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> the acceleration meter won't measure any force because gravity is acting on every part of it uniformly No. The acceleration meter won't measure anything beca

by pdonis 3y ago

> the acceleration meter won't measure any force because gravity is acting on every part of it uniformly No. The acceleration meter won't measure anything because there is nothing to measure. An object in free fall is in free fall; there is no "gravity" acting on it at all. It's just as if the object were floating out in deep space, far from all gravitating bodies. That's the point of the equivalence principle. > If you had an acceleration meter entirely made out of the same magnetic substance, and you brought a magnet near it, would the acceleration meter register anything Yes. Electromagnetism, weak, and strong interactions all make the acceleration meter register nonzero, even if the act "equally" on all parts of an object.

hammock 3y ago
In other words, there is no gravity field, in the way there is an EM field that propagates these forces. Or, the "gravity field" is the fabric of space-time itself
- pdonis 3y ago
  Yes.
BoiledCabbage 3y ago
> The acceleration meter won't measure anything because there is nothing to measure. Put this way, isn't it almost begging the question? In GR the definition of acceleration is movement in contrast with the movement of gravity. If course gravity will never meet this criteria - all movement due to gravity will be aligned with movement due to gravity. If instead we had a universe where instead of all matter having a gravitational effect, it was that all matter had a magnetic effect the we'd see no acceleration due to the magnetic effect and gravity would "produce a field" and cause acceleration in your above examples. You can't use a gravitational biased tool to proclaim gravity is a neutral actor and everything else is a field. It seems like more accurately, everything is "gravitationally charged", so instead we say it warps spacetime, but really is no different.
- pdonis 3y ago
  > Put this way, isn't it almost begging the question? In GR the definition of acceleration is movement in contrast with the movement of gravity. No, it isn't. You have it backwards. The definition of acceleration in GR is proper acceleration, i.e., what an accelerometer reads. The "movement" property is then a consequence of this plus picking an appropriate frame of reference. > If instead we had a universe where instead of all matter having a gravitational effect, it was that all matter had a magnetic effect the we'd see no acceleration due to the magnetic effect Yes, you would, because unlike gravity, magnetism does not obey the equivalence principle, so differently charged objects in the same magnetic field with the same initial conditions can have different motions. With gravity, all objects in the same field with the same initial conditions have the same motion, regardless of their mass. That is why it is possible to model gravity using spacetime curvature, and that property is unique to gravity.
  - lostmsu 3y ago
    I'm sure you are familiar with Kalusa-Klein theory (knowing that it is incomplete and/or doesn't describe our universe), don't you think that motion in that theory is less bound to gravity alone like the parent comment suggests because the notion of proper time is different? I tend to gravitate toward the same line of thinking because of the existence of black hole charge limit.
- lostmsu 3y ago
  You might be interested in https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory
Ma8ee 3y ago
> Yes. 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 genuinely curious how that acceleration meter would work. There won’t be any internal forces as a consequence of the external field and no relative motion.
- pdonis 3y ago
  > I’m genuinely curious how that acceleration meter would work. Look up how the one in your phone works. It reads nonzero when you are standing on Earth because of electromagnetic repulsion between your atoms and the atoms in the floor. > There won’t be any internal forces Yes, there will, because the object's internal state (including its shape, size, and internal stresses) when it is accelerated is different from its shape when it is in free fall. Why? Because the acceleration sets up internal forces in the object that result in a different equilibrium from the one it was in while it was freely falling.
  - Ma8ee 3y ago
    > It reads nonzero when you are standing on Earth because of electromagnetic repulsion between your atoms and the atoms in the floor. It works because there's an external force pushing on the surface of the phone, and not equally on all its parts, which is the scenario we are discussing. > Because the acceleration sets up internal forces in the object that result in a different equilibrium from the one it was in while it was freely falling. And the cause of this is what you fail to explain.
    - johnbcoughlin 3y ago
      I think the burden of explanation is on the great great grandparent post which proposed an accelerometer "entirely made out of the same magnetic field" which would let it satisfy an equivalence principle for "magnetic force".
    - pdonis 3y ago
      > It works because there's an external force pushing on the surface of the phone Which then gets transmitted by the surface of the phone to the rest of the phone. > not equally on all its parts Yes, equally on all its parts, once you take into account that the parts of the phone can exert forces on each other. > the cause of this is what you fail to explain Cause of what? The internal forces? That's obvious: the distances between adjacent atoms change, and the electromagnetic forces between adjacent atoms are distance dependent. In other words, as I have already pointed out, the size and shape of the object changes when it is accelerated (in the sense of proper acceleration), compared to when it is not. Or, to put it another way, the equilibrium state of the object is different when it is accelerated than when it is not, with different inter-atomic distances and therefore different internal forces (as in, nonzero internal forces when accelerated, compared to zero when not).
itishappy 3y ago
> 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.

by pdonis 3y ago

hammock 3y ago

In other words, there is no gravity field, in the way there is an EM field that propagates these forces. Or, the "gravity field" is the fabric of space-time itself

pdonis 3y ago
Yes.

