Time reversal and negative energies in general relativity

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The significance of past-pointing four-vectors and negative energies in general relativity is discussed. The sign of the energy is not absolute, but relative to the four-velocity of the observer, and every particle/observer always measures its own mass as positive. It is shown that the description of the interaction of past-pointing and future-pointing matter in general relativity requires two metric tensors for self-consistency. This aspect of general relativity might account for the observations that led to the proposal of "dark energy" and non-baryonic "dark matter".