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Is there such a thing as an absolutely straight line?
Conceptually, yes. But in reality, the roughness of the paper, or pixels on a screen will add tiny deviations from the absolute. And if that is not enough, there are random quantum fluctuations which will contribute.
Depends. Probably not because nothing is perfect! :) i have to disagree - the sides of crystals are straight ~~~ *Sigh* All lines are straight. A man-made one may not be, but by the correct...
parallel ~~~ Actually, all lines are straight and endless. A ray has one endpoint, is straight, and the other end of the ray is endless. A segment has a defined beginning and end, and is straight.