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

In summary, the conversation discusses the concept of gravity as a force and whether or not it can be sensed. Some argue that it is a real force because it can be measured, while others argue that it is not a force but rather a result of the bending of space-time. The idea of normal force is brought up, with some saying it is a consequence of electrostatic repulsion or exchange interaction, not gravity. The conversation also delves into the effects of gravity on objects in free fall and the difficulty in sensing it due to the even acceleration of all parts of an object.
  • #1
physics.alex
28
0
HI,
According to GR, gravitation is simply the geometry bending. Does this also apply on Earth? We can stick on Earth, does it the force attract us?? Or is a geometry problem as well??

I get confused about this.

Thanks
Alex
 
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  • #2
Regardless of what any theory says - gravity is a force - you can feel it. It just so happens that the 'charge' ( gravitational mass) is the same as the resistance to movement ( inertial mass) so you can make a mathematical model in which the force becomes geometry.
 
  • #3
Thanks!
So, the light bending (near Sun) is due to force basically.

One more question, do we also suffer from gravitation wave on Earth as well??
Alex
 
  • #4
Remember that Newtonian gravity also predicts light bending by the sun.
I suppose if gravitational radiation is out there, we are exposed to it.

There's a good discussion in the concurrent thread 'Is gravity an inertial force ?'.
 
  • #5
Mentz114 said:
Regardless of what any theory says - gravity is a force - you can feel it.
I disagree. You don't feel gravity. What you feel when standing on the surface of the Earth is the normal force propagating through your body. Suppose you take one of those amusement parks rides that take a sudden 1g drop. Right before the bottom falls out on the ride you don't feel anything different than you did 100 meters or so down when you were standing on the solid Earth. When the ride let's loose you no longer feel that the normal force propagating through your body. There is no change in gravitational force, but your equilibrioceptors nonetheless are telling you that something is very, very wrong.

The same goes for accelerometers, and gravimeters, and any other local experiment that purportedly measures gravitation. Those sensors measure everything but gravity. The way to explain this in Newtonian mechanics terms is to mystically invoke gravitation as the only real force that cannot be sensed. The general relativistic explanation is much easier: gravitation is not a real force. Of course you can't sense it.
 
  • #6
If gravity is a force or not is a matter of semantics.

D H said:
The general relativistic explanation is much easier: gravitation is not a real force. Of course you can't sense it.
Of course we can sense it otherwise there would be nothing to measure. Masses attracting, bodies in orbit, time dilation, light bending. There are more things in nature to be sensed than acceleration.

Also, free falling (or floating) objects will experience, albeit extremely small in most cases, tidal forces.
 
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  • #7
DH said:
I disagree. You don't feel gravity. What you feel when standing on the surface of the Earth is the normal force propagating through your body.
I knew someone would. What name would you give the normal force ? I know some forces can be transformed away, but the frame in which it's gone, is itself moving under the influence of ... gravity.

gravitation is not a real force
You are inviting the question "what do you mean by 'real' ?"

PassionFlower said:
Of course we can sense it otherwise there would be nothing to measure. Masses attracting, bodies in orbit, time dilation, light bending. There are more things in nature to be sensed than acceleration.
Agree. The acceleration in the direction of the tangent vector is nothing for a geodesic, but there are still effects orthogonal to the tangent plane.
 
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  • #8
there is a basic law mentz...
any motion is due to a force...
irrespective of the force on the force...
there is my Earth moving round the sun due to gravitational force...
now consider a pendulum covering 360 degrees in one cycle...
the thread here is the motion controller n ultimately the force controller
But still the Earth follows a projectile motion around the Sun... And the pendulum follows rotational motion...
And please a projectile on Earth is also in an infinite frame of ref. still only a force is making it possible...
Gravity is a force... Saying that its a geometrical drama of the space time fabric wud suffice only the definition of force...
 
  • #9
Mentz114 said:
I knew someone would. What name would you give the normal force ?
Depending on who you talk to, the normal force is a consequence of electrostatic repulsion or the exchange interaction. Certainly not gravity. Suppose you push against a wall. The wall exerts a horizontal normal force on you. That is the normal force.

You are inviting the question "what do you mean by 'real' ?"
The opposite of fictitious.
 
  • #10
D H said:
I disagree. You don't feel gravity. What you feel when standing on the surface of the Earth is the normal force propagating through your body. Suppose you take one of those amusement parks rides that take a sudden 1g drop. Right before the bottom falls out on the ride you don't feel anything different than you did 100 meters or so down when you were standing on the solid Earth. When the ride let's loose you no longer feel that the normal force propagating through your body. There is no change in gravitational force, but your equilibrioceptors nonetheless are telling you that something is very, very wrong.

