Research published in Physical Review Letters has brought forward an important new understanding of general relativity laws, and has found some peculiar physics happening inside black holes. Namely, that the direction of time could be flipped within them.

Many physical processes are perfectly symmetric in time. Take a pendulum for example. If somebody shows you a video of a pendulum swinging, you cannot tell if the video is moving forward or backward. But some processes are not symmetric. We can tell that a pendulum will eventually slow down due to friction and we know that it was started at some point, so we can give a temporal direction to physics. The directionality of time and our perception of it was called the "Arrow of Time" by British astronomer Arthur Eddington, and it has been linked to the entropy of the universe.

Entropy is usually described as the measure of the disorder of a system. In a more thermodynamical definition, entropy can be seen as the amount of energy a system has lost forever. In an isolated system (like the universe) entropy is always increasing. Many scientists therefore believe that there must be a link between entropy and our perception of time. This is called the thermodynamical time. If we saw a broken egg reforming and jumping back on the counter, we would know something was wrong.

The entropy of an object is an extensive property (it scales with volume, a bigger object equals higher entropy). However, it was discovered by Stephen Hawking in 1974 that the entropy of a black hole is actually related to the area of the event horizon, a boundary in spacetime that separates an observer from the black hole. It is considered the point of no return: nothing can escape the event horizon. This discovery led to the definition of the holographic principle, which states that all the information regarding a black hole in 3-dimensional space is encoded on its 2-dimensional surface, which is analogous to how a holographic image creates the illusion of a third dimension based on two dimensional projection.

Event horizons arise from general relativity solutions, but the simplest mathematical description of an event horizon leads to a paradox. Event horizons must be aware of all the history of the universe, from the Big Bang to its death. This creates several complications as it assumes that the universe must be deterministic and that past and future are written on the "skin" of black holes. While this paradox doesn’t affect observations and predictions of the physics of black holes, it is clearly a limiting factor in truly understanding how black holes work.

The new research tries to correct this significant gap in black hole physics. The basic idea of this paper is based on the holographic principle. The authors assume that the event horizon is actually a holographic screen, a hypersurface with a specific entropy. You can have two types of holographic screens, past holographic screens and future holographic screens, depending on whether the entropy within the surface is increasing or decreasing.

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

If this law is applied to the universe as a whole, the entropy arrow is consistent with the second law of thermodynamics. Entropy increases, time moves forward. But the application of the law onto black holes produces a curious result. Inside a black hole, entropy decreases (things become more organized) and thus thermodynamical time runs backward.

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

^{Image Credit: An artist's impression of a supermassive black hole by NASA/CXC/M.Weiss, via Flickr. CC BY-NC 2.0}