# 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?

215 views

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?

In: 2

[removed]

[removed]

Short answer: the faster you travel, the slower you perceive time.

As you approach the speed of causality (of light) it comes closer to stopping. The effect of slower time isn’t linear however. 1% to 2% change of speed is miniscule. 90% to 91% I think is like 1/3 (I’m probably off) slower time than normal.

The effect is that assuming 1/3 time, every 40 minutes you experience, a stationary person would have an hour.

Short answer: the faster you travel, the slower you perceive time.

As you approach the speed of causality (of light) it comes closer to stopping. The effect of slower time isn’t linear however. 1% to 2% change of speed is miniscule. 90% to 91% I think is like 1/3 (I’m probably off) slower time than normal.

The effect is that assuming 1/3 time, every 40 minutes you experience, a stationary person would have an hour.

Everyone moves at the same speed through spacetime.

Imagine you’re on a Ferris wheel. The faster you’re moving *horizontally* the slower you move *vertically*, but your total speed is the same throughout.

Spacetime is the same way: The faster you move through the *space* part, the slower you move through the *time* part. Put another way, the more space you experience on your path through spacetime, the less time you experience.

Humans, relative to any useful reference, are effectively stationary, which means we experience basically all that motion through the time component of spacetime. This in turn means we all basically agree on how fast time is passing. If you move fast enough relative to others, that’s no longer the case. From your point of view, time is “compressed”, meaning events seem to occur closer together.

Everyone moves at the same speed through spacetime.

Imagine you’re on a Ferris wheel. The faster you’re moving *horizontally* the slower you move *vertically*, but your total speed is the same throughout.

Spacetime is the same way: The faster you move through the *space* part, the slower you move through the *time* part. Put another way, the more space you experience on your path through spacetime, the less time you experience.

Humans, relative to any useful reference, are effectively stationary, which means we experience basically all that motion through the time component of spacetime. This in turn means we all basically agree on how fast time is passing. If you move fast enough relative to others, that’s no longer the case. From your point of view, time is “compressed”, meaning events seem to occur closer together.

This may be a horrible way to explain it but light has no speed. As far as light is concerned, its trip is instantaneous yet we perceive said trip to have time.

So now imagine you can travel at or near the speed of light from Proxima Centauri. It’s 4.5 light years away. From light’s perspective, its trip is instant and it didn’t age at all. We perceive the time in light hence taking it 4.5 years to get here.

So, a simple thought experiment is: if you were to travel at it near the speed of light, you’d get here real quick per your perception but everyone would be 5 years older when you got here

Edited. I’m being downvoted but this video explains it: https://youtu.be/ACUuFg9Y9dY

This may be a horrible way to explain it but light has no speed. As far as light is concerned, its trip is instantaneous yet we perceive said trip to have time.

So now imagine you can travel at or near the speed of light from Proxima Centauri. It’s 4.5 light years away. From light’s perspective, its trip is instant and it didn’t age at all. We perceive the time in light hence taking it 4.5 years to get here.

So, a simple thought experiment is: if you were to travel at it near the speed of light, you’d get here real quick per your perception but everyone would be 5 years older when you got here

Edited. I’m being downvoted but this video explains it: https://youtu.be/ACUuFg9Y9dY

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)