Gravity-based intelligence theory

The gravity-based intelligence theory is a speculative hypothesis proposed by Ryland Grace and discussed with Rocky, which posits that the intelligence level of a species is fundamentally determined by the gravity of its home planet, as gravity dictates the speed of animal movement and thus the reaction time required for survival. The theory emerges from a broader conversation about why humans and Eridians, despite evolving on separate worlds billions of years apart, possess roughly comparable intelligence. Grace argues that this convergence is not coincidental but is driven by a shared physical constraint: gravity.

Origin and Core Argument

The theory arises during a conversation in the Hail Mary's dormitory, where Grace and Rocky are discussing the nature of intelligence. Grace observes that Eridians are far superior at mathematics and memory, but Rocky counters that "math is not thinking" and "memory is not storage," insisting that the core process of "thinking" — solving problems — operates at the same speed in both species. This prompts Grace to ask why they are not a thousand times smarter or dumber than each other. He then formulates his hypothesis: "Intelligence evolves to give us an advantage over the other animals on our planet. But evolution is lazy. Once a problem is solved, the trait stops evolving." He concludes that both humans and Eridians are "just intelligent enough to be smarter than our planet's other animals." This leads to the central question of why the baseline intelligence required for planetary dominance would be the same on two different worlds.

The Role of Gravity

Grace's answer is gravity. He explains that gravity determines how fast an animal can run: "Higher gravity, more time spent in contact with the ground. Faster movement." The speed of potential threats and prey, therefore, sets the minimum reaction time an animal needs to survive. Intelligence, in this view, is calibrated to this reaction-time requirement. He states, "I think animal intelligence, ultimately, has to be faster than gravity." Rocky immediately identifies a potential flaw in the theory, noting that Erid has "double Earth gravity" yet he and Grace possess the same intelligence. Grace counters that the gravities of Earth and Erid are "so close to the same, astronomically, that the intelligence needed is almost the same." He further speculates that a creature from a planet with one-hundredth of Earth's gravity would likely appear "pretty stupid" to a human, as its environment would not have selected for rapid cognitive processing.

The Self-Sacrifice Corollary

The conversation extends beyond raw cognitive speed to a related behavioral trait: the willingness to die for one's people. Rocky observes that both he and Grace share this instinct, despite evolution's apparent aversion to death. Grace explains this as a species-level benefit: "A self-sacrifice instinct makes the species as a whole more likely to continue." Rocky, however, points out that not all Eridians are willing to die for others, and Grace acknowledges that the same is true for humans. This exchange reinforces the theory's underlying premise that intelligence and social instincts are not monolithic but are shaped by the specific pressures of a planet's environment, with gravity serving as a fundamental, planet-wide constraint that sets the baseline for cognitive and behavioral evolution.

Significance and Narrative Role

The gravity-based intelligence theory serves as a key moment of intellectual bonding between Grace and Rocky, demonstrating that their scientific collaboration extends beyond immediate problem-solving to fundamental questions about the nature of life and the universe. It also provides a thematic counterpoint to the book's central crisis: while Astrophage threatens to destroy both worlds, the theory suggests that the very forces that shaped their respective intelligences — gravity, evolution, and the struggle for survival — are what enable them to work together to find a solution. The theory remains speculative within the narrative, but it is presented as a plausible and elegant explanation for a remarkable coincidence, reinforcing the book's themes of shared physical laws and the common challenges faced by intelligent life across the cosmos.