"Let me get this straight... a thing that looks like a police box, standing in a junkyard, it can move anywhere in time and space?"
"But that's ridiculous!"
Time travel has been at the heart(s) of Doctor Who since the series' very beginnings. But the show's approach to exactly how it works, and what the rules of changing history are, has changed more times than the Doctor's face.
Ten years into the show's run, it was established that the Time Lords have their own Laws of Time, practically all of which the Doctor has broken over the course of his/her adventures: they include a strict policy of non-interference in the history of others, and a rule banning Time Lords from ever crossing over into their own time-stream (i.e. meeting their past self).
But those are all just self-imposed edicts. What are the fundamental rules of time travel, as far as Doctor Who is concerned? And do they make any sense? Let's find out.
"You can't rewrite history!"
In one of Doctor Who's very earliest jaunts into history, 1964's 'The Aztecs', the first Doctor (William Hartnell) reprimands his companion Barbara (Jacqueline Hill) for planning to intervene in the Aztecs' practice of human sacrifice. "You can't rewrite history!" he insists. "Not one line!"
His words here are, of course, open to interpretation. When the Doctor says Barbara "can't" rewrite history, he could either be saying that the act is physically impossible – that time travellers cannot impact events in a way that would change their future – or arguing that she simply shouldn't interfere, because the results could be catastrophic.
Let's be generous and assume that it's the latter, because later Doctor Who makes it very clear that time can be rewritten. Well, sometimes.
The eleventh Doctor (Matt Smith) puts it in blunt terms in the 2010 episode 'Flesh and Stone': "Time can shift. Time can change. Time can be rewritten."
Events in the past can be changed, and this will directly affect the future. 'Pyramids of Mars', a fourth Doctor's outing (portrayed by Tom Baker) from 1975, provides us with a clear illustration of this, with companion Sarah Jane (Elisabeth Sladen) briefly returning to a version of 1980 that's been transformed into an apocalyptic wasteland by the time tinkerings of villain Sutekh in 1911.
"Back to the Future... it's like Back to the Future!"
But what happens to the time-travellers themselves when history is altered?
Well, in 2007's 'The Shakespeare Code', the tenth Doctor (David Tennant) tells Martha (Freema Agyeman) that if the Carrionites succeed in taking over Earth in 1599, she would vanish – Back to the Future-style – because the future she's from would no longer exist.
The recent 'Demons of the Punjab' affirms that direct intervention in someone's lineage could cause them to blink out of existence, with the whole episode hinging on the Doctor allowing Prem (Shane Zaza) to die so that Yaz's grandmother Umbreen (Amita Suman) can later meet Yaz's grandfather, guaranteeing that Yaz (Mandip Gill) will be born.
If Prem lives, we're told, Yaz will blink out of existence.
However, other stories established that time travellers could survive certain changes to history, small alterations that don't impact their being born, since they were 'outside of time'. 'Flesh and Stone' even suggests that they'd be able to remember the original timeline – though the same year's 'Cold Blood' indicates that the more directly the change affects someone's life, the more difficult it is for that person to retain their memories.
Thus, Amy Pond (Karen Gillan) is able to remember the clerics swallowed up by the cracks in time in 'Flesh and Stone', but has to fight to remember her fiancé Rory (Arthur Darvill) after he's similarly written out of history in the later episode.
Still with us? Good.
How, though, does Doctor Who deal with that pitfall of all time-travel stories, the question of why 'the past' is sacrosanct, while 'the future' is malleable? If Barbara couldn't meddle with the Aztecs' history, why is it alright for the Doctor to overthrow the Dalek invasion of Earth in the 22nd century? That's still meddling with established events, right?
"Some things are fixed"
To tackle this problem, Doctor Who has introduced the idea of "fixed points" existing in history – sequences of events that must not be disturbed, or a thread will be pulled that could end up unravelling the universe.
"I'm history to you, [but] you saved me in 2008," Donna (Catherine Tate) challenges the tenth Doctor in 'The Fires of Pompeii'. "Why is that different?"
"Some things are fixed, some things are in flux," he responds, insisting that the 79 AD eruption of Mount Vesuvius which buried Pompeii and its surrounding cities must not be averted.
What is and isn't a fixed point tends to be entirely dependent on the demands of the story, but, importantly, we're assured that the Doctor is able to tell the difference. "I can see what is, what was, what could be, what must not," he tells Donna. "That's the burden of a Time Lord."
Conveniently, most of these fixed points tend to fall in Earth's history, which is how the show excuses fiddling with our future. (Though, to be fair, not all of them – the death of Bowie Base One's entire crew in 2009's 'The Waters of Mars' was also established as a fixed point in the year 2059, one that a reckless Doctor attempted and ultimately failed to defy.)
"It's more like a big ball of wibbly-wobbly, timey-wimey... stuff."
Clearly, like the TARDIS, the question of how time-travel works in Doctor Who is far more expansive than you might suspect.
But these are the essential points to remember...
Time isn't fixed, and you can rewrite history, every single line. In fact, simply by arriving in a new time and place, you've already made an impact.
For the most part, it's safe enough to make small alterations to the past – you can save the odd life here, overthrow the odd corrupt government there, so long as there's no major disruption to history.
It was implied, in episodes like 2005's 'The Unquiet Dead', that changing history was more perilous after the demise of the Time Lords in the wake of the Last Great Time War. This suggests that they were previously able to somehow paper over the cracks caused by minor changes to history, with the same year's 'Father's Day' revealing that they'd long held the Reavers – creatures who fed on time paradoxes – at bay.
However, major alterations should be avoided at all costs. (The Doctor appears to be the arbiter of what does and doesn't count as "major" – the annihilation of Earth in 1985, for example, would disrupt the web of time, as established in that year's 'Attack of the Cybermen'.)
You should also resist making significant alternations to your own personal history. While these might not affect the wider universe, they could impact on your memory, or even, in extreme circumstances, on your very existence.
Other than that, the only periods of history you shouldn't change at all are "fixed points", because any and all changes made to these will set off a chain reaction of sorts that will destabilise all of time.
It's a rather complicated lot of rules to remember. But essentially, any time-traveller in the Who-niverse would do well to mind the Doctor's advice from 'Demons of the Punjab': "Tread softly."
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