I don't know who told you that? Perhaps your forgetting to consider how far we are away from the centre of the Earth? Anyone knows that the further away you move from the centre of gravity, the faster you spin and the greater the centrifical force.
So if you increased the diameter of the Earth by 3 times, the increased centrifical forces would counteract the increase in mass - to what extent i do not know? We could even end up with less gravity!
First of all, centrifical forces also decrease with radius, and while my micrometers/s^2 guess was a little low, the actual numbers are still relatively insignificant; the actual weight difference between the poles and the equator is only 0.5%
if you put two very precise clocks together, and sync them, then put one on a plane, fly it around, the clocks will no longer be in sync when it lands. Theory of Relativity I believe is an attempt to explain this phenomenon. Theory says that for the person, they won't notice the difference who is experiencing the effect...
I'm not exactly sure what causes time differentials, but you hardly need a blackhole to cause differences...
If you had humans living on Mars, I believe they would eventually stop using Earth time, and instead use martian time....Makes you wonder what the life span for a human on mars would be....
An outside observer will view the clock of someone on the plane moving at a rate of (1-v^2/c^2)^.5 relative to their own. At the same time, someone on the plane will see the clocks on the outside moving slower by the same ammount. Basically what this means is that every object in the universe moves through space-time at the speed of light; thus as your spacial velocity increases, your temporal velocity decreases. The tricky part, however, is that all motion is relative, so two people can each say that the other is moving while they themselves are stationary, and laws of phyics will remain valid for either point of view.
Another person that needs a physics refresher. The only thing that affects time is the speed that we are travelling. The faster you go, the slower time progresses. However it's strictly a relative thing. To someone travelling at the speed of light, time would seem to progress normally even though to someone standing still he would appear to be moving much slower. It has no real effect on allowing people to live longer. Under no circumstances does mass have anything to do with it, it's strictly the speed at which we're moving. Go read some Einstein, he's the one who figured it all out. BTW, it's called space-time "continuum".
Actually, mass has a lot to do with it. You see there's this little thing called general relativity which states that gravity is indistiguishable from an accelerating reference plane, and as we learned in special relativity, objects in motion experience time at a slower rate.