Insight about gravity

[Carl-Richard]

There is this idea that gravity is just acceleration. While there is some truth to that, it's obviously not the whole story.

Let's say you were accelerating in a car. One obvious thing is that you'll feel like you're getting heavier (inertia). However, if you're jumping from a cliff and into a lake, for a while you'll feel like you're weightless (you're in "free fall"). The point is that those are two rather different experiences even though both involve some form of acceleration. But it gets deeper, and here is my insight:

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Gravity is not you accelerating towards the Earth – It's the Earth accelerating towards you.

Now, that is only partially true, but it serves as a good opener for the point about reference frames. But first, another common description that is also not very satisfying, is that the earth is "pulling" you using its gravitational pull, because let's say if you were pulled by a car on earth, you would still feel like you're getting heavier (inertia). So the problems with these two descriptions of gravitational "pull" or "acceleration" is that the everyday experience or common conception of them are not very accurate for describing gravity (the lack of feeling of inertia).

So about reference frames: in space, there is no absolute frame of reference for movement. In that sense, when you have two bodies in space entering their respective gravitational fields, they will simply start moving towards each other. It's not that the Earth is "pulling" you, it's not that you're "accelerating" towards the Earth. No – strictly speaking, the distance between the two bodies is decreasing.

But more importantly, from the perspective of what you're actually experiencing on a bodily level, it's as if you're simply floating in space but the Earth is moving/accelerating towards you. From the Earth's perspective, it's as if it's simply floating in space and your body is moving towards it. The nature of this movement can of course be partially described as acceleration, but the point is that it's qualitatively different from other phenomenas involving acceleration.

I tried to keep it as laymen friendly as possible (as that's what I am after all), but I'll appreciate any input from people who are more immersed in physics than me  

[JohnnyAb]

Why would you compare accelerating in a car to free falling? In former gravity clearly isn't the main acting force. Also where did you get an idea that gravity is an acceleration? There is acceleration caused by gravity but these are not the same thing.

[Carl-Richard]

9 hours ago, JohnnyAb said:

Why would you compare accelerating in a car to free falling?

I wasn't comparing them. I was making a distinction. Both involve acceleration but they're still not the same. The car example is only to explain a point.

Edited May 30, 2021 by Carl-Richard

[Carl-Richard]

8 hours ago, Tim Ho said:

According to mordern physic, Gravity bend space around it, or reduce the space between the objects.  There is no frame.  Objects just "fall" into empy space between them.

Only in Classic physic, when objects pull directly on each other, then your claims might be considered.

That is one model, yes. However, Einstein is also using reference frames as a basis for his models, hence the Lorentz factor for estimating time dilation in Special Relativity and the equivalence principle in General Relativity:

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General relativity is based upon the principle of equivalence:[51][52]

There is no experiment observers can perform to distinguish whether an acceleration arises because of a gravitational force or because their reference frame is accelerating.

— Douglas C. Giancoli, Physics for Scientists and Engineers with Modern Physics, p. 155.

The idea that your reference frame is accelerating is exactly what my insight was about: it's what you would experience if you imagine that the Earth is accelerating towards you. In reality, in a sense you are accelerating, but you're also weightless because your entire "frame" is accelerating, not just your body so to speak. It's not so much that your body is accelerating relative to the background than it is your body "+" the background accelerating in unison.

 

10 hours ago, JohnnyAb said:

Also where did you get an idea that gravity is an acceleration?

"Gravity is not a force – it's an acceleration" is a statement made by the yt channel Veritasium, which is obviously just an opener for a deeper explanation, but that is also why I included it here.

If I had to state it again, my point with this topic was to point out the peculiar nature of gravity using laymen terms without copy-pasting some physics textbook. This is just one way to do it.

Edited May 30, 2021 by Carl-Richard

[JosephKnecht]

You might find this insightful. Explained for the layman. 

 

[Carl-Richard]

@JosephKnecht Exactly. He talked about the distinction between knowing about the "force" of gravity (9.81m/s^2 - an acceleration) and knowing the mechanics behind how it works (his examples about free fall). He lost me at the differential geometry jargon though