

Are we getting close to time travel? - coondoggie
http://www.networkworld.com/community/blog/time-travel-may-find-home-large-hadron-collid

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archgoon
>"Because time travel is limited to these special particles, it is not
possible for a man to travel back in time and murder one of his parents before
he himself is born, for example. However, if scientists could control the
production of Higgs singlets, they might be able to send messages to the past
or future,"

This is absurd. If you can send messages back in time, then someone can build
a robot that can accept messages from the future, and kill your parents that
way^. Sending information back in time is a paradox. Period.^^

^An option available to everyone after the robot is built. Maybe this is
actually enough to resolve the issues they're concerned with. I'm somewhat
suspicious.

^^To be clear, I am not saying that the work is wrong or impossible, merely
that their example is less than explanatory about why their theory does not
have the problems of other theories.

~~~
bermanoid
_If you can send messages back in time, then someone can build a robot that
can accept messages from the future, and kill your parents that way^. Sending
information back in time is a paradox. Period_

Not quite. You're neglecting quantum mechanics.

Very roughly speaking, in quantum field theory, the probability of an event
happening is a weighted sum over all the possible ways that it could happen (a
path integral), and (again, speaking roughly) the options that seriously
violate the various conservation laws tend to have negligible contributions.
Presumably the path integral formulation would still hold up even with closed
timelike curves (time travel curves).

An inconsistent state, like one where you kill off your own parents, would
probably exhibit all sorts of self interference that would make its
contribution to the path integral very small, whereas the consistent states
would be much better represented, and much more likely to happen. Weird
"violations" of physics could quite easily present themselves as the most
probable outcomes, especially if the alternative was an inconsistent outcome.

A good way to think about this is in terms of waves on a string, or the "wave"
of an electron in orbit around a nucleus - the only vibration frequencies that
"survive" are the ones that reinforce themselves, and the rest get cancelled
out by interference (in the electron case, because they don't "match up" when
they meet themselves after an orbit). If you think of the state of the
universe as the electron, and the orbit as a loop through the closed timelike
curve, then it's easy enough to imagine quantum mechanics doing all the hard
work for us, and making sure that only consistent outcomes are reinforced,
while all the other ones are destroyed by interference.

~~~
bermanoid
I should point out, though, the fact that it's theoretically conceivable that
the universe could support time travel doesn't necessarily mean that it does,
and I suspect most physicists think it's highly unlikely.

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bermanoid
Here's the article that I assume this is referring to:
<http://arxiv.org/abs/1103.1373>

I hate judging based on stupid journalist writeups of stuff like this, so I'll
actually look through the article, for once.

The first part of the abstract: "We construct a simple class of compactified
five-dimensional metrics which admits closed timelike curves (CTCs), and
derive the resulting CTCs as analytic solutions to the geodesic equations of
motion. The associated Einstein tensor satisfies the null, weak, strong and
dominant energy conditions; in particular, no negative-energy "tachyonic"
matter is required."

Translated for the layman: "We completely arbitrarily and quite deliberately
construct a geometry that allows normal particles to time travel. It turns out
that under this geometry, if a particle time travels, it doesn't have to be
abnormal."

Let's just say I'm not impressed so far...we'll see how the rest of the
article goes...

~~~
bermanoid
Ok, so I gave this a really quick read-through, and as usual the article _way_
overstates what's been done.

I'll try to put this briefly, without too much technical detail.

General relativity permits (and we've always known this) solutions that
involve closed timelike curves (CTCs). Usually, when we look into these
solutions in detail, we find that they require strange sorts of matter to
sustain themselves, or that they're unstable. Hawking proposed that the
universe in some way bans any geometry that would allow these solutions.

The authors here claim that by adding a fifth dimension that's curled up like
a circle, and adding a specific sort of curvature to it (the metric contains a
periodic cross term between the usual time dimension and the curled
dimension), you can get CTCs that are geodesics (i.e. paths that particles
will take with no forces on them), and you don't get weird energy conditions
or anything like that.

Then they go on about the potential consequences _if_ the universe is actually
curved up like this, and so on, but don't offer any particularly compelling
reason to believe that it is.

So basically, they claim to have shown that it's possible for the universe to
be like this, and that no condition we currently know of prohibits it.

Meh. Could be, if we assume that they're right - it's always, of course,
possible that there's some other problem with their solution that wouldn't be
covered by the usual energy conditions, I'm not sure, I've been out of the GR
circuit for a while so I don't know what people currently consider to be the
most reasonable set of constraints that the Real World follows. But even if
they're right, there are plenty of solutions to Einstein's equations that
_could_ have happened and don't, so there's nothing too revolutionary here,
and mentioning a whole lotta shit about branes and the Higgs boson doesn't
change that.

This is still quite useful theoretical physics to do, mind you, but it's
nowhere near as important as the article makes it out to be.

------
civil
Maybe in this context it would be useful to reflect about _time_.

Our life is a continuous concatenating of present moments. And every present
moment is new. But where do they come from? Nobody knows.

I see time as a continuous gift from Someone outside of our physical Universe.
No other explanation is logically possible, because any explanation would come
from _inside_ our Universe -- but fresh new present moments can't be located
_inside_ , otherwise they would not be _new_.

