6 ms·

I had a prof explain it using basic analogy. Forces are things that can move other things. An electrical field can move a metal ball. So too can a metal bat.

I had a prof explain it using basic analogy. Forces are things that can move other things. An electrical field can move a metal ball. So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration. No force is acting upon a ball. Put that metal ball in an eccentric orbit around the earth and it will speed up and slow down during each orbit, but the g-meter will register zero acceleration. The ball falls but is no not accelerated. So gravity is not a force because it doesn't move things. Gravity is something different.

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
- 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
ajross 3y ago
This is a clever analogy, but it's actually a little specious. The reason the "g-meter" (e.g. a weight on a scale) doesn't move in the gravity case is that the weight is affected by the same field. The weight is a weight and feels gravity just like you and everything else in your environment does. But by construction, you're imagining that the scale you have holds a different electrical charge than the object to which it's attached. Which is "normal" according to our everyday experience, but just an artifact of the way charges work on large objects (they distribute themselves on the "outside" of a conductive environment and everything inside tends to have a neutral distribution). But that's just arbitrary. You could equally demand (in your gedankenexperiment, though doing this in practice would be very difficult) that your electrical charge be distributed just like the mass is, in which case the force measured would be zero too.
- User23 3y ago
  As an aside there are real gravimeters that aren’t scales and they are sensitive enough to detect a person walking around the room they are in and snow accumulating on that building’s roof. Yes that’s right they detect the gravitational force exerted by the snow’s mass.
  - pdonis 3y ago
    > Yes that’s right they detect the gravitational force exerted by the snow’s mass. No, they don't. Gravimeters of the type you describe measure the coordinate acceleration of a freely falling test object in the accelerated frame of the gravimeter. In other words, it's the gravimeter (the part that isn't the freely falling test object) that has a force acting on it, which makes it accelerate upward (proper acceleration--an accelerometer attached to the gravimeter reads nonzero), and a freely falling test mass therefore appears to accelerate downward (coordinate acceleration in the frame of the gravimeter), just as if the gravimeter were inside an accelerating rocket out in deep space far from all gravitating bodies. In other words, gravimeters of this type rely on the equivalence principle, which is the same principle that GR uses to justify the statement that gravity is not a force.
  - ben_w 3y ago
hammock 3y ago
> put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration Put an electrical field generator on a falling metal ball and it will detect changing electric field though...
- coldtea 3y ago
  Well, a changing electric field is not a force tho.
nyrikki 3y ago
Gravity in GR is a 'fictitious force' or an apparent force, meaning it is is an artifact of your chosen frame of reference. A feather falling the same speed as an elephant in the Earth reference frame is an example. It is the same as other apperant forces like centrifugal force. Even Newton himself said 'Hypotheses non fingo,' or 'I feign no hypotheses. ' It was a conjecture.
trhway 3y ago
> So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration. That really depends on construction of your g-meter. If instead of mass (ie. gravitational charge) you use electric charge in your accelerometer then that electric charge on the falling ball - ie. moving with acceleration - will generate EM wave thus providing clear detection of acceleration. Wrt. the "boson" - gravity effects propagate with finite speed, i.e. wave, and the neutron in gravitational potential experiment shows that the gravitational potential/energy is quantized, and thus we have wave and quantized nature -> boson (wave packet/quant mediating interaction of a charge with the field).
- ultrafilter 3y ago
  Your charge-based g-meter would also measure zero acceleration in free fall. https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charged_particles_in_a_gravitational_field https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charge...
  - lostmsu 3y ago
    > the radiation goes into a region of spacetime inaccessible to the co-accelerating, supported observer. In effect, a uniformly accelerated observer has an event horizon, and there are regions of spacetime inaccessible to this observer Curious interpretation, but beware this bit wasn't substantiated that well.
moi2388 3y ago
Yeah this never made sense to me. Yes, it detects no acceleration. Great. But we know it IS accelerating; if you fall your speed will increase, this can only occur if you are accelerating?!
- lupire 3y ago
  How would you measure this alleged speed increase, internally? You can't.
  - zackees 3y ago
    [dead]
  - moi2388 3y ago
    I sure can’t. But why would I want to measure speed increase internally, when it’s due to an external field? Quick question, how would I even measure speed itself internally? I thought motion and speed was always measured relative to something else, why would an increase of these properties then have to be internal? Not trying to be a dick, I’m genuinely curious. And yes, I obviously do not know anything about physics. Any explanation or link to a source to help me understand?

