As Doctor Who’s first story editor David Whitaker tells us,
Everyone expects […] to see large, gleaming spaceships orbiting planets. But what if the spaceships were here already, disguised as ordinary artifacts? And what if their occupants were already walking among us, keeping cautiously in the background to avoid notice and suspicion […]?1
We know the TARDIS can reach any planet in space, but have I mentioned that it also travels in time?
Doctor Who confirms its status as science fantasy, and as distinct from hard SF, with every time-travel paradox, conundrum or parallel universe it presents. That is, it avoids relying on a particular theory about the nature of time itself, and in this regard it ducks many thorny conceptual questions about cosmology. ‘In spite of the fact that travel to the past flies in the face of all common sense’, claims the author of one study of the literature of time travel, ‘we find it throughout the literature. Indeed, it is one of the most common fantasies we have.’2
What is time travel? The late David Lewis (1941–2001) satisfies the philosopher’s need for precision when he explains:
Inevitably, it involves a discrepancy between time and time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival […] is the duration of the journey. But if he is a time traveler, the separation in time between departure and arrival does not equal the duration of his journey. He departs; he travels for an hour, let us say; then he arrives. The time he reaches is not the time one hour after his departure. It is later, if he has traveled toward the future; earlier, if he has traveled toward the past.3
The discrepancy Lewis describes is between, on the one hand, the ‘personal time’ that elapses for the traveller from start to finish of his journey and, on the other, the ‘external time’ that defines the continuity of all other (non-time-travel-related) events. In the 25th-anniversary episode ‘Silver Nemesis’, the seventh Doctor provides an excellent example of this discrepancy in answering Ace’s question, ‘How can a statue destroy the world?’ ‘I’ll tell you 350 years ago,’ he replies. Moments later, the TARDIS materialises in Lady Peinforte’s house in 1638.
Despite the fact that time travel violates principles of common sense and produces absurdities of personal time detached from external time, it does demand from its central premise of travel through the time vortex a certain coherency, even if the scientific standards upon which that coherency are (loosely) based have changed radically over the history of the programme. Early in its history, the first Doctor admits, ‘Even after all this time […] I dare not change the course of history’ (‘The Massacre of St Bartholomew’s Eve’), while more recently we have been treated to the ‘riddle, wrapped in a mystery, inside an enigma’ of the 2011 season, in which the eleventh Doctor is killed eight minutes into the première episode, ‘The Impossible Astronaut’. There is a logic to how one could gallivant through time shown to us by Doctor Who, which in the history of SF or science fantasy must be the most sustained meditation on time travel.
The first aired episode of Doctor Who, ‘An Unearthly Child’, introduces us to the science fantasy elements of the show gradually, using a narrative structure unusual for the programme: flashbacks to the uncanny classroom experiences of Ian Chesterton and Barbara Wright regarding their student Susan Foreman. Speaking with Barbara about one of these incidents, Ian says that, the other day, he’d ‘set the class a problem with A, B and C as the three dimensions’. The flashback begins with Susan clearly upset at the assignment:
SUSAN: It’s impossible unless you use D and E!
IAN: D and E? Whatever for? Do the problem that’s set, Susan.
SUSAN: I can’t, Mr. Chesterton! You can’t simply work on three of the dimensions!
IAN: Three of them? Oh. Time being the fourth, I suppose. Then what do you need E for? What do you make the fifth dimension?
SUSAN: (quietly, mysteriously) Space …4
As quaint as Susan’s own theory of time as the Fourth Dimension appears, it is at least an improvement over the naïve view that time is linear and analogous to a stream or river. Treating time as another dimension that interpenetrates and can vary with space is the same thing as accepting that there is no ‘space’ and no ‘time’, but only space-time, that is, ‘reality consists of a single unified space-time, which contains all of the past, present and future […] Time does not flow; time is like space.’5 The Einsteinian unification of space and time, however, raises just as many questions as it solves and the wrinkles of quantum indeterminacy make the question, ‘Is time travel possible?’ a particularly knotty one. As we shall see, even if it were possible, it might not be desirable to head to the past or the future in one’s own personal TARDIS. The outcome could be, as the fifth Doctor’s companion Tegan Jovanka liked to say, ‘ZAP!’
The roots of the metaphysical questions about time and, by extension, the philosopher’s take on the possibility and limits of time travel go back all the way to the beginning. That is, to the beginning of Western thought before Socrates in the Eleatic school of what would become Italy. In the Greek colony of Elea, Parmenides (early fifth century BCE) and his student Zeno (c.490–c.430 BCE) were to set the terms under which Plato, Aristotle, and many others after them would think about the fundamental nature of reality. Parmenides, unlike his fellow pre-Socratics and Socrates himself, wrote down his work, but in verse. This was often the case with the Greeks; for Parmenides the ability to poetise and to reason in flawlessly logical ways were not mutually exclusive abilities. His central insight about the nature of being and the meaning of change has structured the Western view of language and reality for more than two thousand years.
In ‘The Way of Opinion’, the third part of the poem for which he is known, he writes:
Thus, according to opinion, were these things born and now are,
And from now on will grow, and will later perish.
