then why haven't they got a message from the future?

They failed...


BUT according to my own theory, IF they intend to try that, they first have to work out the details and make the effort. They will not get any messages until the intent is set.

Until then, the future (of that project anyway) is unwritten.

well, if they did try, it would of had to of failed... or maybe not, maybe the future cannot send messages back. so they have to wait and see if they get the message... like waiting on an email...lol

They will never get a message if they don't try it, that's for sure.

But first they are trying to create life.... er a tiny big bang... or matter.... or a black hole that will suck in their immediate reality.

they should try to make a white ho... i mean hole

I'm not sure but I think their expectations are that the experiment would send something back in time for a tiny fraction of a second such that it would show up on the instrumentation. Most events in space are power dependent, i.e., the more energy you put in, the bigger result you get out. They are at the beginning of the learning curve of what happens as the tiny big bangs get bigger.

Tiny big bang?...lol

Yeh, that is a contradiction in terms.

But then, size is relative.

Instead of "tiny big bang" they should just call it "tiny bang."

"They" is me and I meant it as a joke.

If we did get a message from the 'future' it would not be from our future but that future another 'line' developed... (like a cross-time future).

Wouldn't do us much good.

OUr future is still writing.

i believe that scenario too... i don't think the grandfather effect is possible

Maybe these scientists can let me know by e-mail when i turn 16 again. Oh how i love living in backwards mode. Especially when it's too hard too prove?

Most serious thought in recent decades has focused on the possibity of time travel in Einstein's spacetime... for a variety of reasons. Einstein's world was of the large, quantum fields are of the very small ... the math didn't seem to have much in common between the two. It would be much more fun to send a human back in time than an electron so one tends to focus on the big. The following paper is a classical treatment of Einstein's theory and has yet to consider the newer concepts of M Theory which is fueling the excitement at CERN. To really "get into" the concept, one needs to place entropy on the table as a concept possibly unique to our universe. Entropy give a direction to time.

Begin Quote:


