Holographic spacetime

This paper is generating quite a bit of controversy. Researchers from MIT, Harvard and Caltech have teamed up with Google Quantum AI to simulate wormhole dynamics on a quantum computer using the holographic correspondence. 

Roughly speaking, holography is the idea that Gravity in N dimensions is mathematically equivalent to Quantum Field Theory (QFT) in N-1 (on the boundary), so one can simulate the dynamics of a classical gravitational system using an analogue non-gravitational quantum system, like a collection of entangled qubits on a quantum processor. This is what the authors have done, which is emphatically not the same as creating a wormhole, like some news outlets have reported.

Media sensationalism notwithstanding, the study is interesting, but not what I wish to talk about right now. The buzz surrounding this publication got me thinking again about the holographic principle and its consequences for our understanding of physics.

For quite some time physicists have tried unifying gravity with quantum mechanics, to no avail. Logically, there are three possible ways to solve the problem: either one of the two theories is fundamental, and the other emerges from it, or there exist a third more fundamental theory which the other two emerge from in different limits (small scale and large scale limit presumably).

Given Bell's theorem, which makes it very difficult to derive quantum correlations from a classical theory, physicsts have tended to prefer the other two approaches to unification. The simplest thing one could try is to derive spacetime as a coarse-grained description of some fundamental QFT. In particular, one could try to derive the (spin-2) graviton as a bound state of elementary spin<2 particles. If such a thing was at all possible, gravity might reasonably emerge as the classical theory of massless spin-2 bound states. 

As it turns out, that's not possible. There's a theorem by Weinberg and Witten which prevents massless spin-2 bound states in Lorentz-covariant relativistic quantum field theories. Essentially, you can't have gravity emerge from quantum fields. So either gravity (and spacetime) is fundamental, or the yet unknown fundamental degrees of freedom don't come in the form of quantum fields.

There's a catch. The Weinberg-Witten theorems makes a tacit assumption, namely that the composite particles live in the same spacetime as the original theory. This is exactly the loophole that the holographic principle exploits to permit such a thing as emergent gravity. In a holographic theory, the quantum fields that give rise to gravity live on the boundary of the ambient spacetime, which emerges as if it were a hologram of the boundary dynamics of the fundamental degrees of freedom.

In a sense, the two descriptions are equivalent: gravity in the bulk and QFT on the boundary. The two cannot be unified in the traditional sense because they are the same. Or put it another (more controversial) way, there's no such thing as gravity: it is simply an emergent phenomenon that springs from the quantum entanglement of small bits of information. 

The ER=EPR conjecture states this idea more formally: entangled particles are dual to wormholes (Einstein-Rosen bridges). This opens the possibility that spacetime connectivity itself might be the result of the entanglement structure of some D<4 quantum field theory. Mark Van Raamsdonk, a physicist who pioneered efforts towards proving the conjecture, came to ICTP once to give a lecture and said something that stuck with me:

Space-time is just a geometrical picture of how stuff in the quantum system is entangled.

According to this view, entanglement and space-time geometry are really the same thing viewed at different scales. We think space and time are fundamental because we see the world in terms on those a-priori categories, but they might be illusions of some more fundamental reality that is beyond our perception (quantum entanglement). 

Modern physics is increasingly moving away from the idea that spacetime is fundamental, and most theories of quantum gravity start from building blocks that are in no way temporal or spatial in nature (to name one example, think about Wolfram's theory of everything, where spacetime emerges from simple computational rules).

The world as humans understand it becomes increasingly remote from experience, increasingly alien ... and increasingly purely mathematical.