A new research paper published in Physical Review Letters has brought forward a significant new understanding of general relativity laws, and has found some strange physics taking place inside black holes. Specifically, that the direction of time could be reversed within them. Several physical procedures are perfectly symmetric in time. Take a pendulum for instance. If someone shows you a video of a pendulum swinging, you cannot differentiate if the video is actually moving forward or backward.

But some processes are not symmetric at all. We can tell that a pendulum will ultimately slow because of friction and we know that it was triggered at some point, so we can give a temporal direction to physics. The directionality of time and our view of it was called the "Arrow of Time" by British astronomer Arthur Eddington, and it has been connected to the entropy of the cosmos.

Event horizons ascend from the mathematics of general relativity, but the simplest mathematical explanation of an event horizon leads to a paradox. Event horizons must contain all the history of the cosmos, from the Big Bang to its death. This generates numerous complications as it assumes that the universe must be deterministic and that past and future are inscribed on the "skin" of black holes. While this paradox doesn’t affect explanations and forecasts of the physics of black holes, it is evidently a limiting factor in correctly understanding how black holes work.

The new research tries to fill this important gap in black hole physics. The elementary notion of this paper is grounded on the holographic principle. According to the author of the paper the event horizon is essentially a holographic screen, a hypersurface with a precise entropy. You can have two kinds of holographic screens, past holographic screens and future holographic screens, liable on whether the entropy inside the surface is increasing or decreasing.

Study author Netta Engelhardt told Phys.org "Holographic screens are in a sense a local boundary to regions of strong gravitational fields. Future holographic screens correspond to gravitational fields which pull matter together... whereas past holographic screens correspond to regions which spread matter out..."

If this principle is applied to the cosmos as a whole, the entropy arrow is steady with the second law of thermodynamics. Entropy increases, time moves forward. But the use of the law onto black holes yields an interesting outcome. Within a black hole, entropy decreases (things become more structured) and thus thermodynamically time runs backward.

Although this is an exciting consequence, the paper is important because it verifies the first area law in general relativity and it might have solved a long-standing problem in black holes physics.

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