And for each of these human beings proposed a distinguishing name.6
According to Parmenides, we all have a basic understanding of change, and it seems that changes really occur, because our experience is not of a static world or an undifferentiated mental life. In order for change to occur, a particular thing – no matter whether a blue whale or an isotope (or a sense impression or idea, for that matter) – must change its state at least fractionally. A stone split in half suffers a major change, and we can no longer say ‘that (whole) stone is still there’; an atom that gives up an electron becomes a ‘cation’, and becomes positively charged. Whatever the changed thing is, it is not what it was before (note that Parmenides is using ‘is’ in the strict sense of numerical identity, not mere qualitative identity, both of which were discussed in Chapter 5). Change requires that ‘what is’ – at least one property of the changing thing – revert to ‘what is not’, and this for Parmenides is an intolerable suggestion. He says, ‘It is necessary to say and to think What-Is, for to-be is / And nothing is not. These things I command you to consider.’7 Something that is ‘X’ cannot become ‘not-X’, but the transformation from ‘X’ into something else (even if that thing is qualitatively similar, like a stone that loses but one atom) is the most basic logical formula for change. Therefore, Parmenides concludes, change itself is an illusion, and the senses that inform us of myriad ‘changes’ are fundamentally defective for gaining knowledge. ‘Coming-into-being’ or ‘going-out-of-existence’ are non-sequiturs. Famously, his student Zeno would show through his ‘paradoxes’ that motion and other forms of change were rationally unsupportable. We cannot aim for rational coherency in our explanations and accept change at the same time. They were quite serious about all this.
Any future metaphysical explanation of the basics of change would have to engage with Parmenides. Aristotle understood there to be a subject of change that contained the potential for transformation. This subject remained the same despite qualitative change, moving from a state of privation of a particular quality in the case of ‘coming-into-being’ or to a state of privation in the case of ‘going-out-of-existence’.8 So, for instance, when Martha Jones is trying to help the crew of the SS Pentalian prevent their ship from falling into the living sun in ‘42’, she needs the answer to the question, ‘Who had more pre-download Number Ones, Elvis Presley or The Beatles?’ She doesn’t know the answer, so she goes from a state of ‘privation’, or lack of this information, to a state of ‘being-Martha-knowing-the-answer’ after calling her mother and getting the answer (it’s Elvis, by the way). Knowing this trivia doesn’t fundamentally change what she is, for Aristotle, so it seems that both ‘what is’ (Martha as a subject of change) and ‘what is not’ (Martha’s knowledge of pre-download pop hits) are needed to explain genuine change.
Aristotle does seem to concede this to Parmenides: once a change has occurred and is in the past, it is unalterable. ‘What was’ and ‘what was not’ in the past were, and were not, necessarily. For Aristotle, this is more a truth from logic and language than from metaphysics. ‘With regard to what is and what has been it is necessary for the affirmation or the negation [of a claim about what happened] to be true or false’, he writes. ‘For if every affirmation or negation is true or false it is necessary for everything either to be the case or not to be the case.’9 Suppose we say, from the vantage point of today, that ‘Rose Tyler’s dad, Pete Tyler, died on 7 November 1987’. This claim is either true or false. If it’s true (which, despite the events of ‘Father’s Day’ [2005], it still is), then its contradiction, ‘It’s not the case that Rose Tyler’s dad, Pete Tyler, died on 7 November 1987’, is false. Note that if Pete were only hurt instead of being killed by the speeding car (or had escaped unscathed), the contradiction would be true and the original statement false. We could extend this sort of reasoning to any claim about anything that has happened in the past: ‘Napoleon died on the island of St Helena in 1821’ (true); or ‘The extended news coverage of JFK’s assassination delayed the broadcast of the first episode of “An Unearthly Child”’ (false). For any statement the truth of which is undetermined, we must, on Aristotle’s view, assume that either it or its contradiction is true.
Now, that’s true of what’s past, but what about the future? We know that Pete Tyler had dreams about his future and he must have made certain statements about things that hadn’t yet happened. Suppose Pete, in early 1987, had said, ‘One day I’ll make millions from my Vitex Lite sales.’ This claim doesn’t seem to be classifiable as true or false, but it could prove to be one or the other years later. In 2006, on ‘Pete’s World’ – the alternate-universe Earth that the tenth Doctor, Mickey Smith and Rose arrive on in ‘Rise of the Cybermen’ – this statement is true. Of course, for alternate Pete, that’s all in the past. But the only reason we can give for why the statement is unclassifiable as true or false in 1987 is Pete Tyler’s own ignorance of the future. In fact, Aristotle makes a very strong claim regarding statements about the future: they are true or false now, but we’re simply not aware of which it is. In fact, these statements were always true or false, whether they were uttered in the past or present, or will be uttered in the future. ‘Again if [a thing] is white now it was true to say earlier that it would be white’, Aristotle tells us,
so that it was always true to say of anything that has happened that it would be so. But if it was always true to say that it was so, or would be so, it could not not be so, or not be going to be so.10
In Aristotle’s defence, it seems to make sense to say that if a statement is worded precisely enough, then it cannot be true at one time and false at another. Precision is key here. If we say ‘At 5.02 p.m. GMT on 22 April 2011, a woman wearing an astronaut’s flight suit on the shore of Lake Silencio, Utah, fires on a person wearing a tweed coat and bow tie’, we would happily say that if all these conditions are met, then the sentence becomes true at 5.02 p.m. GMT on 22 April 2011. Because the statement is precisely indexed in time and space, however, it doesn’t make sense to say that it’s false before 5.02 p.m. on that date. Rather, like Aristotle’s white object, either it must be true or it must be false, no matter when it is uttered. ‘Hence, if in the whole of time the state of things was such that one or the other was true, it was necessary for this to happen, and for the state of things always to be such that everything that happens happens of necessity.’11 Call this the logical necessity of past and future events (or non-events), which stems from the basic roles that truth and falsity play in language and beliefs. Aristotle’s view of logical necessity means that although we may see ‘possibilities’ for a thing to change in more than one way, such possibilities are only apparent, not genuine – one change or the other will occur, and this has always been the case.
