4 ms·

That’s because you aren’t in free fall.

ctdonath 5y ago
If I step off a bridge, why does the distance between me and the very large nearby spherical mass begin decreasing at an, umm, accelerating rate?
- babesh 5y ago
  The surface is accelerating up at you relative to the gravitational field.
  - mrow84 5y ago
    What happens if two people jump off bridges on opposite sides of the earth? How does that work in that interpretation?
    - 317070 5y ago
      Same thing? For each of them, they see themselves non-accelerating with respect to the gravitational field. But they do see the soil accelerating towards them against the gravitational field. Why is the soil not in free-fall? Because it feels normal force of all the earth below. And why is not the whole earth in free fall towards its core? Because it has an electrostatic force (a real one!) which keeps all the atoms and molecules from collapsing on itself. If it would collapse on itself, it would first become a neutron star and then a black hole.
      - mrow84 5y ago
        It still isn’t obvious to me how, if “the surface is accelerating up at you relative to the gravitational field” on both sides of the earth, the earth isn’t pulling itself apart. To be clear, my confusion is about how this interpretation is consistent with the facts, not the facts themselves (I think).
        317070 5y ago
        ah, that is where the curvature of spacetime comes in. That field is curved between both observers on each side of the earth, because there is a massive object between them: the earth. So the field is going radially inwards towards the center of the massive object.
        mrow84 5y ago
        I'm willing to accept it as yet another thing I don't understand, but it still isn't obvious to me how the field being curved solves the issue. Even if we have bent our coordinate system around the earth (which I think is equivalent to what you're saying), how can the two sides of the earth both be accelerating "upwards", or now "outwards", without the earth being pulled apart? Do you know of any diagrams that might illustrate this idea to a layperson?
        Twisol 5y ago
        Layperson here. As I understand it, if there were no bonds or repulsive forces between the atoms that made up Earth, they would all be in free fall, and they would all fall toward the center of the Earth's mass. Since these are inertial paths, there's no force compelling them to fall; they're just following the curvature of spacetime. The electrostatic forces and bonds between atoms are exactly what prevents these atoms from following inertial paths. The atoms act on each other, and collectively produce a force radially outwards from the center of Earth supporting its mass. Earth isn't pulling itself apart because nothing is pulling in the first place. All the mass is pushing against itself, refusing to be packed tighter, despite the flow of spacetime around it drawing it together over time. When we're standing on Earth, our particles are following the same flows of spacetime toward the core of Earth. From our inertial frame, the ground below us is moving toward us, because as noted Earth resists being compacted further. So we meet the surface, we both resist being compacted by the other, and we experience a stable force.
        mrow84 5y ago
        "From our inertial frame, the ground below us is moving toward us" makes a lot of sense to me, but doesn't seem to be quite the same as "the surface is accelerating up at you relative to the gravitational field". The former is with respect to the inertial frame of the faller(s), the latter seemingly with respect to the inertial frame of some shared gravitational field. That said, your description does kind of explain it for me, unless I am misunderstanding, though at the moment I only get it as a "negative" argument: we are being accelerated upwards/outwards with respect to the field's reference frame, because otherwise we would be moving along our natural inertial path. It is still a bit puzzling that if the ground disappears we appear to accelerate, whereas my current reading of what you wrote seems to imply that we would immediately return to our "inertial path". What I'm really wondering is if all of this "acceleration relative to the field" business is a misleading abstraction, and "relative reference frames" fix it (and indeed that was my very primitive understanding of relativity). However, on balance it seems much more likely that I'm simply misunderstanding things.
- gumby 5y ago
  Compared to the radius of the earth the Bridge is on the surface of the earth. In other words you don’t travel far enough / long enough to observe any difference in g.

If I step off a bridge, why does the distance between me and the very large nearby spherical mass begin decreasing at an, umm, accelerating rate?

babesh 5y ago
The surface is accelerating up at you relative to the gravitational field.
- mrow84 5y ago
  What happens if two people jump off bridges on opposite sides of the earth? How does that work in that interpretation?
  - 317070 5y ago
    Same thing? For each of them, they see themselves non-accelerating with respect to the gravitational field. But they do see the soil accelerating towards them against the gravitational field. Why is the soil not in free-fall? Because it feels normal force of all the earth below. And why is not the whole earth in free fall towards its core? Because it has an electrostatic force (a real one!) which keeps all the atoms and molecules from collapsing on itself. If it would collapse on itself, it would first become a neutron star and then a black hole.
    - mrow84 5y ago
      It still isn’t obvious to me how, if “the surface is accelerating up at you relative to the gravitational field” on both sides of the earth, the earth isn’t pulling itself apart. To be clear, my confusion is about how this interpretation is consistent with the facts, not the facts themselves (I think).
      - 317070 5y ago
        ah, that is where the curvature of spacetime comes in. That field is curved between both observers on each side of the earth, because there is a massive object between them: the earth. So the field is going radially inwards towards the center of the massive object.
        mrow84 5y ago
        I'm willing to accept it as yet another thing I don't understand, but it still isn't obvious to me how the field being curved solves the issue. Even if we have bent our coordinate system around the earth (which I think is equivalent to what you're saying), how can the two sides of the earth both be accelerating "upwards", or now "outwards", without the earth being pulled apart? Do you know of any diagrams that might illustrate this idea to a layperson?
        Twisol 5y ago
        Layperson here. As I understand it, if there were no bonds or repulsive forces between the atoms that made up Earth, they would all be in free fall, and they would all fall toward the center of the Earth's mass. Since these are inertial paths, there's no force compelling them to fall; they're just following the curvature of spacetime. The electrostatic forces and bonds between atoms are exactly what prevents these atoms from following inertial paths. The atoms act on each other, and collectively produce a force radially outwards from the center of Earth supporting its mass. Earth isn't pulling itself apart because nothing is pulling in the first place. All the mass is pushing against itself, refusing to be packed tighter, despite the flow of spacetime around it drawing it together over time. When we're standing on Earth, our particles are following the same flows of spacetime toward the core of Earth. From our inertial frame, the ground below us is moving toward us, because as noted Earth resists being compacted further. So we meet the surface, we both resist being compacted by the other, and we experience a stable force.
        mrow84 5y ago
        "From our inertial frame, the ground below us is moving toward us" makes a lot of sense to me, but doesn't seem to be quite the same as "the surface is accelerating up at you relative to the gravitational field". The former is with respect to the inertial frame of the faller(s), the latter seemingly with respect to the inertial frame of some shared gravitational field. That said, your description does kind of explain it for me, unless I am misunderstanding, though at the moment I only get it as a "negative" argument: we are being accelerated upwards/outwards with respect to the field's reference frame, because otherwise we would be moving along our natural inertial path. It is still a bit puzzling that if the ground disappears we appear to accelerate, whereas my current reading of what you wrote seems to imply that we would immediately return to our "inertial path". What I'm really wondering is if all of this "acceleration relative to the field" business is a misleading abstraction, and "relative reference frames" fix it (and indeed that was my very primitive understanding of relativity). However, on balance it seems much more likely that I'm simply misunderstanding things.
gumby 5y ago
Compared to the radius of the earth the Bridge is on the surface of the earth. In other words you don’t travel far enough / long enough to observe any difference in g.