Eli5: How does gravity affect time, dumb it down for me as much as possible
In: 286
Hi /u/JustTransportation51!
While we generally think of gravity as a force pulling us down, this is not how General Relativity (GR) – the bet theory of gravity we currently have – sees it.
According to GR, gravity is the curvature of spacetime. So, in order to understand what this means, we have to back up a little and ask “what is even spacetime”?
Newtonian physics saw space and time separate and unchangeable entities in though physical objects could move. In particular, Newtonian mechanics thought, that all objects moved through time at the same rate.
The theory of relativity, however, discovered that space and time are neither separate nor unchangeable. Rather, phenomena such as [time dilation](https://en.wikipedia.org/wiki/Time_dilation) or [Lorentz contraction](https://en.wikipedia.org/wiki/Lorentz_contraction) meant, that spatial and temporal coordinates could be transformed into one another and depend on the state of the observer. Thus, spacetime – i.e. the unification of space and time into one single object – was required to fully describe these phenomena.
Furthermore, General Relativity discovered, that the geometry of this spacetime was not fixed. Rather, the presence of mass and energy were able to distort spacetime. This distortion of spacetime caused by mass and energy is what we perceive as gravity ¹.
Now we can come back to your question: As gravity is the curvature of spacetime, space and time are affected by gravity. And in particular, strong gravitational fields curve spacetime in such a way, that time is slowed down.
Does this answer your question?
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#Appendix:
¹: To go into some more detail how a curvature of spacetime leads to the phenomenon we perceive as gravity:
To understand how a curvature of spacetime can lead to the effects we observe around us, we have to understand how curved surfaces change the behaviour of straight lines.
First things first: an object that has no force acting on it is force-free. Force-free objects do not accelerate and, therefore, move along straight lines.
In a flat geometry, two straight lines which are parallel at one point will remain parallel for all times. That is, two parallel straight lines will never cross on a flat surface.
So far so intuitive, right?
But what happens, if those straight lines do not move across a flat surface, but instead along a curved surface? We call such straight lines on curved surfaces [geodesics](https://en.wikipedia.org/wiki/Geodesic).
Imagine a [sphere](http://pi.math.cornell.edu/%7Edwh/books/eg99/Ch06/3776c40d.jpg) with two lines perpendicular to the equator. As they are both perpendicular to the same line, they are parallel at that altitude.
Imagine two objects that are moving along the lines perpendicular to the equator. They start out parallel, and move in a straight line upwards. Despite the fact that neither of them is turning, the two objects that started out moving along parallel lines will meet at the north pole. Hence, despite the fact that both objects are force-free at all times, they experience relative acceleration.
Such trajectories, that lead across curved surfaces without turning are called geodesics and they can be thought of as straight lines on curved surfaces. Objects under the influence of gravity follow [geodesics](https://en.wikipedia.org/wiki/Geodesic).
As gravity curves spacetime, geodesics can experience relative acceleration despite the fact, that both objects following said geodesics are force-free. And this relative acceleration of force-free bodies is what Newton mistook for the gravitational force. According to GR, though, there is no force, only curvature which causes force-free objects to move along paths that seem accelerated to outside observers.
This is why gravity is a fictitious force: The reason why two objects in a gravitational field may experience relative acceleration is not a force between them, but geodesic deviation between two force-free objects.
If you have any more specific questions, feel free to ask.
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For a great video on the basics of GR, check out [this](https://www.youtube.com/watch?v=NblR01hHK6U) video by PBS Spacetime.
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