Now that we’ve shown time travel to be, at the very least, philosophically possible we’ve cleared the way for the building of a real time machine. So, put down the book, get out some blue planks of wood and a flashing light, and get to work. Just keep an eye out for Scaroth—you can’t miss him, his face looks like a bowl of green spaghetti with an eye in the middle (which brings me to the question: How was he able to see out of his human disguise with two eye-holes? 27 Mmm . . . now there’s a philosophical question).28
6
Could the Daleks Stop the Pyramids Being Built?
PHILIP GOFF
The Daleks decide to time travel to the past in order to conquer Ancient Egypt, and in the process of doing so they exterminate every single Egyptian slave builder and stop the pyramids from being created. As the result of their evil destruction a new history without pyramids begins to unfold. Our only hope is that the Doctor can go back and foil the Daleks’ evil plan. But will he succeed?
Such a story seems, on the face of it, paradoxical. On the one hand we want to say that the Daleks have made it the case that there were no pyramids in ancient Egypt: they’ve stopped them from being built at all. But on the other hand we want to say that there were Pyramids in ancient Egypt: they existed before the Daleks went back to the time before the pyramids and stopped them from being built. We seem to face a contradiction: it both was and wasn’t the case that there were pyramids in ancient Egypt.
Whether or not we can avoid this paradox depends on our theory of time. Neither of the two most popular philosophical theories of time, presentism and eternalism, are able to make sense of changing the past. However, there is a middle way between these two views, the growing block theory, which allows us to make sense of this possibility of time travelers altering history.
There’s No Time Like the Present
Presentism is the theory of time closest to common sense. According to presentism only events in the present moment exist: only the event of your reading these words, and everything simultaneous with it, are part of reality. Events in the future, such as your finishing this chapter or the setting up of human colonies on Mars, don’t exist (although they perhaps will exist). Events in the past, such as your reading of the first line of this chapter or the Battle of Hastings in 1066, don’t exist (although they did exist). For the presentist, there’s an absolute fact about what time is ‘now’: quite simply ‘now’ is the only time that exists.
If presentism is the correct view of time, then time travel isn’t possible at all. If the past and future don’t exist, then we can’t travel to them (you can’t go to a land that doesn’t exist). If the Doctor lives in a presentist world, then stepping into the TARDIS and setting the co-ordinates for ancient Egypt would be tantamount to committing suicide; he’d be traveling straight into non-existence.
What’s So Special about Now?
Eternalists believe that all events in time exist equally. The Battle of Hastings in 1066, the setting up of human colonies on Mars, and your reading of this chapter, are all equally real. For the presentist ‘now’ is absolute, but for the eternalist ‘now’ is relative. For William the Conqueror, the eleventh century is ‘now’. For the people setting up the first human colonies on Mars, the fifty-first century is ‘now’. For you reading this chapter (unless my reputation survives my death by a bit) the twenty-first century is ‘now’.
For the eternalist, ‘now’ functions a bit like ‘here’. For me as I write this chapter, London is ‘here’. Perhaps you’re reading this in New York, in which case New York is ‘here’ for you (and ‘there’ for me). For the people in Timbuktu, Timbuktu is ‘here’. There’s no absolute fact about where ‘here’ is. God couldn’t look down on space from Heaven, and say, “Ah, there’s where ‘here’ is,” pointing to some specific place slightly left of the Milky Way. Similarly, according to the eternalist, when God looks down on the whole of time, he doesn’t look at the twenty-first century (or any other time) and say, “Oh yes, that’s when ‘now’ is.”
Presentism seems closer to our common-sense picture of time, but eternalism is arguably closer to our science. If there’s an absolute present, as the presentist supposes, then it consists of a large number of events—my writing this chapter in London, Emma in New York getting on a bus, Yujin in Japan eating breakfast—which all happen at the same time. But according to Einstein’s special theory of relativity, for any two events, say my writing this chapter in London and Yujin’s eating breakfast in Japan, there’s no absolute fact of the matter as to whether those two events happened at the same time or at different times. If there’s no absolute fact of the matter as to whether two events happen ‘at the same time’, then there can be no absolute fact of the matter over which events are happening ‘at the present time’. It looks as if Einstein’s theory rules out there being an absolute present, and so is inconsistent with presentism.
