Hello group 
,
Here is a very interesting text about time travel:
http://www.arts.unimelb.edu.au/amu/ucr/student/1996/m.joyce/timetrav.htm
Copyright © Michael Joyce 1996
10 Is time travel to the past logically possible? Is it logically possible
for you to travel back and have a conversation with your former self?
Could you kill your former self? Could you do anything to anyone, or could
they do anything to you, which you had not already done or had done to
you? Would you, or anyone else, in such circumstances, be a free agent?
Introduction
Long a favourite topic of science fiction, the notion of time travel is
one that raises an enormous number of philosophical problems and
quandaries regarding causation, identity and the nature of time itself.
While it is fascinating to study these merely as hypotheticals, research
in the context of relativity has suggested some circumstances under which
time travel, to the past as well as the future, might be possible.
Views of time
Linear
The traditional view of time has been of something flowing inexorably
forwards, and in three distinct stages: past, present and future. The past
is fixed and unchangeable, the future is unwritten and does not exist yet,
and the present is what we are experiencing now. This picture is still the
most intuitive and natural way of viewing time. Since the theory of
relativity has become more widely accepted, time is no longer seen as
being so unique and separate, but as part of a four-dimensional framework.
It is still different to the three space dimensions in many important
aspects, but it can still be altered by factors such as speed and gravity.
It is relativity that gives a little credibility to the possibility of
time travel.1
Many-worlds
Another view of time that I will mention is the "many-worlds" or parallel
universe view, in which a time traveller may not actually be visiting
their own past, but is actually travelling to a universe similar to his or
her original universe. Although it might be seen as an extreme extension
of some principles of quantum mechanics, the many-worlds hypothesis is a
useful way of sidestepping many of the logical and philosophical problems
of time travel. This is often thought of in terms of an extra dimension of
time, with different time-lines branching off because of the different
events in each system.2
Time travel is not a physical impossibility
A good simple description of time travel was given by David Lewis3 as
being when the personal time of the time traveller differs from the global
and historical time of the rest of the world. Lewis makes the analogy of a
personal time as a winding mountain railroad and external time as being
the distance as the crow flies to demonstrate that this does need a second
time dimension, but can fit in with our more usual picture of space-time
as a four-dimensional system. The railroad might take a meandering path,
even crossing over itself, but it is still contained within the same three
dimensions as a straight line between the railroad's origin and
destination. So too, a time traveller's personal time might different to
the global, external time of the rest of the world, but is still contained
within that global time.
Possible types of time travel
Future
The type of time travel that is easiest to explain is travel into the
future, which could be accomplished by a spacecraft travelling at
relativistic speeds4. As the ship gets faster, time dilation slows time on
board the spacecraft relative to the planet it has left behind. For the
viewpoint of those remaining on earth, the astronaut has travelled into
the future. Travel in this manner is not usually thought of as time
travel, since the astronaut's personal time continues normally, but it
would have the same effect as a classic time machine (although it would be
disappointing for the astronaut to learn that there is no reverse gear on
the time machine). It also differs from the normal description of time
travel because, since the time traveller cannot return to his or her
natural time, his or her path does not cross over or duplicate itself.
The "Twins Paradox" described by travelling at relativistic speeds is not
one of time travel, but of time dilation and frames of reference. The
notion of time flowing at different paces is unexpected and difficult, but
not as counter-intuitive and confusing as the possibilty of travelling
backwards in time.
Past
Like travelling into the future, travelling into the past would also
require several gross manipulations and incredible engineering feats in
order to exploit some of the more unusual effects of the theory of general
relativity. Many physicists are willing to ignore the possibility of time
travel to the past because it is so unlikely to occur and the energy
required to complete it render it extremely improbable5.
A number of schemes have been proposed to travel backwards in time. Some
involve a time that loops around itself6, others would use a rotating
black hole7, an accelerated wormhole8 or a cylindrical object so massive
that time would be bent right around and into the opposite direction9.
Although all of them differ to time travel as usually suggested in popular
fiction in that they also involve travel through space, the logical
complications are much the same.
