Gravity seems holographic. What does that mean for reality?

In the realm of physics, a perplexing string of characters called AdS/CFT has captured the attention of researchers for nearly a decade. This shorthand represents a remarkable connection between gravity and quantum mechanics, and the implications of this "anti-de Sitter/conformal field theory" correspondence are truly astonishing.

It suggests that gravity eliminates the distinction between volume and area, a concept known as the holographic principle. This principle has become the most profound proposal in theoretical physics in the past three decades, with foundational papers from the late 1990s garnering tens of thousands of citations.

The holographic principle posits that no matter what occurs within a region of space-time, you can discern the entire contents of the box by repeatedly measuring points on its surface. This idea, while seemingly outrageous, is rooted in thought experiments and mathematics, and it is believed to trace back to the unique nature of gravity.

Unlike other forces, gravity is always positive and has no negative counterpart, allowing researchers to infer the arrangement of matter inside a box from the warping of space-time at its surface.

One of the most compelling pieces of evidence for the holographic principle comes from the study of black holes. In the 1970s, Jacob Bekenstein and Stephen Hawking calculated the entropy of black holes, discovering that it grew in lockstep with the surface area rather than the volume. Leonard Susskind took this finding a step further, proposing that black holes are essentially holograms, with all internal events observable from the surface.

This notion, while initially met with skepticism, has gained widespread acceptance among physicists, who find it the most plausible explanation for the observed behavior of black holes.