In an eternalist world, all times exist and so time travel is possible. All that needs to happen for an eternalist world to contain time travel into the past is that causation sometimes work ‘backwards’: some of the causal processes in that world begin at a later time and end an earlier time). If the Doctor’s operating the TARDIS controls in the twenty-first century causes the TARDIS to appear in the eleventh century, then we can truly say that the world contains time travel into the past.
While eternalism allows for travel into the past, it doesn’t allow for time travelers to change the past. This is because eternalism is a static picture of time, which is to say the facts about time as a whole don’t change. The facts about time as a whole are eternally laid out before God, as it were. If God looks to ancient Egypt, he either does or doesn’t see the pyramids. It can’t be the case that ‘earlier’ God sees the pyramids in ancient Egypt, and ‘later’ God sees no pyramids in ancient Egypt because they’ve been destroyed by the Daleks. There is no ‘earlier’ and ‘later’ in this context, after all God is looking down on the whole of time! The story of the Daleks changing the past by preventing the pyramids from being built is incoherent on the eternalist view of time.
This doesn’t mean that time travelers can’t affect the past, according to the eternalist picture of things. It might be among the eternal facts of time that the Doctor began his life in the far future and then went back in time and helped build the pyramids. We can make coherent sense of this so long as it doesn’t involve a change in the eternal facts of time. Either it’s among the facts of time that the Doctor helped build the pyramids or it isn’t. It can’t ‘earlier’ be the case that the Doctor didn’t help build the pyramids, and ‘later’ the case that the Doctor did help build the pyramids; ‘earlier’ and ‘later’ make no sense when we’re talking about the whole of time. In an eternalist world, time travellers like the Doctor and the Daleks can affect the past, but can’t change the past.
The Growing Block Theory—A Middle Way
The presentist picture doesn’t allow us to go to the past at all, because the past doesn’t exist. The eternalist picture doesn’t allow us to change the past, because the facts of time are unchangeably fixed. What we need is a theory of time in which the past exists, and so is there for us to pop over to, but in which time isn’t static and unchangeable. This is exactly what we get from the growing block view of time, which is a kind of middle way between presentism and eternalism. The basic idea is that events in the past and present exist, but events in the future don’t. The Battle of Hastings and your reading this chapter are part of reality, but the setting up of human colonies on Mars isn’t. The flow of time is a continuous process of new events coming into being and remaining in existence.
The growing block view of time gives us everything we need to make sense of genuinely changing the past. The past exists, so we can get into a TARDIS and go back and visit it. But the events of time aren’t static and unchanging, but flowing and dynamic. When God looks down at the world he witnesses the growing course of history, as more and more events come into existence. Jus
t as in the presentist conception of things, in the growing block world there’s an absolute fact about when ‘now’ is, but for a slightly different reason. For the presentist, ‘now’ is the only time that exists. For the growing block theorist, ‘now’ is not the only time that exists (the past exists too), but ‘now’ is the most recent time to have come into existence. Each “now” is the latest addition to reality.
What happens when the Daleks travel back into the past and exterminate the builders of the pyramids? If causation in the past works as it does in the present, the Daleks’ intervention will create a new causal history without pyramids. Assuming that change happens at exactly the same rate in the past as it does in the present, this new history will replace the old history at exactly the same rate as the absolute present advances. If the absolute present is in the twenty-first century A.D. when the Daleks intervene in history in the twenty-first century B.C., then, by the time the absolute present reaches thirty-first century A.D. (one thousand years later), the new history (the one lacking pyramids) will have advanced to the eleventh century B.C. (one thousand years later, remember B.C. years count backwards!). By the time the absolute present reaches the forty-first century A.D. (another thousand years later), the new history will have advanced to the first century A.D. (another thousand years later). It’s only when the absolute present reaches the sixty-first century A.D. that the new history will reach the twenty-first century A.D.
