
Time Machines - Hooke
http://plato.stanford.edu/entries/time-machine/
======
cogware
This seems like an interesting article, but I found it very difficult to read.
For example, from the introductory paragraph:

This literature equates the possibility of time travel with the existence of
closed timelike curves (CTCs) or worldlines for material particles that are
smooth, future-directed timelike curves with self-intersections.[3] Since time
machines designate devices which bring about the existence of CTCs and thus
enable time travel, the paradoxes of time travel are irrelevant for attempted
“no-go” results for time machines because these results concern what happens
before the emergence of CTCs.[4] This, in our opinion, is fortunate since the
paradoxes of time travel are nothing more than a crude way of bringing out the
fact that the application of familiar local laws of relativistic physics to a
spacetime background which contains CTCs typically requires that consistency
constraints on initial data must be met in order for a local solution of the
laws to be extendable to a global solution.

I'm not sure what the intended audience for this is; combination philosopher
physicists? Given that it's on SEP it seems unreasonable to assume that CTCs
need no other explanation than that they are smooth, future-directed timelike
curves with intersections. I would love it if someone would ELI5 this concept,
though.

~~~
KenoFischer
I'll have a go at defining it, term-by-term,

> smooth

Meaning no abrupt kinks (for a notion of abrupt which is probably not
physically achievable, so don't worry to much about it) or
jumps/discontinuities in the path of a particle, applying to both space and
time

> timelike

roughly speaking, traveling slower than the speed of light

> future-directed

meaning going into the direction of causality

> with intersections

meeting itself again.

In other words, it's a path through space/time on which you experience your
own local time as always running forward, but yet somehow you end up in the
past.

Perhaps an analogy is in order. Suppose we were in 1+1 dimensions, with the
time dimension being compact (closing in on itself). Visualize this as an
infinite cylinder (the infinite axis being space). Now, let's impose a finite
speed of light (for simplicity 1cm/s) and define the direction of causality,
say counter-clockwise. Then, this spacetime has CTC, because we can find a
path that, is smooth, future-directed (going around counter-clockwise) and
timelike (going at angles < 45 degrees compared to the spacial axis). The
simplest such would be a simple circle around the cylinder, but any curve that
matches the above rules and intersects itself would be considered a CTC.

Now, as a final note " a crude way of bringing out the fact that the
application of familiar local laws of relativistic physics to a spacetime
background which contains CTCs typically requires that consistency constraints
on initial data must be met in order for a local solution of the laws to be
extendable to a global solution." means the following. When we usually do
physics, we know what happens locally (e.g. ball get shot at wall, bounces
back), so we simply have to figure out (or decide if we're doing a thought
experiment) where everything is in the universe at some particular time, apply
our rules everywhere and we essentially know what's happening everywhere,
anywhere. This doesn't work any more in the presence of CTC. "Consistency
conditions" basically just means that you have to chose your placement of
objects in such a way that they don't cause any paradoxes (there's a more
technical notion here, but that's essentially what's meant).

~~~
studentrob
If time travel is possible, why is consistency a concern? If one thing changes
at one time, couldn't another change at the same time? Could time travel exist
without our being aware of it by constantly changing our understanding of
events such that there appears to have been no change?

Does consistency mean that some change must cause many other changes in order
to make everything end up in perfect alignment?

~~~
KenoFischer
Consistency here is a mathematical statement. Think about the cylinder
example, and suppose you want to assign a number of each location. Then,
naturally, going once around the cylinder puts you back at the same spot on
the cylinder, so it needs to have the same number. At this point it is useful
to remember that the numbers on the cylinder are fixed. We're already
representing time as the compact dimension of the cylinder, so there's no
extra time dimension.

Now, with this setup, let's imagine living in this 1D world, just sitting
still, reading off numbers. Our life would be cyclic with period cm/((2pi
r)s).

