Making Interstellar Travel Possible

For those of us who keep our eyes fixed to the heavens, Einstein's theory of special relativity has thrilling implications. Namely, the relativity of time and space allows for the possibility of human interstellar travel.

The nearest stars to Earth, the binary stars Proxima and Alpha Centauri, are about four light-years away. In other words, light from these stars, traveling at 300,000 km (186,000 mi) per second, takes four years to reach us. And there are other interesting stars for us to visit beyond our closest neighbors. Over the past ten years, astronomers have discovered more than 125 planets orbiting around stars similar to our sun. They've been able to "see" them by studying the tiny motions that these planets cause on their suns as they move around them.

Among these newly discovered planets, there is a very young one — a planet in orbit around the star CoKu Tau 4, about 420 light-years from Earth. There is even one that astronomers have actually seen directly with the European Southern Observatory Very Large Telescope in Chile. It orbits its sun at some 230 light-years from us.

We may also want to visit the center of our galaxy, which is hidden from our eyes by interstellar dust but visible to our x-ray, infrared, and radio telescopes. Scientists have discovered a super massive black hole there. (A black hole is an object with such strong gravity that not even light can escape.) In the future, human beings may also want to tour our entire galaxy and even visit other galaxies.

However, even if we design a spaceship that can travel at 0.99c, interstellar travel beyond the nearest stars seems impossible for the foreseeable future. Crossing our own galaxy will take more than 100,000 years, and a trip to Andromeda, the nearest galaxy, will take more than 2 million years.

That timeline is accurate for those of us staying behind. But, on the moving ship, time will be dilated. A future spacecraft, using technologies that we haven't even dreamed of, may use an engine that could sustain a constant acceleration of 1 g until the ship reaches relativistic speeds. With such an engine, a trip even to Andromeda may be possible within a human lifetime. For those astronauts, however, returning back home is out of the question. Back on Earth, entire civilizations would've come and gone, while the astronauts who left in their 20s would be only in their 80s.

Table 1 shows several possible trips on a ship constantly accelerating at 1 g. The figure for "Distance in Light-Years" is also the time that would pass on Earth while the ship traveled to its destination.

Table 1: Ship Time for Interstellar Travel at 1 g