Speculative degrees of difficulty

How hard could it be?

Everyone loves a good round of blue-sky speculation and "what if"?  What if people could live for centuries?  What if electricity were too cheap to meter?  What if we could send messages telepathically?    As the wild ideas start flying, it can be hard to remember that some what if's really could happen in our lifetimes and some are, well, just impossible.

With that in mind, here's a sort of Mohs hardness scale (or maybe Beaufort scale) for speculative ideas, using aerospace as a running example (except the last item, where I couldn't come up with a suitable example for aerospace).  The categories here are broad, partly because there's a lot of ground to cover and partly in hopes that it will generally not be too hard to agree what category something fits in.  In other words, I've traded precision for accuracy.  The exact boundaries are not necessarily so important as simply asking what it would actually take to realize a given idea and getting a rough but believable idea of the answer.  Here's my proposed scale:

1. Most people could do it easily.  Example: Making a paper airplane or something else that flies.
2. Many people do it, particularly in richer countries, but at noticeable expense.  Example: Taking a trip on a commercial airliner.
3. Only the richest individuals or smallish corporations could do it.  Example: Orbiting the earth (using someone else's rocket)  (see note a).
4. Generally done by large corporations or small countries (see note b).  Example: Producing a system to put a satellite in orbit [actually, this is level 3 now, thanks to SpaceX. A better example might be manufacturing commercial airliners].
5. Only done by large countries or groups of countries.  Example: Sending an interplanetary probe.
6. Requires bleeding-edge technology in untested combinations and would require a concerted effort by one or more large countries.  Example: Sending a manned interplanetary mission.
7. Requires yet-to-be built technology, but based on known principles.  Example: Getting any macroscopic amount of matter to any star (other than the sun) with travel time under a millennium (see note c).
8. Does not require a new understanding of the universe, but no plausible technology exists, even on paper.  Example: Getting a manned mission to any star (other than the sun) with travel time under a decade in Earth's frame of reference (see note d).
9. Would require a new understanding of the universe, but not logically impossible.  Example: Travel between galaxies on human time scales (see note e).
10. No way.  Logically impossible or in blatant conflict with any reasonable understanding of the universe.  Example: Travel back in time.

Note a: There's a bit of leeway here.  Orbital flights cost tens of millions of dollars.  Not many individuals could afford that, but the very richest are considerably richer than those who could merely afford a single orbital flight.

Note b: "Large" and "small" here refer to economy (say, GDP), not population or area.

Note c: To get to Proxima Centauri in a millennium an object would have to be traveling approximately 1/250th of light speed, or about 1200 km/s relative to Earth.  New Horizons maxed out around 20 km/s after it flew by Jupiter.  A probe with the same mass going 1200 km/s would require 3600 times as much energy.  An ion drive with an exhaust velocity of around 400 km/s -- the one propelling the Dawn spacecraft has more like 30 km/s -- could provide the required acceleration if the thing starts out as 95% fuel, but I'm completely handwaving about the power source.

Note d: Traveling four light-years in ten years implies that relativity will become noticeable for at least part of the trip.  The (true) astronauts on board would experience a somewhat shorter travel time than mission control would.  Going a hundred times faster than the previous example would require 10,000 times as much energy.  Compared to the New Horizons probe, that's 36 million times more energy at the very minimum.  A real manned craft would have to be significantly bigger than New Horizons, even without a propulsion system (most of the New Horizons propulsion system fell away shortly after launch).  It would also be nice to be able to slow down when we got there, and, ideally, turn around and come back.  A factor of a billion is probably more realistic.

Note e: At the very least this would require some form of faster-than-light travel, and not just a little bit faster like the famous neutrinos might or might not have been doing [They weren't, of course].  The Canis Major Dwarf Galaxy, probably our nearest neighbor, is 25,000 light-years away.  To get there in a decade you'd need to be going 2500 times light speed.  The nearest big pretty spiral galaxy, Andromeda, is about a thousand times further still.  See the comments section for a little about why this is probably not level 10, and Wikipedia's article on faster-than-light for a lot more.