Nonetheless, Aristotle thought that the ‘action and deliberation’ of people like us demonstrated that there was genuine ‘possibility’ in the world rather than the ‘necessity’ that characterised the rest of nature. One reason for treating humans as a special case stems from the fact that Aristotle didn’t have access to physical sciences of power and precision as we do today. This caused him to refrain from understanding the human mind or soul as operating according to the same basic principles of nature as those of the body, and so to disagree with later materialists such as Julien Offray de La Mettrie (1709–51), who proclaimed, ‘Let us then conclude boldly that man is a machine, and that in the whole universe there is but a single substance differently modified.’12 La Mettrie, like many other Enlightenment thinkers, found the hypothesis of a ‘single substance’, matter, producing all the changes of nature by its motion and the interaction of its parts, a particularly powerful one. Our talents and moral dispositions, for example, can be understood better as products of nature, he claimed:
Man’s preeminent advantage is his organism. In vain all writers of books on morals fail to regard as praiseworthy those qualities that come by nature, esteeming only the talents gained by dint of reflection and industry. For whence come, I ask, skill, learning, and virtue, if not from a disposition that makes us fit to become skilful, wise, and virtuous? And whence again, comes this disposition, if not from nature? Only through nature do we have any good qualities; to her we owe all that we are.13
On this view, there is nothing mysterious or occult about human action or deliberation; we need only know of the necessary and sufficient conditions that motivate or restrain our decisions and behaviours in order to bring them, too, into the domain of scientific prediction and control. Richard Taylor explains:
[I]t is not hard to suppose […] that everything that happens is wholly determined by what went before it, and hence that whatever happens at any future time is the only thing that can then happen given what precedes it. Or even disregarding that, it seems natural to suppose that there is a body of truth concerning what the future holds, just as there is such truth concerning what is contained in the past, whether or not it is known to any person or even to God, and hence, that everything asserted in that body of truth will assuredly happen, in the fullness of time, precisely as it is described therein.14
Call this causal or physical necessity, what philosophers often discuss under the heading of ‘determinism’. Determinism isn’t just a metaphysical theory; it can be applied, at least in classical Newtonian physics, with this principle: ‘Classical determinism is a logical equivalence between two propositions of Newtonian dynamics with respect to two different instants of time.’15
If this hypothesis about causes and effects is true, then it’s easy to understand why the first Doctor was constantly warning his companions about interference in history. As a history teacher, 1960s companion Barbara Wright finds it compelling not only to participate in history, but also to think about changing it. The Doctor explodes in frustration at Barbara’s attempt to make the Aztecs a kinder, gentler civilisation, telling her, ‘You can’t rewrite history! Not one line!’ (‘The Aztecs’). She’s not only up against the physical necessity of causal chains of influence that deterministically produce both Aztec beauty and tragedy. She’s also colliding with the logically necessary truth of past statements about the Aztecs’ cruelty to their sacrificial victims and the equally logically necessary falsity of statements such as, ‘In 1454, schoolteacher Barbara Wright, posing as the reincarnated priest Yetaxa, put an end to bloody sacrifices among the Aztecs.’16 Barbara’s situation, then, is not that different from the time traveller in the infamous ‘grandfather paradox’, in which a person is displaced into the past fifty or so years with the intention of killing her grandfather, and all before her grandfather has had any children.
Only two pointed questions then remain: from Barbara’s perspective, nothing is physically or logically preventing her from using her power for this end, so why must the evil simply be lived with?17 And if it is genuinely impossible, logically and physically, for her to make a change in history, why is the Doctor so terribly upset?
One possible explanation is that there’s no impossibility to changing the past, but that the Doctor fears drawing the attention of the as-yet-unnamed Time Lords. This is his motivation in ‘Frontios’ when he uncharacteristically refuses to land near one of the last human settlements in the far future, citing the Laws of Time. ‘Now, we mustn’t interfere. Colony’s too new, one generation at the most. The future hangs in the balance.’
The more interesting answer, philosophically speaking, is that the TARDIS repeatedly materialises at the ‘Jonbar Hinges’ of time, a term taken from Jack Williamson’s 1952 collection of time-travel stories The Legion of Time. Jonbar Hinges, as the name suggests, are linchpins of possible change in the sea of temporal necessity; points that are particularly crucial in the grand scheme of things. The fifth Doctor seems to suggest that the human colony in the Veruna system represents one such ‘Hinge’, and it may be that the Aztec capital Tenochtitlan in 1454 is another one. On the other hand, the tenth Doctor adopts quite a different view in ‘The Waters of Mars’: ‘I mean, it’s only a theory, what do I know, but I think certain moments in time are fixed. Tiny, precious moments. Everything else is in flux, anything can happen, but those certain moments, they have to stand.’ If this is correct (and he admits it’s only a theory), then there are apparently ‘Jonbar Struts’ as well as ‘Hinges’, with the vast majority of events in space-time ‘in flux, anything can happen’, implying that they are causally or historically less significant than ‘Jonbar’ events.