Crucially, because they advance at exactly the same rate, the new history will never catch up to the absolute present, and so will never completely ‘write over’ the old history. When the ‘new history’ (the one lacking pyramids) is up to the first century A.D., the second century A.D. onwards will still contain the ‘old history’ (the one where people have historical records of there being pyramids). If there’re no time travelers, the flow of time will be just the flow of new events coming into existence in the present. But if there are time travelers journeying to the past and intervening in the course of history, then the flow of time will also be the flow of new histories slowly replacing old ones (at exactly the same rate as new events come into existence in the present). And so, even if the Doctor is unable to foil the Daleks and stop them preventing the pyramids from being built, it’ll always be possible for him to get back into the TARDIS, return to the absolute present, and find himself in a world which is causally continuous with the old history, containing memories and historical documents which make reference to pyramids.
Of course, this is not how Doctor Who script writers generally present things. Generally, in time travel stories the old history is completely eliminated by intervention in history—in the first Back to the Future film Marty McFly slowly starts to fade as the history containing him is wiped from existence. But this is just because script writers are not thinking carefully enough about the metaphysics of time. If Doctor Who script writers adopted the growing block view of time as I have described it, then they could write time travel stories free from concern about the potential paradoxes of time travel (they wouldn’t, for example, have to resort to avoiding paradox by introducing flying monsters to sterilize wounds in time, as we find in the “Father’s Day” story from 2005).
To Be in the Past or Not to Be in the Past—What Is the Question?
Suppose the Daleks destroy the pyramids, so that they no longer exist in the past. The new history with no pyramids slowly begins to replace the old history with pyramids. But in the absolute present, because the people are causally continuous with the old history (remember, the ever advancing present will always contain people causally continuous with the old history because the new history will never be able to catch up with it), they’ll have historical documents referring to the pyramids. Are those historical documents now incorrect, given that the pyramids are no longer contained in the past? Surely there’s still a sense in which those documents are correct because, although there are no pyramids in the past, there used to be pyramids in the past.
To make sense of the growing block view of time we need to distinguish two senses in which an event can be said to ‘be in the past’. First, an event might ‘be in the past’, in the sense that it occurred in the concretely existing past. In this sense, the pyramids, once the Daleks have destroyed them, cease to ‘be in the past’. Call this sense of being in the past, ‘concrete past existence’. Second, an event might ‘be in the past’ in the sense that it used to exist in the concretely existing past. Even when the Daleks destroy the pyramids, it still remains true that, first time round (as it were), the pyramids were part of the concrete past. Call this sense of being in the past ‘abstract past existence’. Even after the Daleks have destroyed the pyramids, we can say that the historical records of pyramids in the absolute present are correct, because the pyramids still have abstract past existence (even though they lack concrete past existence).
Hold on: Didn’t the Doctor and Rose travel to the year five billion to witness the end of the Earth? (“End of the World,” 2005). It may seem as if the growing block view of time is limited because it can’t allow travel to the future. This is of course true in a sense: we can’t travel to the future of the absolute present, because the future of the absolute present doesn’t exist. However, we need not suppose that our present (2010) is the absolute present. Perhaps the Time Lords of Gallifrey exist (or rather existed before they were annihilated from all of space and time in the Time War) in the absolute present a billion years ahead of our present. In this way we can make sense of the Doctor’s adventures in our future as well as our past.
The Daleks Can Stop the Pyramids from Being Built!
