Bibek Pokharel’s Comps Presentation

How do we know what time it is? We measure time by observing and measuring some change around us. To measure the duration of such a change we need a fundamental unit of time, for instance the second. Measuring the second in a relativistic world requires a careful definition of a duration. We will continue the discussion about measuring time from special relativity (Physics 151) and look at three assumptions we must make in order to define the second – periodicity of a clock, reproducibility of events, and principle of causality (which states that causes precede effects). I will then explain the workings of the Cs-133 atomic clock which is the current basis for defining the second. I will explore the relationship between the principle of causality and the directionality of time (time always moves in the forward direction). Finally, I will explain how causality is related to the initial conditions of the universe and why we remember the past and not the future.