Now that could be a challenge - you would need a minimum of 100,000 years just to cross it, probably closer to a million if traveling at only a fraction of light speed and not following a straight path through the intensely radioactive galactic core. But yeah, I suppose if you had a fleet of rogue planets looping through the galaxy at a substantial fraction of light speed with people breeding as fast as possible and getting off at every star they passed near, you could at least get a decent start.

If on the other hand you assume a few colony ships per century launched from each established colony, and maybe a century for a new colony to get well enough established to start sending out it's own colonists, then things slow down dramatically.

I suppose it would come down to what you meant by "the whole galaxy" - if you're only going to colonize worlds that are easily terraformable (or which w can easily be bioformed to endure) there may only be a few million to reach, if that. On the other hand if you're attempting it in only a few million years you're obviously capable of massive feats of engineering and have mastered the art of artificial ecosystems, so why not colonize all 200 billion stars, or at least the ones that won't explode before you finish?

Can't offer one offhand - there was a TED talk on it not too terribly long ago though, and they may have a few references.

A normal walking pace is relevant because that is what your kinesthetic senses are acclimated to - meanwhile the sorts of acceleration your virtual head is subjected to in a FPS would kill you (exercise for the reader: what is the minimum acceleration involved when standing still one instant and moving at 20+mph 1/60th of a second later?)

> If your inner ear is fucked up due to infection or whatever and it's sending you bad data, you get debilitating loss of balance, motion sickness, etc.
And that is a situation extremely analogous to subjecting yourself to high virtual accelerations while your inner ear is "fucked up" by virtue of reporting the fact that you are in fact remaining stationary.

That the spinning tunnel illusion or it's many variants typically doesn't cause severe vertigo is presumably because your brain is able to compensate for a fixed error - if the speed and axis of rotation were constantly changing I imagine vertigo would kick in quite rapidly.

Hmm, now you've got me curious just what it would take. Let's see... human energy consumption in 2008 ~=144,000 TWh = 5*10^20 Joules
E = 1/2mv^2 (for negligibly relativistic speeds), therefore
m = 2E/v^2. And if we're looking to get to 0.1c that gives us...
m = 2*5e20/(0.1c)^2 = 1,112,650 kg

So given an acceleration system that requires minimal reaction mass, with the amount of energy we consumed in 2008 we could get a 1000 metric ton craft up to around 1/10 lightspeed. Eminently doable if we had the political will to attempt it, and the (few) passengers could reach the nearest stars in a single generation.

Of course for colonization with near-term tech we'd probably want to make large generation ships, which would increase the energy needs considerably. Still, a decent fusion reactor and some powerful ion drives should make it viable, and both of those are currently hovering on the edge of viability. And of course if we ever manage to find/make magnetic micro-singularities then mass-conversion reactors become an easy source of near unlimited energy, and such things become almost inevitable. Hmm, let's see: 5*10^20J/c^2 =~ 5,563 kg. Not too shabby, a 200:1 payload to fuel ratio to accelerate to 0.1c. Of course your reaction mass is going to dwarf that... and sadly Google won't cough up any thrust-to-energy ratios for a "radio drive" - I seem to recall they're expected to be terribly inefficient, but with mass conversion for power the lack of reaction mass might make it an acceptable option.

Hmm, I seem to remember some folks a while back figuring out how to replace the fats in neurons to render a brain transparent. If they could figure out how to do that, even partially, without killing the organism in the process, then it could dramatically increase the power of this tracking.

In fairness fly neurons appear to be far more sophisticated than those of humans - we had the luxury of being able to just keep adding more neurons at minimal cost in order to build more sophisticated structures - as long as those extra neurons on average put more calories in our bellies than they consumed they were a clear win. Flies on the other hand have extremely tight mass and volume constraints, and as a result the individual neurons had to become more sophisticated instead - to the point where an individual fly neuron may serve the function of several independent human neurons (i.e. having multiple independently operating axion&dendrite clusters all connected to the same nucleus), and/or being able to fundamentally alter their responses based on the current activity (flying, feeding, etc) in a sort of "situational multiplexing" of the neuron's functionality - a feature that has no known analogue in mammals.

Well, if neurons worked like transistors, perhaps. However available evidence is that each neuron operates more like an embedded processor, possessing memory and firing in response to some sort of non-trivial analysis of the state of the 7,000 (average for a human) synaptic connections is possesses. That suggests that in order to make an artificial copy of a mind we'd first have to figure out how to emulate individual neurons and possibly even record their internal states. And of course emulate the various types of synapses and their sensitivity to ambient brain chemistry. And the ambient brain chemistry... things could get complicated.

And of course even if you could perfectly emulate a brain there's still the core problem with "uploading" a mind for most purposes (aka immortality) - much like uploading a file you aren't moving it, you're making a copy. Which means that after the upload you'd get out of the scanner and continue on with your normal mortal life while your new mind-twin begins their new potentially immortal one. Or you die in the process. Either way there's not much in it for the original.

Actually they are already beginning to experiment with this - IIRC they've managed to emulate "generic" subsections of rat brain to the point where they can wire in a simulated version to restore much of the functionality lost by destroying the original. Very crude, and I doubt personality would would be preserved even with an exact copy. But undeniably cool.

Actually, the problem with a gaseous environment is not that the molecules are too far apart - in fact you get a (very roughly) comparable frequency of collisions, and they're at higher energies which make reactions more likely. The problem is that as larger molecules form they tend to precipitate out of solution, and in a gas there is insufficient buoyancy to keep them mobile once they've done so. On Earth life likely evolved within the primordial open-faced sandwich on the bottom of tidal pools, borrowing mobility from the surrounding water and structure from the solid substrate. Get rid of either and things become much more difficult, though there's no reason to believe it would be impossible. Get rid of both (such as in the atmosphere of a gas giant where chemistry becomes radically altered at the enormous pressures around the quasi-solid core) and you're in completely unknown territory.

