Currently, the best lower bound achieved on the optimal packing fraction of regular tetrahedra is 85.63%. It has been known since J. Müller, in the fourteen-hundreds, that tetrahedra do not tile space, but an upper bound below 100% (namely 1-(2.6...)10−25) has only recently been reported.
In 2006, Conway and Torquato showed that a packing fraction about 72% can be obtained by constructing a non-Bravais lattice packing of tetrahedra (with multiple particles with generally different orientations per repeating unit), and thus they showed that the best tetrahedron packing cannot be a lattice packing (with one particle per repeating unit such that each particle has a common orientation). These packing constructions almost doubled the optimal Bravais-lattice-packing fraction 36.73% obtained by Hoylman. In 2007 and 2010, Chaikin and coworkers experimentally showed that tetrahedron-like dice can randomly pack in a finite container up to a packing fraction between 75% and 76%. In 2008, Chen made a significant improvement, proposing a structure with a packing fraction of 77.86%. A further improvement was made in 2009 by Torquato and Jiao, who compressed Chen's structure using a computer algorithm to a packing fraction of 78.2021%. Later these same authors obtained a denser random tetrahedron packing with a packing fraction of 82.26% using the same algorithm.
In mid-2009 Haji-Akbari et al. showed, using MC simulations of initially random systems that at packing densities >50% an equilibrium fluid of hard tetrahedra spontaneously transforms to a dodecagonal quasicrystal, which can be compressed to 83.24%. For a periodic quasicrystal approximant with an 82-tetrahedron unit cell, they obtained a packing density as high as 85.03%.
In late 2009, a new, much simpler family of packings with a packing fraction of 85.47% was discovered by Kallus, Elser, and Gravel. These packings were also the basis of a slightly improved packing obtained by Torquato and Jiao at the end of 2009 with a packing fraction of 85.55%, and then by Chen, Engel, and Glotzer in early 2010 with a packing fraction of 85.63%.
Relationship to other packing problems
Because the earliest lower bound known for packings of tetrahedra was less than that of spheres, it was suggested that the regular tetrahedra might be a counterexample to Ulam's conjecture that the optimal density for packing congruent spheres is smaller than that for any other convex body. However, the more recent results have shown that this is not the case.
- Packing problem
- Disphenoid tetrahedral honeycomb - an isohedral packing of irregular tetrahedra in 3-space.
- The triakis truncated tetrahedral honeycomb is cell-transitive and based on a regular tetrahedron.
- Chen, Elizabeth R.; Engel, Michael; Glotzer, Sharon C. (2010). "Dense crystalline dimer packings of regular tetrahedra". Discrete and Computational Geometry 44 (2): 253–280. doi:10.1007/s00454-010-9273-0.
- D. J. Struik (1925). "De impletione loci". Nieuw Arch. Wiskd. 15: 121=134.
- Simon Gravel; Veit Elser; Yoav Kallus (2010). "Upper bound on the packing density of regular tetrahedra and octahedra". Discrete and Computational Geometry. arXiv:1008.2830. doi:10.1007/s00454-010-9304-x.
- Burkard Polster and Marty Ross (2011-03-14). "Do women have fewer teeth than men?". The Age.
- Conway, J. H. (2006). "Packing, tiling, and covering with tetrahedra". Proceedings of the National Academy of Sciences 103 (28): 10612–10617. Bibcode:2006PNAS..10310612C. doi:10.1073/pnas.0601389103. PMC 1502280. PMID 16818891.
- Hoylman, Douglas J. (1970). "The densest lattice packing of tetrahedra". Bulletin of the American Mathematical Society 76: 135–138. doi:10.1090/S0002-9904-1970-12400-4.
- Jaoshvili, Alexander; Esakia, Andria; Porrati, Massimo; Chaikin, Paul M. (2010). "Experiments on the Random Packing of Tetrahedral Dice". Physical Review Letters 104 (18). Bibcode:2010PhRvL.104r5501J. doi:10.1103/PhysRevLett.104.185501. PMID 20482187.
- Chen, Elizabeth R. (2008). "A Dense Packing of Regular Tetrahedra". Discrete & Computational Geometry 40 (2): 214–240. doi:10.1007/s00454-008-9101-y.
- Cohn, Henry (2009). "Mathematical physics: A tight squeeze". Nature 460 (7257): 801–802. Bibcode:2009Natur.460..801C. doi:10.1038/460801a. PMID 19675632.
- Torquato, S.; Jiao, Y. (2009). "Dense packings of the Platonic and Archimedean solids". Nature 460 (7257): 876–879. arXiv:0908.4107. Bibcode:2009Natur.460..876T. doi:10.1038/nature08239. PMID 19675649.
- Torquato, S.; Jiao, Y. (2009). "Dense packings of polyhedra: Platonic and Archimedean solids". Physical Review E 80 (4). arXiv:0909.0940. Bibcode:2009PhRvE..80d1104T. doi:10.1103/PhysRevE.80.041104.
- Haji-Akbari, Amir; Engel, Michael; Keys, Aaron S.; Zheng, Xiaoyu; Petschek, Rolfe G.; Palffy-Muhoray, Peter; Glotzer, Sharon C. (2009). "Disordered, quasicrystalline and crystalline phases of densely packed tetrahedra". Nature 462 (7274): 773–777. arXiv:1012.5138. Bibcode:2009Natur.462..773H. doi:10.1038/nature08641. PMID 20010683.
- Kallus, Yoav; Elser, Veit; Gravel, Simon (2010). "Dense Periodic Packings of Tetrahedra with Small Repeating Units". Discrete and Computational Geometry 44 (2): 245–252. doi:10.1007/s00454-010-9254-3.
- Torquato, S.; Jiao, Y. (2009). "Analytical Constructions of a Family of Dense Tetrahedron Packings and the Role of Symmetry". arXiv:0912.4210 [cond-mat.stat-mech].
- Packing Tetrahedrons, and Closing in on a Perfect Fit, NYTimes
- Efficient shapes, The Economist
- Pyramids are the best shape for packing, New Scientist