Understanding Special Relativity - 2 VCE Physics – Study Design 2016-2021 Time Dilation 2 – What is time dilation?

: from reference frame of platform World-Map : from reference frame of platform All “orange” clocks showing same time – synchronised

t0 t World-Map : from reference frame of platform t > t0 One clock is “proper time” t0 Two clocks (more clocks) measure larger time interval. This is what was “MEASURED” t t > t0 Time interval measured from platform frame is longer This is “Time Dilation”

: what is “seen” by the girl World-Picture : what is “seen” by the girl This is what I “saw”!

t0 t World-Picture : what is “seen” by the girl t0 > t This is what girl “SAW” t0 > t t0 So, time on train “is faster”! Approaching clocks LOOK like they are FAST. This is what I “saw”! t Receding clocks LOOK like they are SLOW. Time dilation equation does NOT tell us what time will LOOK like.

The time dilation equation relates to “measurements” NOT what we “see”.

But… isn’t motion relative? But… time dilation shows time is doing “weird things” (slowing down / speeding up?) on the train and “normal” on the platform? Consider train stationary, platform moving… should be equivalent. How? Let’s take a look…

: from reference frame of train World-Map : from reference frame of train All “blue” clocks showing same time – synchronised

t t0 World-Map : from reference frame of train t > t0 One clock is “proper time” (this time on platform) Two clocks (this time on train) more clocks = larger interval This is what was “MEASURED” t > t0 t0 Time interval measured from train frame is longer Time dilation again!... but this time other way around

Time Dilation (definition) “proper time interval” (single clock) measured as larger from a relatively moving frame of reference (by two clocks).

People do NOT “feel” any different – they do not experience time moving faster or slower.

Summary Time dilation equation: What we measure, NOT what we see. Relates two measurements of time interval between two events. Single clock measurement = “proper time” less clocks = smaller time interval Two clock measurement (a relatively moving frame) = “dilated time” more clocks = larger time interval Time dilation is about an increase (dilation!) of the measurement of a time interval.