Want to visit an interesting exoplanet, or dip dangerously close to a black hole? It is not impossible – there’s no law of physics that forbids humans from traveling through space – but it’s just really, really hard.
Here are some potential ways we could travel amongst the stars, ranked from least to most likely.
Incredibly unlikely: Faster-than-light travel
You can never travel faster than the speed of light. At least, that’s what we understand through Einstein’s special theory of relativity, the revolutionary theory that united space and time can become interchangeable. And while it might be easy to say that a future understanding of physics might do away with that restriction, it could be much harder to put it into practice.
Special relativity is one of the most – if not the most – well-tested theories in all of physics. That’s because special relativity isn’t just a theory, it’s a meta-theory. It’s a set of instructions that help us build other theories of physics. Special relativity teaches us how space and time are connected in a fundamental way. The nature of this connection puts the speed of light as the ultimate speed limit. It’s not just about light or even movement, but about causality itself.
Special relativity lays out the fundamental groundwork for the relationship between past, present, and future. In other words, going faster than light allows for the possibility of going back in time, which does not appear to be allowed in our universe.
Since all other modern physics theories are built on relativity, every time we test one of those theories, we’re also testing relativity. While we could be wrong about the fundamental structure of spacetime, the speed limit of light is unlikely to be dethroned anytime soon.
Mostly unlikely: Wormholes
Related to the restriction of the speed of light is the seeming impossibility of wormholes. Wormholes are shortcuts in space that connect any two points in the universe. These strange objects are a natural prediction of general relativity, Einstein’s theory of how the force of gravity arises out of the bends and warps in spacetime.
General relativity allows for wormholes by bending spacetime in a very peculiar way. But there’s one small caveat: these objects are catastrophically unstable. The moment anything, even a single photon, tries to travel down the throat of the wormhole, it instantly tears itself apart. The only known way to stabilize a wormhole is by introducing a thread of exotic matter. This is matter that has negative mass, which, like time travel, does not appear to be allowed in the universe.
It could be that our future descendants discover an alternative way to stabilize wormholes and allow for interstellar travel. But the amount of time it might take to uncover the necessary breakthroughs in physics might be longer than simply going to the stars ourselves.
Highly unlikely: Generation ships
If we’re stuck without shortcuts or loopholes in physics, or other means to achieve FTL travel, then we’re going to have to take our time. While sending a spacecraft travelling towards another star is not an issue of physics, it does pose loads of engineering challenges. One fanciful idea to travel among the stars is to build generation ships – large, slow-moving vessels where most of the passengers would never live to see their destination, living generation after generation as a self-contained city-vessel that would eventually reach another star.
Technically, humanity is already an interstellar species. Years ago the Voyager 1 spacecraft traveled through the heliopause, the boundary of the solar system, and entered interstellar space. The good news is that it only took a few decades to achieve that feat. The bad news is that it’s just getting started. Even at the incredible speed at over 36,000 miles per hour (57,940 kilometers per hour), if Voyager 1 were headed in the direction of Proxima Centauri (which it is not), our nearest neighbor star at roughly 4.2 light-years away, it would take the spacecraft roughly 40,000 years to reach its destination.
That number of years predates the development of the first cities and the advent of agriculture. So a “generation ship” isn’t just a handful of generations, but hundreds of them, all needing to live self-sufficiently in the voids between stars, with no additional sources of water, fuel, food, or spare parts.
Not impossible, but also highly unlikely.
Mildly Unlikely: Really, really fast
To get to other stars faster, you can’t have a giant lumbering ship. You instead need to be as small as possible. Rockets or other propellants would then get to higher speeds, making the journey as short as possible. Plus, at high speeds the quirk of relativity helps out. Because of the constancy of the speed of light, movement through space is different than movement through time, and the faster an object advances in space, the slower it moves in time. As it approaches the speed of light, a year for the rest of the universe can shrink to months, days, or even minutes.
Unfortunately, these effects only really kick in once an object progresses to more than 90 percent the speed of light, which is something humanity hasn’t come close to achieving. But accelerating particles coming close to the speed of light is something that powerful events in the universe do on the regular, so it’s definitely not impossible.
But those are tiny particles, not comparatively massive spaceships. Pushing something like a human-sized craft to 90 percent the speed of light might require more energy than the Sun produces in a thousand years, but that is an engineering problem, not a physics one.
Likely: We don’t
In the far, far distant future, assuming that our current understanding of physics holds (at least as far as FTL travel and wormholes are concerned), humanity will likely send only a few scant missions to other stars and viable planets. But there’s plenty of room here in the solar system, with hundreds of moons and thousands of asteroids to call home. It’s a big enough place with plenty of mysteries to uncover.
It is home, and there’s no place like it.