Why has nobody unified the four fundamental forces? What are the key differences between them?

Before they were combined to form electromagnetism in the late 19th century, electricity and magnetism were thought to be entirely distinct concepts.

In the middle of the 20th century, the weak interaction and the electromagnetic force were combined to form the electroweak theory.

The strong force is a quantum theory that is compatible with the electroweak theory because both theories are quantum and share similarities. The strong force is actually a residual effect of the color force that binds quarks.

It should be feasible to develop a grand unified theory that combines the color force and the electroweak theory.

It is difficult to unify the other forces with gravity because gravity is a classical theory that describes gravity as an energy-curving space-time phenomenon rather than a force. It is also very difficult to describe gravity in terms of quantum theory.

The four fundamental forces are gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. These forces have different strengths, ranges, and characteristics, making it challenging to unify them into a single framework. Gravity is the weakest force but has an infinite range, while the strong nuclear force is the strongest but operates only over very short distances. The electromagnetic force acts between charged particles and has an infinite range. The weak nuclear force is responsible for certain types of radioactive decay and has a short range. Unifying these forces requires a theory that can adequately explain their behavior across all scales, which is a significant ongoing challenge in theoretical physics.