Twenty-five years ago this month, a conjecture shook the world of theoretical physics. It had the aura of revelation. “At first, we had a magical statement ... almost out of nowhere,” says Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia, Vancouver. The idea, put forth by Juan Maldacena of the Institute for Advanced Study in Princeton, N.J., hinted at something profound: that our universe could be a hologram. Much like a 3-D hologram emerges from the information encoded on a 2-D surface, our universe’s 4-D spacetime could be a holographic projection of a lower-dimensional reality.
Specifically, Maldacena showed that a five-dimensional theory of a type of imaginary spacetime called anti–de Sitter space (AdS) that included gravity could describe the same system as a lower-dimensional quantum field theory of particles and fields in the absence of gravity, called a conformal field theory (CFT). In other words, he found two different theories that could both describe the same physical system, showing that the theories were, in a sense, equivalent—even though they each included different numbers of dimensions, and one factored in gravity where the other didn’t. Maldacena then surmised that this AdS/CFT duality would hold for other pairs of theories, with one having a single extra dimension than the other, possibly even those describing 4-D spacetime like ours.
The conjecture was both intriguing and shocking. How could a theory that included gravity be the same as a theory that had no place for gravity? How could they describe the same universe? But the duality has largely held up. In essence, it argues that the goings-on inside some volume of spacetime that has gravity can be understood by studying the quantum-mechanical behavior of particles and fields at that volume’s surface, using a theory with one less dimension, one in which gravity plays no role. “Sometimes some things are easier to understand in one description than the other, and knowing that you’re really talking about the same physics is very powerful,” says Netta Engelhardt, a theoretical physicist at the Massachusetts Institute of Technology.
To read more, click here.