Flashback to the 80ies: filling space with the first quasicrystals

This post provides a historical and conceptional perspective for the theoretical discovery of non-periodic 3d space-fillings by Peter Kramer, later experimentally found and now called quasicrystals. See also these previous blog entries for more quasicrystal references and more background material here.
The following post is written by Peter Kramer.

Star extension of the pentagon. From Kramer 1982.
Star extension of the pentagon. Fig 1 from
Non-periodic central space filling with icosahedral symmetry using copies of seven elementary cells by Peter Kramer, Acta Cryst. (1982). A38, 257-264

When sorting out old texts and figures from 1981 of mine published in Non-periodic central space filling with icosahedral symmetry using copies of seven elementary cells, Acta Cryst. (1982). A38, 257-264), I came across the figure of a regular pentagon of edge length L, which I denoted as p(L). In the left figure its red-colored edges are star-extending up to their intersections. Straight connection of these intersection points creates a larger blue pentagon. Its edges are scaled up by τ2, with τ the golden section number, so the larger pentagon we call p(τ2 L). This blue pentagon is composed of the old red one plus ten isosceles triangles with golden proportion of their edge length. Five of them have edges t1(L): (L, τ L, τ L), five have edges t2(L): (τ L,τ L, τ2 L). We find from Fig 1 that these golden triangles may be composed face-to-face into their τ-extended copies as t1(τ L) = t1(L) + t2(L) and t2(τ L) = t1(L) + 2 t2(L).

Moreover we realize from the figure that also the pentagon p(τ2 L) can be composed from golden triangles as p(τ2 L) = t1(τ L) + 3 t2(τ L) = 4 t1(L) + 7 t2(L).

This suggests that the golden triangles t1,t2 can serve as elementary cells of a triangle tiling to cover any range of the plane and provide the building blocks of a quasicrystal. Indeed we did prove this long range property of the triangle tiling (see Planar patterns with fivefold symmetry as sections of periodic structures in 4-space).

An icosahedral tiling from star extension of the dodecahedron.
The star extension of the dodecahedron.
Star extension of the dodecahedron d(L) to the icosahedron i(τ2L) and further to d(τ3L) and i(τ5L) shown in Fig 3 of the 1982 paper. The vertices of these polyhedra are marked by filled circles; extensions of edges are shown except for d(L).

In the same paper, I generalized the star extension from the 2D pentagon to the 3D dodecahedron d(L) of edge length L in 3D (see next figure) by the following prescription:

  • star extend the edges of this dodecahedron to their intersections
  • connect these intersections to form an icosahedron

The next star extension produces a larger dodecahedron d(τ3L), with edges scaled by τ3. In the composition of the larger dodecahedron I found four elementary polyhedral shapes shown below. Even more amusing I also resurrected the paper models I constructed in 1981 to actually demonstrate the complete space filling!
These four polyhedra compose their copies by scaling with τ3. As for the 2D case arbitrary regions of 3D can be covered by the four tiles.

Elementary cells The paper models I built in 1981 are still around and complete enough to fill the 3D space.
The four elementary cells shown in the 1982 paper, Fig. 4. The four shapes are named dodecahedron (d) skene (s), aetos (a) and tristomos (t). The paper models from 1981 are still around in 2014 and complete enough to fill the 3D space without gaps. You can spot all shapes (d,s,a,t) in various scalings and they all systematically and gapless fill the large dodecahedron shell on the back of the table.

The only feature missing for quasicrystals is aperiodic long-range order which eventually leads to sharp diffraction patterns of 5 or 10 fold point-symmetries forbidden for the old-style crystals. In my construction shown here I strictly preserved central icosahedral symmetry. Non-periodicity then followed because full icosahedral symmetry and periodicity in 3D are incompatible.

In 1983 we found a powerful alternative construction of icosahedral tilings, independent of the assumption of central symmetry: the projection method from 6D hyperspace (On periodic and non-periodic space fillings of Em obtained by projection) This projection establishes the quasiperiodicity of the tilings, analyzed in line with the work Zur Theorie der fast periodischen Funktionen (i-iii) of Harald Bohr from 1925 , as a variant of aperiodicity (more background material here).


The Nobel Prize 2011 in Chemistry: press releases, false balance, and lack of research in scientific writing

To get this clear from the beginning: with this posting I am not questioning the great achievement of Prof. Dan Shechtman, who discovered what is now known as quasicrystal in the lab. Shechtman clearly deserves the prize for such an important experiment demonstrating that five-fold symmetry exists in real materials.

My concern is the poor quality of research and reporting on the subject of quasicrystals starting with the press release of the Swedish Academy of Science and lessons to be learned about trusting these press releases and the reporting in scientific magazines. To provide some background: with the announcement of the Nobel prize a press release is put online by the Swedish academy which not only announces the prize winner, but also contains two PDFs with background information: one for the “popular press” and another one with for people with a more “scientific background”. Even more dangerously, the Swedish Academy has started a multimedia endeavor of pushing its views around the world in youtube channels and numerous multimedia interviews with its own members (what about asking an external expert for an interview?).

