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
Tl; dr. When you are in motion *the definition of a second changes* so light always looks like it’s traveling the same speed. In these scenario nobody would agree how much time passed.
In all of your examples neither of you experience the *exact* same wait time. For the 777 and SR-71 you would be imperceptibly younger (Like a tiny tiny fraction of one second) but the affect is still there. Like in the SR-71 if you stay at it’s top speed for about 3200 years you would be 1 second younger.
This is all because of one thing. Light *has* to look like its traveling exactly the same speed for every single person. Normally speed is relative.
Like lets say there’s a 3rd person here. They are in a SR-71, you in a 777, and your brother on the ground.
Relative to you the SR-71 might look like its traveling a 1000 mph. Relative to your brother it might look like its traveling 1600 mph. (slightly made up numbers btw)
But light? No so much. It’s going to look like it’s traveling at *c* for everyone. This happens because when you move the definition of a second changes. That’s because speed is distance travelled over time. So how much time passes has to change. And it changes *exactly* enough that light still looks like it’s moving at c. For normal speeds which are soooo slow compared to light this is, like i mentioned, basically imperceptible but it’s still there.
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