapse can be interpreted as the cosmological constant. This constant is associated with a familiar description of dark energy and the universe’s accelerated expansion.

The proposal offers a way for one definite value to emerge from a quantum spread of possible values. That is narrower than predicting the particular small value inferred from observations. The calculation does not establish that nature selects the measured value, or resolve every aspect of the cosmological constant problem.

Measurements of dark energy and cosmic expansion address the universe we actually observe. The collapse study instead asks how a definite parameter could arise in a quantum description. Connecting those tasks requires more than showing that a parameter’s uncertainty can decrease mathematically.

The paper also acknowledges that its framework does not necessarily solve quantum gravity’s problem of time. Its conclusions depend on having a suitable clock variable. Extending the approach to other models may broaden its usefulness, but would not itself establish that spontaneous collapse occurs in the real universe.

The immediate value is a possible connection between collapse dynamics and cosmology. The model can describe a transition without an external observer. It does not predict a distinctive late-universe signal that separates its outcome from an already classical universe. Researchers also cannot rerun cosmic history to compare many outcomes directly.

Repeatable laboratory experiments offer a different route. Collapse models can produce small departures from standard quantum predictions, depending on their details. Researchers look for effects involving interference, motion, heating or radiation. Work with atom interference and quantum sensors illustrates the broader effort to make delicate quantum behavior measurable.

The 2024 paper did not conduct those tests or establish a new experimental detection. Testing a specific collapse prescription requires identifying its predictions and separating them from ordinary noise. Existing experimental constraints on other collapse models cannot automatically be treated as confirmation or rejection of this cosmological proposal.

For now, spontaneous collapse remains a hypothesis. The study shows how it could help a simplified quantum universe acquire a definite geometry. Its lasting contribution is a concrete framework that researchers can examine, extend and challenge, while keeping the distinction between mathematical possibility and demonstrated physics clear.

These resources cover experimental superposition tests, collapse-model constraints and the role of decoherence, listed newest first.

Probing quantum mechanics with nanoparticle matter-wave interferometry
Experiments observe interference in metal nanoparticles, testing quantum behavior at greater mass and complexity. (Nature, January 2026)

Present status and future challenges of non-interferometric tests of collapse models
A review explains how heating, radiation, mechanical systems and other measurements constrain proposed collapse mechanisms. (Nature Physics, February 2022)

Underground test of gravity-related wave function collapse
An underground radiation experiment tests a particular gravity-related collapse model rather than collapse theories as a whole. (Nature Physics, September 2020)

Models of wave-function collapse, underlying theories, and experimental tests
A detailed review introduces dynamical collapse models and their theoretical and experimental challenges. (Reviews of Modern Physics, April 2013)

Decoherence, the measurement problem, and interpretations of quantum mechanics
This review distinguishes environmental decoherence from broader questions about measurement outcomes and interpretation. (Reviews of Modern Physics, February 2005)

Research findings are available online in the Journal of High Energy Physics.

The original story "A new twist on Schrödinger’s Cat could finally unite quantum mechanics and Einstein's relativity" is published in The Brighter Side of News.

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