Possibility

Anything you can imagine is possible.  In fact, we can safely go even further than this cliche, because there are a great deal more things that you can't imagine which are possible than there are things that you can imagine which aren't possible.

Most things that you can imagine could, with sufficient work, feasibly be actualized.  As in, get-out-the-hammer-and-nails, elbow-grease-and-motivation, built, fabricated... created.  Most examples you are thinking of right now to contradict this statement are in the category of things that are possible but not currently imaginable by homo sapiens.  In fact, even the most conceptual and outlandish, existentially absurd things any one of us could imagine might possibly be created from scratch.  There is no reason to suppose, for example, that fundamental qualities of our universe cannot be changed from within it, by conscious will, with a sufficient level of control over sufficiently small things.  We don't even know what those very small things are yet exactly, so any argument that these things are outside of the sphere of willful manipulation is made completely out of ignorance.  The assumption that there are some fundamentally unchangeable parameters of the universe is exactly that - an assumption.  A knee-jerk, pessimistic assumption made out of circumstance, certainly not anything that could be called fact.  The more popular thought pattern seems to go:

1. We don't know exactly what series of complex interactions create the "laws" of physics that we observe in our universe.

2. Our particular species of ape has only learned to meaningfully manipulate its environment at the quantum level.

3. (From 1 and 2): It is overly optimistic to think that it is possible to change basic principles of the universe, even locally and for a short period of time.

Well I say phooey.  Going further, I believe deeply that the "laws" of physics are fundamentally plastic, moldable things, created out of other plastic, moldable things.  I believe we've been creating and manipulating these plastic, moldable things literally forever, and we will continue to, because it's the only plastic moldable toy we have, and we love it, because it is awesome, and it oozes out love, consciousness, subjectivity, and hithertoo untapped qualia from its every seam (not to mention kittens and whiskey ginger!), and we'll always try (successfully) to make it better and better, and we have nothing but time to learn how.  (Of course, by "we" I mean the royal we, consisting of all things and times, in all those universes).

As usual, the only real barrier is culture.  The limiting factors for us are only... the things that we decide are limiting factors!  Don't get me wrong, pessimism, doubts and fear-driven dismissal are all useful tools in many contexts, but not in the context of everything, and what we should do with our existence!  In this sense, we should imagine the most amazing things about our global potential, and only then will those things happen.  We're getting a bit too mature as a culture to keep wrapping our shared disposition in lethargy and dissolution.  It's starting to seem a bit silly.  Plus, we can't afford it!  It makes sense that our culture recapitulates the self-effacing, negativity-biased patterns of its constituent parts - namely brains evolved for survival instead of rationality, and at the whim of nasty, messy, electrochemical deviance - but we can't rest on our laurels with this excuse forever.  We got this, you guys!  We're in a cruise ship floating through space with infinite energy, time and resources, not to mention free will.  We're past the hard part.  We've got radio-controlled science-devices on fucking Mars!  Let's do more of that stuff.  1 - 2 - 3 break!