6.1 A Few Notes On The Meaning of FTL Travel
Description
This article is from the Relativity and FTL Travel FAQ, by Jason W. Hinson jason@physicsguy.com with numerous contributions by others.
6.1 A Few Notes On The Meaning of FTL Travel
Before we begin the discussion, I wanted to go over the basic idea of
what we mean by FTL travel. To do so, we should start by noting that most of
space-time through which we would want to travel is fairly flat. For those
who have not read Part III of this FAQ, that means that special relativity
describes the space-time fairly well without having resorting to general
relativity (which applies when a gravitational field is present). Sources of
gravity are few and far between, and even if you travel "close" to one, it
would have to be a significant source of gravity in order to destroy our
flat space-time approximation. Now, some FTL travel concepts we consider
will involve using certain areas of space-time which are not flat (and I
will go over them when we get there); however, the important thing for us is
that all around these non-flat areas, the space-time can be approximated
fairly well as being flat.
Thus, for our purposes, we can use the following to describe FTL
travel. Consider some observer traveling from point A to point B. At the
same time this observer leaves A, a light beam is sent out towards the
destination, B. This light travels in the area of fairly flat space-time
outside of any effects that might be caused by the method our observer uses
to travel from A to B. If the observer ends up at B in time to see the light
beam arrive, then the observer is said to have traveled "faster than light".
Notice that with this definition we don't care where the observer is
when he or she does the traveling. Also, if some space-time distortion is
used to drive the ship, then even if the ship itself doesn't move faster
than light within that distortion, the ship still travels faster than the
light which is going through the normal, flat space-time that is not
effected by the ship's FTL drive. Thus, this ship still fits our definition
of FTL travel.
So, with this basic definition in mind, let's take a look at the
problems involved with FTL Travel.
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