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Re: Time travel is possible?
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While it is true that by traveling at a speed close to c, the speed
of light, you would indeed end up "later" in the future than you
would have had you traveled the same duration by your watch at a
lower speed. From an external observer's time frame, your journey
takes the same time. Note however that (at least to my knowledge)
this is a relativistic effect and not a directly proportional one,
thus the inverse does not hold to be true. Indeed you do not travel
slower through time by standing still, and you certainly have no way
of traveling backwards in time according to this theory. Thus you
would not meet your future self either as you would effectively just
be traveling into the present anyway, and would have been gone during
the duration of your journey from earth's point of view.
-- Edward.
On 25 Oct 2005, at 25/10/200513:32, Alice Evans wrote:
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DIS@bups.org
** For Your Eyes Only **
Arguably, time travel is possible. For example, an object traveling
at high speeds ages more slowly than a stationary object. This
means that if you were to travel into outer space and return,
moving close to light speed, you could travel thousands of years
into the Earth's future.
Newton's most important contribution to science was his
mathematical definition of how motion changes with time. He showed
that the force causing apples to fall is the same force that drives
planetary motions and produces tides. However, Newton was puzzled
by the fact that gravity seemed to operate instantaneously at a
distance. He admitted he could only describe it without
understanding how it worked. Not until Einstein's general theory of
relativity was gravity changed from a "force" to the movement of
matter along the shortest space in a curved spacetime. The Sun
bends spacetime, and spacetime tells planets how to move. For
Newton, both space and time were absolute. Space was a fixed,
infinite, unmoving metric against which absolute motions could be
measured. Newton also believed the universe was pervaded by a
single absolute time that could be symbolized by an imaginary clock
off somewhere in space. Einstein changed all this with his
relativity theories, and once wrote, !
"Newton, forgive me."
Einstein's first major contribution to the study of time occurred
when he revolutionized physics with his "special theory of
relativity" by showing how time changes with motion. Today,
scientists do not see problems of time or motion as "absolute" with
a single correct answer. Because time is relative to the speed one
is traveling at, there can never be a clock at the center of the
universe to which everyone can set their watches. Your entire life
is the blink of an eye to an alien traveling close to the speed of
light. Today, Newtonian mechanics have become a special case within
Einstein's theory of relativity. Einstein's relativity will
eventually become a subset of a new science more comprehensive in
its description of the fabric of our universe. (The word
"relativity" derives from the fact that the appearance of the world
depends on our state of motion; it is "relative.")
What remains of the killing-your-earlier-self paradox in general
relativistic time travel worlds is the fact that in some cases the
states on edgeless spacelike surfaces are 'overconstrained', so
that one has less than the usual freedom in specifying conditions
on such a surface, given the time-travel structure, and in some
cases such states are 'underconstrained', so that states on
edgeless space-like surfaces do not determine what happens
elsewhere in the way that they usually do, given the time travel
structure. There can also be mixtures of those two types of cases.
The extent to which states are overconstrained and/or
underconstrained in realistic models is as yet unclear, though it
would be very surprising if neither obtained. The extant literature
has primarily focused on the problem of overconstraint, since that,
often, either is regarded as a metaphysical obstacle to the
possibility time travel, or as an epistemological obstacle to the
plausibility of time travel in!
our world. While it is true that our world would be quite
different from the way we normally think it is if states were
overconstrained, underconstraint seems at least as bizarre as
overconstraint. Nonetheless, neither directly rules out the
possibility of time travel.
If time travel entailed contradictions then the issue would be
settled. And indeed, most of the stories employing time travel in
popular culture are logically incoherent: one cannot "change" the
past to be different from what it was, since the past (like the
present and the future) only occurs once. But if the only
requirement demanded is logical coherence, then it seems all too
easy. A clever author can devise a coherent time-travel scenario in
which everything happens just once and in a consistent way. This is
just too cheap: logical coherence is a very weak condition, and
many things we take to be metaphysically impossible are logically
coherent. For example, it involves no logical contradiction to
suppose that water is not molecular, but if both chemistry and
Kripke are right it is a metaphysical impossibility. We have been
interested not in logical possibility but in physical possibility.
But even so, our conditions have been relatively weak: we have
asked only whether !
time-travel is consistent with the universal validity of certain
fundamental physical laws and with the notion that the physical
state on a surface prior to the time travel region be
unconstrained. It is perfectly possible that the physical laws obey
this condition, but still that time travel is not metaphysically
possible because of the nature of time itself. Consider an analogy.
Aristotle believed that water is homoiomerous and infinitely
divisible: any bit of water could be subdivided, in principle, into
smaller bits of water. Aristotle's view contains no logical
contradiction. It was certainly consistent with Aristotle's
conception of water that it be homoiomerous, so this was, for him,
a conceptual possibility. But if chemistry is right, Aristotle was
wrong both about what water is like and what is possible for it. It
can't be infinitely divided, even though no logical or conceptual
analysis would reveal that.
Similarly, even if all of our consistency conditions can be met, it
does not follow that time travel is physically possible, only that
some specific physical considerations cannot rule it out. The only
serious proof of the possibility of time travel would be a
demonstration of its actuality. For if we agree that there is no
actual time travel in our universe, the supposition that there
might have been involves postulating a substantial difference from
actuality, a difference unlike in kind from anything we could know
if firsthand. It is unclear to us exactly what the content of
possible would be if one were to either maintain or deny the
possibility of time travel in these circumstances, unless one
merely meant that the possibility is not ruled out by some
delineated set of constraints. As the example of Aristotle's theory
of water shows, conceptual and logical "possibility" do not entail
possibility in a full-blooded sense. What exactly such a full-
blooded sense would be in!
case of time travel, and whether one could have reason to believe
it to obtain, remain to us obscure.
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