[sandworm101]

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]

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)?

[pdonis]

> 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.

[BoiledCabbage]

> 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]

> 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]

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]

You might be interested in https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory

[Ma8ee]

> 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]

> 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]

> 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.

[pdonis]

> 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).

[johnbcoughlin]

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".

[itishappy]

> 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]

> 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.

[jjk166]

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]

> 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.

[hammock]

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]

Yes.

[Geee]

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]

This isn’t true. Try jumping off a building and you’ll find the acceleration to be quite noticeable ;)

[stetrain]

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.

[ivell]

I did sky diving some years back. I did feel acceleration immediately after the jump for a second, but then afterwards it was as if I was lying on bed. So what was that feeling I experienced (it was similar to maintain giant wheel coming down)

[stetrain]

Going from standing on a surface where you are resisting the force of gravity (like standing on the ground, or in a stable aircraft) to being in free fall definitely has a sensation.

It's a sensation that actually causes a decent percentage of people to feel physically ill for a period after launching to orbit on a rocket.

It's like going over the top of a hill on a roller coaster where you go from being pulled down into your seat to floating up against the restraints.

But once you're falling, or or in orbit (which is also free fall just outside an atmosphere), you don't feel the changes in velocity (acceleration) due to gravity.

[saagarjha]

Yes but the feeling of weightlessness is something you can distinguish!

[stetrain]

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.

[Geee]

If the impact somehow could stop your all cells at the same rate, you wouldn't feel it. Your body gets compressed because of different rates of acceleration.

If aliens have tech which can apply this type of force field / acceleration, then they won't get squished in their spaceships no matter how hard they accelerate. You basically need a large force field like gravity, instead of transferring force via small intermolecular force fields.

[neolefty]

The point is you won't feel it (aside from air resistance) on the way down.

[8note]

The air too, you only feel because the ground and other air is pushing it upwards

[gwd]

(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]

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

[ajross]

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]

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]

> 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]

Rather more pertinently to the question "why is gravity different?", we can measure the time dilation caused by a gravitational potential change of less than 1cm near the surface of the Earth: https://physicsworld.com/a/gravitational-time-dilation-measu...

[nyrikki]

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]

> 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]

Your charge-based g-meter would also measure zero acceleration in free fall.

https://en.wikipedia.org/wiki/Paradox_of_radiation_of_charge...

[lostmsu]

> 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.

[hammock]

> 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]

Well, a changing electric field is not a force tho.

[moi2388]

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]

How would you measure this alleged speed increase, internally?

You can't.

[moi2388]

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?