There was a non-fiction book published a few decades back called "The Millennial Project", detailing a grandiose plan of setting humanity from current civilization into a thriving, space faring one within the next one thousand or so years. Its been so long, and I lost my copy of it, but I swear there was one detail that just never sat well with my (admittedly ignorant) knowledge of physics. The author had a theory of interstellar transportation based upon, essentially, giant rail guns. Star systems would have gigantic-rail-gun-like-structures (GRGLS?) that would be precisely aimed, and would then speed up a payload to nearly the speed of light. The 'projectile', as it were, would travel multiple light years until it passed directly into the 'barrel' (so to speak) of the targeted 'GRGLS' in the destination star system. Now here is where it gets really interesting: from what i recall, the author said the system could be rigged so that the payload was slowed down by 'catching' the energy from its movement, AND that this system could be efficient to within a fraction of a percent. That is, he could use a giant electrically powered gun to shoot out something, have something else catch almost all of that projectiles energy and so, not really need to expend much of it to shoot it back. Some energy would be lost. this, it should be emphasized, was NOT some kind of perfect machine with zero entropy, but he DID say that the energy lost per each time it was used would be very low, less than 1 percent I believe. That never sat well with me. I believe my memory is accurate and the author really did write that (in so many words), but is it really possible for ANY machine to "recycle" energy with such efficiency?