Does Modern Physics Permit the
Operation of Time Machines?
Christian Wuthrich
Department of History and Philosophy of Science
University of Pittsburgh
14 March 2003
Abstract
Recently, many physicists have published on a topic that their predecessors
ruminated over only on weekends: time travel and time machines.
Most of these publications concern the prospect of time travel
or of time machines in classical general relativity. A smaller number
investigates the same issues in quantum gravity and quantum eld theory.
For the purposes of this paper, I will con ne myself to results in
classical general relativity. My ambition will be to communicate these
highly abstract results to a general audience. To this end, I will start
out by explicating the concepts of time travel and of time machines.
Time travel will be associated with the presence of closed causal curves
in a spacetime. A time machine, to put it crudely, is a device that produces
such closed causal curves. The physics literature has evolved
in the absence of any precise delineation of the spacetime structure
that would characterize the operation of these devices. My goals are,
rst, to specify the spacetime structure required to implement a time
machine and, second, to assess an attempted no-go results against
time machines due to Sergei Krasnikov (2002a, 2002b), which seems
to amount to the prohibition of a very general class of such devices. I
argue that this theorem leaves open the possibility of an incremental
time machine, a device that increases the probability of the emergence
of closed timelike curves. I close by discussing some of the technical
diculties arising from such a reformulation of the problem.
I am indebted to Phil Dowe, John Earman, Arthur Fine, Brian Hepburn, John Norton,
Christopher Smeenk, and James Tabery for discussions and comments.
1
1. Introduction. Philosophers have long been intrigued by the prospect
of travelling back in time. They have debated the paradoxical consequences
that arise from this possibility, in particular the infamous grandfather paradox.
1 The grandfather paradox states that a time traveller undertakes the
assassination of his grandfather before Grandfather gets the opportunity to
beget his father, thereby precluding his own birth, and consequently preventing
himself from travelling back in time to execute his mean intention.
In general, discussions of the issue have focused on the agency of bringing
about some event in the past, which then produces some contradiction to
present facts of the matter. The causal loops at stake were usually taken to
involve an agent who initiates a change in the past by means of backward
causation. Until a few years ago, the received view in philosophy maintained
that time travel is therefore not possible on logical or conceptual grounds.
In recent years, also under the impression of an emerging consensus in
the physics community that the possibility of time travel had to be taken seriously,
the front lines of the debate have signi cantly changed. It is widely
believed today that the existence of closed causal loops in itself does not
engender paradox.2 However, the possibility of such loops imposes consistency
constraints on the range of admissible scenarios. The current debate
centres on whether these consistency constraints can be generated from the
resources of physical theories or are dictated by merely logical considerations.
It seems reasonable to surmise that the answer to this question has
to be found in a conceptual analysis of the relevant physical theories.
2. Time travel in Classical General Relativity. The possibility of
time travel (TT) in classical general relativity (CGR) has long been known,
at least since Godel (1949) found a solution of the Einstein eld equations
(EFEs) which contains closed timelike curves (CTCs). CTCs represent permitted
paths for material particles that are continuous, timelike, futuredirected
curves intersecting themselves, thus forming causal loops. These
CTCs, nota bene, emerge as a result of the geometric structure of spacetime
rather than from an action performed by an agent attempting to travel back
in time. I call a spacetime that does not contain CTCs a causal spacetime,
and acausal otherwise. It is important to note that CTCs are not due to
some form of backward causation, but, as they are timelike everywhere, only
require standard forward causation. Thus, many of the original philosophi-
1The loci classici of this debate are Dummett 1964, Lewis 1976, and Mellor 1998,
132-135.
2And that some of the adduced arguments against the admissibility of causal loops are
thus fallacious. See e.g. Berkovitz 2001 and Dowe 2002.
2
cal worries do not apply.
As we have no indication that the spacetime region we inhabit is aicted
by the presence of causal circularities, there is widespread agreement that we
do not live in a globally acausal universe. Most of the spacetimes containing
CTCs, therefore, are not taken to represent a relevant cosmological model.
However, when it comes to astrophysical objects, the situation changes. The
most prominent example of a spacetime representation of such an object,
the Kerr-Newman solution, describes the gravitational collapse of a star and
harbours CTCs in its maximal extension (Hawking and Ellis 1973, section
5.6). The resulting ring singularity has a mass, rotates around its axis of
symmetry, and is electromagnetically charged. Although it is believed that
such objects do indeed exist and that the Kerr-Newman solution thus gains
physical relevance, there remain several challenges to the practicability of
time travel in Kerr-Newman spacetimes, such as the astronomical energy
requirements for a spacecraft scheduled to follow the CTCs.
Because all spacetimes contaminated with CTCs turned out either to
be unphysical entirely or to contain CTCs only in inaccessible regions, the
problem of time travel was largely regarded as academic. All this changed
as a result of two seminal articles by Morris and Thorne in 1988, which
communicated the possibility that an advanced civilisation might produce
CTCs where otherwise none would have existed. Physicists started to take
the possibility of time machines (TMs) seriously and devoted their attention,
as Hawking would put it, `to making the universe safe for historians'. A TM
is a device that creates CTCs in a region to the future of its operation,
where otherwise none would have transpired. Thus, spacetimes that allow
for TMs stand in contrast to spacetimes with `naturally' occurring CTCs,
i.e. physically possible worlds where the time traveller can take advantage
of the pre-existing acausal structure of the spacetime to travel along a CTC
without the need to produce one. TT, therefore, can occur in a much wider
range of scenarios than only as a result of the operation of a TM.
The physicists' attempts to achieve the goal of making the universe safe
for historians focused on extracting no-go theorems for TMs from the resources
of CGR itself, generally by imposing further conditions such as the
weak energy condition (WEC). The most promising of these results was
Hawking's Chronology Protection Theorem (Hawking 1992, 2002).3 Several
other no-go theorems for limited cases have been proved. But, as Earman
et al. (2003) argued, this and other partial no-go theorems have not conclusively
established the impossibility to operate a TM. They propose a
3For a serviceable survey of the results obtained up to 1995, see Earman 1995.
3
necessary condition for the operation of a TM, the so-called `potency condition'
(PC). In section 3, I review PC and other general properties that need
to be satis ed such that an arbitrarily advanced civilisation could operate a
TM.
On 7 August 2002, Krasnikov (2002b; cf. also 2002a) published a new
theorem pertaining to the possibility of TMs. In fact, Krasnikov's theorem
suggests that the PC must be violated in a very general class of spacetimes.
I discuss this novel result, its relevance for TMs, and its implications for the
PC in section 4. This will lead me to reconsider the PC in section 5 and to
conclude that Krasnikov's Theorem does not prohibit a weaker version of
the PC to hold. Such a weaker PC entails that the operator does not have
the full control over her TM. Rather, if a mitigated PC holds, she could only
increase the probability of the emergence of CTCs by operating her TM. In
section 5, I will evaluate the prospect of de nite results in this direction.
3. Time Machines and the Potency Condition. We need to specify
some general features a spacetime must display in order to qualify as a
universe which allows for the operation of TMs. Unfortunately, it is in
principle impossible to list conditions on some nite spacetime region that
are sucient to causally determine the emergence of CTCs. However, there
are necessary conditions that have to be ful lled for the operation of a proper
TM in relativistic spacetimes. To these, I now turn.
First, the spacetime (M; g) must permit a global spacelike hypersurface
 that divides M into a `before' and an `after'. Given the failure of
absolute simultaneity, of course, these temporal relations should not be taken
to imply that all points in  are simultaneous, but rather that the hypersurface
 intersects all future-directed timelike curves (henceforth `chronological
curves'), demarcating a past and a future section. An assignment of past
and future sections of those chronological curves that intersect  is unique
if and only if the curves intersect  only once. A spacelike hypersurface 
is then called a partial Cauchy surface if no causal (i.e. future-directed and
non-spacelike) curve intersects  in more than one point. Apart from insisting
that  be a partial Cauchy surface, we also demand that M is causal
up to, and including, `time' . If this were not the case, then why should
anyone bother to construct a TM? To capture this idea more precisely, some
terminology is in order.
A TM requires more than a spacelike hypersurface. It involves initial
data on  such that their manipulation within some nite and compact region
K|the TM region|of the future domain of dependence D+(), i.e. the
4
region of spacetime for which all causal in
uences emanate from ,4 brings
about the existence of a region where CTCs occur. This region is termed
the chronology-violating domain V . If any causal signals are supposed to
reach V from K at all, then V  J+(K), where J+(U) is the causal future
of U, de ned as the set of all points in M which can be reached from U by
a causal curve in M. (J􀀀(U), the causal past of U, is de ned analogously)
For an illustration, see Fig. 1. Similarly, I(U) denotes the chronological

