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Whether that’s possible depends on subtle details and is a subject of active research, which I studied recently as the time reversal of a collapse to a black hole, with Paul Chesler, a postdoctoral fellow at Harvard’s Black Hole Initiative.

In an antiferromagnet, the flipping of spins associated with time reversal changes the pattern of alternating spins.

Fu, L. & Kane, C. L. Time reversal polarization and a Z2 adiabatic spin pump.

This supports scientist's models for how antimatter should behave. Some of the basic rules of physics predict that antihydrogen and hydrogen would emit the same light spectra — including Einstein's theory of special relativity, which states that light is the fastest moving thing in the universe; the Standard Model, which explains how elementary particles interact; and something called Charge, Parity, and Time Reversal symmetry, or the CPT theorem, which says that energy levels in matter and antimatter should be the same.

In addition, it had been assumed that the laws of nature were, in the terms of science, “invariant under time reversal.”