What Is Possible?

Some lines of thought have come up and might take some time to develop so I might start into something, then break off and return to it later - one of the beauties of blogging. It will be seen that everything written below is relevant to sf in general and to Poul Anderson's works in particular.

As a philosopher, I make a big deal out of two kinds of impossibility. Some people might either not understand this distinction, at least initially, or not see why it matters. In any case, both kinds of impossibility are kinds of impossibility. Let us just stay with that for the time being. 

Where is the line between possible and impossible?

interplanetary travel

slower than light interstellar travel

faster than light interstellar travel

inter-galactic travel, same differences

inter-universal travel

time travel (by this, I mean travel into the real past among events that really happened, not events that look like the past but are malleable by the traveller)

It will be seen that Anderson's works are all over this list. How confident do you feel about proceeding down the list? We know that interplanetary travel is possible because it has been done, albeit on the meanest scale, but what is next?

Brian Cox has mounted a powerful argument that we should draw the line under interplanetary travel, that even STL interstellar travel will not happen and is not happening. I was going to share a video exposition of Cox's argument but it has become unavailable. I very inadequately summarized the argument in Not To Be Pessimistic But. I have more to say about this and will not be alone in that.