Before the internet age journalists got the names of the prize winners, but did not have immediately access to a “ready to print” explanation of the subject at hand. I remember that local journalists would call at the universities and ask a professor who is familiar with the topic for advice or get at least the phone number of somebody familiar with it. Not any more. This year showed that the background information prepared in advance by the committee is taken over by the media outlets basically unchanged. So far it looks as business as usual. But what if the story as told by the press release is not correct? Does anybody still have time and resources for some basic fact checking, for example by calling people familiar with the topic, or by consulting the archives of their newspaper/magazine to dig out what was written when the discovery was made many years ago? Should we rely on the professor who writes the press releases and trust that this person adheres to scientific and ethic standards of writing?

For me, the unfiltered and unchecked usage of press releases by the media and even by scientific magazines shows a decay in the quality of scientific reporting. It also generates a uniformity and self-referencing universe, which enters as “sources” in online encyclopedias and in the end becomes a “self-generated” truth. However it is not that difficult to break this circle, for example by

  1.  digging out review articles on the topic and looking up encyclopedias for the topic of quasicrystals, see for example: Pentagonal and Icosahedral Order in Rapidly Cooled Metals by David R. Nelson and Bertrand I. Halperin, Science 19 July 1985:233-238, where the authors write: “Independent of these experimental developments, mathematicians and some physicists had been exploring the consequences of the discovery by Penrose in 1974 of some remarkable, aperiodic, two-dimensional tilings with fivefold symmetry (7). Several authors suggested that these unusual tesselations of space might have some relevance to real materials (8, 9). MacKay (8) optically Fourier-transformed a two-dimensional Penrose pattern and found a tenfold symmetric diffraction pattern not unlike that shown for Al-Mn in Fig. 2. Three-dimensional generalizations of the Penrose patterns, based on the icosahedron, have been proposed (8-10). The generalization that appears to be most closely related to the experiments on Al-Mn was discovered by Kramer and Neri (11) and, independently, by Levine and Steinhardt (12).
  2. identifying from step 1 experts and asking for their opinion
  3. checking the newspaper and magazine archives. Maybe there exists already a well researched article?
  4. correcting mistakes. After all mistakes do happen. Also in “press releases” by the Nobel committee, but there is always the option to send out a correction or to amend the published materials. See for example the letter in Science by David R. Nelson
    Icosahedral Crystals in Perspective, Science 13 July 1990:111 again on the history of quasicrystals:
    “[…] The threedimensional generalization of the Penrose tiling most closely related to the experiments was discovered by Peter Kramer and R. Neri (3) independently of Steinhardt and Levine (4). The paper by Kramer and Neri was submitted for publication almost a year before the paper of Shechtman et al. These are not obscure references: […]

Since I am working in theoretical physics I find it important to point out that in contrast to the story invented by the Nobel committee actually the theoretical structure of quasicrystals was published and available in the relevant journal of crystallography at the time the experimental paper got published. This sequence of events is well documented as shown above and in other review articles and books.
I am just amazed how the press release of the Nobel committee creates an alternate universe with a false history of theoretical and experimental publication records. It does give false credits for the first theoretical work on three-dimensional quasicrystals and at least in my view does not adhere to scientific and ethic standards of scientific writing.

Prof. Sven Lidin, who is the author of the two press releases of the Swedish Academy has been contacted as early as October 7 about his inaccurate and unbalanced account of the history of quasicrystals. In my view, a huge responsibility rests on the originator of the “story” which was put in the wild by Prof. Lidin, and I believe he and the committee members are aware of their power  since they use actively all available electronic media channels to push their complete “press package” out. Until today no corrections or updates have been distributed. Rather you can watch on youtube the (false) story getting repeated over and over again. In my view this example shows science reporting in its worst incarnation and undermines the credibility and integrity of science.

Quasicrystals: anticipating the unexpected

The following guest entry is contributed by Peter Kramer

Dan Shechtman received the Nobel prize in Chemistry 2011 for the experimental discovery of quasicrystals. Congratulations! The press release stresses the unexpected nature of the discovery and the struggles of Dan Shechtman to convince the fellow experimentalists. To this end I want to contribute a personal perspective:

From the viewpoint of theoretical physics the existence of icosahedral quasicrystals as later discovered by Shechtman was not quite so unexpected. Beginning in 1981 with Acta Cryst A 38 (1982), pp. 257-264 and continued with Roberto Neri in Acta Cryst A 40 (1984), pp. 580-587 we worked out and published the building plan for icosahedral quasicrystals. Looking back, it is a strange and lucky coincidence that unknown to me during the same time Dan Shechtman and coworkers discovered icosahedral quasicrystals in their seminal experiments and brought the theoretical concept of three-dimensional non-periodic space-fillings to live.

More about the fascinating history of quasicrystals can be found in a short review: gateways towards quasicrystals and on my homepage.