= Clock hypothesis
{tag=Proper time}
The <clock hypothesis> says that an ideal clock following a <timelike curve> measures its <proper time>: its instantaneous rate depends on <velocity> through $d\tau=dt\sqrt{1-v^2/c^2}$, with no separate <acceleration> term. <Acceleration> still changes the <world line> and hence the accumulated <proper time>. A physical clock may suffer disturbances under forces or environmental changes, so experiments must distinguish its mechanism's errors from the geometric prediction.
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