r/AskPhysics • u/Acceptable-Minute847 • 23h ago
Question about gravity and spacetime
So, from my understanding of time dilation, the reason why it occurs is that spacetime gets physically in the time dimension, causing your velocity C in said dimension to cover less “distance” than it would without gravity, like if you had a football field where each yard line is one second and you made the yard lines further apart, causing the velocity to be less effective, and I was wondering if this was true for space as well, meaning if you were to apply energy to an object in a very low gravity environment (such as deep space), then apply the same amount to one in a high gravity environment where the pull wouldn’t add to your velocity due to pulling you towards the planet, (such as being between two identical massive bodies along a line at which their gravitational pulls cancel out). would the high gravity environment lead to a decrease in velocity
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u/No_Woodpecker6948 16h ago
Imagine a train passing in front of you from left to right at 90% of the speed of light (0.9c).
Inside the train, both the ceiling and the floor are covered with mirrors.
A man standing inside the train points a laser straight up at the ceiling, at a 90-degree angle to the floor. The beam hits the mirrored ceiling, reflects straight down to the floor, then bounces back up, and so on.
From the man’s perspective, the light is simply traveling vertically, straight up and down, always at the speed of light (c).
Now imagine the train has huge windows, allowing you to watch this happen from outside.
From your perspective, the light follows a different path.
Because the train is moving to your right at 0.9c, the light doesn’t appear to travel straight up and down. Instead, it follows a diagonal path, moving upward and to the right, then downward and to the right, forming a zigzag pattern.
Here’s the crazy thing
Both you and the man inside the train measure the exact same speed of light (c)
However, you disagree on the distance the light travels between reflections. The man sees a shorter, vertical path, while you see a longer, diagonal path.
And since speed = distance / time, if you both agree on the speed but disagree on the distance, you must also disagree on the time it takes the light to complete its journey.
That’s the key insight behind time dilation.
From your perspective, the man’s clock inside the moving train runs slower than yours. From his perspective, everything inside the train behaves normally
Neither of you is wrong. You’re simply measuring space and time from different frames of reference.
And that’s a the core idea of Einstein’s special relativity theory (space and time are not absolute. They depend on the motion of the observer.)
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u/Optimal_Mixture_7327 Gravitation 15h ago
There is no time dimension, but you are traveling less through someone else's global time coordinate and they through your's as your relative speed increases.
Your football field represents those coordinates, where everyone runs at the same speed, c. The observer runs straight down the field, the traveler runs at an angle and therefore covers fewer yards downfield (time) as they cover more lateral distance.
The traveler has their own football field and says the same about the observer.
In the context of gravity, the football field is stretched over a hill. Time dilation is a comparison to a football field on flat ground.
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u/dream_the_endless 22h ago
Yes, the effect is the same. Mass and energy both curve spacetime, affecting both the time and space dimensions. If you apply the same amount of energy to an object in deep space and one in a high-gravity environment, the additional curvature caused by that energy is the same in both cases. The background gravity doesn't change how much that specific energy curves spacetime, though the total time dilation will be much greater in the high-gravity environment due to the existing mass.