Time Travel and Philosophy Dr. Alasdair Richmond A.Richmond@ed.ac.uk

Overview 1) What might :me travel be anyway? 2) Grandfather paradoxes 3) Two senses of change 4) Causal loops 5) Where next?

1) What might time travel be anyway? The key text for this lecture is a paper by American philosopher David Lewis (1941-2001): The Paradoxes of Time Travel, American Philosophical Quarterly, Volume 13, 1976, pp. 145-51 Lewiss 1976 Deni:on: What is :me travel? Inevitably, it involves discrepancy between :me and :me. Any traveler departs and then arrives at his des:na:on; the :me elapsed from departure to arrival (posi:ve, or perhaps zero) is the dura:on of the journey. But if he is a :me traveler, the separa:on in :me between departure and arrival does not equal the dura:on of the journey. (Lewis, 1976, page 145.)

Lewiss deni:on requires a dis:nc:on between two ways of registering :me: external :me and personal :me.

External :me is :me as it is registered by the world at large, whether measured by :des, sun-rises, receding galaxies etc.

Personal :me is :me as registered by the traveller and anything travelling with her, e.g. heart-beat, watch-hands, accumula:on of memories, greying of hairs etc.

For us non-:me-travellers, external :me and personal :me share the same dura:on and direc:on.

A #me travel journey has dierent dura#ons and/or dierent direc#ons in external and personal #me.

Forward :me travel: personal :me and external :me have the same direc5on but dier in dura5on, e.g. journeying from 2013 to 2063 in ve minutes of personal :me. In personal and external :me, a forward :me journey ends a`er it begins.

Backward :me travel: personal :me and external :me dier in direc:on (and maybe dura:on) e.g. journeying in ve minutes of personal :me from 2013 to 1863.

A backward :me journey ends a`er it begins in personal :me but ends before it begins in external :me.

Einsteins Special Theory of Rela:vity (1905) says :me passes at dierent rates in references-frame in rela:ve mo:on.

This :me-dila:on eect allows what Lewiss deni:on would regard as forward :me travel. E.g. if I travel suciently fast rela:ve to the Solar System, I can witness billions of years of the Suns future in mere decades (or less) of my personal :me.

Einsteins General Theory of Rela:vity (1918) allows that :me in dierent reference-frames can dier not only in dura:on but also in direc:on and may allow backward :me travel.

Kurt Gdel (1903-78) in 1949 solved Einsteins eld-equa:ons for a world where its possible to travel between any points in space and :me.

2) Grandfather Paradoxes Lewis thought backward #me travel can be logically possible. However, he did not argue that :me travel is physically possible, s:ll less that :me travel actually occurs.

Lewis thought some backward :me journeys can be consistently described, although :me-travel worlds might seem very weird to us.

But if backward :me travel is possible, what about the notorious Grandfather Paradox?

If you could travel back in :me, could you not then travel back and assassinate one of your Grandfathers as a child, i.e. before he has any chance to become a parent?

If Grandfather was assassinated in infancy, one of your parents wouldnt then exist and neither would you.

But if you dont exist, how do you go back and assassinate Grandfather?

Grandfather seemingly survives and does not survive. So it seems your mission can succeed if and only it fails. Paradox ahoy?

The Grandfather Paradox argument runs like this: 1.If backward :me travel is possible, contradictory outcomes could be created.

2.Contradictory outcomes cannot be created. Backward :me travel is not possible. Assuming reality must be consistent (i.e. no contradic:ons are possible), this argument would be devasta:ng if it was successful. But is it?

Lewis thought contradictory outcomes cannot occur. However, he did not accept that backward :me travel must allow contradictory outcomes.

Lewis emphasises that the term possible can mean dierent things according to context.

Confusion follows if these dierent contexts are not kept clearly dis#nguished and the Grandfather Paradox argument is guilty of such confusion.

In assessing what :me-travellers (or anyone else) can do, we need to consider what is possible rela:ve to a set of facts.

Philosophers talk about compossibility, meaning possibility rela:ve to some states of aairs, facts or circumstances.

Lets assume I am despatched from 2013 on a mission to assassinate Adolf Hitler in Vienna in 1908.

Can my mission succeed? Well, yes and no yes in that Hitler isnt bullet-proof and I am present in his vicinity, but no in that my target doesnt die un:l 1945.

My assassina:ng Hitler in 1908 is compossible with some facts about the set-up (e.g. my gun is working, Hitler is not wearing armour) but not other facts, (like Hitler dying in 1945).

