One of those subjects that generates real 'black and white' responses.
Regarding the 'earth reference point', consider a 50 kt Northerly wind air mass, with two gliders on reciprocal N/S tracks doing 50 kts KIAS (with lateral separation, before the smart arses say they collide!) As they land in a field they encounter a brick wall at the point of touchdown. The one heading North touches down gently from a hover and the pilot steps out unscathed from his intact glider. The southerly heading one disappears into a pile of glassfibre shards and all they find is the pilot's floppy sunhat.
Relevant? Maybe not, but don't tell me an aircraft's total energy is not related/referenced to the earth.
Following on from JF's theory on theories, I once read an article written by a USN Lt. Cmdr. that explained in great detail how it was impossible for a light SE aircraft to carry out a turnback from 500 ft following an EFATO. It had lots of calculations and diagrams to prove the point. Strangely enough, as a 40 hour Bulldog student, I was able to fly quite nice turnbacks from 450 ft and upwards, so I guess that theory (which is still widely held) was wrong!