We’ll return to the Doctor’s pet theory about time, but another reason we might share Barbara’s confusion about the possibility of changing history has to do with her autonomy to act as she sees fit in 1454. While Aristotle might claim that the statement, ‘In 1454, schoolteacher Barbara Wright, posing as the reincarnated priest Yetaxa, put an end to bloody sacrifices among the Aztecs’ is just as false in 1454 as it is in Barbara’s ‘home time’ of 1963, it seems puzzling that this statement would be necessarily false before Barbara acts. In physics, the ‘autonomy principle’ states that ‘it is possible to create in our immediate environment any configuration of matter that the laws of physics permit locally, without reference to what the rest of the universe may be doing’.18 This means that if Barbara is able to convince key Aztec leaders that sacrifices should be stopped, only the local causal laws could prevent her from doing so; something like ‘historical inertia’, a force that keeps chains of causes and effects from being disturbed, as Barbara intends to do, is ruled out.
Surprisingly, however, classical physics says that historical inertia would prevent Barbara from changing history, because the autonomy principle cannot conflict with the ‘consistency principle’, which states that ‘the only configurations of matter that can occur locally are those that are self-consistent globally’.19 There is only one history, one staggeringly complex network of causes and effects linked by ironclad chains of physical necessity, so the Doctor is correct to admonish Barbara, even if his tone seems overzealous. This is similar to Aristotle’s view of the unchanging truth or falsity of any claim about the past. In ‘Father’s Day’, Rose and the ninth Doctor unwisely travel, not once but twice, to the spot in 1987 where her father Pete was killed. Upon their second arrival, Rose rushes past her earlier self to actually save her father’s life. This demonstrates a (brief) triumph of autonomy over consistency, but since her earlier self and the earlier Doctor didn’t experience her rushing into the road to pull Pete away from the speeding car, they vanish, cancelled out by something like the effect of the consistency principle. What is worse, of course, is that Rose’s benevolent intervention in her own history brings a particular nasty manifestation of the consistency principle: the Reapers. ‘Time’s been damaged,’ the Doctor explains, ‘and they’ve come to sterilize the wound.’ Of course, the Reapers are a metaphorical representation of the unusual and perhaps absurd effect that consistency can have in limiting our autonomy to act in what appears to be a determined past (or future). Paul Horwich comments on this, saying:
Time travel would allow one to influence the past but not to change it. Changing the past is indeed logically impossible. However, no such contradictions are involved in the idea of influencing the past […] The same considerations apply to the future […] we can bring about future events, but we cannot bring about an event that will not occur.20
It has been mentioned above that in classical physics, the autonomy principle and the consistency principle couldn’t come into conflict with each other. The fact that only one history exists – and presumably, just one present and one future – rules out interventions in time. If we discover, for example, that it was the escapades of the fifth Doctor in Pudding Lane (‘The Visitation’, 1982) that led to the Great Fire of London in 1666, then that has always been the reason for the fire. The fact that the two principles accord with each other doesn’t rule out time travel, but it does imply that the actions of time travellers would be constrained by what has already occurred – or perhaps that any chrononauts could only be ghostly observers of history, not participants in it, much as the first Doctor and company experience at the beginning of ‘The Space Museum’ when the TARDIS has ‘jumped a time track’.
The possibility of ‘Closed Timelike Curves’ (CTCs), however, changes all this. If we briefly revert to thinking in terms of the naïve view that time is linear, like a stream, our own lives and the course of history seem to unfold in one direction: from past to present to future, from earlier to later (‘time’s arrow’). In Einstein’s description of the universe as a unified ‘space-time’, as mentioned earlier, space and time are one four-dimensional entity in which each point represents a place and time – an event. While the lives of non-time travellers appear to be a straight line burrowed through the block of space-time like a worm’s path through an apple, the ‘worldline’ of their life history cannot simply be an arbitrary path chiselled out through space-time. According to Einstein, the speed of light is a constraining factor on travel throughout space-time, which is the same thing as saying that my path through space-time can be shown to increase in duration along one direction of a possible worldline. Anything that obeys this constraining factor is called ‘timelike’. But space-time could, through the influence of massively gravitational attractors (such as black holes), be distorted in such a way that a worldline could curve back on itself, producing a CTC and effective travel to the past. While it’s not entirely clear whether or not the TARDIS might take advantage of CTCs, it is true that the basis for Gallifrey’s time-travel technology is the ‘Eye of Harmony’, a black hole engineered by the pioneer Omega from a star in the Sector of Forgotten Souls (‘The Three Doctors’; ‘Omega’, Big Finish Audio Production, 2003; ‘Journey to the Centre of the TARDIS’, 2013).
It’s helpful to think of multi-Doctor stories in Doctor Who in terms of worldlines and CTCs. The Doctor’s own ‘personal time’ worldline zigzags through space-time, only coalescing into ‘external time’ at his destination for the length of each adventure. When his worldline doubles backwards (or forwards) on itself so that his location in both space and time are roughly the same in more than one segment of his personal time, he is capable of meeting himself. While this typically occurs as a pretext for having several different incarnations meet each other (as discussed in Chapter 5), every so often the Doctor does literally meet himself in the same incarnation (‘Day of the Daleks’; ‘The Big Bang’; ‘Last Night’, a short included in the 2011 sixth series set on DVD and Blu-Ray). The physics of the Doctor Who universe seem to either rule out this kind of meeting for non-Time Lords, or predict disastrous effects should it occur. This fictive physical law, the ‘Blinovitch Limitation Effect’, was first mentioned in ‘Day of the Daleks’ and attributed to Aaron Blinovitch from his book Temporal Mechanics of 1928, but it is not entirely clear what the law specifies. In ‘Mawdryn Undead’ (1983), two versions of Brigadier Lethbridge-Stewart meet each other aboard a spacecraft trapped in a warp ellipse, triggering an enormous and potentially deadly release of temporal energy. The pseudo-scientific explanation for what occurs when this happens is a ‘shorting out of the time differential’. The way in which Time Lords get around this is rather cleverly explained in the 2007 Children in Need special ‘Time Crash’, scripted by Stephen Moffat, in which the fifth Doctor appears in front of the tenth Doctor after their versions of the TARDIS suffer a temporal collision in the time vortex:
FIFTH DOCTOR: Is there something wrong with you?
