Steiner's name for silk: thread the caterpillar spins along a sunbeam, so a chrysalis is earthly matter wrapped in light.
Spun Light in Anthroposophy is Rudolf Steiner's name for silk, and for the whole chrysalis sheath, read as sunlight drawn down into earthly substance. In the lecture of 8 October 1923 at Dornach, printed in How Spirit Works in Nature (GA 351), Steiner watched the caterpillar spin its thread along a beam of light, laying the fibre out by day in the sun and rolling it up again at night, so that the finished covering repeats the form of the sunbeam. The bearer is the caterpillar, a creature that wants the light it cannot climb to and gives its own body over to it instead. Silk, on this reading, is not a secretion that happens to shine: it is the sunbeam copied in matter, with the sun forces sealed inside the shell until the butterfly is ready to leave. The same picture stands behind silk photonics today, where fibroin cast from Bombyx mori cocoons is made to carry a beam.
Spun light is what Steiner called silk when he spoke to the Goetheanum workmen about butterflies. The caterpillar follows the sunbeam it cannot climb, spins its thread in the pattern of that beam, pausing through the dark and resuming at first light, and hangs the result on a branch. What hangs there, Steiner said, is sunlight held inside earthly matter until the butterfly is ready to leave it.
In Steiner's Own Words
As the caterpillar crawls along, it spins a thread in the pattern of the beam of light. It spins in exact accordance with the beam of light and at night when there is no light it rolls up the thread. It spins it out in the sunlight and rolls it up again at night. In this way it forms its sheath. The caterpillar completely surrenders to the light, it dies in the light. [...] The silkworm spins the silk according to the light, so when you take its silk you can certainly say: This is spun light! Earthly matter is spun in the pattern of light rays, and when you come across a chrysalis you are really seeing pure sunlight spun around this earthly matter in the pattern of the sunbeam.
What it Means Today
Silk became an optical material in the laboratory less than twenty years ago. At Tufts University in Medford, Massachusetts, the group around Fiorenzo Omenetto and David Kaplan learned to dissolve Bombyx mori cocoons, purify the fibroin, and cast it back as clear films. Their 2010 paper in Science, New Opportunities for an Ancient Material, sets out what those films will do: serve as lenses, hold diffraction gratings pressed at nanometre scale against a patterned master, and guide a beam along a waveguide made from a protein a moth secreted. The cocoon Steiner held up in 1923 as a picture is now a light-handling substance that dissolves in water and leaves nothing behind.
Thalira synthesis: the point is not that Steiner forecast photonics, because he did not, but that neither account will treat silk as dead material. Steiner asked what the caterpillar was following while it spun; the Tufts group asked what the finished fibre could still do with light once the caterpillar was gone. Both answers land on the same property, that this thread holds a beam.
The practical move for a reader is Goethean rather than technical. Find a real cocoon, keep a rearing record and note that the thread is laid down through daylight hours and paused in darkness, then hold the shell against a window before deciding whether spun light was only a figure of speech.
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