My killing Hitler in 1908 is clearly inconsistent with his surviving un:l 1945. If death is a one-o opera:on, I cant kill someone in 1908 who survives un:l 1945.

However, my presence in 1908 Vienna is perfectly consistent with Hitlers survival.

On Lewiss analysis, backward #me travel is consistent (i.e. logically possible) if the consequences of a travellers ac#ons are in place in the history whence the traveller departs.

A non-:me-travel example (adapted from Lewis): can I speak Gaelic? In a sense, I can I have a func:oning larynx and have learned at least one language already. But in another sense, I cant speak Gaelic I simply never learned enough of the words, the syntax or the grammar.

But theres no paradox in this case my speaking Gaelic is compossible with some facts but not others.

Its a familiar fact that what is possible rela:ve to one set of facts may not be possible rela:ve to a more inclusive set.

In Lewiss model, each :me happens one and once only. Backward :me travel does not require one and the same moment to happen repeatedly as if Vienna got through 1908 without my presence and then (somehow) I travel back and make 1908 happen again dierently, this :me with me there.

If :me travellers were not present in 1908 then nothing later :mes could do can alter that fact.

But then again if future travellers were present in 1908 then nothing later :mes could do can alter that fact either.

Okay, so backward :me travel is logically possible provided the history arrived in is consistent with the history departed from.

Lewiss analysis might seem to suggest that a :me traveller is completely impotent in the past constrained merely to observe without aec:ng the past in any way.

Worse, it might seem as if :me travel only makes sense if the backward traveller is only present in the past as a totally impalpable ghost.

But this ghost traveller idea rests on a mistake

3) Two senses of changing the past It sounds as if consistency demands that a :me traveller in the past is ulerly powerless.

Put it another way, can a :me traveller change the past without crea:ng paradoxes?

Just as the no:on of possibility needs to be unpacked to prevent ambiguity, so too does the no:on of change.

To adapt a point from Lewis, consider what might be called replacement change and counterfactual change.

A replacement change: a glass is dropped on a concrete oor and shalers. So this change involves an intact glass being replaced by a mass of glass shards.

Lewis thinks concrete objects can suer replacement changes but :mes (past, present or future) cannot.

A counterfactual change: the outcome of the balle of Waterloo on June 18th 1815 was crucially aected by the arrival of a Prussian army under Field Marshall Blcher.

Suppose its true that without Blchers interven:on, the French forces would have triumphed. We could then assert the counterfactual If Blcher had been delayed, Napoleon would have won.

So Waterloo happened only once, and issued in an Allied victory, but Blcher changed the outcome of Waterloo.

It is not that Waterloo originally issued in a French victory, which Blchers arrival somehow replaced.

Rather, Blchers impact can be assessed counterfactually in terms of what would have happened if he hadnt arrived.

Lewis thinks travellers in the past can only change history in the counterfactual sense, i.e. history would have been dierent had the traveller not been present.

Suppose my arrival in 1908 Vienna so startles the young Hitler that he leaps out of the path of an oncoming tram which would otherwise have cut him down. Suppose the correct counterfactual is then If I hadnt appeared when I did, Hitler would have died in 1910. Alas in this case, I have changed history, albeit counterfactually and not how I would have wanted. So travellers can help to make the past what it was without paradox, provided that their impact on past events is counterfactual and not by replacement.

4) Causal Loops Theres another kind of :me travel example that poses problems that dont involve consistency but informa5on.

The cases in ques:on involve causal loops. A causal loop is a chain of events which loops backwards in #me so that an event proves to be among its own causes.

A couple of examples:

Imagine you travel back in :me to 1588 equipped with a copy of Shakespeares complete works.

On arrival, you make the acquaintance of struggling young player, Will Shaxberd (as he then called himself).

You then read Will the following lines: What a piece of work is a man,

How noble in reason,

How innite in faculty,

In form and moving, how express and admirable

In short, you read Will a great speech from Hamlet. Furthermore, you then let Will copy all the contents of the Complete Works that youve brought with you.

Now there is no inconsistency here no Grandfather Paradoxes involved yet there is something odd.

Where does the informa:on come from? Or to put it another way, who writes Hamlet?

Another example (a`er Lewis 1976): youre sirng at home one evening when the telephone rings.

When you answer, an oddly familiar voice says Dont say a word. Write these instruc:ons down and follow them to the leler.

The instruc:ons prove to be those for building and opera:ng a :me machine. You follow the instruc:ons and the machine takes you into the recent past. On arrival, you dial your own phone number and say into the receiver Dont say a word. Write these instruc:ons down and follow them to the leler.

