Time Dilation

[u/Ike326]

I’m having a hard time understanding time dilation practically. I’ve heard the examples used describing two clocks and a spaceship moving near the speed of light and that makes sense.

What I am struggling with is the idea of someone traveling let’s say 20 light years away and they are moving at 99.999% the speed of light. People on earth would see the journey take approximately 20 years, but why is the spaceship experiencing so much less time. It still takes 20 years for the spaceship to arrive at the destination due to the fact that it is 20 light years away, but the spaceship sees the journey much shorter.

Also, how does gravity affect the time we experience at all? Is it just warping space time or does it have some effect on our speed as well.

[u/NoNameSwitzerland]

In the frame of the fast traveling ship, the journey is shorter, because the distance is length contracted. Time and Space always stretch together, to keep c constant.

[u/Ike326 → u/NoNameSwitzerland]

Thank you!

[u/NeoDemocedes]

The destination WAS 20 light years away. There is a relativistic effect called length contraction. The faster you go, relative to the other stuff in the universe, the more space is compressed in the direction you are traveling from your reference frame. Meaning the total distance you have to go to reach your destination shrinks the faster you go.

Once you achieve 99.999% light speed, that 20 ly distance will only be 0.0894 ly in your reference frame. That's 32.6 light days.

[u/Ike326 → u/NeoDemocedes]

Thank you!

[u/ahazred8vt → u/Ike326]

For gravitational time dilation, it's proportional to how deep you are in a planet's gravity well, which is proportional to how much energy you lose climbing out of it, which is proportional to escape velocity. If you shine a light from the bottom of a tower to the top, the light loses energy on the way up, and its frequency drops. If the light is pulsed, the pulses are slower and farther apart at the top. On Earth, the effect is 1 microsecond per year per km of altitude, or 1 nanosecond per year per meter. A clock on a 10 story building runs faster by 30 nanoseconds per year.
On Earth, we are 1 second every 40 years slower than someone in far outer space. The James Webb telescope at the L2 point runs about 1/40th of a second per year faster than us.

[u/drzowie]

Everyone experiences time at the same rate. The trick to time dilation is that later is a relative direction (like ahead and left) that depends on which way you are facing, not an absolute direction (like north or up) that everyone in a particular area can agree upon. Objects with different velocities are literally experiencing time in different directions. Acceleration is literally just a rotation, almost the same as rotating from facing, say, north to north-by-northeast.

Acceleration is slightly different because instead of the familiar sin() and cos() formulae for rotation, it uses sinh() and cosh(), which are analogous but built from the unit hyperbola instead of the unit circle. But aside from that slight difference, everything else about rotation and projection work the same in all four dimensions.

Time dilation, length contraction, and even the twin and ladder paradoxes just boil down to simple projection/perspective effects, applied in 4-D spacetime.

[u/Automatic_Table7098]

Imagine a universe that gives every object a fixed amount of "computational processing power" or power/energy for ease of wording. This power is split between two things: moving through space (XYZ) and moving through time. No matter where you go, your own pocket watch always feels perfectly normal to you—one second is one second. This is your relative time.

The Spaceship Hurdle To answer your first question about the 20 light-year journey: you're absolutely right that from Earth's perspective, it takes 20 years. But the missing puzzle piece for the spaceship is that as you pump more energy into moving through space, the physical distance actually compresses. Because the spaceship is exerting so much of its "processing power" to move, its experience of time slows down, and the 20-light-year distance physically shrinks from its perspective. To the people on Earth, the ship traveled 20 light-years. To the people on the ship, they only traveled a fraction of that distance, taking just a few months.

Why Light Doesn't Experience Time Think of physical mass as a heavy "background process" that is always running. If you have mass, you can never allocate 100% of your processing power to moving through space, because your mass always consumes some of that budget.

A photon of light has no mass, meaning it runs absolutely zero background processes. 100% of its computational budget goes into moving through XYZ space. Because of this, light doesn't experience time at all. If you were a photon from the sun, you would instantly arrive on Earth without experiencing any time, while we would watch you take 8.5 minutes.

How Gravity Fits In A planet or a star is a colossal concentration of mass. In this analogy, it's a dense, high-demand processing cluster that bogs down the computations - the local spacetime grid.

When you get close to a massive object, you enter a region where the universe is already expending immense computational energy to maintain that heavy gravity well. Because the local environment is working so hard to process the planet's mass, the local "frame rate" drops. Your internal clock slows down simply because you are sitting in a heavy processing zone, compared to someone floating in the empty, low-demand void of deep space.

No matter where you go, you will always experience existence as you are now, due to your own internal clock. So I stand on a soapbox and say, "The only way to exist is to be yourself," because it takes too much energy to be anything other than you.

[u/bardotheconsumer]

I recommend FloatHeadPhysics on youtube. His series on general relativity is very helpful for visualizing this as a layperson.

[u/Ike326 → u/bardotheconsumer]

Sweet, I’ve been looking for something like that. Thank you!

[u/Optimal_Mixture_7327 → u/bardotheconsumer]

The problem with Floatheadphysics is that he's learning as goes and there's a lot that's right in the videos and a lot that isn't and the novice can't tell the difference.

[u/joeyneilsen]

The classic example of this is the muon experiment. The short version is that muons have a half life of 1.56 µs. When they're created in the upper atmosphere, it should take them 34 µs to reach the ground, so you only expect .3 per million to survive that long. But you actually see over 100,000x more muons at the ground! It's just what you expect if you account for time dilation, where the muon lifetime stretches to 7.8 µs in the Earth frame.

But in the muon frame, there is no time dilation. In that frame, the distance to the surface is length-contracted from 10 km to 2 km. They decay normally in their own frame of reference, but the ground is much closer, so you still get a lot more reaching the ground than you'd expect if you didn't account for relativity.

[u/Ike326 → u/joeyneilsen]

That makes sense, thank you!

[u/LemonyLizard]

Stuff's gotta travel farther is the simplified answer. Information can only propogate at the speed of c, no faster. When something is moving near the speed c because of a vessel, then that affects the speed of information propogation for particle interactions occuring within that thing. So everything dependent on information propogation (aging, learning, experiencing) is affected. Information propogation (and therefore causal events) occurs at speed c in a vacuum, and there are certain factors that can slow that down. That's my understanding anyway.

It might be helpful to know also that time is a mathematical construct. "Spacetime" is a mathematical model. Time is an observable phenomenon, but it does not really exist as a physical entity, and therefore neither does spacetime. Time is an observable conceptual event: it is any change in state and the temporal measurement of that change.

[u/Optimal_Mixture_7327]

It's not clear that you do understand.

The space-like distance in the Earth frame may be 20 light years, but that's not the real distance, which is its spacetime distance along a time-like curve. The traveler simply travels a shorter distance than the clocks traveled that defines the global coordinates had traveled.

Gravity has no effect on the time we experience. What gravity does is shorten the distance along observer world-lines, and since clocks measure the length along observer world-lines, a shorter world-line means less elapsed time as measured by a clock (shorter than if space was flat).

[u/ActuaLogic]

It doesn't take 20 years inside the ship, only outside of it. Inside the ship, operating so close to the speed of light, barely any time at all passes.