Grabby Aliens — The Model That Predicts When We'll Meet Them
The absence of visible alien empires is the measurement. In 2021, Robin Hanson, Daniel Martin, Calvin McCarter, and Jonathan Paulson built a model where the empty sky puts numbers on when expansionist civilizations appear, how fast they spread, and why humans may be early. The claim is not that aliens are impossible. The claim is sharper: if fast-expanding aliens had reached our part of the universe, galaxies near us would already look different.
The model
The paper separates civilizations into two kinds. Quiet aliens arise, do whatever they do, and do not visibly remake large cosmic volumes. Grabby aliens expand outward at a fraction of light speed and change the territory they enter: star use, galaxy appearance, waste heat, colonization fronts, or some other large-scale signature.
The model has three main knobs:
| Parameter | Meaning | Typical range in the model |
|---|---|---|
| n | number of hard evolutionary steps | about 3 to 9 |
| k | time scale for appearance of grabby civilizations | calibrated from cosmic history |
| s | expansion speed | roughly 0.1c to 0.9c |
The hard-steps idea does most of the work. Earth took about 4.5 billion years to produce a technological species, while the Sun’s long-term habitable window is finite. If key transitions such as abiogenesis, eukaryotes, multicellular life, language, and technology were easy, one might expect them to happen early. They did not. That delay is treated as evidence that several steps are rare enough to act like filters.
What the numbers say
The model’s central estimate places humans early in the cosmic schedule, not late. Grabby civilizations become common later because star formation, planet formation, and biological waiting times create a delayed arrival curve.
A few claims carry the page:
- Median time until humanity meets a grabby expansion front: roughly 200 million to 2 billion years, with about 1.5 billion years often used as the central figure.
- Estimated expansion speed: around half the speed of light in many example runs.
- Hard steps: about 6 in the central case, with plausible values from 3 to 9.
- Current density before grabby civilizations dominate: on the order of one grabby civilization per million galaxies.
- The model predicts no nearby civilization millions or billions of years older than us that has chosen fast cosmic expansion.
This reframes the concept fermi paradox. The silence is not just “nobody is there.” It may be “the first visible expanders are only now beginning to appear, and we are early because late observers get preempted.”
What’s contested
The model is clean because it assumes the thing most in doubt: that advanced civilizations tend to become visibly expansionist. A civilization could stay quiet by choice, physics, coordination failure, ethics, virtual migration, or lack of interest in stars. If that is common, the model still explains the lack of grabby aliens but says less about intelligence itself.
The hard-steps count is also fragile. Earth gives us one data point. Reading six independent filters from one planet’s history may be useful, but it is not a measurement in the way the Hubble constant is a measurement.
The detection claim has a gap too. A galaxy-scale civilization might not look like a science-fiction Dyson swarm. If its waste heat, engineering, or expansion style is subtle, “we see no grabby aliens” becomes weaker than the model needs.
Why this has to do with other realms
Grabby Aliens is a space model built out of philosophy. It depends on anthropic selection: observers only find themselves in places not already swallowed by earlier expansion waves. That makes it a cousin of concept simulation hypothesis, concept great filter, and any argument where the fact that “we are here” becomes data rather than background.
It also touches biology. If the Great Oxygenation Event was one hard step, then a planetary chemistry event 2.4 billion years ago becomes part of a cosmic timing model. The bridge from microbes to galaxies runs through concept great oxygenation event.
Key sources
- Robin Hanson, Daniel Martin, Calvin McCarter, Jonathan Paulson, “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare,” arXiv:2102.01522, 2021 — the core Grabby Aliens paper.
- Brandon Carter, “The Anthropic Principle and its Implications for Biological Evolution,” 1983 — the older hard-steps argument behind the model.
- Robin Hanson, “The Great Filter: Are We Almost Past It?”, 1998 — the classic framing of filters before or after technological civilization.
- Andrew J. Rushby et al., “Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars,” Astrobiology, 2013 — useful for thinking about biological time windows.
- David Kipping, work on hard steps and astrobiological inference, 2020s — to verify: best single source for his critique of hard-step confidence.
Further reading
- concept fermi paradox — the parent puzzle that Grabby Aliens tries to make numerical.
- concept von neumann probes — if expansion happens through self-replicating machines, the speed and visibility assumptions change.
- concept great filter — the model’s hidden engine is a claim about where the filters sit.
- The Anthropic Cosmological Principle by John Barrow and Frank Tipler, 1986 — dense but central for observer-selection arguments.
- Hanson’s Overcoming Bias posts on Grabby Aliens — useful for seeing the model’s assumptions argued in plain language.
See Also
- concept fermi paradox
- concept great filter
- concept von neumann probes
- tech stellar engines
- concept great oxygenation event
- concept simulation hypothesis
Open question
If the first galaxy-scale civilizations decide not to be grabby, would the universe look empty for the same reason, and how would we tell the difference?