TENTH DOCTOR: Oh there it goes! The frowny face, I remember that one!
(He grabs FIVE’s face in both his hands and squishes his cheeks around.) Mind you, bit saggier than it ought to be, hair’s a bit grayer. That’s because of me, though, the two of us together has shorted out the time differential, should all snap back in place when we get you back home. Be able to close that coat again. But never mind that! Look at you! The hat, the coat, the crickety cricket stuff the … stick of celery, yeah. Brave choice celery, but fair play to you – not a lot of men can carry off a decorative vegetable.21
So now we know why each incarnation of the Doctor looks older when he bumps into his future selves. Despite the effectiveness of Blinovitch’s principle as a mechanism of historical inertia – keeping any substantial changes from occurring to history by travelling back in time via CTC or otherwise – in the new version of Doctor Who since 2005, violations of the Limitation Effect seem fairly common. River Song, Amy Pond and Kazran Sardick (‘A Christmas Carol’, 2010) have all met themselves without batting an eye.
‘Time Crash’ is also paradigmatic of another apparent paradox of time travel. What is known as the ‘knowledge paradox’ results when knowledge is gained by something other than typical problem-solving processes; it can also extend to any literature or art carried from the future into the past, where it is ‘created’. The deus ex machina solution to the big problem in ‘Time Crash’ involves just such a paradox:
FIFTH DOCTOR: Supernova and black hole at the exact same instant.
TENTH DOCTOR: Explosion cancels out implosion.
FIFTH: Matter stays constant.
TENTH: Brilliant!
FIFTH: Far too brilliant. I’ve never met anyone else who could fly the TARDIS like that.
TENTH: Sorry, mate, you still haven’t.
FIFTH: You didn’t have time to work all that out; even I couldn’t do it.
TENTH: I didn’t work it out, I didn’t have to.
FIFTH: You remembered.
TENTH: Because you will remember.
FIFTH: You remembered being me, watching you, doing that. You already knew what to do because I watched you do it.
TENTH: Wibbley-wobbley …
FIFTH and TENTH together: Timey-wimey!22
As fascinating as they are, knowledge paradoxes compound the puzzles of time travel with their implications that many pieces of ‘new’ knowledge were never discovered or created at all, but exist because of the looping effects of something like a CTC. In their examination of knowledge paradoxes, Deutsch and Lockwood spell out the problem like this: ‘What is philosophically objectionable here is not that knowledge-bearing artifacts are carried into the past – it is the “free lunch” element. The knowledge required to invent the artifacts must not be supplied by the artifacts themselves.’23
Of course, this is only a paradox if the fifth Doctor’s observations are carried forward, to be used by the tenth Doctor within his own universe; if more than one universe is involved, we have quite a different story entirely.
Multi-world theories (MWTs) are a mainstay of both SF and philosophy. While the term ‘multiverse’ was coined by American philosopher and psychologist William James, the idea’s most famous philosophical proponent was Gottfried Wilhelm Leibniz, whose law of identity we discussed in Chapter 5. In trying to buttress the idea that God freely chose to create the actual world, Leibniz was forced to confront the requirements for a real choice; for him, this implied that God must have had other worlds in mind that could have been created, but were not. ‘Thus was born the concept of the infinite set of logically possible worlds.’24 Leibniz viewed ours as ‘the best of all possible worlds’, as it displays the greatest degree of harmony (the norm most prized by God) between unity and multiplicity in its design and providential course of events. Of course, ‘parallel-world plots are much beloved of science fiction writers, who use the scheme to ask questions about worlds like our own, except that particular possibilities turned out differently’.25 Actual travel to a parallel universe – what I’ll call the phenomenon of ‘paratime’, following SF author H. Beam Piper – requires that worlds very much like our own be not merely logically possible, but also actual.
In this important respect, the final serial of Doctor Who’s seventh season in 1970, ‘Inferno’, is ahead of its time. Ostensibly a cautionary tale about breaching the Earth’s crust in search of alternative fuels, ‘Inferno’ works in both a MWT-based plotline and as a glimpse of the end of the world itself, as a parallel universe’s Project Inferno runs out of control, unleashing the forces of nature in a cataclysm the third Doctor can only barely avoid. Settling into his exile and experimenting with the non-functional TARDIS console, the Doctor manages, entirely by accident, to cross over into the parallel world ‘Republic of Great Britain’: a fascist country in which the royal family had been executed years before. Script editor Terrance Dicks had added the parallel dimension element to Don Houghton’s script for ‘Inferno’ at a late stage, and this inclusion created a path-breaking opportunity for Doctor Who to explore MWT several years before the most significant publications by Brandon Carter and G.F.R. Ellis in the mid-1970s. In particular, the Doctor is fascinated by the differences between events in his own universe and those at the end of the Project Inferno of fascist Britain, running on a faster timetable. ‘Yes, of course, of course,’ he muses after returning from the cataclysm on the alternate Earth. ‘An infinity of universes, ergo an infinite number of choices. So free will is not an illusion after all. The pattern can be changed.’ Of course, it’s not at all clear that this is what the Doctor’s multiple-world experiences prove at all. Rather, what they do show is that, given certain starting conditions, one result will occur, while given sufficiently different conditions, a different result will probably come about. Despite the perverse replication of the Doctor’s friends in the form of Brigade Leader Lethbridge-Stewart and Section Leader Elizabeth Shaw, there are enough significant differences between the ‘parallel’ worlds to challenge the Doctor’s speculation about the significance of free will. ‘Inferno’ tells us more about the working of the aforementioned ‘consistency principle’ in different worlds than it does anything else.
