Time dilation has long been understood, even if its effects are still mind-numbing. It was in 1963 that Carl Sagan laid out the idea of exploiting relativistic effects for reaching other civilizations. In a paper called “Direct Contact Among Galactic Civilizations by Relativistic Interstellar Flight,” Sagan speculated on how humans could travel vast distances, reaching beyond the Milky Way in a single lifetime by traveling close to the speed of light. At such speeds, time for the crew slows even as the millennia pass on Earth. No going home after a journey like this, unless you want to see what happened to your remote descendants in an unimaginable future.

Before Sagan’s paper appeared (Planetary and Space Science 11, pp. 485–98), he sent a copy to Soviet astronomer and astrophysicist Iosif Shklovskii, whose book Universe, Life, Mind had been published in Moscow the previous year. The two men found much common ground in their thinking, and went on to collaborate on a translation and extended revision of the Shklovskii book that appeared as Intelligent Life in the Universe (Holden-Day, 1966).

This one should be on the shelf of anyone tracking interstellar issues. My own battered copy is still right here by my desk, and I haven’t lost the sense of wonder I felt upon reading its chapters on matters like interstellar contact by automatic probes, the distribution of technical civilizations in the galaxy, and optical communications with extraterrestrial cultures.

Much has changed since 1966, of course, and we no longer speculate, as Shklovskii did in this book, that Phobos might be hollow and conceivably of artificial origin (the chapter is, nonetheless, fascinating). But for raw excitement, ponder this Sagan passage on what possibilities open up when you travel close to lightspeed:

If for some reason we were to desire a two-way communication with the inhabitants of some nearby galaxy, we might try the transmission of electromagnetic signals, or perhaps even the launching of an automatic probe vehicle. With either method, the elapsed transit time to the galaxy would be several millions of years at least. By that time in our future, there may be no civilization left on Earth to continue the dialogue. But if relativistic interstellar spaceflight were used for such a mission, the crew would arrive at the galaxy in question after about 30 years in transit, able not only to sing the songs of distant Earth, but to provide an opportunity for cosmic discourse with inhabitants of a certainly unique and possibly vanished civilization.

The songs of distant Earth indeed! An Earth distant not only in trillions of kilometers but in time. Memories of Poul Anderson’s Leonora Christine (from the classic novel Tau Zero) come to mind, and so do Alastair Reynolds’ ‘lighthuggers.’ Could you find a crew willing to leave everything they knew behind to embark on a journey into the future? Sagan had no doubts on the matter:

Despite the dangers of the passage and the length of the voyage, I have no doubt that qualified crew for such missions could be mustered. Shorter, round-trip journeys to destinations within our Galaxy might prove even more attractive. Not only would the crews voyage to a distant world, but they would return in the distant future of their own world, an adventure and a challenge certainly difficult to duplicate.

But while the physics of such a journey seem sound, the problems are obvious, not the least of which is what kind of propulsion system would get you to speeds crowding the speed of light. The Bussard ramjet once seemed a candidate (and indeed, this is essentially what Anderson used in Tau Zero), but we’ve since learned that issues of drag make the concept unworkable and better suited to interstellar braking than acceleration. And then there’s the slight issue of survival, which William Edelstein (Johns Hopkins) and Arthur Edelstein (UCSF) discussed at the recent conference of the American Physical Society (abstract here). The Edelsteins worry less about propulsion and more about what happens when a relativistic rocket encounters interstellar hydrogen.

Figure two hydrogen atoms on average per cubic centimeter of interstellar space, and that average can vary wildly depending on where you are. A relativistic spacecraft encounters this hydrogen in highly compressed form. Travel at 99.999998 percent of the speed of light and the kinetic energy you encounter from hydrogen atoms reaches levels attainable on Earth only within the Large Hadron Collider, once it’s fully ramped up for service. This New Scientist article comments on the Edelstein’s presentation, noting that the crew would be exposed to a radiation dose of 10,000 sieverts within a second at such speeds. Six sieverts is considered a fatal dose.

Traveling near lightspeed seems a poor choice indeed. The Edelsteins calculate that a 10-centimeter layer of aluminum shielding would absorb less than one percent of all this energy, and of course as you add layer upon layer of further shielding, you dramatically increase the mass of the vehicle you are hoping to propel to these fantastic velocities. The increased heat load would likewise demand huge expenditures of energy to cool the ship.

If travel between the stars within human lifetimes is possible, it most likely will happen at much lower speeds. Ten percent of lightspeed gets you to the Centauri stars in forty three years, a long but perhaps feasible mission for an extraordinary crew. If we eventually find shortcuts through space (wormholes) or warp drive a la Miguel Alcubierre, so much the better, but getting too close to lightspeed itself seems a dangerous and unlikely goal.