The unification of forces is the idea that it’s possible to view all of nature’s forces as manifestations of one single, all-encompassing force.
Scientists have made great strides toward the goal of understanding how the forces can be combined. Newton realized in the 17th century that the same gravitational force that describes an apple falling from a tree also describes the moon’s orbit around Earth. Then, in the 19th century, James Clerk Maxwell demonstrated that the electric and magnetic forces are aspects of a single electromagnetic force. Finally, in the 20th century, Steven Weinberg, Abdus Salam and Sheldon Lee Glashow discovered that, at high energies, the electromagnetic and weak force—without which the sun wouldn’t shine—merge into a single electroweak force.
Today, scientists seek to unify this with the strong force—without which the nucleus of an atom wouldn’t hold together—under a Grand Unified Theory. Unfortunately, the predicted energy at which these forces would experimentally combine is about 100 billion times the energy produced by today's most powerful particle accelerators. And even if we succeed in figuring out how the strong force fits into the puzzle, we will still need to look to even higher energies to combine these forces with gravity. But it will be worth it. Understanding whether the known subatomic forces have a common origin is key to creating a Theory of Everything.