As is well known, the Einstein-Hilbert action of general
#relativity, i.e. Einstein
#Gravity, can not be "simply" turned into a
#quantumFieldTheory in the usual perturbative way. Over the decades, this has given rise to dozens of proposed solutions, either by modifying the Einstein-Hilbert action (e.g. introducing a fourth derivative term), or by modifying quantum field theory (e.g. replace it by
#stringTheory ), or by examining the possibility for fully non-perturbative quantum field theory (e.g.
#asymptoticSafety studies via functional renormalization).
One of the approaches that somehow combines the first and second viewpoint is
#causalSetTheory . There, one assumes that spacetime is fundamentally discrete, not like a lattice, but more like a random
#graph and the edges in this graph represent causal relations (i.e. being in the future or past light cone). Viewed from afar, this might look like a 4-dimensional spacetime, but looked closely, there is only discrete structure. The edges (causal connections) do not only join nearest neighbours, but can reach far along the lightcone, this gives rise to non-trivial effects, and it makes it surprisingly hard to simulate such a causal set (I've tried that once, but without much success).
The causal sets community has produced a 1 hour video to explain these concepts to a general audience interested in
#physics https://www.youtube.com/watch?v=5EoZE1rOHAo
mj
Paul Balduf
Dorian Santner 🇦🇹