How Quantum is Life?

1. A Quantum Life, Bounded by Spacetime
   In 1944, Erwin Schrödinger asked: What is life? His answer—that life sustains order by feeding on “negative entropy”—foreshadowed a deeper truth now coming into focus: life doesn’t just obey quantum mechanics; it harnesses it. Photosynthetic complexes perform quantum walks to route energy with near-perfect efficiency. Avian cryptochromes use spin-correlated radical pairs to detect Earth’s magnetic field—a feat inexplicable without entanglement. Enzymes like alcohol dehydrogenase accelerate reactions by orders of magnitude via proton tunneling, confirmed by anomalous kinetic isotope effects.
   Yet these quantum advantages are rare, short-lived, and embedded in molecular architectures that seem “designed” to protect coherence. Why? The standard explanation points to thermal and electromagnetic noise. But there is another influence—universal, unshieldable, and inescapable: gravity.
   This essay argues that gravity, through its coupling to internal energy via spacetime curvature (even in the weak-field limit), imposes a fundamental decoherence channel that constrains the evolutionary design of quantum biological systems. Unlike thermal noise, gravitational dephasing cannot be screened or cooled away. It is a silent boundary written into the fabric of spacetime itself.

2. Gravitational Decoherence: A Semi-Classical Boundary
   General relativity tells us that clocks tick slower in stronger gravitational potentials. When a quantum system has internal energy differences and spatial extension along the gravitational field, this time dilation translates into a relative phase shift between its components. Pikovski et al. (2015) showed that for a system of mass m, vertical separation Δz, and coherence time t, the accumulated gravitational phase is:
   Δϕ_g = (m g Δz t) / ℏ
   Decoherence becomes significant when Δϕ_g ≳ 1 radian. Applying this:
   Enzymes (proton mass m_p, Δz = 5 nm, t = 100 fs): Δϕ_g = 10⁻⁴ rad → negligible.
   Photosynthesis (electron mass m_e, Δz = 5 nm, t < 1 ps): Δϕ_g ≪ 1 → irrelevant.
   Cryptochromes (electron mass m_e, Δz ≈ 10 µm, t = 1–5 µs): Δϕ_g = 0.85 rad → comparable to residual environmental noise.
   Thus, gravity selectively matters only where biology has already pushed coherence to its limits. Intriguingly, avian photoreceptor cells stack cryptochrome-containing discs horizontally, minimizing vertical electron separation—possibly an evolutionary adaptation not just to light or magnetism, but to gravity itself.
   Macroscopic quantum hypotheses (e.g., Orch-OR in microtubules) are ruled out: even under generous assumptions (segment length 50 µm, mass  10⁹ protons), the Diósi–Penrose gravitational self-energy implies a collapse time of  10⁻¹⁴ s—far faster than any biological timescale.

3. Testing the Silent Architect
   To distinguish gravitational decoherence from other noise sources, I propose three tiered experiments:
   Photosynthesis in microgravity (negative control) Perform two-dimensional electronic spectroscopy on Fenna-Matthews-Olson (FMO) complexes aboard the International Space Station (ISS). No change in coherence time is expected—validating the model’s selectivity.
   Simulated radical pairs with controlled Δz Use trapped ions or superconducting qubits to emulate cryptochrome radical pairs. Systematically vary vertical separation (1–100 µm) under 1g versus simulated microgravity. A clear dependence of coherence decay on the product g · Δz · t would confirm the gravitational mechanism.
   Enzymatic kinetics in space Measure kinetic isotope effects (KIEs) for alcohol dehydrogenase on the ISS. While the absolute gravitational effect is small ( 0.01% shift in reaction rate), comparative assays between wild-type enzymes and mutants engineered to reorient the active site (thereby altering Δz) can isolate the g-dependent signal. Modern enzymatic assays routinely resolve rate differences at the 0.1% level.
   These experiments form a falsifiable program grounded in established physics and current technology—a rarity in foundational questions at the gravity–life interface.

4. Implications: A Planetary Quantum Horizon
   If validated, this hypothesis reframes quantum biology as planetary science. Life on Earth operates near a gravitational decoherence threshold. But on Mars (g = 0.38 g_⊕) or Europa (g = 0.13 g_⊕), the same molecular system could sustain longer coherence or require less shielding. The “quantum edge” of life is not universal—it is locally sculpted by spacetime geometry.
   This perspective also hints at a deeper principle: if gravity is emergent from quantum information or thermodynamic structure, as suggested by several approaches to quantum gravity, then its role in decoherence may reflect a more fundamental link between information, coherence, and the arrow of time in living systems. While this essay remains within semi-classical physics, it opens a door to such deeper unifications.

5. Conclusion
   Life exploits quantum mechanics—but not without limits. Thermal noise sets one boundary; electromagnetic shielding, another. Gravity sets a third: silent, universal, and inescapable. By acting as a decoherence channel for vertically extended, long-lived quantum states, Earth’s gravitational field may have shaped the very architecture of quantum-sensitive biomolecules.
   This directly answers the FQXi contest question: Yes, gravity and spacetime shape quantum life—not by enabling it, but by constraining where and how deeply biology can dive into the quantum realm. And in doing so, they suggest a profound truth: to understand life’s quantum potential, we must look not only inward to molecules—but outward to the stars and the gravity that binds them.

Gravitational decoherence model: Pikovski et al., Nature Physics 11, 668 (2015).
Threshold time: T_threshold = ℏ / (m g Δz).
Orch-OR collapse estimated via Diósi–Penrose gravitational self-energy.
Horizontal stacking of retinal discs observed in ultrastructural studies of avian retina.