The planetarium: an advanced civilisation is projecting an empty sky around us so that we believe we are alone

A planetarium shows an audience a sky that is not really there. The planetarium hypothesis says the sky we see is one of these: a civilisation far beyond us has built a false sky around us, whether a shell of some kind around the solar system or a whole artificial universe, that shows us a universe empty of anyone else, either to protect us, to study us, or for reasons we cannot guess. It is the zoo hypothesis with the fence made invisible. The silence is manufactured, and we are the only ones who do not know it.

The novelist Stephen Baxter, who writes science fiction with a strong grounding in physics, put the idea forward in a 2001 paper. He proposed that we may be living in an artificial universe, a kind of virtual-reality planetarium, designed to give us the illusion that the universe is empty, and he treated the question as an engineering problem: how much energy would it take to build such a thing, at different sizes and different levels of quality? The paper's abstract gives his answer. A perfect simulation of present-day human civilisation would be within reach of a civilisation of the highest rank on the scale Nikolai Kardashev drew up, one that commands the energy output of a whole galaxy (Baxter calls it K3). But a single human culture spread across about a hundred light-years would be more than any conceivable generator of virtual realities could handle. On that reckoning a civilisation of that rank could keep a show running around a people of our size, and we would see what we see: stars, planets, galaxies, and no one in them.

The idea is the oldest kind of philosophical worry, that the senses might be deceived by a powerful being, given a physical budget. In Baxter's own version it is a form of the simulation hypothesis, built for one purpose, which is to hide everyone else from us. A lighter version, in which we and the solar system are real and only the sky beyond is faked, would need less than a whole artificial universe, but that is not the version his abstract describes. It differs from the zoo hypothesis in that the zoo keepers merely stay out of sight, whereas here they actively paint the walls.

The bearing on the Drake Equation is that the observations the equation is meant to explain are themselves in question: the factors are left unchanged, and the silence is put down to a deliberate deception. That is why the explanation sits in the "unseen" family and why it is usually mentioned as a limiting case rather than a candidate.

The usual objections are these. The hypothesis cannot be tested from inside, except by finding a flaw in the show, and any flaw found could be dismissed as part of the show. A deception that has to keep pace with better instruments would get more expensive every decade, and there might come a point at which the keepers either let it slip or spend absurd resources to maintain it. And the hypothesis gives no reason why a civilisation would go to such lengths, beyond the reasons given for the zoo. Baxter's abstract points to a limit of its own: a show that has to cover a culture spread over about a hundred light-years is beyond any conceivable generator, so it works only for a people who stay at home. The value of the idea is in showing how little the silence alone can prove, since any observation at all is compatible with a sufficiently capable deceiver.

What it says about the equation

Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:

f_c · The share of intelligent species that make themselves detectable piece 6 of 7 leaves this piece where it is; the silence is about us

The test: what would strengthen or weaken it

Would strengthen it: An observation that behaves like a seam in the sky, such as a region where physical law or the distribution of stars changes inexplicably at a fixed distance from the Sun; evidence that distant observations change in response to what our instruments can resolve.

Would weaken it: Measurements from probes far outside the solar system matching what was predicted from inside it; the sheer improbability of maintaining the deception against ever better instruments; a detected civilisation, which would mean the show had failed.

These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

2026-10-16 Nothing new By hand

Nothing. New Horizons, at about 65 astronomical units in mid-2026, keeps measuring a sky consistent with what was predicted from inside, with dust beyond the Kuiper belt's expected edge the only surprise, and that is a natural one. No new writing on the hypothesis itself.

How seriously it is taken

A thought experiment from a science fiction writer with a physics background, cited in surveys as the extreme case of the 'they are hiding from us' family; nobody defends it as likely, and its value is in making clear what the silence can and cannot prove.

The papers that bear on it

Fermi's Paradox: The Last Challenge for Copernicanism? June 2009 Milan M. Ćirković · logged; moved no range
Are You Living in a Computer Simulation? April 2003 Nick Bostrom · logged; moved no range
The Zoo Hypothesis July 1973 John A. Ball · logged; moved no range

Books, films and other work that take it up

Spin 2005 Book · Robert Charles Wilson

Notes

Checked 2026-10-16 against the British Interplanetary Society's listing of the paper (https://bis-space.com/shop/product/the-planetarium-hypothesis-a-resolution-of-the-fermi-paradox/, reference code 2001.54.210): the citation (Journal of the British Interplanetary Society 54, 210-216, 2001) is correct. Corrected 2026-10-16: the first draft described Baxter's planetarium as a shell around the solar system with real inhabitants inside, said Baxter priced a false sky "around a region the size of the solar system", said a perfect copy of the observable universe was ruled out as needing more energy than the universe holds, and credited Baxter with the objections and the closing judgement. The abstract instead describes an artificial universe, a virtual-reality planetarium; says a perfect simulation of present-day human civilisation is within reach of a K3 (galaxy-scale) civilisation; and says a human culture spanning about a hundred light-years would exceed any conceivable generator. The summary and body now say that. The statements the abstract does not support (the solar-system-sized region, the universe-sized limit, "fool only the instruments", Baxter's own objections and conclusion) were removed or no longer credited to him. Only the abstract was read; the full paper was not reached. The related simulation page's sentence contrasting the two ideas was reworded to match. No change to the id, family, the direction under bears_on, or the would_strengthen and would_weaken lines.

Think this page gets the explanation wrong, or misses an objection? Say so, or read how disagreement gets folded in. The record itself is data/explanations/planetarium.toml in the public repository. This page's address will never change; if the explanation is ever retired, the address will say what replaced it.