[FoRK] [NSG-d] [NSG] Meeting Announcement 09/18

Eugen Leitl eugen at leitl.org
Wed Sep 19 01:09:55 PDT 2012


----- Forwarded message from "Jerry B. Roberts" <jbr at pobox.com> -----

From: "Jerry B. Roberts" <jbr at pobox.com>
Date: Tue, 18 Sep 2012 14:33:19 -0400
To: nsg-d at marshome.org
Subject: Re: [NSG-d] [NSG] Meeting Announcement 09/18
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Reply-To: Nanotechnology Study Group - open discussion <nsg-d at marshome.org>

>> "Warp Drive May Be More Feasible Than Thought, Scientists say."

This analysis is clearly trying to base itself on standard GR.  That means 
it has at its heart the most basic assumption of relativity theory -- 
namely that all inertial frames are fully equivalent.  In turn,  
unfortunately, that means that their drive (like any FTL drive that works 
within relativity) is an actual, realizable way to go back and kill your 
grandmother.  (Accelerate away from the Earth in a direction opposite to, 
say, Sirius.  Do this to high speed by conventional means, but before 
you've gone very far turn on your warp drive toward Sirius.  When you get 
there, turn off warp and use conventional thrust to get going fast away 
from Earth.  Turn on warp to return to Earth.  If inertial frames are 
equivalent, you have to arrive well before you left.  Put it this way:  
Just before you turn on warp, kick Ford and Arthur out the airlock.  If 
they then see you, with respect to themselves, proceed to reach "10 times 
the speed of light", and this happens regardless of what speed you were 
going when you kicked them out, then you can time travel.)

That doesn't have to be a flaw of FTL travel, if local physics actually  
respects a universal frame of reference.  Such a frame -- the average  
motion of matter -- is perhaps the most conspicuous feature of physics  
globally.  Just stand still with respect to the 2.7K background radiation, 
and the universe is of the opinion that you are indeed standing still.  If, 
though, you can detect this frame with an experiment set up inside a 
bounded bit of otherwise empty space, that violates the relativity 
principle.  A real FTL drive would presumably respect (and have to respect) 
the simultaneity defined by this global frame.  The problem with FTL travel 
is thus often overstated:  It's not that FTL means causality collapses; 
it's just that FTL means that /either /1) causality collapses (mightly 
unlikely), or 2) different inertial frames aren't, in fact, /exactly 
/equivalent (much, much more likely).

Nineteenth century physics was very full of confidence in the finality of 
Newtonian mechanics, given its exquisite match to celestial measurements.  
Oops!  The match wasn't absolutely correct; but this necessarily went 
hidden until ways appeared to test things at much greater extremes.  
Current physics is very full of confidence in the finality of relativity 
theory, at least as far as behavior above the quantum scale is concerned.

There is, of course, the possibility that even at the macroscopic scale  
relativity is incomplete in the same sense that Newtonian mechanics was.  
There could be one or more layers of deeper theory that are still not 
quantum-mechanical.  What could be really interesting about the work  
reported in the article, if it's correct, is that it would put a stake in 
the heart of the notion that GR is a full description of macroscopic  
space-time physics.  People have known for a long time that GR allows of  
solutions with macroscopic closed time-like curves.  These have  
tentatively been waved off as a non-fatal problem, based on the hope that 
no initial conditions could ever arise in the real universe that would 
actualize such a curve.  My own gut tells me that -- forget the initial 
conditions -- if the rules by which a state evolves allow paradoxes for 
/any /state that's describable, the theory can't be complete.  But the gut 
argument becomes moot, if you can actually build anything that's FTL!

So there are three possibilities (unless you include killing your grandma):
1) The analysis in the article is just wrong -- it misinterprets the  
physics it's trying to use.
2) The analysis is right, but GR is wrong in a way that defeats the drive.
3) The analysis is right, with GR being wrong in a way that allows their  
design to actually do FTL travel.

Probably in order of decreasing probability.

Also, bear in mind:  Local physics respecting a universal frame doesn't  
mean FTL travel is possible.  It's just a necessary condition, not a  
sufficient one.

Jerry


On 9/17/2012 12:56 PM, Fred Hapgood wrote:
> Meeting notice for 09/18:
>
> These days we are meeting at Tom Yum Koong II, at 1377 Mass. Ave. in
> Arlington, just across the street from the Jade Garden. 7:30 PM.
> http://www.tomyumkoong.net/
>
> Possibly worth discussing:
>
> The most recent issue of Space.com is carrying an article titled "Warp
> Drive May Be More Feasible Than Thought, Scientists say." This sounds
> promising, given that thought seems perfectly feasible.  At least in
> theory.  Anyway, guys at the Johnson Space Center are apparently trying
> to
> build one right now.
>
> http://bit.ly/QwIkFZ
>
>
>
>> <+><+><+><+><+><+><+><+><+><+><+><+>
> Announcement Archive: http://www.pobox.com/~fhapgood/nsgpage.html.
>
> <+><+><+><+><+><+><+><+><+><+><+><+><+>
>
> Legend:
>
> "NSG" expands to Nanotechnology Study Group.
>
> The NSG mailing list carries announcements of these meetings and little
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>
>
>
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> http://www.pobox.com/~fhapgood
>
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