The same goes for accelerometers, and gravimeters, and any other local experiment that purportedly measures gravitation. Those sensors measure everything but gravity. The way to explain this in Newtonian mechanics terms is to mystically invoke gravitation as the only real force that cannot be sensed. The general relativistic explanation is much easier: gravitation is not a real force. Of course you can't sense it.



Although I know this is common explanation given when talking about gravity as force or not, I still have big issue with it.

How can any instrument register any force in free fall when every and each part of it, is subjected to the same acceleration (I know that you will object here that things in free fall are not accelerating, but let's stick to Newtonian view). It simply can't.

Same goes for humans. You can't feel anything if every part of you is accelerated evenly.

So because gravity influence everything proportional to the mass, and that things end up evenly accelerated (F=ma) does not mean that it is not a force.
 
  • #11
Calimero said:
Same goes for humans. You can't feel anything if every part of you is accelerated evenly.

I suggest you try out your theory in an elevator. Tell me if you can feel the push when you are accelerated upwards.
 
  • #12
espen180 said:
I suggest you try out your theory in an elevator. Tell me if you can feel the push when you are accelerated upwards.

Hi But in an elevator every part of you is not accelerated evenly. The acceleration starts at your feet and propagates through your body from there. It is the inertial resistance to this propagating added momentum that causes the "feeling" of force.
No?
 
  • #13
In elevator you can feel push, because elevator is exerting force on your legs which is then transferred through your body (organs, blood, bones, etc..). When you are accelerating due to the gravity you don't feel a thing, because every part of you is accelerated evenly.
 
  • #14
Passionflower said:
If gravity is a force or not is a matter of semantics.


Of course we can sense it otherwise there would be nothing to measure. Masses attracting, bodies in orbit, time dilation, light bending. There are more things in nature to be sensed than acceleration.

Also, free falling (or floating) objects will experience, albeit extremely small in most cases, tidal forces.

Profound agreement on the semantics.

On sensing and measuring it . Couldn't you say that you are actually measuring the effects?
IN principle you can measure all the effects of inertial motion; dilation, contraction etc.
but cannot sense them within a system and we consider them a result of spacetime geometry with no force involved and so unreal.

Are there actual experimental observations of tidal forces?
 
  • #15
Austin0 said:
Hi But in an elevator every part of you is not accelerated evenly. The acceleration starts at your feet and propagates through your body from there. It is the inertial resistance to this propagating added momentum that causes the "feeling" of force.
No?

Calimero said:
In elevator you can feel push, because elevator is exerting force on your legs which is then transferred through your body (organs, blood, bones, etc..). When you are accelerating due to the gravity you don't feel a thing, because every part of you is accelerated evenly.

Exactly. You feel a force when you are accelerated. Therefore, you don't accelerate when you are in free fall. The normal force you feel when standing on Earth is there because you are being accelerated.
 
  • #16
D H said:
Depending on who you talk to, the normal force is a consequence of electrostatic repulsion or the exchange interaction. Certainly not gravity. Suppose you push against a wall. The wall exerts a horizontal normal force on you. That is the normal force.


The opposite of fictitious.

Are you suggesting that the force normal to the ground that you feel standing is due to electrostatic exchange or interaction?
As apposed to the individual particles of your body being in inertial motion on geodesics and meeting resistance from the ground?? This resistance to inertia propagates through your body succeeding altitudes inertially pushing on the proceeding and this is what you feel?
Still a fictitious force of course . But then isn't the normal force exerted by the wall a totally fictitious force??
 
  • #17
Austin0 said:
But then isn't the normal force exerted by the wall a totally fictitious force??
:confused: Why would you think that the normal force is fictitious?
 
  • #18
espen180 said:
Exactly. You feel a force when you are accelerated. Therefore, you don't accelerate when you are in free fall. The normal force you feel when standing on Earth is there because you are being accelerated.

This exemplifies a strange confusion and mixup of Newton and GR within the forum.

Newton------- G is a force ----> -r

GR -------- G is not a force but a geometry.

GR --------- G is a force but -----> +r
 
  • #19
espen180 said:
Exactly. You feel a force when you are accelerated. Therefore, you don't accelerate when you are in free fall. The normal force you feel when standing on Earth is there because you are being accelerated.

That is simply not truth. You feel force when you are accelerated by mechanical means.

