Can Justice Bao Change the Past?
A recent television drama has featured travel through time. Whatever the merits of the drama itself, we may consider the possibilities and limitations of time travel. This article concentrates upon two questions: first, whether it is possible to change historical events, and the philosophical problems involved; and second, whether it is possible to introduce technology into the past, together with the questions arising from such an undertaking.
Interestingly, within the metaphysics of time travel considered here,1 the dialogue quoted from the television drama raises a remarkably perceptive question. For under this metaphysical model, history cannot be changed. It must therefore unfold exactly as recorded — assuming that the historical account supplied by travellers from the future is accurate — without the slightest deviation.
If history must unfold exactly as recorded, what prevents it from being changed?
Suppose A travels into the past intending to murder his grandfather. If he succeeds, a contradiction arises: if his grandfather is killed, neither A's father nor A himself will ever be born. Yet if A never exists, how could he have murdered his grandfather? This is known as the grandfather paradox.
Since the assassination cannot succeed, what exactly prevents it?
Suppose A is an exceptionally skilled marksman, yet a succession of coincidences causes him to miss his target.2 Such coincidences seem to suggest that the possibility of time travel is exceedingly remote.
We shall examine this question more closely.
Beginning with the nature of time itself, we shall first consider the impossibility of changing history and the apparent coincidences involved.
1. Travelling into History
The first part outlines why a time traveller cannot change history. The argument rests upon the dimensionality and metaphysics of time, together with the constraints imposed by the structure of the universe. Examples will be employed to demonstrate the plausibility of this account.
1.1 Travel in One-Dimensional Time
In a model of one-dimensional time, time proceeds along a line. Let us suppose that its direction is towards the right.3
Within such a model, time travel consists in travelling otherwise than according to the ordinary passage of time: (a) moving towards the right faster than time ordinarily passes; or (b) moving towards the left at any speed.
Consider a person swimming in a river. Rather than drifting along at the speed of the current, he may swim downstream faster than the water, or swim against the current.
Alternatively, we may say that, at a particular moment, the time registered by the traveller's watch passes at a different rate from objective time.
Thus, if I travel back five years, I shall emerge from the time machine as the person I am today. I shall not have become five years younger.
1.2 A Single Universe
In a single-universe model, no material object can depart from that universe. We shall assume, as is customary, that this universe possesses three spatial dimensions.
Imagine a river flowing from its source to its mouth, without branches, tributaries, or obstructions.
In a universe with one-dimensional time and a single spatial universe, there can be only one occurrence of any particular place at any particular time. There cannot be another location which is numerically identical in both time and place.
Consider, for example, the North Pole on 1 January 2013. There is only one such point.
Similarly, within a river, a position specified by three measurements — depth, distance from the bank, and distance from the source — identifies one unique point. We might specify a point one foot beneath the surface, three feet from the bank, and one mile from the source.
This is the uniqueness of a spatiotemporal location.
1.3 Time Travel
The time travel discussed in this article is confined to the one-dimensional model of time and the single-universe model described above.
Thus, if I travel back to the Northern Song dynasty, I shall be present in the Northern Song dynasty within our one and only universe — the Northern Song dynasty familiar to us from history.
I shall not have entered the Northern Song dynasty of another universe, nor a Northern Song dynasty existing merely in a storybook or a dream.
1.4 History
If an event occurs at a particular time and place, then, by virtue of the uniqueness of that spatiotemporal location, that event must occur there and then.
We may demonstrate this by considering the contrary.
Suppose an event occurs at place A at time T, but that the event need not occur at place A at time T. Then either time T is not unique, or place A is not unique.
Consider the river again. Suppose there is a fish at a point one foot beneath the surface, three feet from the bank, and one mile from the source.
At that same moment, at precisely that same point, the fish must be there. It cannot both be present and absent.
By the same reasoning, an event which did not occur at a particular time cannot occur at that time.
Consequently, within a single universe possessing one-dimensional time, if we know that a historical event occurred at a particular time, we cannot prevent its occurrence at that time. Conversely, if we know that an event did not occur, we cannot bring about its occurrence at that time.
1.5 History Cannot Be Changed
Ordinarily, to change something is to transform it from one state into another, or to bring about an event which previously had not occurred.
