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> What would one experience travelling through an invisible Krasnikov tube?

A flat piece of paper doesn't _quite_ lie flat, and the walls on your ship are under considerable stress (unless the ship's built to handle being in non-Euclidean geometries). Apart from that, not much; it's just spacetime, albeit a slightly oddly shaped bit of it. (If the tube's too small, your bones might crack, but I'm assuming it's not that small.) This distortion effect would occur more towards the ends of the tube and less in the middle, I think.

Travelling "through" the tube is a bit of a misnomer. You'd travel along the 3-surface of the tube. If you looked sort of sideways of the opening, you'd see light that had "looped round" the tube before reaching your eyes, a little like the effect you get when you put two long parallel mirrors next to each other (but with more dimensions). You'd also be able to see your own ship in the distance if you looked exactly perpendicular to your direction of travel. (This isn't much different to other kinds of wormhole, except perhaps for the length.)

I'm not sure what would happen if you tried to take a wormhole through another wormhole; my poorly-calibrated intuition says it'd be okay if you were taking a small wormhole through a really big wormhole, but moving a wormhole to the mouth of one not much bigger might make one or both of them violently collapse. Make sure to run the numbers before you actually try to do this.

> I just wanted to know if this could really work without causing causality issues?

Without [assuming the chronology protection conjecture](https://scientific-speculation.codidact.com/a/263534/276341)? No. FTL travel necessarily implies closed time-like curves, unless closed time-like curves happen to be impossible due to some as-yet-unknown implication of the True and Complete Laws of the Universe.

Here's an example construction of a time machine:

  * Go from point A to point B really fast, leaving a Krasnikov tube behind you.
  * Go from point B to point A really fast, leaving a second Krasnikov tube behind you.

You're now in the far future of point A… but go back through the tubes, and you're back at A, not too far in the future from when you started. Future people can now travel into the past. Oops.

Unlike Alcubierre drives, Krasnikov tubes _don't_ let you travel back in time to before your time machine was created. However, there's still time travel.