Relativity states that as an object moves faster, it "shrinks" proportionately to the object as it moves faster. Now, once the object approaches light speed, its density becomes greater, and therefore harder and harder to accelerate. It is impossible to reach light speed, because to do so you will have infinite density, which requires an infinite amount of force to accelerate to. This is similar and is linked to the observer seing the object differently because blah blah blah....

I hope I helped