The Aperiodic Monotile

Imagine covering an infinite floor with tiles so that no pattern ever repeats — not in any direction, not at any scale. Mathematicians call this an aperiodic tiling, and creating one is surprisingly easy if you are allowed a handful of different shapes working together. The Penrose tiling, discovered in the 1970s, does it with just two shapes. But that raised an obvious question: could a single shape do it alone?

This is the einstein problem — not after the physicist, but after the German ein Stein, meaning "one stone." For more than 60 years, no one could find such a shape or prove it impossible.

Then in 2023, David Smith, an amateur tile-enthusiast, noticed that an unassuming 13-sided polygon — which he called the "hat" — seemed to cover the plane without ever settling into a repeating rhythm. Together with mathematicians Joseph Samuel Myers, Craig Kaplan and Chaim Goodman-Strauss, he turned that observation into a full proof, publishing it in March 2023. The einstein problem was solved. One stone was all it took.

The demo below places copies of the hat monotile — including reflected copies, since the hat needs its mirror image too — and grows the patch outward. Use the controls to add more tiles and observe the pattern.

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No matter how large you grow the patch, you will never see a translation that maps the tiling onto itself. Every neighbourhood looks locally similar, but zoom out and the arrangement never locks into a repeating grid — that is the essence of aperiodicity. The hat forces quasi-order: almost-pattern without true periodicity, a behaviour closely related to quasicrystals and Penrose-like structures.

Showing that the hat can tile the plane is straightforward — just keep placing tiles and check that they fit. Showing it never tiles periodically is the hard part.

The proof by Smith, Myers, Kaplan and Goodman-Strauss has two main pillars:

• Substitution structure. They showed that every valid hat tiling can be described by a hierarchical inflation rule: small clusters of hats assemble into larger "super-tiles," which in turn assemble into even larger ones, ad infinitum. If a periodic tiling existed, it would have to be compatible with every level of this hierarchy — and that leads to a contradiction.
• Computer-assisted case analysis. The hat has 13 sides and fits its neighbours in finitely many orientations. Using a careful (and computer-verified) enumeration, the authors showed that every local patch of hat tiles extends uniquely to a larger patch, forcing global aperiodicity.

The proof also covered a continuous family of shapes: the hat is one point on a spectrum of "hat-like" tiles parameterised by the ratio of two edge lengths. Almost all of these tiles are aperiodic monotiles too, though the proof is more subtle at the boundary cases.

A closely related result followed shortly after: the "spectre" tile (2023, same authors) achieves aperiodicity without needing mirror-image copies at all — a so-called non-chiral aperiodic monotile.

Unlike NP-complete problems where the difficulty is computational, the difficulty here is logical and combinatorial: there is no short certificate that a tiling never repeats, because periodicity (or its absence) is a property of the entire infinite plane.

Aperiodic tilings are not merely a mathematical curiosity — they show up wherever local order without global repetition matters:

• Quasicrystals: In 1984, Dan Shechtman discovered metallic alloys whose atomic arrangement is aperiodic, with sharp diffraction patterns but no repeating unit cell — exactly the physical counterpart of a Penrose tiling. He won the 2011 Nobel Prize in Chemistry for the discovery. The hat tile adds a new single-shape building block to the theory.
• Architecture and design: Aperiodic patterns appear in Islamic geometric art, Penrose-tiled floors (the Templeton College Oxford floor is a famous example), and modern architectural facades where visual interest without repetition is prized.
• Coding theory and cryptography: Sequences with low autocorrelation — in spirit aperiodic — are used in spread-spectrum communications, radar and pseudorandom number generation.
• Foundations of computation: Wang tiles (square tiles with coloured edges that must match) are closely related. Deciding whether a given finite set of Wang tiles can tile the plane is undecidable — a connection that links tiling theory to the halting problem.

The hat tile also renewed public interest in recreational mathematics: within weeks of the 2023 preprint, crafters were 3D-printing hat coasters, artists were designing hat-tile wallpapers, and journalists were calling David Smith a "retired print technician who may have changed mathematics forever."

The hat tile is a reminder that mathematics still has room for surprises that arrive from unexpected directions — a hobbyist playing with paper cutouts, a question older than most living mathematicians, and a proof that required both deep combinatorial insight and computer assistance.

What makes the result beautiful is not just the shape itself, but what it says about pattern and infinity: a single rule, applied forever, can produce complexity that never settles. There is no periodic skeleton hiding underneath — the aperiodicity is fundamental, not accidental.

And the story is not finished. The spectre tile removed the need for mirror images. Researchers are now asking what other geometric constraints can force aperiodicity, how thin the boundary between periodic and aperiodic can be, and what physical materials might realise hat-like atomic arrangements. One stone opened many doors.

For the algorithmic connections, see P vs NP and the halting problem — tiling questions sit at the crossroads of geometry, logic and the limits of what computers can decide.