**A Couple of Physics Questions**

I've got a couple of physics questions that have been bothering me for the past several years. The first concerns acceleration and the speed of light. While I understand that we can't currently build interstellar or intergalactic spaceships that can approach anywhere near the speed of light due to our being unable to produce sufficient energy over a sustained period of time, one thing I'm not clear on is whether manned spaceflight at near light speed would be impractical due to the effects of acceleration. In other words, assuming we did have a spaceship equipped with a powerful enough engine, if we stuck a crew of astronauts inside, set a course, and accelerated the ship continuously at 1 G, how long would it take for it to approach 90% of the speed of light? I've never taken physics, so I can't work it out on my own. As far as I know, it could take a week or it could take a thousand years. In the latter case, I think we can just give up on manned missions to other stars. Also, I should say that when I talk about time, I mean the time for the astronauts in the ship and not for outside observers still on Earth.

My second question concerns the fourth dimension. What I want to know is simply this: Is it or is it not time? I've heard convincing arguments for both sides. Some people say that it's not literally time. We only think of time as the fourth dimension because of the convention of using it as a fourth parameter to plot the location of a point in space. First you work out its spatial x, y, z coordinates and then use a fourth variable for time if it's not stationary. But I've also heard that just as a sphere is a two-dimensional plane curved in upon itself in the third dimension, so too is our three-dimensional universe curved into a sort of four-dimensional sphere. And spacetime does affect time, especially around black holes. So it would seem that the fourth dimension may actually be time. If this is the case, then I have additional questions:

- Why four? Shouldn't time be the first dimension or the last dimension?
- Would beings in a two-dimensional universe (flatlanders) not be able to experience time, since they are two dimensions away from it?
- Is the fourth dimension the only non-spatial one, or are there a few higher dimensions that also specify some non-spatial attribute? Is there a dimension for chocolate?