How does relativistic time work. I just read Project Hail Mary and the space science went over my head. Why would a person experience less time the faster they go? They kept saying once you get to a certain speed you experience time differently, but what does that mean?
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It means that time quite literally ticks differently from one person to another. It’s not a shared universal constant, despite what our everyday experience might tell us. One second will always tick at a rate of one second *for you*. It’s only when you compare your clock to someone else’s that you would ever notice anything different.
You can think of space and time a bit like a teeter totter, where on the left side you have the three dimensions of space (up / down, front / back, side to side) and on the right side you have the one dimension of time (past to future). The more your motion occurs through space the less it occurs through time, and vise versa. Everything in the universe is in motion through these four dimensions but how much of that motion is through space and how much of that motion is through time is different for everything. For example light, the fastest moving thing in the universe, experiences all of its motion through space and consequently none of its motion through time. From the perspective of a photon it doesn’t experience time, it reaches its destination at the instant of its creation. On the other end of the teeter totter something that is perfectly at rest and sitting still through space is still in motion towards the future – tomorrow – and so it experiences all of its motion through time and none of it through space.
Technically any time you board a plane, get in a car, or even walk to the fridge you’re experiencing time dilation relative to someone sitting still, it’s just happening on scales so insignificantly small that for all practical purposes we ignore it.
The reason *why* can get complicated but it has to do with the fact that light is measured as moving at the same rate of speed for all observers. No matter how fast you’re moving light will always be measured as moving at the speed of light, just the same as if you were standing perfectly still. The only way for that to be true is if the person standing still and the person moving both experienced time at different rates. We’ll both agree that light moves at roughly 300,000 meters in one second, but we won’t agree on when that second worth of time has passed.
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