Grabby Aliens (2021)

Original Article ↗ Hacker News Discussion ↗

A speculative “grabby aliens” model argues that any advanced civilization inclined to expand could eventually colonize vast volumes of the universe, which would explain why we haven’t seen others yet: either we’re among the first, or expansion is much harder than it looks. Commenters examine the model’s core assumptions — rapid interstellar travel, long-lived technological societies, and detectable megastructures — against alternatives such as great filters, self-destruction, low incentives to colonize, or deliberate concealment. Many conclude that our current observational limits and single data point (humanity) make firm answers impossible, but the framework is useful for clarifying what different resolutions of the Fermi paradox would imply for our own future.

Overview of the “grabby aliens” model

  • Assumes a subset of civilizations:
    • Expand outward at a common high fraction of light speed.
    • Dramatically and visibly alter the regions they control.
    • Keep expanding until stopped by similarly “grabby” civilizations.
  • Under these assumptions, such civilizations eventually dominate large cosmic volumes; we don’t see them yet, which constrains their speed, timing, or rarity.

Earliness and the Fermi paradox

  • One line of argument: if any earlier civilization with our tech level tended to expand, we’d see them; we don’t, so either:
    • We’re unusually early/rare.
    • We’re bad at detecting them.
    • Interstellar colonization is effectively impossible or unattractive.
    • Humans are somehow atypical (e.g., uniquely expansionist or uniquely not).
  • Some see “we’re early” as plausible; others argue it makes us “special” and distrust that assumption.
  • Several comments question anthropic/“mediocrity principle” reasoning and point out we’re using a sample size of one.

Feasibility of interstellar expansion and great filters

  • Skeptical points:
    • Interstellar travel may be technically possible but never cheap or safe enough for galaxy-filling colonization.
    • Civilizations may stabilize once local resources allow very high standards of living; no strong pressure to expand.
    • Technological civilizations may self-destruct or collapse (war, WMDs, resource depletion) before large-scale expansion.
  • More optimistic/neutral points:
    • Automation and advanced energy could drastically change cost constraints.
    • Slow but steady methods (light sails, nuclear sails, von Neumann probes, stellar engines) might suffice over millions of years.

Detection limits and megastructures

  • Our ability to detect ETI is described as very limited:
    • Radio signals decay rapidly; only very powerful, nearby, or deliberately targeted transmissions are detectable.
    • Most stars and potential anomalies (e.g., Dyson spheres, IR halos from asteroid mining) haven’t been systematically checked.
  • Some argue Dyson spheres or large-scale industrial activity should be obvious in infrared; others counter that:
    • Waste could be minimized or tightly directed (no “obvious” excess heat).
    • Many natural objects (brown dwarfs, dust, rogue planets) complicate interpretation.
  • Dark matter as “alien energy capture” is rejected as inconsistent with its observed gravitational behavior.

Alternative explanations: dark forest, zoo, simulation, non-grabby civs

  • Dark forest: any visible civilization is destroyed by others using relativistic projectiles; everyone stays quiet.
  • Zoo/preservation ideas: advanced civilizations deliberately hide or avoid interfering with young ones; possibly using sophisticated heat/EM masking.
  • Simulation ideas: we might be in a simulation; doesn’t by itself explain the absence of visible aliens, since other simulated beings could exist too.
  • Non-grabby or harmonious civilizations:
    • Expansionist, resource-maximizing behavior might be self-limiting or filtered out.
    • Technological progress could depend on global cooperation that warlike species rarely achieve.

Critiques of the model and reasoning style

  • Several see the model as highly assumption-driven (constant expansion, clear visibility, universal behavior) and “1960s sci-fi” flavored.
  • Concerns about:
    • Extrapolating galactic-scale behavior from short human history.
    • Assuming aliens share human-style incentives (growth, conquest, empire).
    • Treating speculative constructs (“loud,” “grabby”) as if they were empirically grounded.
  • Others defend it as a first-pass, deliberately simplified model that generates testable predictions and clarifies which assumptions matter, not a final answer.