Hi guys,

Time travel is actually possible, as long as you are consistent (i.e.
Novikov self-consistency principle).  Please consider the argument for
it, beginning at:

http://www.reddit.com/r/askscience/comments/hr4x2/physicists_what_do_you_think_of_the_following/

Continue the discussion there at reddit if you would like.

Thank you!
F.H.


On Fri, Jun 3, 2011 at 12:09 PM, Travis Garrett
<travis.garr...@gmail.com> wrote:
> Hi Roc,
>
>   Sure.  Let me go ahead and start by assuming that we need to exist
> in an environment that began in a state of low entropy (so that life
> can evolve during the "increasing entropy phase" - I could also
> examine this assumption, but that's another discussion...).  GR then
> does some interesting things.  First, gravity in GR couples to energy
> and momentum, and everything has energy and momentum, so, er, it
> couples to everything (binding them all together like the one ring I
> suppose).  It can thus essentially "get everybody on the same page"
> when things are starting out - forcing "everybody" (all the particle
> species) to "pay attention" and synchronize their behavior...
>
>  GR can then do something quite cool.  If you feed the Einstein
> equations with a scalar field that happens to have much more potential
> energy than kinetic energy, then the spacetime responds by growing
> exponentially (i.e. the curvature is in the time direction - the
> spatial directions are driven to be very flat (i.e. the angles inside
> a triangle add up to 180 degrees), with the overall scale factor
> growing exponentially (i.e. the overall size of the triangle is
> growing exponentially in time)).  Thus, consider some complex universe
> with a lot of entropy.  Entropy is an extensive quantity, and thus if
> we consider some tiny volume element dV then there can't be much
> "stuff" inside dV, and therefore there is very little entropy inside
> dV.  If we can get a scalar field inside that dV to satisfy the
> condition that its potential energy is much larger than its kinetic
> energy, then blammo, we get inflation and that dV region can grow
> larger than our Hubble volume in a tiny fraction of a second (and then
> scalar field can decay, ending inflation, to be followed by a
> "standard" big bang...).
>
>  It is by no means an open and shut case - there are lots of details
> to be filled in - but I think the overall picture makes a lot of
> sense...
>
>    Sincerely,
>           Travis
>
> On Jun 2, 6:35 am, Roc <roc...@gmail.com> wrote:
>> nice answer.
>> could you elaborate on this, though?
>>
>> Why then should spacetime be curved?  There are at least 2 good reasons:
>>
>> 1) it allows for a big bang to happen, thus "starting things off" in a state
>>
>> > of low entropy.
>>
>> thanks
>
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