philosophy paper
Valerielee
ThePossibilityofTimeTravel.pdf
1/18/2018
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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)?
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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.
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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
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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
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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)
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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)
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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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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
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.
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• 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.
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• 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:
https://www.youtube.com/watch?v=630FHQHdMww
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