It's also worth mentioning that gas-versus-liquid has little to do with distance between molecules except at a given pressure, the phase is determined by the nature of the weak intermolecular bonds. The gas deep within a gas giant could be far denser than water, but the immense pressure and temperature maintain it in a gaseous state, smoothly transitioning to liquid as you go deeper. Or perhaps not - high-pressure chemistry is still a very young field and we keep discovering surprising things.

Actually if I remember my Dune correctly water was once plentiful on Arrakis, but was locked away deep underground by the larval sand trout in order to provide a more hospitable environment for their adult form, the sand worms.

Also, if plate tectonics stops that means our planet's core has cooled to the point where it can no longer provide a strong magnetosphere, at which point the solar wind will begin stripping away our atmosphere, boiling away the oceans in the process as the air pressure drops, and leaving erosion to be a process fueled primarily my meteorite impacts. Much as is believed to have happened to Mars some time in the last few tens of millions of years.

Actually, there's some argument that tidal flats were important in the evolution of early life, as were the amplified plate tectonics (life probably evolved around underwater volcanic vents. Not to mention the fact that having a large moon kneading the planet has dramatically slowed the cooling of the core, maintaining a strong magnetic field for far longer than would otherwise have been possible. Which in turn allows an atmosphere to be retained. Mars has plenty of gravity to retain an atmosphere, and it could conceivably have hosted an Earthlike ecosystem as recently as 10 million years ago. But then it's core cooled to far, shutting down its magnetic dynamo and allowing the solar wind to strip away whatever atmosphere it had, boiling away any substantial surface water in the process and carrying that away as well (the boiling point falls with decreased air pressure).

Also, just FYI, fusion only happens naturally in stars - hydrogen and oxygen react chemically to form water in a completely unrelated process. And incidentally free oxygen is reactive enough that it's presence is a likely indicator of life - something needs to produce it faster than it can get re-bound into oxides, but it's definitely not necessary for life-as-we-know-it to exist: Life on Earth all evolved from anaerobic ancestors, and most of it was eradicated by the toxic oxygen released by the first photosynthesizing bacteria. To this day most life on the planet is indifferent to oxygen, it's only a miniscule fraction of those forms colonizing the outermost surface of that has evolved to harness the toxic substance for fuel.

And how would you recommend we look for life of a kind we have no understanding of? We're still trying to figure out how to detect life-much-as-we-know-it if it's not jumping up and down and screaming (metaphorically of course). An example of a much easier problem: Suppose I know with absolute certainty that there's a specific thing in the room with you. Given only that much information, do you suppose you can identify it? Now identify the other specific thing that I suspect is also in the room. That's the life-as-we-don't-know-it challenge.

We don't even know for sure that it arose on this one. Panspermia could easily have spread the seeds of life throughout our galaxy, in which case the relevant question is how hospitable is the planet to the sorts of extremophiles that could survive the journey? And how genetically diverse and evolutionarily flexible is the life that survived the trip, especially in regards to stabilizing and fertilizing the planetary ecosystem it finds itself in?

Well, for starters if we discovered a convincing atmospheric evidence of life then I would bet that launching a gravitational-lens telescope for a closer look would become a major priority. Even it if could only resolve planetary surface features to a few meters the spectroscopic data alone would provide enormous amount of information, including a *lot* of information on the specific local biochemistry. A great deal of the science we're only beginning to do on our own planet thanks to orbital monitoring could also be done to an exoplanet with the help of a gravitational telescope. How useful that knowledge might be is difficult to judge beforehand, but at a minimum it would give us a much better idea of how common life is in the universe. And perhaps more to the point - until we've got that level of magnification it would be virtually impossible to begin determine whether or not the life was potentially intelligent unless we were lucky enough to be able to establish communication.

And of course once we have a gravitational telescope there's no telling what *else* we might see with it. Granted at 550+AU from the sun it would be constrained to examining a tiny fragment of the night sky, but we would be able to see further and with far more detail than any telescope to come before it. Quite likely we could look all the way to the edge of the potentially visible universe, to the first moments after the universe became transparent. And of course we'd be seeing everything more recent in completely unprecedented levels of detail as well. ~14 billion years of astrophysics on display, much of it at far greater detail than what our best telescopes can currently resolve around even the nearest stars. If that doesn't reveal a potentially ground-breaking secret or two about the universe I don't know what will.

As for the moon landing - in and of itself I would say its significance is more symbolic than practical, but symbolism can be extremely powerful - after all one of the primary things that separate our species from other animals is the facility with which we can harness symbolic thought. On a more mundane level rising to the challenge spawned several other benefits - dramatically stimulating the evolution of the transistor to a viable earth-side technology for example. Introducing photos of the Earth from space, and the unifying awareness that helped promote. Not to mention the frustration it promotes in space enthusiasts to think that we were on the cusp of spreading out into our solar system before the collapse of the Soviet Union removed the political will for such morale-boosting endeavors. The baton may have been dropped for a generation, but it is being picked up again, and you can thank that in part to the fact that we proved it could be done before losing our will to do so.

>still leaves us with too many to ever hope of getting to

Pessimist. If we develop interstellar travel, even at small fractions of light speed, remain expansionistic, and avoid completely eradicating ourselves or transcending as a species we could colonize the whole friggin galaxy in only a few billion years.

Or maybe you meant "we" in a personal sense in which case yeah, barring the surprise development of feasible near-instantaneous (in ship-time of course) travel, we have absolutely no hope of visiting more than the planets in our own system and maybe those of one other star.