And the rest of the Cosmos?I read an interesting article in yesterday's NY Times. Basically it said that because the universe is expanding (and thanks to dark energy, it is expanding at an accelerating rate), at some point in the far future the "observable universe" will be reduced to just our gravity-bound Local Group of galaxies, as everything else will have gone over the cosmic horizon, so to speak.
To astronomers in that far future, the universe will seem very much like how astronomers saw the universe before the 1920s: just the Milky Way surrounded by static, infinite darkness.
Of course, this is 100 billion years from now. I think it's safe to assume that astronomy will have advanced somewhat beyond the methods and instruments of today.
According to the Heisenberg Principle of Uncertainty, knowledge is limited. There are certain types of phenomena that you cannot observe (an electron's orbital position in an atom) without interfering with the event itself. Again, to use the electron example, to obtain the particle's exact position, you must bounce a photon off of it and alter it's speed and energy. To meaure its speed, you must refrain from knowing the position. To fix both the speed and location of the electron specifically is theoretically impossible. The observation of photons is another case in point. Light exhibits properties of both a particle (photon) and a wave. Which properties it demonstrates oftentimes seems to depend upon the equipment used to observe it.
On the macroscopic level of experience, other sets of problems arise as bodies move at relative velocities approaching the speed of light -- namely, time dilation. A traveler moving close to the speed of light relative to the earth, there will be vast discrpencies in the experience of time between the traveler and a "stationary" observer on earth. At high speeds, the theory of relativity precludes agreement in observation/knowledge.