4 ms·

> Electromagnetism, weak, and strong interactions all make the acceleration meter register nonzero, even if the act "equally" on all parts of an object. I'm st

> Electromagnetism, weak, and strong interactions all make the acceleration meter register nonzero, even if the act "equally" on all parts of an object. I'm struggling to wrap my head around this assertion. If all parts of the object are acted upon "equally" (why is this in quotes?) where would this acceleration come from?

pdonis 3y ago
> If all parts of the object are acted upon "equally" (why is this in quotes?) where would this acceleration come from? It is proper acceleration, not coordinate acceleration. An object can have nonzero proper acceleration even if none of its parts are in relative motion. Geometrically, proper acceleration corresponds to path curvature of the worldlines of the atoms in the object. "No relative motion" means all the worldlines of the object's atoms have the same path curvature (modulo some technicalities that don't really matter here). It does not require that that path curvature be zero. Physically, a typical accelerometer works by measuring the internal stresses that are set up in an object when it is accelerated. These stresses put the object into a different equilibrium state than it was in when it was freely falling: the object's size and shape can change. For typical solid objects at typical Earthbound accelerations these changes are too small for us to see with our unaided senses--but sensitive instruments like accelerometers can detect them.
- deleted 3y ago
  [deleted]
- jjk166 3y ago
  But that path curvature of worldlines is in spacetime, the field of gravity. If you chose your worldline along the electromagnetic field instead, you'd see no "proper" acceleration when being pulled by the electromagnetic field and you would see "proper" acceleration due to gravity. And to develop internal stresses, there needs to be some difference in the forces acting upon different parts of the body. Again, you are arbitrarily using free fall as your choice of a default state, against which you are comparing. If you instead chose being stationary on the ground, which would correspond to when you are in electromagnetic free fall, you would register an acceleration when in gravitational free fall.
  - pdonis 3y ago
    > that path curvature of worldlines is in spacetime It's the path curvature of worldlines in spacetime. > the field of gravity Only if the spacetime is curved. But worldlines can have path curvature even in flat spacetime. > If you chose your worldline along the electromagnetic field instead, you'd see no "proper" acceleration when being pulled by the electromagnetic field and you would see "proper" acceleration due to gravity. There is no such thing as "worldline along the electromagnetic field" as you appear to be using the term. A charged object in an electromagnetic field will have nonzero proper acceleration, as measured by an accelerometer. That's an invariant prediction; there is no alternate model in which it's any different. > to develop internal stresses, there needs to be some difference in the forces acting upon different parts of the body No, there doesn't. There just needs to be a difference in the shape and size of the body, from its state when under no external forces. It is true that, if the body is large enough, the distribution of internal stresses in the body might not be uniform when the body reaches its equilibrium state under some externally applied force. For example, if the object is tall enough, the internal pressure at the top will be measurably less than the internal pressure at the bottom. But this is not due to any difference in external force being applied to the object. It's due to how the object adjusts itself to be in equilibrium under the applied external force. > If you instead chose being stationary on the ground, which would correspond to when you are in electromagnetic free fall, you would register an acceleration when in gravitational free fall. Again, this is just wrong. There is neither a valid theoretical model, nor any experimental data, to support this claim.
    - jjk166 3y ago
      You are grossly misunderstanding what I am saying, and I don't know how better to explain it.
      - deleted 3y ago
        [deleted]
      - pdonis 3y ago
        As far as I can tell, you are claiming that there is a viable model in which, for example, a charged object moving in an electromagnetic field would have "zero proper acceleration" while an uncharged object moving solely under gravity, would have "nonzero proper acceleration". But such a model can't be right, because "proper acceleration" is a direct observable, and our direct observation is the other way around.

pdonis 3y ago

> If all parts of the object are acted upon "equally" (why is this in quotes?) where would this acceleration come from? It is proper acceleration, not coordinate acceleration. An object can have nonzero proper acceleration even if none of its parts are in relative motion. Geometrically, proper acceleration corresponds to path curvature of the worldlines of the atoms in the object. "No relative motion" means all the worldlines of the object's atoms have the same path curvature (modulo some technicalities that don't really matter here). It does not require that that path curvature be zero. Physically, a typical accelerometer works by measuring the internal stresses that are set up in an object when it is accelerated. These stresses put the object into a different equilibrium state than it was in when it was freely falling: the object's size and shape can change. For typical solid objects at typical Earthbound accelerations these changes are too small for us to see with our unaided senses--but sensitive instruments like accelerometers can detect them.

deleted 3y ago
[deleted]
jjk166 3y ago
But that path curvature of worldlines is in spacetime, the field of gravity. If you chose your worldline along the electromagnetic field instead, you'd see no "proper" acceleration when being pulled by the electromagnetic field and you would see "proper" acceleration due to gravity. And to develop internal stresses, there needs to be some difference in the forces acting upon different parts of the body. Again, you are arbitrarily using free fall as your choice of a default state, against which you are comparing. If you instead chose being stationary on the ground, which would correspond to when you are in electromagnetic free fall, you would register an acceleration when in gravitational free fall.
- pdonis 3y ago
  > that path curvature of worldlines is in spacetime It's the path curvature of worldlines in spacetime. > the field of gravity Only if the spacetime is curved. But worldlines can have path curvature even in flat spacetime. > If you chose your worldline along the electromagnetic field instead, you'd see no "proper" acceleration when being pulled by the electromagnetic field and you would see "proper" acceleration due to gravity. There is no such thing as "worldline along the electromagnetic field" as you appear to be using the term. A charged object in an electromagnetic field will have nonzero proper acceleration, as measured by an accelerometer. That's an invariant prediction; there is no alternate model in which it's any different. > to develop internal stresses, there needs to be some difference in the forces acting upon different parts of the body No, there doesn't. There just needs to be a difference in the shape and size of the body, from its state when under no external forces. It is true that, if the body is large enough, the distribution of internal stresses in the body might not be uniform when the body reaches its equilibrium state under some externally applied force. For example, if the object is tall enough, the internal pressure at the top will be measurably less than the internal pressure at the bottom. But this is not due to any difference in external force being applied to the object. It's due to how the object adjusts itself to be in equilibrium under the applied external force. > If you instead chose being stationary on the ground, which would correspond to when you are in electromagnetic free fall, you would register an acceleration when in gravitational free fall. Again, this is just wrong. There is neither a valid theoretical model, nor any experimental data, to support this claim.
  - jjk166 3y ago
    You are grossly misunderstanding what I am saying, and I don't know how better to explain it.
    - deleted 3y ago
      [deleted]
    - pdonis 3y ago
      As far as I can tell, you are claiming that there is a viable model in which, for example, a charged object moving in an electromagnetic field would have "zero proper acceleration" while an uncharged object moving solely under gravity, would have "nonzero proper acceleration". But such a model can't be right, because "proper acceleration" is a direct observable, and our direct observation is the other way around.