Having read further down and under the context of the Fermi problem, I think the idea is that the general limitations (on the first question) are more due to engineering than due to particle physics, relativity, and so on. Allow me to explain.
Relativity sets a limit on information propagation at the speed of light. More specifically, in physics they talk about waves having a phase velocity (which can be arbitrarily large) and a group velocity. The group velocity refers to the information carrying content of the wave, and this speed is always strictly limited to less than c. Since things like light waves are the fastest means of communication, this sets your upper bound according to the current state of the art.
But in reality, if you were expanding outwards from a point in space, you would need more than just a light wave. If you were a person, you would need a ship. In a more imaginative case, even if they could decode your body into pure information content and then broadcast that signal at light speed across the universe, it would be good for nothing unless you had something that could reconstruct you on the other side. But I'm guessing you'd already thought about this.
In a ship then, accelerating to high speeds in a straight line for a macroscopic object like a spaceship isn't that hard, theoretically. The hard part would be in things like maneuvering and radiation shielding. It takes a lot of energy to turn your trajectory if you're a massive object moving at significant fractions of c. I haven't calculated it but that's just my intuition.
If you did something like accelerating on straight line trajectories (or geodesics, or what have you) and decelerating in straight lines, going from point to point around obstacles, then you obviously accumulate more delay time in a different fashion. In any case, there's these engineering difficulties that are practical issues to expansion.
Maybe you'd rather imagine cellular automata or some kind of machine construction proceeding outwards at light speed, rather than more conventional ideas. In this case, it would be something radically different from what currently exists, so one can only speculate. The hard part might be the following. If you are building yourself outwards at the speed c, then you are also colliding with whatever is in front of you at speed c, and this would most likely cause your total destruction.
This is of course assuming that the machine would require a conventional structure in the form of nanorobots, gelatin, or basically anything with nontrivial mass distributions.
You might instead try to compromise in the following way. Namely, you expand at some high speed, say 0.5 c, where you can still shoot (e.g.) space nukes out in front of you at another 0.5 c, to attempt to vaporize or sufficiently distillate the obstacles before you hit them. And so on and so forth....
We have recently obtained evidence that a number of people, some with quite interesting backgrounds and areas of expertise, find LessWrong an interesting read but find limited opportunities to contribute.
This post is an invitation to engage, in relative safety but just a little beyond saying "Hi, I'm a lurker". Even that little is appreciated, to be sure, and it's OK for anyone who feels the slightest bit intimidated to remain on the sidelines. However, I'm confident that most readers will find it quite easy to answer at least the first of the following questions:
...and possibly these follow-ups:
Comments like the following, from the "Attention Lurkers" thread, suggest untapped resources:
My own area of professional expertise is computer programming - perhaps one of the common sub-populations here. I'm also a parent, and have been a beneficiary of prenatal diagnosis (toxoplasmosis: turned out not to be a big deal, but it might have been). My curiosity is often engaged by what goes on "behind the scenes" of the professions I interact with as an ordinary citizen.
Yes, I would be quite interested in striking up a conversation about applying the tools discussed here to prenatal diagnosis; or in a conversation about which conceptual tools that I don't know about yet turn out to be useful in dealing with the epistemic or ethical issues in prenatal diagnosis.
Metaphorically, the intent of this post is to provide a marketplace. We already have the "Where are we?" thread, which makes it easier for LessWrongers close to each other to meet up if they want to. ("Welcome to LessWrong" is the place to collect biographical information, but it specifically emphasizes the "rationalist" side of people, rather than their professional knowledge.)
In a similar spirit, please post a comment here offering (or requesting) domain-specific insights. My hunch is, we'll find that even those of us in professions that don't seem related to the topics covered here have more to contribute than they think; my hope is that this comment thread will be a valuable resource in the future.
A secondary intent of this post is to provide newcomers and lurkers with one more place where contributing can be expected to be safe from karma penalties - simply answer one of the questions that probably comes up most often when meeting strangers: "What do you do?". :)
(P.S. If you've read this far and are disappointed with the absence of any jokes about "yet another fundamental question", thank you for your attention, and please accept this apophasis as a consolation gift.)