A revised version of quantum mechanics that incorporates gravity might finally reach one of physics’ major aims and expose the fundamental uncertainty of time.
At times, effort on a problem leads to the realization that the approach has been reversed. Consider forcing a large old piano through a narrow entrance. After repeated attempts at turning, dismantling parts or pushing hard, success remains impossible. The solution appears when a suitable space is built around the piano in its current position.
Physicists now face a comparable shift. For many years the standard method toward a complete theory has been to fit our leading description of gravity inside quantum mechanics. Because quantum theory already accounts well for the remaining three fundamental forces, the strategy seemed logical. Yet nearly a century later gravity still resists inclusion.
A small number of researchers therefore propose the opposite step. They argue that altering the equations of quantum mechanics to accommodate gravity can clarify how the probabilistic behavior of particles produces ordinary experience.
Several experiments are now being prepared, using levitating objects, glowing surfaces, oscillating pendulums and precise clocks. These tests aim to reveal the limits of quantum behavior and point toward a fuller description of nature. “This is like entering unknown waters: we have no clear direction,” says physicist Angelo Bassi of the University of Trieste. “Yet by exploring what may be the wrong path, we might uncover the correct one.”
Everyday objects appear definite: books rest on shelves, clocks advance steadily and a cat is clearly alive. At atomic scales, however, properties such as position are described only by probabilities. Quantum mechanics predicts the chance of finding a particle in any given location, yet the actual outcome of a measurement remains undetermined until it occurs. Prior to observation the system exists in a superposition of all possibilities, represented by a wave function.
Two central difficulties follow. First, the mechanism by which the probabilistic quantum realm yields definite classical outcomes remains unexplained. Second, this probabilistic framework conflicts with Einstein’s classical theory of gravity. Attempts to express gravity in quantum terms have produced elaborate models such as string theory that are difficult to test.
A common view holds that everything is ultimately quantum. Nevertheless, a century after its formulation, physicists continue to seek a consistent account. “Something additional must be involved, and we need to identify it,” Bassi notes. “The key is to test quantum mechanics at its boundaries.”
One approach examines superposition, in which a particle occupies two separate locations simultaneously until measured. Interference patterns confirm the mixed state, yet measurement reduces it to a single outcome.
Interpretations differ on what triggers this reduction. The many-worlds view holds that all outcomes occur in separate realities, while the Copenhagen view treats the mathematics as sufficient.
Another set of proposals looks for a physical cause. In the 1980s Ghirardi, Rimini and Weber suggested an unknown process that forces wave functions to collapse. Later, Lajos Diósi and Roger Penrose proposed that gravity itself induces the collapse, implying that gravity prevails over quantum superposition.