If an object is square at one moment and round at another, it must have undergone a change between those moments, since its states at the two times differ.
Change is therefore understood in relation to time.
Time itself, accordingly, cannot be changed in this sense. Historical events, each occurring at a particular time, likewise cannot be changed.
When we travel into history, we cannot change history. Nevertheless, our actions have consequences. They influence events, but they do not alter a history which was previously otherwise.
If I travel to the Northern Song dynasty, then I have always been present in the Northern Song dynasty. That is to say, throughout the period of my visit, the Northern Song dynasty has always included my presence.
My presence certainly influences its history. Yet this is not a change, for change would require a transition from one state to another — for instance, from a Northern Song dynasty in which I was absent to one in which I was present.
To put the matter simply, we influence the present, but we do not change the present in this sense, because the present has not previously been otherwise.
1.6 Strange Coincidences
Now suppose someone unfamiliar with the philosophy of time travel attempts to change history.
A travels into the past intending to assassinate his younger self.
As he fires his gun, a bird flies past and intercepts the bullet.
He practises throwing knives, but the younger A narrowly escapes.
He then attempts to attack with his bare hands, only to step upon a banana skin and fall.
Again and again, he fails.
We know that A cannot succeed. We therefore know that something must prevent the assassination.
But can A really encounter so many banana skins?
There appear to be rather too many coincidences.
Some philosophers have argued that, if time travellers existed, an extraordinary number of coincidences would have to occur. Time travel therefore appears highly improbable, perhaps even impossible.4
We shall examine this objection in the next part.
2. Coincidences Beget Coincidences
This part argues that coincidences are not brought about by time travel itself.
I shall first use two examples to suggest that the apparent coincidences arise not from time travel, but from circumstances which are themselves even more extraordinary.
I shall then examine the motivation for deliberately arranging circumstances in which such coincidences would have to occur, arguing that this motivation is itself unusual and highly improbable.
I shall contend that, in the ordinary course of events, we cannot reasonably and deliberately plan an action during time travel which would require the occurrence of such coincidences.
2.1 Cause and Effect
Let us begin with an obvious example illustrating why the apparent coincidences are not caused by time travel.
The collapse of the World Trade Center in New York following the impact of the second aircraft depended upon the first aircraft's not having brought down the building, and upon the second aircraft's subsequently striking it.
Suppose I travel back to a time before the attacks of 11 September 2001 and tell the people around me that aircraft will strike the buildings. The attacks then occur as I predicted.
Do I possess the ability to foresee the future, or do I know what will happen because the events have already occurred?
Clearly, the latter.
I do not know what will happen in 2050, but I know perfectly well what happened in 2001.
I know because the events of 2001 have already taken place.
Similarly, A's failure to assassinate his grandfather is not something imposed by time travel as a preventive force.
A does not fail to murder his grandfather because he already knows that the attempt will fail.
Rather, because A fails to murder his grandfather, A is subsequently able to be born.
It is therefore from A's existence that we know that, should he attempt to murder his grandfather, he must fail.5
2.2 Coincidences Require Coincidences
But do incidents involving banana skins and similar accidents not require an excessive number of coincidences?
Not necessarily.
To persist in attempting something whilst knowing that the attempt is bound to fail involves a contradictory state of mind.
If a person believes that an action must fail, he cannot reasonably and sincerely attempt to perform it.
If I believe that I cannot possibly touch my foot with my nose, I shall not sincerely attempt to do so.
I might, perhaps, try to lick something on the ground and accidentally touch my foot with my nose. But that would be an accident, not a genuine and reasonable attempt to accomplish the original task.6
One might suggest that scientists would be interested in discovering precisely why such attempts must fail.
They might therefore travel repeatedly into the past and attempt to kill their younger selves by various means, hoping to investigate the details of each failure.
Yet an attempted murder is a serious matter.
If I can discover where my younger self was on 1 January 1990 — thereby enabling me to travel to that location and attempt an assassination — I have little reason to suppose that I would have forgotten what happened on that day.
A near-fatal attack upon oneself would surely be remembered for a lifetime.
It would be unreasonable to remember exactly where one was on that day whilst remembering nothing of such an extraordinary incident.
