1/18/2018 1 Phil 2: Puzzles and Paradoxes Prof. Sven Bernecker University of California, Irvine The Possibility of Time Travel Is Time Travel Possible? • Is time travel logically possible or does it involve contradictions? • Is it possible to traverse, say, 100 years in external time in just 10 minutes of personal time (as is the case in forward time travel)? • Is it possible that later moments in personal time correspond to earlier moments in external time (as is the case in backward time travel)? 2 McTaggart on time travel: • Only the B-series of time, which holds that all temporal positions (in A-series terminology ”past,“ “present“ and “future“) are equally real, allows for travel into the past and future. • If the A-series of time is true, then time travel moves the temporal position called “present“ either backward (in the case of travel into the past) or foreward (in the case of travel into the future). But it would not be possible to travel into a temporal position other than the present. 3 Even if we assume the B-series of time, there are three reasons to be skeptical of the possibility of time travel: - Backward causation - Causal loops - Grandfather paradox 4 LiYuxi LiYuxi LiYuxi 1/18/2018 2 Backward Causation Worry: Time travel into the past necessarily involves backward causation with respect to external time. Example: The traveler punches her face before she departs and causes her eye to blacken centuries ago. The idea of the effect preceding its cause (in external time) is incoherent. Therefore, time travel is incoherent. Q: Is the idea of the effect preceding its cause (in external time) coherent? A: It depends on one‘s theory of causation 5 The Main Theories of Causation: 1. Regularity Theory of Causation: An event a of type A causes an event b of type B if a and b actually occur and A-type events are regularily followed by B- type events. (David Hume) 2. Cause as INUS Condition: An event a causes an event b, if a is an insufficient but necessary part of a complex condition, which is unnecessary but sufficient to bring about event b. (John Mackie) 6 3. Probabilistic Theory of Causation: An event a causes an event b, if, given the occurrence of a, the probability of the occurrence of b is higher than the probability of the occurrence of b would have been if a had not occurred. (Hans Reichenbach) 4. Causation as Counterfactual Dependence: An event a causes an event b, a had not occurred, then b would not have occurred. (David Lewis) 7 Upshot: • Of the four main theories of causation only the regularity theory assumes that an effect must be preceded by its cause. • The INUS condition, the probabilistic theory and the counterfactual theory of causation are compatible with backward causation. 8 1/18/2018 3 Causal Loops • Causal Loop: A closed causal chain in which some of the causal links are normal in direction and others are reversed. ...

1. 1/18/2018
1
Phil 2: Puzzles and Paradoxes
Prof. Sven Bernecker
University of California, Irvine
The Possibility of
Time Travel
Is Time Travel Possible?
• Is time travel logically possible or does it involve
contradictions?
• Is it possible to traverse, say, 100 years in external time in
just 10
minutes of personal time (as is the case in forward time travel)?
• Is it possible that later moments in personal time correspond
to
earlier moments in external time (as is the case in backward
time
travel)?

2. 2
McTaggart on time travel:
• Only the B-series of time, which holds that all temporal
positions
(in A-series terminology ”past,“ “present“ and “future“) are
equally
real, allows for travel into the past and future.
• If the A-series of time is true, then time travel moves the
temporal position called “present“ either backward (in the case
of
travel into the past) or foreward (in the case of travel into the
future). But it would not be possible to travel into a temporal
position other than the present.
3
Even if we assume the B-series of time, there are three
reasons to be skeptical of the possibility of time travel:
- Backward causation
- Causal loops
- Grandfather paradox
4

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1/18/2018
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Backward Causation
Worry: Time travel into the past necessarily involves backward
causation with respect to external time. Example: The traveler
punches
her face before she departs and causes her eye to blacken
centuries
ago. The idea of the effect preceding its cause (in external time)
is
incoherent. Therefore, time travel is incoherent.
Q: Is the idea of the effect preceding its cause (in external time)
coherent?
A: It depends on one‘s theory of causation

4. 5
The Main Theories of Causation:
1. Regularity Theory of Causation:
An event a of type A causes an event b of type B if a and b
actually occur and A-type events are regularily followed by B-
type events. (David Hume)
2. Cause as INUS Condition:
An event a causes an event b, if a is an insufficient but
necessary part of a complex condition, which is unnecessary
but sufficient to bring about event b. (John Mackie)
6
3. Probabilistic Theory of Causation:
An event a causes an event b, if, given the occurrence of a,
the probability of the occurrence of b is higher than the
probability of the occurrence of b would have been if a had
not occurred. (Hans Reichenbach)
4. Causation as Counterfactual Dependence:
An event a causes an event b, a had not occurred, then b

5. would not have occurred. (David Lewis)
7
Upshot:
• Of the four main theories of causation only the regularity
theory assumes that an effect must be preceded by its cause.
• The INUS condition, the probabilistic theory and the
counterfactual theory of causation are compatible with
backward causation.
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1/18/2018
3
Causal Loops
• Causal Loop: A closed causal chain in which some of the
causal
links are normal in direction and others are reversed.
• The effect of a causal loop is also its cause.
If A causes B and B also causes A, then

