Henrik Vestergaard <vestergaard_henr@gmx.net> wrote:
> Per Rønne wrote:
> > Henrik Vestergaard <vestergaard_henr@gmx.net> wrote:
> >
> >>> Har du ingen logisk sans? Alvidenhed kan ikke fungere med begrænsinger i
> >>> alvidenheden ...
> >> Gud kan sagtens vide *alt* hvad der foregår NU.
> >>
> >> Men det betyder ikke at Han på forhånd ved hvordan du og jeg vil handle
> >> forud for handlingen.
> >
> > Men så vil han ikke være alvidende ...
> >
> >> Det er ligesom dér friheden til at vælge kommer ind i billedet...
> >
> > Nej. Måske ville det hjælpe dig lidt, sådan logisk, at læse lidt science
> > fiction om tidsrejser?
>
> Jeg er helt enig med dig. Dine tanker hjælpes bedst af science fiction.
> Det satte vist tingene lidt i perspektiv.
>
> Tak fordi du sagde det bedre end jeg selv kunne have postuleret.
Det ville nu også have hjulpet, hvis du vidste mere om moderne fysik.
Tiden er skam ikke sådan som vi intuitivt opfatter den, og for en
almægtig gud vil den naturligvis ikke være en begrænsning.
<
http://en.wikipedia.org/wiki/Time_travel>
- hvorfra jeg citerer den /videnskabeligt funderede/ del [fiktionen
kommer bagefter]:
Time travel in fact
Some theories, most notably special and general relativity, suggest that
suitable geometries of spacetime, or certain types of motion in space,
may allow time travel into the past and future if these geometries or
motions are possible. Concepts that aid such understanding include the
closed timelike curve.
Albert Einstein's special theory of relativity (and, by extension, the
general theory) very explicitly permits a kind of time dilation that
would ordinarily be called time travel. The theory holds that, relative
to a stationary observer, time appears to pass more slowly for
faster-moving bodies: for example, a moving clock will appear to run
slow; as a clock approaches the speed of light its hands will appear to
nearly stop moving. The effects of this sort of time dilation are
discussed further in the popular "twin paradox".
A second, similar type of time travel is permitted by general
relativity, where a distant observer sees time passing more slowly for a
clock at the bottom of a deep gravity well, and a clock lowered into a
deep gravity well and pulled back up will indicate that less time has
passed compared to a stationary clock that stayed with the distant
observer.
These effects allow "time travel" only toward the future: never
backward. This is not typical of the "time travel" featured in science
fiction, and there is little doubt surrounding its existence. "Time
travel" will hereafter refer to travel with some degree of freedom into
the past or future.
Many in the scientific community believe that time travel is highly
unlikely. This belief is largely due to Occam's Razor. Any theory which
would allow time travel would require that issues of causality be
resolved. What happens if you try to go back in time and kill your
grandfather?—see grandfather paradox. Also, in the absence of any
experimental evidence that time travel exists, it is theoretically
simpler to assume that it does not happen. Indeed, Stephen Hawking once
suggested that the absence of tourists from the future constitutes a
strong argument against the existence of time travel—a variant of the
Fermi paradox, with time travelers instead of alien visitors. However,
assuming that time travel cannot happen is also interesting to
physicists because it opens up the question of why and what physical
laws exist to prevent time travel from occurring.
The "presentist" view
Some theorists have argued that the matter of the universe only exists
in the present moment. Thus, if one were to travel back from the
'present' to an earlier time, none of the material universe would be
found there, because it will have remained in the present: the traveller
alone is the only part of the universe to have gone back to the earlier
time. In terms of a 4-dimensional spacetime, the traveller (or, more
generally the atomic particles that comprise the traveller) would have
travelled 'back' to an area of spacetime corresponding to an earlier
value of 't'; but none of the other particles that form the universe
will have done so, so the traveller finds precisely nothing when
arriving back at the earlier time. This viewpoint eliminates all of the
supposed paradoxes about time travel.
