In this thought experiment, my twin brother and I are both turning 20 at the airport.
At midnight on our birthday, we are both exactly age 20 years.
He stays put while I get on a 777 and fly around the world. The flight takes me 24 hours and so he waits 24 hours. I arrive and we are both age 20 years plus 24 hours.
If I instead get on an SR-71 and fly around the world at 3x speed of the 777, the flight takes me 8 hours so he waits 8 hours. I arrive and we are both age 20 years plus 8 hours. Clearly, we are both younger in this scenario than the first one.
If I got onto a super plane flying at 0.99x light speed and fly around the world, the flight takes me 1 second. Since I’m so fast, he should also only wait one second. Intuitively, I’m back and we’re both 20 years and 1 second old.
But my understanding of time dilation is that I’m 20 years and 1 second old when I’m back, but he would be much older since I was almost going at light speed.
Why is that? My flight and his wait time should both be much much shorter since I was flying much much faster.
In: Physics
For the first two experiments, you and your brother will agree on the time taken, to reasonable accuracy. For the last one, you will not agree. Your brother will say that the trip took ten times longer than you think it took. You will both have accurate clocks that support your opinion. That’s the dilation effect.
This effect has been actually measured, in ordinary airplanes. However, it requires a super accurate atomic clock to measure the tiny difference. {[Reference to the experiment in 1971](https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment )}
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