Want to visit an interesting exoplanet or get dangerously close to a black hole? It's not impossible (there's no law of physics that prohibits humans from traveling through space), but it is very, very difficult.
Here are some possible ways we could travel between 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 space and time together can become interchangeable. And while it may be easy to say that a future understanding of physics could remove that restriction, it could prove much harder to put into practice.
Special relativity is one of the most tested theories (if not the most tested) in all of physics. That's because it's not 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 places the speed of light as the ultimate speed limit. It's not just about light or even motion, it's about causality itself.
Special relativity lays down the fundamental foundations for the relationship between past, present and future. In other words, traveling faster than light allows for the possibility of going back in time, something that does not seem to be allowed in our universe.
Since all other theories of modern physics are based on relativity, every time we test one of those theories, we are also testing relativity. While we may be wrong about the fundamental structure of space-time, the speed limit of light is unlikely to be dethroned any time soon.
Very unlikely: wormholes
Related to the restriction of The speed of light is the apparent 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 from the curvatures and warps of space-time.
General relativity allows for the existence of wormholes by warping space-time in a very peculiar way. But there is one small caveat: these objects are catastrophically unstable. The moment anything, even a single photon, tries to pass through the throat of the wormhole, it is instantly torn 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 seem to be allowed in the universe.
It may be that our future descendants discover an alternative way to stabilize wormholes and enable interstellar travel, but the time it would take to discover the necessary advances in physics could be longer than it would take to simply go to the stars ourselves.
Very unlikely: generational ships
If we have no shortcuts or loopholes in physics, or other means of achieving faster travel, then we will have to take our time. While sending a spacecraft to another star is not a matter of physics, it does pose many engineering challenges. One outlandish idea for traveling between stars is to build generation ships: large, slow-moving ships in which most of the passengers would never live to see their destination, and would live on for generation after generation as a self-contained city-ship that would eventually reach another star.
Technically, humanity is already an interstellar species. It has been years since the Voyager 1 spacecraft Voyager 1 has passed 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 accomplish that feat. The bad news is that this is just the beginning. Even at the incredible speed of more than 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 neighboring star about 4.2 light-years away, it would take the spacecraft about 40,000 years to reach its destination.
That number of years predates the development of the first cities and the advent of agriculture. A “generation ship” is therefore not made up of a handful of generations, but of hundreds of them, all of them needing to live self-sufficiently in the spaces between the stars, without additional sources of water, fuel, food or spare parts.
It's not impossible, but it's also very unlikely.
Slightly unlikely: very, very fast
In order to reach other stars faster, you cannot rely on a gigantic, heavy ship; you need to be as small as possible. Rockets or other propellants would reach higher speeds, which would make the trip as short as possible. In addition, at high speeds, the peculiarity of relativity helps. Due to the constancy of the speed of light, movement through space is different from movement through time, and the faster an object moves through space, the slower it moves in time. As it approaches the speed of light, a year for the rest of the universe can be reduced to months, days, or even minutes.
Unfortunately, these effects only manifest themselves when an object reaches more than 90 percent of the speed of light, something humanity has not come close to achieving. But accelerating particles approaching the speed of light is something powerful phenomena in the universe do regularly, so it's definitely not impossible.
But these are tiny particles, not comparatively huge spaceships. Propelling a human-sized craft to 90 percent of the speed of light might require more energy than the Sun produces in a thousand years, but that's an engineering problem, not a physics one.
Likely: We don't
In the very, very distant future, assuming our current understanding of physics holds up (at least as far as extraterrestrial travel and wormholes are concerned), humanity will likely send only a few viable missions to other stars and 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's my home and there is no place like it.
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