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The history of science is littered with occasions when the scientific community felt it was on the verge of completing the big picture; One theory that could explain all physical phenomena.

Perhaps this hope was the truly lasting contribution of Newton's great work. Whilst his specific equations have been largely superseded, the audacious idea of presenting a handful of axioms and principles from which the whole dynamic universe could be determined has been the goal of the many scientifically realist physicists and cosmologists ever since.

Frontiers this week looks at the latest candidates for a "Theory of Everything" (TOE).

Einstein spent the second half of his career trying to unify the forces of nature into one comprehensible system. At that time the known forces of nature that he regarded as fundamental were electromagnetism and gravity. He was unsuccessful in his quest, but could he have succeeded if he had started with today's knowledge?

The physics community today is mostly agreed that the "big TOE" challenge lies in unifying Einstein's general theory of relativity (gravity) with quantum field dynamics (the aspect of quantum mechanics that links the other three fundamental forces).

There has been much excitement that string theory might be the secret - that our three or four dimensional perceptions could be mere projections of an underlying ten-dimensional reality, through which the various interactions between space and matter propagate.

But as Peter Evans discovers, this Holy Grail for cosmologists and particle physicists continues to prove elusive. Are we as close to knowing everything as some would like us to believe?