Because they convert the necessities of our world into what seem to be new possibilities, alternate universes have their drawbacks as well as their insights. In the first episode to feature MWT in the new Doctor Who, ‘Rise of the Cybermen’, the tenth Doctor counsels his companions Rose and Mickey, ‘Parallel world, it’s like a gingerbread house. All those temptations calling out.’ If there are an infinite number of parallel universes, each related to the next but different in subtle ways, then this fact could even be subversive of morality in general, As Bud Foote points out by claiming that if all possibilities exist somewhere, choice becomes meaningless: ‘Plot, like morality, depends upon acts having consequences.’26 But the likelihood of just such an infinite number of alternatives is precisely what the multiple-worlds interpretation of quantum mechanics implies.
Among other interpretations, MWT as it ties into quantum mechanics was first proposed by Hugh Everett in 1957, and it challenges the definition of determinism we encountered earlier in this chapter: ‘Classical determinism is a logical equivalence between two propositions of Newtonian dynamics with respect to two different instants of time.’27 At the quantum level, no logical, one-to-one equivalence between propositions in Newtonian physics – most famously, those concerning the speed and velocity of a particle à la Heisenberg’s indeterminacy principle – will necessarily obtain in every case. This means that certain observations cannot be predicted absolutely, so physicists – beginning with Everett – concluded that in such cases they could specify only a range of possible observations, each with a different probability. Of course, in the long run, determinism still obtains: ‘as applied to the multiverse, quantum theory is deterministic – it predicts the subjective probability of each outcome by prescribing the proportion of universes in which that outcome occurs’.28 According to the ‘many-worlds’ interpretation of quantum mechanics, each of these possible observations corresponds to a different universe. SF movies, television and literature have exploited this insight to speculate that, for any contingent change that may be resolved one way or another, a duplicate universe is created in which the unactualised resolution becomes actual.
This is the premise of the intriguing tale ‘Turn Left’ (2008). In this episode, the apparently trivial decision of Donna Noble (Catherine Tate) to pursue a different job opportunity six months before meeting the Doctor prompts her to take a right rather than left turn at a traffic intersection. A mysterious fortune-teller from the planet Shan Shen prompts her to change her own past after she separates from the Doctor to sightsee a bit. ‘Turn right, and never meet that man. Turn right, and change the world,’ the fortune-teller urges her compellingly. For most of the rest of the episode, the audience is left in the dark as to whether or not Donna’s right turn is actually a change to established history or whether she is merely dreaming or hallucinating.
Although the ‘turn right’ parallel world is created by Donna’s mundane decision, the episode ingeniously explores what would have happened had she not met the Doctor as she did in ‘The Runaway Bride’. Writer Russell T. Davies kills the Doctor off right away, an unfortunate victim of his own tactics against the Racnoss queen in that episode. Although Donna is confused about what is going on in and around Chiswick, it’s made clear to the viewer that without her involvement with the Doctor – and their efforts to save people from the Judoon (‘Smith and Jones’, 2007), the crash of the starship Titanic (‘Voyage of the Damned’), the Adipose (‘Partners in Crime’), and the Sontarans (‘The Sontaran Strategem’/‘The Poison Sky’, both 2008) – the world is a much worse place. The ‘turn right’ universe shows us a Britain decimated and its economy gutted by alien invasions and radioactivity from the Titanic crash, its immigrant population forced into labour camps, and even the stars vanishing from the night sky. Only Donna – with the help of Rose Tyler and UNIT – can restore the proper chain of events.
When the situation is resolved and history is on the ‘right track’ again, the Doctor explains that Donna’s memory has been the victim of a powerful giant ‘Time Beetle’. The creature apparently feeds off the energy created by a time differential, but has to produce these by changing one person’s history – in this case, Donna’s:
DOCTOR: Just got lucky, this thing. It’s one of the Trickster’s Brigade. Changes a life in tiny little ways. Most times, the universe just compensates around it, but with you? Great big parallel world.
DONNA: Hold on. You said parallel worlds are sealed off.
DOCTOR: They are. But you had one created around you. Funny thing is, seems to be happening a lot to you.29
The reason why Donna is special is revealed in the ensuing story featuring Davros and the Daleks, but ‘Turn Left’ is a brilliant extrapolation of MWT. It demonstrates how the ‘principle of consistency’ of natural laws produces different outcomes when one key decision – or one key player, the Doctor – is changed or removed from the equation.
Many-worlds quantum mechanics also provides an explanation for how the ‘autonomy principle’ could override the ‘consistency principle’, which, as discussed above, would never conflict within the world of classical Newtonian physics. Recall that even travelling back into the past by using a closed timelike curve would not necessarily allow someone like Donna to change the past, as logical and physical necessity would have it. But using a CTC in a situation in which quantum indeterminacy leads to the creation of a parallel universe means that when Donna goes backward in time, she doesn’t emerge in her own past, but instead in that of a roughly parallel universe (call this the ‘Trickster’ universe). It may have been true in her ‘home’ universe that the Doctor didn’t die in his confrontation with the Racnoss queen, and so events could not be different. But in the Trickster universe, Donna has an open future (the universe was just split off from her ‘home’ universe) and, in many ways, is a free agent, since she doesn’t even hail from this parallel world. In such a case, her autonomy to change the future as she sees fit would trump the normal constraint of the consistency principle. Donna could even send her loud-mouthed mother Sylvia packing without worrying about the fact that she never did so in her home universe – in the Trickster universe, that hasn’t happened yet.
