“The very closest stars would require many years to visit, even traveling at the speed of light, which is impossible according to Einstein’s theory of relativity. Today’s fastest spaceships would require 200,000 years to travel to Alpha Centauri, our closest bright star. The energy required to send a hundred colonists to another star, as Frank Drake has pointed out, would be enough to meet the energy needs of the entire United States over a human lifetime. And these estimates are regarding nearby stars. When we consider the distances across the entire galaxy, and between galaxies, interstellar travel seems absolutely untenable.” –David E. Fisher
So you’ve got dreams of interstellar travel. do you? And you don’t want to wait for multiple generations to come and go before we get there; you want to make it happen in a single human lifetime. The most straightforward strategy of all is to accelerate in one direction at 1g for some time, turn around mid-flight, and decelerate so that you’ll reach your destination at a reasonably low speed.
With current technology, this is woefully impossible, as the energies required — as well as the fuel masses you’d need — are simply too great. But with antimatter technology, accelerating close to the speed of light is a real possibility. Reaching it or exceeding it, though? Special relativity simply won’t allow it.