It’s a bitter pill to swallow, but it’s starting to look as if Einstein might just have been right about that speed of light thing. From apparently superluminal radio sources in deep space, to the neutrinos that were supposed to be arriving ahead of schedule at the Grand Sasso experiment in Italy, every apparent exception to Einstein’s ultimate speed law has turned out to be a phantom. Even in the quantum realm, where entangled particles seem to communicate with each other instantaneously across any distance, no useful information is shared at anything other than the speed of light.

This is a particular blow if you happen to enjoy the galaxy-spanning fantasies of Star Trek, Star Wars and the like. Dreams of warp drives and hyperspace are just that — dreams.

But that doesn’t mean that interstellar travel is itself a fantasy. Travel below the speed of light — but as arbitrarily close to it as you wish — is perfectly feasible. Of course, that isn’t the same thing as saying it will be easy. Imagining the technologies that will carry us to the stars places us in roughly the same position as Leonard da Vinci, when he first sketched the concept behind the helicopter. Getting close to the speed of light will demand energies and materials beyond anything within our current capabilities. Chemical rockets certainly won’t cut it, nor the ion propulsion systems employed by some spacecraft. Even dauntingly difficult fusion power wouldn’t be up to the job. But perhaps harnessing antimatter, or the power of miniature black holes, may just give us the tools. At the moment we can make antimatter in tiny quantities, but not nearly enough to fuel a ship. And while we suspect that miniature black holes may exist, and may even be produced in high-energy physics experiments like the Large Hadron Collider, so far they’ve proven elusive.

That said, any civilization willing to contemplate an interstellar expedition at close to the speed of light might also settle for something half as fast, or a quarter as fast. It would just be a question of waiting a bit longer for the news. At 10 percent of the speed of light, an expedition could reach the nearest star within 50 years. Such a mission could be achieved using fusion technologies which aren’t too far beyond those now on the drawing boards, although slowing down at the other end does add to the difficulty. Still, 50 years is a long time by any measure. The astronaut Scott Kelly has just embarked on a one-year expedition to the International Space Station, and no one has yet spent longer than 14 consecutive months in space. Clearly we have some way to go before we can contemplate decades-long interstellar missions. But current space missions already have planning and execution cycles spanning many decades, so an interstellar mission lasting 50 years does not seem entirely outlandish.

But having the technology is only half the answer: we also need the collective will to embark on such a costly, time-consuming enterprise. We need an objective, a destination close enough to achievable, but also sufficiently tantalizing to galvanize our efforts. For most of our history, worlds beyond our own solar system were no more than hypothetical entities — few astronomy textbooks published more than 20 years ago even discuss the possibility of other planets.  

Lately, though, we have gone from knowing nothing about such worlds, to being overwhelmed with the pace of discovery. It now appears that alien solar systems are commonplace, with more than 1,000 stars now known to host planets. Our understanding of these distant locales is slowly improving, and before long we should have at least hints about the conditions and chemistry on some of these worlds. Astronomers have already figured out the expected signatures of oceans, landmasses, icecaps and atmospheric chemistry — now all they need is sufficiently good data to tease out these buried signals. Finding an Earth-like planet — or at least a world with a good chance of hosting life — would surely provide a decisive spur to interstellar exploration.

Be careful what you wish for, though, because an increase in our knowledge of other solar systems could have exactly the opposite effect — dampening our enthusiasm, not stoking it. Such worlds may not be as enticing as we now imagine. While it would be nice to find life-bearing planets, gorgeous blue-green marbles with oceans and vegetation, they may be vanishingly rare. Perhaps the majority of worlds will fall into a handful of similar categories, with very few surprises — rocks, gas giants, hot Jupiters — endless reiterations of the same few themes. While this data will vastly enrich our knowledge of the surrounding cosmos, it may have a chilling effect on our aspirations. After all, why cross light years to end up at a place much the same as the one you left? It could even be one explanation of the Fermi paradox, the puzzling absence of interstellar travelers in our own vicinity. Perhaps alien civilizations learn enough about their place in the galaxy that they lose the imperative to engage in physical exploration and colonization.

On the other hand, we won’t know until we look. Perhaps our telescopes will find something that demands detailed examination from close quarters — more than can ever be achieved by remote observation. We may send machines, or people, or perhaps by then the distinction between humans and artificial intelligences simply won’t matter. My suspicion is that, for one reason or another, we will eventually launch an interstellar expedition. But like Leonardo’s helicopter, it may bear only the slightest resemblance to our present-day imaginings.