K (TM region)
'
&
$
%
V
@
@
@
@
@
@
@
@
@
@
􀀀
􀀀
􀀀
􀀀
􀀀
􀀀
􀀀
􀀀
􀀀
􀀀
J+(K)
Figure 1: Basic structure of a TM spacetime.
future (past) of U de ned as the set of all points inMwhich can be reached
from U by a chronological curve in M (set of all points in M from which U
can be reached by a chronological curve in M). In what follows, I want to
make the following explicit
Assumption 1. No CTCs exist in I􀀀().
A sucient condition for the operation of a TM would be the time traveller's
ability to bring about the emergence of CTCs in a causally deterministic
way, i.e. such that all causal curves that reach V intersect . Clearly,
4The past domain of dependence D􀀀() is de ned analogously. The domain of dependence
D() is de ned as D+() [ D􀀀().
5
the above requirement that V  J+(K) does not preclude the possibility
that causal in
uences from outside J􀀀(V ) \  penetrate V . Causal in
uences
on V emanating from outside  are avoided if and only if V  D+(),
which cannot be the case since V contains CTCs and  is a partial Cauchy
surface. Therefore, there exist causal curves that reach V and yet do not
intersect . Although it may thus become a daunting task to de ne su-
cient criteria for the operation of a TM, it is possible to nd more stringent
necessary conditions.
The possibility of creating CTCs as an unavoidable result of the operation
of a TM in K in terms of causal determinism is hence precluded.
However, there must be some sense in which the operation of a TM brings
about the emergence of CTCs, for otherwise the device could not rightfully
be called a TM. The task at hand, then, is to nd the strongest sense in
which the appearance of CTCs can be due to the operation of a TM in K
and to de ne a necessary condition that has to be met in order for the scenario
to qualify as a case of a TM in this sense. Earman et al. (2003) have
suggested such a necessary criterion, viz. what they dub the
Potency Condition (PC). In a TM scenario, every smooth, maximal,
hole-free extension of D+() contains CTCs. These extensions must be solutions
of the Einstein eld equations (EFEs) and satisfy energy conditions.
The PC requirements for admissible extensions as speci ed are widely
agreed upon as watermarks of physical plausibility. In particular, the hole
freeness is important as it prevents additional causal in
uences from reaching
V apart from those from outside of J􀀀(V ) \  we had to accept earlier.
Hole freeness assures the absence of tears in the spacetime fabric and thus
minimises the causal in
uences on V not emanating from .
4. Krasnikov's No-Go Theorem. The evolution of a spacetime in CGR
is fundamentally non-unique. Into whatever extension U0 ) U the initial
spacetime U evolves, there always exists a spacetime U00|and in fact, in-
nitely many|which also represents a possible evolution of U. The question
of no-go theorems can thus be reformulated as the search for general classes
of spacetimes for which all maximal extensions contain CTCs, or at least all
maximal extensions satisfying certain criteria. Krasnikov (2002a, 2002b) has
recently proved a theorem suggesting that whatever happens in the causal
region of a spacetime M, it can always evolve according to a maximal extension
where V = ;, thereby violating PC and disallowing TMs.
That the operation of a TM is indeed impossible is suggested by a recent
theorem due to Krasnikov (2002b):
6
Krasnikov's Theorem (KT). Any spacetime U has a maximal extension
Mmax such that all closed causal curves in Mmax (if they exist there) are
con ned to the chronological past of U.
In other words, there is always a possible evolution of a causal spacetime
beyond the Cauchy horizon such that it preserves its causal nature. KT
entails that any set of initial data on  can be evolved without creating
CTCs. Therefore, in any causal spacetime, the laws of motion, which, for
their validity inside a region U, do not depend on anything beyond U, are
compatible with any initial data in this causal region. If a TM is understood
as a device which necessarily has to comply with PC, then KT eectively
claims to prohibit their operation (Wuthrich 2002).
While it does not outlaw TT altogether, KT prohibits the TM operator
to bring about an inevitable appearance of CTCs to the future of . There
may still pop up, at some point into the future of , CTCs. Typically,
a causal spacetime can still evolve into one of in nitely many extensions
containing CTCs. Nothing was said as to which one of these extensions
the initial spacetime will actually evolve into. KT only asserted that there
always exists the possibility that the operation of a TM fails.
Thus, CTCs can emerge, as Krasnikov puts it, `spontaneously', as opposed
to the `arti cially crafted' CTCs. The problem is that this is true
in spite of the locally causal character of the dynamical laws. Arntzenius
and Maudlin (2000) have argued that TT generically results not in contradictions
in V nor in constraints on the initial data on , but rather in
an underdetermination of what happens beyond the future Cauchy horizon
by the initial data. The task at hand, it seems, should be to try and nd
natural laws or additional conditions that restrict the class of admissible
extensions beyond the future Cauchy horizon and whose validity can be
independently con rmed. But this is not what KT achieves: rather than
con ning the range of possibilities, it shows that no setting of initial data
on  can necessitate the emergence of CTCs.
What should we make of Krasnikov's claim that his result shows that
PC must be violated? Surely, the possibility that the time traveller can
comply with PC is saved if it turns out that there are cases in which all
CTC-free maximal extensions of D() violate one or several of the further
requirements of PC. The TM operator could maintain her hope on the basis
that the spacetimes Krasnikov considers are only minimally speci ed in that
they have to be manifolds with Lorentz metrics. In other words, how can it
be ascertained that if D() is hole-free, and it satis es the EFEs and energy
conditions, that at least one of its evolutions-sans-CTCs does as well?
