The law of spectral return:

"The light of those inconceivably distant stars in the Milky Way takes seventy thousand years to reach the earth; if there were a telescope powerful enough to bring the surface of one into view, the events we could observe, however much they appeared to be taking place before our eyes, would be things that had already been past for seventy thousand years.  The idea that shook him with its terrifying simplicity was that in the infinite expanse of the universe every event that had ever occurred must be preserved somewhere, as an image embalmed in light.  'Therefore,' he reasoned, 'there must exist the possibility—even if it is beyond the power of man—of bringing back the past?'"
—Gustav Meyrink, The Green Face