as i understand this - in space you have no friction, so you just gain more & more speed. so why can't we reach a near to light speed?
what is the problem?
a power source? the rockets ran out of fuel so the gaining of speed is slower?
That's a large part of it. Acceleration requires thrust, thrust requires fuel, and you need to carry that fuel with you. The more you need to accelerate, the more fuel you need to carry, and that added mass makes it harder to accelerate. So it's a diminishing-returns situation; it's just not practical to carry enough fuel to accelerate you to a sizeable fraction of lightspeed. Especially since your effective mass would increase if you got really close to lightspeed, making it exponentially harder to accelerate further, though you'd have to get close to 90% of c
for that to become a noticeable factor.
This is why many proposals for starships are for designs that don't carry fuel with them. The first such proposal was the interstellar ramjet, which collected hydrogen from the interstellar medium (ISM), refueling itself as it went. The problem with that, though, turned out to be that its magnetic fields would create drag against the ISM and slow it down. Now there's a lot of theoretical work done with sailships, such as lightsails accelerated by lasers back in the Sol system, or magnetic sails acelerated by particle beams. This way, the source of power is back home and the ship itself can be much lighter.
The other problem is that the closer you get to c
, the more hazardous spaceflight gets. The starlight and background radiation from ahead is blueshifted to become far more intense and deadly, and oncoming specks of dust would hit you with the force of nuclear warheads. (Kinetic energy is mass times velocity squared, so as you go faster, the amount of energy something hits you with goes up considerably.)