If I remember correctly the authors stated it was based off of the theoretical limits of efficiency for a reaction mass drive (e.g. if we could turn mass into a

If I remember correctly the authors stated it was based off of the theoretical limits of efficiency for a reaction mass drive (e.g. if we could turn mass into acceleration with very little waste energy). Which is a really great conceit in my opinion because it stays hard sci-fi where it matters (e.g. besides the aliens) while still being close enough to our time to be interesting.