6. A causes A. So A has no origin. Loop!
• Worry: Time travel into the past allows for causal loops.
Causal
loops are impossible. Therefore, time travel is impossible.
9
• If time travel is possible, then some things in the present can
cause some
things in the past. But all the things in the past together cause
all the
things in the present. Loop!
• 1st example: A time traveler steals a time machine from the
local
museum in order to make his trip into the past and then donates
the time
machine to the same museum at the end of the trip (i.e., in the
past). In
this case the machine itself is never built by anyone – it simply
exists.
• 2nd example: a man receives plans for building a time
machine from his
future self. In the future, he sends the same information back to
his past
self. No one invented the time machine.

7. 10
• Q: Are causal loops impossible? If they are, then travel into
the past
must be impossible too.
• A: Causal loops are strange but there is no reason to think that
they
are impossible. We assume the possibility of many uncaused
and
inexplicable events. Examples: God, the Big Bang, the infinite
past
of the universe, etc.
• If we have no complete explanation for the existence of any
causal
sequence (closed or linear), then we have no good reason to
reject
the possibility of causal loops. In other words, may have to
accept
spontaneous creation of objects.
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Add’l. Video about Causal Loops
• Video about causal loops:

8. https://www.youtube.com/watch?v=630FHQHdMww
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https://www.youtube.com/watch?v=630FHQHdMww
1/18/2018
1
Phil 2: Puzzles and Paradoxes
Prof. Sven Bernecker
University of California, Irvine
The Grandfather
Paradox
Grandfather Paradox
• Grandfather paradoxes arise whenever a time traveler goes
into
the past and prevents an event that is a pre-condition of the
traveler making the backward time-journey in the first place.
• Example: You have just finished building a time machine. You
use this time machine to travel back twenty-four hours, cut the
power-supply to your own laboratory so you can’t finish the

9. time
machine that lets you go back in time to cut the power-supply ...
2
• Another example: A time traveler goes back in time and kills
his
grandfather before the grandfather has fathered children. If the
grandfather dies at this point, then one of the time traveler’s
parents
never exists. Hence the traveler can’t be born and travel back to
kill
the grandfather ... and so on.
• Worry: It is impossible to kill your own grandfather because it
would
violate the law of non-contradiction – the grandfather would
somehow
both survive to become a parent and not survive. If
contradictory
situations are impossible, and if time travelers could create
contradictory situations, then time travel is impossible.
3
• A backward time traveler

10. seems to be able to change the
past. But it is impossible to
change the past. But since the
past cannot be changed, time
travel is impossible.
1) By ordinary standards of ability, the backward time traveler
can kill his grandfather
2) But the time traveler cannot kill his grandfather. The
grandfather lived, so to kill him would be to change the past.
It is logically impossible to change the past.
3) Premises (1) and (2) contradict each other
C) Therefore, backward time travel is impossible
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David Lewis’s
Solution
of the
Grandfather Paradox
• Premises (1) and (2) are both true. They do not contradict one
another. The term “can“ in both premises mean different things.
The Grandfather paradox rests on an equivocation about the
meanng of “can.“
• When we say that somebody “can” do something, we mean
that
they have the capacity, holding certain things fixed. Which
things we hold fixed will depend upon context.

12. 5
• Unlike dogs and cats, I can speak a foreign language, like
Urdu.
So there is a sense in which I can speak Urdu. But don’t take me
as your translator when you go to Pakistan, because I can’t
speak
Urdu. I have never learned it. When we say “Sven can speak
Urdu", we are only holding fixed my brain’s linguistic
potentialities
— we are saying that Sven could learn it.
• When we say “Sven can’t speak Urdu", we are holding fixed
my
actual knowledge of languages — we are saying that he doesn’t
currently speak it.

13. • What I can do, relative to one set of facts, I cannot do, relative
to
another set of facts.
6
• Similarly, holding fixed only the past up to the attempt to kill
the grandfather, the time traveler can kill his grandfather.
However, holding fixed the past following the attempt to kill
the grandfather, the time traveler cannot kill his grandfather.
There is no paradox.
• The argument on slide #5 simply equivocates with respect to
“can.”
7
• Q: What will actually prevent the time traveler from killing
his grandfather?

14. • A: The failure to kill his grandfather will be caused by strange
coincidences:
His gun jams, a noise distracts him, he slips on a banana peel,
he
accidentally shoots someone else, etc.
• To render backward time travel possible we have to assume
that a time
traveler cannot change the past even though he can participate
in the
past. A time traveler cannot do anything that did not actually
happen. But he
can be amongst the people who did make the past the way it
was. The
freedom of action of a backward time traveler is severely
limited.
8

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¿Parallel Worlds?
• Imagine that whenever a person travels into the past, the
universe
splits into parallel worlds. If this occurs, there is no longer a
contradiction between the grandfather both existing and not
existing.
The grandfather may exist in one world but not in the other.
• Parallel worlds allow for coherent scenarios in which the past
is
changed
• But this is no longer time travel; it is inter-world travel.

