what fits at the centre is the corpus's anchor: three published 256-bit hashes, 768 bits. each bit is one copper atom, written in the atom's mass. copper has two stable isotopes: Cu-63 is a 0, Cu-65 is a 1. neither ever decays, so the corpus never fades. copper packs in a face-centred cube 3.615 Å on a side. 768 atoms, stacked around one centre atom, finish on a part-filled layer. the next complete layer comes at 791, so the core is 791 atoms: 768 carry the corpus and 23 sites are spare. one layer of gold, about 400 atoms, hugs it shut. gold's cube is 12.8% wider than copper's, so the shell sits strained, and a real one has to grow on slowly. the dot is 3.3 nm across. one extra electron costs 0.86 eV, 33 times what heat carries at room temperature. so the dot holds −1, 0 or +1 without spilling: that's the real version of the gold+/silver− beacon. its energy levels sit only about 8 meV apart. above about 91 K heat smears them into a metal; below that it is a true quantum dot. reading the isotopes atom by atom is possible in principle with an atom probe, but today's instruments catch only part of the atoms. writing 791 chosen isotopes into a 3-D cluster is beyond any tool yet, so the page shows the arrangement, not a made one. drag the dot to turn it.