In Ensteins theroy of gravity.when a cannon ball is fired up,it

[DrGreg]

It is worth pointing out that if gravitons exist they relate to gravity waves which occur only when there is a change in gravity. When gravity remains constant over time there are no gravity waves and therefore no gravitons (if gravitons exist at all).

 

[IcedEcliptic]

It is worth pointing out that if gravitons exist they relate to gravity waves which occur only when there is a change in gravity. When gravity remains constant over time there are no gravity waves and therefore no gravitons (if gravitons exist at all).

Does the existence of G-Waves necessitate gravitons?

 

[D H]

Now when the object falls to the surface of the Earth and stops, it experiences a real force when resting on the surface, because an accelerometer attached to it shows a non-zero acceleration reading. When it stops on the surface of the Earth, it is no longer following a geodesic and it experiences gravity as a real force.

No! It experiences the normal force as a real force. The normal force is an electromagnetic phenomenon. Gravitation in general relativity is not a real force, a real force defined here as "something that causes an accelerometer to read other than zero."

In short, gravity is a real force acting on an object when the object is not following a geodesic, but when the object is following a geodesic (free-falling) the force of gravity is not real.

That is overly convoluted and wrong. A real force is required to make an object not follow a geodesic, and that real force is not gravitation.

Consider the Vomit Comet, an airplane used by NASA to make astronaut candidates and others lose the contents of their stomachs. Suppose you want to estimate the plane's trajectory based on some initial position and velocity plus accelerometer readings taken at regular intervals throughout the flight. You will need dv/dt to construct that estimated trajectory. One problem here: The accelerometer does not measure dv/dt, ever. It measures the non-gravitational component of dv/dt. To get dv/dt you will need to estimate the gravitational acceleration.

 

[D H]

Does the existence of G-Waves necessitate gravitons?

No. It is general relativity that necessitates gravitational waves. By way of analogy, Maxwell's equations necessitated that electromagnetic waves emanate from an accelerating charged particle. That photons are the quantum of the electromagnetic interaction came well after the development of Maxwell's equations. The specific way in which gravitons mediate gravitation, and whether gravitons exist at all, is yet to be developed.

 

[IcedEcliptic]

No. It is general relativity that necessitates gravitational waves. By way of analogy, Maxwell's equations necessitated that electromagnetic waves emanate from an accelerating charged particle. That photons are the quantum of the electromagnetic interaction came well after the development of Maxwell's equations. The specific way in which gravitons mediate gravitation, and whether gravitons exist at all, is yet to be developed.

Good, I was becoming very confused with the introduction of gravitons here.

 

[latter]

DB WROTE
As we walk on Earth an accelerating force of ~9.8 m/s^2 is acting upon us, keeping us grounded. This is because we are forced to be following a geodesic (a straight line). As our planet is curved, our geodesic path is a curve aswell. The bigger the spherical planet (as are all) the stronger the force must be to keep its atmosphere at a geodesic pace. It's the curved path in space time that creates the acceleration.

is this correct i am coming a little confused about whether there is a force that keeps us on Earth or not.by a force i mean something like gravitons or something equal.
if it is a force like this i don't see how Einstiens gravity works for if you move away from Earth slightly you don't need force but on the Earth a real force is needed or not?

 

[yuiop]

On the note of gravitons, are they really expected? Could that not be an area where GR and QM just don't get along.

I don't know much about gravitons and nor does anyone else, it seems. In QM all forces except gravitaional force are explained by particles and it would be "nice" if the same was true for gravity. If gravitons do not exist, then that is one less area for GR and QM not to get along in. If gravitons do exist, then presumably either GR or QM will have to be modified to make them compatible, or they are already compatible and we just have not made the connection yet ... or there is duality like we have for the wave and particle behavior of elementary particles.

Now when the object falls to the surface of the Earth and stops, it experiences a real force when resting on the surface, because an accelerometer attached to it shows a non-zero acceleration reading. When it stops on the surface of the Earth, it is no longer following a geodesic and it experiences gravity as a real force.

No! It experiences the normal force as a real force. The normal force is an electromagnetic phenomenon. Gravitation in general relativity is not a real force, a real force defined here as "something that causes an accelerometer to read other than zero."

I thought we were saying the same thing. Maybe I am missing something. Please elaborate.

In short, gravity is a real force acting on an object when the object is not following a geodesic, but when the object is following a geodesic (free-falling) the force of gravity is not real.

That is overly convoluted and wrong. A real force is required to make an object not follow a geodesic, and that real force is not gravitation.

Maybe what I said is overly convoluted but I am not convinced it is outright wrong. If the real force (as measured by an accelerometer) acting an object resting on a table or rolling down an incline is not due to gravitation, please tell me what it is due to?

<EDIT>

Gravitation in general relativity is not a real force, a real force defined here as "something that causes an accelerometer to read other than zero."

O.K. Let us consider an accelerometer laying on a table. It shows an non zero acceleration and therefore a real force is acting on it. The direction of the acceleration is upwards and so the accelerometer is indicating the reaction force of the table on the accelerometer. Is that what you are getting at? The accelerometer shows the real reaction force that reacts in response to the pseudo force of gravity?

 

[D H]

Maybe what I said is overly convoluted but I am not convinced it is outright wrong. If the real force (as measured by an accelerometer) acting an object resting on a table or rolling down an incline is not due to gravitation, please tell me what it is due to?

The normal force is not gravitational. It is a manifestation of electrostatic repulsion. It is the normal force that the accelerometer is measuring, not gravitation.

 

[yuiop]

The normal force is not gravitational. It is a manifestation of electrostatic repulsion. It is the normal force that the accelerometer is measuring, not gravitation.

O.K. it does look like you are talking about electrostatic reaction force of the intermolecular forces between the molecules that make up the table, so I think I now understand what you are getting at. So... a particle follows a geodesic unless a force acts upon it and in the case of a particle resting on a surface the force is provided by the surface.