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## Tiffany Chang

on 24 March 2016

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#### Transcript of The Grandfather Paradox

... is the incompatibility of the following claims, each of which seem plausible:
(1) Tim can kill his grandfather.
(2) Tim cannot kill his grandfather.
(He has the ability and all the needed tools)
(If he did, he wouldn't have ever existed!)
David Lewis
Peter van Inwagen
Changing the Past
'S can do x'
=
S's doing x is compossible with a set of facts F
"Tim can kill his grandfather"
"Tim cannot kill his grandfather"
Tim's killing grandfather is
compossible
with the facts F1: Tim has a gun, bullets, training, and good eye-sight
Tim's killing grandfather is
not compossible
with the facts F2: Grandpa does not die on that day and later has a child
worlds in which Tim kills his grandfather
worlds in which the following facts obtain:
Tim has a gun
Tim has bullets
Tim is well trained
Tim has good eye-sight
worlds in which Tim kills his grandfather
worlds in which the following facts obtain:
Grandpa does not die that day
Grandpa lives long enough to have a child
An Argument for the
Impossibility of Changing the Past
If changing the past is possible, then it is possible for some time
t
to be
F
and for someone to cause
t
to be
~F
.
Necessarily, if some time
t
is
F
and
t
is caused to be
~F
, then
t
is
F
and
~F
.
Necessarily, nothing can be
F
and
~F
.
So, changing the past is not possible.
[Any change involves an entity being one way and then another, incompatible, way]
[Normally, change occurs over time. But changing the past involves changing what's true at a single time. Thus we get a genuine instantiation of incompatible properties.]
[Leibniz's Indiscernibility of Identicals]
Hypertime allows the possibility of changing the past without violating Leibniz's Law, like so:
time
hypertime
HT-1
HT-2
1947
2014
grandpa
Tim
Premise 2 of the Impossibility Argument is false
This undermines claim 2 of the Grandfather Paradox
Another possible scenario...
2014
2004
1947
HT-1
HT-2
HT-2'
HT-5
HT-2'
75 yrs old
8 yrs old
75 yrs old
85 yrs old
18 yrs old
HT-4
= grandpa
= tim
1996
HT-3
Tim doesn't kill grandpa in 1947. At (HT-3, 1996) Tim hyper-was born (again). At (HT-4, 2004) Tim hyper-was 8 years old (again).
Objections to van Inwagen's Account
1
2
3
Time travel will involve annihilating everyone else!
We might get stuck in a loop!
Why the Growing Block View? This model will also work for Eternalism and Presentism...
On the GBV, future objects don't yet exist. If Tim travels to 1947, then you and I don't (yet) exist.
Suppose you travel to 5 minutes ago but end up in outer space and die...
12:00
12:05
2024
2014
1947
The Eternalist's Account
HT-1
HT-2
HT-2'
HT-3
The Presentist's Account
1947
HT-2'
Note that in addition to presentism, growing block theory, and eternalism about
times
, we also have presentism, growing block theory, and eternalism about
hyper-times
; these views about times and hyper-times can be mixed and matched...
2014
HT-2
HT-1
(+ hyper-eternalism)
(+ hyper-presentism)
presentism
growing block
eternalism
hyper-presentism
hyper-growing block
hyper-eternalism
Full transcript