A new theory of quantum gravity is providing promising results which link gravity and Einstein's General Relativity. It is called CDT (Causal Dynamical Triangulation). I like it because it's the first theory of quantum gravity and space-time of which I can actually grasp the mathematics and understand the theory, and it takes the classical differential manifold of space-time and its metric properties into account.
The theory is intuitive, non-perturbative (it isn't predicated upon the assumption of strings or membranes or extra dimensions or supersymmetry or anything like that), deterministic (and thus has very few 'knobs' that can be twiddled to skew the results), preserves the arrow of time (which very few other theories can do) and thus causality, on a large scale reproduces Einstein's classical theory while also accurately describing the quantum structure of space-time at the Planck scale (which thus far no other theory can do) and doesn't invent new tools to describe the structure of the universe, instead relying upon known and well-researched principles such as geometric triangulation, superposed quantum amplitudes and causality.
And just as reality does, eigenstates which do not reflect reality are excluded or "collapsed", wave-function collapse in mathematical form of acausal bifurcations. Hence it can not only arrive at Einstein's General Relativity equation on large scales, it can arrive at a path-integral solution to Shrodinger's equation for a single particle moving non-relativistically in space while also preserving the full quantum state of the single-particle system. Thus the theory can also work with relativistically-moving particles, virtual particles, gravity itself, etc. In fact, with sufficient computing power, it can model black holes and the beginning of the universe.
It finds that space-time at the Planck scale is flat (as many physicists now state) and 2 dimensional, but as one 'zooms out', one finds that space-time is actually curved and 4 dimensional.
Ironically, other theories of quantum gravity a priori assumed a flat 4 dimensional Minkowski space-time manifold in which the space-time background was 'frozen' or fixed, and attempted to formulate how particles would interact within that flat space-time... and they thus discounted gravity itself as a change in the structure of space-time! Their originating premise was incorrect, so the theories arrive at incorrect solutions. To compensate, they added strings or membranes or extra dimensions, complicating their theories beyond comprehension. :facepalm:
An analogy is that we've been looking at a large geodesic dome consisting of many flat panels linked together... we've been studying the surface of one single panel because we lacked sufficient computing power and mathematical aptitude to expand the view, and thus we've erroneously believed that space-time is flat (because the theories thus far put forth assumed it was flat from the get-go). But Causal Dynamical Triangulation shows the universe on a large scale to be curved.
https://arxiv.org/pdf/hep-th/0509010.pdf
There are several (hour+ long) podcast lectures from the Second Erlangen Fall School on Quantum Geometry 2013/2014 which describe the CDT theory in detail, for those interested.
The theory is intuitive, non-perturbative (it isn't predicated upon the assumption of strings or membranes or extra dimensions or supersymmetry or anything like that), deterministic (and thus has very few 'knobs' that can be twiddled to skew the results), preserves the arrow of time (which very few other theories can do) and thus causality, on a large scale reproduces Einstein's classical theory while also accurately describing the quantum structure of space-time at the Planck scale (which thus far no other theory can do) and doesn't invent new tools to describe the structure of the universe, instead relying upon known and well-researched principles such as geometric triangulation, superposed quantum amplitudes and causality.
And just as reality does, eigenstates which do not reflect reality are excluded or "collapsed", wave-function collapse in mathematical form of acausal bifurcations. Hence it can not only arrive at Einstein's General Relativity equation on large scales, it can arrive at a path-integral solution to Shrodinger's equation for a single particle moving non-relativistically in space while also preserving the full quantum state of the single-particle system. Thus the theory can also work with relativistically-moving particles, virtual particles, gravity itself, etc. In fact, with sufficient computing power, it can model black holes and the beginning of the universe.
It finds that space-time at the Planck scale is flat (as many physicists now state) and 2 dimensional, but as one 'zooms out', one finds that space-time is actually curved and 4 dimensional.
Ironically, other theories of quantum gravity a priori assumed a flat 4 dimensional Minkowski space-time manifold in which the space-time background was 'frozen' or fixed, and attempted to formulate how particles would interact within that flat space-time... and they thus discounted gravity itself as a change in the structure of space-time! Their originating premise was incorrect, so the theories arrive at incorrect solutions. To compensate, they added strings or membranes or extra dimensions, complicating their theories beyond comprehension. :facepalm:
An analogy is that we've been looking at a large geodesic dome consisting of many flat panels linked together... we've been studying the surface of one single panel because we lacked sufficient computing power and mathematical aptitude to expand the view, and thus we've erroneously believed that space-time is flat (because the theories thus far put forth assumed it was flat from the get-go). But Causal Dynamical Triangulation shows the universe on a large scale to be curved.
https://arxiv.org/pdf/hep-th/0509010.pdf
There are several (hour+ long) podcast lectures from the Second Erlangen Fall School on Quantum Geometry 2013/2014 which describe the CDT theory in detail, for those interested.