Why you shouldn’t stress over boundary duals

Despite its age and numerous successes, the principle of holography is often misconstrued either as a mere offshoot of string theory or as being based on a series of mysterious identifications of black hole physics with thermodynamics. In particular, any intuition one could hope to gain from these perspectives is often hidden by ambiguous constructions or obscure observations. Ultimately, it may well be that such discomfort is more due to a lack of understanding rather than anything intrinsic. Nonetheless, it would be nice to have a viewpoint that is not hindered by the mentioned problems.

In this pedagogical talk, I will attempt to present one viewpoint based on an incredible (underappreciated?) feature of classical gravity theories - that any notion of energy in such theories must live on a boundary. I will begin by motivating and illustrating this feature in a couple of classic setups, then sketch why we believe this should hold in more generic settings. Following this, I will outline a more ambitious construction which aims to unearth a boundary stress tensor for a certain class of metrics known as Locally Asymptotically Anti-de Sitter spacetimes. Almost magically, we will find several notions from quantum field theories emerging in this very classical setup, demonstrating (hopefully!) that holography truly is something far more than a mere playground for string theory. Time permitting, I will conclude with a birds-eye view of ongoing research detailing how such stress tensor constructions could be extended to the significantly murkier setting of asymptotically flat spacetimes.