OK, I'll try to answer some of these big questions. I'm sure some others will add to or correct my comments:

1) Yes, the speed of light can be measured. It's fast but it can be measured experimentally. Why it's 300,000 km/s (186,000 miles/sec) and not some other value, I don't know. Note: these figures are for the speed of light in a vacuum. It is slower in other mediums.

2) Why do moving objects gain mass? This is a tricky one. Einstein's theory of relativity predicts this and experiments demonstrate it. I don't understand the mechanism myself but, hey, it works. And that's the main thing that stops things moving at c as they'd need infinite energy to accelerate them to that speed.

3) I think 1/3c is probably quoted as a point when there are noticeable effects on mass. Any motion will produce an increase in mass but at the speeds we're familiar with, it is a very small effect.

4) This last question is one of the many ways in which relativity is counter-intuitive. When two objects approach each other, the velocities are not added like in classical mechanics. Another way to look at it is when your spaceship is moving at 90% of lightspeed and you turn the headlights on, do they send light out at 190% lightspeed? Well, no. Everything is based on what frame of reference you use. Again, it's a tricky one to explain and I'm sure someone else can explain better how it works. But I think your headlights will be red-shifted so that they won't have a velocity greater than c.

Note: there is only one way I know of that any velocity can exceed c and that's the expansion of the universe itself that, in theory, can be at any speed and, as you get further from us, the universe can be expanding at c or beyond.

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Spike
:)