Some things are impossible because they violate fundamental laws of the universe, as far as we know. The theory of relativity says that neither matter nor information can travel faster than light. Matter because an object reaches infinite mass at the speed of light. (Though the recent measurement of neutrinos apparently traveling faster than light remains to be explained, most physicists suspect it reflects a subtle error, not an overthrow of the theory of relativity.) Information because that would reverse the order of cause and effect for some observers, effectively enabling time travel and violating everything we think we know about how nature operates. Other things are impossible, or at least extremely difficult, because of practical or engineering limitations rather than fundamental ones.
Travel to the stars has both kinds of constraints. The fundamental one is that no spacecraft can reach, let alone exceed, the speed of light. But that speed of 300,000 km/second is so far beyond what our spacecraft now typically achieve – 16 km/second or less at launch – that it is not currently relevant to a discussion of interstellar travel, though it may be in the future.
A speed of a few km/second has allowed us to send people to the Moon, and probes to Mars, Saturn, and more, and so it might seem that we can reach the stars, too. But travel to the stars is qualitatively different. This was convincingly expressed at the first conference devoted to the possibility of reaching the stars, the 100 Year Starship (100YSS) event held in Orlando, Florida, which I attended last September.
Sponsored by DARPA (Defense Advanced Research Projects Agency) of the U.S. Department of Defense, 100YSS asked whether, within the next century, we can build a spacecraft capable of reaching the stars. DARPA does not necessarily see direct military use for interstellar travel, but it has often invested in unconventional ideas that yielded benefits for the military and for civil society. A starship project would have an excellent chance of producing valuable new science and technology.
- Neanderthals were ancient mariners
- SpaceX on the launchpad of a new era in space transport