An observer in \(S\) moving relative to the clock measures a time interval \(\Delta{t}\) for one tick of the light clock. An observer in \(S'\) who is at rest with respect to the clock measures the time interval for one tick to be \(\Delta{t_{\mathrm{proper}}}\), regardless of whether the observer and clock are moving with respect to some other observer. Equation 25-10 shows that \(\Delta{t}\) and \(\Delta{t_{\mathrm{proper}}}\) are not equal: An observer in the \(S\) frame sees time running at a different rate than an observer in the \(S'\) frame. This is astounding!