
Alcubierre drive - angrygoat
https://en.wikipedia.org/wiki/Alcubierre_drive
======
BlackFly
There was a time where relativists realized that instead of solving the
Einstein equations subject to initial conditions coupled to some matter stress
energy terms, they could instead fully specify what spacetime looks like, ram
it through the Einstein equations and get a stress energy tensor on the right
hand side.

The Einstein equations became a recipe generator: if you want this spacetime,
then you need this matter/energy.

Unfortunately, all the interesting spacetimes (such as the Alcubierre drive)
require exotic matter, matter unlike anything we have ever seen. Other
spacetimes people were interested in generating were stable wormholes. If we
could ever produce such matter then we could start some spacetime
engineerinng. Meanwhile, we are left with these oddities.

I heard that Alcubierre somewhat regrets having his name on this spacetime as
he gets contacted by all kinds of Star Trek fans and other people who believe
they can produce the drive. He has done all kinds of other quality research in
General Relativity, but this is what he will be famous for.

~~~
Razengan
> _If we could ever produce such matter then we could start some spacetime
> engineering._

Imagine the exotic "pollution" caused by civilizations which are advanced
enough to tinker with the fabric of spacetime, but not advanced enough to
understand (or care about) the long-term consequences of that tinkering.

Destabilizing solar systems or even entire galaxies, maybe even violating
causality..

Imagine the home (and only) planets of younger, insignificant civilizations
being caught up in those cataclysms, all traces of their existence wiped from
cosmic history in what might amount to the equivalent of a traffic accident or
"oil spill" of a greater civilization.

..maybe that has already happened and some of the leviathanic voids we've
observed were a result of that? :)

[0]
[https://en.wikipedia.org/wiki/Void_(astronomy)](https://en.wikipedia.org/wiki/Void_\(astronomy\))

~~~
jerf
"Imagine the exotic "pollution" caused by civilizations which are advanced
enough to tinker with the fabric of spacetime, but not advanced enough to
understand (or care about) the long-term consequences of that tinkering"

That's fun science fiction, as others have pointed out, but in terms of real
science, at the moment that's just a sort of subtle way of anthropomorphizing
the universe and projecting human experience far, far beyond where it is
applicable. Einstein's equations for spacetime evolution don't have anything
that looks like "pollution" in them. There's no "stress term" for "space time
that has been put through the wringer and is broken now". Black holes do not
seem to leave "broken space" behind them or anything like that; indeed,
"leaving broken space 'behind' them" would be an incredibly ill-defined term.
In what preferred reference frame does your "pollution" take place in?

The only real concern is if there is some way to violate the conservation of
energy, as real space does seem to do that at large scales at the moment. In
that case someone might explode something, potentially large enough to make a
mess of a galaxy or more. But we don't have even a hypothetical example of how
to do that; just because the universe is violating energy conservation does
not mean there's any conceivable organization of energy we can create that
will do it, if the reason conservation is being violated is the injection of
energy from some other effect or something.

~~~
repolfx
By violating conservation of energy are you referring to dark matter?

~~~
antidesitter
Dark energy. Generally speaking, energy is not conserved in general
relativity:
[http://www.preposterousuniverse.com/blog/2010/02/22/energy-i...](http://www.preposterousuniverse.com/blog/2010/02/22/energy-
is-not-conserved/).

------
nabla9
Template for discussion:

A: It requires hypothetical exotic matter. It's just theoretical thought
experiment.

B: counterargument: The existence of exotic matter is not theoretically ruled
out, so it could be real.

A: That's not a good argument. Something completely hypothetical invented so
that it allows what you want to happen not ruled out is not good argument for
relevance.

B: platitude counterargument: "If an elderly but distinguished scientist says
that something is possible, he is almost certainly right; but if he says that
it is impossible, he is very probably wrong." \- Arthur C. Clarke. MOT.

~~~
trhway
the best counterargument for me is that remote galaxies run away from us with
the speeds larger than c. So space expansion and the speed faster than c as a
result is possible. Without exotic matter, etc. Now there is just an
engineering task of implementing it as a practical space ship drive.

The arguments against FTL are based on fixed spacetime frames of references.
Which is obviously not-applicable to the situation where the spacetime is what
you actively manipulate. Spacetime manipulating approaches don't violate SR as
nothing is moving faster than c wrt. local space. The same like with those
remote galaxies - while running faster than c from us they don't move faster
than c wrt. their local space.

