I’ve never been clear on this point: I understand Einstein tried to find some unified theory that connects what physicists consider the four forces of the universe: weak force, strong force, electromagnetic force, and gravity. I always thought he never found such a theory. Talking to a close friend who is an astrophysicist, he says Einstein DID find his unified theory. Last, when reading some books on the subject, it sounds like the experts make an exception for gravity because it doesn’t quite behave like the other three. So, is it true that Einstein found his unified theory? And, if so, did he bend the rules in his favor for what he could not explain about gravity? What’s the SD on this?
The Unified Theory combines relativity and electromagnetism.
The Grand Unified Theory combines electromagnetism, and the weak and strong forces.
The Theory of Everything combines all four fundamental forces, including gravity, so it would combine relativity and quantum mechanics.
The terms are sometimes used loosely, but they are very different in concept.
Einstein did not discover a unified theory.
The nuclear forces were not thought of until the 1930s, so for the first 10-15 years of his quest Einstein was seeking to unify only gravity and electromagnetism. I am not sure his work in the area is now considered useful.
Einstein’s search for a unified field theory was considered a waste of time by nearly all physicists of his time, and the search failed during his lifetime.
However, Einstein’s concept of unifying the forces of physics is now a part of mainstream physics, so ultimately the consensus is that he had the right idea but was not pursuing the right solution.
There may be some parts of Einstein’s work that is or will be useful, but the actual theory would be independent of his work.
Just to clarify this: He was trying to unify electromagnetism with gravity. This has still not been done. What has been cone was unify electromagnetism with the strong and weak nuclear forces that Einstein would not have known about till the 1930s. Whether his work was a waste of time is debatable. If you know where you are going, it is not research, it is development. Not to say that the latter is useless, but only to emphasize that most real research is unsuccessful, but the effort spent is not wasted since you discover where the answer isn’t. Einstein had an important idea but was not able to push it through.
Electromagnetism has been unified with the Weak Nuclear Force, but not the Strong Nuclear Force. The last cogs of “Electroweak” were published in the early 1970s by Steven Weinberg and Gerard 't Hooft (following up on work by Sheldon Glashow and Abdus Salam). Since then there has been no further unification, although EW has been put into a working relationship with SNF in a theory called the “Standard Model”.
The whole idea of unification far predated Einstein; it arose with Maxwell’s unification of electricity/light and magnetism. Further unification was an obvious step requiring no genius to perceive. Solution, however, had eluded the greatest geniuses of the field for over 80 years.
Arguably even earlier with the notion that gravitational force as demonstrated by things near the Earth’s surface falling down and the gravitation force as demonstrated by planets orbiting the sun is actually the same force. If we didn’t already know this, it certainly wouldn’t be obvious.
IIRC Unification in the sense being discussed here refers to a description of all the most fundamental natural forces by one set of equations. Did that idea predate Maxwell? Newton of course solved the gravitational problem you pose. He was also did seminal famous work on light and optics, but I do not recall him or anyone else proposing that equations might be found which described both gravity and light.
I would say that the first unified force theory was Archimedes’ unification of the forces of gravity and levity, which were prior to that time viewed as distinct.
In any event, where we stand now is that electromagnetism and the weak force are completely unified; there are some tantalizing hints that suggest that the electroweak force can be unified with the strong force, and a fair number of models on how to do it, but none have yet been experimentally verified; and there are some pie-in-the-sky hopes for unifying gravity with the others, but no real reason beyond wishful thinking to even believe it’s possible at all.
There was, at one point, serious work on trying to unify gravity and electromagnetism via something called the Kaluza-Klein model, and Einstein might have indirectly had something to do with that. But it turns out that while Kaluza-Klein can unify gravity with something that superficially resembles electromagnetism, the details are irreconcilably different. Kaluza-Klein did open up some ways of thinking that led, eventually, to the string model and its variants, which might be able to unify all four forces (if anything can), but the Kaluza-Klein model itself is dead.