relativity - Doesn't Warp theory violate causality?

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. But doesn't it violate causality? Say, we make warp drive possible and travel towards Alpha Centauri (4.22 light years away) in warp speed and reach there, say, within a year (+/- few months) and blow it up. Now using Special Relativity, we could devise a frame of reference wherein the observer would see the Star blow up before we ever left planet Earth. Wouldn't that violate causality and make warp speeds unattainable?

Answer

This is more of a meta answer, since it isn't really Physics, but it got a bit long to put in a comment. You say:

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws.

You need to think about precisely what this statement means. If we take the Alcubierre drive as an example it is a perfectly legitimate solution to Einstein's equation. So if General Relativity is a Law of Physics then the Alcubierre drive indeed doesn't violate any laws of physics, and it can be used to create closed time-like curves with the loss of causality that this implies.

But there are two problems with saying the Alcubierre drive doesn't violate any laws of physics:

Firstly the Alcubierre drive requires a ring of exotic matter to work, and exotic matter violates a number of energy conditions. If these count as laws of physics (and so far observation suggests they do) then the Alcubierre drive does violate laws of physics.

Secondly most of us believe that general relativity is an approximate theory that breaks down under a number of conditions (specifically in the quantum regime). It's been argued that when you take quantum effects into account a closed timelike curve causes an instability that destroys it. All this is speculative since we have no theory of quantum gravity, but if true it also means that the Alcubierre drive does violate laws of physics.

So when you see a bald statement like xxx doesn't violate laws of Physics, the statement is meaningless unless you specify the assumptions it is based on.