Although any conjecture about such advanced civilizations is a matter of sheer speculation, one can still use the laws of physics to place upper and lower limits on these civilizations. In particular, now that the laws of quantum field theory, general relativity, thermodynamics, etc. are fairly well-established, physics can impose broad physical bounds which constrain the parameters of these civilizations.

This question is no longer a matter of idle speculation. Soon, humanity may face an existential shock as the current list of a dozen Jupiter-sized extra-solar planets swells to hundreds of earth-sized planets, almost identical twins of our celestial homeland. This may usher in a new era in our relationship with the universe: we will never see the night sky in the same way ever again, realizing that scientists may eventually compile an encyclopedia identifying the precise co-ordinates of perhaps hundreds of earth-like planets.

Today, every few weeks brings news of a new Jupiter-sized extra-solar planet being discovered, the latest being about 15 light years away orbiting around the star Gliese 876. The most spectacular of these findings was photographed by the Hubble Space Telescope, which captured breathtaking photos of a planet 450 light years away being sling-shot into space by a double-star system.

But the best is yet to come. Early in the next decade, scientists will launch a new kind of telescope, the interferome try space telescope, which uses the interference of light beams to enhance the resolving power of telescopes.

For example, the Space Interferometry Mission (SIM), to be launched early in the next decade, consists of multiple telescopes placed along a 30 foot structure. With an unprecedented resolution approaching the physical limits of optics, the SIM is so sensitive that it almost defies belief: orbiting the earth, it can detect the motion of a lantern being waved by an astronaut on Mars!

The SIM, in turn, will pave the way for the Terrestrial Planet Finder, to be launched late in the next decade, which should identify even more earth-like planets. It will scan the brightest 1,000 stars within 50 light years of the earth and will focus on the 50 to 100 brightest planetary systems.

All this, in turn, will stimulate an active effort to determine if any of them harbor life, perhaps some with civilizations more advanced than ours.

Although it is impossible to predict the precise features of such advanced civilizations, their broad outlines can be analyzed using the laws of physics. No matter how many millions of years separate us from them, they still must obey the iron laws of physics, which are now advanced enough to explain everything from sub-atomic particles to the large-scale structure of the universe, through a staggering 43 orders of magnitude.