ABSTRACT
Our knowledge of the physical universe beyond the earth comes almost entirely from the electromagnetic radiation received by our eyes or our manmade sensors. The environs of the earth, which we can explore directly, constitutes perhaps 10−58 times the volume of the universe. In our lifetime, mankind has seen the extension of the universe that can be physically explored with space probes to the distance of the Solar System’s furthest planets. Human exploration thus far is limited to the moon, a tiny step on the cosmic scale. Although it is now feasible to consider unmanned space probes that will reach out to the nearest stars, it is still true that, in the foreseeable future, mankind will be limited to the observation of the radiations from distant sources as the sole means of exploring the distant cosmos.
