How do we know that gravity is not an accelerated expanding of space?
It seems to do almost same thing to interacting objects.
“it’s complicated” comment is not welcome.
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Can you explain more what you mean? Gravity tends to bring things closer together, so wouldn’t it be a contraction of space, if anything, rather than an expansion?
And how does that work when I feel pressure from the soles of my feet against the ground? Is there space expansion or contraction at the interface between my feet and the ground, and that’s why I feel pressure?
> Eli5 How do we know that gravity and acceleration is two different thing?
Because hitting the gas in a sportscar doesn’t magically produce gravity.
First off, what the sam hill is “an accelerated expanding of space”?
Second, space doesn’t move with gravity. Gravity distorts space but we can see relativistic effects due to speed regardless of gravitational effects.
At first glance, you might be able to attribute the effects of gravity to some kind of acceleration through space. But that theory breaks down pretty quickly once you start considering stuff like other celestial bodies. If gravity is just us accelerating through space, then why does gravity also pull us towards other massive objects when we get close to them?
The short answer is they aren’t different, see: the Equivalence Principle.
The long answer is… it’s complicated. Locally they are identical but a gravitational field will exhibit properties that an otherwise accelerating frame of reference does not: e.g. tidal forces. This equivalence stems from the observed fact that gravitational and inertial mass appear to be identical in value, possibly not by coincidence.
Based on our observations and scientific understanding, we know that gravity and acceleration are two different things because they have distinct effects on interacting objects, even though they may seem similar at times.
I think what you’re confusing are forces. Things accelerate because of forces applied to them. Without context, one force would appear and could be confused for another force, if all you see is the result (acceleration)
You’re actually asking a pretty good question here, and there’s a lot of misunderstanding happening in the responses — Partly because people are answering confidently with incomplete information, but also because you’re speculating a lot in your questions with specifics that are not correct.
But to get to (what I believe is) the heart of your question: You are correct that from a mathematical perspective the feeling of weightlessness that one experiences during free fall is identical to the feeling of weightlessness of floating freely in space. And likewise the feeling of weight on Earth’s surface is identical to the feeling of being acted on by a constant force (such as being in a rocket perpetually accelerating at 9.8m/s^2 in space). This thought is actually one of the foundations of Einstein’s General Relativity, and what makes it different from the earlier theory of gravity posited by Newton.
In a Newtonian perspective, a falling object has a force acting on it, and one resting on the ground is at rest. This is the framing that most of the other commenters are using in their replies to you. But in General Relativity, we change our perspective such that being in free fall is to be an “inertial frame of reference,” which basically means there are no forces acting on you. This (counterintuitively) means that to be “resting” on the ground is better described as having the Earth exerting a pushing force on you.
The explanation for this difference is that in General Relativity, the presence of mass bends spacetime. In a perfectly flat spacetime, you and the Earth might move in parallel straight lines, never getting any closer to each other. But in the curved spacetime surrounding the Earth, those straight lines now converge toward each other. This means that as time passes, if you are not acted on by an outside force, you are guaranteed to get closer to the Earth. We perceive this as an acceleration because we approach the Earth at a faster rate as we get closer to it and the curvature gets more pronounced.
Eventually, our straight line (which is trying to go to the Earth’s center of gravity) is interrupted by the solid surface of the Earth. Thus the solid ground beneath our feet can be said to be “pushing” us off trajectory. We experience this pushing as weight — a constant force pushing on the bottom of our feet against our “desired” movement toward the interior of the planet.
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