We’ve covered methods of interstellar travel that are achievable with presently understood technology, such as light sails and generation ships. We’ve also looked at methods that we can’t do yet but might be able to soon, such as Bussard ramjets and suspended animation. Now it’s time for the things we can imagine but which we might never be able to do.
The first thing that comes to mind is faster than light travel. According to Einstein’s General Theory of Relativity, nothing can traverse normal space faster than the speed of light. Even the effects of gravity propagate at that speed. Since Relativity has passed all the tests thrown at it so far, we have to accept its limitations. Fortunately Relativity also offers some possible ways around them.
One is the so-called Warp Drive. Put simply, instead of going from one point to another in space, we would warp space itself, bringing the destination closer. It would be something like surfing a wave of warped space, while never actually exceeding the speed of light within the wave. One problem is it requires manipulating exotic matter with negative mass.
Another possibility is to use a wormhole. Some solutions of Einstein’s equation allow for the existence of wormholes which connect two distant regions of space. You would enter the wormhole and exit it many light years away. Again, negative mass, which might not be physically possible, is required.
There are even more exotic suggestions. A sail could use existing radiation in space by eliminating its pressure on one side, resulting in propulsion from the pressure on the other side. We could do something similar with gravity, so it would pull us in only one direction.
Quantum physics suggest that the vacuum of space, after everything else has been removed, all the matter and light and heat, still has some energy left. If we could tap into that energy we wouldn’t need any other source.
Finally we come to teleportation. If we could find a way to turn physical objects into information then we could transmit the data at the speed of light and re-assemble the objects at the destination. This would require astronomical computing power.
Any way we look at it, interstellar travel is going to be extremely difficult.