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

[latter]

i did ask many times whether it stopped.i did think it did stop myself but there we go

IcedEcliptic wrote
If you place a ball on a hill, and halfway down I stop it with my foot, when I move my foot the ball continues to fall, because of the slope of the hill.

so my question now is why is the ball in this example .not stopped.or is it stopped
to me i would say the ball has stopped .And as i asked .Is it that in Einstiens gravity that if something truly stops it can not remove by gravity alone.
if this is true then the ball if it has stopped shouldn't move(due to gravity) when you remove your foot.
i mean the ball is stopped isn't it.this time

 

[yuiop]

i did ask many times whether it stopped.i did think it did stop myself but there we go


so my question now is why is the ball in this example .not stopped.or is it stopped
to me i would say the ball has stopped .And as i asked .Is it that in Einstiens gravity that if something truly stops it can not remove by gravity alone.
if this is true then the ball if it has stopped shouldn't move(due to gravity) when you remove your foot.
i mean the ball is stopped isn't it.this time

Well this is where it gets a little complicated, so bear with me. When an object is free-falling there are no real forces acting on it. (You can attach an accelerometer to it and see that the accelerometer always reads zero.) When I say free-falling, this includes the motion of the objects upwards and the motion of the object on its way back down (and the bit inbetween). Now although the motion can be up or down for a free-falling object, the coordinate acceleration is always downwards, on the way up , at the top of its trajectory and on the way back down. Even when the object appears momentarily stationary at the top it is always accelerating from the point of view of the observer on the Earth (even though its proper acceleration is always zero).

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. If the object is now released again down a mine shaft, it once again feels no real force (continues along a geodesic) and it appears to accelerate from the point of view of the observer that stays at the top of the mine shaft (coordinate acceleration). This observer at the top experiences a real force (he can feel the force of gravity on his feet and measure it with an accelerometer) and he can account for the motion of the falling ball in terms of his own acceleration just like the observer in the accelerating rocket in the previous post.

Note that the free-falling ball at the top of its trajectory, when it appears to stop, never measures any acceleration on an accelerometer attached to it, while when the ball is resting on the surface of the Earth an accelerometer does record a non-zero acceleration reading. You should be able to see now that "stopped" at the top of its trajectory is different from stopped when at rest on the ground.

When a ball is rolling down a hill, it also experiences a real force, because its geodesic is straight down so its diagonal path down the hill is not following a geodesic. It is only when the ball is exactly following its geodesic that it feels no real force.

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.

 

[latter]

thanks that will take me a long time to work threw.i i suspect your right.i may never be able to tell ,if i can't work it out.anyway thanks

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

now if i had know this i would of not posted this problem i think.
strange that i don't remmeber reading this.when you say gravity is a real force do you mean it suddenly has gravitons or something equal.
or just that objects have a feeling of a force.
to me gravity only has a real force if it has gravitons or something equal.

gravity is a real force acting on an object when the object is not following a geodesic,

.
if it doesn't suddenly have gravitons then how does it transmitt or pull a object that is on a hill(not following geodesic) but not moving in the first place.
sorry to re ask this you may have answered this above but i am rarther desperate to get this point only.

 

[yuiop]

thanks that will take me a long time to work threw.i i suspect your right.i may never be able to tell ,if i can't work it out.anyway thanks


now if i had know this i would of not posted this problem i think.
strange that i don't remmeber reading this.when you say gravity is a real force do you mean it suddenly has gravitons or something equal.
or just that objects have a feeling of a force.
to me gravity only has a real force if it has gravitons or something equal.
.
if it doesn't suddenly have gravitons then how does it transmitt or pull a object that is on a hill(not following geodesic) but not moving in the first place.
sorry to re ask this you may have answered this above but i am rarther desperate to get this point only.

Well, I can't answer your question as well as you would like, because there is no generally accepted theory for gravitons. General Relativity is not formulated in terms of gravitons, but there are a lot of (very clever) people trying to work out how a graviton based theory could be made compatible with GR, but as far as I know, they have not succeeded yet.

I guess the nearest I can come to an answer, is to say that a real force (proper force) is when an object has a feeling of force that can be measured by an accelerometer. You are right that it is not generally made clear that force of gravity on a stationary object is real.

Maybe the way gravitons interact with objects that are following geodesics, is different to how they interact with objects that are not following geodesics, perhaps in a way similar to how an accelerating observer sees Unrah radiation while an non-accelerating observer does not (See http://en.wikipedia.org/wiki/Unruh_effect ), but that is speculation and within the rules of this forum we probably shouldn't go there.

 

[IcedEcliptic]

Kev, thanks for your answers, you said this better than I could (post 32).

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

 

[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.