How do you know how to build an :me machine? Well, your later self knows how because you remember hearing the instruc:ons as your earlier self.

In turn, your earlier self knows how because of being instructed by your later self.

But where does the informa:on come from? Perhaps surprisingly, Lewiss answer is: there is no answer.

The instruc:ons for building a :me machine in Lewiss case, or the text of Hamlet as above, in a real sense do not come from anywhere they simply are.

On the face of it, this no answer answer isnt very sa:sfactory. Surely we have stories for the origins of informa:on and informa:on just doesnt spring into being from nothing?

Well, Lewis says, it is important to dis:nguish between asking where an event comes from and asking where an en:re chain of events comes from. The rst ques:on is perfectly sensible but the second maybe less so.

When it comes to explaining an event, we can usually appeal to some earlier event.

I am alive now in part because of facts about my parents. Those facts in turn reect facts about human evolu:on, the origins of the Milky Way and the history of the universe But where does the whole chain come from?

In eect, Lewis says, causal chains come in three kinds: 1. Innite linear 2. Finite linear 3. Finite non-linear

Innite linear: each event has a cause, and those causes in turn have causes, and so on ad innitum. The chain as a whole has no beginning but recedes to innity.

Finite linear: some chains terminate in events that are causes but that do not themselves have causes. Quantum physics and Big Bang Cosmology take very seriously the idea that such uncaused chains occur. Stephen Hawking: asking what is before the Big Bang is like asking what lies to the North of the North Pole.

The nite non-linear case is that involving a causal loop, i.e. where a causal chain loops back on itself so that an event is (partly) a cause for itself.

Lewiss point is that all three kinds of chain are equally mysterious when it comes to their ul:mate origin.

A causal loop is no more (or less) hard to explain than any other kind of causal chain.

5) Where next? There are lots and lots of philosophical :me travel ques:ons we havent looked at and which I can only men:on in passing.

For one thing, :me travel poses some interes:ng problems in persistence and iden:ty.

Consider the Lewis telephone call case again: here, we seemingly have two versions of the same person exis:ng in dierent places at the same (external) :me.

How might #me travel confer the power of biloca#on?

Lots of interes:ng philosophical ques:ons arise from the physics of :me travel. What sort of physical laws might occur in a world that permiLed #me travel? Physicist David Deutsch and philosopher Michael Lockwood argue that physically realis:c :me travel can only occur if there are many histories. They argue thus: backward-travelling systems face curious restric:ons on their ac:ons. Otherwise physically possibly set-ups yield impossible outcomes.

Autonomy: the causal powers of a physical system should reect only local facts about that system and should not depend on the state of the universe as a whole.

Therefore, physically realis:c :me travellers must also make a transi:on into a dierent branch of history.

So I can travel to 1908 and kill Hitler the Hitler I kill resides in one branch and the Hitler from my history resides in another.

No paradox because the Hitler who dies in 1908 and the one who dies in 1945 are dis:nct individuals in dierent branches.

Is #me travel between branching histories really #me travel?

Stephen Hawking famously asked If :me travel is possible, where are the :me travellers?

In a similar spirit, philosopher Paul Horwich has argued that if backward :me travel did occur, we would know about it because :me travellers from the future would trail long chains of unlikely coincidences in their wakes.

Would #me travellers from the future give themselves away?

Work by philosopher John Earman suggests that if physical :me travel mechanisms were constructed, they would be uncontrollable. Once constructed, it would be physically impossible to predict what they would do.

When physicists discuss :me travel, they tend to talk about closed :melike curves (CTCs)

CTC: a path through space:me that returns to the very point whence it departed but never exceeds the speed of light.

The model universes described by Kurt Gdel are innite space-:mes that have CTCs through their every point.

It remains unclear whether the successful union of general rela:vity and quantum mechanics (the long-sought theory of quantum gravity) will allow CTCs.

A CTC-generator is not a vehicle, a travelling device like those familiar from c:on, but rather a region of curved space-:me.

Hawkings Where are the travellers? problem threatens some scenarios more than others.

If our universe were an innite, CTC-lled, Gdel universe, the observed absence of :me travellers would be very puzzling.

However, physicists also discuss localised CTC-generators. Local CTC-devices all have the feature that they facilitate access to the past only from the rst moment when a CTC is generated. Suppose the rst CTC-generator ever made comes on-line in 2015 later :mes could use it to travel back to 2015 but no :mes before that could be accessed.

So maybe :me travel is possible but not just yet

For a full list of references to works cited, plus some quotes and diagrams, please see the hand-out that accompanies this lecture.

Thank you very much.