In physics there are basically two different types of Unified Field Theories. There is the Quantum Mechanics version. And the Einstein version. The Quantum Mechanics version has to do with particle physics in which there is an attempt to describe all fundamental forces and the relationships between elementary particles in terms of a single theoretical framework. There are many problems with the Quantum Mechanics version of the Unified Field Theory.
The Standard model defines particle interactions as forces. This means that electromagnetism has been reformulated into a particle to particle interaction. The strong and weak forces are also particle to particle interactions. Gravity on the other hand is a field theory. I feel strongly that particle interactions should not be classified as forces. Only fields emerging from a continuous medium should be classified as forces. This means that gravity will never be joined to the other so called forces unless Gravity theory is converted into a Quantum Field theory. This means that particles must somehow acquire their mass from interacting with some other particle. I believe that this theoretical model is incorrect. This aspect of Quantum Theory will never work because the source cause of gravity is not going to be due to a particle to particle interaction. According to the Theory of Super Relativity the source of gravity is due to a particle to space, (or aether) interaction.
To support my point for the last 80 or so years the best minds in physics have attempted to prove the existence of the graviton and have failed to do so. Instead of conceding that the graviton particle does not exist they continue to look for it. Additional evidence that supports my point is that again the best minds in physics have also attempted no less than 28 times to develop a quantum theory of gravity and they have failed 28 consecutive times. Still with all of this failure to confirm Quantum Theory they continue to try and use the same QM model.
This famous quote by an unknown person definitely is applicable to the QM Theory and the Standard Model. “Insanity is doing the same thing over and over again and expecting different results.”
In other words modern day scientists have repeatedly tried to prove something that is not true. Gravity does not happen as a result of a particle to particle interaction. Gravity happens as a result of being a field of force that exists within and is consubstantial with the fundamental field which is the aether. Gravity is a contraction of space.
There are some physicists that are beginning to have doubts about Quantum Mechanics.
The extreme weakness of quantum gravitational effects now poses some philosophical problems; maybe nature is trying to tell us something new here: maybe we should not try to quantize gravity. Richard Feynman What if Quantum Theory is wrong? This is the soft underbelly of the whole project of quantum gravity. If Quantum Theory is wrong, then trying to combine it with gravity will have been a huge waste of time. Lee Smolin
So to answer your question, “Why is it hard to unify gravity?”
The answer is clear. Scientists for the last 80 or so years have been trying to prove something that is not true. No amount of attempts using the most brilliant mathematical formalism will ever lead to proving something that is fundamentally untrue. The false doctrines of Quantum Theory has led science away from discovering what is true. That is why it is so hard to unify gravity to the rest of the QM Standard Model.
On the other hand
The other Unified Field Theory concept was created by Albert Einstein. In the mid 19th century. James Clerk Maxwell formulated the first field theory which was very successful. He managed to show a relationship between moving electrons which cause magnetic fields to appear and between moving magnetic fields that cause electrons to move in conductors. He also established that light is an electromagnetic wave and travels at a specific rate of motion. It is important to note that Maxwell was a believer in the underlying medium called the aether. This belief played a major role in the development of the Theory of Electromagnetism. The aether theory also played a major role in the development of the Lorentz Transformations that were used by Einstein in his theory of Special Relativity.
The next field theory that was developed happened in the early part of the 20th century and was created by Albert Einstein. "
It was called the theory of General Relativity and it was a theory of gravitation. The two theories were not connected or unified by a mechanical method. Later on in Albert Einstein’s career he attempted to construct a unified field theory in which electromagnetism and gravity would emerge as aspects of a single fundamental field. It is also important to note that at the beginning of Einstein’s career and at the end of Einstein’s career, he also believed in the foundational medium called the aether.
The evidence to support what I just said comes from a speech Einstein made in Leiden to honor his mentor and friend Hendrick Antoon Lorentz. Lorentz was the strongest supporter of the Aether theory.
Ether and the Theory of Relativity
The entire speech discusses the history and theory of the aether and how it relates to General Relativity. This is a quote from the last paragraph.
“Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.”
So here we can see in the end Einstein was convinced that an aether must exist.
The Real Grand Unified Field Theory is then properly defined as electromagnetism and gravity emerging as aspects of the single fundamental aether field.
In the theory of Super Relativity I deconstructed the model so the the unification of the three fundamental forces could be properly identified. Once this was accomplished the unification of gravity, magnetism and electrostatic field are exposed as being three different aspects of the fundamental aether field which is a quasi-elastic-solid. The various fields are different deformation types (bendings of space) which transform and define the metric of space.
The unification of the three forces is accomplished via the motion of unbalanced electromagnetic charges that move in an accelerated motion. Whenever this happens a real physical contraction of space occurs in and around the accelerating wave-particles.
In my book Master of Reality I clearly define the three forces of nature and then unite them via the motion of particle-waves that deform the aether in three different ways. Basically I provide the entire solution for the Unify Field Theory problem in my book.