
Some Emerging Possibilities For Space Propulsion - mike_esspe
http://www.nasa.gov/centers/glenn/technology/warp/possible.html
======
Steuard
Speaking as a professional physicist, this page feels awfully light on actual
information. And as far as I can tell, that's merited: there's not a lot of
solid science discussed on this page, or even especially tantalizing ideas for
what might come next.

They spend a lot of time discussing quantum vacuum energy, but the one short
note about its "propulsion implications" includes the comment that "Those
particular gravity theories are still up for debate." That's perhaps generous:
quantum gravity isn't far from my field, and I've never heard of this take on
the origin of gravity and inertia.

I'm also rather puzzled by the quote "Although gravity’s effects on
electromagnetism and spacetime have been observed, the reverse possibility, of
using electromagnetism to affect gravity, inertia, or spacetime is unknown."
That's really not at all true. Our models of gravity all include very well-
defined effects of electromagnetism on spacetime: the energy density in those
fields causes space to curve, just like any other form of mass or energy.
Hypothesizing anything else would require fundamental changes to the theory.
That's entirely possible! But it's misleading to characterize the current
state of affairs as having an unfilled _gap_ like this.

So yeah. I'd love a good spacedrive as much as the next guy. But this article
doesn't do much to make me optimistic about it.

~~~
mrfusion
How do these "legitimate" ideas for FTL deal with causality? I've always
wondered that.

People always use causality to argue against FTL, but then readily acknowledge
that wormholes or Albicurie might allow it. I'm not getting that disconnect?

~~~
Steuard
Here's a link to a good discussion of some related issues by cosmologist Sean
Carroll:

[http://www.preposterousuniverse.com/blog/2011/09/24/can-
neut...](http://www.preposterousuniverse.com/blog/2011/09/24/can-neutrinos-
kill-their-own-grandfathers/)

(It's in the context of that mistaken "neutrinos move faster than light" claim
from a few years back, but the arguments are general. Bob Geroch's article
mentioned near the end is pretty much the gold standard for dealing with these
issues as far as I know.)

I really need to sit down at some point and figure out how Alcubierre "warp"
spacetimes would relate to causality. It may be a moot point: I've heard
people claim that one feature of Alcubierre's solutions in General Relativity
is that they are "eternal", in the sense that there's no obvious way to "turn
on" a warp bubble around some given region or to "turn off" the warp once you
reach some destination. If that's true, it wouldn't be useful for sending
information, which might be all that's needed to preserve causality.

As for wormholes or similarly strange spacetime topologies, those really do
open a can of worms for causality. There's some interesting physics/philosophy
involved, and I'm not sure that we've got any solid answers. (I have my own
strong suspicions about what the answer looks like, but I won't try to explain
them here.) But that may also be moot: every known wormhole solution in
relativity requires matter with properties entirely different than anything
ever observed in the universe, so we won't be testing it out any time soon.

~~~
mrfusion
Thanks! Could you explain a bit more about how Alcubierre can't be used to
send information?

Wouldn't a spaceship traveling FTL be able to carry information?

~~~
Steuard
You know what? I take that back. At some point, I'd had the impression that
Alcubierre's solution required a fixed velocity (so you couldn't start or stop
the spaceship), but evidently that's not the case. It gets to be problematic
after all! :)

------
sehugg
For something a bit more realistic, we are still studying nuclear propulsion
in fits and starts since we halted its development in the 60s:

[http://www.nasa.gov/topics/technology/features/ntrees.html#....](http://www.nasa.gov/topics/technology/features/ntrees.html#.U5cQ6i8ikUs)

------
madaxe_again
Here's an interesting Wired interview with Podkletnov on his anti-gravity work
from '97 :
[http://archive.wired.com/wired/archive/6.03/antigravity.html...](http://archive.wired.com/wired/archive/6.03/antigravity.html?pg=1&topic=&topic_set=)

------
eudox
I honestly can't imagine why NASA even acknowledges the idea of FTL travel -
Is there some kind of propeller beanie PAC forcing their hand?

They should just have a page that says: "FTL is impossible, but that's fine,
once you get used to the way things are."

~~~
TheCraiggers
>They should just have a page that says: "FTL is impossible, but that's fine,
once you get used to the way things are."

No offense intended to you, but I feel this is a disastrously negative
attitude to have. Except where morals dictate otherwise (which are outside the
scope of science anyway) nothing is banned from exploration. Suggesting
otherwise implies that we know all there is to know about a subject; to me,
this is analogous to willful ignorance. Science is dedicated to removing
ignorance, not enforcing it.

~~~
crusso
_nothing is banned from exploration_

Nothing is banned, per se... but there's just too much search space to sift
through to not be smart about where you look.

~~~
yazaddaruvala
I don't know about you, but for me, some of the best things I've ever found
were in places I shouldn't have been looking.

------
thearn4
NASA used to have a small program to evaluate concepts for breakthrough
propulsion, which ended around 2002.

[http://www.grc.nasa.gov/WWW/bpp/](http://www.grc.nasa.gov/WWW/bpp/)

~~~
chroem
Why not a fission fragment rocket? We could send a probe to the centauri
system with current technology.

[https://en.wikipedia.org/wiki/Fission-
fragment_rocket](https://en.wikipedia.org/wiki/Fission-fragment_rocket)

------
jcfrei
I would be much more excited if NASA resumed the research on NERVA
([http://en.wikipedia.org/wiki/NERVA](http://en.wikipedia.org/wiki/NERVA)). 50
years later a nuclear propulsion drive still seems like the most promising and
cost-efficient way to explore our solar system.

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sdegutis
_" Theory for a faster-than-light "warp drive" consistent with general
relativity."_

For real?

~~~
madaxe_again
[http://en.wikipedia.org/wiki/Alcubierre_drive](http://en.wikipedia.org/wiki/Alcubierre_drive)

~~~
crusso
The only bright spot in that article is that the math doesn't contradict
general relativity. Everything else points to its being a curiosity that would
disappear if GR were more integral with Quantum Mechanics.

------
misterfusion
A real-life warp drive can be practically possible by employing phased
standing waves.
[http://www.youtube.com/watch?v=6zh9abFF3ZE](http://www.youtube.com/watch?v=6zh9abFF3ZE)

------
throwwit
The submitter should have tacked on (2007).

