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
You might be tripping up on “the flight takes me 1 second.” It takes 1 second to who? There is no absolute frame of reference, which is the point of relativity: both of you are experiencing time at different rates.
If he thinks it took you 1 second to make the .99c trip, then he’s aged 1 second. But *you* would have aged a fraction of a second.
If *you* think the flight took you 1 second, then you’ve aged 1 second, but *he* would have aged around 10 seconds.
There isn’t an objective “the flight *actually* took X seconds” because any third-party observer would also be experiencing time at their own relative rate.
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