young people can comprehend." Byrne – who, like Anderson, did not have a scientific background – found the central concept hard to grasp: "Once I got the go-ahead I suddenly realised I knew very little about the theory of relativity ... I went out and read Relativity for the Layman, and realised I was in deep trouble because there were so many aspects of it." Struggling with the plot, Anderson and Byrne engaged Professor John G. Taylor of the University of London, an expert on black holes, as a scientific adviser. However, according to Byrne, "there wasn't an awful lot [Taylor] could do. In the end the story simplified itself down by necessity. I gave an illustration of the Doppler shift, and gave an idea of how planets destroy themselves, and how they can become a black hole. Then we tried to duplicate the effect of sending people into the black hole." Despite the focus on relativity, the special also discusses time dilation: an effect whereby time decelerates at a rate proportional to an object's acceleration. For the crew of Altares, which travels at light speed, the journey to Alpha Centauri takes only a few years; during this time, whole decades pass on Earth. Recognising that the child characters Jane and David would serve to represent the young audience's curiosity and ask questions on its behalf, Heinemann wanted them to be "just like real children" rather than "[coming] across like kids do on Saturday morning programmes." Byrne wrote the dialogue so that the characters give only partial explanations of the phenomena that they encounter, the aim being to encourage the audience to study the topics in their own time and develop their research skills. Casting Most of the cast had appeared in earlier Anderson productions. Nick Tate had played regular character Alan Carter in Space: 1999, while Brian Blessed and Joanna Dunham had both appeared as guest characters on the same series – Blessed in the episode "Death's Other Dominion", Dunham in "Missing Link". The credits billed Blessed first, Dunham second and Tate third. Don Fellows had played an uncredited role in Space: 1999s first episode, "Breakaway". Ed Bishop had voiced Captain Blue in the puppet series Captain Scarlet and the Mysterons before taking on the role of Ed Straker in Anderson's first live-action series, UFO. Neither Katharine Levy nor Martin Lev had substantial acting experience, though the filming at