I've just read an interesting, but ultimately unsatisfying article in New Scientist about string theory and loop quantum gravity and how these two theories might agree with each other. This agreement may be a great mathematical achievement, but it is only that, because neither theory is testable.
I have blogged about string theory before (here). It imagines every particle in nature is made of a string (in 11-dimensions) and the waves on the string determine the properties of the particle. I admire its ambition, since it tries to explain all the particles, including the graviton, the particle assumed to be responsible for gravity, and tries to be a theory of everything, but it is really a theory of nothing, since it has so many variations you can pick whatever version agrees with what you are looking at, and it makes no specific testable predictions. The one sort-of prediction made, supersymmetry, has now been falsified by the LHC (see here).
Loop quantum gravity is the other popular theory and it is simpler and bolder. A great simplification of Einstein was that he made space-time dependent on the mass within it. A bit like making the stage one of the actors in a play. He did this because space-time is something you cannot directly see anyway, so it's fair game for tweaking and this process means that general relativity is neatly 'background independent': the background space-time is determined by the mass. Loop quantum gravity continues this simplification by saying that spacetime is quantised and so, as in commercial airflight, there is a minimum distance you can travel. Loop quantum gravity is neat but has not yet made a good testable prediction. In the article they claim bouncing black holes might be a test, and there are a lot of 'may's and 'might's, but this is not the same as a controllable lab test: how can you be sure you are seeing a bouncing black hole from afar and not a million other possibilities?
Neither of these theories address the huge observations anomalies we can see including anomalous galaxy rotation and cosmic acceleration which are crying out for attention. Both theories focus on the big bang and distant black holes, as if they are afraid of a more down-to-Earth test. Common sense says we need to learn to fix the bathroom tap (eg: galaxy rotation, flybys, emdrive) before we tackle the plumbing on Pluto (eg: the big bang and black holes).
There is a theory that in some sense looks a bit like both these, but it has not come from a theoretical approach. It has come from paying attention to the anomalous observations that the mainstream ignore. This theory is MiHsC/quantised inertia/horizon mechanics (three names, take your pick!). In this theory, incomplete as yet, particle properties (inertial mass) depend on whether the Unruh waves they can see fit inside horizons. This is similar to string theory's waves on strings, but without needing to invent new waves and seven new dimensions! Quantised inertia also has the background independence of loop quantum gravity in that the behaviour of masses determines their space: an observer's acceleration creates horizons that determine what space is for that observer and that leads back to mass. Plus quantised inertia has no lack of tests, predicting galaxy rotation, its redshift dependence and cosmic acceleration perfectly and simply.
In summary, the New Scientist article is interesting and informative, but far too theoretical, as is all of mainstream physics. Too much theory is a mistake: history shows that new physics always comes from thinking about new observations, because the cosmos' imagination is far better than man's.
Cartright, J., 2017. When loops become strings. New Scientist, 11th March 2017.