Is Gravity Only a Force on Earth or a Geometry Problem as Well?

[Dale]

Regardless of what any theory says - gravity is a force - you can feel it.

No you can't. At least, not in a uniform field.

EDIT: Never mind, I see that I am 25 posts too slow But I second D H et al.

 

[Dale]

COuldnt you look at particle collisions and accelerations as transfers of momentum with conservation but without any need for a concept of force?

You can do Lagrangian and Hamiltonian mechanics using conservation and symmetry concepts and without needing the concept of forces.

 

[Calimero]

A measuring device of an electrostatic field probes the field with a known charge and measures the force acting on it, for example using a spring. It is possible to construct a similar apparatus for probing the gravitational field using a spring and a known mass, but it will not work. you will have created an accelerometer, which is unable to differenciate between gravitational acceleration and motional acceleration (the two are equivalent in GR). Therefore, such an apparatus will measure that it is being accelerated away from the center of the Earth when you hold it, standing on the surface.

You are failing to show how that differs from charge in electrostatic field.

My point is: If you are talking about forces, then gravity is force.

 

[espen180]

You are failing to show how that differs from charge in electrostatic field.

The very important difference is that charges of different magnitudes interract with the EM-field, getting accelerations of different magnitude, while masses of different magnitude interract with the gravitational field, gaining equal acceleration. The fact that the magnitude of a mass is irrelevant to its acceleration is the big difference.

 

[Calimero]

The fact that the magnitude of a mass is irrelevant to its acceleration is the big difference.

No, it is not. It only means that gravitational mass to inertial mass ratio is constant.

 

[Dale]

Hi Calimero, I am with espen180 on this. The fact that the gravitational force is proportional to the mass is what allows it to be removed by choice of reference frame. Notice, that the centrifugal and Coriolis forces in a rotating reference frame are also proportional to mass.

 

[Calimero]

Hi Calimero, I am with espen180 on this. The fact that the gravitational force is proportional to the mass is what allows it to be removed by choice of reference frame. Notice, that the centrifugal and Coriolis forces in a rotating reference frame are also proportional to mass.

I don't argue against that.
If we are talking about forces, then gravitational force does not differ from electrostatic force (assuming attractive charges), other then having constant gravitational/inertial mass ratio, unlike charge/mass ratio which may, or may be not constant in the case of electrostatic force.

I objected against arguments that accelerometers do not register acceleration in free fall, and that one can't feel anything other then weightlessness while in free fall. You really can't exclude gravity from being force based on that.

 

[espen180]

I don't argue against that.
If we are talking about forces, then gravitational force does not differ from electrostatic force (assuming attractive charges), other then having constant gravitational/inertial mass ratio, unlike charge/mass ratio which may, or may be not constant in the case of electrostatic force.

I objected against arguments that accelerometers do not register acceleration in free fall, and that one can't feel anything other then weightlessness while in free fall. You really can't exclude gravity from being force based on that.

EM and gravity are different on a much higher level.

The gravitational field can be made to disappear by a coordinate transformation - The EM-field cannot.
Gravity is caused by the stress-energy tensor, a 2-rank tensor - EM is caused by the four-current, a 1-rank tensor
Gravity influences space and time (universal dimensional properties) - EM influences charges and currents (individual particle properties)

The fact that E^2=\left(pc\right)^2+\left(mc^2\right)^2 means that everything that has either mass, momentum or energy is influenced by and sources gravity.

GM takes things like accelerometer measurements very seriously. If you don't rely on your measurements and observations, what can you rely on?

 

[Calimero]

EM and gravity are different on a much higher level.

The gravitational field can be made to disappear by a coordinate transformation - The EM-field cannot.
Gravity is caused by the stress-energy tensor, a 2-rank tensor - EM is caused by the four-current, a 1-rank tensor
Gravity influences space and time (universal dimensional properties) - EM influences charges and currents (individual particle properties)

The fact that E^2=\left(pc\right)^2+\left(mc^2\right)^2 means that everything that has either mass, momentum or energy is influenced by and sources gravity.

Well thanks for pointing that out. I assumed that you will understand context in which I meant that they don't differ.

GM takes things like accelerometer measurements very seriously. If you don't rely on your measurements and observations, what can you rely on?

We are here arguing semantics. Point is: you can talk about gravity as a force, and easily explain why accelerometers do not register acceleration. That is all.

 

[espen180]

We are here arguing semantics. Point is: you can talk about gravity as a force, and easily explain why accelerometers do not register acceleration. That is all.

Well of course you can say gravity is a force and accelerates objects in GR - in a reference frame that is itself accelerating.

 

[Dale]

I assumed that you will understand context in which I meant that they don't differ.

I don't understand that either. They seem to have much more in common with the centrifugal and Coriolis forces to me.

Do you also intend to admit centrifugal and Coriolis forces as forces? If not then on what basis do you exclude them and admit gravity?

 

[Austin0]

No, the contact forces are quite real.


The contact force from the walls of a centrifuge is just as real as the contact force pushing on you in an elevator.

Gravity is a special case. In Newtonian physics, gravity is a real force; in GR, it's an inertial force. But the contact force between objects is real.

Having given some thought I think I may have found a point of possible agreement.

COnsidering a collision interaction between two atoms;
Viewing from the transfer of momentum perspective the leading electron shells begin the transfer. The momentum of the nucleii causes them to continue forward until the electrostatic and internal atomic forces causes them to decrease and then cease net forward motion and the trailing shells are continuing under inertia to the same point . At some point the internal forces are sufficient to overcome the state of compression and stress and cause motion in the opposite direction which proceeds through the same sequence of transference as the internal equilibrium and symetry are re-established ultimately resulting in a reciprocal inertial coordinate path .
SO I can accept the totally valid logic and possible reality of your view; that the net reciprocal motion could result from the forces within. The relevant part of which is the repulsive aspect.
SO the equal reaction results from internal compression and expansion ,[not totally disimilar to a bouncing rubber ball].
SO I can agree with you
At the same time I don't think this possibility negates the possible validity of the view that these forces are simply acting as a medium for the transference of momentum.
And that momentum itself may prove to be an actual form of energy not just an equivalence or relative meeasurement. AN internal vibrational frequency which is mutually transferred through interaction.

If we look at electron or photon interactions it seems quite possible that the reactions are fundamentally due to wave interference . Possibly the waves passing through each other and determining the new paths through forward interference.
Certainly it seems strange to posit photon interactive path changes through a concept of a negative or repulsive force doesn't it?? ANd in this context a photon may not have mass but it does have momentum and so is not neccessarilly fundamentally different from an electron.
Thanks for your input.

 

[Austin0]

Well of course you can say gravity is a force and accelerates objects in GR - in a reference frame that is itself accelerating.

Doesnt it also accelerate things in a reference frame which is inertial but a rest wrt the center of gravity? Say a satellite in geo-synchronous orbit?