philosophy discussion
Valerielee
HowTimeTravelWorks1.pdf
1/18/2018
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Phil 2: Puzzles and Paradoxes
Prof. Sven Bernecker
University of California, Irvine
How Time Travel
Works
David Lewis (1941 – 2001)
Lewis was an American
philosopher who taught most
of his life at Princeton. He has
made important contributions
in philosophy of language,
philosophy of mind,
metaphysics, epistemology,
and philosophical logic.
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Not Time Travel
• Virtual. One enters a highly realistic, interactive virtual reality
simulator in which some past era has been recreated down to
the finest detail.
• Crystal. One looks into a crystal ball and sees what happened at
some past time, or will happen at some future time.
• Dateline. One boards and airplane, departs at 5pm on Monday,
flies for twelve hours, and arrives at 5pm on Tuesday.
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• Coma. One is in a coma for a number of years and then
awakes, at which point it seems from one's own point of
view that hardly any time has passed.
• Cryogenics. One is cryogenically frozen for hundreds of
years. Upon being woken, it seems from one's own point of
view that hardly any time has passed.
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Time Travel Scenarios
• Doctor. Doctor Who steps into a machine in 2015. Observers
outside the machine see it disappear. Inside the machine, time
seems to Doctor Who to pass for ten minutes. Observers in
1984 (or 3072) see the machine appear out of nowhere.
Doctor Who steps out.
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• Leap. The time traveler takes hold of a special device (or
steps into a machine) and suddenly disappears; he
appears at an earlier (or later) time. Unlike in Doctor, the
time traveler experiences no lapse of time between his
departure and arrival: from his point of view, he
instantaneously appears at the destination time.
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• Gödel. The time traveler steps into an ordinary rocket ship (not a
special time machine) and flies off on a certain course. At no
point does he disappear (as in Leap)—yet because of the
structure of spacetime (as conceived in the General Theory of
Relativity), the traveler arrives at a point in the past (or future) of
his departure. (Compare the way in which someone can travel
continuously westwards, and arrive to the east of his departure
point, thanks to the overall curved structure of the surface of the
earth.)
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• Einstein. The time traveler steps into an ordinary rocket
ship and flies off at high speed on a round trip. When he
returns to Earth, thanks to certain effects predicted by the
Special Theory of Relativity, only a very small amount of
time has elapsed for him—he has aged only a few
months—while a great deal of time has passed on Earth: it
is now hundreds of years in the future of his time of
departure.
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What is Time Travel?
• A traveler departs and then arrives at his destination. The time
elapsed from departure to arrival is the duration of the journey. If
he is a time traveler, the separation in time between departure and
arrival does not equal the duration of his journey.
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• Let’s distinguish between external time and the personal time
of the time traveler. Personal time is that which is measured by
the traveler’s wristwatch. His journey takes, say, an hour of his
personal time. But the arrival is more than an hour after the
departure in external time, if he travels toward the future; or the
arrival is before the departure in external time if he travels
toward the past.
• Time travel takes place when there is a discrepancy between
external time and personal time. Arrival and departure are
separated by two unequal amounts of time.
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• The distinction between personal and external time is not that
between two dimensions of time.
• Personal time is not really time, but it plays the role in the time
traveler‘s life that time plays in the life of a common person.
• The personal identity of the time traveler consists in the mental
and/or bodily continuity with respect to personal time.
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Time Travel Scenarios
• Given the definition of time travel, Coma, Virtual, Crystal and
Dateline are clearly not cases of time travel.
• Doctor and Leap clearly are cases of time travel.
• It is unclear whether Cryogenics is a case of time travel.
• It is unclear whether Gödel and Einstein are cases of time travel.
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