"I could be bounded in a nutshell and
count myself a king of infinite space."
For a few brief moments the other day, I convinced myself that interstellar travel was not as unlikely as I have generally assumed. (UFOs, therefore, would be correspondingly more likely to be controlled by aliens.) Skimming the relevant Wikipedia pages, I learned of a clever scheme whereby we could travel to the stars at a constant 1g acceleration, thus providing perfect simulated gravity and, due to the miracle of time dilation, make it back and forth to almost anywhere in the galaxy within a comfortable lifetime. (Aliens, meanwhile, beginning on a planet with, say, three times the Earth's gravity, who could therefore comfortably live in a starship accelerating at 3g, and with a lifespan, just as arbitrarily assumed to be three times our own, might view interstellar distances with significantly less awe than we do.)
But then I found Nathan Geffen's entirely convincing demolition of the idea. Unless his math is simply wrong (I haven't made the effort to check), it would seem that even under the most optimistic assumptions about our future ability to convert matter into energy we would need a fuel load that weighs 4.5 times the total weight of the ship. (Yes, that's a paradox. It's like requiring the astronauts to be one fifth their own weight or something.) As Geffen puts it:
We ... have to conclude that using onboard fuel, it is theoretically impossible to get to Proxima Centauri. It doesn't matter what we set the spacecraft mass to, the ratio remains the same. So even for unmanned spaceflight a 1g acceleration to our nearest stellar neighbour is impossible.
I'm a big supporter of this kind of level-headed disciplining of our science-fiction-addled imaginations. Once you do the math, something that seemed like just a future away becomes a straight-up impossibility. So I was amused to find Geffen imagining what he calls a "not altogether far-fetched possibility" to give us (and "hard" sci-fi authors) a bit of hope:
…one day it will be possible to download the human brain, to something as small as a microchip, with durability of hundreds if not thousands of years. Perhaps if this is the future evolution of our species, we will be able to travel in this form to distant stars without being too concerned about the time it takes.
I'm sure I've seen someone somewhere do an analysis of the energy required to completely "read" the contents of an entire human brain, and the time that is subsequently required to transfer that information to some other device (like a chip). It is similar to the energy requirements needed to make a Star Trek transporter. Geffen understands the physical constraints of the universe in regards to matter and motion, but he has to learn to apply the same kind of thinking to information. Once he does so, he'll realize that the brain-in-a-chip scenario is perhaps more far-fetched than 1g acceleration to the stars. There's more going on in the nutshell of our skulls than we sometimes imagine.