Abstract
Signal propagation considered in the previous chapter does not exhaust all types of motion which one might call superluminal. Imagine, for example, that we plan an expedition to Deneb. The distance d from the Sun to Deneb, determined from the parallax, is about \(1500\,\)ly. So, a pessimist could think that the expedition, if it is scheduled to start in \(t(s)=2100\,\)CE, will reach the destination no sooner than in \(t(f)=3600\,\)CE and its report will be received on the Earth no sooner than in 
CE, which makes the whole enterprise meaningless. Suppose now that the successfully completed expedition returns to the home port at 2016CE. Undoubtedly, such a trip would deserve the name ‘superluminal’.
The question of interstellar travel under present conditions of physical theory is ... uh ... vague.
Alfred Lanning in [1]
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Notes
- 1.
Originally, the requirements were much more restrictive [136].
- 2.
Of course, it only seems that the sphere encloses the mouth: the real boundary of the throat is a pair of spheres, each around its own mouth.
- 3.
The metric originally considered by Alcubierre [3] is slightly different from (9), which was proposed in [29] (in particular, the light cones in U are tilted forward), but the principle of operation is the same.
- 4.
To which in this context it is customary to refer since [136].
- 5.
Though a pair of tubes can combine into a time machine [47].
- 6.
This reasoning is not rigorous. As of today the conjecture that any shortcut violates the WEC is not proven (see [121, 140], though).
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Krasnikov, S.V. (2018). Shortcuts. In: Back-in-Time and Faster-than-Light Travel in General Relativity. Fundamental Theories of Physics, vol 193. Springer, Cham. https://doi.org/10.1007/978-3-319-72754-7_3
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