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?

Question

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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Answer 1

i think your question is the why, and the most simple answer is that light always seems to be going the same speed no matter how you travel in relation to the light source.

you’re in a car going 50 miles an hour and you see another car also going fifty miles an hour, directly towards you. the apparent speed of that car is 100 mph, because you’re both moving toward each other.

light doesn’t work that way. if you’re going .5C (the speed of light in a vacuum) toward a flashlight and you somehow clock the speed of the light coming towards you, you won’t clock it at 1.5C. it’s still going 1C. your speed doesn’t add to it.

the speed of light is also the speed of causality. since nothing can go faster than light, nothing can affect something else faster than light (the classic trollscience thing where two people have a pole that’s one light year long, you will **not** be able to poke him instantaneously with it, that would violate causality.)

so when you put all that together, since the speed of causality doesn’t increase or decrease relative to your speed the way the speed of a car would, you have to infer that your perception of the flow of time outside your spaceship would have to slow down. keeping time constant would require your perceived speed of light to change.

edit: or wait no, i got it backwards. outside perception of your time slows down. which for you means more time passes outside your spaceship than appears to pass inside it. which means you could do a loop around the solar system at some fraction of lightspeed, perceive it as say one year, but get back to earth and its been ten years since you left.

and practical experiments have shown that the inference is correct, this does actually happen (e.g. gps satellites have to correct for time dilation or they’ll get out of synch with clocks on earth)