wait a second. time travel (forward & backward) is permitted by physics my friend. Read up!
that is possible but I think you are talking about technology to build a machine
if that's plausible then howcome, we havent had any visitors from the future yet
that is a plausible argument (and i think put forward by a physicist 'where are all the time travelers') .. but does not cover all the aspects. There are 2 theories that answer that question
Time travel is only possible towards future and is one way travel .
Time travel can be backward but is not possible beyond the time when you built the time machine. So if machine was built last week, you can go back till that point only and not beyond that