Could there be particles so energetic they have yet to be produced in our particle accelerators? Perhaps the next generation of powerful detectors will reveal such unusual entities.

Other revolutionary schemes involve modifying the law of gravity itself. Could it be that both Newton and Einstein—the greatest geniuses in physics—were wrong about the nature of the gravitational force? Perhaps their portraits of gravity, like unfinished masterpieces, require extra flourishes.

Yet another option involves transforming one or more of nature’s constants into a variable. A group of physicists recently speculated that the speed of light could vary over time. Other “variable constant” ideas involve slowly changing values of the fine-structure constant (the parameter governing the strength of electromagnetic interactions), the gravitational constant, and even mass itself.

Finally, some of the most promising approaches for explaining the cosmological mysteries postulate the existence of a fifth dimension beyond ordinary space and time. The fifth dimension arises as a means of unifying all known forces of nature into a single theory. Although its origins date back to the early days of Einstein’s general theory of relativity, it has recently been revived in methods for unification called supergravity, string theory, and M-theory.

Traditionally, if a fifth dimension exists, physicists have imagined it to be so small that it could hardly be detected. However, many contemporary approaches envision a large extra dimension, one comparable in scale to conventional space and time. In such a case the fifth dimension could influence the dynamics of the universe and possibly explain why it is accelerating. Moreover, if celestial mechanics is truly five dimensional, the Big Bang need not have been the beginning of time. Rather, it could have been a transition between different cosmic eras. Perhaps the actual cosmos is eternal and its finite age only an illusion wrought by the limitations of our senses.

Indeed, even with the best of all possible astronomical devices, much about the universe could well remain mysterious. Our place in