A free falling body doesn’t experience gravity but is moving relative to a stationary observer. This observer experiences gravity. – check.
An accelerating body experiences “gravity” and a stationary body observed by the accelerating body appears to be in motion. – check.
There’s a Veritasium video that covers this: [Why Gravity is NOT a Force](https://youtu.be/XRr1kaXKBsU).
Here’s where it gets a bit mind-bendy though. If the surface of the earth is stationary, why is it pushing up on our feet like the floor of an accelerating rocket would? Is it because although it is stationary relative to our space, it *is* moving through TIME?
In: 0
It’s pretty complicated yes.
Spacetime is curved by gravity. In order to be stationary in curved spacetime you’d have to be accelerating in a universe that appears flat.
>Is it because although it is stationary relative to our space, it *is* moving through TIME?
Kind of yeah, but the important part is that it’s moving through curved spacetime. If you’re in space far away from earth you’re also moving through time without accelerating.
You’re always moving at c, if you’re stationary you’re moving at c in the time direction, if you move quickly through space you move slower through time. If you are within curved spacetime then a part of your movement through time becomes a movement through space because space and time are interlinked
The details are hard to ELI5 and involve a lot of heavy math.
Maybe I can find some “general relativity exlained in simple terms” but I guess they’ll also just state those facts and can’t explain the why. The why is simply “because that’s the only way it can both be true that movement is always relative and the speed of light is a universal limit”. There is no other way to make the math work out with both of these staying true
The idea that gravity isn’t really a force is a pretty high-level concept that I wouldn’t recommend for an ELI5 understanding of gravity. But if you do want to run with it, here’s a simple (and true-ish, before you get too deep into general relativity) statement to help get your head around it:
The Earth’s orbit around the Sun is a straight line.
Not in space as we perceive it, of course, but in spacetime, it is (sort of) a straight line. This is (sort of) true because of the way spacetime is curved near big heavy objects like the Sun. Things travel in straight lines when there are no forces acting on them, so the Earth orbits the Sun because there aren’t any forces acting on it (remember: gravity isn’t a force in this setup). Note, though, that the Earth’s orbit is only straight in spacetime, and only if it goes at the specific speed that the Earth is moving at. A slower or faster orbit in the same physical path would not be a straight line in spacetime.
Once you’ve got your head around that, you might be able to see how a satellite orbiting the Earth is moving in a straight line, and a bowling ball dropped from the Leaning Tower of Pisa is moving in a straight line (if we ignore air resistance in both cases), but a human standing still on the surface of the Earth is *not* moving in a straight line. Therefore, the human must have a force pushing on them. That force, by the way, is the force of the ground trying to expand against the pressure of the human and the dirt being jammed together by proximity. It’s typically called the “normal force” in Newtonian physics, because it acts “normal” (perpendicular) to the surface of the ground.
That’s it – both you and the Earth are moving through time and your paths are intersecting.
Everything in the universe is naturally moving through space and time in a straight line at a constant rate. Gravity warps space and time, bending and curving it such that objects moving straight ahead see their path curve. It’s still a straight line, but it’s a straight line in curved spacetime. We call this a geodesic.
In order for an object to deviate from its geodesic requires some kind of force. When the thrusters of a rocket activate the rocket begins to accelerate, and the floor of the rocket begins pushing you upwards. The force of the rocket pushing you upwards is forcing you to deviate from the geodesic path you would have been taking had the rocket not been there.
So how does this relate to gravity?
Imagine an apple falling from a tree and onto the ground. The apple, which was previously motionless, begins moving through the three dimensions of space until it eventually hits the ground, only accelerating once it actually reaches the ground. How is that possible though? If gravity isn’t a force how was the apple able to begin moving in the first place?
Well remember, even though the apple was previously motionless through space it was always still moving forward through time. Once the apple releases from the tree it continues to move forward in time, following a path from present to future. Because gravity curves space *and time* towards the Earth’s center of mass the apple, over time, will begin moving closer and closer towards the ground. Even though the apple reaches the ground it still wants to continue to follow this geodesic path but the Earth’s surface is hard and it pushes against the apple, preventing it from following its natural path. It’s for this reason that the apple doesn’t register an acceleration until it reaches the ground – while falling it was following it’s natural path through spacetime. The Earth, which is also moving forward through time, is pushing against the apple and moving it off it’s natural trajectory.
Latest Answers