[btilly]

If you enjoy such questions, I highly recommend https://www.amazon.com/Indistinguishable-Magic-Robert-L-Forw....

The best speed for interstellar travel with technologies that current theory says should be within our reach can be achieved with a vehicle with a light sail pushed by a giant laser, that is powered by solar power. There is even a way to brake it when it reaches the target star. I forget what the predicted velocity was though.

This technology is basically the same as one that the Moties developed in the story, The Mote in God's Eye.

[DiggyJohnson]

Slow down by pulling on the fishing line tied to the back of it, carefully.

[btilly]

It slows down by releasing a large light sail in front of it, designed to reflect light back to a much smaller light sail behind it. The laser then pushes the large sail away, and as the sail goes it pushes the smaller sail (and ship) back. This leaves the ship at moderate speed relative to the new star, and a large sail traveling very, very quickly beyond it.

We do not yet have this technology. But we can show that it is plausible.