Despite time-traveling constantly, the contemporary Time Lords on Doctor Who seldom run into their counterparts from the ‘70s, ‘80s, or ‘90s. In the real world, it makes sense: Many of the old actors aren’t alive. But the in-universe reason why David Tennant would never cross paths with William Hartnell makes even more sense. And it’s all about an exploding star.
Right now, it’s simply convenient for Doctor Who to be in a somewhat straight line. Otherwise, Christopher Eccleston would be getting into fights with Peter Capaldi and Matt Smith would be comparing ties with Colin Baker. The paradoxes would be unbelievable and the casting impossible.
But, Reddit user SteamDelta posted a theory yesterday explaining why it is that Time Lords who travel with a TARDIS always meet in themselves the “correct” order while other characters in Doctor Who who also time travel pop up in any order they please.
First introduced in the Fourth Doctor adventure “The Deadly Assassin” and featured heavily in the Eighth Doctor’s 1996 Doctor Who TV movie, the TARDIS harnesses something called “The Eye of Harmony.” This is the TARDIS’s power source and is described by Matt Smith’s Eleventh Doctor in “Journey to the Center of the TARDIS” as an “exploding star in the act of becoming a black hole … You rip the star from its orbit; suspend it in a permanent state of decay.”
SteamDelta poses that the Eye of Harmony — which originally existed as the sustaining power for the Doctor’s home planet of Gallifrey, and which all TARDISes are linked to — cannot cross its own time stream. So, if the Doctor is in the TARDIS — or if any Time Lord is in any TARDIS — and a year passes for him, a year has also passed on Gallifrey. Which somehow still works, even though Gallifrey was sort of destroyed in the Time War.
This is why each version the Master and the Doctor always meet on the same timeline in which they match up with each other’s “current” incarnations. Meanwhile, time travelers who don’t have a TARDIS at their disposal — such as the Face of Boe or River Song — are rarely in sync with what’s going on in the Doctor’s timeline. If the Doctor and the Master exclusively used the “cheap and dirty” time travel of the vortex manipulators, they might meet versions of themselves out of order. But if they use a TARDIS, then they’ll always match up.
Then again, maybe we don’t need the Eye of Harmony. Maybe it’s just as easy to say that the Doctor not running into himself is just random chance. After all, it’s a big universe. But where’s the fun in that?