The marvelous telephone and television network that has now enmeshed the whole world, making all men neighbors, cannot be extended into space. It will never be possible to converse with anyone on another planet. Even with today's radio equipment, the messages will take minutes--sometimes hours--on their journey, because radio and light waves travel at the same limited speed of 186, 000 miles a second. Twenty years from now you will be able to listen to a friend on Mars, but the words you hear will have left his mouth at least three minutes earlier, and your reply will take a corresponding time to reach him. In such circumstances, an exchange of verbal messages is possible--but not a conversation. To a culture which has come to take instantaneous communication for granted, as part of the very structure of civilized life, this 'time barrier' may have a profound psychological impact. It will be a perpetual reminder of universal laws and limitations against which not all our technology can ever prevail. For it seems as certain as anything can be that no signal--still less any material object--can ever travel faster than light. The velocity of light is the ultimate speed limit, being part of the very structure of space and time. Within the narrow confines of the solar system, it will not handicap us too severely. At the worst, these will amount to twenty hours--the time it takes a radio signal to span the orbit of Pluto, the outer most planet. It is when we move out beyond the confines of the solar system that we come face to face with an altogether new group of cosmic reality. Even today, many otherwise educated men--like those savages who can count to three but lump together all numbers beyond four--cannot grasp the profound distinction between solar and stellar space. The first is the space enclosing our neighboring worlds, the planets, the second is that which embraces those distant suns, the stars, and it is literally millions of times greater. There is not such abrupt change of scale in the terrestrial affairs. Many conservative scientists, shocked by these cosmic gulfs, have denied that they can ever be crossed. Some people never learn, those who sixty years ago laughed at the possibility of flight, and ten years ago laughed at the idea of travel to the planets, are now quite sure that the stars will always be beyond our reach. And again they are wrong, for they have failed to grasp the great lesson of our age--that if something is possible in theory, and no fundamental scientific laws oppose its realization, then sooner or later it will be achieved. One day we shall discover a really efficient means of propelling our space vehicles. Every technical device is always developed to its limit and the ultimate speed for spaceships is the velocity of light. They will never reach that goal, but they will get very near it. And then the nearest star will be less than five years of voyaging from the earth. For light to travel across the solar system, it will take ______.