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But the acceleration meter won't measure any force because gravity is acting on every part of it uniformly. If you had an acceleration meter entirely made out

by gwd 3y ago

But the acceleration meter won't measure any force because gravity is acting on every part of it uniformly. 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, or would it read zero acceleration, since all parts of it were being acted on uniformly (and thus didn't "notice" any acceleration)?

gwd 3y ago
(Replying to myself) I guess one answer to this is that particles which are (supposedly?) massless, like the photon, are affected by the space-time warping effects of gravity. A parallel construction wouldn't be true of magnetism or an electric field. Furthermore, when we detect gravity waves, they come at the same time as corresponding gamma ray bursts; since the gamma rays are affected by the space-time warping effects of gravity, this means that the gravity waves themselves are affected by the space-time warping effects of gravity. So gravity probably is something else. But who knows!
- lupire 3y ago
  That could also be explained by light having momentum which is affected by (but does not generate) the force of gravity acts like mass.
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.
Geee 3y ago
Yeah, if every part of human body is accelerated equally, there's no way to feel anything (you could see it). No matter how hard you get hit or thrown around, you can't measure it internally. Each cell can only feel their neighbors, and there's no internal stress / force between cells.
- saagarjha 3y ago
  This isn’t true. Try jumping off a building and you’ll find the acceleration to be quite noticeable ;)
  - stetrain 3y ago
    Only because you feel the air rushing past, and the impact with the ground. If you do this in a capsule where the air is moving with you, and avoid hitting the ground, we call the sensation "weightlessness" or "zero g", like is experienced by astronauts in orbit. Gravity is absolutely acting on astronauts orbiting the earth, at nearly the same strength as if they were standing on the ground. Depending on the shape of the orbit their linear speed may be increasing or decreasing, and they are definitely experiencing directional acceleration as their path bends in a circle around the Earth. But internally there is no bodily sensation of acceleration. It feels the same as floating, or free fall without the air rushing past.
    - saagarjha 3y ago
      Yes but the feeling of weightlessness is something you can distinguish!
      - stetrain 3y ago
        But you don't feel the force of acceleration. Floating weightless and motionless in space feels the same as being in an elliptical orbit around a planet.
        lupire 3y ago

by gwd 3y ago

gwd 3y ago

(Replying to myself) I guess one answer to this is that particles which are (supposedly?) massless, like the photon, are affected by the space-time warping effects of gravity. A parallel construction wouldn't be true of magnetism or an electric field. Furthermore, when we detect gravity waves, they come at the same time as corresponding gamma ray bursts; since the gamma rays are affected by the space-time warping effects of gravity, this means that the gravity waves themselves are affected by the space-time warping effects of gravity. So gravity probably is something else. But who knows!

lupire 3y ago
That could also be explained by light having momentum which is affected by (but does not generate) the force of gravity acts like mass.

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.

Geee 3y ago

Yeah, if every part of human body is accelerated equally, there's no way to feel anything (you could see it). No matter how hard you get hit or thrown around, you can't measure it internally. Each cell can only feel their neighbors, and there's no internal stress / force between cells.

saagarjha 3y ago
This isn’t true. Try jumping off a building and you’ll find the acceleration to be quite noticeable ;)
- stetrain 3y ago
  Only because you feel the air rushing past, and the impact with the ground. If you do this in a capsule where the air is moving with you, and avoid hitting the ground, we call the sensation "weightlessness" or "zero g", like is experienced by astronauts in orbit. Gravity is absolutely acting on astronauts orbiting the earth, at nearly the same strength as if they were standing on the ground. Depending on the shape of the orbit their linear speed may be increasing or decreasing, and they are definitely experiencing directional acceleration as their path bends in a circle around the Earth. But internally there is no bodily sensation of acceleration. It feels the same as floating, or free fall without the air rushing past.
  - saagarjha 3y ago
    Yes but the feeling of weightlessness is something you can distinguish!
    - stetrain 3y ago
      But you don't feel the force of acceleration. Floating weightless and motionless in space feels the same as being in an elliptical orbit around a planet.
      - lupire 3y ago