When do quantum systems source gravity and how can we test it?

Gravity is the force that keeps our feet on the ground and holds the planets in their orbits, but it is also the most stubborn mystery in modern physics. For over a century, scientists have tried to unite two great theories: general relativity, which describes gravity as the smooth bending of space and time by massive objects, and quantum mechanics, which describes the chaotic, probabilistic behavior of the tiniest particles. The problem is that these two theories speak different languages. When physicists try to combine them, the math often breaks down, producing infinite values and nonsensical results. This has led many to believe that gravity itself must be made of tiny, discrete particles, just like light is made of photons. However, there is another possibility that has long been dismissed: that gravity remains a smooth, classical force, but it only "turns on" when quantum systems interact with their surroundings in a specific way.

Francisco Pipa, a researcher at the University of Queensland, proposes a new way to think about this puzzle. His work suggests that gravity does not exist as a constant field generated by every quantum particle at all times. Instead, he argues that a quantum system only sources a gravitational field—meaning it only creates a gravitational pull—when it participates in a specific chain of interactions that leads to a definite outcome. In the quantum world, particles often exist in a state of superposition, where they are in multiple places or states at once. Pipa suggests that as long as a particle remains in this isolated, superposed state, it does not generate gravity. It is only when the particle interacts with other matter in a way that causes "decoherence"—a process where the environment forces the particle to choose a single, definite state—that it begins to act as a source of gravity. This idea challenges the standard view that mass always creates gravity, regardless of whether that mass is isolated or interacting.

The paper outlines a framework where gravity emerges from a chain of cause-and-effect events. Pipa calls these chains "stable determination chains." Imagine a sequence where one quantum system interacts with another, creating a record of that interaction, which then allows a third system to interact, and so on. It is only within these chains, where information is passed along and definite outcomes are established, that the gravitational field appears. If a system is completely isolated from such chains, it evolves without generating its own gravity, even if it has mass. This approach avoids the need to quantize gravity into particles. Instead, it treats gravity as a conditional phenomenon that arises from the way quantum matter settles into definite states through interaction.

To test this radical idea, Pipa proposes several experiments that could distinguish his theory from existing ones. One major proposal involves the Bose-Marletto-Vedral experiment, a setup designed to see if gravity can entangle two particles. In this experiment, two masses are placed in a superposition of different locations. If gravity is a quantum force, it should be able to link the two masses together, creating a shared quantum state. Pipa's theory offers the statement for the null hypothesis in this experiment: it predicts that because the masses are isolated and not part of a decoherence-inducing chain, they should not source a gravitational field at all. Therefore, they should not entangle, and the experiment should show no gravitational effect. If the experiment does show entanglement, Pipa's theory would be ruled out. However, if the masses remain unentangled, it would support the idea that gravity is conditional.

The paper also suggests a more direct test using quantum circuits. Researchers could build a circuit where they control exactly when a system enters a "determination chain." By isolating a system and then selectively allowing it to interact with a chain of other systems, they could turn the system's gravitational field on and off. If the theory is correct, the system should only produce a measurable gravitational pull when it is actively part of the chain. This would allow scientists to use quantum computers to control the very act of sourcing gravity, a capability that has never been demonstrated before.

Pipa's work also addresses the nature of the universe on a grand scale. If gravity only arises from these chains of interaction, then the vacuum of space, which is devoid of such interactions, would not have a gravitational field of its own. This leads to a new way of understanding the cosmological constant, the mysterious force driving the expansion of the universe. The paper suggests that the value of this constant might not be fixed but could change over time as the number of interactions in the universe changes. This could explain why dark energy is becoming progressively weaker, a phenomenon that current theories struggle to explain.

The strength of this proposal lies in its ability to resolve long-standing paradoxes without inventing new particles or forces. It suggests that the "weirdness" of quantum mechanics and the "smoothness" of gravity are not in conflict; rather, gravity is simply the result of quantum systems becoming definite through interaction. The paper does not claim to have proven this theory yet. Instead, it provides a clear set of rules and predictions that can be tested in the laboratory. By proposing specific experiments, such as the controlled quantum circuit and the modified Bose-Marletto-Vedral setup, Pipa offers a path forward for scientists to determine whether gravity is a fundamental force that exists everywhere, or a conditional effect that only appears when the quantum world interacts with itself.

The implications of this work are profound. If gravity is conditional, it means that the fabric of spacetime is not a static stage upon which events play out, but a dynamic structure that emerges from the interactions of matter. It suggests that the universe is not filled with a constant gravitational field, but rather that gravity is a phenomenon that spreads through a network of interactions, much like a signal passing through a chain of people. This view could solve the problem of singularities, the infinitely dense points at the center of black holes. If gravity only exists where interactions occur, then in the deepest core of a black hole, where interactions might cease, gravity would simply disappear, leaving a flat, empty space instead of a singularity.

Ultimately, this paper invites us to rethink the relationship between the quantum world and the cosmos. It proposes that the force that shapes the stars and galaxies is not a constant background presence, but a consequence of the way matter settles into reality. By focusing on the conditions under which gravity appears, Pipa offers a fresh perspective that could bridge the gap between the very small and the very large. The next step is to build the experiments that can confirm or deny these ideas, turning a theoretical possibility into a physical reality.