~~~
KenoFischer
Let me try to phrase this a different way. Suppose the universe is a database,
where (t,x,y,z) are the unique keys. For every key there is some value of
"physical reality" at that point in spacetime. All the original statement is
saying that if you want to generate such a database by starting with only some
of the entries and applying the laws of physics, then in the presence of CTC,
there are some very strict conditions on what those initial rows can be if you
don't want to run into a situation where you'd want to assign two different
values to the same key (e.g. by getting to the same point forwards in time and
backwards in time).

~~~
logfromblammo
Perhaps the database administrator chose the unique keys (observer_id,
subjective_observer_time).

Then, the (t,x,y,z) coordinates could all be parametric functions of
observer_id and subjective_observer_time. If you follow a CTC and meet your
younger self, then convince your younger self to not follow the CTC, you may
simply be reassigning your younger self to a new observer_id, rather than
creating a paradox that destroys the database integrity.

As long as all observers obey the rules of causality locally, your elder time-
twin will never remember meeting their own elder time-twin, and your younger
time-twin will never remember _not_ meeting their own elder time-twin. The CTC
has no way of knowing whether anyone ever followed it or not, because it isn't
an observer. So if you ever meet yourself, you're no longer meeting yourself,
because the person you meet is from that moment no longer you. If you do
anything at all within the past half of your own light cone that interrupts
causality, you are just creating a new observer.

So the photograph doesn't change itself, like in Back to the Future. Marty
can't erase himself, but he could erase all references to himself from the
historical record, except for whatever he carried through the CTC. He could
then take a DNA test that proves him without a doubt to be the child of two
people who cannot recall ever even touching one another. Even if he mostly
restores causality, upon reversing through another CTC to a time before he
entered the first CTC, there will be a _different_ third child of his parents
living with them when he gets back--someone who might not ever traverse a CTC.

If that's how it works, time travelers could do no worse than completely erase
themselves from all historical records and the memories of every living
person. They would be considered people who never previously existed, who
spontaneously appeared, complete with memories of future events that might
never come to pass. In that case, almost _anything even remotely plausible_
could come out of _either_ end of a CTC.

------
joe_the_user
Hmm,

The thing about human beings - or goal-seeking subjects, is that they imagine
a series of possible future lines and choose those lines they want to have
happen.

If you put a human being on one of these closed timelike curves (CTCs), then
the human does their usual human-thing, the human analyzes several lines and
acts to cause the one they want to occur to actually happen. Only their action
could causing a timeline that they are already a part of to change. Which is
where the grandfather paradox might come in - the situation of the grandfather
paradox being not just that a person might kill their own grandfather but
rather that they might _want to_ kill Hitler or whoever's grandfather to keep
something they don't like from being true about their present world (and
humans generally have something they don't like about the world).

Of course various devices could be evoked to deal with this - the circuit
could be too long to influence or by some magic the things you do to stop the
undesirable event actually cause the event (or oppositely for the desired
event) - over and over again in the time loop. Science Fiction authors have
dealt with along with other scenarios.

But the problem is the "magic", (the coincidence or force that stymies human
intentions) is required for every instance of comprehensible time travel, a
situation that intuitively seems very unstable as a state-of-the-world.

Thus I think the element that's incompatible with closed timelike curves is
intentional thinking itself.

~~~
studentrob
> Thus I think the element that's incompatible with closed timelike curves is
> intentional thinking itself.