And you are not accelerated when you are standing on earth, because it would mean that work is being done on you, and that some energy is being transfered. Work is force times distance, and since distance is zero, work is also zero.
 
  • #20
Austin0 said:
This exemplifies a strange confusion and mixup of Newton and GR within the forum.

Newton------- G is a force ----> -r

GR -------- G is not a force but a geometry.

GR --------- G is a force but -----> +r

In GR, gravity is a ficticious force, or psuedo-force. The gravitational field can be made to vanish by a choice of coordinates. In an accelerating reference frame, like that of the surface of the Earth, gravity accelerates things and is a force. But in an intertial or locally inertial frame, gravity does not accelerate things and is not a force. Therefore, it is on equal standing as the centrifugal force.

Calimero said:
That is simply not truth. You feel force when you are accelerated by mechanical means.

And you are not accelerated when you are standing on earth, because it would mean that work is being done on you, and that some energy is being transfered. Work is force times distance, and since distance is zero, work is also zero.

Distance is a coordinate dependent quantity, and so is work.
 
  • #21
Doc Al said:
:confused: Why would you think that the normal force is fictitious?

Because the force involved is being exerted by your muscles. The force you feel is meeting the constraint of the wall .

Isn't this equivalent to the fictitious centripetal force encountered by particles accelerated by the fictitious centrifugal force in a centrifuge when they meet the constraint of the wall of the machine??
So isn't the wall equivalent to the bounds of the centrifuge?

I have a question: COuldnt you look at particle collisions and accelerations as transfers of momentum with conservation but without any need for a concept of force?
As inertial paths with only relative coordinate changes in velocity and direction.
Equivalent to a particle in inertial motion in curved spacetime with only relative coordinate changes in velocity??
The concept of Force would then be a useful bookkeeping procedure but essentially fictitious . The same as Newtonian gravity, centrifugal etc.
Thanks
 
  • #22
Austin0 said:
Because the force involved is being exerted by your muscles. The force you feel is meeting the constraint of the wall .
Sounds pretty real and non-fictitious to me! The normal force is a contact force between two surfaces--very real in any frame.

Isn't this equivalent to the fictitious centripetal force encountered by particles accelerated by the fictitious centrifugal force in a centrifuge when they meet the constraint of the wall of the machine??
The centripetal force is quite real. (The centrifugal force is a fictitious artifact of viewing things from a rotating frame.)
 
  • #23
espen180 said:
In GR, gravity is a ficticious force, or psuedo-force. The gravitational field can be made to vanish by a choice of coordinates. In an accelerating reference frame, like that of the surface of the Earth, gravity accelerates things and 1 )is a force. But in an intertial or locally inertial frame, gravity does not accelerate things and is not a force. Therefore, it is on equal standing as the centrifugal force.
QUOTE]

1) Did you mean to say here ficticious force?

So are you saying that; the surface of the Earth is accelerating, even though it is not moving let alone changing in velocity, on the basis of a hypothetical ficticious force?
 
  • #24
Austin0 said:
So are you saying that; the surface of the Earth is accelerating, even though it is not moving let alone changing in velocity, on the basis of a hypothetical ficticious force?

The surface of the Earth is indeed accelerating in space-time. Why? Because it deviates from geodesic motion. Free fall, that is.

Newton's first law states that when there are no forces on an object, it travels in a straight line. This is exactly what is meant when one says gravity is a geometric artifact. Gravity is not a force, so objects in free fall move in straight lines through space-time. Mass and energy curves space-time, so straight lines in space-time appear curved in space.
 
  • #25
=Doc Al;2785692]Sounds pretty real and non-fictitious to me! The normal force is a contact force between two surfaces--very real in any frame.
I understand what you are saying but simple contact between surfaces does not involve force. It is only when momentum is added to one surface or the other that there is resistance. I understand the third law and that in principle the force is equal and opposite but in actual practice in this example the muscles provide the momentum and the wall provides an overwelming inertia. Just as in the accelerating elevator the motor provides the muscle and momentum and we as passengers provide inertia which we feel as force.
Isn't what is experienced in both cases ficticious?


The centripetal force is quite real. (The centrifugal force is a fictitious artifact of viewing things from a rotating frame.)

In a centrifuge of course the bounding walls are real as is the momentum imparted to the contents but is the force any more real than in the elevator?
The centripetal force of Newtonian G acting against inertia to hold the planets in orbit is now viewed as ficticious. No?
 
