@book{
  author = "Einstein, Albert, 1879-1955",
  title = "Äther und relativitäts-theorie : Rede gehalten am 5. Mai 1920 an der Reichs-Universität zu Leiden",
  year: 1920
  key = "Ether (Space)",
  key = "Relativity (Physics)",
  publisher = "J. Springer",
  address = "Berlin",
}