Time

Time runs slower for objects moving fast.

Time runs slower for objects closer to a strong gravitational pull.

True things:

Say you have two clocks that keep accurate time. If you leave one at home and take one with you on a round-trip airplane flight, the clock you bring with you on the trip will be behind the one you left at home when you get back.

If you take the same two clocks, put one on top of a mountain and one down by the beach (assuming no weird gravitational pull anomalies based on geographical location), the clock on the beach will be behind the one on the mountain. While that example is extreme, you can replicate scaled results simply by placing one on a table and one on the floor.

The mountain example works for Mount Everest.

https://ascentdescentadventures.com/blog/is-gravity-different-at-the-top-of-everest/
https://www.physicsclassroom.com/class/circles/lesson-3/the-value-of-g

• At sea level, the value of g is approximately 9.806 m/s².
• At the summit of Mount Everest, this value drops to around 9.773 m/s².
• At twice the distance from the center of the Earth, g is approximately 2.45 m/s².

These clocks running slower or faster experiences always depend on two relative points in space. If you are an observer in intergalactic space (between galaxies) and one on Earth, the clocks are running very differently. There is a measurable difference if you are a person on Pluto vs. someone on Earth. It is not the insane levels shown on Interstellar, but it will tick by differently.

The Speed of Light

The last thing that is often tricky to comprehend is how things moving at the speed of light observe time. It has been shown using equations that anything with measurable mass cannot move at precisely light speed. There is a problem with mass and energy when you reach that c speed. However, if you could move at that speed, you could travel anywhere in the universe instantly for yourself.

The math shows that time would stop for a mass moving at the speed of light. If you wanted to travel 500 light-years at instant light speed, stop, and return at the same rate, it would take you 0 seconds, but the Earth would have experienced 1,000 years. Going to instant light speed is currently impossible, let alone changing direction 180 degrees in an instant.

Most of our ambitious plans to send probes to nearby stars use laser energy to speed objects up to insane speeds to shorten the time required to get there. If we could get a probe to 80% of the speed of light, the probe would experience 3/5ths of a year for every 1 year we sat here and waited for it to return the signal.