  • #26
espen180 said:
In GR, gravity is a ficticious force, or psuedo-force. The gravitational field can be made to vanish by a choice of coordinates. In an accelerating reference frame, like that of the surface of the Earth, gravity accelerates things and is a force. But in an intertial or locally inertial frame, gravity does not accelerate things and is not a force. Therefore, it is on equal standing as the centrifugal force.

Distance is a coordinate dependent quantity, and so is work.



Suppose we have some force measuring device in electrostatic field. Imagine that every part of that device has charge directly proportional to its mass. Now, field is large enough that we can assume that parts of our device are equidistant from opposite charge. Would it be able to register any force? So electrostatic force is not a force? What is force?

If it comes to that that real force is something that is acting only on some parts of object, and that pseudo-forces are acting homogeneously throughout object, I really don't see any difference between the two.
 
  • #27
Austin0 said:
I understand what you are saying but simple contact between surfaces does not involve force. It is only when momentum is added to one surface or the other that there is resistance. I understand the third law and that in principle the force is equal and opposite but in actual practice in this example the muscles provide the momentum and the wall provides an overwelming inertia. Just as in the accelerating elevator the motor provides the muscle and momentum and we as passengers provide inertia which we feel as force.
Isn't what is experienced in both cases ficticious?
No, the contact forces are quite real.

In a centrifuge of course the bounding walls are real as is the momentum imparted to the contents but is the force any more real than in the elevator?
The contact force from the walls of a centrifuge is just as real as the contact force pushing on you in an elevator.
The centripetal force of Newtonian G acting against inertia to hold the planets in orbit is now viewed as ficticious. No?
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.
 
  • #28
espen180 said:
The surface of the Earth is indeed accelerating in space-time. Why? Because it deviates from geodesic motion. Free fall, that is.

Newton's first law states that when there are no forces on an object, it travels in a straight line. This is exactly what is meant when one says gravity is a geometric artifact. Gravity is not a force, so objects in free fall move in straight lines through space-time. Mass and energy curves space-time, so straight lines in space-time appear curved in space.

I am aware of the basics of the geometric (geodesic) concept. In fact I can see reason for the abolishment of the term force outside of electrodynamics and atomic structure.
Acceleration would have the purely kinematic interpretation of a change in velocity except in the case of accelerations through electromagnetic fields with no direct interaction of particles..
I understand every thing you have said and would agree within your context.
I am talking about the inconsistancy of interpretation and meaning of the terms force and acceleration.
Re; the surface of the Earth acceleration has no motion. In Einsteins elevator it has real motion. In both cases then the force must be considered ficticious to be consistent but many consider the force in the elevator real and the force of acceleration of the Earth surface ficticious. Etc. etc,...
 
  • #29
Calimero said:
Suppose we have some force measuring device in electrostatic field. Imagine that every part of that device has charge directly proportional to its mass. Now, field is large enough that we can assume that parts of our device are equidistant from opposite charge. Would it be able to register any force? So electrostatic force is not a force? What is force?

If it comes to that that real force is something that is acting only on some parts of object, and that pseudo-forces are acting homogeneously throughout object, I really don't see any difference between the two.

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.
 
  • #30
Austin0 said:
I am aware of the basics of the geometric (geodesic) concept. In fact I can see reason for the abolishment of the term force outside of electrodynamics and atomic structure.
Acceleration would have the purely kinematic interpretation of a change in velocity except in the case of accelerations through electromagnetic fields with no direct interaction of particles..
I understand every thing you have said and would agree within your context.
I am talking about the inconsistancy of interpretation and meaning of the terms force and acceleration.
Re; the surface of the Earth acceleration has no motion. In Einsteins elevator it has real motion. In both cases then the force must be considered ficticious to be consistent but many consider the force in the elevator real and the force of acceleration of the Earth surface ficticious. Etc. etc,...

It's all relative. :wink:

In GR, Newton's second law is upheld, when there is an acceleration, there is a force. Whether or not there is an acceleration depends on the reference frame.

In Einstein's elevator, the man inside is in full right to consider himself and the elevator stationary (no motion). Someone falling towards Earth can also consider himself stationary (or in intertial motion), and the people on Earth are accelerating towards him.

Anyway, I think there is a misunderstanding here. When I say that the people on Earth's surface are accelerating, I mean accelerating through Space-Time, not neccesarily in space. The two are very different.
 
  • #31
Mentz114 said:
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 :redface: But I second D H et al.
 
  • #32
Austin0 said:
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.
 
  • #33
espen180 said:
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.
 
  • #34
Calimero said:
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.
 
  • #35
espen180 said:
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.
 

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