Stephen Moffat’s ‘Whoniverse’ has, ever since the landmark episode ‘Blink’, been a universe depicted not as structured by a ‘web of time’ but by ‘wibbley-wobbley, timey-wimey stuff’. Not only does this explicit effort to disavow a theory of time and time travel smack of the Romanticism we discussed all the way back in Chapter 1; it works surprisingly well with what quantum physics tells us about the nature of time. The Doctor’s own theory about the nature of time is fundamentally different from most of the views we have examined so far in this chapter. We must assume that it is informed by intellectual intuitions about time, change and history similar to those discussed throughout the book; in the Doctor’s own words from ‘Time and the Rani’, he has ‘a unique conceptual understanding of the properties of time’. The tenth Doctor’s theory from ‘The Waters of Mars’, recall, is roughly encapsulated in the ideas that ‘certain moments in time are fixed’, while ‘everything else is in flux, anything can happen’. We took to calling fixed moments in time – like the destruction of Bowie Base One – ‘Jonbar Struts’. Although our favourite Time Lord doesn’t here mention anything like ‘Jonbar Hinges’, it’s intimated that there are such things in tales such as ‘Genesis of the Daleks’, ‘Frontios’ and ‘Attack of the Cybermen’.
Causal determinism refutes the Doctor’s theory because, in such a world, every event is a Jonbar Strut. It cannot stomach, in principle, the idea that ‘anything can happen’. Similarly, the attribution of logical necessity to all timelessly true and timelessly false statements, which we saw was Aristotle’s move, cannot be reconciled with ‘everything in flux’, unless this flux is mere appearance, merely the way the universe seems to work according to the uneducated or ignorant.
But can history be rewritten? The Doctor seems to have changed his mind about this since ‘The Massacre of St Bartholomew’s Eve’. Elements of his theory are present in the fan-favourite episode ‘Blink’, starting with the Doctor’s description of the Weeping Angels to police detective Billy Shipton:
DOCTOR: Fascinating race, the Weeping Angels. The only psychopaths in the universe to kill you nicely. No mess, no fuss, they just zap you into the past and let you live to death. The rest of your life used up and blown away in the blink of an eye. You die in the past, and in the present they consume the energy of all the days you might have had. All your stolen moments. They’re creatures of the abstract. They live off potential energy.
BILLY: What in God’s name are you talking about?
MARTHA: Trust me. Just nod when he stops for breath.30
The Weeping Angels, also known as ‘Lonely Assassins’, are, like the Silence from the 2011 series, Doctor Who monsters for the quantum age. Their threat of ‘living you to death’ is unique, but, as Michelle Saint and Peter A. French comment, ‘being transported back in time is a far cry from being torn to pieces or having one’s brain stuffed into a metallic shell’.31 When being gazed upon, the Angels are ‘quantum-locked’ and cannot move, a conceit aimed at capitalising on the ‘observer effect’ in quantum mechanics, which dictates that precise measurements of extremely small particles cannot be made without affecting the systems of which they are a part. When not thus ‘turned to stone’, the Angels can move instantaneously and without effort, much like the phenomenon of ‘quantum tunnelling’. The science fantasy dimension of Doctor Who is uniquely situated to bring out the horrifying implications of a scientific discovery such as this, which tells us that ‘potential barriers in the quantum world are fuzzy. They do inhibit the motion of particles, but they are not the solid, impenetrable boundaries of the Newtonian world.’32 Since the Weeping Angels possess these kinds of powers, why isn’t the Doctor more worried about the threat they pose on Earth? With each human that they touch, he faces the possibility of yet another Barbara Wright cast back into history, trying to rewrite the past.
Quantum mechanics (and the Doctor’s own theory of time) suggest that while there is considerable order in the universe – particularly at the level of the ‘middle-sized dry goods’ that we interact with on a daily basis – there is far more room for the malleability of past, present and future than logical necessity or causal determinism would suggest. A robust determinism would fail in the face of quantum indeterminacy, leaving us with descriptions of causes and effects in terms of very high probabilities, not necessities. As mentioned earlier, only at the broadest possible level – across the multiverse of quantum-linked worlds – would determinism obtain, because quantum mechanics become the new ‘consistency principle’ for every world. Meantime, criticisms of Aristotelian logical necessity could note that the level of precision that we can achieve in describing logically necessary truths and falsities about ‘middle-sized dry goods’ simply doesn’t obtain at the quantum level.
The tenth Doctor explains in this first Weeping Angel episode that ‘people assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint, it’s more like a big ball of wibbly-wobbly, timey-wimey stuff’. This may be his effort to explain something that is beyond human ken, but one is tempted to say that in dialogue such as this, the tenth and eleventh Doctors often parody the programme’s own tendency to substitute pseudo-scientific gobbledygook for genuine explanations. In ‘Blink’, the Doctor tells Billy Shipton:
This is my timey-wimey detector. It goes ding when there’s stuff. Also, it can boil an egg at thirty paces, whether you want it to or not, actually, so I’ve learned to stay away from hens. It’s not pretty when they blow.
