Energia - Grandeza intervalar ou absoluta?

Neste paper muito interessante (e razoávelmente legível), consegui esclarecer uma dúvida que coloquei no fórum de Física da UFF:

Gravity and its Mysteries: Some Thoughts and Speculations
Authors: A. Zee
(Submitted on 14 May 2008 (v1), last revised 28 Jul 2008 (this version, v2))

Abstract: I gave a rambling talk about gravity and its many mysteries at Chen-Ning Yang's 85th Birthday Celebration held in November 2007. I don't have any answers.

Gravity, knowing about everything, is the only interaction sensitive to a shift of the Lagrangian by an additive constant. In classical physics, additive constants do not affect the equation of motion. In quantum mechanics, experiments typically measure only energy differences Delta E and not the energies themselves. The Casimir effect measures the change in vacuum energy Delta E before and after the mirrors are introduced, not the vacuum energy itself (as is sometimes erroneously stated.) But gravity knows about the vacuum energy 1/2 hw!

Is the zero point energy 1/2 hw real? I should think so, since it comes directly from the uncertainty principle. The textbook demonstration of reality is of course the liquidity of helium at zero temperature, but in fact, during the early days of quantum mechanics, many of the greats were skeptical. At the 1913 Solvay Congress Einstein declared that he did not believe in zero point energy, writing to Ehrenfest that the concept was “dead as a door nail.”

Pauli also had his doubts, but the experiment gamma + H2 → H + H convinced him. He was apparently the first to worry about the gravitational effect of the zero point energy filling space. He used for M the classical radius of the electron and concluded that the resulting universe “could not even reach to the moon!” With the passage of time people found “better” things to worry about and the issue was forgotten until Zel’dovich raised it again in the late sixties.