The equivalence of time travel and faster-than-light travel
If one were able to move information or matter from one point to another
faster than light, then according to special relativity, there would be
an observer who sees this transfer as allowing information or matter to
travel into the past. Additionally, faster than light travel along
suitable paths would correspond to travel backward in time as seen by
all observers. This results simply from the geometry of spacetime and
the role of the speed of light in that geometry.
Special spacetime geometries
The general theory of relativity extends the special theory to cover
gravity, describing it in terms of curvature in spacetime caused by
mass-energy and the flow of momentum. General relativity describes the
universe under a system of "field equations," and there exist solutions
to these equations that permit what are called "closed time-like
curves," and hence time travel into the past. The first and most famous
of these was proposed by Kurt Gödel, but all known current examples
require the universe to have physical characteristics that it does not
appear to have. Whether general relativity forbids closed time-like
curves for all realistic conditions is unknown. Most physicists believe
that it does, largely because assuming some principle against time
travel prevents paradoxical situations from occurring.
Using wormholes
A proposed time-travel machine using a wormhole would (hypothetically)
work something like this: A wormhole is created somehow. One end of the
wormhole is accelerated to nearly the speed of light, perhaps with an
advanced spaceship, and then brought back to the point of origin. Due to
time dilation, the accelerated end of the wormhole has now experienced
less subjective passage of time than the stationary end. An object that
goes into the stationary end would come out of the other end in the past
relative to the time when it enters. One significant limitation of such
a time machine is that it is only possible to go as far back in time as
the initial creation of the machine; in essence, it is more of a path
through time than it is a device that itself moves through time, and it
would not allow the technology itself to be moved backwards in time.
This could provide an alternative explanation for Hawking's observation:
a time machine will be built someday, but has not yet been built, so the
tourists from the future cannot reach this far back in time.
According to current theories on the nature of wormholes, creating a
wormhole of a size useful for a person or spacecraft, keeping it stable,
and moving one end of it around would require significant energy, many
orders of magnitude more than the Sun can produce in its lifetime.
Construction of a wormhole would also require the existence of a
substance known as "exotic matter", which, while not known to be
impossible, is also not known to exist in forms useful for wormhole
construction (but see for example the Casimir effect). Therefore it is
unlikely such a device will ever be constructed, even with highly
advanced technology. On the other hand, microscopic wormholes could
still be useful for sending information back in time.
Matt Visser argued in 1993 that the two mouths of a wormhole with such
an induced clock difference could not be brought together without
inducing quantum field and gravitational effects that would either make
the wormhole collapse or the two mouths repel each other. [1] Because of
this, the two mouths could not be brought close enough for causality
violation to take place. However, in a 1997 paper, Visser hypothesized
that a complex "Roman ring" (named after Tom Roman) configuration of an
N number of wormholes arranged in a symmetric polygon could still act as
a time machine, although he concludes that this is more likely than not
a flaw in classical quantum gravity theory rather than proof that
causality violation is possible. [2]
Another approach — attributed to Frank Tipler, but invented
independently by Willem Jacob van Stockum [3] in 1936 and Kornel Lanczos
[4] in 1924 — involves a spinning cylinder. If a cylinder is long, and
dense, and spins fast enough about its long axis, then a spaceship
flying around the cylinder on a spiral path could travel back in time
(or forward, depending on the direction of its spiral). However, the
density and speed required is so great that ordinary matter is not
strong enough to construct it. A similar device might be built from a
cosmic string, but none are known to exist, and it does not seem to be
possible to create a new cosmic string.
Physicist Robert Forward noted that a naïve application of general
relativity to quantum mechanics suggests another way to build a time
machine. A heavy atomic nucleus in a strong magnetic field would
elongate into a cylinder, whose density and "spin" are enough to build a
time machine. Gamma rays projected at it might allow information (not
matter) to be sent back in time. However, he pointed out that until we
have a single theory combining relativity and quantum mechanics, we will
have no idea whether such speculations are nonsense.
