

Why Do Time Travel Suicides Get Botched? - hhm
http://plato.stanford.edu/entries/time-travel-phys/#2

======
Electro
The reason people think there are time travel paradoxes are largely because
people think "going back in time" actually moves time backward. If you know
the position of every atom in the universe you can predict the future and the
past, and if you set every atom to how it was 100 years ago you've essentially
'traveled back in time', however continuity runs along just as normal because
time hasn't been effected just matter.

That's the newtonian model of time travel. In the quantum model of time
travel, causality doesn't really matter because it happens anyway and nothing
stops it; on the macroscopic scale things might get iffy, but as wormholes are
predicted to happen randomly in the subatomic scale it might be possible to
create one large enough for X-ray scale data transmission (which could hold
phenominal data amounts) and you could send design blueprints to the 1940's
and tell them how to build advanced weapons or technology with creatable parts
back then.

The final form of time travel is the multiverse version, which in time travel
is perfectly acceptable because despite going back in time you never kill YOUR
grandfather you kill a duplicate you didn't come from.

I did all the research for this for a short story I wrote, sadly nothing ever
happened to it. Perhaps when I get my site up again, I should consider
uploading it. Oh well.

~~~
crescendo
I've always thought that the "Back to the Future" style of time travel would
be a direct violation of the first law of thermodynamics.

Let's say that you invent a time machine and go back to time (t) in the past.
Now, if someone at time (t - 1 hour) measured the entire energy in the
universe and arrived at figure (e), when he measures again at time (t + 1
hour) the total energy in the universe will have increased by a factor of
(<your mass at time of departure from present> * c^2). Energy will have been
created.

I suppose you could look at it in terms of an equilibrium over all time--i.e.
if you take eternity as a whole, the disappearance of your energy from the
future would cancel out its appearance at time (t). But it would make these
laws basically useless, I think, if you have to take into account any future
or past events and how they might affect your current measurements.

~~~
Electro
That's one of the newtonian problems yes as time is essentially irrelevant you
have one amount of energy 1 second after the big bang as you do 1 second
before the end of the universe.

However, you can fix this problem. Causality won't be violated if the area you
take the energy from is sufficiently far away from where you're acting. If I
take mass from the Alpha Centauri system to solve the energy problem 5 years
ago, if I just live out 5 years causality gets violated as our light cones
merge.

If I take the energy from a star 6 billion light years away, I can go back in
time 5,999,999,999.999 years and causality won't be violated as said star
couldn't have had an effect on Earth for me to remove.

Essentially this is like time travel in a bubble.

As for "Back to the Future" style, that's actually between theories. Doc talks
about multiple timelines, essentially describing the Multiverse model; I think
Biff picks up a gambling book from the future and instead of being Marty's
families' stooge, he becomes a rich bastard and like owns the planet or
something odd. However, people disappearing from the timeline and Marty
getting weak when he was unlikely to be born is more Newtonian as in a
multiverse you can't effect your own past.

It's possible it's Quantum Time Travel, as it's predicted some particles act
in time loops; essentially they ditch our time and go back a year and then
carry on like normal and no one even notices, because they're actually there 1
second after they go back in time but we can't tell they're actually 1 year
and 1 second older not just 1 second older. It's very odd.

In Quantum, violating the law of thermodynamics (AFAIK) doesn't seem to have
an effect. As far as I understood what I read your latter assumption seems to
be on par. However, I think if you sufficiently altered the entropy of a
system, that's when causality would kick in. Merely having more mass in a
system doesn't make much different, as long as that small amount of mass
doesn't start causing asteroids to crash into the planet and black holes to
form... or the death of your grandfather. Like the predicted particles, going
back in time doesn't cause buildings to collapse or people to die, so the
universe lets it slide. I think if we went sending neutron bombs back in time
to wipe out Hitler, then there would be major causality problems for multiple
reasons namely there'd never have been a Hitler to nuke!

So overall I'd say the BttF style of time travel was obviously the writers'
confusion between two distinct theories of time travel. Multiverse fits the
universal effects, but quantum fits the characters' effects.

~~~
derefr
> in a multiverse you can't effect your own past.

But there can be events where every single version of you is guaranteed to
Infinite-Hotel to the universe next door and kill Grandfather[n+1]--basically
the same thing, as _your_ grandfather does indeed end up dead.

------
henning
As long as we're thinking about the physics of things straight out of science
fiction (that have been entertained by reputable physicists as at least not
violating the known laws of physics), there was a paper submitted to the arXiv
about how, unfortunately, warp/faster-than-light travel requires unphysical
amounts of energy, meaning it's effectively impossible. Bummer.
<http://arxiv.org/abs/gr-qc/9702026>

------
pixcavator
I found the beginning of the article especially interesting. The emphasis on
continuous transformations seems out of place though. Why not discrete? For
example, at time A I receive a coin from the future, flip it, then send it
back to time A. Question: is it heads or tails?

~~~
derefr
I suppose this is a simplified explanation of the dial they mention later on,
which they declare impossible.