Information
Another type of time travel that might be much more possible, although a
little less dramatic, would involve tachyons. Tachyons are controversial
particles that travel faster than light, and, as a consequence of their
superluminal speed, backwards in time10. An ability to control these would
lead to the much the same philosophical problems of causation as a person
actually travelling backwards. While there would be no problems of
physically meeting oneself in a previous time, it might be possible to
communicate with oneself with tachyons, or even to use those tachyons to
cause (or at least attempt to cause) some events in the past.
Meeting a previous self
One scenario of backwards time travel that frequently raises concerns is
that of a time traveller travelling back and meeting themselves at an
earlier stage in life, often with the intention of giving themselves
advice in order to improve their lives or avoid a particularly traumatic
event. The first issue here is of the problem of duplication: is it
possible to have two "copies" of the same person at the same place and the
same time, and if so what is the difference, if any between the two
people? The second is the problem of the inconsistencies between the
traveller's memory of the past and the events that transpire once he or
she goes back to the past.
In David Lewis' description of time travel, the time traveller's world
line might have discontinuities in relation to the rest of the "normal"
timeline11, so he has to go to great pains to explain the continuing
identity of a time traveller as not only having the same physical
characteristics as the time traveller's previous existencein their
personal time, but also an element of causal connection. The methods of
time travel mentioned above involving travel through space as well do not
have these discontinuites, so personal identity is easier to ascertain.
Identity
If a time traveller did travel back to a time that he or she had already
lived through, would we be able to say that the two people are one and the
same? There would obviously be some physical differences in age and
appearance and mental differences in the older time traveller's extra
memories and experiences, but these differences would not normally lead us
to make a distinction between a young and old version of an everyday
non-time-travelling person as being different people. Although time travel
seems more complicated, there is no reason why a time traveller's personal
time line could not cross over itself like a mountain railroad crosses
over itself on a bridge.
Inconsistency with the past
The second problem of meeting oneself as one travels back in time is
really just a simpler version of the grandfather paradox detailed below.
If the time traveller does meet with him or herself in the past, then it
follows that the meeting will be a part of his or her own memories and
past. Unless the two-dimensional time framework is introduced, the time
traveller must have a memory of being visited by an older version of
himself in his or her youth. The possibility of a time traveller acting in
a way that was inconsistent with the known past raises a few interesting
questions.
The Grandfather Paradox
As the most extreme example of changing the past, a hypothetical is often
envisaged whereby the time traveller visits the past with the intention of
killing his or her own grandfather or grandmother so that the time
traveller will never be born. This is usually known as the Grandfather
Paradox12. Numerous variations on the theme exist, but the basic question
remains the same: if time travel to the past were possible, would it be
possible to change the past, and moreover to change it in such a way as to
make the time traveller's existence impossible?
The simplest way to avoid this paradox is by invoking the many-universes
theory of time. In this case, the time traveller actually kills the person
corresponding to his ancestor in a different time, leaving his own
ancestor (and thus himself) untouched13.
To many, the two-dimensional-time solution is a cop-out, since the time
traveller is not visiting his or her own past, just a past that is
remarkably similar to it. Even though the time traveller can perform
actions that are inconsistent with the past as he or she know it, these
actions will still have to be consistent with the timeline of that
particular universe.
Lewis reminds us not to confuse "logically impossible" with physically
incapable14. If we look at the time traveller as just another occupant of
that time armed with a gun and all the skill required to kill his target,
we would not say that it is impossible for him to complete the task.
However, this ignores the important fact of the time traveller's past. We
know that the time traveller's grandfather did not die at that stage, so
the assassin could not have succeed. We can say the same thing of any past
event: John Hinckley could have killed Ronald Reagan when he shot him in
1981, but didn't.
Doing anything else inconsistent with history
The case of killing your own grandmother or grandfather is a particularly
interesting one because it involves not only a contradiction with the past
as the time traveller knows it happens, but also a paradox that shows why
such an inconsistency is illogical. Although the existence of the time
traveller's grandfather is of obvious importance to the time traveller, it
is not significantly different from any other act that is inconsistent
with the time traveller's past, whether he or she knows about it or not.
The time traveller's past exists not just for the time traveller, but also
for everyone else alive in his or her "natural" time. If the time
traveller killed someone else or gave them the blueprints for an
invention, the records of the time traveller's natural time would have to
show events that correspond with the actions of the time traveller in the
past, regardless of whether the time traveller knew if that was how things
would pan out or not.