Moreover, even if only a few attempts were made, the traveller ought to remember the corresponding events from childhood and thus already know that the attempts could not succeed.7
2.3 Coincidences Remain Rare
Someone might object that changing history need not involve anything so dramatic as an assassination.
A small alteration would still be an alteration, and a minor incident need not be memorable for a lifetime.
Perhaps I have simply forgotten what happened on a particular day.
I might therefore return to that day, attempt to change history, and investigate the circumstances which prevent me from succeeding.
Yet an extension of the preceding argument reveals the difficulty with this proposal.
Changing history is not itself a particular physical action.
To change history, one would first have to know what the time traveller originally did in the past, and then perform a different action.
Suppose I return to yesterday intending to change history. What I must do is act differently from what I already know the time-travelling version of myself did yesterday.
But if I do not know what my time-travelling self did, I cannot deliberately perform an action different from it.
On the other hand, if I do know what my time-travelling self did, then I already know the circumstances which prevented me from changing history.
The experiment would merely seek to discover something I already knew.
Such an unreasonable motivation appears itself to be an unusual circumstance capable of producing the apparent coincidences.
3. Justice Bao and Cowpox Vaccination? Nobita's Time Machine?
In ordinary experience, a cause occurs either before its effect or simultaneously with it.8
Time therefore appears to possess a direction: the temporal ordering of cause and effect does not ordinarily reverse itself.
In our experience, a causal chain proceeds in one direction and does not form a circle.
If time travel is possible, however, it appears that we may be able to produce causal loops.
We must therefore ask two questions.
First, are causal loops philosophically acceptable?
Second, can time travel occur without producing any causal loop at all?
This part examines the relationship between causal loops and time travel, considering various arguments concerning their possibility.
We shall first introduce causal loops and their apparent importance in time travel. We shall then revisit several familiar examples of different kinds of causal loop, before considering their possibility and philosophical acceptability.
3.1 Causal Loops
A causal loop is a closed causal chain in which some causal connections run forwards in time and others backwards.9
Broadly speaking, a simple causal loop consists of a sequence of causally connected events in which each event is both a cause and an effect of the other events within the sequence.
For example, A causes B, B causes C, and C in turn causes A.
Together, A, B, and C form a causal loop.
Some causal loops may have an external cause which brings the loop into existence. For example, X causes A, which causes B, which causes C, which in turn causes A; yet X is not itself an effect of any event within the loop.
Other causal loops have no external initiating cause.
Such a loop is not brought into existence by any event outside itself. For the purposes of the present discussion, we may regard it as having always existed, its constituent events perpetually giving rise to one another.
Since time travel involves returning to the past, whilst ordinary causal relations generally involve causes preceding their effects, time travel can give rise to causal loops.
Let us consider some examples.
3.2 Nobita and the Time Machine
Where did Nobita's time machine come from?
The adult Nobita sent Doraemon, together with the time machine, back to the days of his younger self.
But where did the adult Nobita obtain his time machine?
Here I shall invent a continuation of the story.
Perhaps it was simply the very same machine that Nobita possessed as a boy, still in working order after all those years.
But who, then, invented the time machine?
We do not know.
In this example, the adult Nobita's possession of the time machine causes the young Nobita to possess it. Yet the young Nobita's possession of the machine also enables the adult Nobita to possess it.
This is a causal loop involving a physical object.
Consider a second example.
An inventor has dreamt of becoming a great inventor since childhood.
One day, when he is ten years old, he discovers an old notebook upon his desk. It contains designs for numerous advanced devices.
He copies the contents into a new notebook and throws the old one away.
Over the next ten years, he uses the designs to construct various inventions, including a time machine.
At the age of twenty, he looks back fondly upon the curiosity of his childhood.
He therefore enters his time machine and travels ten years into the past.
There he places the notebook which he has used for the past ten years upon the desk, secretly hoping that his ten-year-old self will discover it.
In this example, the notebook itself does not travel around the causal loop, since the younger inventor throws away the old notebook each time.
What circulates within the loop is the information contained in the notebook — particularly the design of the time machine.10
Such causal loops, lacking an external originating cause, appear to involve objects or information without an identifiable source.
We shall now consider whether this is philosophically acceptable.
3.3 Can You Accept It?
Lewis11 observes that, although we have no experience of eternal causal loops, their existence is nevertheless logically possible.
