If the universe's space-time is positively curved, like the surface of a sphere, parallel beams would come together.
如果宇宙的时空像球体表面一样正向弯曲，平行的光线就会交合在一起。
A two-dimensional model of such a universe would look like a sphere. It's impossible to have parallel geodesics (straight lines on a curved surface) — the two lines will cross at some point.
这样的2维宇宙模型就像是球体，不可能出现平行的测地线(测地线是曲面上的直线)——两直线必然在某点相交。
The parallel milling is adopted to curved surface NC machining. The row spacing has an effect on surface roughness of workpiece.
采用平行铣削对曲面进行数控加工时，行距对工件的表面粗糙度有一定的影响。
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