MathTrek

Lu's determination to find out took him on a journey through hundreds of photographs of Islamic architecture in the libraries at Harvard—and now it's landed him an article in Science.

Archway from the Darb-i Imam shrine in Isfahan, Iran, which was built in 1453 C.E. The larger pentagons outlined in pale blue were constructed using a large-scale girih tile pattern, and the small white pentagons were constructed using a small-scale girih tile pattern. Image courtesy of K. Dudley and M. Elliff.

The only mathematical tools the builders had available to them were straightedge and compass. Theoretically, all these patterns could be made by drawing the lines directly onto the buildings.

But Lu noticed that the patterns were astonishingly perfect, even over very large areas. If the builders had been scribing the patterns directly on a wall, Lu expected the patterns to accumulate small errors that would be detectable on really big walls.

But he didn't see any errors. So he figured that they must have had some tricks to guide the pattern making, and he decided to figure out what they were.

He had a clue where to look from his undergraduate research. The patterns on the Islamic buildings reminded him of Penrose tiles, which are two simple geometric shapes, usually a kite and a dart or a fat and a skinny rhombus (diamond). When laid down in a tiling, these pairs of tiles can cover a plane in a pattern that never repeats.

Kite and dart Penrose tiles. Wikipedia

As a Penrose tiling spreads across a larger and larger surface, the ratio between the numbers of each type of tile approaches the golden ratio. The golden ratio (or mean) is the irrational number 1.618 . . . .

Penrose tilings also have fivefold rotational symmetry, the same kind of a symmetry that a five-pointed star has. If you rotate the whole pattern by 72 degrees, it looks just the same.

A Penrose tiling made up of fat and skinny diamonds. Wikipedia

For his undergraduate thesis, Lu had looked for examples in the physical world of quasicrystals, materials that are thought to have crystal structures that are three-dimensional versions of a Penrose tiling. Physical quasicrystals have remarkable properties. For example, metal quasicrystals don't conduct heat very well, and a company is now developing a tough but slippery nonstick coating from quasicrystals.

The patterns on Islamic buildings had lots of pentagons and decagons and stars, geometric figures with fivefold symmetry. Lu immediately thought of Penrose tiles.

"I see a fivefold pattern and my eyes light up, and I try to decompose it into tiles," he says.

Lu returned to Harvard and studied photos, trying to deconstruct the patterns. He found a picture of a 15th-century architectural scroll from Istanbul, the Topkapi scroll, which was "like the AutoCAD manual for ancient times," Lu says.

The main, dark pattern of red and blue lines was very complex and nonrepeating. But underneath, he saw a fainter red pattern that broke the design up into five decorated tiles: a decagon, a pentagon, a hexagon, a bowtie, and a rhombus.

A 15th-century Timurid-Turkmen scroll now held by the Topkapi Palace Museum in Istanbul. The faint reddish lines outline the shapes of the underlying tiles. One example of each shape has been shaded in the picture. Peter J. Lu

He had hit paydirt. It was just like a Penrose tiling.

The five decorated shapes. Peter J. Lu

When Lu looked at photographs of Islamic buildings, he found that he could break the patterns on their surfaces up into the same shapes, even though the shapes often weren't immediately visible. "I couldn't sleep for days," he said. "I skipped Christmas break to work on it."

Lu suggests that Islamic architects used these shapes, which he calls girih tiles, to scribe the patterns onto the walls. That would explain how they tiled large surfaces with such precision.

An archway in the Sultan's Lodge in the Green Mosque in Bursa, Turkey from 1424. Image courtesy of W.B. Denny

Although the underlying shapes aren't obvious in the final design, this diagram shows how the pattern from the Sultan's Lodge archway was constructed using girih tiles. Image courtesy of W.B. Denny

Lu also figured out that the girih tiles could be broken up into the kites and darts of Penrose tiles. When he divided the tiles in this way, one building, the Darb-i Imam shrine, had a near-perfect Penrose tiling. The shrine was built in 1453, and it would be another 500 years before the mathematics behind Penrose tiles was developed.

The Darb-i Imam shrine was particularly remarkable because it showed girih tile patterns at two different scales, so that large girih tiles were broken up into smaller girih tiles. In principle, by repeatedly scaling up the tiling in this way, they could have covered an arbitrarily large wall with a Penrose tiling.

Lu has a history of finding math wherever he looks. In 2006, he turned his attention to the fossil record, creating a mathematical model that demonstrated that Earth's biosphere recovered from mass extinctions more quickly than people had thought. He published the result in the Proceedings of the National Academy of Sciences.

And in 2004, he landed his first publication in Science when he noticed that the spiral patterns in a Chinese jade ring from 500 B.C.E. were perfect Archimedes spirals—and showed that ancient Chinese technology must have been far more advanced than previously thought in order to produce such a ring.

Now all he needs to do is to finish his dissertation.

References:

Goho, A. 2005. In the buff: Stone Age tools may have derived luster from diamond. Science News 167(Feb. 19):116. Available to subscribers at http://www.sciencenews.org/articles/20050219/fob3.asp.

Lu, P., and P. Steinhardt. 2007. Decagonal and quasi-crystalline tilings in medieval Islamic architecture. Science 315(Feb. 23):1106-1110. Available at http://www.physics.harvard.edu/~plu/publications/Science_315_1106_2007.pdf.

Lu, P.J., M. Yogo, and C.R. Marshall. 2006. Phanerozoic marine biodiversity dynamics in light of the incompleteness of the fossil record. Proceedings of the National Acadamies of Science 103(Feb. 21):2736-2739. Available at http://www.physics.harvard.edu/~plu/publications/PNAS_103_2736_2006.pdf.

Lu, P.J. 2004. Early precision compound machine from ancient China. Science 304(June 11):1638. Available at http://www.physics.harvard.edu/~plu/publications/Science_304_1638_2004.pdf.

Peter J. Lu has a Web site at http://www.peterlu.org/.

You can learn more about Penrose tilings at http://en.wikipedia.org/wiki/Penrose_tiling.