re: When, if ever will we have spacecraft that can travel at light speed?

I think 0.2ls would be a sweet spot in terms of being possible in the next 100 years. At even a fifth the speed of light and you are having to have serious relativistic thought go into your journey. Whatever propulsion method is used, assuming it has an optimum powerband, will still require half of the trip to accelerate before the second half being used to slow down.

Ok, so we could send a probe to the nearest star in a little over 20 years, what happens to a potential traveler experiencing the time dilation? Does less time pass if you are going that speed, in relation to an observer from Earth? As in, if a person could experience light speed travel, the trip would seen instantaneous, whereas one oberving this traveler would still experience the light years encurred?