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
Everything in the universe is actually traveling at the same rate through spacetime, it’s a property called your “four-velocity”. And this four-velocity has the same magnitude at all times. The concept of movement is actually just rotation of this vector. When you’re sitting still relative to another object, you’re basically traveling entirely in the time direction and not at all in the space direction. But when you increase your speed in the traditional sense, what you’re doing is rotating the direction of this fixed magnitude four-velocity so that you start traveling a little in space as well
and not as much in time. Photons are completely rotated and travel only in the space direction, which is why they are said to not experience time. For a free photon in deep space that won’t hit anything, the entire life of the universe is instantaneous. So naturally if you find yourself traveling near the speed of light, you will find yourself similarly rotated almost entirely in the space direction and hardly at all in the time direction.
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