The problem with unification may lie in our ideas of what the forces are and how they interact with matter.  Relativity tells us gravity is a warping of spacetime, but the other three forces are different.  Electromagnetism, strong nuclear, and weak nuclear forces are typically referred to as fields.  A field is simply an area in which a specific type of particle will experience a force.  For example, in an electromagnetic field, any charged particle such as an electron will experience a force in accordance with the strength of the field and the particle’s orientation relative to the field.  You can picture a field as a region of space that is populated with a sea of messenger particles.  These messenger particles interact with matter and tell it how to behave.  The specific type of particles involved depends on the force in question.  Photons are the messenger particles of the electromagnetic force.  The weak nuclear force is mediated by W⁺, W^-, and Z bosons, and it acts on atomic nuclei.  Strong nuclear force is mediated by eight varieties of gluons that act on quarks and nucleons.

            Many physicists, including Einstein, believed that there should be a link between electromagnetism and gravity.  They are the only two forces with an infinite range, and they both obey the inverse square law.  This means that their strength is inversely proportional to the square of the distance between the bodies being acted upon.  Gravity and electromagnetism have the same fundamental mathematical format, so it is only natural to think they might be related.  Oddly enough, this has been one of the most difficult steps to take.  Gravity and electromagnetism may share the same basic mathematical formulation, but they behave quite differently from each other.  By the early 1980s, progress in the standard model of Quantum Mechanics had started to slow.  Physicists began to look new ways to solve old problems, and Kaluza’s idea of a fifth dimension was resurrected.

Just as gravity is warping of spacetime in four dimensions, electromagnetism can be described as warping occurring in the fifth dimension.  Gravity in the fifth dimension behaves just like electromagnetism in the standard four dimensions.  From here, the natural progression is to add more dimensions and see what happens.  At this point, the precise shape of the curled dimension starts to come into play, and herein lies the problem.  Nobody knows just what shape it should be.  The possibilities have been narrowed down to a type of shape known as a Calabai-Yau space, but that still leaves a huge (possibly infinite) number of specific shapes.  The mathematics required to determine the exact shape are just too complex to be solved right now.  Although the exact shape is not known, it is possible to make rough estimates of the effects of the extra dimensions.  It seems that if there is seven curled dimensions, all four fundamental forces could be accounted for.  In the coming few years, we will probably see great advances in the physics of curled dimensions.  When the topology of these hidden dimensions is better understood, their true significance will become more clear.

There are currently three main theories that employ curled dimensions, Superstring Theory, Eleven-Dimensional Supergravity, and the most promising of them all, M-Theory.  As it stands now, Superstring Theory and Eleven-Dimensional Supergravity appear to be separate parts of M-Theory.  When the math behind M-Theory can be worked out further, it will probably add another layer of unification to modern physics.  All three theories employ a total of eleven dimensions; three extended, one curled, and one time dimension, to explain the four fundamental forces and the three families of elementary particles.  The current theories are very different from the original Kaluza-Klein theory, but they could never exist without the willingness of two scientists to challenge not only the general opinion at the time, but the way our universe appears to us. 

In lieu of a final paragraph in which all is revealed and resolved, I am forced to leave you with a bit of uncertainty, which I suppose is strangely appropriate for an article about theoretical physics.