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Assuming what? We know the earth is dense because it has a lot of gravity. Yet we don't fully understand gravity, and have admitted other things can be indistinguishable from gravity. If we don't fully understand gravity, or what causes it, and don't know what's in our earth, how it it we can be so sure it is density causing it? I tried to move on on this point, but no one could assure me ---density = gravity. 100%, without exception, in theory or observation. Because if we can make any exception, I want to have a look at maybe making one too.
Density IS not gravity. Massive bodies exhibit gravitational attraction across a gradient determined by density.
The conventional way of conceptualising gravity is to view space as a rubber sheet onto which we put on object, like a ball. The object will create a dent in the sheet. Similarly, massive objects in space "bend" that space such that other objects slide down the gradient towards them.
Where density enters the picture is that it determines how steep the gradient is. A very heavy, very small, ballbearing on the rubber sheet would make a deep, steep depression, while a large snowball might make a wide, shallow depression.
Hence, we can calculate the total mass of the earth from knowing how big the earth is, and observing much it influences other bodies such as the moon.
Now "other" things can be subjectively indistinguishable from gravity, but this does not mean they are the same thing as gravity. That is, what gravity DOES is cause two massive objects to accelerate toward one another, according to the gradient discussed above. ANY OTHER EFFECT that causes you to accelerate will "feel" like gravity, but will not BE gravity. So when you are in a lift that goes down, you start falling toward the centre of the earth - but so is the lift in which you are standing. So the experience that you FEEL is that gravity has lessened, but this is only an illusion cause by acceleration.
similarly, we can cxreate "pseudogravity" in space by spinning a ring or cylinder; if it were wide enough, you could walk on the inside of the ring as if under gravity, which is "attracting" out away from the centre of the cylinder. But this is not real gravity - its just a form of acceleration frustrated by the surface on which you walk. And that will be clear when you throw a ball "up" and find that it does not come "down" where you are standing, because the surface you are on has rotated away while the ball was in flight.
The experience of gravity is that of acceleration, and so other htings can "feel" like gravity without being gravity.
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