BoiledCabbage 3y ago

> The acceleration meter won't measure anything because there is nothing to measure. Put this way, isn't it almost begging the question? In GR the definition of acceleration is movement in contrast with the movement of gravity. If course gravity will never meet this criteria - all movement due to gravity will be aligned with movement due to gravity. If instead we had a universe where instead of all matter having a gravitational effect, it was that all matter had a magnetic effect the we'd see no acceleration due to the magnetic effect and gravity would "produce a field" and cause acceleration in your above examples. You can't use a gravitational biased tool to proclaim gravity is a neutral actor and everything else is a field. It seems like more accurately, everything is "gravitationally charged", so instead we say it warps spacetime, but really is no different.

pdonis 3y ago
> Put this way, isn't it almost begging the question? In GR the definition of acceleration is movement in contrast with the movement of gravity. No, it isn't. You have it backwards. The definition of acceleration in GR is proper acceleration, i.e., what an accelerometer reads. The "movement" property is then a consequence of this plus picking an appropriate frame of reference. > If instead we had a universe where instead of all matter having a gravitational effect, it was that all matter had a magnetic effect the we'd see no acceleration due to the magnetic effect Yes, you would, because unlike gravity, magnetism does not obey the equivalence principle, so differently charged objects in the same magnetic field with the same initial conditions can have different motions. With gravity, all objects in the same field with the same initial conditions have the same motion, regardless of their mass. That is why it is possible to model gravity using spacetime curvature, and that property is unique to gravity.
- lostmsu 3y ago
  I'm sure you are familiar with Kalusa-Klein theory (knowing that it is incomplete and/or doesn't describe our universe), don't you think that motion in that theory is less bound to gravity alone like the parent comment suggests because the notion of proper time is different? I tend to gravitate toward the same line of thinking because of the existence of black hole charge limit.
lostmsu 3y ago
You might be interested in https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory

Ma8ee 3y ago

> Yes. 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 genuinely curious how that acceleration meter would work. There won’t be any internal forces as a consequence of the external field and no relative motion.

pdonis 3y ago
> I’m genuinely curious how that acceleration meter would work. Look up how the one in your phone works. It reads nonzero when you are standing on Earth because of electromagnetic repulsion between your atoms and the atoms in the floor. > There won’t be any internal forces Yes, there will, because the object's internal state (including its shape, size, and internal stresses) when it is accelerated is different from its shape when it is in free fall. Why? Because the acceleration sets up internal forces in the object that result in a different equilibrium from the one it was in while it was freely falling.
- Ma8ee 3y ago
  > It reads nonzero when you are standing on Earth because of electromagnetic repulsion between your atoms and the atoms in the floor. It works because there's an external force pushing on the surface of the phone, and not equally on all its parts, which is the scenario we are discussing. > Because the acceleration sets up internal forces in the object that result in a different equilibrium from the one it was in while it was freely falling. And the cause of this is what you fail to explain.
  - johnbcoughlin 3y ago
    I think the burden of explanation is on the great great grandparent post which proposed an accelerometer "entirely made out of the same magnetic field" which would let it satisfy an equivalence principle for "magnetic force".
  - pdonis 3y ago
    > It works because there's an external force pushing on the surface of the phone Which then gets transmitted by the surface of the phone to the rest of the phone. > not equally on all its parts Yes, equally on all its parts, once you take into account that the parts of the phone can exert forces on each other. > the cause of this is what you fail to explain Cause of what? The internal forces? That's obvious: the distances between adjacent atoms change, and the electromagnetic forces between adjacent atoms are distance dependent. In other words, as I have already pointed out, the size and shape of the object changes when it is accelerated (in the sense of proper acceleration), compared to when it is not. Or, to put it another way, the equilibrium state of the object is different when it is accelerated than when it is not, with different inter-atomic distances and therefore different internal forces (as in, nonzero internal forces when accelerated, compared to zero when not).

itishappy 3y ago

> 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.