7
In spite of the lack of a conclusive result, there seem to be no problems
with respect to hole freeness. The concept of causal convexity of spacetimes
central to Krasnikov's proof seems to bar the danger of holes appearing in
the spacetime fabric. Also with respect to EFEs and energy conditions,
Krasnikov's result seems to dash the hopes of our TM operator. If we
impose local requirements on the metric by specifying the properties of the
stress-energy tensor, then for any manifold U with a metric that satis es
these requirements, there is a maximal extension such that all CTCs are
con ned to the chronological past of U and this maximal extension also
satis es the same local conditions on its metric.5 In particular, if (U; gjU)
satis es EFEs and energy conditions, then it can be extended to a CTC-free
spacetime that does so as well. Thus, KT establishes the violation of PC.
5. Incremental Time Machines. What retreat strategies are open to
a would-be TM operator in the aftermath of KT? Now that KT has eectively
dismantled her hopes to necessitate the appearance of CTCs, the TM
operator nds herself in dire need of realigning the conceptual support for
her business. First, it turned out to be impossible to uniquely determine an
extension containing CTCs by setting the initial data on  appropriately,
because any initial data uniquely determine (up to a dieomorphism) an
extension only within the domain of dependence D() and the causality
violation region V lies without this domain. Next, the prospect of suitably
manipulating the initial data on  such as to restrict the range of admissible
extensions to those which contain CTCs was shattered by KT. But, as was
stated in the previous section, nothing so far precludes the emergence of
spontaneous CTCs.
In order to stem the spontaneous appearance of CTCs in future extensions
into which a causal spacetime with suitable physical properties can
evolve, one would have to appeal to|and prove|robust no-go theorems.
Lacking these, the TM operator may still identify loopholes to be exploited
to her end. An obvious attempt to save her from bankruptcy is to reformulate
PC into a somewhat less rigid criterion which has to be met in order
for her craft to qualify as a TM. De ne an incremental time machine (ITM)
as a device that creates a tendency of a spacetime to evolve towards an
extension with CTCs. Creating a tendency of a spacetime to evolve into
CTC-containing extensions means to increase the `measure' of evolutions
with non-empty causality-violating regions V . The PC would then have to
5I wish to thank Serguei Krasnikov for a personal communication which helped me to
appreciate this point.
8
be restated accordingly as a
Mitigated Potency Condition (MPC). The operation of an ITM must
increase the measure of those extensions of D() containing CTCs among
the set of all suitable extensions. Admissible extensions must be smooth,
maximal, hole-free, and satisfy EFEs and WEC in order to be suitable.
By construction, KT does not have any implications for MPC over and
above the acknowledgment that the operation of an ITM cannot increase the
`probability' that CTCs emerge in the extension to 1.6 Intuitively, it should
be possible to operate an ITM, if only for the reason that the choice of
initial data on  should aect in some way how history unfolds. Also, MPC
is not an unreasonable criterion for the operation of a device that quali es
as a time machine. After all, even advanced technology may fail and regular
aircrafts do not, unfortunately, always reach their destination. In this sense,
it seems too demanding to require that CTCs transpire in any evolution as
the result of the operation of a time machine. If the device would be potent
enough to reliably create CTCs in almost all cases, obtaining risk capital to
start a time travel agency should not be a problem.
The most powerful insights could be gained, of course, if we developed
an understanding of the physical mechanisms associated with the production
of CTCs. Such a theory of time machines would provide probabilities
as a measure of how likely some manipulations of the initial data are to produce
CTCs. Such a theory would have to allow for physical probabilities as
opposed to measures over non-stochastically produced evolutions of D().
In order for these probabilities to be calculable, the mechanism which is
unleashed by the operation of an ITM and which conveys the causal signal
into the future extension of D() must be known. If we do not want to open
a Pandora's box of counterfactual discourse, this mechanism must provide
a calculable alteration of the relevant probabilities. However, the prospects
of this path seem rather dim.
Although knowledge of the mechanics of an ITM may be practically
unattainable, de nite results may still be obtained by determining the measure
of the set of extensions harbouring CTCs. Ideally, measures of this set
could be compared between two possible worlds, in one of which the ITM
operates while in the other it does not. The great advantage of this approach
is the weakness of the former: it requires no understanding of the details of
the operation of an ITM.
6It still could, however, increase the measure to one as the CTC-free extensions might
be of measure zero.
9
The rst problem, then, is to nd the set of admissible extensions for any
given causal spacetime. The main diculty in this respect derives from the
non-linearity of the EFEs which has so far frustrated attempts to nd their
analytic solution. The self-interaction of the gravitational eld instils this
non-linearity even in the absence of other elds. In other words, in CGR,
the underlying spacetime cannot considered as a xed background on which
the eld evolves. Rather, the joint system of a eld-cum-spacetime must
co-evolve. But as long as the most general solution of the EFEs is unknown,
it is unclear how the set of admissible evolutions could be established.
The second diculty concerns the assignment of measures to the `space'
of admissible extensions, once we have found this space. Unfortunately, there