~~~
raattgift
> space expansion and the speed faster than c as a result is possible. Without
> exotic matter, etc.

Except that in order to get the observables right, dark energy is required to
keep the galaxy clusters running away from each other faster over time. It's a
heavy price that no physical cosmologist is really happy about, but the
concordance cosmology gets its name from how it is explicitly constructed to
match ("concord with") observation (and it is consequently updated from time
to time).

It is not really surprising that some energy is required to keep the warp
bubble from dissipating away from its compact region of spacetime,
particularly since the Alcubierre model is a vacuum spacetime, so there's no
mass in the middle to keep the bubble local to it (or source it, or however
you want to look at it). He says so at the end of his paper[1]. It's important
to remember that there is no observational evidence supporting Alcubierre's
metric; it concords with nothing. However, one can still attempt to answer the
questions he himself raised, and Lobo & Visser do just that in their 2004
paper[2], where they note that the geometry of the Alcubierre metric requires
an even-more-unusual-than-dark-energy stabilizing term even at arbitrarily low
warp-bubble velocities.

\- --

[1]
[https://doi.org/10.1088/0264-9381/11/5/001](https://doi.org/10.1088/0264-9381/11/5/001)
[https://arxiv.org/abs/gr-qc/0009013](https://arxiv.org/abs/gr-qc/0009013)
(short, not especially difficult, fewer than 20 equations, 4 refs) See the
text bracketing eqns (18) & (19)

[2]
[https://doi.org/10.1088/0264-9381/21/24/011](https://doi.org/10.1088/0264-9381/21/24/011)
[https://arxiv.org/abs/gr-qc/0406083](https://arxiv.org/abs/gr-qc/0406083)
(twice as long, but mostly developed through more than 120 equations, 38 refs
incl. [1])

------
no1youknowz
AsteronX is an awesome channel and goes into the mechanics of space travel [0]
and then two videos on the Alcubierre-Froning Warp drive [1], [2].

[0]:
[https://www.youtube.com/channel/UC2kkCGRqZWaSIK3BmLC8vaw/vid...](https://www.youtube.com/channel/UC2kkCGRqZWaSIK3BmLC8vaw/videos)

[1]:
[https://www.youtube.com/watch?v=n38aP2tROBo](https://www.youtube.com/watch?v=n38aP2tROBo)

[2]:
[https://www.youtube.com/watch?v=M3fw2wQlWL4](https://www.youtube.com/watch?v=M3fw2wQlWL4)

------
geraltofrivia
Issac arthur has a fascinating video on the topic, that I recommend to anyone
who's interested in the idea:
[https://youtu.be/yCCsmxGjEV0](https://youtu.be/yCCsmxGjEV0)

~~~
russdill
He ties it in well with his Fermi paradox videos. If such a thing is possible,
it makes the stakes of the Fermi paradox much higher. We aren't just wondering
why we don't see other intelligent life in our galaxy, but why we don't see it
in a much larger region of the Universe.

------
paulhallett
The frame shift drive in the ELITE video game series is based on this
technology:
[https://www.reddit.com/r/EliteDangerous/comments/2nqtzp/fram...](https://www.reddit.com/r/EliteDangerous/comments/2nqtzp/frame_shift_drive_science/)

------
pcnix
For anyone interested in fiction based on ideas like this, check out the Three
Body Problem series. I'll avoid talking about it further to keep from spoiling
it.

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deepnotderp
Didn't they propose to elide the use of exotic matter with a Casimir vacuum.

EDIT: it says that in the wikipedia article too

------
yannk
It reminds me of Hyperion's Cantos' "Hawking drive"

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jlebrech
how about we just figure out how to get as close as possible to 1C?

~~~
russdill
Without a preferential reference frame, that's a nonsensical question. You can
define your velocity to be arbitrarily close to C just be defining in relation
to which inertial reference frame.

~~~
nategri
I would point out that this is "nonsensically pedantic." Clearly the frames of
note are the one you begin your acceleration in and the one you end your
acceleration in.

~~~
russdill
The question isn't phrased that way, its "as close as possible", implying that
there is some physical limitation that's passed by traveling 99.9% c vs 99.99%
c.

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zunzun
It would be _way_ cool if one of these could be installed in an old VW Beetle.
Totally excellent fuel economy!