Free will
The worrying aspect of looking at time travel in this way is that it
eliminates free will for the time traveler. Although the time traveller
believes that he or she is able to kill a particular person, they are
destined to fail since the events of that time have already been fixed,
regardless of whether the time traveller knows of them or not.
We think of the future as being variable and determined by the actions we
take in the present. The presence of a time traveller in our time would
shatter this illusion. Our future is the time traveller's past. If he
cannot change it, then neither can we. Whatever is written in the history
books of the time traveller is what we in the present will do.
If the future is fixed as well as the present, we might look at time as
like a movie film strip. To everyday people, the film moves past the
projector at a steady rate and we see only the present. We know what has
already happened in the movie, but can only guess at what will happen
next. To a time traveller, the movie is unravelled and spread out on the
floor. The whole of the movie is fixed, regardless of which frame the time
traveller believes he or she is in. At no stage can anyone alter the
course of events once the film has been processed and fixed in its nature.
In a two-dimensional description of time, there is no paradox in a time
traveller killing what appears to be their own ancestor, so it at first
looks as though free will still exists. Although the time traveller might
seem to be able to do whatever they want, whatever they do will still have
to be consistent with the events that transpire within the timeline that
the time traveller now occupies.
Imagine two unrelated time travellers both travelling to the same
timeline: one is travelling to his or her own past (Traveller-A) and the
other to a time-line that is merely similar to his or her own past
(Traveller-B). Traveller-A will be in exactly the same situation as if he
or she were in a one-dimensional time situation, and will be unable to do
anything inconsistent with his or her own past. Similarly, Traveller-B
will also be unable to do anything that would be inconsistent with
Traveller-A's past. Since the events forward of the time when Traveller-A
and Traveller-B meet are fixed for that timeline, the situation is no
different to a one-dimensional time situation.
Changing the past in fiction
Depictions of time travel in science fiction commonly allow the
protagonists to intervene with and interfere with the past, although the
travellers usually take enormous pains to avoid doing so. Two-dimensional
depictions of time apart, the most logical of these depictions is in
stories where the memories and records of the past that the time
travellers have change to reflect the past: usually instantaneously, but
sometimes as a flow-on effect of the change. Such a notion is plausible,
so long as the changes they make do not logically rule out the fact that
the travellers were able to make these changes. In such a situation, free
will is preserved in all aspects but those that will make it impossible
for the travellers to exist at that point in time. Of course, the problem
with this is in determining what changes would rule out this existence:
killing your own grandfather is obvious, but what of merely preventing two
of your distant ancestors from meeting, or preventing the time machine
from being invented?
The more usual story depicted in science fiction is one where the
traveller's past does change, but he or she retains her memory of the past
as it was. This story is inconsistent from a logical and philosophical
point of view, but makes for a more interesting and conventionally
comprehensible narrative.
Conclusions
For a time traveller, the distinctions between past, present and future
disappear, so that the future may be fixed and unchangeable. If this is
the case, free will is gone too, not only for the time traveller, but also
for all the other inhabitants of his or her past. This conclusion is so
extreme that we may be better to reject the notion of time travel and keep
our precious free will than to accept this consequence of the general
theory of relativity.
References
Davies, Paul (1995) About Time Penguin Books
Horwich, P. (1987) Asymmetries in Time: Problems in the Philosophy of
Science, MIT Press, US
Lewis, D.K. (1976): 'The Paradoxes of Time Travel', American Philosophical
Quarterly 13
pp 145-152. Reprinted in Source Materials 136-220, 1996
Footnotes
Davies, Paul (1995) About Time Penguin Books, p 33
Lewis, D.K. (1976): 'The Paradoxes of Time Travel', American
Philosophical Quarterly 13
pp 145-152. Reprinted in Source Materials 136-220, 1996 p 231
Ibid p 137
Davies p 234
Ibid p 244
Ibid p 246
Ibid p 244
Ibid p 246
Ibid p 245
Ibid p 234
Lewis p 136
Lewis p 141-146
Ibid p 146
Ibid p 143
Copyright © Michael Joyce 1996.Created by Michael Joyce
mjoyce at connexus dot apana dot org dot au
Last Modified October 25, 1996
<This message has been edited by Marcelo (edited 24 June 2000).>