If only the present moment exists, then the Daleks couldn’t travel to the past and stop the pyramids being built, because the past no longer exists. If the past, present, and future all exist equally, then it could be among the eternal facts of time that the Daleks traveled to ancient Egypt, but they couldn’t stop the pyramids being built (given that they were built), because the facts of time as a whole are eternal and unchanging. If the past and the present exist, but the future doesn’t, then the Daleks can travel to ancient Egypt (because it’s there to travel to), and they can also change it (because time in such a world is dynamic and flowing rather than static and unchanging). The growing block theory of time allows us to make sense of the Daleks stopping the pyramids from being built. But don’t panic; even if the growing block theory is true, I’m sure the Doctor won’t let them get away with it!
7
The Doctor on Reversed Causation and Closed Causal Chains
WILLIAM EATON
The Doctor is usually too occupied with practical concerns to participate in any serious metaphysics, and justifiably so. But in “Blink” (2007), the practical concerns themselves require the Doctor to present some of his views on the nature of time, time travel, and causation. The Doctor is trapped, without his TARDIS, in 1969, and the TARDIS itself is in danger of being destroyed, taking the Earth’s solar system along with it. The Doctor’s only chance to retrieve the TARDIS and prevent the calamity is to convince a woman in 2007 named Sally Sparrow to send it back to him. In the course of doing this he explains the nature of time. The explanation is brief and, due to his pressing predicament, left unjustified. All we get concerning the Doctor’s philosophy of time are the following fragmentary claims:People don’t understand time. It’s not what you think it is. [It’s] complicated, very complicated. People assume that time is a straight progression of cause to effect. But from a non-linear, non-subjective viewpoint, it’s more like a big ball of wibbly-wobbly, timey-wimey stuff. (“Blink,” 2007)
This is all we get, the Doctor doesn’t have time to elaborate—and as we’ll see, there’s a sense in which it isn’t possible for him to elaborate. The statement is so brief one might hesitate to describe it as a coherent philosophy of time at all. But I think the Doctor says just enough to make describing his philosophy of time possible. His statements are open to a number of interpretations, but David Lewis pro
vides a model by which we can develop the statements into an insightful theory of time, which many philosophers today might agree with.
Let’s start by considering Lewis’s philosophy of time. According to Lewis, the universe is a four-dimensional manifold of events. There’re the normal three spatial dimensions of height, width, and depth, and there’s also one temporal dimension. This view is sometimes called four-dimensionalism. It’s actually very similar to the original model H.G. Wells himself describes in The Time Machine. People usually move in one direction in time, but it’s at least possible to move in both.
In his article, “The Paradoxes of Time Travel,” Lewis’s goal is to show that time travel is logically possible, which means the apparent paradoxes raised by time travel can be solved. According to Lewis’s view there’s only one temporal dimension that, like the spatial dimensions, is more or less fixed. This can be contrasted with other views of time travel, less plausible in my opinion, in which events, such as decisions or actions, can branch or split, creating multiple temporal dimensions.
Anatomy of a Time Traveler
To explain the possibility of time travel, Lewis introduces the important distinction between external time and personal time. By external time Lewis means the complete, objective temporal dimension comprising the entire history of the universe. Personal time shouldn’t be seen as a separate temporal dimension existing outside of external time. It’s composed, rather, of the events that comprise the personal history of the time traveler. It might be helpful to think of personal time as the time measured by the traveler’s watch. For example, imagine a time traveler, perhaps Martha Jones, who lives normally for twenty-seven years and time travels for the first time in 2007, after she turns twenty-eight. If she travels back in time to 1969 then parts of the twenty-eighth year of her life actually occur before she was born. As Lewis puts it:A time traveler, like anyone else, is a streak through the manifold of space-time, a whole composed of stages located at various times and places. But he is not a streak like other streaks. If he travels toward the past he is a zig-zag streak, doubling back on himself. If he travels toward the future, he is a stretched-out streak. And if he travels either way simultaneously, so that there are no intermediate stages between the stage he departs and the stage he arrives and his journey has zero duration, then he is a broken streak. (“The Paradoxes of Time Travel,” p. 69)
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