is not much in the literature that would help in assigning such measures.
Most of the attempts to de ne canonical measures over sets of solutions
focus on causally well-behaved spacetimes, such as the Friedmann-Lema^tre-
Robertson-Walker cosmological models.7 Mostly, these measures have been
designed to deal with the `
atness problem' of standard cosmology in an
attempt to avoid in
ationary scenarios. However, extant results in this
eld can hardly be applied to the present purpose as they only extend to
a particular parameter family of well-understood solutions. Without the
most general analytic solution at hand and thus lacking the parameters over
which these solutions range, introducing measures to the space of all possible
solutions of the EFEs will be astronomically dicult if possible at all.
In principle, results can be obtained without reference to measures.
Rather than measuring extensions with CTCs, we could count them. Presumably,
a theorem analogous to KT may be found which shows that it is
always possible to nd an extension which respects certain local conditions
while displaying CTCs. Maybe we can establish a theorem of `parallel existence'
according to which there exists, for every causally virtuous extension,
an extension with CTCs. Take, for instance, any `clean' extension received
with the help of KT. It seems that any one of these extension could be
infected e.g. with a Deutsch-Politzer gate, thus producing CTCs (Deutsch
1991; Politzer 1992). However, such limited theorems could at best serve
to strengthen our intuitions. In order to obtain more conclusive statements
about the genericity of causal and acausal extensions, one would have to
establish theorems asserting the open density of one of the two families of
extensions, causal or acausal, thus showing that it is of measure 1, while its
complement is `nowhere dense' and therefore of zero measure. Such a proof,
7Hawking and Page 1988; Cho and Kantowski 1994; Coule 1995. I wish to thank Chris
Smeenk for references and helpful comments on this topic.
10
however, would again require a metrical structure on the space of extensions.
Unfortunately, to sum up, unlike in the case of the original PC where
knowledge of the mechanism of the TM was not necessary, the notions of
ITM and MPC remain rather vague as long as we are ignorant of how an
ITM can be realised in terms of calculable physical processes and how these
processes interact with the spacetime structure. Also, as long as no canonical
measure over extensions can be introduced, assessing the prospect of an ITM
remains a daunting task. I see no obvious way out of the quandary.
6. Conclusions. The fact that CGR allows for evolutions that host CTCs
has renewed interest among physicists and philosophers of science in the
possibility of time travel and time machines. In the present paper, I have
discussed the possibility of the operation of a device that produces CTCs in
the light of recent results.
According to PC, such a device can be dubbed a TM if it brings about the
appearance of CTCs in all its future extensions by manipulation of the initial
data on a (global) partial Cauchy surface that delimits the causal past of
the time travelling age. A recent theorem due to Krasnikov, however, shows
that for a very general class of scenarios, the emergence of CTCs can be circumvented.
He proved that any spacetime can be maximally extended such
that all CTCs, if they transpire at all, are con ned to the chronological past
of the spacetime. The result was a violation of the PC in our typical TM
scenario. However, the time traveller can retreat to a mitigated PC according
to which the operation of an incremental TM increases the likelihood
that any physically suitable extension of the domain of dependence of 
contains CTCs. The assessment of MPC, however, faces diculties regarding
the de nition (and justi cation) of a measure on the space of extensions
as well as the challenge to nd an intelligible, i.e. calculable, physical mechanisms
that would amount to an ITM. But without having overcome these
obstacles, there is little hope of a fruitful investigation of ITMs.
References
[1] Arntzenius, Frank and Tim Maudlin (2000), `Time Travel and Modern
Physics', in Edward N. Zalta (ed.), Stanford Encyclopedia of Philosophy.
http://plato.stanford.edu/entries/time-travel-phys/
[2] Berkovitz, Joseph (2001), `On Chance in Causal Loops', Mind 110:
1-23.
11
[3] Cho, H. T. and Ronald Kantowski (1994), `Measure on a Subspace of
FRW Solutions and \the
atness problem" of Standard Cosmology',
Phys. Rev. D50: 6144-6149.
[4] Coule, David H. (1995), `Canonical Measures and the Flatness of a
FRW Universe', Class. Quant. Grav. 12: 455-469.
[5] Deutsch, David (1991), `Quantum Mechanics near Closed Timelike
Lines', Phys. Rev. D44: 3197-3217.
[6] Dowe, Phil (2002), `Causal Loops and the Independence of Causal
Facts', Phil. Sci. 68 (Proceedings): S89-97.
[7] Dummett, Michael (1964), `Bringing About the Past', Phil. Rev. 73:
338-359.
[8] Earman, John (1995), `Outlawing Time Machines: Chronology Protection
Theorems', Erkenntnis 42: 125-139.
[9] Earman, John, Christopher Smeenk and Christian Wuthrich (2003),
`Take a Ride on a Time Machine', to appear in R. Jones and P.
Ehrlich (eds.), Reverberations of the Shaky Game: Festschrift for
Arthur Fine. Oxford: Oxford University Press (forthcoming). phil
sci-archive.pitt.edu/documents/disk0/00/00/09/65/index.html.
[10] Godel, Kurt (1949), `An Example of a New Type of Cosmological Solutions
of Einsteins Field Equations of Gravitation', Rev. Mod. Phys.
21: 447-450.
[11] Hawking, Stephen W. (1992), `Chronology Protection Conjecture',
Phys. Rev. D46: 603-611.
[12] ||| (2002), `Chronology Protection: Making the World Safe for Historians',
in Stephen W. Hawking et al. (eds.), The Future of Spacetime.
New York: W. W. Norton, 87-108.
[13] Hawking, Stephen W. and George F. R. Ellis (1973), The Large Scale
Structure of Space-Time. Cambridge: Cambridge University Press.
[14] Hawking, Stephen W. and Don N. Page (1988), `How Probable is In-