Yet the apparent existence of causal loops, together with the common assumption that time travel would probably produce them, makes time travel more difficult for many people to accept.
Suppose I travel today into the year 1990.
Even if I do not directly influence my younger self, or anything in his immediate surroundings, it would be difficult — perhaps impossible — to demonstrate that my presence in 1990 had absolutely no influence upon anything which would subsequently affect me.
After all, the world is a small place. Indirect influence is certainly possible.
But suppose we construct a space divided by a thin membrane.
Objects may pass from the left side to the right, but never from the right to the left.
We may then arrange for a time traveller to move from left to right, travelling into the past at the very instant he passes through the membrane and arrives on the right-hand side.
Because of the membrane, no causal loop can arise: once I have reached the right-hand side, I can never influence anything on the left, including my own earlier self.12
Thus, even if time travel in an unrestricted spatial model would give rise to causal loops, at least indirectly, time travel does not necessarily entail their existence.
Furthermore, causal loops need not be unacceptable.
Even within a model without spatial restrictions, we may accept the possibility of causal loops.
'Where did the universe come from?'
Perhaps we have no answer to the question of the universe's origin.
Similarly, we need not necessarily possess an answer to the question of how a causal loop began.
It may be that some things exist without requiring a cause at all, and that we have simply not yet encountered such things.
Conclusion
In summary, we have insufficient reason to reject the possibility of time travel merely because it may give rise to causal loops.
I maintain that time travel is logically possible. Moreover, within a single universe possessing one-dimensional time, history cannot be changed, and we should not expect time travel to produce an extraordinary abundance of coincidental events.
Contemporary discussions of time travel have increasingly involved questions of causation,13 the metaphysics of time,14 the philosophy of physics,15 and the philosophy of science.
A more thorough examination of these questions would, I fear, lie beyond my abilities.
I therefore welcome correction from those whose learning and understanding exceed my own.
1 David Lewis, 'The Paradoxes of Time Travel'; Nicholas J. J. Smith, 'Bananas Enough for Time Travel?', British Journal for the Philosophy of Science, vol. 48, pp. 364–367. ⇑
2 Lewis, 'The Paradoxes of Time Travel', pp. 148–149. ⇑
3 This article does not consider universes possessing additional time-like dimensions, in which temporal passage might resemble a plane or two parallel lines. See, for example, Jack Meiland, 'A Two-dimensional Passage Model of Time for Time Travel', Philosophical Studies, vol. 26, pp. 153–173; and, on 'hypertime', Geoffrey Goddu, 'Time Travel and Changing the Past', Ratio, vol. 16 (2003), pp. 16–32. ⇑
4 Paul Horwich, Asymmetries in Time (Cambridge, Massachusetts: MIT Press, 1988), Chapter 7. ⇑
5 Smith, 'Bananas Enough for Time Travel?'. ⇑
6 Ibid. ⇑
7 Ibid. ⇑
8 Some philosophers accept the possibility of backward causation; see, for example, Michael Dummett, 'Bringing about the Past', The Philosophical Review, vol. 73 (1964), pp. 338–359. Others reject it; see, for example, David Hume, A Treatise of Human Nature. In any case, it is generally accepted that ordinary experience supplies no clear instances of effects preceding their causes. ⇑
9 Lewis, 'The Paradoxes of Time Travel'. ⇑
10 Richard Hanley, 'No End in Sight: Causal Loops in Philosophy, Physics and Fiction', Synthese, vol. 141 (2004), pp. 123–152. ⇑
11 Ibid., referring to Lewis, 'The Paradoxes of Time Travel'. ⇑
12 Bradley Monton, 'Time Travel Without Causal Loops', The Philosophical Quarterly, vol. 59 (2008), pp. 54–67. ⇑
13 Lewis may be said to have exerted a considerable influence upon philosophical discussions of this subject for some twenty or thirty years. ⇑
14 Huw Price has written extensively on this subject, and his works, ranging from introductory discussions to academic studies, are well worth consulting. ⇑
15 John Abbruzzese, 'On Using the Multiverse to Avoid the Paradoxes of Time Travel', Analysis, vol. 61 (2001), pp. 36–38. ⇑