Parallel universe As Sergeant John Benton once observed , a parallel universe was " some sort of mirror universe , like in that " Star Trek " episode where Spock had a beard . " Amused , called Benton ' s assessment " succinct . " His own belief about parallel universes was more complex ; to him , a parallel Earth occupied " the same space - time coordinates as this Earth , but in a different dimension . Sideways in time , if you like , rather than forward or back . " ( PROSE : " The Face of the Enemy " ) Physics . The Third Doctor once described travel between parallel universes as " like travelling sideways " rather than on a nature course . ( TV : " Inferno " ) As the Fifth Doctor once explained to Turlough , parallel universes were a specialised form of alternate universe — " alternative universes [ that existed ] in parallel with each other " — but the terms weren ' t complete synonyms . A parallel universe had no impact upon another parallel universe ; it was " separated only by a millisecond of time and a nanometre of space , without ever having contact . " Conversely , a fully alternate universe was when , because of the inherent weakness at the point that one universe split from the other , one timeline actually destroyed the other . ( PROSE : " Imperial Moon " ) At least some parallel universes were created as a consequence of people making different decisions — the Tenth Doctor explained to Jackie Tyler that " every single decision we make creates a parallel existence " . ( TV : " Doomsday " ) The Tenth Doctor also said that there were " billions of parallel universes all stacked up against each other . The Void is the space in between , containing absolutely nothing . " ( TV : " Army of Ghosts " ) However , the Eighth Doctor noted that such significant events remained indeterminate until the act of observing them forced the universe to " choose " the outcome , with the new timeline only splitting if absolutely necessary and transit between the universes normally being very difficult . ( PROSE : " Time Zero " ) Time behaved differently in some parallel universes compared to N - Space . In one universe the Inferno Project was more advanced . ( TV : " Inferno " ) Pete ' s World was three years ahead , ( TV : " Doomsday " ) while William Blake spent a few days in the Spiral Yssgaroth and returned to Earth seventy years later than when he left . ( PROSE : " The Pit " , " The Book of the War " ) At one point , travel between parallels was made relatively simple by the Time Lord s - although TARDISes were still equipped with security protocols to stop them travelling into other universes on a regular basis ( TV : " Inferno " , PROSE : " The Quantum Archangel " ) - but , according to the Tenth Doctor , travel to parallel universes became almost completely impossible after the Last Great Time War , and could only happen under extraordinary circumstances . ( TV : " Rise of the Cybermen " ) Examples . In July 1963 , the Fifth Doctor visited a parallel universe in which World War II was still being fought . While there , he met Angus Goodman , who became his companion . ( COMIC : " Lunar Lagoon " , " 4 - Dimensional Vistas " , " The Moderator " ) In the 1970s , the Third Doctor visited a parallel universe in which the United Kingdom was a fascist Republic ruled by an alternate Doctor . ( TV : " Inferno " , PROSE : " " ) The Fourth Doctor visited a universe in which the Roman Empire spanned the galaxy and was beginning to conquer other universes . ( COMIC : " Doctor Who and the Iron Legion " ) One parallel universe was lifeless and empty due to the destruction of Logopolis in 1981 . ( PROSE : " The Quantum Archangel " ) In 1997 , Carbury was invaded by knight s from a parallel universe which bore similarities to Arthurian legend . ( TV : " Battlefield " ) The Eighth Doctor eventually returned to this world - before this invasion from their perspective - when he unintentionally disabled some of the TARDIS ' s security protocols , allowing him to set up the events that would lead to his past self thwarting their earlier invasion . ( PROSE : " One Fateful Knight " ) The Sixth Doctor and Evelyn Smythe travelled to a parallel universe in which the Doctor actually enjoyed his exile on Earth with the Brigadier . ( AUDIO : " The 100 Days of the Doctor " ) In 2007 , the Tenth Doctor , Rose Tyler and Mickey Smith visited a universe in which Rose ' s father was still alive and the Cybermen were created on Earth by John Lumic . In this universe , the United Kingdom was instead the People ' s Republic of Great Britain led by the President of Great Britain . ( TV : " Rise of the Cybermen " ) The Torchwood Institute in this universe had managed to develop a technology that allowed quick travel between their dimension and N - Space through the breach created in the Void by the Dalek Void Ship , but as an unintended side - effect , travelling in this manner further opened the breach , to a point where it started threatening the existence of that universe . When the breach was closed by the Doctor , this method of travel was rendered unusable . ( TV : " Doomsday " ) In 2008 , Ianto Jones accepted the offer of Mairwyn . He did not die when the 456 invaded and Mairwyn enslaved Earth . The two later visited the primary universe to observe Ianto ' s funeral . ( COMIC : " Shrouded " ) A