Using Quantum Entanglement
Quantum-mechanical phenomena such as quantum teleportation, the EPR
paradox, or quantum entanglement might appear to create a mechanism that
allows for faster-than-light (FTL) communication or time travel, and in
fact some interpretations of quantum mechanics such as the Bohm
interpretation presumes that some information is being exchanged between
particles instantaneously in order to maintain correlations between
particles. This effect was referred to as "spooky action at a distance"
by Einstein.
Nevertheless, the rules of quantum mechanics curiously appear to prevent
an outsider from using these methods to actually transmit useful
information, and therefore do not appear to allow for time travel or FTL
communication. The fact that these quantum phenomena apparently do not
allow FTL/time travel is often overlooked in popular press coverage of
quantum teleportation experiments. The assumption that time travel or
superluminal communications is impossible allows one to derive
interesting results such as the no cloning theorem, and how the rules of
quantum mechanics work to preserve causality is an active area of
research.
The possibility of paradoxes
The Novikov self-consistency principle and recent calculations by Kip S.
Thorne indicate that simple masses passing through time travel wormholes
could never engender paradoxes—there are no initial conditions that lead
to paradox once time travel is introduced. If his results can be
generalised, they would suggest, curiously, that none of the supposed
paradoxes formulated in time travel stories can actually be formulated
at a precise physical level: that is, that any situation you can set up
in a time travel story turns out to permit many consistent solutions.
The circumstances might, however, turn out to be almost unbelievably
strange.
Parallel universes might provide a way out of paradoxes. Everett's
many-worlds interpretation of quantum mechanics suggests that all
possible quantum events can occur in mutually exclusive histories. These
alternate, or parallel, histories would form a branching tree
symbolizing all possible outcomes of any interaction.
In a 2005 paper, Professor Daniel Greenberger of City University of New
York and Karl Svozil of the Vienna University of Technology proposed
that quantum theory gives a model for time travel without paradoxes. [5]
In quantum theory observation causes possible states to 'collapse' into
one measured state; hence, the past observed from the present is
deterministic (it has only one possible state), but the present observed
from the past has many possible states until our actions cause it to
collapse into one state. Our actions will then be seen to have been
inevitable.
Since all possibilities exist, any paradoxes can be explained by having
the paradoxical events happening in a different universe. This concept
is most often used in science-fiction. However, in actuality, physicists
believe that such interaction or interference between these histories is
not possible (see Chronology protection conjecture).
A further suggestion related to paradoxes suggests that time travel will
never exist, even if theoretically possible. The reasoning is that as
long as time travel exists, history will change, and will only become
static when a timeline is reached in which no time travel exists and
thus no further changes can be made. Assuming there is only a single
dimension of time, the timeline we perceive must be the one that exists
after all changes (if any) are made, and thus we will never perceive the
invention of time travel, since it will have already destabilised itself
out of the timeline by the time we would have reached it.
Time travel and the direction of time
The notion of time travel (either towards the future or towards the
past) tacitly assumes that there exists a direction of time, the
direction from the past to the future. On the other hand, the direction
of time (or the arrow of time) may not be a fundamental intrinsic
property of time, but rather could be viewed as an emergent property
traceable to the fact that we live in a universe in which the entropy
increases with time. In this view, as the direction of time is not
fundamental, the notion of time travel is also not fundamental. Without
a fundamental notion of time travel there can be no fundamental problems
with time travel. Without an intrinsic direction of time, time can be
viewed as a "static" coordinate similar to other spacetime coordinates.
From this point of view, the Novikov self-consistency principle is a
tautology, a demand that hardly needs to be questioned, which
automatically prevents causal paradoxes.
Time travel and the anthropic principle
It has been suggested by physicists such as Max Tegmark that the absence
of time travel and the existence of causality may be due to the
anthropic principle. The argument is that a universe which allows for
time travel and closed time-like loops is one in which intelligence
could not evolve because it would be impossible for a being to sort
events into a past and future or to make predictions or comprehend the
world around them.
--
Per Erik Rønne
http://www.RQNNE.dk