16. 9 10
Add’l Videos about the
Grandfather Paradox
• Short video about the grandfather paradox:
https://www.youtube.com/watch?v=Y6RjjaEy59I
• Slightly longer video about the grandfather paradox
https://www.youtube.com/watch?v=M8oITAoaCr4
• Video explaining Lewis’s solution to the grandfather paradox:
https://www.youtube.com/watch?v=_oZhA5cxUxs
11
https://www.youtube.com/watch?v=Y6RjjaEy59I
https://www.youtube.com/watch?v=M8oITAoaCr4
https://www.youtube.com/watch?v=_oZhA5cxUxs

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1
Phil 2: Puzzles and Paradoxes
Prof. Sven Bernecker
University of California, Irvine
Fatalism, Determinism
and Free Will
What‘s the Issue?
• We take ourselves to have free will. Any argument for the
conclusion that
we don’t have free will – that we never really choose between
actions we
are able to do, that nothing is really up to us – would undermine

18. a view of
the world. Hence such an argument will count as a paradox.
• One challenge to free will comes from fate.
2
“The fatalist, then, is someone who believes that whatever
happens is and
always was unavoidable. He thinks that it is not up to him what
will happen
a thousand years hence, next year, tomorrow, or the very next
moment.”
(Taylor, p. 55).
Voluntary vs. Intentional Action
• Intentional action is goal-oriented action. But an intentional
action need not reach its goal, i.e., be successful.
• The motive behind an intentional action need not be rational.

19. • Whether someone acts intentionally may depend on how the
action is described. E.g., Oedipus intentionally killed the old
man. But Oedipus did not intentionally kill his father.
Nevertheless, the old man was his father.
3
• Being a fee agent is being an agent capable of influencing
the world; being the source of ones own actions.
• Usually voluntary actions are intentional. But some
voluntary actions are unintentional – those one does not do
on purpose, and which can be inhibited with an effort.
• Freedom of will vs. freedom of action: An agent can
possess free will without also having freedom of action.
(Whether one can have freedom of action without free will
is controversial.)
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Free Will
• The will is free if it is not predetermined/ caused by
antecedent factors.
• Free will means that we are self-determined, not (ultimately)
subject to
forces outside of our control - it means, we could have chosen
otherwise.
• Subject S‘s will is free with respect to performing action A, if
and only if
S could have chosen to do other than A.

21. • Free will is the ability to choose to do otherwise in the same
circumstances.
5
• “To say that one has free will is to say that
when one decides among forks in the road
of time (or, more prosaically, when one
decides what to do), one is at least
sometimes able to take more than one of
the forks…One has free will if sometimes
more than one of the forks in the road of
time is ‘open’ to one.” (Peter van Inwagen)
• “One lacks free will if on every occasion on
which one must make a decision only one
of the forks before one -- of course it will be
the fork one in fact takes -- is open to one.”
(Peter van Inwagen)
6
Relevance of Free Will

22. • We feel that we are free, that we are originators of our own
actions.
• Free will is a condition on the moral responsibility for our
actions.
• Free will is a condition on desert for one's accomplishments
(why
sustained effort and creative work are praiseworthy).
• Free will is a condition on the autonomy and dignity of
persons
• Free will is a condition on the value we accord to love and
friendship.
7
Causal Determinism
• Causal determinism is the view that the state of the world at a

23. given time determines the state of the world at the next moment.
• Every event that occurs, including human action, is entirely
the
result of earlier causes.
• The state of the universe plus the laws of nature determine a
single unique future.
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1/18/2018
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• What is the state of the universe? It is a complete description
of everything occurring at this exact moment:
- The way my shirt is wrinkled right now.
- The position of your fingers on your desk
- The position of all the atoms and molecules in the room

24. - The mental states each of you have right now
- Everything happening on this planet
- Everything occurring in the universe at this exact moment
• Examples of laws of nature: law of gravitation, law of
conservation of energy, etc.
9
Causal Determinism vs. Fatalism
• Causal determinism is the view that every event that occurs,
including human action, is entirely the result of earlier causes.
Note
that some of these earlier causes may be free choices.
• Fatalism is the view that whatever happens now and will
happen in
the future happens necessarily. Because the present and future

25. is
fixed, pre-ordained, our choices about what to do in a situation
are
inconsequential. E.g., it was true 100 years ago that you would
sit in
this lecture hall today.
10
• Note: Fatalism leaves it open
whether there is any purpose
guiding our fates. Fatalism also
leaves it open whether anyone
knows our fates.