Physicists have demonstrated that the wave function describing the entire universe's quantum state cannot be determined through any experiment, even from a limited set of possibilities. Researchers Eddy Keming Chen and Roderich Tumulka argue this reveals a fundamental limit in quantum physics. Their work challenges assumptions about empirical knowledge in cosmology.
In quantum physics, objects like electrons are described by wave functions that encode their states, allowing predictions of experimental outcomes. Extending this to the cosmos, as suggested by figures like Stephen Hawking, implies the universe itself has a wave function. However, Eddy Keming Chen at the University of California, San Diego, and Roderich Tumulka at the University of Tübingen in Germany have shown that this universal wave function remains inaccessible.
Their proof relies on quantum statistical mechanics and the immense complexity of the wave function, which involves vast numbers of parameters or high-dimensional states. Starting from a set of plausible wave functions for the universe, the researchers calculated that no quantum-allowed measurement provides enough information to identify the correct one.
"The wave function of the universe is like a cosmic secret that physics itself conspires to keep," Chen explains. "We can know enormously much about how the universe behaves, yet remain fundamentally uncertain of which quantum state it is in."
Tumulka adds, "Any measurement that’s allowed according to the rules of quantum mechanics will give us very limited information about the wave function of the universe. It’s impossible to determine the wave function of the universe with any useful accuracy."
This finding builds on prior theoretical models but emphasizes the role of observations, which previous work overlooked. JB Manchak at the University of California, Irvine, notes parallels with limitations in general relativity and observes that quantum theory was not designed for cosmic scales. Sheldon Goldstein at Rutgers University views the wave function as a theoretical tool; multiple candidates may yield similar results in calculations, mitigating the issue.
Philosophically, the result cautions against strict positivism, where untestable statements are dismissed. "Certain things actually exist out there in reality, but we cannot measure them," Tumulka says. Emily Adlam at Chapman University sees it as support for quantum interpretations focusing on observer perspectives rather than a single objective reality.
The study appears in the British Journal for the Philosophy of Science (DOI: 10.1086/740609). Chen and Tumulka plan to apply their insights to smaller large-scale systems using techniques like shadow tomography.