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【Note】 Hyperbolic Angle (16:44) In fact, ???? in space and ???? in space-time should not be expressed with the same symbol like this, and this is strictly speaking wrong. The angle in space-time is a 'hyperbolic angle', and this is defined differently from the angle in space. However, the purpose of this video is not to go into such details, but to induce an intuitive understanding of 'space-time rotation' in a short time, so all angles, regardless of space or space-time, are symbolized as '????'.