ation?', Nucl. Phys. B298: 789-809.
[15] Krasnikov, Serguei V. (2002a), `The Time Travel Paradox', Phys. Rev.
D65: 064013. gr-qc/0109029.
12
[16] ||| (2002b), `No Time Machines in Classical General Relativity',
Class. Quant. Grav. 19: 4109-4129. gr-qc/0111054.
[17] Lewis, David (1976), `The Paradoxes of Time Travel', in Philosophical
Papers, Volume II. New York: Oxford University Press (1986), 67-80.
[18] Mellor, D. Hugh (1998), Real Time II. London and New York: Routledge.
[19] Morris, Michael S. and Kip S. Thorne (1988), `Wormholes in Spacetime
and Their Use for Interstellar Travel: A Tool for Teaching General
Relativity', Am. J. Phys. 56: 395-412.
[20] Morris, Michael S., Kip S. Thorne, and Ulvi Yurtsever (1988), `Wormholes,
Time Machines, and the Weak Energy Condition', Phys. Rev.
Lett. 61: 1446-1449.
[21] Politzer, H. David (1992), `Simple Quantum Systems in Spacetimes
with Closed Timelike Curves', Phys. Rev. D46: 4470-4476.
[22] Wuthrich, Christian (2002), `Incremental Time Machines', MA Comprehensive
Paper, University of Pittsburgh (unpublished).
13

