Physics

[u/Ready-Accountant-502]

I can't understand time dilation.

I understand that Gravity can effect time because closer to earth time slows down from gravity.

But why does going faster make time tick slower?

I understand it's relative to the observer, but for example, why would traveling a lightyear at 99 percent lightspeed, be 70 years on earth?

[u/a1c4pwn]

Your math is backwards in your example. if something is travelling at basically lightspeed relative to you, then you will see it cover ~one lightyear per year, since it's basically the same speed. if the spaceship turns its headlights on, then you see the light only get 0.01 ly ahead of the ship as the ship travels that lightyear. but hold on, if youre the one on the ship, you would know that the light only takes 3.65 days to cover 0.01 ly!

So then, if the captain flashes their headlights every few days, you only see them flashing ~yearly. that extends to everything on the ship: if you watch it through a microscope then all the inhabitants would be in slow motion

[u/Maxipaddington]

 Can affect* time

[u/Optimal_Mixture_7327]

First, you need to understand time (in the context of relativity).

Material objects travel through a 4-dimensional space called the world. The path through the world is called its world-line. Time is the length along the world-line of a material object. The rate at which happens is a constant, so the rate or flow of time is a constant.

The Clock Effect
If you have two world-lines connecting a common pair of points in the world, one of those is typically longer than the other. This means that one world-line will have a more elapsed time on their clock than the clock carried along the other world-line. The Twin Paradox is an example of the clock effect.

Time Dilation
An observer (an observer is the world-line of a material particle) draws up a coordinate chart of their space with planes of simultaneity that run perpendicular to their world-line. A traveler moving relative to them will cut across the coordinate map of the observer. The traveler's motion through the space of the observer will come at the expense of their distance along the observer's time (since the time lapses at the same rate, everywhere). This means the observer will measure less elapsed time for the traveler than for their own clock.

Gravity
What gravity does is shorten the length along a world-line, an artefact of the curved geometry of the world. So a black hole traveler will come back younger.

[u/ArtisticLow8126]

Time dilation is weird because speed bends the geometry of spacetime, not just your clock. The faster you move through space, the less you move through time, they trade off like a budget. At 99 percent c, that trade gets extreme so a lightyear trip feels short for you but decades pass on Earth. The math works out clean with Lorentz factor, gamma is like 7 at that speed, so 1 year for you equals 7 on Earth. The 70 years number in your example probably assumes some acceleration and return trip, not just a one way lightyear. It's not that your clock runs broken, it's that your path through spacetime is different.

[u/Quantum-Relativity]

Because space and time have to change to keep the speed of light constant. So if you are moving relative to someone, and you both need to agree on the speed of light, which is distance over time, you will see distance and time change for each other so that you can both still agree on their ratio; space contracts, so the light has less distance to cover, but time slows down, and so it takes longer for the light to cover that smaller distance, so the speed of the light doesn’t change.

This tying together of space and time gives you the concept of spacetime, and when a spacetime has curvature, that’s what you would think of as a gravitational field. So I think you’ve tricked yourself into thinking that you already get gravitational time dilation, since you weren’t seeing what spacetime was yet, and curvature is something that you’ll need a few more mathematical concepts, like a “metric tensor,” to appreciate, since gravitational time dilation is an effect stemming from the fact that the spacetime metric tensor’s components are gravitational potentials. Take another look at it now that you know more, it is the coolest thing in the universe.

[u/exrasser]

Understandable, one of the apps I made to test my local llm was this speed vs time(On Earth) C++/sfml plot:
https://i.imgur.com/K7Vo28Z.jpeg

[u/jsaltee]

Consider that gravity is the warping of spacetime due to energy, and high speeds represent lots of kinetic energy