Re: Tracking

"Erik Max Francis" <max@xxxxxxxxxxx> wrote in message
news:2a-dnVKsu5YJaNLZnZ2dnUVZ_tGdnZ2d@xxxxxxxxxxxxxxxx
Logan Kearsley wrote:

But it takes finite time for that image to move all the way along the
path.
Thus, a delay in seeing where the ship actually is.
The drive doesn't have to be instantaneous, anything that emits light
(or
some other signal travelling below or at c) in normal space, while
itself
moving faster than light, will exhibit the same behavior of always
producing
an image moving away from you, but with different time lags for
different
speeds.

But he's talking about determining in which direction the ship is
travelling, not where it ends up.

I'm not entirely sure which 'he' you're referring to, so I'll respond for
both the cases of Wayne Throop and Major Oz. Pay attention to which ever one
is appropriate.
WT: Even so, since the image is always of a ship moving away from you, you
can't tell what direction the ship is moving in unless you know at least one
of where it started or where it ends up.
MO: That wasn't specified. The original post just said 'tracking'. In my
idiolect, that encompasses figuring out where it's going / has ended up, not
just in which direction it went.

I could imagine some sort of gravitational effect that would track the
ships
position instaneously. That's easier for me to imagine than something
that
emits sub-light gravitational waves, actually, as doppler shift would
seem
to produce waves of imaginary (heh, punny) wavelength without some sort
of
preferred frame defined.

Gravitational waves travel at c. Non-gravitational disturbances in the
gravitational field, just like non-electromagnetic disturbances in an
electromagnetic field, travel at c. So if general relativity is
anything like correct, a gravitational detector won't be able to detect
positions instantaneously.

That's why I didn't say 'gravitational waves'. Just some unspecified
gravitational effect. We've already broken the universe by assuming an
unspecified form of FTL; what's to stop this particular unspecified form of
FTL from producing instantaneous changes to the gravitational field at a
distance? It may be less physically plausible, but at least its easier for
me to wrap my brain around than imaginary wavelengths are.

I've had a thought: something like an Alcubierre drive could leave a light
trail without having to worry about preferred frames. The extreme tidal
forces involved would wreak havoc on any matter that got in the way, and
radiation could be emitted omnidirectionally by the constituent particles
getting back together again (or failing to do so, as the case may be), with
the frame defined by the bit of matter that just got run over.

-l.
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