I recently attended a lecture by esteemed physicist Prof Clifford Will in honour of Einstein's landmark contributions in understanding how gravity works. The title of the lecture was "was Einstein right?", now that's a catchy title! However, being relatively (no pun intended) up to date in the field I was trying to see whether there will be any new information around that's not caught the media attention. And, I'll say, some things were quite surprising, other things, merely consolidating.
As far as theories go, General Relativity has stood the test of time to describe (and predict) relatively accurately, many phenomena that are observed in the universe. Some of the parameters that the theory predicts come out extremely precisely. Prof Will spent quite some time explaining some experiments that have really honed in on the precision of many astronomical measurements to confirm many of Einstein's predictions. Recent measurements of gravitational waves which led to the Nobel Prize in 2017 and the discovery of the Higg's Boson (and subsequent Nobel Prize in 2013), have contributed to a renewed excitement in the whole field.
In the lecture by Prof Will he spoke about a whole host of added effects that General Relativity predicted but were too difficult to measure, until recently. These do not discredit the theory at all, but make small corrections to Relativity; think of them as small perturbations to the theory. This is done in many areas of science: you take some standard model for something and make small corrections to account for some factor and refine the theory over time. I'll give a layman's example:
You input your current and final destination on Google maps (or Waze... or take your pick) to find out how long it will take to get from X to Y by car. "Simple" says Waze, "take the shortest path and the speed limits of those roads and calculate the travel time"... "hold on a minute" says Waze, and will then adjust this time for actual average speed, traffic, unexpected events, road closures, cars stopped in the road, accidents, police presence, donkey in road, etc. etc. So you have a general rule (accounting for distance, velocity, time), but then many perturbations to make the final result more accurate (correction: you cannot currently enter 'donkey' in to Waze).
So, it seems, that General Relativity, can now account for frame dragging, rotating bodies, wobbling/spinning giant bodies, and others. But, does this fundamentally change anything in Relativity itself? Probably not, says Prof Will. What is remarkable however, is how Relativity has stood the test of time and, as far as I can tell, also stands up to the predicted perturbations that the theory could never measure when it was initially formulated.
This is a key ingredient in science: predictive power. A good theory needs to be able to predict the outcomes, and path the way, for future observations and discoveries. So in this regard, General Relativity is doing pretty well. Are there many unanswered questions? Plenty. Is General Relativity the final stop? Probably not.
I went up to Prof. Will after the lecture and asked him whether there were other contending theories that could usurp Relativity, or totally change our paradigm (much like Einstein overhauled Gravity, in the wake of Newton). Prof Will was unsure. However, there are plenty of very very (and also VERY) unknown things in the universe to date. There is currently no knowing how big a part those things will play in the sum total of our understanding of the universe at large. Some players include: quantum gravity, dark matter, dark energy, the edges of the observable universe, inflation, black holes, etc. We cant know, maybe you do?!
In any case, one very very (and may I say VERY) subsidiary positive that has come out of all of this is that major blockbuster films like Interstellar can make excellent graphic work of gravitational lensing around supermassive black holes (see image)!