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Sorry, this paper won't format cleanly.

Here is a link to the above article on the Internet. It can be read easier.


http://aardvark.ucsd.edu/grad_conference/wuthrich.pdf

Thanks, I forgot to post the link. It seemed short enough to just post.

ummm... formatting is not my main problem with that... need a layman's copy...

I have heard that the way UFO's travel across the Galaxy is by use of such devices that create a "V" (violating domain) or wormhole in which they travel from place to place.

I've heard that reentry is tricky.

i'm still trying to figure wormholes out... there must be a time limit ( in light years)on them, because too much can happen in a million years... either that or what they see before they leave is different than what they see when they get here...

I have a hard time getting all their abbreviations memorized.

K= time machine region
D= domain of dependence
D+() =the region of spacetime for which all causal in uences emanate from

V = the chronology violating domain
J+(U) = the causal future of U

CTC =close timelike curves

M = spacetime

etc etc etc...

I am assuming U= universe?

My conclusion is that within this particular dimensional universe, time travel in the traditional scientific sense is not possible.

Inter-dimensional travel is another story. But I think it would involve changing the structure of matter itself.

...like the transporter on the Star Trec show.

But you would have to be able to change the structure of your body and recreate it when you arrive at your destination. But that is only if you are wanting to take your physical body with you.

The other way would be like the quantum leap show were the persons point of observation (or soul)is transported to another time and inhabits a waiting body at that destination.

I have read about this kind of multi-dimensional travel in many sci-fi books. The traveler is transported without a body.... and has to be placed in a body when he arrives at his destination.