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

If time dilation is all about “measurements” is Time Dilation “real”?

Is it real? Real impacts e.g. GPS Thought Experiment – Twin Paradox

clock at either end of platform one-clock on another train Clocks and trains one-clock on train X X A B clock at either end of platform Y Y one-clock on another train total X + Y time is less than same interval measured on platform

Space ships and planets one-clock on spaceship X X A clock on each planet B Y Y one-clock on another spaceship total X + Y time is less than same interval measured by planets

Re-using the same clock one-clock on spaceship X X A clock on each planet B X X same spaceship total X time is less than same interval measured by planets

Twins One twin on spaceship X X One twin on planet A A B X X Twin returns Twin on spaceship is younger on return!

Objection / Paradox Motion is relative: Look at it from reference frame of Twin on spaceship Twin on planet ages less!

Twins One twin on spaceship X One twin on planet A A Twin on planet is younger on return?

Resolution Situation is not symmetrical Cannot construct one-clock / two clock in same way Twin on spaceship changes reference frame (accelerates at turnaround)

CAREFUL Acceleration is NOT “central”: if the spaceship just accelerated out and back without the long journey there would not be much ageing ageing is approximately just the one-clock two-clock ageing calculate difference in age with time-dilation equation (+/- “insignificant bits”) Twin paradox “resolved” as situation is NOT symmetrical One twin remains in same frame of reference Other twin changes frame of reference OK… so this requires an acceleration but can be “minimised” and is “not the cause” of the ageing (though “cause” depends on interpretation)

Clearing up misconceptions SR CAN deal with accelerations and CAN deal with Twin Paradox Even the +/- little bit… calculus GR is NOT required to “resolve” Twin Paradox GR is NOT SR + acceleration GR = SR + Gravity Nice discussion (Scientific American): How does relativity theory resolve the Twin Paradox? Good detailed / technical discussion: Physics FAQ (Baez) – Twin Paradox

Actual Twin Experiment Hafele-Keating Atomic clocks sent around the world on airliners Does require GR correction, gravitational field – low vs high altitude.