Interesting. I think I followed you.. Expanding on that,

We do not know for certain whether intentional thinking exists or if
everything is fated. We do not know if true randomness exists. Our intuition
says both intentional thinking and randomness do exist, yet there may or may
not ever be a science that proves this.

~~~
joe_the_user
Actually,

I think science, quantum mechanic in particular, points to hard randomness
being an inherent part of the physical laws.

On the other hand, I don't think you need any really specific physical laws to
have intentional thinking - I would say that any computer program that works
with tree search, alphoGo or even much simpler alpha-beta pruners, is going to
be engaging in intentional thinking on a level where they act in the present
to attain their most desirable outcome in the future - and if that future
somehow loops back to the present then the only stable situation is one where
the intentional-thinker's efforts are thwarted or otherwise stabilized - ie,
the only stable solution is artificially constructed for the instance of the
closed causation loop.

------
dclowd9901
It's cool to read something that feels like a genuine and academic stab at
time travel. At the same time, it's so brutally academic I feel like I need a
doctorate just to consume it.

------
Natanael_L
Most of these timeline variants assume that the past state of all particles
exists in the past (making up something of a trace/ribbon to today's particle
state), along the time dimension up to the current point. Kind of like a movie
tape, except that they often assume a single-timeline self-consistent
causality must exist, such that any version of events created in any timeline
must be possible to arrive at without the time travel event to have happened.
In other words, the trace can't be broken for any particle in the time
traveler's past, or must be restored before the point of the time travel.

Obviously that excludes time travel within your "light cone" from even being
possible, as going back will sever at least one particle's "causality trace"
within your own past.

More reasonable assumptions are for example that you don't need to fold
timelines together, so those branched time travel graphs made for just about
every time travel movie would be what the actual time would look like, so only
the time traveler's _local causality_ matters (any change he makes in his past
in his light cone does not affect him, it happens in a timeline parallel to
the one he came from).

Another is one where you essentially have a data tape where the the current
time is like a writer head moving along the tape, writing down what happens.
Time travel is to jump to another point and rewriting what happened. Multiple
write heads may or may not be possible (likely impossible if you assume
locally reversing the flow of time is possible, otherwise you'll get a real
big mess).

Then there's the one where time is a dimension like usual, but particles do
not leave a trace in their past - going back or forth by jumping with a time
machine puts you in empty vacuum with no end, not even with any cosmic
background radiation (that too moves forward in time with all other matter).
Perhaps gravity waves from the past would still be visible?

------
nxzero
Theory trumps reality in this example, since the assumption is that the
timeline would center around humanity, which it might or might not. For
example, say a time machine was created on a planet for which interplanetary
travel was impossible. Time would be in constrained to that planet and would
likely appear as a black hole to any observer outside its sphere of influence;
meaning any matter entering that space would be transformed to such a degree
it would be relatively speaking unreconizable and unobservable due to the
constant flux of time relative to the speed of light.

------
poelzi
Every model that does not have a proper arrow of time, and therefor prevents
time-travel backwards, is fundamentally broken. It either breaks causality or
energy conservation and therefor simply does not relate to our reality. I'm so
happy that the model I adopted disallows time travel (this does not mean that
your time can't be slowed down, but you simply can't travel backwards).

~~~
nuncanada
Agreed. But such a reasonable assumption would inviabilize philosophers
writing so many papers and books... What model did you adopt?

~~~
poelzi
Basic Structures of Matter - Supergravitation Unified Theory by Dr. Stoyan
Sarg.

[http://www.amazon.com/Basic-Structures-Matter-
Supergravitati...](http://www.amazon.com/Basic-Structures-Matter-
Supergravitation-Unified/dp/1412083877)

Uses the most minimal basic assumptions and fits experimental data very well.
Of course, the book analyzes many parts simplified, therefor small differences
to the observed values.

We have a long term plan to implement a numeric simulator to get better values
and check the model in more details. Also, there are some questions that can't
be answered otherwise - but those are corner cases in the universe (merger of
anti-matter galaxy with matter one - the result is different in this model)

------
dmh2000
i heard a discussion once on time machines that had an interesting slant. (on
the Art Bell show). The guest proposed 'what if you could be frozen in a
capsule, then woken up 1000 years later, with no ill affects. Isn't that
effectively a (future only) time machine? How is it different from a machine
that somehow teleported you to the future? In that case where are you in the
intervening 1000 years?'.

~~~
chillacy
You can get something similar by going at near the speed of light. I'm sure
future spacefaring generations will figure that one out.

------
cjslep
The important question is: are we on the alpha or beta world line? El Psy
Kongroo.

~~~
TeMPOraL
I don't know, but I'll keep drinking Dr Pepper until someone finds out. :).

~~~
Kristine1975
I don't think it's wise to wait until the 2030s to see if CERN takes over the
world. We must start operation Mímisbrunnr now! _whooshing lab coat_

