
Traveling back in time could be possible - jurjenh
https://medium.com/starts-with-a-bang/how-traveling-back-in-time-could-really-physically-be-possible-b212517d01c9
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smallnamespace
Note that under our current understanding of General Relativity, wormholes
will rapidly collapse under their own gravity so quickly that not even a
single photon would be able to traverse it because it closes off.

There's a theoretical way to stabilize it: put 'exotic matter' in the middle.
Unlike all other matter, exotic matter has a negative energy density. This is
the same sort of matter that would be needed to create an Alcubierre warp
drive.

Unfortunately, we don't have any idea how to go about creating exotic matter.
The closest we've come is the Casimir effect, when a region between two very
close conductive plates will in some ways act as if there were negative
energy.

[1]
[http://www.bibliotecapleyades.net/ciencia/negativeenergy/neg...](http://www.bibliotecapleyades.net/ciencia/negativeenergy/negativeenergy.htm)

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sullyj3
> Unfortunately, we don't have any idea how to go about creating exotic matter

A priori, I don't see any reason to believe that creating it should be
possible. It's never been observed, and I'm assuming there aren't any models
which predict any methods by which it might come into existence. That's as
good as saying it's made up to me.

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MiddleEndian
I broke my rule of not reading medium articles, but how exactly does this
prevent the grandfather paradox? Once you're back in time you can still stop
yourself in the future from entering the worm hole...

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pavel_lishin
Where's the paradox? Given that a person has already appeared out of nowhere -
matter/energy has been created - I'd say that having someone with identical
genetic make-up to them running around is minor stuff.

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MiddleEndian
I exist in 2017. I go to 1978. I kill my other form before 2017 is reached.
Therefore I cannot have traveled to 1978. What has occurred at each particualy
moment is no longer consistent in a way that is clearly comprehensible by
humans.

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kolinko
You have it backward.

The only reason you managed to live into 2017 is because all the
assassinations on your grandparents failed.

The only thing that will happen if you try to go into 1978 is that you will
discover why you failed.

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MiddleEndian
Hahaha sure but this scifi scenario is orthogonal to the wormhole time travel
mechanism.

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SolarNet
It's not clear what all the rules are and how this prevents the grandfather
paradox.

What ever mechanism that prevents the paradoxical violations also makes this
not very exciting (as far as time travel goes): you have to spend the "40
years" between you visiting one end of the wormhole and you entering the other
at near-luminal velocity (hence you would only perceive it as 1 anyway). It's
not so much "time travel" as an instant return ticket (where only your
personal real time is spent, assuming your ship is exactly as fast as the one
that carried the wormhole).

Which does actually have an interesting effect if you make them in pairs. It
makes books like The Commonwealth Saga technically almost viable (e.g. where
they build networks of portals and then just run trains through them non-stop
to travel between worlds). Except that instead of using portals to explore new
worlds, they would have to send a probe to those planets with a wormhole on
board, at near the speed of light. And trips might cost you a bit of relative
time to make a trip and return (e.g. the difference between the speed of light
and the speed the probe delivered the wormhole from both portals added
together would be how far in the relative future a trip would send you).

This all of course requires exotic matter which probably doesn't exist; which
is why this whole thing probably doesn't work in the first place (e.g. why
stable wormholes probably don't exist). Still might make it's own excellent
sci-fi book if someone ran with the constrains of this, and it would
technically be harder sci-fi than most.

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pfarnsworth
I don’t see how time travel doesn’t violate the law of conservation of energy
or conservation of mass. That seems pretty fundamental to me, so it would
require converting an extremely large amount of energy into the mass that is
going back in time.

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Gravityloss
What if you can only send and receive information?

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rurban
Theoretically yes, but what they didn't tell you is that all those particles
travelling through will completely lose all their information and order how
they belong together. So please dear sci-fi authors stop with these ridiculous
plots.

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alistproducer2
If I read that correctly, you are never able to travel back further than the
creation of the wormhole.

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sullyj3
Sounds like the movie Primer

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matt_wulfeck
The real question I have is how would you ever stabilize something as powerful
as a black hole.

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ssijak
universe> ssh god@blackhole-42

universe@backhole-42> configure

universe@backhole-42> set stable MAX_VALUE

universe@backhole-42> commit

..changes successfully committed to blackhole-42

universe@backhole-42> exit

universe>

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wallace_f
It seems we have laws of physics covering everything from Newtonian mechanics,
relativity and quantum mechanics... Is there any law stating time travel must
go one direction? Has it ever been proposed?

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BlaXpirit
Second law of thermodynamics
[https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)](https://en.wikipedia.org/wiki/Entropy_\(arrow_of_time\))

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matt_wulfeck
> _The Second Law of Thermodynamics states that the state of entropy of the
> entire universe, as an isolated system, will always increase over time_

My understanding (which is _very_ small) is that the multiverse theory takes
care of not violating this law, since it’s not an isolated system.

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PhilWright
And....there is no evidence at all that there is another Universe other than
ours. Once you have proven the existence of a multiverse we can then take
theories that require it seriously. Until then it is pure fantasy.

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snissn
> If this negative mass/energy matter exists

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true_tuna
Once

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hanoz
Travelling backwards in time is not possible, it's obviously not possible, and
it's a wonder that the idea gets given such credence in educated circles,
apart from it being a nice thought and a diverting intellectual exercise.

~~~
Shikadi
Actually, time travel is real. My name is John Titor, from the year 2036. I'm
looking for an old IBM 5100, does anyone here know where I can find one? The
future of the world depends on it!