parallel universe , where Earth was part of a meta - government called the United Federation of Planets , was joined with the Doctor ' s at one point , resulting in the Fourth Doctor coming in contact with the Federation in the 23rd century and the Eleventh Doctor in the 24th century , although this combination was undone when the Doctor and the " Enterprise " - D defeated the Borg - Cybermen alliance that caused this invasion . ( COMIC : " Assimilation² " ) In one universe , the Doctor stayed on Gallifrey and became an author . ( AUDIO : " Auld Mortality " , " A Storm of Angels " , PROSE : " The Innocents " ) In another , the Doctor regenerated into a female incarnation to escape the Time Lords . ( AUDIO : " Exile " ) Bernice Summerfield was once accidentally ' abducted ' by an alternate version of the Doctor from a world where he was exiled to Earth in 1997 rather than the 1970s ; after this universe had been subject to a major war , the Doctor had hoped to use Benny ' s accumulated artron energy to escape his dying universe , but instead he dragged her into his world . ( AUDIO : " The Library in the Body " ) In one universe , the Valeyard won the trial and successfully obtained the Sixth Doctor ' s remaining regeneration s . ( AUDIO : " He Jests at Scars . . . " ) In another universe , Julia Caesar became empress of the Roman Empire and had her life prolonged by an alien technology . By the 1900s , the Roman Empire owned sixty percent of Earth and Caesar was still alive . America and Australia were populated by " savages " . During an unknown incident , this universe ' s version of the Sixth Doctor lost one of his eyes . Melanie Bush was a slave . ( PROSE : " Spiral Scratch " ) Melanie Bush met a version of herself from a universe where Silurian s never went into hibernation . In this universe , she was Melanie Baal , a Human - Silurian hybrid . ( PROSE : " Spiral Scratch " ) In one universe , the Doctor believed that the ends justified the means . ( AUDIO : " Full Fathom Five " ) When Sabbath ' s actions caused the barriers between various parallel realities to break down ( PROSE : " Time Zero " ) , the Doctor visited several different worlds trying to restore the prime reality , including a world where a group had been formed specifically to kill Sabbath ( PROSE : " The Infinity Race " ) , a world where the computer was never invented as part of the efforts of that world ' s Sabbath to ' protect ' his Earth ( PROSE : " The Domino Effect " ) , a world where the mysterious Eternine s sought to drain Earth ' s energy to save themselves ( PROSE : " Reckless Engineering " ) , and a world where time - travel tours were founded while Earth faced invasion by an alternate race of Martians ( PROSE : " The Last Resort " ) . " All thirteen " incarnations of the Doctor came together to save Gallifrey in the Last Great Time War by freezing it in a single moment in time , in a " parallel pocket universe " . ( TV : " The Day of the Doctor " ) Donna ' s World was a " great big parallel world " created around Donna Noble by a Time Beetle , where the Tenth Doctor died facing the Racnoss under the River Thames in 2007 , and was subsequently unable to save the people inside Royal Hope Hospital who were taken to the Moon in 2008 , or the Earth from being attacked by a replica of the " Titanic " , the Adipose or ATMOS in 2008 - 09 . When the Donna of this world travelled back in time to make her younger self turn left at a junction on Little Sutton Street , both the Doctor and Donna believed that this world ceased to exist . ( TV : " Turn Left " ) Miscellany . Fitz Kreiner once told Trix MacMillan that " the future " was the best place to shop for music , since one could pick up an artist ' s complete run . Parallel universes , however , complicated this theory : " You think you ' ve got all the Beatle albums until you ' ve been to a few of those . " When later called upon to sing in a bar , Fitz briefly considered using one of the " parallel universe Beatles songs , " since no one in the prime universe would recognise them . However , when Trix pointed out that one of these songs was merely " Jealous Guy " with different lyrics , he quickly abandoned the idea of using these songs as " cheating . " ( PROSE : " The Gallifrey Chronicles " ) Chronovore s feasted on parallel universes , but their meals were infrequent because they were banished to Calabi - Yau Space , which provided them no access to the timelines and parallels of the normal universe . once released the Chronovores to N - Space , where they threatened to eat away all realities . ( PROSE : " The Quantum Archangel " ) In all alternate universes with Melanie Bush , she grew up with her sister Anabel . In the prime universe , Melanie accidentally killed Anabel when they were children . ( PROSE : " Spiral Scratch " ) As colloquialism . The term was used sometimes in an inexact way , simply to indicate the unlikelihood of an event . Claudia Bruderbakker once told Peri Brown , for instance , that if her stepmother Joan Bruderbakker had been responsible for a particular event , she ' d " entered a parallel universe " . Peri , familiar with parallel universes , briefly considered the genuine possibility of another universe being created , but then realised that Claudia was speaking figuratively . ( PROSE : " Synthespians™ " )