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
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

ajross 3y ago

This is a clever analogy, but it's actually a little specious. The reason the "g-meter" (e.g. a weight on a scale) doesn't move in the gravity case is that the weight is affected by the same field. The weight is a weight and feels gravity just like you and everything else in your environment does. But by construction, you're imagining that the scale you have holds a different electrical charge than the object to which it's attached. Which is "normal" according to our everyday experience, but just an artifact of the way charges work on large objects (they distribute themselves on the "outside" of a conductive environment and everything inside tends to have a neutral distribution). But that's just arbitrary. You could equally demand (in your gedankenexperiment, though doing this in practice would be very difficult) that your electrical charge be distributed just like the mass is, in which case the force measured would be zero too.

User23 3y ago
As an aside there are real gravimeters that aren’t scales and they are sensitive enough to detect a person walking around the room they are in and snow accumulating on that building’s roof. Yes that’s right they detect the gravitational force exerted by the snow’s mass.
- pdonis 3y ago
  > Yes that’s right they detect the gravitational force exerted by the snow’s mass. No, they don't. Gravimeters of the type you describe measure the coordinate acceleration of a freely falling test object in the accelerated frame of the gravimeter. In other words, it's the gravimeter (the part that isn't the freely falling test object) that has a force acting on it, which makes it accelerate upward (proper acceleration--an accelerometer attached to the gravimeter reads nonzero), and a freely falling test mass therefore appears to accelerate downward (coordinate acceleration in the frame of the gravimeter), just as if the gravimeter were inside an accelerating rocket out in deep space far from all gravitating bodies. In other words, gravimeters of this type rely on the equivalence principle, which is the same principle that GR uses to justify the statement that gravity is not a force.
- ben_w 3y ago

hammock 3y ago

> put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration Put an electrical field generator on a falling metal ball and it will detect changing electric field though...

coldtea 3y ago
Well, a changing electric field is not a force tho.

nyrikki 3y ago

Gravity in GR is a 'fictitious force' or an apparent force, meaning it is is an artifact of your chosen frame of reference. A feather falling the same speed as an elephant in the Earth reference frame is an example. It is the same as other apperant forces like centrifugal force. Even Newton himself said 'Hypotheses non fingo,' or 'I feign no hypotheses. ' It was a conjecture.

trhway 3y ago

> So too can a metal bat. put a g-meter on the ball and it will register acceleration as force is applied to the ball. Gravity is different. Put a g-meter on a falling metal ball and it will detect zero acceleration. That really depends on construction of your g-meter. If instead of mass (ie. gravitational charge) you use electric charge in your accelerometer then that electric charge on the falling ball - ie. moving with acceleration - will generate EM wave thus providing clear detection of acceleration. Wrt. the "boson" - gravity effects propagate with finite speed, i.e. wave, and the neutron in gravitational potential experiment shows that the gravitational potential/energy is quantized, and thus we have wave and quantized nature -> boson (wave packet/quant mediating interaction of a charge with the field).

ultrafilter 3y ago
Your charge-based g-meter would also measure zero acceleration in free fall. https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charged_particles_in_a_gravitational_field https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charge...
- lostmsu 3y ago
  > the radiation goes into a region of spacetime inaccessible to the co-accelerating, supported observer. In effect, a uniformly accelerated observer has an event horizon, and there are regions of spacetime inaccessible to this observer Curious interpretation, but beware this bit wasn't substantiated that well.

moi2388 3y ago

Yeah this never made sense to me. Yes, it detects no acceleration. Great. But we know it IS accelerating; if you fall your speed will increase, this can only occur if you are accelerating?!

lupire 3y ago
How would you measure this alleged speed increase, internally? You can't.
- zackees 3y ago
  [dead]
- moi2388 3y ago
  I sure can’t. But why would I want to measure speed increase internally, when it’s due to an external field? Quick question, how would I even measure speed itself internally? I thought motion and speed was always measured relative to something else, why would an increase of these properties then have to be internal? Not trying to be a dick, I’m genuinely curious. And yes, I obviously do not know anything about physics. Any explanation or link to a source to help me understand?