Quantum mechanics and Einstein’s theory of general relativity are two of the most successful theories in modern physics. Quantum mechanics describes matter and energy at the smallest scales, while general relativity explains gravity, the motion of stars and black holes, and the expansion of the universe.
Despite their extraordinary success, the two theories are not fully compatible. For decades, physicists have searched for a theory of quantum gravity that could unite quantum mechanics and general relativity into a single, coherent description of nature.
A major challenge for quantum gravity
Most physicists expect gravity to eventually follow the rules of quantum mechanics. However, combining the two theories creates profound conceptual challenges.
Quantum mechanics allows objects to exist in a superposition of multiple locations at the same time. This counterintuitive behavior has been demonstrated in experiments involving atoms and even small pieces of metal. General relativity, by contrast, describes gravity as a property of space-time itself. Gravitational wave detectors have confirmed that space-time can bend, curve, and carry waves across the universe.
As a result, many physicists have proposed that the space-time surrounding a quantum object could also exist in multiple “states” simultaneously.
But what would such a phenomenon actually mean in an experiment?
Researchers from Kyushu University, the University of Waterloo, and Stockholm University may have found part of the answer. Their findings were published in npj Quantum Information.
When quantum gravity can appear classical
The researchers developed a theoretical framework showing that many situations commonly described as “gravitational quantum superpositions” may also be interpreted in another way.
In these scenarios, quantum particles can remain in a superposition while moving through ordinary gravity and classical space-time. This interpretation does not require gravity itself to display quantum behavior.
“Many researchers have proposed experiments that could reveal the quantum nature of gravity,” explains Joshua Hu, associate professor at Kyushu University’s Institute for Advanced Study and lead author of the study. “We found that some of these scenarios can be understood from two equally valid perspectives. One describes gravity as being in a quantum superposition, while the other describes quantum particles moving through a conventional gravitational field.”
The research team refers to this concept as the “theory of relativity of superposition of space and time.”
One way to understand the idea is to imagine two maps that depict the same landscape using different projections. The maps may look different, but both represent the same underlying terrain. Similarly, some physical situations that appear to involve quantum gravity may instead be described using classical gravity and space-time, provided that the motion of each particle is represented by the correct quantum state.
Clarifying ambiguity in quantum gravity experiments
The study does not prove that gravity is classical, nor does it disprove the possibility of quantum gravity.
Instead, it highlights an important ambiguity in experiments designed to test whether gravity has quantum properties. In some cases, observations that seem to provide evidence for quantum gravity could be explained without requiring gravity itself to behave according to quantum mechanics.
“Our study does not show that these experiments rule out quantum gravity,” says study co-author Magdalena Zych of Stockholm University. “Rather, it helps identify which experimental signatures truly require a quantum description of gravity and which could arise from more familiar physics. This distinction will be essential when designing future experiments.”
The search for definitive evidence of quantum gravity
Although this research addresses some of the deepest questions in physics, studying fundamental laws has often led to important practical technologies.
GPS navigation, lasers, and modern electronic devices all grew out of advances connected to quantum physics and Einstein’s theory of gravity.
The most immediate benefit of this new framework is that it gives researchers a clearer roadmap for testing quantum gravity. By identifying which observations can distinguish between classical and quantum descriptions of gravity, the study helps narrow the search for conclusive evidence.
“Understanding how gravity and quantum mechanics fit together is one of the greatest challenges in physics,” Foo concludes. “Before we can test the quantum nature of gravity, we must first determine what evidence would demonstrate that we have found it. Our research helps clarify that question.”
Source: www.sciencedaily.com