~~~
jordan0day
I'm not sure if your post is meant to be read as philosophical/theological or
as your actual interpretation of the way time exists in our universe.

If it's the latter, you might find sections of RobotRollCall's first comment
in this reddit discussion enlightening:
[http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl...](http://www.reddit.com/r/askscience/comments/fjwkh/why_exactly_can_nothing_go_faster_than_the_speed/)

If it's the former, well, I guess you're probably right. Nobody knows. At
least, nobody thinking about it in a purely philosophical/theological context.

------
TanisDLC
An "instance in time" is simply the state of all energy at any one moment. I
say "all energy" because our reality is comprised of 1) energy in many
different forms and 2) space - the container of the energy.

To travel back in time would be to reverse the course of all energy - from the
perspective of a segment of the energy. Given the total amount of energy in
our reality - that's not very likely.

------
bhousel
This is just silly. If time travel were possible, it would have been invented
already in the future and our world would be a very different place.

~~~
arethuza
What about limited forms of time travel such as the one in the film Primer -
where they have a device that does indeed allow travel through time but only
back to the point where it was turned on and only at the normal rate (i.e. it
takes an hour subjectively to go back an hour in time).

~~~
bermanoid
In the article under discussion, this is not what the authors are talking
about: they're literally proposing a model under which every time you traverse
the extra spatial dimension, you end up at an earlier point in time. So the
reach of that communication would be, in theory, unlimited.

------
binarymax
Very interesting but its bad speculation to say we can send any type of
message this way. Messages sent to the past have already been received. So
unless scientists have been getting messages from their future experiments...

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seanalltogether
How does someone measure that a particle has been sent to the past?

~~~
hammock
You build a mechanism now that sends a particle to the future upon receipt of
a particle from the future, then later on you send a particle back in time to
that mechanism.

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GeoffreyHull
Theoretically, traveling forward in time has already been figured out. Its
just a matter of traveling REALLY fast. Traveling backwards seems to be the
biggest issue...

------
taocoyote
I have come from the future to tell you that time travel is not possible.

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hellweaver666
Why do people bother researching into time travel? Even if it's possible, it's
potentially a massively dangerous act.

Nobody should have the power to alter events outside of the present in any
way, shape or form. Period.

~~~
cryptoz
> events outside of the present

I hate to break this to you, but there's no such thing that you can describe
as simply as "the present". Since we're talking about time travel here, it's
important to remember that relativity is real, and that every single atom in
your body exists at a different "present".

Our GPS satellites are time travellers. Their distance away from Earth is so
great that we must compensate for their different perception of time, or our
reported locations would be way off.

You should not be so absolutist in your views, especially when you don't have
an accurate understanding of time.

I also take issue with you suggesting that nothing dangerous should be
"bothered" with in research. Look where the Manhattan project got us: We have
safe nuclear reactors all over the planet, we have sent a few spaceships
_outside the solar system_ (Voyager 1 and 2, for example) that run on nuclear
reactors. We know that nuclear research presents great danger, but the rewards
are so huge it's worth it.

All knowledge is worth some danger, especially the truly incredible knowledge
like finding out that our understanding of physics is incorrect, or building
spaceships that leave the solar system.

~~~
dexen
You laid it out pretty clearly, but one detail ought to be corrected --
Voyagers etc. use Radioisotope Thermoelectric Generators rather than reactors.

The difference is, there is no chain reaction in RTGs; the natural spontaneous
fission is relied on. The construction (lack of neutron moderator) prevents
chain reaction for safety reasons -- no chance of runaway reaction. On the
other hand, it means the thermal output cannot be regulated at all, and drops
steadily in parallel to the half-decay time.

The RTGs are so safe they even used miniature ones in some heart pacemakers
implanted years ago; they are still going [1].

\--

[1]
[http://duckduckgo.com/?q=nuclear+pacemaker&v=](http://duckduckgo.com/?q=nuclear+pacemaker&v=)

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maeon3
The faster you travel, the faster time goes relative to things stationary to
you it could be possible that some particles are spinning off at some multiple
of the speed of light, and is aging much faster than we are, and having spent
millions of years existing in the space next to the collision, evaporates or
flips into another place by chance.

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andymatic
No.

