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
stop thinking about it on a linear line
instead, conceptualize yourself as travelling through both space *and* time simultaneously.
we are all *always* travelling at c through *spacetime*, the faster you travel through one dimension, the slower you travel through the other. and vice versa. so if you’re sitting absolutely still, you are moving through time the fastest. if you are going very fast in space, time is slow for you.
you can visualize this relationship with a spacetime diagram, here’s a couple of videos that explain it, including a physical demonstration
the problem with your examples is that your first two are nowhere near fast enough to notice the discrepancy, and the last one is nowhere near long enough to notice the discrepancy. misleading you into falling back on your medium-sized intuition, when you’re in fact dealing with extremes.
a better real example would be actual measured differences in how quickly time passes between on-the-ground and in-orbit clocks (and people!), even though you have to go faster than a bullet to stay in orbit, the difference at the end is still *tiny*.
a better conceptualization would be if you travelled very far away, let’s say 1 light year away (arbitrary number). if you went at the speed of a commercial jet it would take you however long to get there and back, and it’s slow enough that both you and the person on the ground would subjectively experience *basically* the same amount of time elapsed. but if you were going at close to the speed of light, it would still take you something like 2 lightyears to get there and back from the ground observer’s point of view, but to you the distance is much shorter so it takes you significantly less time to make the same trip. then you end up back home much younger than your sibling.
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