This story and later Moffat-scripted episodes like ‘The Big Bang’ and ‘The Wedding of River Song’ (2011) insinuate that when changes to history occur, even massive changes, the effects are felt instantaneously and the shift typically goes unnoticed except for one central character. In ‘The Big Bang’, this is young Amelia Pond, while in ‘The Wedding of River Song’, it’s the bearded Doctor in the ‘5.02’ paratime universe. The Doctor is often shown to be resistant to the effect of major, or even catastrophic, changes to the fabric of space-time – what he calls an ‘eye of the storm effect’. In ‘The Time Monster’ (1972), the Master’s experiments with TOMTIT produce a ‘crack in time’ with any number of freak effects, and in ‘Cold Blood’ the Doctor resists the inimical effects of a similar crack (which persists throughout the episodes of that year’s series) that erases Rory Williams from time (only the second of his many deaths!). Rory’s effacement from Amy Pond’s memory splits the difference between an instantaneous causal ‘quantum shift’ and the slow fading-out of the no-longer-actual future (sometimes called the ‘Back to the Future’ effect); in the time remaining to him, the Doctor urges Amy to keep him in memory:
DOCTOR: Keep him in your mind. Don’t forget him. If you forget him, you’ll lose him forever.
AMY: When we were on the Byzantium, I still remembered the Clerics because I am a time traveller now, you said.
DOCTOR: They weren’t part of your world. This is different. This is your own history changing.
AMY: Don’t tell me it’s going to be okay. You have to make it okay.
DOCTOR: It’s going to be hard, but you can do it, Amy. Tell me about Rory, eh? Fantastic Rory. Funny Rory. Gorgeous Rory. Amy, listen to me. Do exactly as I say. Amy, please. Keep concentrating. You can do this.
AMY: I can’t.33
However, Amy’s fighting a losing battle against quantum field theory, classical physics and brain biochemistry combined. ‘Quantum entanglement’, is the feature of quantum mechanics that poses the greatest challenge to the tradition of metaphysics, because it fundamentally undermines our notion of cause and effect as requiring a necessary connection of transfer of energy over relatively short distances. When objects as small as electrons or photons – or as large as carbon buckyballs or tiny diamonds – have interacted with each other, they become ‘entangled’, or share one or more of the same quantum states. These states can be seen to vary correlatively in both objects, despite distances between them; the objects seem to ‘communicate’ with each other, even though there is no energy transfer between them. Although in reality it is small-scale experiments which produce observable entanglements under laboratory conditions, a large-scale change of particles at one point in the universe could create a ‘ripple’ effect in their entangled partners elsewhere in the cosmos, producing a seemingly instantaneous change of great magnitude.
To produce one of these quantum shifts that reorder reality, action on the part of the time traveller in the past or future would not be sufficient; after all, any decision that a non-time traveller makes, any action they take, by definition creates change in their world, the material aspect of which will always involve quantum mechanical transformations. It is the fact that the time traveller doesn’t belong – according to the consistency principles of the world and time they’ve arrived in – that makes all the difference. For physicist Stephen Hawking, this fact is enough to prove that time travel isn’t possible. Hawking has claimed that something like historical inertia – but with a valid scientific explanation – could prevent time travel into the past in the first place:
There seems to be a Chronology Protection Agency that makes the world safe for historians by preventing travel into the past. What happens is that the uncertainty principle implies that space-time is full of pairs of particles and antiparticles that appear together, move part, and come back together again, and annihilate. These particles and antiparticles are said to be virtual because one does not normally notice their existence and one cannot observe them with a particle detector. However, if space-time is warped so that particles can come back to earlier points in their histories, the density of virtual particles will go up because one could have many copies of a given particle at the same time. This extra density of virtual particles would either distort space-time so much that it was not possible to go back in time, or it would cause space-time to come to an end in a singularity, like the Big Bang or the Big Crunch.34
Such an event seems to have occurred when the eleventh Doctor piloted the Pandorica into his own exploding TARDIS in the finale to the 2010 series. Having transmitted the Pandorica’s restoration field to every particle of space and time simultaneously to produce ‘Big Bang Two’, the Doctor has either restored the universe before the stars went out, or has created a new and very similar one. This latter option might have prompted some viewers to ask, ‘Is the Doctor now god?’ A number of long-time Doctor Who fans, however, were satisfied to ask, ‘Does this mean that Matt Smith is now the first Doctor?’
The Doctor has repeatedly asserted that fundamental changes have come about in the multiverse now that the Time Lords have no sway over it. Is it possible that when we compare classic Who’s vague presuppositions about ‘linear time’ or the ‘web of time’ to post-2005 Who, the lack of a ‘Chronology Protection Agency’ makes all the difference? And if the warping of space-time is minimal – if it’s only the Doctor whipping about the universe and a few other misguided mad scientists whose experiments produce ripples in time ‘up to point four on the Bocher scale’ (‘The Two Doctors’), what effect would that produce? The entanglement between philosophy, quantum mechanics and Doctor Who has only begun, and it is impossible to see precisely where it may take us.
One fact – take it as frightening or invigorating as you please – does remain clear: with the new wrinkles on the viability of time travel put by quantum theory, it’s possible that the past is already being changed rather frequently by our reality’s equivalent of Time Lords and their dastardly foes. We would probably be completely unaware of this if instantaneous quantum shifts occur, changing histories and memories in a flash through quantum entanglement. Perhaps history is being changed right now – on Wimbledon Common and the Eye of Orion, in Tooting Bec or Charing Cross Underground Station or Totter’s Lane, perhaps in a town called Mercy. As the Doctor reminds us: ‘The universe is big. It’s vast and complicated and ridiculous, and sometimes, very rarely, impossible things just happen and we call them miracles, and that’s the theory. Nine hundred years, never seen one yet, but this would do me’ (‘The Pandorica Opens’, 2010).
Anything could happen.