Oskar Klein, a Swedish mathematician, proposed a possible solution to the obvious flaw in Kaluza’s theory.  Klein suggested that an extra dimension could exist in our universe, provided that it was curled up into a small enough space to escape ordinary detection.  The concept of a curled up dimension sounds a bit odd at first, but it’s actually fairly familiar.  Picture an ordinary drinking straw.  If you view the straw from far away, it looks like a line.  All you can see is the length of the straw, it appears to have no width at all.  When you get closer to the straw, you can see two distinct dimensions-- the extended dimension of the straw’s length, and the smaller circular dimension of its width.  This is an example of a curled dimension.  From a distant vantage point, the straw appears to be a one-dimensional line, but up close it appears to be a two-dimensional cylinder.  This is not a perfect analogy for our universe, but it is a good place to start.  The major importance of Klein’s postulate is that it is now possible for Kaluza’s theory to have real, physical meaning.  Kaluza’s fifth dimension could simply be curled up so small that we had never noticed it.  Theories involving extra, curled-up dimensions have come to be known as Kaluza-Klein theories.  Once again, major developments in Quantum Mechanics pushed these theories into the background.

Unified theories are a bit of a holy grail in physics.  A true unified force theory would be able to explain all four of the known fundamental forces (gravity, electromagnetic, weak nuclear, and strong nuclear) under the same mathematical framework.  Symmetry and unification are very alluring concepts for scientists.  Physicists want to find that all forces are different manifestations of the same thing, and through the last century science has discovered several strong indicators of such an underlying unity.  In 1855, James Clerk Maxwell found that electricity and magnetism are not merely related to each other, but are fundamentally the same.  This unified force is known as electromagnetism.  Maxwell found that electromagnetic radiation traveled in waves, with the electric and magnetic waves lying on planes perpendicular to each other.  Years later Sheldon Glashow, Steven Weinberg, and Abdus Salam were awarded the Nobel Prize for discovering a deep connection between the weak nuclear force and electromagnetism.  Salam and Glashow were able to show that at high enough energies, electromagnetism and weak nuclear force would merge into what is known as the electroweak force.  There have been great advances in the quest to unify the electroweak force with strong nuclear force.  Already it seems that there is a connection between the two, but no strong mathematical framework is in place yet.  The combination of the electroweak and strong nuclear forces is known as grand unification.  Although a grand unified theory seems plausible, it is still beyond the current reaches of science.

Amidst all these developments in unified field theory, one thing remained a mystery—gravity.  Newton was the first to describe the effects of gravity.  He developed the first mathematical relation of mass to the force of gravity, but was unable to describe what gravity was.  Einstein was the first to describe the nature of gravity, and in doing so completely reshaped our ideas about space and time.  General Relativity is Einstein’s theory of gravity.  It relates the effects of gravity to a warping of spacetime, the multidimensional fabric of the universe.  The standard analogy is that of a bowling ball placed on a rubber membrane.  The membrane deforms the most at the center of the mass, and the deformity becomes less pronounced as one moves away from the ball.  With this analogy, it is easy to see how an object’s gravitational field can influence other objects.  They simply follow the shortest path on a warped background.  The flaw in this analogy is that the warping of the rubber sheet is caused by earth’s gravity, whereas the warping of spacetime is gravity.  The details of General Relativity are unnecessary at this point, but suffice to say it has stood the test of experiment time and time again.